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Lupe
Lupe
2021
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.

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