Inhalt des Dokuments
|Henrike Schmies, Arno Bergmann, Jakub
Drnec, Guanxiong Wang, Detre Teschner, Stefanie Kühl, Daniel J. S.
Sandbeck, Serhiy Cherevko, Martin Gocyla, Meital Shviro, Marc Heggen,
Vijay Ramani, Rafal E. Dunin-Borkowski, Karl J. J. Mayrhofer, and
Unravelling Degradation Pathways of Oxide-Supported Pt Fuel Cell Nanocatalysts under In Situ Operating Conditions
Adv. Energy Mater. 8 (4), 1-13
Knowledge of degradation pathways of catalyst/support ensembles aids the development of rational strategies to improve their stability. Here, this is exemplifed using indium tin oxide (ITO)-supported Platinum nanoparticles as
electrocatalysts at fuel cell (FC) cathodes under degradation protocols to mimic operating conditions in two potential regimes. The evolution of crystal structure, composition, crystallite and particle size is tracked by in situ X-ray techniques (small and wide angle scattering), metal dissolution by in situ scanning ﬂow cell coupled with mass spectrometry (SFC ICP-MS) and Pt surface morphology by advanced electron microscopy. In a regular FC operation regime, Pt poisoning rather than Pt particle growth, agglomeration, dissolution or detachment was found to be the likely origin of the observed degradation and ORR activity losses. In the start-up regime degradation is actually suppressed and only minor losses in catalytic activity are observed. The presented data thus highlight the excellent nanoparticle stabilization and corrosion resistance of the ITO support, yet point to a degradation pathway involving Pt surface modifcations by deposition of sub-monolayers of support metal ions. The identifed degradation pathway
of the Pt/oxide catalyst/support couple contributes to our understanding of cathode electrocatalysts for polymer electrolyte fuel cells (PEFC).
|Stefan Rudi, Detre Teschner, Vera Beermann, Walid Hetaba,
Lin Gan, Chunhua Cui, Manuel Gliech, Robert Schlögl, and Peter
pH-Induced versus Oxygen-Induced Surface Enrichment and Segregation Effects in Pt–Ni Alloy Nanoparticle Fuel Cell Catalysts
ACS Catal. 7 (9), 6376-6384
We present a voltammetric, spectroscopic, and atomic-scale microscopic study of how initial interfacial contact with high- and low-pH electrolytes affects the surface voltammetry, near-surface composition, CO binding, and electrocatalytic oxygen reduction reaction (ORR) of dealloyed Pt–Ni alloy nanoparticles deployed in fuel cells. The first contact of the catalyst with the electrolyte is critical for the evolution of the catalytically active surface structure, yet still insufficiently understood. Counter to chemical intuition, we find that voltammetric activation protocols in both pH 1 and pH 13 electrolytes result in similarly Ni-depleted surfaces with similar near-surface Ni/Pt ratios to a 2.5 nm depth, yet vastly different ORR reactivities. On the basis of our combined voltammetric, scanning transmission electron microscopy with the spectroscopic mapping by energy dispersive X-ray (STEM-EDX) microscopic and X-ray photoelectron spectroscopy (XPS) analysis, we conclude that oxygen-saturated alkaline electrolytes causes a strong surface segregation of the more oxophilic Ni component toward the particles surface, however in distinctly different ways depending on the pretreatment pH. Data suggest a controlling role of the initial thickness of the Ni-depleted Pt shell for the catalysis-driven segregation process. We analyze and discuss how such subtle differences in initial surface composition can unfold such dramatic subsequent variations in ORR activity as a function of pH. Our findings have practical bearing for the design of active Pt bimetallic ORR catalysts for alkaline exchange membrane fuel cells. If the non-noble oxophilic Pt alloy component is insoluble in the alkaline electrolyte, our results call for an imperative acid-pretreatment to avoid surface blocking by oxygen-induced segregation. If the non-noble oxophilic Pt alloy component is soluble in an alkaline electrolyte, acid or alkaline, even nonpretreated Pt alloy catalyst may be employed.
Mistry, Rulle Reske, Peter Strasser, Beatriz Roldan Cuenya|
Size-dependent reactivity of gold-copper bimetallic nanoparticles during CO2 electroreduction
Catalysis Today 288, 30-36
New catalysts are needed to achieve lower overpotentials and higher faradaic efficiency for desirable products during the electroreduction of CO2. In this study, we explore the size-dependence of monodisperse gold-copper alloy nanoparticles (NPs) synthesized by inverse micelle encapsulation as catalysts for CO2 electroreduction. X-ray spectroscopy revealed that gold-copper alloys were formed and were heavily oxidized in their initial as prepared state. Current density was found to increase significantly for smaller NPs due to the increasing population of strongly binding low coordinated sites on NPs below 5 nm. Product analysis showed formation of H-2, CO, and CH4, with faradaic selectivity showing a minor dependence on size. The selectivity trends observed are assigned to reaction-induced segregation of gold atoms to the particle surface and altered electronic or geometric properties due to alloying.
|Alexander Bagger, Wen Ju, Ana Sofia
Varela, Peter Strasser, Jan Rossmeisl|
Single site porphyrine-like structures advantages over metals for selective electrochemical CO2 reduction
Catalysis Today 288, 74-78
Currently, no catalysts are completely selective for the electrochemical CO2 Reduction Reaction (CO2RR). Based on trends in density functional theory calculations of reaction intermediates we find that the single metal site in a porphyrine-like structure has a simple advantage of limiting the competing Hydrogen Evolution Reaction (HER). The single metal site in a porphyrine-like structure requires an ontop site binding of hydrogen, compared to the hollow site binding of hydrogen on a metal catalyst surface. The difference in binding site structure gives a fundamental energy-shift in the scaling relation of similar to 0.3 eV between the COOH* vs. H* intermediate (CO2RR vs. HER). As a result, porphyrine-like catalysts have the advantage over metal catalyst of suppressing HER and enhancing CO2RR selectivity.
Gong, Xuecheng Cao, Fan Li, Yue Gong, Lin Gu, Rafael Gregorio Mendes,
Mark H. Rummeli, Peter Strasser, and Ruizhi Yang|
PdAuCu Nanobranch as Self-Repairing Electrocatalyst for Oxygen Reduction Reaction
ChemSusChem 10 (7), 1469-1474
During start-up and shut-down operations of fuel cells, high potential is inevitably experienced at cathode, which leads to the deterioration of the oxygen reduction electrocatalyst. The design of catalysts that can repair themselves under severe conditions has been identified as a primary challenge for fuel cells. Herein, self-supported PdAuCu branched nanostructure is synthesized by a hydrothermal method. By smartly utilizing the high-potential treatment, the activity of PdAuCu is significantly enhanced owing to the synergistic effect between the Pd and Cu generated by such treatment. Moreover, the high activity of PdAuCu can be well maintained by repeating the high-potential treatment. We hence propose this catalyst as a “self-repairing” catalyst in a broad sense.
Velasco-Velez, K. Skorupska, E. Frei, Yu-Cheng
Huang, Chung-Li Dong, Bing-Jian Su, Cheng-Jhih
Hsu, Hung-Yu Chou, Jin-Ming Chen, P. Strasser, R.
Schloögl, A. Knop-Gericke, and C.-H. Chuang |
The Electro-Deposition/Dissolution of CuSO4 Aqueous Electrolyte Investigated by In Situ Soft X‑ray Absorption Spectroscopy
J. Phys. Chem. B 122 (2), 780-787
The electrodeposition nature of copper on a gold electrode in a 4.8 pH CuSO4 solution was inquired using X-ray absorption spectroscopy, electrochemical quartz crystal microbalance and thermal desorption spectroscopy techniques. Our results point out that the electrodeposition of copper prompts the formation of stable oxi-hydroxide species with a formal oxidation state Cu+ without the evidence of metallic copper formation (Cu0). Moreover, the subsequent anodic polarization of Cu2Oaq yields the formation of CuO, in the formal oxidation state Cu2+, which is dissolved at higher anodic potential. It was found that the dissolution process needs less charge than that required for the electrodeposition indicating a non-reversible process most likely due to concomitant water splitting and formation of protons during the electrodeposition.
|Alexander Bagger, Wen Ju, Ana Sofia
Varela, Peter Strasser, and Jan Rossmeisl |
Electrochemical CO2 Reduction: A Classification Problem
ChemPhysChem 18 (22), 3266-3273
In this work, we propose four non-coupled binding energies of intermediates as descriptors, or "genes", for predicting the product distribution in CO2 electroreduction. Simple reactions can be understood by the Sabatier principle (catalytic activity vs. one descriptor), while more complex reactions tend to give multiple very different products and consequently the product selectivity is a more complex property to understand. We approach this, as a logistical classification problem, by grouping metals according to their major experimental product from CO2 electroreduction: H2 , CO, formic acid and beyond CO* (hydrocarbons or alcohols). We compare the groups in terms of multiple binding energies of intermediates calculated by density functional theory. Here, we find three descriptors to explain the grouping: the adsorption energies of H*, COOH*, and CO*. To further classify products beyond CO*, we carry out formaldehyde experiments on Cu, Ag, and Au and combine these results with the literature to group and differentiate alcohol or hydrocarbon products. We find that the oxygen binding (adsorption energy of CH3 O*) is an additional descriptor to explain the alcohol formation in reduction processes. Finally, the adsorption energy of the four intermediates, H*, COOH*, CO*, and CH3 O*, can be used to differentiate, group, and explain products in electrochemical reduction processes involving CO2 , CO, and carbon-oxygen compounds.
|Vera Beermann, Martin Gocyla, Stefanie Kuehl, Elliot
Padgett, Henrike Schmies, Mikaela Goerlin, Nina Erini, Meital Shviro,
Marc Heggen, Rafal E. Dunin-Borkowski, David Muller, and Peter
Tuning the Electrocatalytic Oxygen Reduction Reaction Activity and Stability of Shape-Controlled Pt–Ni Nanoparticles by Thermal Annealing. Elucidating the Surface Atomic Structural and Compositional Changes
J. Am. Chem. Soc. 139 (46), 16536-16547
Shape-controlled octahedral Pt-Ni alloy nanoparticles exhibit remarkably high activities for the electroreduction of molecular oxygen (ORR), which makes them fuel cell cathode catalysts with exceptional potential. To unfold their full and optimized catalytic activity and stability, however, the nanooctahedra require post-synthesis thermal treatments, which alter the surface atomic structure and composition of the crystal facets. Here, we address and strive to elucidate the underlying surface chemical processes using a combination of ex situ analytical techniques with in situ transmission electron microscopy (TEM), in situ x-ray diffraction (XRD), and in situ electrochemical Fourier transformed infra-red (FTIR) experiments. We present a robust fundamental correlation between annealing temperature and catalytic activity, where a ~25x higher ORR activity than commercial Pt/C (2.7 A mgPt -1 at 0.9 VRHE) was reproducibly observed at annealing at 300 °C. The electrochemical stability, however, peaked without any and at the most severe heat treatments at 500 °C. Aberration-corrected scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDX) in combination with in situ electrochemical CO stripping/FTIR data revealed subtle, but important differences in the formation and chemical nature of Pt rich and Ni rich surface domains in the octahedral (111) facets. Estimating trends in surface chemisorption energies from in situ electrochemical CO/FTIR investigations suggested that balanced annealing generates an optimal degree of Pt surface enrichment, while the others exhibited mostly Ni rich facets. The insights from our study are quite generally valid and aid in developing suitable post synthesis thermal treatments for other alloy nanocatalysts as well.
|Wen Ju, Alexander Bagger, Guang-Ping Hao, Ana Sofia
Varela, Ilya Sinev, Volodymyr Bon, Beatriz Roldan Cuenya, Stefan
Kaskel, Jan Rossmeisl & Peter Strasser|
Understanding activity and selectivity of metal-nitrogen-doped carbon catalysts for electrochemical reduction of CO2
Nature Communications 8 (1), 1-8
Direct electrochemical reduction of CO2 to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO2-consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO2 to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M–N x moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe–N–C and especially Ni–N–C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M–N x moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M–N x moieties and it provides predictive guidelines for the rational design of selective carbon-based CO2 reduction catalysts.
|Toshinari Koketsu, Jiwei Ma, Benjamin
J. Morgan, Monique Body, Christophe Legein, Walid Dachraoui, Mattia
Giannini, Arnaud Demortière, Mathieu Salanne, François Dardoize,
Henri Groult, Olaf J. Borkiewicz, Karena W. Chapman, Peter
Strasser and Damien Dambournet |
Reversible magnesium and aluminium ions insertion in cation-deficient anatase TiO2
Nature Materials 16, 1142-1148
In contrast to monovalent lithium or sodium ions, the reversible insertion of multivalent ions such as Mg2+ and Al3+ into electrode materials remains an elusive goal. Here, we demonstrate a new strategy to achieve reversible Mg2+ and Al3+ insertion in anatase TiO2, achieved through aliovalent doping, to introduce a large number of titanium vacancies that act as intercalation sites. We present a broad range of experimental and theoretical characterizations that show a preferential insertion of multivalent ions into titanium vacancies, allowing a much greater capacity to be obtained compared to pure TiO2. This result highlights the possibility to use the chemistry of defects to unlock the electrochemical activity of known materials, providing a new strategy for the chemical design of materials for practical multivalent batteries.
|Xingli Wang, Ana Sofia Varela, Arno Bergmann, Stefanie
Kühl, and Peter Strasser|
Catalyst Particle Density Controls Hydrocarbon Product Selectivity in CO2 Electroreduction on CuOx
ChemSusChem 10 (22), 4642–4649
A key challenge of the carbon dioxide electroreduction (CO2RR) on Cu-based nanoparticles is its low faradic selectivity toward higher-value products such as ethylene. Here, we demonstrate a facile method for tuning the hydrocarbon selectivities on CuOx nanoparticle ensembles by varying the nanoparticle areal density. The sensitive dependence of the experimental ethylene selectivity on catalyst particle areal density is attributed to a diffusional interparticle coupling that controls the de- and re-adsorption of CO and thus the effective coverage of COad intermediates. Thus, higher areal density constitutes dynamically favored conditions for CO re-adsorption and *CO dimerization leading to ethylene formation independent of pH and applied overpotential.
|Nina Erini, Vera Beermann, Martin Gocyla, Manuel Gliech,
Marc Heggen, Rafal E. Dunin Borkowski, and Peter Strasser
The Effect of Surface Site Ensembles on the Activity and Selectivity of Ethanol Electrooxidation by Octahedral PtNiRh Nanoparticles
Angew. Chem. 56 (23), 6533-6538
Direct ethanol fuel cells are attractive power sources based on a biorenewable, high energy-density fuel. Their efficiency is limited by the lack of active anode materials which catalyze the breaking of the C−C bond coupled to the 12-electron oxidation to CO2. We report shape-controlled PtNiRh octahedral ethanol oxidation electrocatalysts with excellent activity and previously unachieved low onset potentials as low as 0.1 V vs. RHE, while being highly selective to complete oxidation to CO2. Our comprehensive characterization and in situ electrochemical ATR studies suggest that the formation of a ternary surface site ensemble around the octahedral Pt3Ni1Rhx nanoparticles plays a crucial mechanistic role for this behavior.
|Mikaela Görlin, Jorge Ferreira de
Araujo, Henrike Schmies, Denis Bernsmeier, Sören Dresp, Manuel
Gliech, Zenonas Jusys, Petko Chernev, Ralph Kraehnert, Holger Dau, and
Tracking Catalyst Redox States and Reaction Dynamics in Ni-Fe Oxyhydroxide Oxygen Evolution Reaction Electrocatalysts: The Role of Catalyst Support and Electrolyte pH
J. Am. Chem. Soc. 139 (5), 2070–2082
Ni–Fe oxyhydroxides are the most active known electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes and are therefore of great scientific and technological importance in the context of electrochemical energy conversion. Here we uncover, investigate, and discuss previously unaddressed effects of conductive supports and the electrolyte pH on the Ni–Fe(OOH) catalyst redox behavior and catalytic OER activity, combining in situ UV–vis spectro-electrochemistry, operando electrochemical mass spectrometry (DEMS), and in situ cryo X-ray absorption spectroscopy (XAS). Supports and pH > 13 strongly enhanced the precatalytic voltammetric charge of the Ni–Fe oxyhydroxide redox peak couple, shifted them more cathodically, and caused a 2–3-fold increase in the catalytic OER activity. Analysis of DEMS-based faradaic oxygen efficiency and electrochemical UV–vis traces consistently confirmed our voltammetric observations, evidencing both a more cathodic O2 release and a more cathodic onset of Ni oxidation at higher pH. Using UV–vis, which can monitor the amount of oxidized Ni+3/+4 in situ, confirmed an earlier onset of the redox process at high electrolyte pH and further provided evidence of a smaller fraction of Ni+3/+4 in mixed Ni–Fe centers, confirming the unresolved paradox of a reduced metal redox activity with increasing Fe content. A nonmonotonic super-Nernstian pH dependence of the redox peaks with increasing Fe content—displaying Pourbaix slopes as steep as −120 mV/pH—suggested a two proton–one electron transfer. We explain and discuss the experimental pH effects using refined coupled (PCET) and decoupled proton transfer–electron transfer (PT/ET) schemes involving negatively charged oxygenate ligands generated at Fe centers. Together, we offer new insight into the catalytic reaction dynamics and associated catalyst redox chemistry of the most important class of alkaline OER catalysts.
|Tobias Reier, Hong Nhan Nong, Detre Teschner, Robert
Schlögl, and Peter Strasser|
Electrocatalytic Oxygen Evolution Reaction in Acidic Environments – Reaction Mechanisms and Catalysts
Adv. Energy Mater. 7 (1), 1-18
The low efficiency of the electrocatalytic oxidation of water to O2 (oxygen evolution reaction-OER) is considered as one of the major roadblocks for the storage of electricity from renewable sources in form of molecular fuels like H2 or hydrocarbons. Especially in acidic environments, compatible with the powerful proton exchange membrane (PEM), an earth-abundant OER catalyst that combines high activity and high stability is still unknown. Current PEM-compatible OER catalysts still rely mostly on Ir and/or Ru as active components, which are both very scarce elements of the platinum group. Hence, the Ir and/or Ru amount in OER catalysts has to be strictly minimized. Unfortunately, the OER mechanism, which is the most powerful tool for OER catalyst optimization, still remains unclear. In this review, we first summarize the current state of our understanding of the OER mechanism on PEM-compatible heterogeneous electrocatalysts, before we compare and contrast that to the OER mechanism on homogenous catalysts. Thereafter, an overview over monometallic OER catalysts is provided to obtain insights into structure-function relations followed by a review of current material optimization concepts and support materials. Moreover, missing links required to complete the mechanistic picture as well as the most promising material optimization concepts are pointed out.
Özer, Camillo Spöri, Tobias Reier, and Peter Strasser|
Iridium(111), Iridium(110), and Ruthenium(0001) Single Crystals as Model Catalysts for the Oxygen Evolution Reaction: Insights into the Electrochemical Oxide Formation and Electrocatalytic Activity
ChemCatChem 9 (4), 597–603
We report a comparative study on the influence of generic electrochemical activation–oxidation protocols on the resulting surface oxides of Ir(1 1 1) and (1 1 0) and Ru(0 0 0 1) single crystals and their electrocatalytic reactivity for the oxygen evolution reaction. Well-defined single-crystal electrodes were prepared in a custom-made chamber that combines inductive thermal annealing and electrochemistry. The clean surfaces were analyzed for their electrocatalytic oxygen evolution activities and oxidation behavior. Three different oxidation protocols were used, which revealed a strong activity dependence on the duration and upper potential limit of the electrochemical oxidation. The resulting changes of the surface were followed by using cyclic voltammetry and impedance spectroscopy. Important differences between the two faces of Ir in terms of surface morphology of the formed oxide were identified, which allowed us to draw conclusions for preferable crystal faces in nanoparticle catalysts.
|Denis Bernsmeier, Michael Bernicke, Erik Ortel, Arno
Bergmann, Andreas Lippitz, Jörg Nissen, Roman Schmack, Peter
Strasser, Jörg Polte, and Ralph Kraehnert |
Nafion-Free Carbon-Supported Electrocatalysts with Superior Hydrogen Evolution Reaction Performance by Soft Templating
ChemElectroChem 4, 221-229
Efficient water electrolysis requires electrode coatings with high catalytic activity. Platinum efficiently catalyzes the hydrogen evolution reaction in acidic environments, but is a rare and expensive metal. The activity achieved per metal atom can be increased if small Pt particles are dispersed onto electrically conductive, highly accessible and stable support materials. However, the addition of Nafion, a typical binder material used in the manufacture of electrode coatings, can decrease catalytic activity by the blocking of pores and active surface sites. A new approach is reported for the direct synthesis of highly active Nafion-free Pt/C catalyst films consisting of small Pt nanoparticles supported in size-controlled mesopores of a conductive carbon film. The synthesis relies on the co-deposition of suitable Pt and C precursors in the presence of polymer micelles, which act as pore templates. Subsequent carbonization in an inert atmosphere produces porous catalyst films with controlled film thickness, pore size and particle size. The catalysts clearly outperform all Nafion-based Pt/C catalysts reported in the literature, particularly at high current densities.
|Camillo Spöri, Jason Tai Hong Kwan,
Arman Bonakdarpour, David Wilkinson and Peter Strasser|
The Stability Challenges of Oxygen Evolving Electrocatalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation
Angew. Chem. Int. Ed. 56 (22), 5994-6021
The electrochemical oxygen evolution reaction (OER) is an important reaction in the industrial production of numerous inorganic chemicals. However, over the last decade it has also received growing attention as scalable proton- and electron-providing process for use in the production of solar fuels, such as hydrogen, hydrocarbons, or alcohols carried out in water- and CO2-electrolyzers. To operate these devices efficiently and economically , active and stable electrocatalysts are required. While advances have been made in understanding and tuning OER efficiency and activity, the stability of OER catalysts and the reduction of their degradation continue to be major challenges. While most stability studies limit themselves to short-term testing in idealized three-electrode set ups, a much stronger focus on advancing our understanding of degradation of OER catalysts in realistic Membrane Electrode Assemblies (MEAs) with industrial current densities, is critically needed. This review addresses the technical challenges, their scientific basis, as well as recent progress and the road ahead regarding stability and degradation of OER catalysts operating at electrolyzer anodes in acidic MEA environments. First, we start clarifying the complexity associated with the term "catalyst stability", cover today's performance targets and outline major catalyst degradation mechanisms and their mitigation strategies. Then we evaluate suitable in-situ experimental methods to get insight into catalyst degradation and describe achievements in tuning OER catalyst stability. Finally, we highlight the importance of identifying universal figures of merit for stability and develop a comprehensive accelerated life test (ALT) that would yield comparable performance data across labs and catalyst types. As a whole, this review will help to disseminate and highlight the important relations between structure, composition and stability of OER catalysis under different operating conditions.
KontaktProf. Dr. Peter Strasser
Institut für Chemie
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+49 (0)30 314 22239
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