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Lupe
2019

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

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

Electrochimica Acta, 313, 1-15

doi.org/10.1016/j.electacta.2019.05.011

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

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

Energy Storage Materials, 25, 154-163

doi.org/10.1016/j.ensm.2019.10.019

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

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

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

doi.org/10.1016/j.coelec.2019.10.011

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

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

Nature Nanotechnology, 14, 1063–1070

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

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

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

Nano Lett., 2019, 19, 6876−6885

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

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

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

ACS Catal. 2019, 9, 1283−1288

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

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

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

ACS Catal. 2019, 9, 3764−3772

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

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

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

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

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

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

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

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

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

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

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

ACS Catal. 2019, 9, 7270−7284

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

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

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

ACS Catal. 2019, 9, 6653−6663

doi.org/10.1021/acscatal.9b00648

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

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

Energy Environ. Sci., 12, 2476-2485

DOI: 10.1039/c9ee01185d

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

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

ACS Energy Lett., 2019, 4, 1663−1671

DOI: 10.1021/acsenergylett.9b01049

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

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

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

DOI:10.1021/acssuschemeng.8b05106

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

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

ACS Catal. 9, 1482−1488

DOI:10.1021/acscatal.8b02846

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

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

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

DOI:10.1021/acs.jpcc.8b08794

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

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

Chem. Commun. 55, 818-821

DOI:10.1039/C8CC06410E

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We present an unusual, yet facile, strategy towards formation of physically mixed Ni–Fe(OxHy) oxygen evolution electrocatalysts. We use in situ X-ray absorption and UV-vis spectroscopy, and high-resolution imaging to demonstrate that physical contact between two inferior Ni(OH)2 and Fe(OOH) catalysts self-assemble into atomically intermixed Ni–Fe catalysts with unexpectedly high activity.
Yancai Yao, Sulei Hu, Wenxing Chen, Zheng-Qing Huang, Weichen Wei, Tao Yao, Ruirui Liu, Ketao Zang, Xiaoqian Wang, Geng Wu, Wenjuan Yuan, Tongwei Yuan, Baiquan Zhu, Wei Liu, Zhijun Li, Dongsheng He, Zhenggang Xue, Yu Wang, Xusheng Zheng, Juncai Dong, Chun-Ran Chang, Yanxia Chen, Xun Hong, Jun Luo, Shiqiang Wei, Wei-Xue Li, Peter Strasser, Yuen Wu and Yadong Li

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

Nature Catalysis 2, 304-313

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

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

Direct Electrolytic Splitting of Seawater: Opportunities and Challenges

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

DOI:10.1021/acsenergylett.9b00220

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

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

Catalysis Today 328, 8-14

DOI: 10.1016/j.cattod.2018.10.011

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

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

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

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

DOI: 10.1039/c8ta11298c

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

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

ACS Catal. 9 (2), 1283–1288

DOI: 10.1021/acscatal.8b03734

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

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

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

DOI:10.1039/C8EE02662A

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

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

ChemComm 55 (8), 818-821

DOI: 10.1039/c8cc06410e  

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

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