2019

Matthias Kroschel, Arman Bonakdarpour, Jason Tai Hong Kwan, Peter Strasser and David P. Wilkinson

Analysis of oxygen evolving catalyst coated membranes with different current collectors using a new modified rotating disk electrode technique

Electrochimica Acta, 313, 1-15

doi.org/10.1016/j.electacta.2019.05.011

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For the first time, the oxygen evolution reaction (OER) behavior of commercial catalyst-coated membranes (CCMs) has been studied using an in-house modified rotating disk electrode (MRDE) tip capable of reaching current densities of 2 A cm2. The electrochemical results are comparable with a full PEM electrolysis cell. The kinetic analysis reveals Tafel slopes of about 50 and 60 mV dec1 for all temperatures with the PEM electrolysis hardware and the MRDE, respectively. The activation energy of the OER obtained with the MRDE and the PEM electrolysis cell are 75 and 68 kJ mol1, respectively. Detailed cyclic voltammetric measurements of Ir-based CCMs were performed to examine the influence of operating temperature, Ti-based current collectors, the scan rate on the adsorbed charge, and the electro kinetics of the OER. Six different expanded metal Ti current collectors were analyzed in sulfuric acid and their impact on the CCM charging at various temperatures were  examined in order to calculate the electrode's inner and outer charge. It was observed that the voltammetric charge is proportional to the coverage of
the CCM by the current collector. The MRDE tool presented here is an ideal tool for electrochemical characterization of CCMs and current collectors and allows for an economical and accelerated screening of these important PEM electrolyzer components without the requirement of full cell testing.

Toshinari Koketsu, Jiwei Ma, Benjamin J. Morgan, Monique Body, Christophe Legein, Pooja Goddard, Olaf J. Borkiewicz, Peter Strasser, Damien Dambournet

Exploiting cationic vacancies for increased energy densities in dual-ion batteries

Energy Storage Materials, 25, 154-163

doi.org/10.1016/j.ensm.2019.10.019

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Dual-ion Li–Mg batteries offer a potential route to cells that combine desirable properties of both single-ion species. To maximize the energy density of a dual-ion battery, we propose a strategy for achieving simultaneous intercalation of both ionic species, by chemically modifying the intercalation host material to produce a second, complementary, class of insertion sites. We show that donor-doping of anatase TiO2 to form large numbers of cationic vacancies allows the complementary insertion of Liþ and Mg2þ in a dual-ion cell with a net increase in cell energy density, due to a combination of an increased reversible capacity, an increased operating voltage, and a reduced polarization. By tuning the lithium concentration in the electrolyte, we achieve full utilization of the Ti4þ/Ti3þ redox couple with excellent cyclability and rate capability. We conclude that native interstitial sites preferentially accommodate Liþ ions, while Mg2þ ions occupy single-vacancy sites. We also predict a narrow range of electrochemical conditions where adjacent vacancy pairs preferentially accommodate one ion of each species, i.e., a [LiTi þ MgTi] configuration. These results demonstrate the implementation of additional host sites such as cationic sites as an effective approach to increase the energy density in dual-ion batteries.

Lujin Pan, Sebastian Ott, Fabio Dionigi, Peter Strasser

Current challenges related to the deployment of shape-controlled Pt alloy oxygen reduction reaction nanocatalysts into low Pt-loaded cathode layers of proton exchange membrane fuel cells

Current Opinion in Electrochemistry, 2019, 18, 61-71

doi.org/10.1016/j.coelec.2019.10.011

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The reduction of the amount of platinum used in proton exchange membrane fuel cell cathodes at constant power density helps lower the cell stack cost of fuel cell electric vehicles. Recent screening studies using the thin film rotating disk electrode technique have identified an ever-growing number of Pt-based nanocatalysts with oxygen reduction reaction Pt-mass activities that allow for a substantial projected decrease in the geometric platinum loading at the cathode layer. However, the step from a rotating disk electrode test to a membrane electrode assembly test has proved a formidable task. The deployment of advanced, often shape-controlled dealloyed Pt alloy nanocatalysts in actual cathode layers of proton exchange membrane fuel cells has remained extremely challenging with respect to their actual catalytic activity under hydrogen/oxygen flow, their hydrogen/air performance at high current densities, and their morphological stability under prolonged fuel cell operations. In this review, we discuss some of these challenges, yet also propose possible solutions to understand the challenges and to eventually unfold the full potential of advanced Pt-based alloy oxygen reduction reaction catalysts in fuel cell electrode layers.

Xingli Wang, Jorge Ferreira de Araújo, Wen Ju, Alexander Bagger, Henrike Schmies, Stefanie Kühl, Jan Rossmeisl and Peter Strasser

Mechanistic reaction pathways of enhanced ethylene yields during electroreduction of CO2–CO co-feeds on Cu and Cu-tandem electrocatalysts

Nature Nanotechnology, 14, 1063–1070

www.nature.com/articles/s41565-019-0551-6

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Unlike energy efficiency and selectivity challenges, the kinetic effects of impure or intentionally mixed CO2 feeds on the catalytic reactivity of the direct electrochemical CO2 reduction reaction (CO2RR) have been poorly studied. Given that industrial CO2 feeds are often contaminated with CO, a closer investigation of the CO2RR under CO2/CO co-feed conditions is warranted. Here, we report mechanistic insights into the CO2RR reactivity of CO2/CO co-feeds on Cu-based nanocatalysts. Kinetic isotope-labelling experiments—performed in an operando differential electrochemical mass spectrometry capillary flow cell with millisecond time resolution—showed an unexpected enhanced production of C2H4, with a yield increase of almost 50%, from a cross-coupled 12CO2–13CO reactive pathway. The results suggest the absence of site competition between CO2 and CO molecules on the reactive surface at the reactant-specific sites. The practical significance of sustained local interfacial CO partial pressures under CO2 depletion is demonstrated by metallic/non-metallic Cu/Ni–N-doped carbon tandem catalysts. Our findings show the mechanistic origin of improved C2 product formation under co-feeding, but also highlight technological opportunities of impure CO2/CO process feeds for H2O/CO2 co-electrolysers.

F. Dionigi, C. Cesar Weber, M. Primbs, M. Gocyla, A. Martinez Bonastre, C. Spöri, H. Schmies,E. Hornberger, S. Kühl, J. Drnec, M. Heggen, J. Sharman, R. Edward Dunin-Borkowski and P. Strasser

Controlling Near-Surface Ni Composition in Octahedral PtNi(Mo) Nanoparticles by Mo Doping for a Highly Active Oxygen Reduction Reaction Catalyst

Nano Lett., 2019, 19, 6876−6885

pubs.acs.org/doi/10.1021/acs.nanolett.9b02116

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We report and study the translation of exceptionally high catalytic oxygen electroreduction activities of molybdenum-doped octahedrally shaped PtNi(Mo) nanoparticles from conventional thin-film rotating disk electrode screenings (3.43 ± 0.35 A mgPt−1 at 0.9 VRHE) to membrane electrode assembly (MEA)-based single fuel cell tests with sustained Pt mass activities of 0.45 A mgPt−1 at 0.9 Vcell, one of the highest ever reported performances for advanced shaped Pt alloys in real devices. Scanning transmission electron microscopy with energy dispersive X-ray analysis (STEM-EDX) reveals that Mo preferentially occupies the Pt-rich edges and vertices of the element-anisotropic octahedral PtNi particles. Furthermore, by combining in situ wide-angle X-ray spectroscopy, X-ray fluorescence, and STEM-EDX elemental mapping with electrochemical measurements, we finally succeeded to realize high Ni retention in activated PtNiMo nanoparticles even after prolonged potential-cycling stability tests. Stability losses at the anodic potential limits were mainly attributed to the loss of the octahedral particle shape. Extending the anodic potential limits of the tests to the Pt oxidation region induced detectable Ni losses and structural changes. Our study shows on an atomic level how Mo adatoms on the surface impact the Ni surface composition, which, in turn, gives rise to the exceptionally high experimental catalytic ORR reactivity and calls for strategies on how to preserve this particular surface composition to arrive at performance stabilities comparable with state-of-the-art spherical dealloyed Pt core−shell catalysts.

Lei Han, Yanyan Sun, Shuang Li, Chong Cheng, Christian E. Halbig, Patrick Feicht, Jessica Liane Hübner, Peter Strasser and Siegfried Eigler

In-Plane Carbon Lattice-Defect Regulating Electrochemical Oxygen Reduction to Hydrogen Peroxide Production over Nitrogen-Doped Graphene

ACS Catal. 2019, 9, 1283−1288

pubs.acs.org/doi/10.1021/acscatal.8b03734

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Carbon-based materials are considered to be active for electrochemical oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2) production. Nevertheless, less attention is paid to the investigation of the influence of in-plane carbon lattice defect on the catalytic activity and selectivity toward ORR. In the present work, graphene precursors were prepared from oxo-functionalized graphene (oxo-G) and graphene oxide (GO) with H2O2 hydrothermal treatment, respectively. Statistical Raman spectroscopy (SRS) analysis demonstrated the increased in-plane carbon lattice defect density in the order of oxo-G, oxo-G/H2O2, GO, GO/H2O2. Furthermore, nitrogen-doped graphene materials were prepared through ammonium hydroxide hydrothermal treatment of those graphene precursors. Rotating ring-disk electrode (RRDE) results indicate that the nitrogen-doped graphene derived from oxo-G with lowest in-plane carbon lattice defects exhibited the highest H2O2 selectivity of >82% in 0.1 M KOH. Moreover, a high H2O2 production rate of 224.8 mmol gcatalyst–1 h–1 could be achieved at 0.2 VRHE in H-cell with faradaic efficiency of >43.6%. Our work provides insights for the design and synthesis of carbon-based electrocatalysts for H2O2 production.

Andreas Glüsen, Fabio Dionigi, Paul Paciok, Marc Heggen, Martin Müller, Lin Gan, Peter Strasser, Rafal E. Dunin-Borkowski and Detlef Stolten

Dealloyed PtNi-Core−Shell Nanocatalysts Enable Significant Lowering of Pt Electrode Content in Direct Methanol Fuel Cells

ACS Catal. 2019, 9, 3764−3772

pubs.acs.org/doi/10.1021/acscatal.8b04883

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Direct methanol fuel cells (DMFCs) have the major advantage of the high energy density of the methanol (4.33 kWh/l) they use as a liquid fuel, although their costs remain too high due to the high quantity of Pt needed as a catalyst for oxygen reduction in the presence of methanol. Pt–Ni core–shell catalysts are promising candidates for improved oxygen reduction kinetics as shown in hydrogen fuel cells. The novelty in this work is due to the fact that we studied these catalysts in DMFC cathodes where oxygen must be reduced and membrane-permeating methanol oxidized at the same time. In spite of many attempts to overcome these problems, high amounts of Pt are still required for DMFC cathodes. During measurements over more than 3000 operating hours, the performance of the core–shell catalysts increased so substantially that a similar performance to that obtained with five times the amount of commercial platinum catalyst was achieved. While catalyst degradation has been thoroughly studied before, we showed here that these catalysts exhibit a self-protection mechanism in the DMFC cathode environment and prolonged operation is actually beneficial for performance and further stability due to the formation of a distinct Pt-rich shell on a PtNi core. The catalyst was analyzed by transition electron microscopy to show how the catalyst structure had changed during activation of the core–shell catalyst.

Juan-Jesus Velasco-Velez, Travis Jones, Dunfeng Gao, Emilia Carbonio, Rosa Arrigo, Cheng-Jhih Hsu, Yu-Cheng Huang, Chung-Li Dong, Jin-Ming Chen, Jyh-Fu Lee, Peter Strasser, Beatriz Roldan Cuenya, Beatriz Roldan Cuenya, Axel Knop-Gericke and Cheng-Hao Chuang

The Role of the Copper Oxidation State in the Electrocatalytic Reduction of CO2 into Valuable Hydrocarbons

ACS Sustainable Chem. Eng., 7, 1, 1485-1492

pubs.acs.org/doi/10.1021/acssuschemeng.8b05106

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Redox-active copper catalysts with accurately prepared oxidation states (Cu0, Cu+, and Cu2+) and high selectivity to C2 hydrocarbon formation, from electrocatalytic cathodic reduction of CO2, were fabricated and characterized. The electrochemically prepared copper-redox electro-cathodes yield higher activity for the production of hydrocarbons at lower oxidation state. By combining advanced X-ray spectroscopy and in situ microreactors, it was possible to unambiguously reveal the variation in the complex electronic structure that the catalysts undergo at different stages (i.e., during fabrication and electrocatalytic reactions). It was found that the surface, subsurface, and bulk properties of the electrochemically prepared catalysts are dominated by the formation of copper carbonates on the surface of cupric-like oxides, which prompts catalyst deactivation by restraining effective charge transport. Furthermore, the formation of reduced or partially reduced copper catalysts yields the key dissociative proton-consuming reactive adsorption of CO2 to produce CO, allowing the subsequent hydrogenation into C2 and C1 products by dimerization and protonation. These results yield valuable information on the variations in the electronic structure that redox-active copper catalysts undergo in the course of the electrochemical reaction, which, under extreme conditions, are mediated by thermodynamics, but critically, kinetics dominate near the oxide/metal phase transitions.

Yanyan Sun, Luca Silvioli, Nastaran Ranjbar Sahraie, Wen Ju, Jingkun Li, Andrea Zitolo,Shuang Li, Alexander Bagger, Logi Arnarson, Xingli Wang, Tim Moeller, Denis Bernsmeier,Jan Rossmeisl, Fredé ric Jaouen and Peter Strasser

Activity−Selectivity Trends in the Electrochemical Production of Hydrogen Peroxide over Single-Site Metal−Nitrogen−Carbon Catalysts

J. Am. Chem. Soc. 2019, 141, 12372−12381

pubs.acs.org/doi/10.1021/jacs.9b05576

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Nitrogen-doped carbon materials featuring atomically dispersed metal cations (M–N–C) are an emerging family of materials with potential applications for electrocatalysis. The electrocatalytic activity of M–N–C materials toward four-electron oxygen reduction reaction (ORR) to H2O is a mainstream line of research for replacing platinum-group-metal-based catalysts at the cathode of fuel cells. However, fundamental and practical aspects of their electrocatalytic activity toward two-electron ORR to H2O2, a future green “dream” process for chemical industry, remain poorly understood. Here we combined computational and experimental efforts to uncover the trends in electrochemical H2O2 production over a series of M–N–C materials (M = Mn, Fe, Co, Ni, and Cu) exclusively comprising atomically dispersed M–Nx sites from molecular first-principles to bench-scale electrolyzers operating at industrial current density. We investigated the effect of the nature of a 3d metal within a series of M–N–C catalysts on the electrocatalytic activity/selectivity for ORR (H2O2 and H2O products) and H2O2 reduction reaction (H2O2RR). Co–N–C catalyst was uncovered with outstanding H2O2 productivity considering its high ORR activity, highest H2O2 selectivity, and lowest H2O2RR activity. The activity–selectivity trend over M–N–C materials was further analyzed by density functional theory, providing molecular-scale understandings of experimental volcano trends for four- and two-electron ORR. The predicted binding energy of HO* intermediate over Co–N–C catalyst is located near the top of the volcano accounting for favorable two-electron ORR. The industrial H2O2 productivity over Co–N–C catalyst was demonstrated in a microflow cell, exhibiting an unprecedented production rate of more than 4 mol peroxide gcatalyst–1 h–1 at a current density of 50 mA cm–2.

Ana Sofia Varela, Wen Ju, Alexander Bagger, Patricio Franco, Jan Rossmeis and Peter Strasser

Electrochemical Reduction of CO2 on Metal-Nitrogen-Doped Carbon Catalysts

ACS Catal. 2019, 9, 7270−7284

pubs.acs.org/doi/10.1021/acscatal.9b01405

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The electrochemical CO2 reduction reaction (CO2RR) is a promising technology for converting waste CO2 into chemicals which could be used as feedstock for the chemical industry or as synthetic fuels. The technological viability of this process, however, is contingent on finding affordable and efficient catalysts. Recently, carbon-based solid state catalyst materials containing small amounts of nitrogen and transition metals (MNC) have emerged as a selective and cost-efficient alternative to noble metal catalysts for the direct electrochemical reduction of CO2 into CO. In addition, other products have also been reported, including formic acid and methane. In this Perspective, we offer a focused discussion of recent advances in the field of MNC catalysts for the CO2RR. The different factors which control the catalytic performance of MNC toward the CO2RR are discussed in this Perspective. We focus on density functional theory-guided experimental studies aiming to elucidate key experimental parameters and molecular descriptors that control the activity and selectivity of this class of materials. We close addressing the remaining challenges and take a look forward into future studies.

Camillo Spöri, Pascal Briois, Hong Nhan Nong, Tobias Reier, Alain Billard, Stefanie Kühl, Detre Teschner and Peter Strasser

Experimental Activity Descriptors for Iridium-Based Catalysts for the Electrochemical Oxygen Evolution Reaction (OER)

ACS Catal. 2019, 9, 6653−6663

doi.org/10.1021/acscatal.9b00648

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Recent progress in the activity improvement of anode catalysts for acidic electrochemical water splitting is largely achieved through empirical studies of iridium-based bimetallic oxides. Practical, experimentally accessible, yet general predictors of catalytic OER activity have remained scarce. This study investigates iridium and iridium–nickel thin film model electrocatalysts for the OER and identifies a set of general ex situ properties that allow the reliable prediction of their OER activity. Well-defined Ir-based catalysts of various chemical nature and composition were synthesized by magnetron sputtering. Correlation of physicochemical and electrocatalytic properties revealed two experimental OER activity descriptors that are able to predict trends in the OER activity of unknown Ir-based catalyst systems. More specifically, our study demonstrates that the IrIII+- and OH-surface concentration of the oxide catalyst constitute closely correlated and generally applicable OER activity predictors. On the basis of these predictors, an experimental volcano relationship of Ir-based OER electrocatalysts is presented and discussed.

Vera Beermann, Megan E.Holtz, Elliot Padgett, Jorge Ferreira de Araujoa, David A. Muller and Peter Strasser

Real-time imaging of activation and degradation of carbon supported octahedral Pt–Ni alloy fuel cell catalysts at the nanoscale using in situ electrochemical liquid cell STEM

Energy Environ. Sci., 12, 2476-2485

DOI: 10.1039/c9ee01185d

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Octahedrally shaped Pt–Ni alloy nanoparticles on carbon supports have demonstrated unprecedented electrocatalytic activity for the oxygen reduction reaction (ORR), sparking interest as catalysts for low-temperature fuel cell cathodes. However, deterioration of the octahedral shape that gives the catalyst its superior activity currently prohibits the use of shaped catalysts in fuel cell devices, while the structural dynamics of the overall catalyst degradation are largely unknown. We investigate the time-resolved degradation pathways of such a Pt–Ni alloy catalyst supported on carbon during cycling and startup/shutdown conditions using an in situ STEM electrochemical liquid cell, which allows us to track changes happening over seconds. Thereby we can precisely correlate the applied electrochemical potential with the microstructural response of the catalyst. We observe changes of the nanocatalysts’ structure, monitor particle motion and coalescence at potentials that corrode carbon, and investigate the dissolution and redeposition processes of the nanocatalyst under working conditions. Carbon support motion, particle motion, and particle coalescence were observed as the main microstructural responses to potential cycling and holds in regimes where carbon corrosion happens. Catalyst motion happened more severely during high potential holds and sudden potential changes than during cyclic potential sweeps, despite carbon corrosion happening during both, as suggested by ex situ DEMS results. During an extremely high potential excursion, the shaped nanoparticles became mobile on the carbon support and agglomerated facet-to-facet within 10 seconds. These experiments suggest that startup/shutdown potential treatments may cause catalyst coarsening on a much shorter time scale than full collapse of the carbon support. Additionally, the varying degrees of attachment of particles on the carbon support indicates that there is a distribution of interaction strengths, which in the future should be optimized for shaped particles. We further track the dissolution of Ni nanoparticles and determine the dissolution rate as a function of time for an individual nanoparticle – which occurs over the course of a few potential cycles for each particle. This study provides new visual understanding of the fundamental structural dynamics of nanocatalysts during fuel cell operation and highlights the need for better catalyst-support anchoring and morphology for allowing these highly active shaped catalysts to become useful in PEM fuel cell applications.

Wen Ju,Alexander Bagge,Xingli Wang,Yulin Tsai,Fang Luo,Tim Möller,Huan Wang,Jan Rossmeis,Ana Sofia Varela and Peter Strasser

Unraveling Mechanistic Reaction Pathways of the Electrochemical CO₂ Reduction on Fe−N−C Single-Site Catalysts

ACS Energy Lett., 2019, 4, 1663−1671

DOI: 10.1021/acsenergylett.9b01049

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We report a joint experimental−computational mechanistic study of electrochemical reduction of CO2 to CH4, catalyzed by solid-state Fe−N−C catalysts, which feature atomically dispersed,catalytically active Fe−Nx sites and represent one of the very rare examples of solid, non-Cu-based electrocatalysts that yield hydrocarbon products. Work reported here focuses on the identification of plausible mechanistic pathways from CO2 to various C1 products including methane. It is found that Fe−Nx sites convert only CO2, CO, and CH2O into methane, whereas CH3OH appears to be an end product. Distinctly different pH dependence of the catalytic CH4 evolution from CH2O in comparison with that of CO2 and CO reduction indicates differences in the proton participation of ratedetermining steps. By comparing the experimental observations with density functional theory derived free energy diagrams of reactive intermediates along the CO2 reduction reaction coordinates, we unravel the dominant mechanistic pathways and roles of CO and CH2O during the catalytic CO2-to-CH4 cascades and their rate-determining steps. We close with a comprehensive reaction network of CO2RR on single-site Fe−N−C catalysts, which may prove useful in developing efficient, non-Cubased catalysts for hydrocarbon production.

Velasco-Velez, J. J.; Jones, T.; Gao, D.; Carbonio, E.; Arrigo, R.; Hsu, C. J.; Huang, Y. C.; Dong, C. L.; Chen, J. M.; Lee, J. F.; Strasser, P.; Cuenya, B. R.; Schlog, R.; Knop-Gericke, A.; Chuang, C. H.

The Role of the Copper Oxidation State in the Electrocatalytic Reduction of CO2 into Valuable Hydrocarbons

ACS Sustainable Chem. Eng. 7 (1), 1485–1492

DOI:10.1021/acssuschemeng.8b05106

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Redox-active copper catalysts with accurately prepared oxidation states (Cu0, Cu+, and Cu2+) and high selectivity to C2 hydrocarbon formation, from electrocatalytic cathodic reduction of CO2, were fabricated and characterized. The electrochemically prepared copper-redox electro-cathodes yield higher activity for the production of hydrocarbons at lower oxidation state. By combining advanced X-ray spectroscopy and in situ microreactors, it was possible to unambiguously reveal the variation in the complex electronic structure that the catalysts undergo at different stages (i.e., during fabrication and electrocatalytic reactions). It was found that the surface, subsurface, and bulk properties of the electrochemically prepared catalysts are dominated by the formation of copper carbonates on the surface of cupric-like oxides, which prompts catalyst deactivation by restraining effective charge transport. Furthermore, the formation of reduced or partially reduced copper catalysts yields the key dissociative proton-consuming reactive adsorption of CO2 to produce CO, allowing the subsequent hydrogenation into C2 and C1 products by dimerization and protonation. These results yield valuable information on the variations in the electronic structure that redox-active copper catalysts undergo in the course of the electrochemical reaction, which, under extreme conditions, are mediated by thermodynamics, but critically, kinetics dominate near the oxide/metal phase transitions.

Cheonghee Kim, Tim Möller, Johannes Schmidt, Arne Thomas and Peter Strasser

Suppression of Competing Reaction Channels by Pb Adatom Decoration of Catalytically Active Cu Surfaces During CO2 Electroreduction

ACS Catal. 9, 1482−1488

DOI:10.1021/acscatal.8b02846

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The direct electrochemical conversion of carbon dioxide to chemicals and fuels is of fundamental scientific and technological interest. The control of the product selectivity, expressed in terms of the Faradaic efficiency, has remained a great challenge. Herein, we describe a surface-electrochemical synthetic strategy to tune the electrochemical CO2 reduction selectivity and yield by controlled suppression of the hydrogen evolution reaction (HER) reaction channel, resulting in increased Faradaic efficiencies for fuels and chemicals. We demonstrate that bimetallic catalysts consisting of only minute submonolayer amounts of Pb adatoms deposited on Cu surfaces exhibit and maintain unusually high selectivities for formate (HCOO–) over a large range of overpotentials. The bimetallic adatom electrodes were prepared using underpotential electrodeposition (UPD), which is able to precisely control the adatom coverage. While as little as 0.16 ML Pb surface adatoms on a polycrystalline Cu surface boosted the observed Faradaic HCOO– product selectivity 15 times, the 0.78 ML Pb/Cu catalyst showed the most favorable ratio of HCOO–/H2 production rate thanks to the effective suppression of the HER combined with a partial (−1.0 to −1.1 V vs RHE) enhancement of the HCOO– production. We argue that the favorable product efficiency is caused by selective adatom poisoning on the strongest binding hydrogen adsorption sites; in addition, electronic effects of Pb adatoms change the chemisorption of reactive intermediates. Our study reveals synthetic access to tailored selective bimetallic copper catalysts for the electrochemical CO2 reduction and demonstrates the enormous effect of even minute amounts of surface adatoms on the product spectrum.

Zarko P.Jovanov, Jorge Ferreira de Araujo, Shuang Li and Peter Strasser

Catalyst Preoxidation and EDTA Electrolyte Additive Remedy Activity and Selectivity Declines During Electrochemical CO2 Reduction

J Phys Chem C 123 (4), 2165-2174.

DOI:10.1021/acs.jpcc.8b08794

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Surplus electricity obtained from renewable energy sources requires suitable large-scale storage. Future CO2 electrolysis devices promise to offer a convenient route to store large quantities of excess electricity in the form of synthetic fuels, such as methane, ethylene, or oxygenates. In this work, we explored the strategies to support long-term stability in electrocatalytic CO2 conversion to CO, hydrocarbons, and alcohols. We show how electrochemical preoxidation of copper electrodes used as catalyst may ensure longer-lasting activity and selectivity in CO2 reduction. We demonstrate that EDTA as an electrolyte additive can be a realiable impurity scavenger, especially combined with a catalyst pretreatment at anodic potentials. An unprecedented rate retention after 20 h electrolysis on a model polycrystalline copper was 91% for ethylene in the case when both anodic surface preoxidation and sufficient additive are applied. Additionally, we established that EDTA exhibits an additional role to being exclusively a chelating agent. We propose that local pH stabilization and increased CO2 concentration near the electrode surface also contribute to long-term stability and improved CO2 reduction selectivities.wo inferior Ni(OH)2 and Fe(OOH) catalysts self-assemble into atomically intermixed Ni–Fe catalysts with unexpectedly high activity.

Gorlin, M.; Chernev, P.; Paciok, P.; Tai, C. W.; Ferreira de Araujo, J.; Reier, T.; Heggen, M.; Dunin-Borkowski, R.; Strasser, P.; Dau, H.

Formation of unexpectedly active Ni–Fe oxygen evolution electrocatalysts by physically mixing Ni and Fe oxyhydroxides

Chem. Commun. 55, 818-821

DOI:10.1039/C8CC06410E

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We present an unusual, yet facile, strategy towards formation of physically mixed Ni–Fe(OxHy) oxygen evolution electrocatalysts. We use in situ X-ray absorption and UV-vis spectroscopy, and high-resolution imaging to demonstrate that physical contact between two inferior Ni(OH)2 and Fe(OOH) catalysts self-assemble into atomically intermixed Ni–Fe catalysts with unexpectedly high activity.

Yancai Yao, Sulei Hu, Wenxing Chen, Zheng-Qing Huang, Weichen Wei, Tao Yao, Ruirui Liu, Ketao Zang, Xiaoqian Wang, Geng Wu, Wenjuan Yuan, Tongwei Yuan, Baiquan Zhu, Wei Liu, Zhijun Li, Dongsheng He, Zhenggang Xue, Yu Wang, Xusheng Zheng, Juncai Dong, Chun-Ran Chang, Yanxia Chen, Xun Hong, Jun Luo, Shiqiang Wei, Wei-Xue Li, Peter Strasser, Yuen Wu and Yadong Li

Engineering the electronic structure of single atom Ru sites via compressive strain boosts acidic water oxidation electrocatalysis

Nature Catalysis 2, 304-313

DOI: doi.org/10.1038/s41929-019-0246-2

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Single-atom precious metal catalysts hold the promise of perfect atom utilization, yet control of their activity and stability remains challenging. Here we show that engineering the electronic structure of atomically dispersed Ru1 on metal supports via compressive strain boosts the kinetically sluggish electrocatalytic oxygen evolution reaction (OER), and mitigates the degradation of Ru-based electrocatalysts in an acidic electrolyte. We construct a series of alloy-supported Ru1 using different PtCu alloys through sequential acid etching and electrochemical leaching, and find a volcano relation between OER activity and the lattice constant of the PtCu alloys. Our best catalyst, Ru1–Pt3Cu, delivers 90 mV lower overpotential to reach a current density of 10 mA cm−2, and an order of magnitude longer lifetime over that of commercial RuO2. Density functional theory investigations reveal that the compressive strain of the Ptskin shell engineers the electronic structure of the Ru1, allowing optimized binding of oxygen species and better resistance to over-oxidation and dissolution.

Sören Dresp, Fabio Dionigi, Malte Klingenhof, and Peter Strasser

Direct Electrolytic Splitting of Seawater: Opportunities and Challenges

ACS Energy Lett., 2019, 4 (4), pp 933–942

DOI:10.1021/acsenergylett.9b00220

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Hot, coastal, hyper-arid regions with intense solar irradiation and strong on- and off-shore wind patterns are ideal locations for the production of renewable electricity using wind turbines or photovoltaics. Given ample access to seawater and scarce freshwater resources, such regions make the direct and selective electrolytic splitting of seawater into molecular hydrogen and oxygen a potentially attractive technology. The key catalytic challenge consists of the competition between anodic chlorine chemistry and the oxygen evolution reaction (OER). This Perspective addresses some aspects related to direct seawater electrolyzers equipped with selective OER and hydrogen evolution reaction (HER) electrocatalysts. Starting from a historical background to the most recent achievements, it will provide insights into the current state and future perspectives of the topic. This Perspective also addresses prospects of the combination of direct seawater electrolysis with hydrogen fuel cell technology (reversible seawater electrolysis) and discusses its suitability as combined energy conversion–freshwater production technology.

Minoo Tasbihi, Michael Schwarze, Miroslava Edelmannová, Camillo Spöri, Peter Strasser, Reinhard Schomäcker

Photocatalytic reduction of CO₂ to hydrocarbons by using photodeposited Pt nanoparticles on carbon-doped titania

Catalysis Today 328, 8-14

DOI: 10.1016/j.cattod.2018.10.011

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Photocatalytic reduction of CO2 with H2O was performed in a top-irradiation stainless-steel photoreactor with Pt/C-TiO₂ as the photocatalyst. Pt/C-TiO₂ photocatalysts with different amount of Pt (0.5–3.0 wt.%) were synthesized by the photodeposition method and were characterized in detail by X-ray powder diffraction (XRD), nitrogen physisorption measurement (BET), UV–vis diffuse reflectance spectroscopy, inductively coupled plasma optical emission spectrometry (ICP-OES), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoelectrochemical measurements. Results revealed the photocatalytic reduction of CO₂ increased by loading Pt on the surface of C-TiO₂. The main reaction product was methane (CH₄), however, hydrogen (H₂) and carbon monoxide (CO) were also detected. The highest yields of CH₄, H₂, and CO were achieved in the presence Pt/C-TiO₂ with a nominal loading of 0.88 wt.%, resulting from the efficient interfacial transfer of photogenerated electrons from C-TiO₂ to Pt as it is evidenced from photoelectochemical measurements

S. Kühl, M. Gocyla, H. Heyen, S. Selve, M. Heggen, R. E. Dunin-Borkowski and P. Strasser

Concave curvature facets benefit oxygen electroreduction catalysis on octahedral shaped PtNi nanocatalysts

J. Mater. Chem. A 7 (3), 1149-1159

DOI: 10.1039/c8ta11298c

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Studies that demonstrated enhanced electrocatalytic oxygen reduction activities on octahedral PtNi nanocatalysts have routinely motivated and explained their data by the structure-sensitivity on PtNi alloy surfaces in general, more specifically by the favourable performance of the annealed Pt3Ni(111) single crystal surface with a monoatomic Pt skin layer. In this contribution, we challenge this view and show that imperfect Ni-enriched {111} nanofacets with concave Pt curvature catalytically outperform flat, well-alloyed, locally ordered {111} nanofacets. To achieve this, we investigate the geometric, compositional, and morphological structure on the ensemble and on the individual particle level of PtNi alloy nano-octahedra. In particular, we track the correlations of these parameters after thermal annealing and link them to their catalytic activity. The level of local compositional and structural disorder appears to be a reliable descriptor and predictor for ORR reactivity – at least within a family of catalysts. Under annealing up to 300°C concave Pt {111} While facets, with partially flat Ni facets remained most prevalent, resulting in nanoparticles with pronounced elemental anisotropy. At higher annealing temperature, concave Pt morphologies gave way to cuboctahedra with healed flat {111} and {100} alloy facets. The imperfect concave nano-octahedral catalysts with enhanced local disorder invariably outperformed more ordered particles, counter to conventional wisdom, yet lacked behind in morphological stability. Faceted PtNi nano-cuboctahedra emerging at 400°C ultimately offered the most reasonable balance between moderate high activity combined with good morphological stability. This is why we proposed these nanooctahedra as the future shaped Pt alloy PEM cathode fuel cell catalyst of choice. While the present results do not invalidate the exceptional oxygen reduction activity of perfect Pt3Ni(111) “skin” single crystal surfaces, it adds a new important perspective on a decade old puzzle about structure-activity relations of PtNi octahedral nanocrystals.

Lei Han, Yanyan Sun, Shuang Li, Chong Cheng, Christian Halbig, Patrick Feicht, Jessica Liane Hübner, Peter Strasser, and Siegfried Eigler

In-Plane Carbon Lattice-Defect Regulating Electrochemical Oxygen Reduction to Hydrogen Peroxide Production over Nitrogen-Doped 3 Graphene

ACS Catal. 9 (2), 1283–1288

DOI: 10.1021/acscatal.8b03734

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Carbon-based materials are considered to be active for electrochemical oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2) production. Nevertheless, less attention is paid to the investigation of the influence of in-plane carbon lattice defect on the catalytic activity and selectivity toward ORR. In the present work, graphene precursors were first prepared from oxo-functionalized graphene (oxo-G) and graphene oxide (GO) with H2O2 hydrothermal treatment, respectively. Statistical Raman spectroscopy (SRS) analysis demonstrated the increased in-plane carbon lattice defect density in the order of oxo-G, oxo-G/H2O2, GO, GO/H2O2. Furthermore, nitrogen-doped graphene materials were prepared through ammonium hydroxide hydrothermal treatment of those graphene precursors. Rotating ring-disk electrode (RRDE) results indicate that the nitrogen-doped graphene derived from oxo-G with lowest in-plane carbon lattice defects exhibited the highest H2O2 selectivity of >82% in 0.1 M KOH. Moreover, high H2O2 production rate of 224.8 mmol gcatalyst-1 h-1 could be achieved at 0.2 VRHE in H-cell with faradaic efficiency of >43.6%. Our work provides new insights for the design and synthesis of carbon-based electrocatalysts for H2O2 production. 

Tim Möller, Wen Ju, Alexander Bagger, Xingli Wang, Fang Luo, Trung Ngo Thanh, Ana Sofia Varela, Jan Rossmeisl and Peter Strasser

Efficient CO2 to CO electrolysis on solid Ni–N–C catalysts at industrial current densities

Energy Envriron Sci. 12 (2), 640-647

DOI:10.1039/C8EE02662A

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The electrochemical CO2 reduction reaction (CO2RR) to pure CO streams in electrolyzer devices is poised to be the most likely process for near-term commercialization and deployment in the polymer industry. The reduction of CO2 to CO is electrocatalyzed under alkaline conditions on precious group metal (PGM) catalysts, such as silver and gold, limiting widespread application due to high cost. Here, we report on an interesting alternative, a PGM-free nickel and nitrogen-doped porous carbon catalyst (Ni–N–C), the catalytic performance of which rivals or exceeds those of the state-of-the-art electrocatalysts under industrial electrolysis conditions. We started from small scale CO2-saturated liquid electrolyte H-cell screening tests and moved to larger-scale CO2 electrolyzer cells, where the catalysts were deployed as Gas Diffusion Electrodes (GDEs) to create a reactive three-phase interface. We compared the faradaic CO yields and CO partial current densities of Ni–N–C catalysts to those of a Ag-based benchmark, and its Fe-functionalized Fe–N–C analogue under ambient pressures, temperatures and neutral pH bicarbonate flows. Prolonged electrolyzer tests were conducted at industrial current densities of up to 700 mA cm−2. Ni–N–C electrodes are demonstrated to provide CO partial current densities above 200 mA cm−2 and stable faradaic CO efficiencies around 85% for up to 20 hours (at 200 mA cm−2), unlike their Ag benchmarks. Density functional theory-based calculations of catalytic reaction pathways help offer a molecular mechanistic basis of the observed selectivity trends on Ag and M–N–C catalysts. Computations lend much support to our experimental hypothesis as to the critical role of N-coordinated metal ion, Ni–Nx, motifs as the catalytic active sites for CO formation. Apart from being cost effective, the Ni–N–C powder catalysts allow flexible operation under acidic, neutral, and alkaline conditions. This study demonstrates the potential of Ni–N–C and possibly other members of the M–N–C materials family to replace PGM catalysts in CO2-to-CO electrolyzers.

Mikaela Görlin, Petko Chernev, Paul Paciok, Cheuk-Wai Tai, Jorge Ferreira de Araújo, Tobias Reier, Marc Heggen, Rafal Dunin-Borkowski, Peter Strasser and Holger Dau  

Formation of unexpectedly active Ni–Fe oxygen evolution electrocatalysts by physically mixing Ni and Fe oxyhydroxides  

ChemComm 55 (8), 818-821

DOI: 10.1039/c8cc06410e  

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We present an unusual, yet facile, strategy towards formation of physically mixed Ni–Fe(OxHy) oxygen evolution electrocatalysts. We use in situ X-ray absorption and UV-vis spectroscopy, and highresolution imaging to demonstrate that physical contact between two inferior Ni(OH)₂ and Fe(OOH) catalysts self-assemble into atomically intermixed Ni–Fe catalysts with unexpectedly high activity.

2018

Cheonghee Kim, Fabio Dionigi, Vera Beermann, Xingli Wang, Tim Möller and Peter Strasser

Alloy Nanocatalysts for the Electrochemical Oxygen Reduction (ORR) and the Direct Electrochemical Carbon Dioxide Reduction Reaction (CO2RR)

Adv. Mater., 31, 1805617

DOI: http://10.1002/adma.201805617

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In the face of the global energy challenge and progressing global climate change, renewable energy systems and components, such as fuel cells and electrolyzers, which close the energetic oxygen and carbon cycles, have become a technology development priority. The electrochemical oxygen reduction reaction (ORR) and the direct electrochemical carbon dioxide reduction reaction (CO2RR) are important electrocatalytic processes that proceed at gas diffusion electrodes of hydrogen fuel cells and CO2 electrolyzers, respectively. However, their low catalytic activity (voltage efficiency), limited long‐term stability, and moderate product selectivity (related to their Faradaic efficiency) have remained challenges. To address these, suitable catalysts are required. This review addresses the current state of research on Pt‐based and Cu‐based nanoalloy electrocatalysts for ORR and CO2RR, respectively, and critically compares and contrasts key performance parameters such as activity, selectivity, and durability. In particular, Pt nanoparticles alloyed with transition metals, post‐transition metals and lanthanides, are discussed, as well as the material characterization and their performance for the ORR. Then, bimetallic Cu nanoalloy catalysts are reviewed and organized according to their main reaction product generated by the second metal. This review concludes with a perspective on nanoalloy catalysts for the ORR and the CO2RR, and proposes future research directions.

Ebru Özer, Ilya Sinev, Andrea M. Mingers, Jorge Araujo, Thomas Kropp, Manos Mavrikakis, Karl J. J. Mayrhofer, Beatriz Roldan Cuenya and Peter Strasser

Ir-Ni Bimetallic OER Catalysts Prepared by Controlled Ni Electrodeposition on Ir_poly and Ir(111)

Surfaces 1 (1), 165-186

DOI: 10.3390/surfaces1010013

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The alteration of electrocatalytic surfaces with adatoms lead to structural and electronic modifications promoting adsorption, desorption, and reactive processes. This study explores the potentiostatic electrodeposition process of Ni onto polycrystalline Ir (Ir_poly) and assesses the electrocatalytic properties of the resulting bimetallic surfaces. The electrodeposition resulted in bimetallic Ni overlayer (OL) structures and in combination with controlled thermal post-deposition annealing in bimetallic near-surface alloys (NSA). The catalytic oxygen evolution reaction (OER) activity of these two different Ni-modified catalysts is assessed and compared to a pristine, unmodified Ir_poly. An overlayer of Ni on Ir_poly showed superior performance in both acidic and alkaline milieu. The reductive annealing of the OL produced a NSA of Ni, which demonstrated enhanced stability in an acidic environment. The remarkable activity and stability improvement of Ir by Ni modification makes both systems efficient electrocatalysts for water oxidation. The roughness factor of Ir_poly is also reported. With the amount of deposited Ni determined by inductively coupled plasma mass spectrometry (ICP-MS) and a degree of coverage (monolayer) in the dependence of deposition potential is established. The density functional theory (DFT) assisted evaluation of H adsorption on Ir_poly enables determination of the preferred Ni deposition sites on the three low-index surfaces (111), (110), and (100).

Ebru Özer, Zarina Pawolek, Stefanie Kühl, Hong Nhan Nong, Benjamin Paul, Sören Selve, Camillo Spöri, Cornelius Bernitzky and Peter Strasser

Metallic Iridium Thin-Films as Model Catalysts for the Electrochemical Oxygen Evolution Reaction (OER)—Morphology and Activity

Surfaces 1 (1), 151-164

DOI: 10.3390/surfaces1010012

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Iridium (Ir) oxide is known to be one of the best electrocatalysts for the oxygen evolution reaction (OER) in acidic media. Ir oxide-based materials are thus of great scientific interest in current research on electrochemical energy conversion. In the present study, we applied Ir metal films as model systems for electrochemical water splitting, obtained by inductive heating in a custom-made setup using two different synthesis approaches. X-ray photoelectron spectroscopy (XPS) and selected area electron diffraction (SAED) confirmed that all films were consistently metallic. The effects of reductive heating time of calcined and uncalcined Ir acetate films on OER activity were investigated using a rotating disk electrode (RDE) setup. The morphology of all films was determined by scanning electron microscopy (SEM). The films directly reduced from the acetate precursor exhibited a strong variability of their morphology and electrochemical properties depending on heating time. The additional oxidation step prior to reductive heating accelerates the final structure formation.

Hong Nhan Nong, Tobias Reier, Hyung-Suk Oh, Manuel Gliech, Paul Paciok, Thu Ha Thi Vu, Detre Teschner, Marc Heggen, Valeri Petkov, Robert Schlögl, Travis Jones  and Peter Strasser

A unique oxygen ligand environment facilitates water oxidation in hole-doped IrNiO_x core–shell electrocatalysts

Nat. Catal. 1, 841-851

DOI: 10.1038/s41929-018-0153-y

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The electro-oxidation of water to oxygen is expected to play a major role in the development of future electrochemical energy conversion and storage technologies. However, the slow rate of the oxygen evolution reaction remains a key challenge that requires fundamental understanding to facilitate the design of more active and stable electrocatalysts. Here, we probe the local geometric ligand environment and electronic metal states of oxygen-coordinated iridium centres in nickel-leached IrNi@IrO_x metal oxide core–shell nanoparticles under catalytic oxygen evolution conditions using operando X-ray absorption spectroscopy, resonant high-energy X-ray diffraction and differential atomic pair correlation analysis. Nickel leaching during catalyst activation generates lattice vacancies, which in turn produce uniquely shortened Ir–O metal ligand bonds and an unusually large number of d-band holes in the iridium oxide shell. Density functional theory calculations show that this increase in the formal iridium oxidation state drives the formation of holes on the oxygen ligands in direct proximity to lattice vacancies. We argue that their electrophilic character renders these oxygen ligands susceptible to nucleophilic acid–base-type O–O bond formation at reduced kinetic barriers, resulting in strongly enhanced reactivities.

Henrike Schmies, Elisabeth Hornberger, Björn Anke, Tilman Jurzinsky, Hong Nhan Nong, Fabio Dionigi, Stefanie Kühl, Jakub Drnec, Martin Lerch, Carsten Cremers, and Peter Strasser

Impact of Carbon Support Functionalization on the Electrochemical Stability of Pt Fuel Cell Catalysts 

Chem. Mater. 30 (20), 7289-7295

DOI: 10.1021/acs.chemmater.8b03612

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Nitrogen-enriched porous carbons have been discussed as supports for Pt nanoparticle catalysts deployed at cathode layers of polymer electrolyte membrane fuel cells (PEMFC). Here, we present an analysis of the chemical process of carbon surface modification using ammonolysis of preoxidized carbon blacks, and correlate their chemical structure with their catalytic activity and stability using in situ analytical techniques. Upon ammonolysis, the support materials were characterized with respect to their elemental composition, the physical surface area, and the surface zeta potential. The nature of the introduced N-functionalities was assessed by X-ray photoelectron spectroscopy. At lower ammonolysis temperatures, pyrrolic-N were invariably the most abundant surface species while at elevated treatment temperatures pyridinic-N prevailed. The corrosion stability under electrochemical conditions was assessed by in situ high-temperature differential electrochemical mass spectroscopy in a single gas diffusion layer electrode; this test revealed exceptional improvements in corrosion resistance for a specific type of nitrogen modification. Finally, Pt nanoparticles were deposited on the modified supports. In situ X-ray scattering techniques (X-ray diffraction and small-angle X-ray scattering) revealed the time evolution of the active Pt phase during accelerated electrochemical stress tests in electrode potential ranges where the catalytic oxygen reduction reaction proceeds. Data suggest that abundance of pyrrolic nitrogen moieties lower carbon corrosion and lead to superior catalyst stability compared to state-of-the-art Pt catalysts. Our study suggests with specific materials science strategies how chemically tailored carbon supports improve the performance of electrode layers in PEMFC devices.

Yanyan Sun, Shuang Li, Zarko Petar Jovanov, Denis Bernsmeier, Huan Wang, Benjamin Paul, Xingli Wang, Stefanie Kehl, and Peter Strasser

Structure, Activity, and Faradaic Efficiency of Nitrogen Doped Porous Carbon Catalysts for Direct Electrochemical Hydrogen Peroxide Production 

ChemSusChem 11 (19), 3388-3395

DOI: 10.1002/cssc.201801583

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Carbon materials doped with nitrogen are active catalysts for the electrochemical two-electron oxygen reduction reaction (ORR) to hydrogen peroxide. Insights into the individual role of the various chemical nitrogen functionalities in the H₂O₂ production, however, have remained scarce. Here, we explore a catalytically very active family of nitrogen-doped porous carbon materials, prepared by direct pyrolysis of ordered mesoporous carbon (CMK-3) with polyethylenimine (PEI). Voltammetric rotating ring-disk analysis in combination with chronoamperometric bulk electrolysis measurements in electrolysis cells demonstrate a pronounced effect of the applied potentials, current densities, and electrolyte pH on the H₂O₂ selectivity and absolute production rates. H₂O₂ selectivity up to 95.3 % was achieved in acidic environment, whereas the largest H₂O₂ production rate of 570.1 mmol g^-1 catalyst  h^-1 was observed in neutral solution. X-ray photoemission spectroscopy (XPS) analysis suggests a key mechanistic role of pyridinic-N in the catalytic process in acid, whereas graphitic-N groups appear to be catalytically active moieties in neutral and alkaline conditions. Our results contribute to the understanding and aid the rational design of efficient carbon-based H₂O₂ production catalysts.

Chang Hyuck Choi, Hyung-Kyu Lim, Min Wook Chung, Gajeon Chon, Nastaran Ranjbar Sahraie, Abdulrahman Altin, Moulay Tahar Sougrati, Lorenzo Stievano, Hyun Seok Oh, Eun-Soo Park, Fang Luo, Peter Strasser, Goran Dražić, Karl Mayrhofer, Hyungjun Kim and Frederic Jaouen 

The Achilles' heel of iron-based catalysts during oxygen reduction in acidic medium

Energy Environ. Sci. 11 (11), 3176-3182

DOI: 10.1039/C8EE01855C

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For catalyzing dioxygen reduction, iron-nitrogen-carbon (Fe-N-C) materials are today the best candidates to replace platinum in proton-exchange membrane fuel cell (PEMFC) cathodes. Despite tremendous progress in their activity and site-structure understanding, improved durability is critically needed but challenged by insufficient understanding of their degradation mechanisms during operation. Here, we show that FeN_xC_y moieties in a representative Fe-N-C catalyst are structurally stable but electrochemically unstable when exposed in acidic medium to H₂O₂, the main oxygen reduction reaction (ORR) byproduct. We reveal that exposure to H₂O₂ leaves iron-based catalytic sites untouched but decreases their turnover frequency (TOF) via oxidation of the carbon surface, leading to weakened O₂-binding on iron-based sites. Their TOF is recovered upon electrochemical reduction of the carbon surface, demonstrating the proposed deactivation mechanism. Our results reveal for the first time a hitherto unsuspected key deactivation mechanism during ORR in acidic medium. This study identifies the N-doped carbon surface as Achilles' heel during ORR catalysis in PEMFCs. Observed in acidic but not in alkaline electrolyte, these insights suggest that durable Fe-N-C catalysts are within reach for PEMFCs if rational strategies minimizing the amount of H₂O₂ or reactive oxygen species (ROS) produced during ORR are developed.

Manuel Gliech, Malte Klingenhof, Mikaela Görlin, Peter Strasser
 
Supported metal oxide nanoparticle electrocatalysts: How immobilization affects catalytic performance
 
Applied Catalysis A, General 568, 11-15
 
DOI: 10.1016/j.apcata.2018.09.023
 
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Active and stable metal oxide nanoparticles supported on high surface area carriers (supports) play an important role in electrochemical energy conversion applications, for instance as anode electrocatalysts for the oxygen evolution reaction (OER) in water electrolyzers. While past studies most often focused on the activity and stability of the active oxide phase along with the surface area and durability of the support material of the combined catalyst/support couple, the influence of the immobilization method on its performance has been widely overlooked. Here, we emphasize the potential and limitations of the presented support methods and evaluate their applicability by means of controlling the metal loading, particle size and the accessibility of surface sites. Further, we present a technique applicable for tuning the loading of the metal oxide catalyst up to 20 wt. % avoiding agglomeration. We also establish a correlation between metal oxide loading and mass-based oxygen evolution activity.

Xiangjun Zheng, Jiao Wu, Xuecheng Cao, Janel Abbott, Chao Jin, Haibo Wang, Peter Strasser, Ruizhi Yang, Xin Chen, Gang Wu

N-, P-, and S-doped graphene-like carbon catalysts derived from onium salts with enhanced oxygen chemisorption for Zn-air battery cathodes

Applied Catalysis B: Environmental 241, 442–451

DOI: 10.1016/j.apcatb.2018.09.054

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Compared to currently studied metal-based catalysts, metal-free heteroatom-doped carbon catalysts have many advantages including no issues of degradation and contamination from metal dissolution. Relying on single type of doping usually cannot yield optimal electronic and geometric structures favorable for the oxygen reduction reaction (ORR). Herein, heteroatom N, P, and S simultaneously doped graphene-like carbon (NPS-G) was successfully synthesized from onium salts by a facile one-step pyrolysis method. The resulting metal-free NPS-G catalyst with optimized N, P, and S contents exhibits enhanced catalytic activity towards the ORR in alkaline media, relative to any single doping. In particular, this metal-free catalyst shows an encouraging half-wave potential (E_1/2=0.857 V) comparable to that of metal-based catalysts. It also demonstrates excellent electrochemical stability and methanol tolerance. This catalyst was further studied as a cathode in a primary Zn-air battery, showing exceptional open-circuit voltage (1.372 V) and power density (0.151W cm−2). The NPS-G cathode delivers a specific capacity of 686 mA h g_Zn^-1 at a current density of 10 mA cm⁻² while utilizing 82.2% of the theoretical capacity (835 mA h g_Zn^-1). The origin of high activity associated with various heteroatom dopings is elucidated through X-ray photoelectron spectroscopy analysis and density functional theory studies. The enhanced chemisorption of oxygen species (*OOH, *O and *OH) onto the dopants of the NPS-G catalysts reduces charge transfer resistance and facilitate the ORR. The porous 2D structure also contributes to the increase of active site density and facile mass transport.

Elisabeth Hornberger, Arno Bergmann, Henrike Schmies, Stefanie Kühl, Guanxiong Wang, Jakub Drnec, Daniel J. S. Sandbeck, Vijay Ramani, Serhiy Cherevko, Karl J. J. Mayrhofer, and Peter Strasser

In Situ Stability Studies of Platinum Nanoparticles Supported on Ruthenium−Titanium Mixed Oxide (RTO) for Fuel Cell Cathodes

ACS Catal. 8 (10), 9675-9683

DOI: 10.1021/acscatal.8b02498

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Using a variety of in situ techniques, we tracked the structural stability and concomitantly the electrocatalytic oxygen reduction reaction (ORR) of platinum nanoparticles on ruthenium− titanium mixed oxide (RTO) supports during electrochemical accelerated stress tests, mimicking fuel cell operating conditions. High-energy X-ray diffraction (HE-XRD) offered insights in the evolution of the morphology and structure of RTO-supported Pt nanoparticles during potential cycling. The changes of the atomic composition were tracked in situ using scanning flow cell measurements coupled to inductively coupled plasma mass spectrometry (SFCICP- MS). We excluded Pt agglomeration, particle growth, dissolution, or detachment as cause for the observed losses in catalytic ORR activity. Instead, we argue that Pt surface poisoning is the most likely cause of the observed catalytic rate decrease. Data suggest that the gradual growth of a thin oxide layer on the Pt nanoparticles due to strong metal−support interaction (SMSI) is the most plausible reason for the suppressed catalytic activity. We discuss the implications of the identified catalyst degradation pathway, which appear to be specific for oxide supports. Our conclusions offer previously unaddressed aspects related to oxide-supported metal particle electrocatalysts frequently deployed in fuel cells, electrolyzers, or metal−air batteries.
 

Arno Bergmann, Travis E. Jones, Elias Martinez Moreno, Detre Teschner, Petko Chernev, Manuel Gliech, Tobias Reier, Holger Dau and Peter Strasser
 

Unified structural motifs of the catalytically active state of Co(oxyhydr)oxides during the electrochemical oxygen evolution reaction
 
Nature Catalysis 1, 711-719
 
DOI: 10.1038/s41929-018-0141-2
 
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Efficient catalysts for the anodic oxygen evolution reaction (OER) are critical for electrochemical H₂ production. Their design requires structural knowledge of their catalytically active sites and state. Here, we track the atomic-scale structural evolu- tion of well-defined CoO_x(OH)_y compounds into their catalytically active state during electrocatalytic operation through oper- ando and surface-sensitive X-ray spectroscopy and surface voltammetry, supported by theoretical calculations. We find clear voltammetric evidence that electrochemically reducible near-surface Co³⁺–O sites play an organizing role for high OER activity. These sites invariably emerge independent of initial metal valency and coordination under catalytic OER conditions. Combining experiments and theory reveals the unified chemical structure motif as μ₂-OH-bridged Co^2+/3+ ion clusters formed on all three- dimensional cross-linked and layered CoO_x(OH)_y precursors and present in an oxidized form during the OER, as shown by ope- rando X-ray spectroscopy. Together, the spectroscopic and electrochemical fingerprints offer a unified picture of our molecular understanding of the structure of catalytically active metal oxide OER sites.

Juan-Jesus Velasco-Vélez, Katarzyna Skorupska, Elias Frei, Yu-Cheng Huang, Chung-Li Dong, Bing-Jian Su, Cheng-Jhih Hsu, Hung-Yu Chou, Jin-Ming Chen, Peter Strasser, Robert Schlögl, Axel Knop-Gericke, and Cheng-Hao Chuang

The Electro-Deposition/Dissolution of CuSO₄ Aqueous Electrolyte Investigated by In Situ Soft X‑ray Absorption Spectroscopy  

J. Phys. Chem. B 122 (2), 780-787

DOI: 10.1021/acs.jpcb.7b06728  

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The electrodeposition nature of copper on a gold electrode in a 4.8 pH CuSO₄ solution was inquired using X-ray absorption spectroscopy, electrochemical quartz crystal microbalance, and thermal desorption spectroscopy techniques. Our results point out that the electrodeposition of copper prompts
the formation of stable oxi-hydroxide species with a formal oxidation state Cu⁺ without the evidence of metallic copper formation (Cu⁰). Moreover, the subsequent
anodic polarization of Cu₂O_aq yields the formation of CuO, in the formal oxidation state Cu²⁺, which is dissolved at higher anodic potential. It was found that the
dissolution process needs less charge than that required for the electrodeposition indicating a nonreversible process most likely due to concomitant water splitting and formation of protons during the electrodeposition.  

Vera Beermann, Stefanie Kühl, Peter Strasser

Tuning the Catalytic Oxygen Reduction Reaction Performance of Pt-Ni Octahedral Nanoparticles by Acid Treatments and Thermal Annealing

J. Electrochem. Soc 165 (15), J3026-J3030

DOI:10.1149/2.0051815jes

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Shape controlled octahedral Pt-Ni alloy nanoparticles are promising oxygen reduction reaction (ORR) electrocatalysts for cathodes of low temperature Polymer Electrolyte Membrane fuel cells. Organic surfactants are used in order to control and tune particle composition, size, shape, and the distribution on the support material. Such methods request intense post-synthesis cleaning, or annealing procedures in order to remove the ligands, demanding for simpler cleaning and activation procedures. Here, we explore the effect of an acetic acid treatment of as-prepared Pt-Ni particles, applied prior to annealing. The resulting nanoparticles underwent an electrochemical surface characterization and were investigated in terms of their ORR activities and electrochemical long-term stability. After acid treatment the particles exhibit a Pt-rich surface, which changed slightly during annealing at 300◦C but drastically to a more homogeneous alloy after annealing at 500◦C due to Ni surface segregation. Besides changes in the (sub-)surface Pt and Ni composition, the octahedral shape did not survive the 500◦C treatment. An improved ORR activity was obtained after annealing at 300◦C. Our insights into effects and benefits of the described post-synthesis treatments aid our general understanding, but also may help improve the practical design of suitable treatment protocols of this class of ORR catalyst. 

Ana Sofia Varela, Wen Ju, and Peter Strasser 

 
Molecular Nitrogen–Carbon Catalysts, Solid Metal Organic Framework Catalysts, and Solid Metal/Nitrogen-Doped Carbon (MNC) Catalysts for the Electrochemical CO₂ Reduction 
 
 

Adv. Energy Mater. 8 (30), 1-35
 
DOI: 10.1002/aenm.201703614
 
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The CO₂ electrochemical reduction reaction (CO₂RR) is a promising technology for converting CO₂ into chemicals and fuels, using surplus electricity from renewable sources. The technological viability of this process, however, is contingent on finding affordable and efficient catalysts. A range of materials containing abundant elements, such as N, C, and non-noble metals, ranging from well-defined immobilized complexes to doped carbon materials have emerged as a promising alternative. One of the main products of the CO₂RR is CO, which is produced on these catalysts with selectivities comparable to those of noble metal catalysts. Furthermore, other valuable products, such as formic acid, hydrocarbons, and alcohols, have also been reported. The factors that control the catalytic performance of these materials, however, are not
yet fully understood. A review of recent work is presented on heterogeneous nitrogen-containing carbon catalysts for the CO₂RR. The synthesis and characterization of these materials as well as their electrocatalytic performance are discussed. Combined experimental and theoretical studies are included to bring insight on the active sites and the reaction mechanism. This knowledge is key for developing optimal catalyst materials that meet the requirement in terms of activity, selectivity, and stability needed for commercial applications. 

Ebru Özer, Benjamin Paul, Camillo Spöri, Peter Strasser
 
Coupled Inductive Annealing-Electrochemical Setup for Controlled Preparation and Characterization of Alloy Crystal Surface Electrodes 
 
Small Methods, 3, 1800232
 
 

DOI: 10.1002/smtd.201800232
 
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The present versatile multifunctional electrochemical crystal preparation-test station features capabilities for controlled thermal annealing of any type of (binary) metallic or oxidic catalyst or support crystal surfaces, in particular single crystals. The setup enables rapid inductive heating of the electrodes to temperatures up to 1900 °C under precise infrared temperature control in controlled gas atmospheres. The constant overpressure inside the cell prevents the ambient atmosphere to permeate and contaminate the pro- cess. Finely adjusted sensors afford accurate and instant information on the electrodes’ surface temperature. The subsequent cooling process in argon atmosphere proceeds without any treatment in water. After annealing, the crystals inside the chamber can immediately be subjected to any type of electrochemical deposition, modification, or characterization in two distinct three-electrode chambers, again without any exposure to air. Both chambers feature pump-controlled supply and withdrawal of liquid electrolyte. A special characteristic is the inert-gas flow inversion capability to transfer electrodes into separate portable inert-gas chambers. Design details are presented and their functionality during thermal preparation and electrochemical voltam- metry of Ir(111) catalysts for the oxygen evolution reaction are demonstrated. The station provides a crystal handling environment superior to the classic flame annealing approaches, yet to constitute a cost-effective alternative to vacuum chambers. 

Jiwei Ma, Toshinari Koketsu, Benjamin. J. Morgan, Christophe Legein, Monique Body, Peter Strasser Damien Dambournet 

 
Controlled hydroxy-fluorination reaction of anatase to promote Mg²⁺ mobility in rechargeable magnesium batteries 
 
Chem. Commun. 54 (72), 10080-10083
 
 

DOI: 10.1039/c8cc04136a
 
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In anatase TiO₂, substituting oxide anions with singly charged (F,OH) anions allows the controlled formation of cation vacancies, which act as reversible intercalation sites for Mg²⁺. We show that ion-transport (diffusion coefficients) and intercalation (reversible capacity) properties are controlled by two critical parameters: the vacancy concentration and the local anionic environment. Our results emphasise the complexity of this behaviour, and highlight the potential benefits of chemically controlling cationic-defects in electrode materials for rechargeable multivalent-ion batteries. 

Raphaël Chattot, Olivier Le Bacq, Vera Beermann, Stefanie Kühl, Juan Herranz, Sebastian Henning, Laura Kühn, Tristan Asset, Laure Guétaz, Gilles Renou, Jakub Drnec, Pierre Bordet, Alain Pasturel, Alexander Eychmüller, Thomas J. Schmidt, Peter Strasser, Laetitia Dubau and Frédéric Maillard 

 
Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis 
 
Nature Materials 17, 827-833
 
DOI: 10.1038/s41563-018-0133-2
 
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Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic perfor- mance and stability criteria required for the development of low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly chal- lenging. Here, we propose ‘surface distortion’ as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-the-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under a simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts. 

Toshinari Koketsu, Chao Wu, Yunhui Huang, Peter Strasser 

 
A high-performance Te@CMK-3 composite negative electrode for Na rechargeable batteries 
 
J. Appl. Electrochem. 48 (11), 1265-1271
 
DOI: 10.1007/s10800-018-1249-4 
 
 
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We report a new class of high-capacity chalcogen–carbon composite negative electrodes for Na rechargeable batteries, consisting of tellurium-infiltrated ordered mesoporous carbon CMK-3. Its unparalleled electric conductivity makes Te a promising electrode material with high-capacity utilization. The rechargeable cell Na/Te@CMK-3, using a carbonate-based electrolyte, exhibited a large stable capacity of ~ 320 mA h (g-Te)⁻¹ at 0.2 C with an excellent rate capability (55% of the theoretical specific capacity at 2 C-rate), and long-term cyclability (>500 cycles), as well as 100% coulombic efficiency. Our study evidences the great potential of mesoporous carbon-encapsulated Te materials concepts as a new class of high- performance chalcogen-based electrode materials for Na rechargeable batteries. 

Sören Dresp and Peter Strasser
 
Non-Noble Metal Oxides and their Application as Bifunctional Catalyst in Reversible Fuel Cells and Rechargeable Air Batteries 
 
 

ChemCatChem 10 (18), 4162-4171
 
 

DOI: 10.1002/cctc.201800660
 
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We report on a comprehensive structural and electrocatalytic reactivity study of a diverse set of non‐noble monometallic and bimetallic Fe, Mn, Co, and Ni ‐based oxide bifunctional ORR and OER electrocatalysts. To assess their catalytic activity and suitability for bifunctional operation in a consistent manner, we introduce and apply a standardized successive electrochemical testing protocol. Correlations are established between bifunctional activity and structure, by which the materials are classified. The large set of tested catalyst materials in this study enabled us to unravel entire reactivity trends across material groups and to make conclusions as to their suitability for reversible operating oxygen electrode applications. Our analysis reveals both beneficial synergistic effects of MnFe and MnCo based catalysts towards the oxygen reduction reaction (ORR) as well as favorable trends of NiFe based materials towards the oxygen evolution reaction (OER). We visualize synoptic activity trends in so‐called “double overpotential” diagrams to elucidate easily the underlying activity trends. The highest bifunctional activity was found for a novel mixed spinel phase of Co and Mn and the highest OER performance was demonstrated for a mixed metal NiFe layered double hydroxide catalysts, from which practical guidance for the design of bifunctional fuel cell or metal‐air battery electrodes ensues.

Hongyu Gong, Shan Lu, Peter Strasser, Ruizhi Yang

Highly efficient AuNi-Cu₂O electrocatalysts for the oxygen reduction and evolution reactions: Important role of interaction between Au and Ni engineered by leaching of Cu₂O 

Electrochimica Acta 283, 1411-1417 
 

DOI: 10.1016/j.electacta.2018.07.083
 
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Electrochemical oxygen reduction reaction (ORR) during discharge and oxygen evolution reaction (OER) during charge are the key electrode processes for rechargeable metal-air batteries. In this article, we report a high-performance self-supported AuNi-Cu₂O hybrid, which works as a bi-functional electro- catalyst for ORR and OER. The as-prepared AuNi-Cu₂O hybrid exhibits excellent catalytic activity, the ORR activity of which is superior to that of commercial Pt/C, and the OER activity of which is close to that of commercial IrO₂/C. Moreover, AuNi-Cu₂O hybrid exhibits better durability than commercial Pt/C and IrO₂/C. The excellent catalytic performance of AuNi-Cu₂O could be attributed to the increase of active sites and the strong “mutual” interaction between Au and Ni induced by leaching of Cu₂O 

Denis Bernsmeier, Michael Bernicke, Roman Schmack, René Sachse, Benjamin Paul, Arno Bergmann, Peter Strasser, Erik Ortel, and Ralph Kraehnert

Oxygen Evolution Catalysts Based on Ir–Ti Mixed Oxides with Templated Mesopore Structure: Impact of Ir on Activity and Conductivity

ChemSusChem 11 (14), 2367-2374

DOI: 10.1002/cssc.201800932

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The efficient generation of hydrogen via water electrolysis requires highly active oxygen evolution catalysts. Among the active metals, iridium oxide provides the best compromise in terms of activity and stability. The limited availability and usage in other applications demands an efficient utilization of this precious metal. Forming mixed oxides with titania promises improved Ir utilization, but often at the cost of a low catalyst surface area. Moreover, the role of Ir in establishing a sufficiently conductive mixed oxide has not been elucidated so far. We report a new approach for the synthesis of Ir/TiOxmixed‐oxide catalysts with defined template‐controlled mesoporous structure, low crystallinity, and superior oxygen evolution reaction (OER) activity. The highly accessible pore system provides excellent Ir dispersion and avoids transport limitations. A controlled variation of the oxides Ir content reveals the importance of the catalysts electrical conductivity: at least 0.1 S m−1 are required to avoid limitations owing to slow electron transport. For sufficiently conductive oxides a clear linear correlation between Ir surface sites and OER currents can be established, where all accessible Ir sites equally contribute to the reaction. The optimized catalysts outperform Ir/TiOx materials reported in literature by about a factor of at least four.

Martin Gocyla, Stefanie Kuehl, Meital Shviro, Henner Heyen, Soeren Selve, Rafal E. Dunin-Borkowski, Marc Heggen, and Peter Strasser

Shape Stability of Octahedral PtNi Nanocatalysts for Electrochemical Oxygen Reduction Reaction Studied by in situ Transmission Electron Microscopy

ACS Nano 12 (6), 5306-5311

DOI: 10.1021/acsnano.7b09202

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Octahedral faceted nanoparticles are highly attractive fuel cell catalysts as a result of their activity for the oxygen reduction reaction (ORR). However, their surface compositional and morphological stability currently limits their long-term performance in real membrane electrode assemblies. Here, we perform in situ heating of compositionally segregated PtNi_1.5 octahedral nanoparticles inside a transmission electron microscope, in order to study their compositional and morphological changes. The starting PtNi_1.5 octahedra have Pt-rich edges and concave Ni-rich {111} facets. We reveal a morphological evolution sequence, which involves transformation from concave octahedra to particles with atomically flat {100} and {111} facets, ideally representing truncated octahedra or cuboctahedra. The flat {100} and {111} facets are thought to comprise a thin Pt layer with a Ni-rich subsurface, which may boost catalytic activity. However, the transformation to truncated octahedra/cuboctahedra also decreases the area of the highly active {111} facets. The morphological and surface compositional evolution, therefore, results in a compromise between catalytic activity and morphological stability. Our findings are important for the design of more stable faceted PtNi nanoparticles with high activities for the ORR.

Sladjana Martens, Ludwig Asena, Giorgio Ercolano, Fabio Dionigi, Chris Zalitis, Alex Hawkins, Alejandro Martinez Bonastre, Lukas Seidl, Alois C. Knoll, Jonathan Sharman, Peter Strasser, Deborah Jones, Oliver Schneider 

A comparison of rotating disc electrode, floating electrode technique and membrane electrode assembly measurements for catalyst testing

Journal of Power Sources 392, 274-287

DOI: doi.org/10.1016/j.jpowsour.2018.04.084

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The development of new catalysts for low temp. fuel cells requires accurate characterization techniques to evaluate their performance.  As initially only small amts. of catalyst are available, preliminary screening must rely on suitable test methods.  In this work, using a carbon supported platinum benchmark catalyst, the rotating disc electrode (RDE) technique was revisited in order to develop a detailed testing protocol leading to comparable results between different labs.  The RDE results were validated by comparison with data measured both in proton exchange membrane single cells and via the relatively new floating electrode technique.  This method can be operated with small amts. of catalyst but does not suffer from low limiting currents and allows prediction of high current capability of newly developed catalysts.  Different durability testing protocols were tested with all three methods.  Such protocols need to be able to introduce changes in the ref. catalyst, but must not be too harsh as otherwise they cannot be applied to alloy catalysts. In all protocols an upper potential limit of 0.925 V was used, as this produced degrdn. in the chosen benchmark catalyst, but still represents realistic conditions for alloy catalysts.

Nathaniel Leonard, Wen Ju, Ilya Sinev, Julian Steinberg, Fang Luo, Ana Sofia Varela, Beatriz Roldan Cuenya and Peter Strasser  

The chemical identity, state and structure of catalytically active centers during the electrochemical CO2 reduction on porous Fe–nitrogen–carbon (Fe–N–C) materials 

Chem. Sci. 9 (22), 5064-5073

DOI: 10.1039/c8sc00491a

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We report novel structure–activity relationships and explore the chemical state and structure of catalytically active sites under operando conditions during the electrochemical CO₂ reduction reaction (CO₂RR) catalyzed by a series of porous iron–nitrogen–carbon (FeNC) catalysts. The FeNC catalysts were synthesized from different nitrogen precursors and, as a result of this, exhibited quite distinct physical properties, such as BET surface areas and distinct chemical N-functionalities in varying ratios. The chemical diversity of the FeNC catalysts was harnessed to set up correlations between the catalytic CO₂RR activity and their chemical nitrogen-functionalities, which provided a deeper understanding between catalyst chemistry and function. XPS measurements revealed a dominant role of porphyrin-like Fe–N_x motifs and pyridinic nitrogen species in catalyzing the overall reaction process. Operando EXAFS measurements revealed an unexpected change in the Fe oxidation state and associated coordination from Fe²⁺ to Fe¹⁺. This redox change coincides with the onset of catalytic CH4 production around -0.9 VRHE. The ability of the solid state coordinative Fe¹⁺–N_x moiety to form hydrocarbons from CO2 is remarkable, as it represents the solid-state analogue to molecular Fe1+ coordination compounds with the same catalytic capability under homogeneous catalytic environments. This finding highlights a conceptual bridge between heterogeneous and homogenous catalysis and contributes significantly to our fundamental understanding of the FeNC catalyst function in the CO2RR.

Sören Dresp, Fabio Dionigi, Stefan Loos, Jorge Ferreira de Araujo, Camillo Spöri, Manuel Gliech, Holger Dau, and Peter Strasser

Direct Electrolytic Splitting of Seawater: Activity, Selectivity, Degradation, and Recovery Studied from the Molecular Catalyst Structure to the Electrolyzer Cell Level  

Adv. Energy Mater. 8 (22), 1-11

DOI:10.1002/aenm.201800338

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Seawater electrolysis faces fundamental chemical challenges, such as the suppression of highly detrimental halogen chemistries, which has to be ensured by selective catalyst and suitable operating conditions. In the present study, nanostructured NiFe-layered double hydroxide and Pt nanoparticles are selected as catalysts for the anode and cathode, respectively. The seawater electrolyzer is tested successfully for 100 h at maximum current densities of 200 mA cm⁻² at 1.6 V employing surrogate sea water and compared to fresh water feeds. Different membrane studies are carried out to reveal the cause of the current density drop. During long-term dynamic tests, under simulated day-night cycles, an unusual cell power performance recovery effect is uncovered, which is subsequently harnessed in a long-term diurnal day-night cycle test. The natural day-night cycles of the electrolyzer input power can be conceived as a reversible catalyst materials recovery treatment of the device when using photovoltaic electricity sources. To understand the origin of this reversible recovery on a molecular materials level, in situ extended X-ray absorption fine structure and X-ray near-edge region spectra are applied.

Frédéric Jaouen, Deborah Jones, Nathan Coutard, Vincent Artero, Peter Strasser, Anthony Kucernak

Toward Platinum Group Metal-Free Catalysts for Hydrogen/Air Proton-Exchange Membrane Fuel Cells: Catalyst activity in platinum-free substitute cathode and anode materials

Johnson Matthey Technol. Rev. 62 (2), 231–255 

DOI: 10.1595/205651318x696828

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The status, concepts and challenges toward catalysts free of platinum group metal (pgm) elements for proton-exchange membrane fuel cells (PEMFC) are reviewed. Due to the limited reserves of noble metals in the Earth’s crust, a major challenge for the worldwide development of PEMFC technology is to replace Pt with pgm-free catalysts with sufficient activity and stability. The priority target is the substitution of cathode catalysts (oxygen reduction) that account for more than 80% of pgms in current PEMFCs. Regarding hydrogen oxidation at the anode, ultralow Pt content electrodes have demonstrated good performance, but alternative non-pgm anode catalysts are desirable to increase fuel cell robustness, decrease the H₂ purity requirements and ease the transition from H₂ derived from natural gas to H₂ produced from water and renewable energy sources.

Laura Carolina Pardo, Nastaran Ranjbar Sahraie, Julia Melke, Patrick Elsässer, Detre Teschner, Xing Huang, Ralph Kraehnert, Robin J. White, Stephan Enthaler, Peter Strasser, and Anna Fischer  

Polyformamidine-Derived Non-Noble Metal Electrocatalysts for Effcient Oxygen Reduction Reaction

Adv. Funct. Mater. 28 (22), 1-13

DOI: 10.1002/adfm.201707551

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A facile approach for the template‐free synthesis of highly active non‐noble metal based oxygen reduction reaction (ORR) electrocatalysts is presented. Porous Fe−N−C/Fe/Fe3C composite materials are obtained by pyrolysis of defined precursor mixtures of polyformamidine (PFA) and FeCl3 as nitrogen‐rich carbon and iron sources, respectively. Selection of pyrolysis temperature (700–1100 °C) and FeCl3 loading (5–30 wt%) yields materials with differing surface areas, porosity, graphitization degree, nitrogen and iron content, as well as ORR activity. While the ORR activity of Fe‐free materials is limited (i.e., synthesized from pure PFA), a huge increase in activity is observed for catalysts containing Fe, revealing the participation of the metal dopant in the construction of active electrocatalytic sites. Further activity improvement is achieved via acid‐leaching and repeated pyrolysis, a result which is attributed to the creation of new active sites located at the surface of the porous nitrogen‐doped carbon by dissolution of the Fe and Fe3C nanophases. The best performing catalyst, which was synthesized with a low Fe loading (i.e., 5 wt%) and at a pyrolysis temperature of 900 °C, exhibits high activity, excellent H2O selectivity, extended stability, in both basic and acidic media as well as a remarkable tolerance toward methanol.

Ana Sofia Varela, Matthias Kroschel, Nathaniel D. Leonard, Wen Ju, Julian Steinberg, Alexander Bagger, Jan Rossmeisl, and Peter Strasser

pH Effects on the Selectivity of the Electrocatalytic CO2 Reduction on GrapheneEmbedded Fe-N-C Motifs: Bridging Concepts between Molecular Homogeneous and Solid-State Heterogeneous Catalysis
 
ACS Energy Lett. 3 (4), 812-817

DOI:10.1021/acsenergylett.8b00273

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The electrochemical CO₂ reduction reaction (CO₂RR) is a promising route for converting CO₂ and excess renewable energy into valuable chemicals and synthetic fuels. Recently, carbon-based solid materials containing dopant-levels of transition metal and nitrogen (M–N–C) have emerged as a cost-effective, energy-efficient catalyst for the direct coreduction of CO₂ and H₂O to CO. Fe–N–C catalysts are particularly interesting as they can reduce CO further to hydrocarbons. Despite these promising reports, the influence of the reaction conditions on the catalytic performance of Fe–N–C catalysts has not been addressed. Here, we study the role of the electrolyte on the CO₂RR selectivity. Unlike hydrogen or methane generation, catalytic CO production is independent of the electrolyte pH on the normal hydrogen electrode potential scale, suggesting a decoupled elementary proton–electron transfer mechanism for CO formation. The similarity between this heterogeneous charge-transfer reaction mechanism and that of molecular metal–nitrogen porphyrin-type macrocyclic complexes strongly suggests that the carbon-embedded FeN_x motifs of the solid-state electrocatalyst act as the primary catalytically active center and illustrates yet another example of unifying concepts between molecular and solid-state catalysis.

Yanyan Sun, Ilya Sinev, Wen Ju, Arno Bergmann, Sören Dresp, Stefanie Kühl, Camillo Spöri, Henrike Schmies, Huan Wang, Denis Bernsmeier, Benjamin Paul, Roman Schmack, Ralph Kraehnert, Beatriz Roldan Cuenya, and Peter Strasser

Efficient Electrochemical Hydrogen Peroxide Production from Molecular Oxygen on Nitrogen-Doped Mesoporous Carbon Catalysts

ACS Catal. 8 (4), 2844-2856

DOI: 10.1021/acscatal.7b03464

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Electrochemical hydrogen peroxide (H₂O₂) production by two-electron oxygen reduction is a promising alternative process to the established industrial anthraquinone process. Current challenges relate to finding cost-effective electrocatalysts with high electrocatalytic activity, stability, and product selectivity. Here, we explore the electrocatalytic activity and selectivity toward H₂O₂ production of a number of distinct nitrogen-doped mesoporous carbon catalysts and report a previously unachieved H₂O₂ selectivity of ∼95–98% in acidic solution. To explain our observations, we correlate their structural, compositional, and other physicochemical properties with their electrocatalytic performance and uncover a close correlation between the H₂O₂ product yield and the surface area and interfacial zeta potential. Nitrogen doping was found to sharply boost H₂O₂ activity and selectivity. Chronoamperometric H₂O₂ electrolysis confirms the exceptionally high H₂O₂ production rate and large H₂O₂ faradaic selectivity for the optimal nitrogen-doped CMK-3 sample in acidic, neutral, and alkaline solutions. In alkaline solution, the catalytic H₂O₂ yield increases further, where the production rate of the HO₂^– anion reaches a value as high as 561.7 mmol g_catalyst^–1 h^–1 with H₂O₂ faradaic selectivity above 70%. Our work provides a guide for the design, synthesis, and mechanistic investigation of advanced carbon-based electrocatalysts for H₂O₂ production.

Peter Strasser, Manuel Gliech, Stefanie Kuehl and Tim Moeller

Electrochemical processes on solid shaped nanoparticles with defined facets

Chem. Soc. Rev. 47 (3), 715-735

DOI: 10.1039/c7cs00759k

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This 2007 Chemistry Nobel prize update covers scientific advances of the past decade in our understanding of electrocatalytic processes on surfaces of nanoscale shape-controlled polyhedral solids. It is argued that the field of chemical reaction processes on solid surfaces has recently been paying increasing attention to the fundamental understanding of electrified solid–liquid interfaces and toward the operando study of the minute fraction of catalytically active, structurally dynamic non-equilibrium Taylor-type surface sites. Meanwhile, despite mounting evidence of acting as structural proxies in some cases, the concept of catalytic structure sensitivity of well-defined nanoscale solid surfaces continues to be a key organizing principle for the science of shape-controlled nanocrystals and, hence, constitutes a central recurring theme in this review. After addressing key aspects and recent progress in the wet-chemical synthesis of shaped nanocatalysts, three areas of electrocatalytic processes on solid shape-controlled nanocrystals of current scientific priority are discussed in more detail: the oxygen electroreduction on shape-controlled Pt–Ni polyhedra with its technological relevance for low temperature fuel cells, the CO2 electroreduction to hydrocarbons on Cu polyhedra and the puzzling interplay between chemical and structural effects, and the electrocatalytic oxygen evolution reaction from water on shaped transition metal oxides. The review closes with the conclusion that Surface Science and thermal catalysis, honored by Ertl’s Nobel prize a decade ago, continue to show major repercussions on the emerging field of Interface Science.

Nathaniel D. Leonard, Stephan Wagner, Fang Luo, Julian Steinberg, Wen Ju, Natascha Weidler, Huan Wang, Ulrike I. Kramm, and Peter Strasser  

Deconvolution of Utilization, Site Density, and Turnover Frequencyof Fe-Nitrogen-Carbon Oxygen Reduction Reaction Catalysts Prepared with Secondary N‑Precursors  

ACS Catal. 8 (3), 1640-1647

DOI: 10.1021/acscatal.7b02897  

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Metal–nitrogen–carbon (MNC) catalysts represent a potential means of reducing cathode catalyst costs in low temperature fuel cell cathodes. Knowledge-based improvements have been hampered by the difficulty to deconvolute active site density and intrinsic turnover frequency. In the present work, MNC catalysts with a variety of secondary nitrogen precursors are addressed. CO chemisorption in combination with Mössbauer spectroscopy are utilized in order to unravel previously inaccessible relations between active site density, turnover frequency, and active site utilization. This analysis provides a more fundamental description and understanding of the origin of the catalytic reactivity; it also provides guidelines for further improvements. Secondary nitrogen precursors impact quantity, quality, dispersion, and utilization of active sites in distinct ways. Secondary nitrogen precursors with high nitrogen content and micropore etching capabilities are most effective in improving catalysts performance.