2017 

Henrike Schmies, Arno Bergmann, Jakub Drnec, Guanxiong Wang, Detre Teschner, Stefanie Kühl, Daniel J. S. Sandbeck, Serhiy Cherevko, Martin Gocyla, Meital Shviro, Marc Heggen, Vijay Ramani, Rafal E. Dunin-Borkowski, Karl J. J. Mayrhofer, and Peter Strasser

Unravelling Degradation Pathways of Oxide-Supported Pt Fuel Cell Nanocatalysts under In Situ Operating Conditions  

Adv. Energy Mater. 8 (4), 1-13

DOI: 10.1002/aenm.201701663

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Knowledge of degradation pathways of catalyst/support ensembles aids the development of rational strategies to improve their stability. Here, this is exemplifed using indium tin oxide (ITO)-supported Platinum nanoparticles as
electrocatalysts at fuel cell (FC) cathodes under degradation protocols to mimic operating conditions in two potential regimes. The evolution of crystal structure, composition, crystallite and particle size is tracked by in situ X-ray techniques (small and wide angle scattering), metal dissolution by in situ scanning flow cell coupled with mass spectrometry (SFC ICP-MS) and Pt surface morphology by advanced electron microscopy. In a regular FC operation regime, Pt poisoning rather than Pt particle growth, agglomeration, dissolution or detachment was found to be the likely origin of the observed degradation and ORR activity losses. In the start-up regime degradation is actually suppressed and only minor losses in catalytic activity are observed. The presented data thus highlight the excellent nanoparticle stabilization and corrosion resistance of the ITO support, yet point to a degradation pathway involving Pt surface modifcations by deposition of sub-monolayers of support metal ions. The identifed degradation pathway
of the Pt/oxide catalyst/support couple contributes to our understanding of cathode electrocatalysts for polymer electrolyte fuel cells (PEFC).

Stefan Rudi, Detre Teschner, Vera Beermann, Walid Hetaba, Lin Gan, Chunhua Cui, Manuel Gliech, Robert Schlögl, and Peter Strasser

pH-Induced versus Oxygen-Induced Surface Enrichment and Segregation Effects in Pt–Ni Alloy Nanoparticle Fuel Cell Catalysts

ACS Catal. 7 (9), 6376-6384

DOI: 10.1021/acscatal.7b00996

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We present a voltammetric, spectroscopic, and atomic-scale microscopic study of how initial interfacial contact with high- and low-pH electrolytes affects the surface voltammetry, near-surface composition, CO binding, and electrocatalytic oxygen reduction reaction (ORR) of dealloyed Pt–Ni alloy nanoparticles deployed in fuel cells. The first contact of the catalyst with the electrolyte is critical for the evolution of the catalytically active surface structure, yet still insufficiently understood. Counter to chemical intuition, we find that voltammetric activation protocols in both pH 1 and pH 13 electrolytes result in similarly Ni-depleted surfaces with similar near-surface Ni/Pt ratios to a 2.5 nm depth, yet vastly different ORR reactivities. On the basis of our combined voltammetric, scanning transmission electron microscopy with the spectroscopic mapping by energy dispersive X-ray (STEM-EDX) microscopic and X-ray photoelectron spectroscopy (XPS) analysis, we conclude that oxygen-saturated alkaline electrolytes causes a strong surface segregation of the more oxophilic Ni component toward the particles surface, however in distinctly different ways depending on the pretreatment pH. Data suggest a controlling role of the initial thickness of the Ni-depleted Pt shell for the catalysis-driven segregation process. We analyze and discuss how such subtle differences in initial surface composition can unfold such dramatic subsequent variations in ORR activity as a function of pH. Our findings have practical bearing for the design of active Pt bimetallic ORR catalysts for alkaline exchange membrane fuel cells. If the non-noble oxophilic Pt alloy component is insoluble in the alkaline electrolyte, our results call for an imperative acid-pretreatment to avoid surface blocking by oxygen-induced segregation. If the non-noble oxophilic Pt alloy component is soluble in an alkaline electrolyte, acid or alkaline, even nonpretreated Pt alloy catalyst may be employed.

Hemma Mistry, Rulle Reske, Peter Strasser, Beatriz Roldan Cuenya

Size-dependent reactivity of gold-copper bimetallic nanoparticles during CO2 electroreduction

Catalysis Today 288, 30-36

DOI: 10.1016/j.cattod.2016.09.017

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New catalysts are needed to achieve lower overpotentials and higher faradaic efficiency for desirable products during the electroreduction of CO2. In this study, we explore the size-dependence of monodisperse gold-copper alloy nanoparticles (NPs) synthesized by inverse micelle encapsulation as catalysts for CO2 electroreduction. X-ray spectroscopy revealed that gold-copper alloys were formed and were heavily oxidized in their initial as prepared state. Current density was found to increase significantly for smaller NPs due to the increasing population of strongly binding low coordinated sites on NPs below 5 nm. Product analysis showed formation of H-2, CO, and CH4, with faradaic selectivity showing a minor dependence on size. The selectivity trends observed are assigned to reaction-induced segregation of gold atoms to the particle surface and altered electronic or geometric properties due to alloying.

Alexander Bagger, Wen Ju, Ana Sofia Varela, Peter Strasser, Jan Rossmeisl

Single site porphyrine-like structures advantages over metals for selective electrochemical CO₂ reduction

Catalysis Today 288, 74-78

DOI: 10.1016/j.cattod.2017.02.028

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Currently, no catalysts are completely selective for the electrochemical CO2 Reduction Reaction (CO2RR). Based on trends in density functional theory calculations of reaction intermediates we find that the single metal site in a porphyrine-like structure has a simple advantage of limiting the competing Hydrogen Evolution Reaction (HER). The single metal site in a porphyrine-like structure requires an ontop site binding of hydrogen, compared to the hollow site binding of hydrogen on a metal catalyst surface. The difference in binding site structure gives a fundamental energy-shift in the scaling relation of similar to 0.3 eV between the COOH* vs. H* intermediate (CO2RR vs. HER). As a result, porphyrine-like catalysts have the advantage over metal catalyst of suppressing HER and enhancing CO2RR selectivity.

Hongyu Gong, Xuecheng Cao, Fan Li, Yue Gong, Lin Gu, Rafael Gregorio Mendes, Mark H. Rummeli, Peter Strasser, and Ruizhi Yang

PdAuCu Nanobranch as Self-Repairing Electrocatalyst for Oxygen Reduction Reaction

ChemSusChem 10 (7), 1469-1474

DOI: 10.1002/cssc.201700008

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During start-up and shut-down operations of fuel cells, high potential is inevitably experienced at cathode, which leads to the deterioration of the oxygen reduction electrocatalyst. The design of catalysts that can repair themselves under severe conditions has been identified as a primary challenge for fuel cells. Herein, self-supported PdAuCu branched nanostructure is synthesized by a hydrothermal method. By smartly utilizing the high-potential treatment, the activity of PdAuCu is significantly enhanced owing to the synergistic effect between the Pd and CuII generated by such treatment. Moreover, the high activity of PdAuCu can be well maintained by repeating the high-potential treatment. We hence propose this catalyst as a “self-repairing” catalyst in a broad sense.

J. J. Velasco-Velez, K. Skorupska, E. Frei, Yu-Cheng Huang, Chung-Li Dong, Bing-Jian Su, Cheng-Jhih Hsu, Hung-Yu Chou, Jin-Ming Chen, P. Strasser, R. Schloögl, A. Knop-Gericke, and C.-H. 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 Cu2Oaq 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 non-reversible process most likely due to concomitant water splitting and formation of protons during the electrodeposition.

Alexander Bagger, Wen Ju, Ana Sofia Varela, Peter Strasser, and Jan Rossmeisl

Electrochemical CO₂ Reduction: A Classification Problem  

ChemPhysChem 18 (22), 3266-3273

DOI: 10.1002/cphc.201700736

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In this work, we propose four non-coupled binding energies of intermediates as descriptors, or "genes", for predicting the product distribution in CO2 electroreduction. Simple reactions can be understood by the Sabatier principle (catalytic activity vs. one descriptor), while more complex reactions tend to give multiple very different products and consequently the product selectivity is a more complex property to understand. We approach this, as a logistical classification problem, by grouping metals according to their major experimental product from CO2 electroreduction: H2 , CO, formic acid and beyond CO* (hydrocarbons or alcohols). We compare the groups in terms of multiple binding energies of intermediates calculated by density functional theory. Here, we find three descriptors to explain the grouping: the adsorption energies of H*, COOH*, and CO*. To further classify products beyond CO*, we carry out formaldehyde experiments on Cu, Ag, and Au and combine these results with the literature to group and differentiate alcohol or hydrocarbon products. We find that the oxygen binding (adsorption energy of CH3 O*) is an additional descriptor to explain the alcohol formation in reduction processes. Finally, the adsorption energy of the four intermediates, H*, COOH*, CO*, and CH3 O*, can be used to differentiate, group, and explain products in electrochemical reduction processes involving CO2 , CO, and carbon-oxygen compounds.

Vera Beermann, Martin Gocyla, Stefanie Kuehl, Elliot Padgett, Henrike Schmies, Mikaela Goerlin, Nina Erini, Meital Shviro, Marc Heggen, Rafal E. Dunin-Borkowski, David Muller, and Peter Strasser
 
Tuning the Electrocatalytic Oxygen Reduction Reaction Activity and Stability of Shape-Controlled Pt–Ni Nanoparticles by Thermal Annealing. Elucidating the Surface Atomic Structural and Compositional Changes
 
J. Am. Chem. Soc. 139 (46), 16536-16547
 
DOI: 10.1021/jacs.7b06846
 
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Shape-controlled octahedral Pt-Ni alloy nanoparticles exhibit remarkably high activities for the electroreduction of molecular oxygen (ORR), which makes them fuel cell cathode catalysts with exceptional potential. To unfold their full and optimized catalytic activity and stability, however, the nanooctahedra require post-synthesis thermal treatments, which alter the surface atomic structure and composition of the crystal facets. Here, we address and strive to elucidate the underlying surface chemical processes using a combination of ex situ analytical techniques with in situ transmission electron microscopy (TEM), in situ x-ray diffraction (XRD), and in situ electrochemical Fourier transformed infra-red (FTIR) experiments. We present a robust fundamental correlation between annealing temperature and catalytic activity, where a ~25x higher ORR activity than commercial Pt/C (2.7 A mgPt -1 at 0.9 VRHE) was reproducibly observed at annealing at 300 °C. The electrochemical stability, however, peaked without any and at the most severe heat treatments at 500 °C. Aberration-corrected scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDX) in combination with in situ electrochemical CO stripping/FTIR data revealed subtle, but important differences in the formation and chemical nature of Pt rich and Ni rich surface domains in the octahedral (111) facets. Estimating trends in surface chemisorption energies from in situ electrochemical CO/FTIR investigations suggested that balanced annealing generates an optimal degree of Pt surface enrichment, while the others exhibited mostly Ni rich facets. The insights from our study are quite generally valid and aid in developing suitable post synthesis thermal treatments for other alloy nanocatalysts as well.

Wen Ju, Alexander Bagger, Guang-Ping Hao, Ana Sofia Varela, Ilya Sinev, Volodymyr Bon, Beatriz Roldan Cuenya, Stefan Kaskel, Jan Rossmeisl & Peter Strasser

Understanding activity and selectivity of metal-nitrogen-doped carbon catalysts for electrochemical reduction of CO₂

Nature Communications 8 (1), 1-8

DOI: 10.1038/s41467-017-01035-z

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Direct electrochemical reduction of CO2 to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO2-consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO2 to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M–N x moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe–N–C and especially Ni–N–C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M–N x moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M–N x moieties and it provides predictive guidelines for the rational design of selective carbon-based CO2 reduction catalysts.

Toshinari Koketsu, Jiwei Ma, Benjamin J. Morgan, Monique Body, Christophe Legein, Walid Dachraoui, Mattia Giannini, Arnaud Demortière, Mathieu Salanne, François Dardoize, Henri Groult, Olaf J. Borkiewicz, Karena W. Chapman, Peter Strasser and Damien Dambournet  

Reversible magnesium and aluminium ions insertion in cation-deficient anatase TiO₂

Nature Materials 16, 1142-1148

DOI:10.1038/nmat4976

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In contrast to monovalent lithium or sodium ions, the reversible insertion of multivalent ions such as Mg²⁺ and Al³⁺ into electrode materials remains an elusive goal. Here, we demonstrate a new strategy to achieve reversible Mg²⁺ and Al³⁺ insertion in anatase TiO2, achieved through aliovalent doping, to introduce a large number of titanium vacancies that act as intercalation sites. We present a broad range of experimental and theoretical characterizations that show a preferential insertion of multivalent ions into titanium vacancies, allowing a much greater capacity to be obtained compared to pure TiO2. This result highlights the possibility to use the chemistry of defects to unlock the electrochemical activity of known materials, providing a new strategy for the chemical design of materials for practical multivalent batteries.

Xingli Wang, Ana Sofia Varela, Arno Bergmann, Stefanie Kühl, and Peter Strasser

Catalyst Particle Density Controls Hydrocarbon Product Selectivity in CO2 Electroreduction on CuOx

ChemSusChem 10 (22), 4642–4649

DOI: 10.1002/cssc.201701179
 
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A key challenge of the carbon dioxide electroreduction (CO2RR) on Cu-based nanoparticles is its low faradic selectivity toward higher-value products such as ethylene. Here, we demonstrate a facile method for tuning the hydrocarbon selectivities on CuO_x nanoparticle ensembles by varying the nanoparticle areal density. The sensitive dependence of the experimental ethylene selectivity on catalyst particle areal density is attributed to a diffusional interparticle coupling that controls the de- and re-adsorption of CO and thus the effective coverage of CO_ad intermediates. Thus, higher areal density constitutes dynamically favored conditions for CO re-adsorption and *CO dimerization leading to ethylene formation independent of pH and applied overpotential.

Nina Erini, Vera Beermann, Martin Gocyla, Manuel Gliech, Marc Heggen, Rafal E. Dunin Borkowski, and Peter Strasser 

The Effect of Surface Site Ensembles on the Activity and Selectivity of Ethanol Electrooxidation by Octahedral PtNiRh Nanoparticles

Angew. Chem. 56 (23), 6533-6538

DOI: 10.1002/ange.201702332

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Direct ethanol fuel cells are attractive power sources based on a biorenewable, high energy-density fuel. Their efficiency is limited by the lack of active anode materials which catalyze the breaking of the C−C bond coupled to the 12-electron oxidation to CO₂. We report shape-controlled PtNiRh octahedral ethanol oxidation electrocatalysts with excellent activity and previously unachieved low onset potentials as low as 0.1 V vs. RHE, while being highly selective to complete oxidation to CO₂. Our comprehensive characterization and in situ electrochemical ATR studies suggest that the formation of a ternary surface site ensemble around the octahedral Pt₃Ni₁Rh_x nanoparticles plays a crucial mechanistic role for this behavior.

Mikaela Görlin, Jorge Ferreira de Araujo, Henrike Schmies, Denis Bernsmeier, Sören Dresp, Manuel Gliech, Zenonas Jusys, Petko Chernev, Ralph Kraehnert, Holger Dau, and Peter Strasser

Tracking Catalyst Redox States and Reaction Dynamics in Ni-Fe Oxyhydroxide Oxygen Evolution Reaction Electrocatalysts: The Role of Catalyst Support and Electrolyte pH

J. Am. Chem. Soc. 139 (5), 2070–2082

DOI: 10.1021/jacs.6b12250

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Ni–Fe oxyhydroxides are the most active known electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes and are therefore of great scientific and technological importance in the context of electrochemical energy conversion. Here we uncover, investigate, and discuss previously unaddressed effects of conductive supports and the electrolyte pH on the Ni–Fe(OOH) catalyst redox behavior and catalytic OER activity, combining in situ UV–vis spectro-electrochemistry, operando electrochemical mass spectrometry (DEMS), and in situ cryo X-ray absorption spectroscopy (XAS). Supports and pH > 13 strongly enhanced the precatalytic voltammetric charge of the Ni–Fe oxyhydroxide redox peak couple, shifted them more cathodically, and caused a 2–3-fold increase in the catalytic OER activity. Analysis of DEMS-based faradaic oxygen efficiency and electrochemical UV–vis traces consistently confirmed our voltammetric observations, evidencing both a more cathodic O₂ release and a more cathodic onset of Ni oxidation at higher pH. Using UV–vis, which can monitor the amount of oxidized Ni^+3/+4in situ, confirmed an earlier onset of the redox process at high electrolyte pH and further provided evidence of a smaller fraction of Ni^+3/+4 in mixed Ni–Fe centers, confirming the unresolved paradox of a reduced metal redox activity with increasing Fe content. A nonmonotonic super-Nernstian pH dependence of the redox peaks with increasing Fe content—displaying Pourbaix slopes as steep as −120 mV/pH—suggested a two proton–one electron transfer. We explain and discuss the experimental pH effects using refined coupled (PCET) and decoupled proton transfer–electron transfer (PT/ET) schemes involving negatively charged oxygenate ligands generated at Fe centers. Together, we offer new insight into the catalytic reaction dynamics and associated catalyst redox chemistry of the most important class of alkaline OER catalysts.

Tobias Reier, Hong Nhan Nong, Detre Teschner, Robert Schlögl, and Peter Strasser

Electrocatalytic Oxygen Evolution Reaction in Acidic Environments – Reaction Mechanisms and Catalysts

Adv. Energy Mater. 7 (1), 1-18

DOI: 10.1002/aenm.201601275

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The low efficiency of the electrocatalytic oxidation of water to O₂ (oxygen evolution reaction-OER) is considered as one of the major roadblocks for the storage of electricity from renewable sources in form of molecular fuels like H₂ or hydrocarbons. Especially in acidic environments, compatible with the powerful proton exchange membrane (PEM), an earth-abundant OER catalyst that combines high activity and high stability is still unknown. Current PEM-compatible OER catalysts still rely mostly on Ir and/or Ru as active components, which are both very scarce elements of the platinum group. Hence, the Ir and/or Ru amount in OER catalysts has to be strictly minimized. Unfortunately, the OER mechanism, which is the most powerful tool for OER catalyst optimization, still remains unclear. In this review, we first summarize the current state of our understanding of the OER mechanism on PEM-compatible heterogeneous electrocatalysts, before we compare and contrast that to the OER mechanism on homogenous catalysts. Thereafter, an overview over monometallic OER catalysts is provided to obtain insights into structure-function relations followed by a review of current material optimization concepts and support materials. Moreover, missing links required to complete the mechanistic picture as well as the most promising material optimization concepts are pointed out.

Ebru Özer, Camillo Spöri, Tobias Reier, and Peter Strasser

Iridium(111), Iridium(110), and Ruthenium(0001) Single Crystals as Model Catalysts for the Oxygen Evolution Reaction: Insights into the Electrochemical Oxide Formation and Electrocatalytic Activity

ChemCatChem 9 (4), 597–603

DOI: 10.1002/cctc.201600423

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We report a comparative study on the influence of generic electrochemical activation–oxidation protocols on the resulting surface oxides of Ir(1 1 1) and (1 1 0) and Ru(0 0 0 1) single crystals and their electrocatalytic reactivity for the oxygen evolution reaction. Well-defined single-crystal electrodes were prepared in a custom-made chamber that combines inductive thermal annealing and electrochemistry. The clean surfaces were analyzed for their electrocatalytic oxygen evolution activities and oxidation behavior. Three different oxidation protocols were used, which revealed a strong activity dependence on the duration and upper potential limit of the electrochemical oxidation. The resulting changes of the surface were followed by using cyclic voltammetry and impedance spectroscopy. Important differences between the two faces of Ir in terms of surface morphology of the formed oxide were identified, which allowed us to draw conclusions for preferable crystal faces in nanoparticle catalysts.

Denis Bernsmeier, Michael Bernicke, Erik Ortel, Arno Bergmann, Andreas Lippitz, Jörg Nissen, Roman Schmack, Peter Strasser, Jörg Polte, and Ralph Kraehnert

Nafion-Free Carbon-Supported Electrocatalysts with Superior Hydrogen Evolution Reaction Performance by Soft Templating

ChemElectroChem 4, 221-229

DOI: 10.1002/celc.201600444

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Efficient water electrolysis requires electrode coatings with high catalytic activity. Platinum efficiently catalyzes the hydrogen evolution reaction in acidic environments, but is a rare and expensive metal. The activity achieved per metal atom can be increased if small Pt particles are dispersed onto electrically conductive, highly accessible and stable support materials. However, the addition of Nafion, a typical binder material used in the manufacture of electrode coatings, can decrease catalytic activity by the blocking of pores and active surface sites. A new approach is reported for the direct synthesis of highly active Nafion-free Pt/C catalyst films consisting of small Pt nanoparticles supported in size-controlled mesopores of a conductive carbon film. The synthesis relies on the co-deposition of suitable Pt and C precursors in the presence of polymer micelles, which act as pore templates. Subsequent carbonization in an inert atmosphere produces porous catalyst films with controlled film thickness, pore size and particle size. The catalysts clearly outperform all Nafion-based Pt/C catalysts reported in the literature, particularly at high current densities.

Camillo Spöri, Jason Tai Hong Kwan, Arman Bonakdarpour, David Wilkinson and Peter Strasser

The Stability Challenges of Oxygen Evolving Electrocatalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation

Angew. Chem. Int. Ed. 56 (22), 5994-6021

DOI: 10.1002/anie.201608601

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The electrochemical oxygen evolution reaction (OER) is an important reaction in the industrial production of numerous inorganic chemicals. However, over the last decade it has also received growing attention as scalable proton- and electron-providing process for use in the production of solar fuels, such as hydrogen, hydrocarbons, or alcohols carried out in water- and CO2-electrolyzers. To operate these devices efficiently and economically , active and stable electrocatalysts are required. While advances have been made in understanding and tuning OER efficiency and activity, the stability of OER catalysts and the reduction of their degradation continue to be major challenges. While most stability studies limit themselves to short-term testing in idealized three-electrode set ups, a much stronger focus on advancing our understanding of degradation of OER catalysts in realistic Membrane Electrode Assemblies (MEAs) with industrial current densities, is critically needed. This review addresses the technical challenges, their scientific basis, as well as recent progress and the road ahead regarding stability and degradation of OER catalysts operating at electrolyzer anodes in acidic MEA environments. First, we start clarifying the complexity associated with the term "catalyst stability", cover today's performance targets and outline major catalyst degradation mechanisms and their mitigation strategies. Then we evaluate suitable in-situ experimental methods to get insight into catalyst degradation and describe achievements in tuning OER catalyst stability. Finally, we highlight the importance of identifying universal figures of merit for stability and develop a comprehensive accelerated life test (ALT) that would yield comparable performance data across labs and catalyst types. As a whole, this review will help to disseminate and highlight the important relations between structure, composition and stability of OER catalysis under different operating conditions.

2016

Peter Strasser

Free Electrons to Molecular Bonds and Back: Closing the Energetic Oxygen Reduction (ORR)–Oxygen Evolution (OER) Cycle Using Core–Shell Nanoelectrocatalysts

Acc. Chem. Res.49 (11),  2658–2668

DOI: 10.1021/acs.accounts.6b00346

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Nanomaterial science and electrocatalytic science have entered a successful “nanoelectrochemical” symbiosis, in which novel nanomaterials offer new frontiers for studies on electrocatalytic charge transfer, while electrocatalytic processes give meaning and often practical importance to novel nanomaterial concepts. Examples of this fruitful symbiosis are dealloyed core–shell nanoparticle electrocatalysts, which often exhibit enhanced kinetic charge transfer rates at greatly improved atom-efficiency. As such, they represent ideal electrocatalyst architectures for the acidic oxygen reduction reaction to water (ORR) and the acidic oxygen evolution reaction from water (OER) that require scarce Pt- and Ir-based catalysts. Together, these two reactions constitute the “O-cycle”, a key elemental process loop in the field of electrochemical energy interconversion between electricity (free electrons) and molecular bonds (H₂O/O₂), realized in the combination of water electrolyzers and hydrogen/oxygen fuel cells.
In this Account, we describe our recent efforts to design, synthesize, understand, and test noble metal-poor dealloyed Pt and Ir core–shell nanoparticles for deployment in acidic polymer electrolyte membrane (PEM) electrolyzers and PEM fuel cells. Spherical dealloyed Pt core–shell particles, derived from PtNi₃ precursor alloys, showed favorable ORR activity. More detailed size–activity correlation studies further revealed that the 6–8 nm diameter range is a most desirable initial particle size range in order to maximize the particle Ni content after ORR testing and to preserve performance stability. Similarly, dealloyed and oxidized IrO_x core–shell particles derived from Ni-rich Ir–Ni precursor particles proved highly efficient oxygen evolution reaction (OER) catalysts in acidic conditions. In addition to the noble metal savings in the particle cores, the Pt core–shell particles are believed to benefit in terms of their mass-based electrochemical kinetics from surface lattice strain effects that tune the adsorption energies and barriers of elementary steps. The molecular mechanism of the kinetic benefit of the dealloyed IrO_x particle needs more attention, but there is mounting evidence for ligand hole effects in defect-rich IrO_x shells that generate preactive oxygen centers.

Stefanie Kühl, Peter Strasser

Oxygen Electrocatalysts on Dealloyed Pt Nanocatalysts

Top. Catal. 59 (17), 1628-1637

DOI: 10.1007/s11244-016-0682-z

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We review the fundamental principles, the preparation and catalytic performance of dealloyed Pt core–shell electrocatalysts for the electroreduction of molecular oxygen. This reaction is key to the efficiency of all fuel cell cathodes, as the oxygen electrocatalysis exhibits much larger kinetic overpotentials compared to typical fuel cell anode reactions. We discuss structural surface lattice strain in metal overlayers and show that they serve as models for nanostructured core–shell catalysts. We address preparation pathways with particular emphasis on the dealloying routes. Trends in reactivity of different dealloyed Pt core–shell catalysts are compared with a focus on the dealloyed Pt–Ni alloy system. Size effects are discussed. Practical catalytic performance data in automotive fuel cells and under automotive fuel cell conditions is provided and contrasted to other state-of-art catalyst concepts. This review concludes that dealloyed Pt core–shell cathode catalysts are currently the most attractive commercialization candidate for automotive applications.

Hyung-Suk Oh, Hong Nhan Nong, Tobias Reier, Arno Bergmann, Manuel Gliech, Jorge Ferreira de Araujo, Elena Willinger, Robert Schlögl, Detre Teschner, and Peter Strasser

Electrochemical Catalyst Support Effects and Their Stabilizing Role for IrOx Nanoparticle Catalysts during the Oxygen Evolution Reaction

J. Am. Chem. Soc. 138 (38), 12552–12563

DOI: 10.1021/jacs.6b07199

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Redox-active support materials can help reduce the noble-metal loading of a solid chemical catalyst while offering electronic catalyst–support interactions beneficial for catalyst durability. This is well known in heterogeneous gas-phase catalysis but much less discussed for electrocatalysis at electrified liquid–solid interfaces. Here, we demonstrate experimental evidence for electronic catalyst–support interactions in electrochemical environments and study their role and contribution to the corrosion stability of catalyst/support couples. Electrochemically oxidized Ir oxide nanoparticles, supported on high surface area carbons and oxides, were selected as model catalyst/support systems for the electrocatalytic oxygen evolution reaction (OER). First, the electronic, chemical, and structural state of the catalyst/support couple was compared using XANES, EXAFS, TEM, and depth-resolved XPS. While carbon-supported oxidized Ir particle showed exclusively the redox state (+4), the Ir/IrO_x/ATO system exhibited evidence of metal/metal–oxide support interactions (MMOSI) that stabilized the metal particles on antimony-doped tin oxide (ATO) in sustained lower Ir oxidation states (Ir^3.2+). At the same time, the growth of higher valent Ir oxide layers that compromise catalyst stability was suppressed. Then the electrochemical stability and the charge-transfer kinetics of the electrocatalysts were evaluated under constant current and constant potential conditions, where the analysis of the metal dissolution confirmed that the ATO support mitigates Ir^z+ dissolution thanks to a stronger MMOSI effect. Our findings raise the possibility that MMOSI effects in electrochemistry—largely neglected in the past—may be more important for a detailed understanding of the durability of oxide-supported nanoparticle OER catalysts than previously thought.

Rosa M. Arán-Ais, José Solla-Gullón, Martin Gocyla, Marc Heggen, Rafal E. Dunin-Borkowski, Peter Strasser, Enrique Herrero, Juan M. Feliu

The effect of interfacial pH on the surface atomic elemental distribution and on the catalytic reactivity of shape-selected bimetallic nanoparticles towards oxygen reduction

Nano Energy 27, 390-401

DOI: 10.1016/j.nanoen.2016.07.024

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The effect of interfacial pH during the surface cleaning of shape-selected PtNi nanoparticles was investigated. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) elemental mapping techniques were used to analyze the morphology and composition of the particles at the nanoscale. The particles show similar atomic compositions for both treated samples but different elemental distribution on the surface of the nanooctahedra. X-ray photoelectron spectroscopy (XPS) analysis confirmed different surface compositions and the presence of different oxidation states species at the outer part of the nanoparticles. In addition, we compare characteristic voltammetric profiles of these nanocatalysts when immersed in three different aqueous supporting electrolytes (H₂SO₄, HClO₄ and NaOH). The behavior of the bimetallic nanoparticles towards adsorbed CO oxidation has been analyzed and compared with that observed after surface disordering of the same catalysts. The electrocatalytic activity of these nanoparticles has been also tested for the electroreduction of oxygen showing high specific and mass activity and better catalytic performance than pure Pt shaped nanoparticles. The different treatments applied to the surface of the nanocatalysts have led to remarkably different catalytic responses, pointing out the outstanding importance of the control of the surface of the alloyed shape-selected nanoparticles after their synthesis and before their use as electrocatalysts.

Fabio Dionigi, Peter Strasser

NiFe-Based (Oxy)hydroxide Catalysts for Oxygen Evolution Reaction in Non-Acidic Electrolytes

Adv. Energy Mater. 6 (23), 1-20

DOI: 10.1002/aenm.201600621

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NiFe-based (oxy)hydroxides are highly active catalysts for the oxygen evolution reaction in alkaline electrolyte solutions. These catalysts can be synthesized in different ways leading to nanomaterials and thin films with distinct morphologies, stoichiometries and long-range order. Notably, their structure evolves under oxygen evolution operating conditions with respect to the as-synthesized state. Therefore, many researchers have dedicated their efforts on the identification of the catalytic active sites employing in operando experimental methods and theoretical calculations. These investigations are pivotal to rationally design materials with outstanding performances that will constitute the anodes of practical commercial alkaline electrolyzers. The family of NiFe-based oxyhydroxide catalysts reported in recent years is addressed and the actual state of the research with special focus on the understanding of the oxygen-evolution-reaction active sites and phase is described. Finally, an overview on the proposed oxygen-evolution-reaction mechanisms occurring on NiFe-based oxyhydroxide electrocatalysts is provided.

Hemma Mistry, Ana Sofia Varela, Cecile S. Bonifacio, Ioannis Zegkinoglou, Ilya Sinev, Yong-Wook Choi, Kim Kisslinger, Eric A. Stach, Judith C. Yang, Peter Strasser and Beatriz Roldan Cuenya

Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene

Nature Communications 7 (12123), 1-8

DOI: 10.1038/ncomms12123

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There is an urgent need to develop technologies that use renewable energy to convert waste products such as carbon dioxide into hydrocarbon fuels. Carbon dioxide can be electrochemically reduced to hydrocarbons over copper catalysts, although higher efficiency is required. We have developed oxidized copper catalysts displaying lower overpotentials for carbon dioxide electroreduction and record selectivity towards ethylene (60%) through facile and tunable plasma treatments. Herein we provide insight into the improved performance of these catalysts by combining electrochemical measurements with microscopic and spectroscopic characterization techniques. Operando X-ray absorption spectroscopy and crosssectional scanning transmission electron microscopy show that copper oxides are surprisingly resistant to reduction and copperþ species remain on the surface during the reaction. Our results demonstrate that the roughness of oxide-derived copper catalysts plays only a partial role in determining the catalytic performance, while the presence of copper⁺ is key for lowering the onset potential and enhancing ethylene selectivity.

Peter Strasser, Stefanie Kühl

Dealloyed Pt-based Core-Shell Oxygen Reduction Electrocatalysts

Nano Energy 29, 166-177

DOI: 10.1016/j.nanoen.2016.04.04

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Dealloyed Pt core-shell nanoparticles constitute the most active and stable bimetallic oxygen reduction catalysts for low-temperature fuel cells. Here, we review recent advances on their preparation, structural characterization, and electrocatalytic performance. Starting with bimetallic metal overlayer model systems, for which we illustrate fundamental principles of the ORR activity enhancements of dealloyed nanoparticles, we discuss progress in our understanding of structure-activity relations of dealloyed nanoparticle catalysts, both in idealized liquid-electrolyte cell formats and more realistic Membrane Electrode Assemblies (MEAs).

Sören Dresp, Fang Luo, Roman Schmack, Stefanie Kühl, Manuel Gliech and Peter Strasser

An efficient bifunctional two-component catalyst for Oxygen Reduction and Oxygen Evolution in reversible fuel cells, electrolyzers and rechargeable air electrodes

Energy Environ. Sci. 9, 2020-2024

DOI: 10.1039/C6EE01046F

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We report on a non-precious, two-phase bifunctional oxygen reduction and evolution (ORR, OER) electrocatalyst with previously unachieved combined roundtrip catalytic reactivity and stability for use in oxygen electrodes of unitized reversible fuel cell/electrolyzers or rechargeable metal-air batteries. The combined OER,ORR overpotential, total , at 10 mA cm-2 was a record low value of 0.747 V. Rotating Ring Disk Electrode (RRDE) measurements revealed a high faradaic selectivity for the 4 electron pathways, while subsequent continuous MEA tests in reversible electrolyzer cells over 24 h confirmed the excellent catalyst reactivity rivaling the state-of-art combination of Iridium (OER) and Platinum (ORR). 

Lin Gan, Stefan Rudi, Chunhua Cui, Marc Heggen, Peter Strasser

Size-Controlled Synthesis of Sub-10 nm PtNi 3 Alloy Nanoparticles and their Unusual Volcano-Shaped Size Effect on ORR Electrocatalysis

Small 12 (23), 3189–3196

DOI: 10.1002/smll.201600027

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Dealloyed Pt bimetallic core–shell catalysts derived from low-Pt bimetallic alloy nanoparticles (e.g, PtNi₃) have recently shown unprecedented activity and stability on the cathodic oxygen reduction reaction (ORR) under realistic fuel cell conditions and become today’s catalyst of choice for commercialization of automobile fuel cells. A critical step toward this breakthrough is to control their particle size below a critical value (≈10 nm) to suppress nanoporosity formation and hence reduce significant base metal (e.g., Ni) leaching under the corrosive ORR condition. Fine size control of the sub-10 nm PtNi 3 nanoparticles and understanding their size dependent ORR electrocatalysis are crucial to further improve their ORR activity and stability yet still remain unexplored. A robust synthetic approach is presented here for sizecontrolled PtNi 3 nanoparticles between 3 and 10 nm while keeping a constant particle composition and their size-selected growth mechanism is studied comprehensively. This enables us to address their size-dependent ORR activities and stabilities for the first time. Contrary to the previously established monotonic increase of ORR specific activity and stability with increasing particle size on Pt and Pt-rich bimetallic nanoparticles, the Pt-poor PtNi 3 nanoparticles exhibit an unusual “volcano-shaped” size dependence, showing the highest ORR activity and stability at the particle sizes between 6 and 8 nm due to their highest Ni retention during long-term catalyst aging. The results of this study provide important practical guidelines for the size selection of the low Pt bimetallic ORR electrocatalysts with further improved durably high activity.

Prashanth W. Menezes, Arindam Indra, Arno Bergmann, Petko Chernev, Carsten Walter, Holger Dau, Peter Strasser and Matthias Driess

Uncovering the prominent role of metal ions in octahedral versus tetrahedral sites of cobalt–zinc oxide catalysts for efficient oxidation of water

J. Mater. Chem. A 4, 10014–10022

DOI: 10.1039/c6ta03644a

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The fabrication and design of earth-abundant and high-performance catalysts for the oxygen evolution reaction (OER) are very crucial for the development and commercialization of sustainable energy conversion technologies. Although spinel catalysts have been widely explored for the electrochemical oxygen evolution reaction (OER), the role of two geometrical sites that influence their activities has not been well established so far. Here, we present more effective cobalt–zinc oxide catalysts for the OER than ‘classical’ Co₃O₄. Interestingly, the significantly higher catalytic activity of ZnCo₂O₄ than that of Co₃O₄ is somewhat surprising since both crystallize in the spinel-type structure. The reasons for the latter remarkable difference of ZnCo₂O₄ and Co₃O₄ could be deduced from structure–activity relationships of the bulk and near-surface of the catalysts using comprehensive electrochemical, microscopic and spectroscopic techniques with a special emphasis on the different roles of the coordination environment of metal ions (octahedral vs. tetrahedral sites) in the spinel lattice. The vital factors influencing the catalytic activity of ZnCo₂O₄over Co₃O₄ could be directly attributed to the higher amount of accessible octahedral Co³⁺ sites induced by the preferential loss of zinc ions from the surface of the ZnCo₂O₄catalyst. The enhanced catalytic activity is accompanied by a larger density of metal vacancies, defective sites and hydroxylation. The results obtained here clearly demonstrate how a surface structural modification and generation of defects of catalysts can enhance their OER performance.

Mikaela Görlin, Petko Chernev, Jorge Ferreira de Araújo, Tobias Reier, Sören Dresp, Benjamin Paul, Ralph Krähnert, Holger Dau, and Peter Strasser

Oxygen Evolution Reaction Dynamics, Faradaic Charge Efficiency, and the Active Metal Redox States of Ni−Fe Oxide Water Splitting Electrocatalysts

J. Am. Chem. Soc. 138(17), 5603–5614

DOI: 10.1021/jacs.6b00332

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Mixed Ni−Fe oxides are attractive anode catalysts for efficient water splitting in solar fuels reactors. Because of conflicting past reports, the catalytically active metal redox state of the catalyst has remained under debate. Here, we report an in operando quantitative deconvolution of the charge injected into the nanostructured Ni−Fe oxyhydroxide OER catalysts or into reaction product molecules. To achieve this, we explore the oxygen evolution reaction dynamics and the individual faradaic charge efficiencies using operando differential electrochemical mass spectrometry (DEMS). We further use X-ray absorption spectroscopy (XAS) under OER conditions at the Ni and Fe K-edges of the electrocatalysts to evaluate oxidation states and local atomic structure motifs. DEMS and XAS data consistently reveal that up to 75% of the Ni centers increase their oxidation state from +2 to +3, while up to 25% arrive in the +4 state for the NiOOH catalyst under OER catalysis. The Fe centers consistently remain in the +3 state, regardless of potential and composition. For mixed Ni_100−xFe_x catalysts, where x exceeds 9 atomic %, the faradaic efficiency of O2 sharply increases from ∼30% to 90%, suggesting that Ni atoms largely remain in the oxidation state +2 under catalytic conditions. To reconcile the apparent low level of oxidized Ni in mixed Ni−Fe catalysts, we hypothesize that a kinetic competition between the (i) metal oxidation process and the (ii) metal reduction step during O2 release may account for an insignificant accumulation of detectable high-valent metal states if the reaction rate of process (ii) outweighs that of (i). We conclude that a discussion of the superior catalytic OER activity of Ni−FeOOH electrocatalysts in terms of surface catalysis and redox-inactive metal sites likely represents an oversimplification that fails to capture essential aspects of the synergisms at highly active Ni−Fe sites.

Fabio Dionigi, Tobias Reier, Zarina Pawolek, Manuel Gliech, and Peter Strasser

Design Criteria, Operating Conditions, and Nickel–Iron Hydroxide Catalyst Materials for Selective Seawater Electrolysis

ChemSusChem 9 (9), 962–972

DOI: 10.1002/cssc.201501581

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Seawater is an abundant water resource on our planet and its direct electrolysis has the advantage that it would not compete with activities demanding fresh water. Oxygen selectivity is challenging when performing seawater electrolysis owing to competing chloride oxidation reactions. In this work we propose a design criterion based on thermodynamic and kinetic considerations that identifies alkaline conditions as preferable to obtain high selectivity for the oxygen evolution reaction. The criterion states that catalysts sustaining the desired operating current with an overpotential <480 mV in alkaline pH possess the best chance to achieve 100 % oxygen/hydrogen selectivity. NiFe layered double hydroxide is shown to satisfy this criterion at pH 13 in seawater-mimicking electrolyte. The catalyst was synthesized by a solvothermal method and the activity, surface redox chemistry, and stability were tested electrochemically in alkaline and near-neutral conditions (borate buffer at pH 9.2) and under both fresh seawater conditions. The Tafel slope at low current densities is not influenced by pH or presence of chloride. On the other hand, the addition of chloride ions has an influence in the temporal evolution of the nickel reduction peak and on both the activity and stability at high current densities at pH 9.2. Faradaic efficiency close to 100 % under the operating conditions predicted by our design criteria was proven using in situ electrochemical mass spectrometry.

Manuel Gliech, Arno Bergmann, Camillo Spöri and Peter Strasser

Synthesis–structure correlations of manganese–cobalt mixed metal oxide nanoparticles

Journal of Energy Chemistry 25, 278-281

DOI: 10.1016/j.jechem.2016.01.002

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Mixed metal oxides in the nanoscale are of great interest for many aspects of energy related research topics as water splitting, fuel cells and battery technology. The development of scalable, cost-efficient and robust synthetic routes toward well-defined solid state structures is a major objective in this field. While monometallic oxides have been studied in much detail, reliable synthetic recipes targeting specific crystal structures of mixed metal oxide nanoparticles are largely missing. Yet, in order to meet the requirements for a broad range of technical implementation it is necessary to tailor the properties of mixed metal oxides to the particular purpose. Here, we present a study on the impact of the nature of the gas environment on the resulting crystal structure during a post-synthesis thermal heat treatment of manganese-cobalt oxide nanoparticles. We monitor the evolution of the crystal phase structure as the gas atmosphere is altered from pure nitrogen to synthetic air and pure oxygen. The particle size and homogeneity of the resulting nanoparticles increase with oxygen content, while the crystal structure gradually changes from rocksalt-like to pure spinel. We find the composition of the particles to be independent of the gas atmosphere. The manganese-cobalt oxide nanoparticles exhibited promising electrocatalytic activity regarding oxygen evolution in alkaline electrolyte. These findings offer new synthesis pathways for the direct preparation of versatile utilizable mixed metal oxides.

Hemma Mistry, Ana Sofia Varela, Stefanie Kühl, Peter Strasser and Beatriz Roldan Cuenya

Nanostructured electrocatalysts with tunable activity and selectivity

Nature Reviews Materials 1, 16009

DOI: 10.1038/natrevmats.2016.9

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The field of electrocatalysis has undergone tremendous advancement in the past few decades, in part owing to improvements in catalyst design at the nanoscale. These developments have been crucial for the realization of and improvement in alternative energy technologies based on electrochemical reactions such as fuel cells. Through the development of novel synthesis methods, characterization techniques and theoretical methods, rationally designed nanoscale electrocatalysts with tunable activity and selectivity have been achieved. This Review explores how nanostructures can be used to control electrochemical reactivity, focusing on three model reactions: O₂ electroreduction, CO₂ electroreduction and ethanol electrooxidation. The mechanisms behind nanoscale control of reactivity are discussed, such as the presence of
low-coordinated sites or facets, strain, ligand effects and bifunctional effects in multimetallic materials. In particular, studies of how particle size, shape and composition in nanostructures can be used to tune reactivity are highlighted.

Ana Sofia Varela, Wen Ju, Tobias Reier, and Peter Strasser

Tuning the Catalytic Activity and Selectivity of Cu for CO₂ Electroreduction in the Presence of Halides

ACS Catal.6, 2136–2144

DOI: 10.1021/acscatal.5b02550

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In the present study we demonstrate that the activity and selectivity of copper during CO₂ electrochemical reduction can be tuned by simply adding halides to the electrolyte. Comparing the production rate and Faradaic selectivity of the major products as a function the working potential in the presence of Cl^–, Br^–, and I^–, we show that the activity and selectivity of Cu depends on the concentration and nature of the added halide. We find that the addition Cl^– and Br^– results in an increased CO selectivity. On the contrary, in the presence of I^– the selectivity toward CO drops down and instead methane formation is enhanced up to 6 times compared with the halide-free electrolyte. Even though Br^– and I^– can induce morphology changes of the surface, the modification in the catalytic performance of Cu is mainly attributed to halides adsorption on the Cu surface. We hypothesizes that the adsorption of halides alters the catalytic performance of Cu by increasing the negative charge on the surface according to the following order: Cl^– < Br^– < I^–. In the case of adsorbed I^–, the induced negative charge has a remarkably positive effect favoring the protonation of CO. These results present an easy way to enhance CH₄ production during the CO2RR on Cu. Furthermore, understanding this effect can contribute to the design of new and more efficient catalysts.

Vera Beermann, Martin Gocyla, Elena Willinger, Stefan Rudi, Marc Heggen, Rafal E. Dunin-Borkowski, Marc-Georg Willinger, and Peter Strasser

Rh-Doped Pt–Ni Octahedral Nanoparticles: Understanding the Correlation between Elemental Distribution, Oxygen Reduction Reaction, and Shape Stability

Nano Lett. 16, 1719−1725

DOI: 10.1021/acs.nanolett.5b04636

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Thanks to their remarkably high activity toward oxygen reduction reaction (ORR), platinum-based octahedrally shaped nanoparticles have attracted ever increasing attention in last years. Although high activities for ORR catalysts have been attained, the practical use is still limited by their long-term stability. In this work, we present Rh-doped Pt–Ni octahedral nanoparticles with high activities up to 1.14 A mg_Pt^–1 combined with improved performance and shape stability compared to previous bimetallic Pt–Ni octahedral particles. The synthesis, the electrocatalytic performance of the particles toward ORR, and atomic degradation mechanisms are investigated with a major focus on a deeper understanding of strategies to stabilize morphological particle shape and consequently their performance. Rh surface-doped octahedral Pt–Ni particles were prepared at various Rh levels. At and above about 3 atom %, the nanoparticles maintained their octahedral shape even past 30 000 potential cycles, while undoped bimetallic reference nanoparticles show a complete loss in octahedral shape already after 8000 cycles in the same potential window. Detailed atomic insight in these observations is obtained from aberration-corrected scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) analysis. Our analysis shows that it is the migration of Pt surface atoms and not, as commonly thought, the dissolution of Ni that constitutes the primary origin of the octahedral shape loss for Pt–Ni nanoparticles. Using small amounts of Rh we were able to suppress the migration rate of platinum atoms and consequently suppress the octahedral shape loss of Pt–Ni nanoparticles.

Frédéric Hasché, Mehtap Oezaslan, Peter Strasser, Tim-Patrick Fellinger

Electrocatalytic hydrogen peroxide formation on mesoporous non-metal nitrogen-doped carbon catalyst

Journal of Energy Chemistry 25, 251-257

DOI: 10.1016/j.jechem.2016.01.024

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Direct electrochemical formation of hydrogen peroxide (H₂O₂) from pure O₂ and H₂ on cheap metal-free earth abundant catalysts has emerged as the highest atom-efficient and environmentally friendly reaction pathway and is therefore of great interest from an academic and industrial point of view. Very recently, novel metal-free mesoporous nitrogen-doped carbon catalysts have attracted large attention due to the unique reactivity and selectivity for the electrochemical hydrogen peroxide formation. In this work, we provide deeper insights into the electrocatalytic activity, selectivity and durability of novel metal-free mesoporous nitrogen-doped carbon catalyst for the peroxide formation with a particular emphasis on the influence of experimental reaction parameters such as pH value and electrode potential for three different electrolytes. We used two independent approaches for the investigation of electrochemical hydrogen peroxide formation, namely rotating ring-disk electrode (RRDE) technique and photometric UV–VIS technique. Our electrochemical and photometric results clearly revealed a considerable peroxide formation activity as well as high catalyst durability for the metal-free nitrogen-doped carbon catalyst material in both acidic as well as neutral medium at the same electrode potential under ambient temperature and pressure. In addition, the obtained electrochemical reactivity and selectivity indicate that the mechanisms for the electrochemical formation and decomposition of peroxide are strongly dependent on the pH value and electrode potential.

Mikaela Görlin, Manuel Gliech, Jorge Ferreira de Araújo, Sören Dresp, Arno Bergmann, Peter Strasser

Dynamical changes of a Ni-Fe oxide water splitting catalyst investigated at different pH

Catalysis Today 262, 65–73

DOI: 10.1016/j.cattod.2015.10.018

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Mixed Ni-Fe oxide electrocatalysts have shown high catalytic activity for the oxygen evolution reaction (OER) in alkaline electrolyte. Fundamental research on mixed Ni-Fe OER catalysts has largely focused on high pH, while the OER activity near neutral pH has remained poorly studied. Here we review the activity of an amorphous mixed Ni-Fe oxyhydroxide catalyst supported on carbon (NiFeO_x/C) in 0.1 M KOH pH 13, in 0.1 M borate buffer (B_i) pH 9.2, and in 0.1 M phosphate buffer (P_i) pH 7.0. The OER catalytic performance was found to decrease in the order of pH 13 > pH 9.2 > pH 7. In contrast to pH 13 and 9.2, the catalyst cycled in pH 7 showed an instantaneous decrease in OER activity and a simultaneous loss of the Ni(OH)₂/NiOOH redox peak. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) showed the formation of crystalline areas upon CV cycling, which appeared more Ni enriched after cycling in pH 7. Deactivated electrodes cycled in pH 13 recovered the OER activity along with a partial reappearance of the Ni redox peak when subsequently cycled in pH 13. SEM-EDX spectroscopy confirmed compositional changes in the bulk during cycling at different pH, with an extensive leaching of Ni in pH 7. Our study provides new insight into the OER activity upon exposure to different electrolyte conditions, which unveils a highly dynamic Ni-Fe oxide framework. 

Lin Gan, Marc Heggen, Chunhua Cui, and Peter Strasser

Thermal Facet Healing of Concave Octahedral Pt–Ni Nanoparticles Imaged in Situ at the Atomic Scale: Implications for the Rational Synthesis of Durable High-Performance ORR Electrocatalysts

ACS Catal. 6 (2), 692–695

DOI: 10.1021/acscatal.5b02620

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We performed in situ transmission electron microscopy of phase-segregated octahedral Pt–Ni alloy fuel cell nanocatalysts under thermal annealing to study their morphological stability and surface compositional evolution. The pristine octahedral Pt–Ni nanoparticles (NPs) showed Pt-rich corners/edges and slightly concave Ni-rich {111} facets. Time-resolved image series unequivocally revealed that upon annealing up to 500 °C, the Pt-rich surface atoms at the corners/edges diffused onto and subsequently covered the concave Ni-rich {111} surfaces, leading to perfectly flat Pt-rich {111} surfaces with Ni-rich subsurface layers. This was further corroborated by in situ aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy. Our results propose a feasible approach to construct shaped Pt alloy nanoparticles with Pt-rich {111} surfaces and Ni-rich subsurface layers that are expected to be catalytically active and stable for the oxygen reduction reaction, thus providing important implications for rational synthesis of durably highly active shaped Pt alloy fuel cell electrocatalysts.

Hemma Mistry, Farzad Behafarid, Rulle Reske, Ana Sofia Varela, Peter Strasser, and Beatriz Roldan Cuenya

Tuning Catalytic Selectivity at the Mesoscale via Interparticle Interactions

ACS Catal. 6, 1075−1080

DOI: 10.1021/acscatal.5b02202

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The selectivity of heterogeneously catalyzed chemical reactions is well-known to be dependent on nanoscale determinants, such as surface atomic geometry and composition. However, principles to control the selectivity of nanoparticle (NP) catalysts by means of mesoscopic descriptors, such as the interparticle distance, have remained largely unexplored. We used well-defined copper catalysts to deconvolute the effect of NP size and distance on product selectivity during CO₂ electroreduction. Corroborated by reaction-diffusion modeling, our results reveal that mesoscale phenomena such as interparticle reactant diffusion and readsorption of intermediates play a defining role in product selectivity. More importantly, this study uncovers general principles of tailoring NP activity and selectivity by carefully engineering size and distance. These principles provide guidance for the rational design of mesoscopic catalyst architectures in order to enhance the production of desired reaction products.

Ana Sofia Varela, Matthias Kroschel, Tobias Reier, Peter Strasser

Controlling the selectivity of CO₂ electroreduction on copper: The effect of the electrolyte concentration and the importance of the local pH

Catalysis Today 260, 8–13

DOI: 10.1016/j.cattod.2015.06.009

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In the present study we demonstrate that the activity and selectivity of copper during the CO₂ electrochemical reduction can be tuned by changing the concentration of the bicarbonate electrolyte. Comparing the absolute formation rate and Faradaic selectivity of H₂, CH₄, CO, and C₂H₄ as a function of the applied electrode potential, we show that variations in the bulk buffer capacities of the electrolyte have substantial impact on absolute product formation rates and relative faradic selectivity. We find that high concentrations of bicarbonate improve the overall Faradaic CO₂ electroreduction activity, largely due to higher absolute formation rates of H₂ and CH₄. In lower-concentrated bicarbonate electrolytes with their lower overall activity, the selectivity toward ethylene was drastically enhanced. Following earlier theoretical work, we hypothesize the pH near the copper electrode interface to largely account for the observed effects: diluted KHCO₃ solutions allow for more alkaline local pH values during CO₂ electroreduction. Our study highlights the controlling role of the interfacial pH on the product distribution during CO₂ reduction over a wide electrode potential range.

Panagiotis Trogadas, Vijay Ramani, Peter Strasser, Thomas F. Fuller, and Marc-Olivier Coppens

Hierarchically Structured Nanomaterials for Electrochemical Energy Conversion

Angew. Chem. Int. Ed. 55, 122–148

DOI: 10.1002/anie.201506394

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Hierarchical nanomaterials are highly suitable as electrocatalysts and electrocatalyst supports in electrochemical energy conversion devices. The intrinsic kinetics of an electrocatalyst are associated with the nanostructure of the active phase and the support, while the overall properties are also affected by the mesostructure. Therefore, both
structures need to be controlled. A comparative state-of-the-art review of catalysts and supports is provided along with detailed synthesis methods. To further improve the design of these hierarchical nanomaterials, in-depth research on the effect of materials architecture on reaction and transport kinetics is necessary. Inspiration can be derived
from nature, which is full of very effective hierarchical structures. Developing fundamental understanding of how desired properties of biological systems are related to their hierarchical architecture can guide the development of novel catalytic nanomaterials and natureinspired electrochemical devices.