2021

Philipp Hauke, Malte Klingenhof, Xingli Wang, Jorge Ferreira de Araújo, Peter Strasser

Efficient electrolysis of 5-hydroxymethylfurfural to the biopolymer-precursor furandicarboxylic acid in a zero-gap MEA-type electrolyzer

Cell Reports Physical Science

DOI: 10.1016/j.xcrp.2021.100650

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Replacement of today’s established chemical production processes by “green” sustainable alternatives has become a scientific priority. The oxidative conversion of 5-hydroxymethylfurfural (5-HMF) to the biopolymer component 2.5-furandicarboxylic acid paired with green electrolytic hydrogen production is a promising emerging green process. Here, we present a family of active selective and stable interlayer anion-tuned NiX (X = Fe, Mn, Co, V) bimetallic-layered, double-hydroxide catalysts for the selective oxidation of 5-HMF to 2.5-furandicarboxylic acid in a zero-gap MEA-type electrolyzer. We report that tuning the structural interlayer distance of the catalyst by anion exchange gives rise to previously unachieved catalytic performance for the anodic production of the biopolymer building block. Operando differential electrochemical mass spectrometry analysis reveals the electrode window for the perfectly selective HMF conversion. The role of the catalyst dopants, their real surface areas, the stability of the catalytic interface, and aspects of its favorable techno-economics are discussed.

Sudarshan Vijay, Wen Ju, Sven Brückner, Sze-Chun Tsang, Peter Strasser and Karen Chan

Unified mechanistic understanding of CO2 reduction to CO on transition metal and single atom catalysts

Nature Catalysis

DOI: 10.1038/s41929-021-00705-y

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CO is the simplest product from CO2 electroreduction (CO2R), but the identity and nature of its rate-limiting step remain controversial. Here we investigate the activity of transition metals (TMs), metal–nitrogen-doped carbon catalysts (MNCs) and a supported phthalocyanine, and present a unified mechanistic picture of the CO2R to CO for these catalysts. Applying the Newns–Andersen model, we find that on MNCs, like TMs, electron transfer to CO2 is facile. We find CO2* adsorption to generally be limiting on TMs, whereas MNCs can be limited by either CO2* adsorption or by the proton–electron transfer reaction to form COOH*. We evaluate these computed mechanisms against pH-dependent experimental activity measurements on the CO2R to CO activity. We present a unified activity volcano that includes the decisive CO2* and COOH* binding strengths. We show that the increased activity of MNC catalysts is due to the stabilization of larger adsorbate dipoles, which results from their discrete and narrow d states.

Cornelia Broicher, Malte Klingenhof, Marvin Frisch, Sören Dresp, Nikolas Mao Kubo, Jens Artz, Jörg Radnik, Stefan Palkovits, Anna Katharina Beine, Peter Strasser and Regina Palkovits

Particle size-controlled synthesis of high-performance MnCo-based materials for alkaline OER at fluctuating potentials

Catalysis Science & Technology

DOI:10.1039/d1cy00905b

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For the large-scale generation of hydrogen via water electrolysis the design of long term stable and active catalysts for the oxygen evolution reaction (OER) remains a key challenge. Most catalysts suffer from severe structural corrosion that becomes even more pronounced at fluctuating potentials. Herein, MnCo based cubic particles were prepared via a hydrothermal approach, in which the edge length of the micron-sized particles can be controlled by changing the pH value of the precursor solution. The cubes are composed of varying amounts of MnCo2O4, CoCO3 and a mixed (Mn/Co)CO3 phase. Structure–activity relationships were deduced revealing a volcano-type behavior for the intrinsic OER activity and fraction of spinel oxide phase. A low overpotential of 0.37 V at 10 mA cm−2 and a stability of more than 25 h was achieved in 1.0 M KOH using a rotating disc electrode (RDE) setup. The best performing catalyst material was successfully tested under dynamic process conditions for 9.5 h and shows a superior catalytic activity as anode for the overall water splitting in an electrolyser setup in 1.0 M KOH at 333 K compared to a reference NiCo-spinel catalyst.

Christian Hoffmann, Jessica Hübner, Franziska Klaucke, Natasa Milojevic, Robert Mueller, Maximilian Neumann, Joris Weigert, Erik Esche, Mathias Hofmann, Jens-Uwe Repke, Reinhard Schomäcker, Peter Strasser, and George Tsatsaronis

Assessing the Realizable Flexibility Potential of Electrochemical Processes

Industrial & Engineering Chemistry Research

DOI:10.1021/acs.iecr.1c01360

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Demand response is a viable concept to deal with and benefit from fluctuating electricity prices and is of growing interest to the electrochemical industry. To assess the flexibility potential of such processes, a generic, interdisciplinary methodology is required. We propose such a methodology, in which the electrochemical fundamentals and the theoretical potential are determined first by analyzing strengths, weaknesses, opportunities, and threats. Afterward, experiments are conducted to determine selectivity and yield under varying loads and to assess the additional longterm costs associated with flexible operation. An industrial-scale electrochemical process is assessed regarding its technical, economic, and practical potential. The required steps include a flow sheet analysis, the formulation and solution of a simplified model for operation scheduling under various business options, and a dynamic optimization based on rigorous, dynamic process models. We apply the methodology to three electrochemical processes of different technology readiness levelsthe syntheses of hydrogen peroxide, adiponitrile, and 1,2-dichloroethane via chloralkali electrolysisto illustrate the individual steps of the proposed methodology.

Chengyi Lu, Meng Tian, Xiangjun Zheng, Chaohui Wei, Mark H. Rummeli, Peter Strasser, Ruizhi Yang

Cotton pad derived 3D lithiophilic carbon host for robust Li metal anode: In-situ generated ionic conductive Li3N protective decoration

Chemical Engineering Journal

DOI:10.1016/j.cej.2021.132722

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Lithium metal anode is considered as one of the most promising candidates for the next-generation batteries with high specific energy density. However, several thorny problems encompassing uncontrollable Li dendritic growth and wild volume variation during cycling, accompanied by the short lifespan and alarming safety concerns, have hindered the commercial viability of Li metal-based batteries. In this contribution, we designed a Li composite anode fabricated via Li infusion into N, O co-doped and Ag coated 3D carbon host from simple treatments of commercial cotton pads, referred as Ag-NOCP@Li. The incorporation of multi lithiophilic atoms can significantly improve the affinity of 3D carbon host towards Li. More importantly, during molten Li infiltration, the composite anode can in-situ generate Li3N decoration, an excellent Li+ conductor and electron insulator. The first-principles calculations further revealed that the active sites for the Li3N generation most likely are pyrrolic nitrogen sites. The Li3N with favorable mechanical strength and ultra-fast Li+ diffusion rate can effectively boost the kinetics of Li transport and redox, as well as inhibit the dendritic generation. Besides, the Ag-NOCP with hierarchical pores and multi-microchannel within the nanofibers, allows the rapid Li+ diffusion and buffers the volume change over long cycling. Therefore, such Ag-NOCP@Li electrode could maintain a stable cycling for 1400 h at 1.0 mA cm− 2/1.0 mAh cm− 2. The full cells using Ag-NOCP@Li anode paired with LiFePO4 and LiNi0.5Co0.2Mn0.3O2 cathodes, displayed impressive long-term cyclic stability up to 400cycles at 0.5 and 1.0C, respectively. This work paves new way for rational design of 3D lithiophilic host towards durable Li anode.

Raphaël Chattot, Isaac Martens, Marta Mirolo, Michal Ronovsky, Florian Russello, Helena Isern, Guillaume Braesch, Elisabeth Hornberger, Peter Strasser, Eric Sibert, Marian Chatenet, Veijo Honkimäki, and Jakub Drnec

Electrochemical Strain Dynamics in Noble Metal Nanocatalysts

J. Am. Chem. Soc.

DOI:10.1021/jacs.1c06780

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The theoretical design of effective metal electrocatalysts for energy conversion and storage devices relies greatly on supposed unilateral effects of catalysts structure on electrocatalyzed reactions. Here, by using high-energy X-ray diffraction from the new Extremely Brilliant Source of the European Synchrotron Radiation Facility (ESRF-EBS) on device-relevant Pd and Pt nanocatalysts during cyclic voltammetry experiments in liquid electrolytes, we reveal the near ubiquitous feedback from various electrochemical processes on nanocatalyst strain. Beyond challenging and extending the current understanding of practical nanocatalysts behavior in electrochemical environment, the reported electrochemical strain provides experimental access to nanocatalysts absorption and adsorption trends (i.e., reactivity and stability descriptors) operando. The ease and power in monitoring such key catalyst properties at new and future beamlines is foreseen to provide a discovery platform toward the study of nanocatalysts encompassing a large variety of applications, from model environments to the device level.

Tim Moeller, Trung Ngo Thanh, Xingli Wang, Wen Ju, Zarko Jovanov and Peter Strasser

The product selectivity zones in gas diffusion electrodes during the electrocatalytic reduction of CO2

Energy & Environmental Science

DOI:10.1039/d1ee01696b

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Here we report on the most prominent factors influencing the performance of a Cu-based CO2 electrolyzer operating at high currents. Using a flow-electrolyzer design where CO2 gas feed passes directly through the electrode interacting with the Cu catalyst layer, we observed that the selectivity of the electrochemical CO2 reduction in (bulk) pH neutral media can greatly be influenced by adjusting the structure of the electrode. In this, the variations in catalyst loading and ionomer content can profoundly affect the selectivity of CO2RR. We explore the hypothesis that this originates from the overall mass transport variations within the porous catalytic layer of the gas diffusion electrode. As further evidence for this, apart from the CO2 electrolysis results, we propose a special method to benchmark the reactant mass transport in flow-cells using oxygen reduction reaction (ORR) limiting current measurements. Our analysis suggests that a restriction of mass transport is highly desirable due to its connection to a local alkalization and corresponding suppression of pH-dependent reaction products, given the absence of local CO2 concentration limitations. We further show how the electrode structure can be used to push the observed catalytic CO2 reduction selectivity either towards C1 or C2+ products, dependent on the ionomer content and catalyst loading in a cathodic current range of 50 to 700 mA cm2. Measurements at various KHCO3 electrolyte concentrations agree with the notion of the local pH dictating the overall selectivity and point towards the presence of pronounced concentration gradients within the system. Overall, our work suggests that the differences in electrocatalytic CO2 reduction selectivity at high currents (in a range of pH neutral buffering electrolytes) largely originate from the local concentration gradients defined by the initial catalyst ink formulation and architecture of the catalytic layer, both of which represent a powerful tool for optimization in the production of selected value-added products.

Elisabeth Hornberger, Valentina Mastronardi, Rosaria Brescia, Pier Paolo Pompa, Malte Klingenhof, Fabio Dionigi, Mauro Moglianetti and Peter Strasser

Seed-Mediated Synthesis and Catalytic ORR Reactivity of Facet-Stable, Monodisperse Platinum Nano-Octahedra

ACS Applied Energy Materials

DOI:10.1021/acsaem.1c01696

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Shape-selective, sub-10 nm-sized metal nanoparticles are of high fundamental and practical interest in catalysis and electrocatalysis, where the surface structure dictates the kinetic properties of the nanomaterials. Unlike their bimetallic analogues, the synthesis of size-controlled, pure Pt octahedral nanocatalysts has remained a formidable chemical challenge. In bimetallic shaped systems, however, the benefit of shape is often convoluted with surface composition in complex ways. In the present work, a seedtemplated approach is presented for the preparation of ultrasmall octahedral platinum nanoparticles (Pt NPs), harnessing the effect of monocrystalline anisotropic seeds and strict control of the reduction rate and other physicochemical parameters while avoiding polymers, surfactants, and organic solvents. The procedure yields previously elusive 6.7 nm, strictly single-crystal, Pt NPs with partially truncated octahedral shape and prevalent extended {111} surface facets. Electrochemical measurements using rotating disk electrodes in an acid electrolyte revealed a much higher electrochemical active surface area (ECSA) over the state-of-the-art octahedral Pt NPs, which is ascribed to small-sized, poison-free, and preferentially {111} orientated facets. The dramatic kinetic benefit for the oxygen reduction reaction (ORR) of the octahedral shape over spherical particle shapes of same size is convincingly demonstrated. More important for practical applications is the fact that the intrinsic specific ORR activity is about 2.4-fold higher than commercial optimized spherical Pt NPs deployed in fuel cell cathodes at comparable ORR stability. In doing this analysis, we validate the voltammetric correspondence between Pt single crystals and Pt nanoparticulate materials and highlight the kinetic benefits of limiting the proportion of {100} facets. Prolonged suppression of {100} facet growth in octahedral Pt catalysts is the reason for the unusually high specific activity and fair stability and calls for their integration and testing as cathode catalysts in fuel cell membrane electrode assemblies.

Sebastian Ott, Andreas Bauer, Fengmin Du,Tuan Anh Dao, Malte Klingenhof, Alin Orfanidi and Peter Strasser

Impact of Carbon Support Meso‐Porosity on Mass Transport and Performance of PEMFC Cathode Catalyst Layers

ChemCatChem

DOI:10.1002/cctc.202101162

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The analysis of the impact of the cathode catalyst layer pore structure on the membrane electrode assembly (MEA) cell performance of a PEMFC is presented. In this study, a pristine CMK-3 catalyst carbon support material with well-defined pore structure in the 3–6 nm range together with two nitrogendoped variants is analyzed against a commercial carbon black to achieve a better understanding of catalyst layer porosityperformance relations. We used chemically N-doped CMK-3 catalyst to learn more about the effect of N-doped porous catalyst supports on the concomitant transport properties and PEMFC cell performance. Chemical treatment using cyanamide was conducted to introduce a variety of N-functionalities. A detailed in-situ electrochemical investigation was combined with N2-physisorption analysis. Based on their structural properties, the mesopore fractions and pore openings display a major role for reducing oxygen transport resistance and enhance Pt accessibility. We find that hierarchically ordered mesoporosity is superior to disordered porosity with prevalent micropore character: Analysis including adsorption electrochemical active surface area (ECSA), Pt-accessibility, ionomer coverage, pore geometry, proton resistivity and transport loss we conclude the importance of a well-defined mesoporous structure for its cell performance.

Yanyan Sun, Shlomi Polani1, Fang Luo, Sebastian Ott, Peter Strasser & Fabio Dionigi

Advancements in cathode catalyst and cathode layer design for proton exchange membrane fuel cells

Nature Communications

DOI:10.1038/s41467-021-25911-x

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Proton exchange membrane fuel cells have been recently developed at an increasing pace as clean energy conversion devices for stationary and transport sector applications. High platinum cathode loadings contribute significantly to costs. This is why improved catalyst and support materials as well as catalyst layer design are critically needed. Recent advances in nanotechnologies and material sciences have led to the discoveries of several highly promising families of materials. These include platinum-based alloys with shape-selectednanostructures, platinum-group-metal-free catalysts such as metal-nitrogen-doped carbon materials and modification of the carbon support to control surface properties and ionomer/catalyst interactions. Furthermore, the development of advanced characterization techniques allows a deeper understanding of the catalyst evolution under different conditions. This review focuses on all these recent developments and it closes with a discussion of future research directions in the field.

Shlomi Polani, Katherine E. MacArthur, Malte Klingenhof, Xingli Wang, Paul Paciok, Lujin Pan, Quanchen Feng, Attila Kormányos, Serhiy Cherevko, Marc Heggen, and Peter Strasser

Size and Composition Dependence of Oxygen Reduction Reaction Catalytic Activities of Mo-Doped PtNi/C Octahedral Nanocrystals

Acs Catalysis

DOI:10.1021/acscatal.1c01761

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A variety of synthesis protocols for octahedral PtNi nanocatalysts have led to remarkable improvements in platinum mass and specific activities for the oxygen reduction reaction. Nevertheless, the values achieved are still only one tenth of the activity measured from Pt3Ni single-crystal (111) surfaces. These particles lose activity during potential cycling, primarily because of Ni leaching and subsequent loss of shape. Here, we present the syntheses and high catalytic oxygen reduction reaction activities of molybdenum-doped PtNi octahedral catalysts with different sizes (6−14 nm) and compositions. We show that the Mo-doped, Nirich, PtNi octahedral catalysts exhibit enhanced stability over their undoped counterpart. Scanning transmission electron microscopy with energy-dispersive Xray analysis reveals the particular elemental distribution for the size and composition of the different catalysts. By combining high-resolution compositional analysis with electrochemical measurements and online inductively coupled plasma mass spectrometry, it was possible to correlate the size, morphology, and composition with the oxygen reduction reaction activities before and after accelerated stress tests. The octahedral catalysts show high electrochemical surface areas and increasing specific activity with increasing surface area of the (111) facets and Ni content, leading to high mass activities. These results demonstrate the advantages of increasing the (111) surface area and Ni content of PtNi nano-octahedral catalysts to improve the performance and stability for the oxygen reduction reaction.

Meng Tian, Chaohui Wei, Zhihui Sun, Ruizhi Yang, Peter Strasser

Unraveling the lithiophilic nature of heteroatom-doped carbons for efficient oxygen reduction in LieO(2) batteries

Carbon

DOI:10.1016/j.carbon.2021.03.002

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Nonaqueous rechargeable lithium-oxygen batteries are promising candidates of the next generation batteries. However, the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) seriously impede their practical implementations. Herein, heteroatom-doping strategies with respect to lithiophilicity chemistry are investigated to improve the catalytic performance of carbon materials during ORR process through first-principles calculations. The binding energy, electronegativity, electric dipole moment and lithium diffusion barrier are found to be key factors that govern the lithiophilicity of doped heteroatoms. Among all non-metallic elements, pyrrolic nitrogen, carboxylic group oxygen and ketone group oxygen doped graphene nanoribbons exhibit improved thermodynamics and kinetics of ORR, giving rise to uniform deposition of lithium oxides for further favorable OER. This work provides insights into the mechanisms of ORR as well as the formation of lithium oxides and a guidance to design an effectively catalytic cathode for lithium-oxygen batteries.

Zhihui Sun, Xuecheng Cao, Meng Tian, Kai Zeng, Yongxiang Jiang, Mark H. Rummeli, Peter Strasser, and Ruizhi Yang

Synergized Multimetal Oxides with Amorphous/Crystalline Heterostructure as Efficient Electrocatalysts for Lithium-Oxygen Batteries

Advanced Energy Materials

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High theoretical specific energy of rechargeable lithium–oxygen (Li–O2) batteries makes them very promising in the development of long driving range electric vehicles and energy storage on large-scale. However, the large polarization and poor cycling stability associated with insufficient catalytic cathodes and the insulating nature of discharge products limit their practical applications. Here, the fabrication of a trimetallic CoFeCe oxide with an amorphous/crystalline heterostructure acting as an electrocatalyst for the Li–O2 battery cathode is reported. The best-performing CoFeCe oxide cathode manages to deliver an initial discharge capacity of 12 340 mAh g−1, while maintaining an impressively enhanced cyclic stability over 2900 h at 100 mA g−1. As revealed by combined experimental results and density functional theory (DFT) analysis, synergistic interaction between oxide components, amorphous–crystalline domains, unique heterostructure with minimized lattice mismatch, and the enhanced adsorption of the key intermediate LiO2 are critical factors in boosting the electrocatalytic activity of CoFeCe toward the formation of decomposable Li2O2. This work offers a new insight to rationally design and synthesize an effective multimetal oxide electrocatalyst for the Li–O2 battery cathode.

Laura C. Pardo Pérez, Detre Teschner, Elena Willinger, Amandine Guiet, Matthias Driess, Peter Strasser, and Anna Fischer

In Situ Formed "Sn1-XInX@In1-YSnYOZ" Core@Shell Nanoparticles as Electrocatalysts for CO2 Reduction to Formate

Advanced Functional Materials

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Electrochemical reduction of CO2 (CO2RR) driven by renewable energy has gained increasing attention for sustainable production of chemicals and fuels. Catalyst design to overcome large overpotentials and poor product selectivity remains however challenging. Sn/SnOx and In/InOx composites have been reported active for CO2RR with high selectivity toward formate formation. In this work, the CO2RR activity and selectivity of metal/metal oxide composite nanoparticles formed by in situ reduction of bimetallic amorphous SnInOx thin films are investigated. It is shown that during CO2RR the amorphous SnInOx pre-catalyst thin films are reduced in situ into Sn1–XInX@In1–YSnYOz core@shell nanoparticles composed of Sn-rich SnIn alloy nanocores (with x < 0.2) surrounded by InOx-rich bimetallic InSnOx shells (with 0.3 < y < 0.4 and z ≈ 1). The in situ formed particles catalyze the CO2RR to formate with high faradaic efficiency (80%) and outstanding formate mass activity (437 A gIn+Sn−1 @ −1.0 V vs RHE in 0.1 m KHCO3). While extensive structural investigation during CO2RR reveals pronounced dynamics in terms of particle size, the core@shell structure is observed for the different electrolysis conditions essayed, with high surface oxide contents favoring formate over hydrogen selectivity.

Woong Hee Lee, Young-Jin Ko, Jung Hwan Kim, Chang Hyuck Choi, Keun Hwa Chae, Hansung Kim, Yun Jeong Hwang, Byoung Koun Min, Peter Strasser & Hyung-Suk Oh

High crystallinity design of Ir-based catalysts drives catalytic reversibility for water electrolysis and fuel cells

Nature Communications

DOI:10.1038/s41467-021-24578-8

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The voltage reversal of water electrolyzers and fuel cells induces a large positive potential on the hydrogen electrodes, followed by severe system degradation. Applying a reversible multifunctional electrocatalyst to the hydrogen electrode is a practical solution. Ir exhibits excellent catalytic activity for hydrogen evolution reactions (HER), and hydrogen oxidation reactions (HOR), yet irreversibly converts to amorphous IrOx at potentials > 0.8 V/RHE, which is an excellent catalyst for oxygen evolution reactions (OER), yet a poor HER and HOR catalyst. Harnessing the multifunctional catalytic characteristics of Ir, here we design a unique Ir-based electrocatalyst with high crystallinity for OER, HER, and HOR. Under OER operation, the crystalline nanoparticle generates an atomically-thin IrOx layer, which reversibly transforms into a metallic Ir at more cathodic potentials, restoring high activity for HER and HOR. Our analysis reveals that a metallic Ir subsurface under thin IrOx layer can act as a catalytic substrate for the reduction of Ir ions, creating reversibility. Our work not only uncovers fundamental, uniquely reversible catalytic properties of nanoparticle catalysts, but also offers insights into nanocatalyst design.

Sören Dresp, Fabio Dionigi, Malte Klingenhof, Thomas Merzdorf, Henrike Schmies, Jakub Drnec, Agnieszka Poulain, and Peter Strasser

Molecular Understanding of the Impact of Saline Contaminants and Alkaline pH on NiFe Layered Double Hydroxide Oxygen Evolution Catalysts

ACS Catal.

DOI:10.1021/acscatal.1c00773

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NiFe layered double hydroxides (LDHs) are among the most active electrocatalysts for alkaline oxygen evolution reaction (OER) and OER selective seawater oxidation. These promising applications call for a fundamental understanding of the catalyst/electrolyte interaction, which is challenging to investigate during operation conditions. This work reports an operando structure−reactivity analysis of NiFe LDH as the electrocatalyst for the OER in alkaline and alkalinized NaCl electrolytes, by combining operando wide-angle X-ray scattering (WAXS) and electrochemical characterization. The operando results showed that higher pH values lead to a higher percentage of the OER active γ-NiFe LDH in the composition of the catalyst layer, larger Ni redox peaks, and higher OER activity. The addition of 0.5 M NaCl to moderate alkaline electrolytes (0.1−0.5 M KOH) also leads to larger Ni redox features and higher activity but appears to limit the percentage of γ-NiFe LDH during the OER in comparison to the corresponding NaCl-free electrolytes. Interestingly, a higher KOH concentration (1.0 M KOH, pH 14) could compensate this structural effect aligning the percentage of OER-active γ-NiFe LDH in both NaCl-free and NaCl-containing electrolytes. Additional scan rate investigations showed a strong correlation of the electrochemical accessibility of NiFe LDH with its history, scan rate, and NaCl addition. In particular, the faster and more effective break-in process induced by NaCl addition is proposed as the origin of the enhanced activity at low pH, despite the lower γ-phase percentage.

Camillo Spöri, Lorenz J. Falling, Matthias Kroschel, Cornelius Brand, Arman Bonakdarpour, Stefanie Kühl, Dirk Berger, Manuel Gliech, Travis E. Jones, David P. Wilkinson, and Peter Strasser

Molecular Analysis of the Unusual Stability of an IrNbOx Catalyst for the Electrochemical Water Oxidation to Molecular Oxygen (OER)

ACS Appl Mater Interfaces

DOI:10.1021/acsami.0c12609

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Adoption of proton exchange membrane (PEM) water electrolysis technology on a global level will demand a significant reduction of today’s iridium loadings in the anode catalyst layers of PEM electrolyzers. However, new catalyst and electrode designs with reduced Ir content have been suffering from limited stability caused by (electro)chemical degradation. This has remained a serious impediment to a wider commercialization of larger-scale PEM electrolysis technology. In this combined DFT computational and experimental study, we investigate a novel family of iridium−niobium mixed metal oxide thin-film catalysts for the oxygen evolution reaction (OER), some of which exhibit greatly enhanced stability, such as minimized voltage degradation and reduced Ir dissolution with respect to the industry benchmark IrOx catalyst. More specifically, we report an unusually durable IrNbOx electrocatalyst with improved catalytic performance compared to an IrOx benchmark catalyst prepared in-house and a commercial benchmark catalyst (Umicore Elyst Ir75 0480) at significantly reduced Ir catalyst cost. Catalyst stability was assessed by conventional and newly developed accelerated degradation tests, and the mechanistic origins were analyzed and are discussed. To achieve this, the IrNbOx mixed metal oxide catalyst and its water splitting kinetics were investigated by a host of techniques such as synchrotron-based NEXAFS analysis and XPS, electrochemistry, and ab initio DFT calculations as well as STEM-EDX cross-sectional analysis. These analyses highlight a number of important structural differences to other recently reported bimetallic OER catalysts in the literature. On the methodological side, we introduce, validate, and utilize a new, nondestructive XRF-based catalyst stability monitoring technique that will benefit future catalyst development. Furthermore, the present study identifies new specific catalysts and experimental strategies for stepwise reducing the Ir demand of PEM water electrolyzers on their long way toward adoption at a larger scale.

Fabio Dionigi, Jing Zhu, Zhenhua Zeng, Thomas Merzdorf, Hannes Sarodnik, Manuel Gliech, Lujin Pan, Wei-Xue Li, Jeffrey Greeley, and Peter Strasser

Intrinsic Electrocatalytic Activity for Oxygen Evolution of Crystalline 3d-Transition Metal Layered Double Hydroxides

Angew Chem Int Ed

DOI:10.1002/anie.202100631

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Layered double hydroxides (LDHs) are among the most active and studied catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, previous studies have generally either focused on a small number of LDHs, applied synthetic routes with limited structural control, or used non-intrinsic activity metrics, thus hampering the construction of consistent structure–activity-relations. Herein, by employing new individually developed synthesis strategies with atomic structural control, we obtained a broad series of crystalline a-MA(II)MB(III) LDH and b-MA(OH)2 electrocatalysts (MA = Ni, Co, and MB = Co, Fe, Mn). We further derived their intrinsic activity through electrochemical active surface area normalization, yielding the trend NiFe LDH > CoFe LDH > Fe-free Co-containing catalysts > Fe-Co-free Nibased catalysts. Our theoretical reactivity analysis revealed that these intrinsic activity trends originate from the dual-metal-site nature of the reaction centers, which lead to compositiondependent synergies and diverse scaling relationships that may be used to design catalysts with improved performance.

Jessica Hübner, Benjamin Paul, Aleksandra Wawrzyniak and Peter Strasser

Polymer electrolyte membrane (PEM) electrolysis of H2O2 from O2 and H2O with continuous on-line spectrophotometric product detection: Load flexibility studies

Journal of Electroanalytical Chemistry

DOI:10.1016/j.jelechem.2021.115465

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Hydrogen peroxide (H2O2) is a green oxidant, widely used in industry. The electrochemical production of H2O2 via the two-electron oxygen reduction reaction (2eORR) is a promising approach for the replacement of the industrial anthraquinone process because it opens the possibility to produce hydrogen peroxide using renewable energies. Classical off-line characterize techniques for the detection of hydrogen peroxide are not suitable for dynamic conditions. To quantify the efficiency of the peroxide formation we developed a novel on-line analysis setup based on the spectrophotometric analysis with titanium oxysulfate, which allows the determination of hydrogen peroxide continuously at the moment it is formed. With our setup, the concentration is continuously measured and the influence of varying process parameter can be directly monitored. We validated the setup with load flexibility experiments, were rectangular current load steps by 33.3% above and below a reference load were performed within an electrolyzer. The evolution of the hydrogen peroxide concentration was directly measurable within 35 s and 95% of the main H2O2 concentration was detectable after 2 min. The applied load steps led to no visible performance decrease of the electrolyzer and faraday efficiencies of > 98% with a maximum production rate of 122.4 mg cm−2h−1 were reached in 0.1 M H2SO4 + 0.05 M K2SO4 for current densities up to −200 mA cm−2.

Jorge Ferreira de Araújo, Fabio Dionigi, Thomas Merzdorf, Hyung-suk Oh and Peter Strasser

Evidence of Mars-Van-Krevelen Mechanism in the Electrochemical Oxygen Evolution on Ni Based Catalysts

Angewandte Chemie International Edition, 2021

DOI: 10.1002/anie.202101698

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Water  oxidation  is  a  crucial  reaction  for  renewable  energy  conversion  and  storage. Among  the  alkaline  oxygen  evolution  reaction  (OER)  catalysts,  NiFe  based oxyhydroxides show the highest  catalytic activity. However, the details of their OER mechanism are still unclear, due to the elusive nature of the OER intermediates. Here by using a novel differential electrochemical mass spectrometry (DEMS) cell interface, we performed  water  isotope-labelled  experiments  in 18O-labelled  alkaline  electrolyte  on Ni(OH)2 and NiFe layered double hydroxide nanocatalysts. Our experiments confirm the occurrence of Mars-van-Krevelen lattice oxygen evolution reaction mechanism in both catalysts  to  various  degrees,  which  involves  the  coupling  of  oxygen  atoms  from  the catalyst  and  the  electrolyte.  The  quantitative  charge  analysis  suggests  that  the participating lattice oxygen atoms belong exclusively to the catalyst surface, confirming DFT computational  hypotheses. Also,  DEMS data  suggest  a  fundamental  correlation between the magnitude of the lattice oxygen mechanism and the faradaic efficiency of oxygen controlled by pseudocapacitive oxidative metal redox charges.

Yanyan Sun, Shuang Li, Benjamin Paul, Lei Han, Peter Strasser

Highly efficient electrochemical production of hydrogen peroxide over nitrogen and phosphorus dual-doped carbon nanosheet in alkaline medium

Journal of Electroanalytical Chemistry, 2021

DOI: 10.1016/j.jelechem.2021.115197

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Electrochemical two-electron oxygen reduction reaction (ORR) is a promising green method for hydrogen peroxide (H2O2) production, but the H2O2 selectivity and productivity in alkaline medium have remained
too low for economic viability of the process. In the present work, we reported the design and preparation of nitrogen and phosphorus dual-doped carbon nanosheet (NPCNS) through direct pyrolysis of supramolecular aggregates that resulted from the cross-linking reaction between nitrogen bio-functional groups of chitosan and phosphoric groups of phytic acid. The resultant NPCNS catalysts exhibited very high electrochemical ORR activity and selectivity toward H2O2 production in alkaline medium due to the unique 2D nanostructure and the synergistic effect between nitrogen and phosphorus dopant. High practical H2O2 production rate of 223.4 mm ol gcatayst−1 h−1 with high faradaic efficiency of 80% could be also achieved in homemade H-cell, indicating the potential applications of H2O2 in the waste water treatment, pulp and paper industry.

Camillo Spoeri, Cornelius Brand, Matthias Kroschel and Peter Strasser

Accelerated Degradation Protocols for Iridium-Based Oxygen Evolving Catalysts in Water Splitting Devices

J. Electrochem. Soc., 168, 034508

DOI:10.1149/1945-7111/abeb61

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Hydrogen production by proton exchange membrane (PEM) water electrolysis is among the promising energy storage solutions to buffer an increasingly volatile power grid employing significant amounts of renewable energies. In PEM electrolysis research, 24-hour galvanostatic measurements are the most common initial stability screenings and up to 5,000 h are used to assess extended stability, while commercial stack runtimes are within the 20,000-50,000 h range. In order to obtain stability data representative of commercial lifetimes with significantly reduced test duration an accelerated degradation test (ADT) was suggested by our group earlier. Here, we present a study on the broad applicability of the suggested ADT in RDE and CCM measurements and showcase the advantage of transient over stationary operation for enhanced catalyst degradation studies. The suggested ADT-1.6V protocol allows unprecedented, reproducible and quick assessment of anode catalyst long-term stability, which will strongly enhance degradation research and reliability. Furthermore, this protocol allows to bridge the gap between more fundamental RDE and commercially relevant CCM studies.

Xingli Wang, Katharina Klingan, Malte Klingenhof, Tim Möller, Jorge Ferreira de Araújo, Isaac Martens, Alexander Bagger, Shan Jiang, Jan Rossmeisl, Holger Dau and Peter Strasser

Morphology and mechanism of highly selective Cu(II) oxide nanosheet catalysts for carbon dioxide electroreduction

Nat Commun, 2021, 12, 794

DOI: 10.1038/s41467-021-20961-7

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Cu oxides catalyze the electrochemical carbon dioxide reduction reaction (CO2RR) to hydrocarbons and oxygenates with favorable selectivity. Among them, the shape-controlled Cu oxide cubes have been most widely studied. In contrast, we report on novel 2-dimensional (2D) Cu(II) oxide nanosheet (CuO NS) catalysts with high C2+ products, selectivities (> 400 mA cm−2) in gas diffusion electrodes (GDE) at industrially relevant currents and neutral pH. Under applied bias, the (001)-orientated CuO NS slowly evolve into highly branched, metallic Cu0 dendrites that appear as a general dominant morphology under electrolyte flow conditions, as attested by operando X-ray absorption spectroscopy and in situ electrochemical transmission electron microscopy (TEM). Millisecond-resolved differential electrochemical mass spectrometry (DEMS) track a previously unavailable set of product onset potentials. While the close mechanistic relation between CO and C2H4 was thereby confirmed, the DEMS data help uncover an unexpected mechanistic link between CH4 and ethanol. We demonstrate evidence that adsorbed methyl species, *CH3, serve as common intermediates of both CH3H and CH3CH2OH and possibly of other CH3-R products via a previously overlooked pathway at (110) steps adjacent to (100) terraces at larger overpotentials. Our mechanistic conclusions challenge and refine our current mechanistic understanding of the CO2 electrolysis on Cu catalysts.

Fang Luo, Stephan Wagner, Ichiro Onishi, Sören Selve, Shuang Li, Ju Wen, Huan Wang, Julian Steinberg, Arne Thomas, Ulrike I. Kramm, Peter Strasser

Surface Sites Density and Utilization of Precious Group Metal (PGM)-free Fe-NC and FeNi-NC Electrocatalysts for the Oxygen Reduction Reaction

Chem. Sci., 2021, 12, 384

DOI: 10.1039/D0SC03280H

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Pyrolyzed Iron-based precious group metal (PGM)-free nitrogen-doped single site carbon catalysts (Fe-NC) are possible alternatives to Platinum-based carbon catalysts for the oxygen reduction reaction (ORR). Bimetallic PGM-free M1M2-NC catalysts and their active sites, however, have been poorly studied to date. The present study explores the active accessible sites of mono- and bimetallic Fe-NC and FeNi-NC catalysts. Combining CO cryo chemisorption, X-ray absorption and 57Fe Mössbauer spectroscopy, we evaluate the number and chemical state of metal sites at the surface of the catalysts along with an estimate of their dispersion and utilization. Fe L3,2-edge X-ray adsorption spectra, Mössbauer spectra and CO desorption all suggested an essentially identical nature of Fe sites in both monometallic Fe-NC and bimetallic FeNi-NC; however, Ni blocks the formation of active sites during the pyrolysis and thus caused a sharp reduction in the metal accessible site density, while with only a minor direct participation as catalytic site in the final catalyst. We also use the site density utilization factor, Øsurface/bulk, as a measure of the metal site dispersion in a PGM-free ORR catalysts. Øsurface/bulk enables a quantitative evaluation and comparison of distinct catalyst synthesis routes in terms of their ratio of metal accessible site. It gives guidance for further optimization of the accessible site density of M-NC catalysts. 

Malte Klingenhof, Philipp Hauke, Sven Brückner, Sören Dresp, Elisabeth Wolf, Hong Nhan Nong, Camillo Spöri, Thomas Merzdorf, Denis Bernsmeier, Detre Teschner, Robert Schlögl,and Peter Strasser

Modular Design of Highly Active Unitized Reversible Fuel Cell Electrocatalysts

ACS Energy Lett., 6, 177−183

DOI: 10.1021/acsenergylett.0c02203

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A modular, multicomponent catalyst design principle is introduced and exemplified using a three-component, oxygen reduction reaction/oxygen evolution reaction (ORR/OER) catalyst designed for the oxygen electrode of unitized reversible fuel cells (URFCs). The catalyst system exhibited unprecedented catalytic performance in liquid electrolyte and in single unitized reversible fuel cell tests. The distinct components, each active for either ORR or OER, are prepared and optimized independently of each other and physically mixed during electrode preparation. The new modular URFC catalyst, Cu-α-MnO2/XC-72R/NiFe-LDH, combined a carbon-supported, Cu-stabilized α-MnO2 ORR catalyst with a NiFe-LDH OER catalyst and displayed improved activity and stability under URFC cycling compared to platinum group metal references. Stepwise modular optimization of the carbon and the interlayer anions of the OER component led to a further improved derivative, Cu-α-MnO2/O-MWCNTs/NiFe-LDH-Cl–. This URFC catalyst outperformed all previous materials in terms of its combined overpotential ηORR-OER and performance stability in the rotating disk electrode (RDE) scale. Its single-cell performance is analyzed and discussed.

2020

Hong Nhan Nong, Lorenz J. Falling, Arno Bergmann, Malte Klingenhof, Hoang Phi Tran, Camillo Spöri, Rik Mom, Janis Timoshenko, Guido Zichittella, Axel Knop-Gericke, Simone Piccinin, Javier Pérez-Ramírez, Beatriz Roldan Cuenya, Robert Schlögl, Peter Strasser, Detre Teschner & Travis E. Jones

Key role of chemistry versus bias in electrocatalytic oxygen evolution

Nature 2020, 587, 408-413

DOI: 10.1038/s41586-020-2908-2

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The oxygen evolution reaction has an important role in many alternative-energy schemes because it supplies the protons and electrons required for converting renewable electricity into chemical fuels. Electrocatalysts accelerate the reaction by facilitating the required electron transfer, as well as the formation and rupture of chemical bonds. This involvement in fundamentally different processes results in complex electrochemical kinetics that can be challenging to understand and control, and that typically depends exponentially on overpotential. Such behaviour emerges when the applied bias drives the reaction in line with the phenomenological Butler–Volmer theory, which focuses on electron transfer, enabling the use of Tafel analysis to gain mechanistic insight under quasi-equilibrium or steady-state assumptions. However, the charging of catalyst surfaces under bias also affects bond formation and rupture, the effect of which on the electrocatalytic rate is not accounted for by the phenomenological Tafel analysis and is often unknown. Here we report pulse voltammetry and operando X-ray absorption spectroscopy measurements on iridium oxide to show that the applied bias does not act directly on the reaction coordinate, but affects the electrocatalytically generated current through charge accumulation in the catalyst. We find that the activation free energy decreases linearly with the amount of oxidative charge stored, and show that this relationship underlies electrocatalytic performance and can be evaluated using measurement and computation. We anticipate that these findings and our methodology will help to better understand other electrocatalytic materials and design systems with improved performance.

René Sachse, Mika Pflüger, Juan-Jesús Velasco-Vélez, Mario Sahre, Jörg Radnik, Michael Bernicke, Denis Bernsmeier, Vasile-Dan Hodoroaba, Michael Krumrey, Peter Strasser, Ralph Kraehnert, Andreas Hertwig

Assessing Optical and Electrical Properties of Highly Active IrOx Catalysts for the Electrochemical Oxygen Evolution Reaction via Spectroscopic Ellipsometry

ACS Catal., 10, 13058-13074

DOI: 10.1021/acscatal.0c03800

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Efficient water electrolysis requires highly active electrodes. The activity of corresponding catalytic coatings strongly depends on material properties such as film thickness, crystallinity, electrical conductivity, and chemical surface speciation. Measuring these properties with high accuracy in vacuum-free and non- destructive methods facilitates the elucidation of structure−activity relationships in realistic environments. Here, we report a novel approach to analyze the optical and electrical properties of highly active oxygen evolution reaction (OER) catalysts via spectroscopic ellipsometry (SE). Using a series of differently calcined, mesoporous, templated iridium oxide films as an example, we assess the film thickness, porosity, electrical resistivity, electron concentration, electron mobility, and interband and intraband transition energies by modeling of the optical spectra. Independently performed analyses using scanning electron microscopy, energy-dispersive X-ray spectroscopy, ellipsometric porosimetry, X-ray reflectometry, and absorption spectroscopy indicate a high accuracy of the deduced material properties. A comparison of the derived analytical data from SE, resonant photoemission spectroscopy, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy with activity measurements of the OER suggests that the intrinsic activity of iridium oxides scales with a shift of the Ir 5d t2g sub-level and an increase of p−d interband transition energies caused by a transition of μ1-OH to μ3-O species.

Sara E. Renfrew, David E. Starr, and Peter Strasser

Electrochemical Approaches toward CO2 Capture and Concentration

ACS Catal. 2020, 10, 13058−13074

DOI:10.1021/acscatal.0c03639

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Carbon capture and concentration of low partial pressure CO2 in air and flue gas is a key step in carbon abatement strategies. Traditional CO2 capture methods employ temperature or pressure swings; however, electrochemical swings, in which an applied potential modulates nucleophilicity, are also possible to mediate the capture and release of CO2. In contrast to the breadth of electrochemical CO2 reduction research, electrochemically mediated CO2 capture and concentration is an emerging field. Although some aspects are reminiscent of those in CO2 reduction, like local pH gradients and (bi)carbonate equilibria, ultimately electrochemical CO2 capture and concentration poses its own unique challenges that will benefit from insights from intercalative batteries, redox flow batteries, and biomimetic/-inspired design, among other fields. After an introduction to carbon capture and current chemical strategies, this Review highlights promising emerging electrochemical methods to enable CO2 capture and concentration; specifically discussed are organic redox, transition metal redox, and pH swings. It closes with an outlook and discussion of future research challenges for electrochemically mediated capture.

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

Anisotropy of Pt Nanoparticles on Carbon- and Oxide-Support and Their Structural Response to Electrochemical Oxidation Probed by in situ Techniques

Physical Chemistry Chemical Physics, 22, 22260

doi.org/10.1039/D0CP03233F

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Identifying the structural response of nanoparticle-support ensembles to the reaction conditions is essential to determine their structure in the catalytically-active state as well as to unravel possible degradation pathways. In this work, we investigate the (electronic) structure of carbon- and oxide-supported Pt nanoparticles during electrochemical oxidation by in situ X-ray diffraction, absorption spectroscopy as well as the Pt dissolution rate by in situ mass spectrometry. We prepared ellipsoidal Pt nanoparticles by impregnation of carbon and titanium-based oxide support as well as spherical Pt nanoparticles on an indium-based oxide support by a surfactant-assisted synthesis route. During electrochemical oxidation, we show that the oxide-supported Pt nanoparticles resist (bulk) oxide formation and Pt dissolution. The lattice of smaller Pt nanoparticles exhibits a size-induced lattice contraction in the as-prepared state with respect to bulk Pt but it expands reversibly during electrochemical oxidation. This expansion is suppressed for the Pt nanoparticles with bulk-like relaxed lattice. We could correlate the formation of d-band vacancies in the metallic Pt with the Pt lattice expansion. The PtOx formation is strongest for platelet-like nanoparticles and we explain this with a higher fraction of exposed Pt(100) facets. Of all investigated nanoparticle-support ensembles, the structural response of RuO2/TiO2-supported Pt nanoparticles is the most promising with respect to their morphological and structural integrity under electrochemical reaction conditions.

Kai Zeng, Xiangjun Zheng, Cong Li, Jin Yan, Jing-Hua Tian, Chao Jin, Peter Strasser, and Ruizhi Yang

Recent Advances in Non-Noble Bifunctional Oxygen Electrocatalysts toward Large-Scale Production

Advanced Functional Materials, 2020

doi.org/10.1002/adfm.202000503

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The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial reactions in energy conversion and storage systems including fuel cells, metal–air batteries, and electrolyzers. Developing low‐cost, high‐efficiency, and durable non‐noble bifunctional oxygen electrocatalysts is the key to the commercialization of these devices. Here, based on an in‐depth understanding of ORR/OER reaction mechanisms, recent advances in the development of non‐noble electrocatalysts for ORR/OER are reviewed. In particular, rational design for enhancing the activity and stability and scalable synthesis toward the large‐scale production of bifunctional electrocatalysts are highlighted. Prospects and future challenges in the field of oxygen electrocatalysis are presented.

Seongkoo Kang, Kyle G. Reeves, Toshinari Koketsu, Jiwei Ma, Olaf J. Borkiewicz, Peter Strasser, Alexandre Ponrouch, and Damien Dambournet

Multivalent Mg2+, Zn2+ and Ca2+ Ion Intercalation Chemistry in a Disordered Layered Structure

ACS Appl. Energy Mater. 2020

doi.org/10.1021/acsaem.0c01530

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The development of practical multivalent-ion batteries critically depends on the identification of suitable positive electrode materials. To gain a better understanding of the intercalation chemistry of multivalent ions, model frameworks can be used to study the distinct specificities of possible multivalent ions, thus expanding our knowledge on the emerging “Beyond Li battery” technology. Here, we compare the intercalation chemistry of Mg2+, Zn2+ and Ca2+ ions into a disordered layered-type structure featuring water interlayers and cationic vacancies as possible host sites. The thermodynamics of cation-inserted reactions performed on the model structure indicated that these reactions are thermodynamically favourable with Zn2+ being the least stable ion. Galvanostatic measurements confirmed that the structure is inactive toward Zn2+ intercala-tion while Mg2+ can be reversibly inserted (0.37 Mg2+ per formula unit) with minor changes of the atomic arrangement as demonstrated by pair distribution function analysis. Moreover, we demonstrate that non-solvated Mg2+ was intercalated in the structure. Finally, the intercalation of Ca2+ performed at 100 °C with Ca(BF4)2 in propylene carbonate, induced the col-lapse of the layered structure releasing water molecules that contribute to the degradation of the electrolyte as revealed by the presence of CaF2 at the electrode level. The decomposition of the structure led to the formation of an electrochemically active phase featuring strong long-range disorder, yet short-range order close to that found in perovskite structures, particu-larly with corner-shared TiO6 octahedra. We, hence, hypothesize that defective CaTiO3-based perovskite could be explored as viable cathode materials for rechargeable Ca-based batteries.

Friedemann Hegge, Florian Lombeck, Edgar Cruz Ortiz, Luca Bohn, Miriam von Holst, Matthias Kroschel, Jessica Hübner, Matthias Breitwieser, Peter Strasser, and Severin Vierrath

Efficient and stable low iridium-loaded anodes for PEM water electrolysis made possible by nanofiber interlayers

ACS Appl. Energy Mater.,2020

doi.org/10.1021/acsaem.0c00735

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Significant reduction of the precious metal catalyst loading is one of the key challenges for the commercialization of proton-exchange membrane water electrolyzers. In this work we combine IrOx nanofibers with a conventional nanoparticle-based IrOx anode catalyst layer. With this hybrid design we are able to reduce the iridium loading by more than 80 % while maintaining performance. In spite of an ultra-low overall catalyst loading of 0.2 mgIr/cm², a cell with a hybrid layer shows similar performance compared to a state-of-the-art cell with a catalyst loading of 1.2 mgIr/cm² and clearly outperforms identically loaded reference cells with pure IrOx nanoparticle and pure nanofiber anodes. The improved performance is attributed to a combination of good electric contact and high porosity of the IrOx nanofibers with high surface area of the IrOx nanoparticles. Besides the improved performance, the hybrid layer also shows better stability in a potential cycling and a 150h constant current test compared to an identically loaded nanoparticle reference.

Robert Marić, Christian Gebauer, Markus Nesselberger, Frédéric Hasché and Peter Strasser

Towards a Harmonized Accelerated Stress Test Protocol for Fuel Starvation Induced Cell Reversal Events in PEM Fuel Cells: The Effect of Pulse Duration

J. Electrochem. Soc., 167, 124520

iopscience.iop.org/article/10.1149/1945-7111/abad68/pdf

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Global  fuel  starvation  is  an  undesired  event  during  fuel  cell  operation  that  results  in  serious degradations  at  the  anode  catalyst  layer  caused  by  the  concomitant  reversal  of  the  cell potentials.  Several  groups  have  therefore  intensified  their  research  efforts  towards  the implementation  of  suitable  diagnostic  tools  and  accelerated  stress  test  (AST)  protocols  that mimic  cell  reversal  events.  However,  the  current  number  of  different  test  protocols  requires consolidation and harmonization to define durability targets towards cell reversal tolerance and to  benchmark  newly  developed  materials.  To  create  a  basis  for  harmonization,  this  study examines the difference between pulsed and quasi-continuous AST protocols at the catalyst-coated  membrane  level.  Utilizing  a  single-cell  setup  combined  with  an  on-line  mass spectrometer,  a  2.5-fold  increase  in  the  carbon  corrosion  rates  were  found  for  short-pulsed compared to long-lasting cell reversal events. The enhanced corrosion was associated with a 2.2-fold  higher  loss  of  electrochemically  active  surface  area  and  a  15  %  higher  reduction  in anode catalyst layer thickness. By contrast, the overall cell performance decreased additionally by 40–50 mV for samples under long-lasting cell reversal events. The decay is mainly driven by  an  increased  ohmic  resistance,  presumably  originating  from a  more  pronounced  surface oxide formation on the carbon support.

Xiangjun Zheng, Xuecheng Cao, Zhihui Sun, Kai Zeng, Jin Yan, Peter Strasser, Xin Chen, Shuhui Sun, Ruizhi Yang

Indiscrete metal/metal-N-C synergic active sites for efficient and durableoxygen electrocatalysis toward advanced Zn-air batteries

Applied Catalysis B: Environmental, 272, 118967

doi.org/10.1016/j.apcatb.2020.118967

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Carbon has been deemed promising electrocatalyst for oxygen reduction/evolution reaction (ORR/OER). However, most carbon materials are not stable in highly oxidative OER environments. Herein, nitrogen (N) and transition metal (TM) co-doped carbon nanosheets hybridizing with transition metal (TM/TM-N-C, TM = Fe, Co, Ni) are developed from biomass lysine by employing a NaCl template and molten-salt-promoted graphitization process. Among the as-synthesized TM/TM-N-C, the Ni/Ni-N-C with Ni nanocubes embedded in carbon demonstrates an excellent ORR-OER stability during the potential of 0.06–1.96 V. The rechargeable Zn-air battery with the fabricated Ni/Ni-N-C as the cathode catalyst produces a low voltage gap of 0.773 V, which is only slightly increased by 5 % after 150 cycles testing. Combined experimental and theoretical studies reveal that the exceptional activity and ORR-OER wide potential durability of Ni/Ni-N-C can be ascribed to highly active Ni-N4-C configuration, synergistic effect between Ni and Ni-N4-C, carbon nanosheets structure and formation of stable Ni3+-N for protecting carbon from oxidation.

Tim Möller, Fabian Scholten, Trung Ngo Thanh, Ilya Sinev, Janis Timoshenko, Xingli Wang, Zarko Jovanov, Manuel Gliech, Beatriz Roldan Cuenya, Ana Sofia Varela, and Peter Strasser

Electrocatalytic CO2 Reduction on CuOx Nanocubes: Tracking the Evolution of Chemical State, Geometric Structure, and Catalytic Selectivity using Operando Spectroscopy
 
Angewandte Chemie, 132, 2-12

doi.org/10.1002/ange.202007136

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The direct electrochemical conversion of carbon dioxide (CO2) into multi‐carbon (C2+) products still faces fundamental and technological challenges. While facet‐controlled and oxide‐derived Cu materials have been touted as promising catalysts, their stability has remained problematic and poorly understood. The present work uncovers changes in the chemical and morphological state of supported and unsupported Cu2O nanocubes during operation in low‐current H‐Cells and in high‐current Gas Diffusion Electrodes (GDEs) using neutral pH buffer conditions. While unsupported nanocubes achieved a sustained C2+ faradaic efficiency of around 60% for 40h, the dispersion on a carbon support sharply shifted the selectivity pattern towards C1 products. Operando XAS and time‐resolved electron microscopy revealed the degradation of the cubic shape and, in the presence of a carbon support, the formation of small Cu‐seeds during the surprisingly slow reduction of bulk Cu2O. Here, the initially (100)‐rich facet structure has presumably no controlling role on the catalytic selectivity, whereas the oxide‐derived generation of under‐coordinated lattice defects, as revealed by the operando Cu‐Cu coordination numbers, can support the high C2+ product yields.

Woong Hee Lee, Jaekyung Yi, Hong Nhan Nong, Peter Strasser, Keun Hwa Chae, Byoung Koun Min, Yun Jeong Hwang, and Hyung-Suk Oh

Electroactivation-induced IrNi Nanoparticles under Different pH Conditions for Neutral Water Oxidation

Nanoscale, 12, 14903-14910

DOI:10.1039/D0NR02951C

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The electrochemical oxidation processes can affect electronic structure and activate catalytic performance of preciousmetal and transition-metal based catalysts for oxygen evolution reaction (OER). Also there are emerging requirements to develop OER electrocatalysts in various pH condition in order to couple with different reduction reactions. Herein, we studied the pH effect on electroativation of IrNi alloy nanoparticles supported on carbon (IrNi/C) and the activated IrNiOx/C evaluated the electrocatalytic activities for water oxidation in a neutral condition. In addition, their electronic structures and atomic arrangement were analyzed in-situ/operando X-ray absorption spectroscopy (XAS) and identical location transmission electron microscopy technique showing reconstruction of the metal elements during electroactivation due to their different stabilities depending on the electrolyte pH. IrNiOx/C activated under neutral pH conditions showed a mildly oxidized thin IrOx shell. Meanwhile, IrNiOx/C activated in acidic and alkaline electrolytes showed Ni-leached IrOx and Ni-rich IrNiOx surfaces, respectively. Especially, the surface of IrNiOx/C activated in alkaline condition shows IrOx with high d-band hole and NiOx with high oxidation state leading excellent OER catalytic activity in neutral media (η = 384 mV at 10 mA cm–2) where much lower OER activity was reported compared with alkaline or acid condition. Our results, which showed that electrochemically activated catalysts under different pH conditions exhibit a unique electronic structure by modifying the initial alloy catalyst, can be applied for the design of catalysts suitable for various electrochemical reactions.

Fabio Dionigi and Peter Strasser

ATOMIC-SCALE STRUCTURAL CHANGES IN OCTAHEDRAL PtNi NANOPARTICLE CATALYSTS FOR HYDROGEN FUEL CELL CATHODES

ESRF Highlights 2019, 147

www.esrf.eu/home/UsersAndScience/Publications/Highlights/esrf-highlights-2019.html

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Octahedral PtNi nanoparticles are promising catalysts for the oxygen reduction reaction in fuel cell applications. The structural changes associated with Ni leaching during operation have been investigated by in-situ wide-angle X-ray  scattering  (WAXS).  Atomic  Ni  losses,  correlating  to  expansion  of  the  crystal  lattice  parameters,  largely affect the activity.

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 Li+ and Mg2+ 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 Ti4+/Ti3+ redox couple with excellent cyclability and rate capability. We conclude that native interstitial sites preferentially accommodate Li+ ions, while Mg2+ 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 ​+ ​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.

Hadla Ferreira, Martin Gocyla, Hadma Ferreira, Rennan Araujo, Caio Almeida, Marc Heggen, Rafal  Dunin-Borkowski, Katlin Eguiluz, Peter Strasser, and Giancarlo R. Salazar-Banda

A Comparative Study of the Catalytic Performance of Pt-Based Bi and Trimetallic Nanocatalysts Towards Methanol, Ethanol, Ethylene Glycol, and Glycerol Electro-Oxidation

J. Nanosci. Nanotechnol., 20, 10

doi.org/10.1166/jnn.2020.18559

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Carbon-supported platinum is used as an anode and cathode electrocatalyst in low-temperature fuel cells fueled with low-molecular-weight alcohols in fuel cells. The cost of Pt and its low activity towards the complete oxidation of these fuels are significant barriers to the widespread use of these types of fuel cells. Here, we report on the development of PtRhNi nanocatalysts supported on carbon made using a reduction chemistry method with different atomic rates. The catalytic activity of the developed catalysts towards the electro-oxidation of methanol, ethanol, ethylene glycol, and glycerol in acidic media was studied. The obtained catalysts performances were compared with both commercial Pt/C and binary Pt75Ni25/C catalyst. The nanostructures were characterized, employing inductively coupled plasma optical emission spectrometer, X-ray diffraction, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The binary catalyst presents a mean particle size of around 2 nm. Whereas the ternary catalysts present particles of similar size and with some large alloy and core–shell structures. The alcohol oxidation onset potential and the current density measured after 3600 s of chronoamperometry were used to classify the catalytic activity of the catalysts towards the oxidation of methanol, ethanol, ethylene glycol, and glycerol. The best PtRhNi/C catalyst composition (i.e., Pt43Rh43Ni14/C) presented the highest activity for alcohols oxidation compared with all catalysts studied, indicating the proper tuning composition influence in the catalytic activity. The enhanced activity of Pt43Rh43Ni14/C can be attributed to the synergic effect of trimetallic compounds, Pt, Ni, and Rh.

Fang Luo, Aaron Roy, Luca Silvioli, David A. Cullen, Andrea Zitolo, Moulay Tahar Sougrati, Ismail Can Oguz, Tzonka Mineva, Detre Teschner, Stephan Wagner, Ju Wen, Fabio Dionigi, Ulrike I. Kramm, Jan Rossmeisl, Frédéric Jaouen and Peter Strasser

P-block single-metal-site tin/nitrogen-doped carbon fuel cell cathode catalyst for oxygen reduction reaction

Nature Materials, 2020

doi.org/10.1038/s41563-020-0717-5

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This contribution reports the discovery and analysis of a p-block Sn-based catalyst for the electroreduction of molecular oxygen in acidic conditions at fuel cell cathodes; the catalyst is free of platinum-group metals and contains single-metal-atom actives sites coordinated by nitrogen. The prepared SnNC catalysts meet and exceed state-of-the-art FeNC catalysts in terms of intrinsic catalytic turn-over frequency and hydrogen–air fuel cell power density. The SnNC-NH3 catalysts displayed a 40–50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Additional benefits include a highly favourable selectivity for the four-electron reduction pathway and a Fenton-inactive character of Sn. A range of analytical techniques combined with density functional theory calculations indicate that stannic Sn(iv)Nx single-metal sites with moderate oxygen chemisorption properties and low pyridinic N coordination numbers act as catalytically active moieties. The superior proton-exchange membrane fuel cell performance of SnNC cathode catalysts under realistic, hydrogen–air fuel cell conditions, particularly after NH3 activation treatment, makes them a promising alternative to today’s state-of-the-art Fe-based catalysts.

Elisabeth Hornberger, Henrike Schmies, Benjamin Paul, Stefanie Kühl and Peter Strasser

Design and Validation of a Fluidized Bed Catalyst Reduction Reactor for the Synthesis of Well-Dispersed Nanoparticle Ensembles

J. Electrochem. Soc., 167, 114509

doi.org/10.1149/1945-7111/aba4eb

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Pt-based nanoparticles supported on carbon materials are state-of-the-art electrocatalysts for proton exchange membrane fuel cells (PEMFCs). Interparticle distance and particle size of the supported nanoparticles play a crucial role for the catalyst's performance. The synthesis approach of wet impregnation and thermal reduction in a regular static packed-bed tube furnace often results in poorly distributed Pt particles in terms of interparticle distance and particle size. Here, we report on a fluidized bed gas reduction reactor for the preparation of supported well-dispersed nanoparticles. To validate the reactor, we compared and contrasted Pt nanoparticle ensembles supported on Vulcan XC 72R prepared using a conventional, horizontal static packed bed tube furnace, and using our novel vertical fluidized bed reduction reactor. The catalysts were physico-chemically characterized and electrochemically tested with respect to their electrocatalytic oxygen reduction reaction reactivity using rotating disk electrode (RDE) experiments. Our results demonstrate that the nanoparticle samples prepared in the customized fluidized bed reduction reactor showed significantly superior mono-dispersion and more homogeneously spatial distribution that resulted in improved electrochemical stability.

Yanyan Sun, Lei Han, Peter Strasser

A Comparative Perspective of Electrochemical and Photochemical Approaches for Catalytic H2O2 Production

Chemical Society Reviews, 49, 6605-6631

DOI: 10.1039/D0CS00458H

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Hydrogen peroxide (H2O2) has a wide range of important applications in various fields including chemical industrial, environmental remediation, and sustainable energy conversion/storage. Nevertheless, the stark disconnect between today’s huge market demand and the historical unsustainability of the currently-used, industrial anthraquinone-based production process is promoting extensive research on the development of efficient, energy-saving and sustainable methods for H2O2 production. Among several sustainable strategies, H2O2 production via the electrochemical and photochemical routes has shown a particular appeal, because only water, O2, and solar energy/electricity are involved during the whole process. In the past few years, considerable efforts have been devoted to the development of advanced electrocatalysts and photocatalysts for an efficient and scalable H2O2 production with high efficiency and stability. In this review, we compare and contrast the two distinct, yet inherently closely linked catalytic processes, before we detail recent advances in the design, preparation, and applications of different H2O2 catalyst systems from the viewpoint of electrochemical and photochemical approach. We close with a balanced perspective on remaining future scientific and technical challenges and opportunities.

Damien Dambournet, Christophe Legein, Ben Morgan, Monique Body, Olaf Borkiewicz, Franck Fayon, vincent sarou-kanian, Jiwei Ma, Peter Strasser, Toshinari Koketsu, Wei Xiankui, Marc Heggen

Atomic Insights into Aluminium‐Ion Insertion in Defective Anatase for Batteries

Angewandte Chemie, 2020

onlinelibrary.wiley.com/doi/abs/10.1002/ange.202007983

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Aluminium batteries constitute a safe and sustainable high–energy‐density electrochemical energy‐storage solution. Viable Al‐ion batteries require suitable electrode materials that can readily intercalate high‐charge Al 3+ ions. Here, we investigate the Al 3+ intercalation chemistry of anatase TiO 2 and how chemical modifications influence the accommodation of Al 3+ ions. We use fluoride‐ and hydroxide‐doping to generate high concentrations of titanium vacancies. The coexistence of these hetero‐anions and titanium vacancies leads to a complex insertion mechanism, attributed to three distinct types of host sites: native interstitials sites, single vacancy sites, and paired vacancy sites. We demonstrate that Al 3+ induces a strong local distortion within the modified TiO 2 structure, which affects the insertion properties of the neighbouring host sites. Overall, specific structural features induced by the intercalation of highly‐polarizing Al 3+ ions should be considered when designing new electrode materials for multivalent batteries.

Mathias Primbs, Yanyan Sun, Aaron Roy, Daniel Malko, Asad Mehmood, Moulay-Tahar Sougrati, Pierre-Yves Blanchard, Gaetano Granozzi, Tomasz Kosmala, Giorgia Daniel, Plamen Atanassov, Jonathan Sharman, Christian Durante, Anthony Kucernak, Deborah Jones, Frederic Jaouen and Peter Strasser

Establishing reactivity descriptors for platinum group metal (PGM)-free Fe–N–C catalysts for PEM fuel cells

Energy Environ. Sci., 2020

pubs.rsc.org/en/content/articlelanding/2020/ee/d0ee01013h

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We report a comprehensive analysis of the catalytic oxygen reduction reaction (ORR) reactivity of four of today's most active benchmark platinum group metal-free (PGM-free) iron/nitrogen doped carbon electrocatalysts (Fe–N–Cs). Our analysis reaches far beyond previous such attempts in linking kinetic performance metrics, such as electrocatalytic mass-based and surface area-based catalytic activity with previously elusive kinetic metrics such as the active metal site density (SD) and the catalytic turnover frequency (TOF). Kinetic ORR activities, SD and TOF values were evaluated using in situ electrochemical NO2− reduction as well as an ex situ gaseous CO cryo chemisorption. Experimental ex situ and in situ Fe surface site densities displayed remarkable quantitative congruence. Plots of SD versus TOF (“reactivity maps”) are utilized as new analytical tools to deconvolute ORR reactivities and thus enabling rational catalyst developments. A microporous catalyst showed large SD values paired with low TOF, while mesoporous catalysts displayed the opposite. Trends in Fe surface site density were linked to molecular nitrogen and Fe moieties (D1 and D2 from 57Fe Mössbauer spectroscopy), from which pore locations of catalytically active D1 and D2 sites were established. This cross-laboratory analysis, its employed experimental practices and analytical methodologies are expected to serve as a widely accepted reference for future, knowledge-based research into improved PGM-free fuel cell cathode catalysts.

Woong Hee Lee, Young-Jin Ko, Yongjun Choi, Si Young Lee, Chang Hyuck Choi, Yun Jeong Hwang, Byoung Koun Min, Peter Strasser, Hyung-Suk Oh

Highly selective and scalable CO2 to CO - Electrolysis using coral-nanostructured Ag catalysts in zero-gap configuration

Nano Energy, 76, 105030

www.sciencedirect.com/science/article/abs/pii/S2211285520306078

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The direct electroreduction of CO2 to pure CO streams has attracted much attention for both academic research and industrial polymer synthesis development. Here, we explore catalytically very active, coral-structured Ag catalyst for the generation of pure CO from CO2-feeds in lab-bench scale zero-gap CO2 electrolyzer. Coral-shaped Ag electrodes achieved CO partial current densities of up to 312 mA cm−2, EECO of 38%, and FECO clearly above 90%. In-situ/operando X-ray Absorption Spectroscopy revealed the sustained presence of Ag+ subsurface species, whose local electronic field effects constitute likely molecular origins of the favorable experimental kinetics and selectivity. In addition, we show how electrode flooding in zero-gap CO2 electrolyzer compromises efficient CO2 mass transfer. Our studies highlight the need for a concomitant consideration of factors related to intrinsic catalytic activity of the active phase, its porous structure and its hydrophilicity/phobicity to achieve a sustained high product yield in AEM zero-gap electrolyzer.

Fabio Dionigi, Zhenhua Zeng, Ilya Sinev, Thomas Merzdorf, Siddharth Deshpande, Miguel Bernal Lopez, Sebastian Kunze, Ioannis Zegkinoglou, Hannes Sarodnik, Dingxin Fan, Arno Bergmann, Jakub Drnec, Jorge Ferreira de Araujo, Manuel Gliech, Detre Teschner, Jing Zhu,Wei-Xue Li, Jeffrey Greeley, Beatriz Roldan Cuenya and Peter Strasser

In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution

Nature Communications, 11, 2522

doi.org/10.1038/s41467-020-16237-1

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NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure M-M centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs.

Sören Dresp,   Trung Ngo Thanh,   Malte Klingenhof,   Sven Brueckner,   Philipp Hauke  and  Peter Strasser  

Efficient direct seawater electrolysers using selective alkaline NiFe-LDH as OER catalyst in asymmetric electrolyte feeds

Energy Environ. Sci., 13, 1725-1729

doi.org/10.1039/D0EE01125H

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Direct seawater electrolysis faces fundamental catalytic and process engineering challenges. Here we demonstrate a promising seawater electrolyser process using asymmetric electrolyte feeds.
We further investigated the faradaic O2 efficiency of NiFe-LDH in alkalinized Cl-containing electrolytes in comparison to commercial IrOx-based catalysts. Other than IrOx, NiFe-LDH prevents the
oxidation of Cl

 and appears highly selective for the oxygen evolution reaction in alkalinized seawater even at cell potentials beyond 3.0 Vcell.

Woong Hee Lee, Hong Nhan Nong, Chang Hyuck Choi, Keun Hwa Chae, Yun Jeong Hwang, Byoung Koun Mina, Peter Strasser and Hyung-Suk Oh

Carbon-Supported IrCoOx nanoparticles as an efficient and stable OER electrocatalyst for practicable CO2 electrolysis

Applied Catalysis B: Environmental, 269, 118820

doi.org/10.1016/j.apcatb.2020.118820

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The development of an efficient and stable oxygen evolution reaction (OER) electrocatalyst operating under pH-neutral conditions is vital for the realization of sustainable CO2 reduction reaction (CO2RR) systems in the future. For commercializing this system, it is also important to be able to use general-purpose water as an electrolyte. Here, we explore, characterize and validate a new IrCoOx mixed metal oxide as efficient and stable OER catalyst, before we investigate and proof its suitability as counter electrode to a CO2RR cathode operating under pH-neutral conditions. More specifically, carbon-supported IrCoOx core-shell nanoparticles exhibited a highly efficient OER catalytic activity and stability compared to state-of-art reference IrOx catalysts in CO2-saturated 0.5 M KHCO3 tap-water. IrCoOx/C also exhibited a significantly improved electrochemical oxidation and corrosion resistance than IrOx, resulting in a beneficial suppression of Ir dissolution. The application of IrCoOx/C in the CO2 electrolyzer displayed superior CO space-time yields over prolonged electrolyzer tests.

Manuel Gliech, Mikaela Görlin, Martin Gocyla, Malte Klingenhof, Arno Bergmann, Sören Selve, Camillo Spöri, Marc Heggen, Rafal E. Dunin-Borkowski, Jin Suntivich and Peter Strasser

Solute Incorporation at Oxide–Oxide Interfaces Explains How Ternary Mixed‐Metal Oxide Nanocrystals Support Element‐Specific Anisotropic Growth

Adv. Funct. Mater., 30, 1909054

onlinelibrary.wiley.com/doi/full/10.1002/adfm.201909054

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Fundamental understanding of anisotropic growth in oxide nanocrystals is crucial to establish new synthesis strategies and to tailor the nanoscale electronic, magnetic, optical, and electrocatalytic properties of these particles. While several growth investigations of metal alloy nanoparticles have been reported, mechanistic studies on the growth of ternary oxide materials are still missing. This work constitutes the first study on the evolution of anisotropic growth of manganese–cobalt oxide nanoparticles by monitoring the elemental distribution and morphology during the particle evolution via scanning transmission electron microscopy–X‐ray spectroscopy. A new growth mechanism based on a “solution‐solid‐solid” pathway for mixed manganese cobalt oxides is revealed. In this mechanism, the MnO seed formation occurs in the first step, followed by the surface Co enrichment, which catalyzes the growth along the <100> directions in all the subsequent growth stages, creating rod, cross‐, and T‐shaped mixed metal oxides, which preferentially expose {100} facets. It is shown that the interrelation of both Mn and Co ions initializes the anisotropic growth and presents the range of validity of the proposed mechanism as well as the shape‐determining effect based on the metal‐to‐metal ratio.

Wenming Tong, Mark Forster, Fabio Dionigi, Sören Dresp, Roghayeh Sadeghi Erami, Peter Strasser, Alexander J. Cowan and Pau Farràs


Electrolysis of low-grade and saline surface water

Nature Energy, 5, 367-377

doi.org/10.1038/s41560-020-0550-8

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Powered by renewable energy sources such as solar, marine, geothermal and wind, generation of storable hydrogen fuel through water electrolysis provides a promising path towards energy sustainability. However, state-of-the-art electrolysis requires support from associated processes such as desalination of water sources, further purification of desalinated water, and transportation of water, which often contribute financial and energy costs. One strategy to avoid these operations is to develop electrolysers that are capable of operating with impure water feeds directly. Here we review recent developments in electrode materials/catalysts for water electrolysis using low-grade and saline water, a significantly more abundant resource worldwide compared to potable water. We address the associated challenges in design of electrolysers, and discuss future potential approaches that may yield highly active and selective materials for water electrolysis in the presence of common impurities such as metal ions, chloride and bio-organisms.

Sebastian Ott, Alin Orfanidi, Henrike Schmies, Björn Anke, Hong Nhan Nong, Jessica Hübner, Ulrich Gernert, Manuel Gliech, Martin Lerch and Peter Strasser

Ionomer distribution control in porous carbonsupported catalyst layers for high-power and low Pt-loaded proton exchange membrane fuel cells

Nature Materials, 19, 77–85

www.nature.com/articles/s41563-019-0487-0

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The reduction of Pt content in the cathode for proton exchange membrane fuel cells is highly desirable to lower their costs. However, lowering the Pt loading of the cathodic electrode leads to high voltage losses. These voltage losses are known to originate from the mass transport resistance of O2 through the platinum–ionomer interface, the location of the Pt particle with respect to the carbon support and the supports’ structures. In this study, we present a new Pt catalyst/support design that substantially reduces local oxygen-related mass transport resistance. The use of chemically modified carbon supports with tailored porosity enabled controlled deposition of Pt nanoparticles on the outer and inner surface of the support particles. This resulted in an unprecedented uniform coverage of the ionomer over the high surface-area carbon supports, especially under dry operating conditions. Consequently, the present catalyst design exhibits previously unachieved fuel cell power densities in addition to high stability under voltage cycling. Thanks to the Coulombic interaction between the ionomer and N groups on the carbon support, homogeneous ionomer distribution and reproducibility during ink manufacturing process is ensured.

Hong Nhan Nong, Hoang Phi Tran, Camillo Spöri, Malte Klingenhof, Lorenz Frevel, Travis Jones, Thorsten Cottre, Bernhard Kaiser, Wolfram Jaegermann, Robert Schlögl, Detre Teschner and Peter Strasser

The Role of Surface Hydroxylation, Lattice Vacancies and Bond Covalency in the Electrochemical Oxidation of Water (OER) on Ni-Depleted Iridium Oxide Catalysts

Zeitschrift für Physikalische Chemie, 234, 787-812

doi.org/10.1515/zpch-2019-1460

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The usage of iridium as an oxygen-evolution-reaction (OER) electrocatalyst requires very high atomefficiencies paired with high activity and stability. Our efforts during the past 6 years in the Priority Program 1613 funded by the Deutsche Forschungsgemeinschaft (DFG) were focused to mitigate the molecular origin of kinetic overpotentials of Ir-based OER catalysts and to design new materials to achieve that Ir-based catalysts are more atom and energy efficient, as well as stable. Approaches involved are: use of bimetallic mixed metal oxide materials where Ir is combined with cheaper transition metals as starting materials, use of dealloying concepts of nanometer sized core-shell particle with a thin noble metal oxide shell combined with a hollow or cheap transition metal-rich alloy core, and use of corrosion-resistant high-surface-area oxide support materials. In this mini review, we have highlighted selected advances in our understanding of Ir–Ni bimetallic oxide electrocatalysts for the OER in acidic environments.