In the search for rational design strategies for oxygenevolutionreaction (OER) catalysts, linking the catalyst structure to activityand stability is key. However, highly active catalysts such as... Show moreIn the search for rational design strategies for oxygenevolutionreaction (OER) catalysts, linking the catalyst structure to activityand stability is key. However, highly active catalysts such as IrO x and RuO x undergostructural changes under OER conditions, and hence, structure-activity-stabilityrelationships need to take into account the operando structure ofthe catalyst. Under the highly anodic conditions of the oxygen evolutionreaction (OER), electrocatalysts are often converted into an activeform. Here, we studied this activation for amorphous and crystallineruthenium oxide using X-ray absorption spectroscopy (XAS) and electrochemicalscanning electron microscopy (EC-SEM). We tracked the evolution ofsurface oxygen species in ruthenium oxides while in parallel mappingthe oxidation state of the Ru atoms to draw a complete picture ofthe oxidation events that lead to the OER active structure. Our datashow that a large fraction of the OH groups in the oxide are deprotonatedunder OER conditions, leading to a highly oxidized active material.The oxidation is centered not only on the Ru atoms but also on theoxygen lattice. This oxygen lattice activation is particularly strongfor amorphous RuO x . We propose that thisproperty is key for the high activity and low stability observed foramorphous ruthenium oxide. Show less
Creating active and stable electrodes is an essential step toward efficient and durable electrolyzers. To achieve this goal, understanding what aspects of the electrode structure dictate activity... Show moreCreating active and stable electrodes is an essential step toward efficient and durable electrolyzers. To achieve this goal, understanding what aspects of the electrode structure dictate activity and catalyst dissolution is key. Here, we investigate these aspects by studying trends in the activity, stability, and operando structure of iridium oxides during the oxygen evolution reaction. Using operando X-ray photoelectron and X-ray absorption spectroscopy, we determined the near-surface structure of oxides ranging from amorphous to crystalline during the reaction. We show that applying oxygen evolution potentials universally yields deprotonated mu(2)-O moieties and a mu(1)-O/mu(1)-OH mixture, with universal deprotonation energetics but in different amounts. This quantitative difference mainly results from variations in deprotonation depth: surface deprotonation for crystalline IrO2 versus near-surface deprotonation for semicrystalline and amorphous IrOx. We argue that both surface deprotonation and subsurface deprotonation modify the barrier for the oxygen evolution and Ir dissolution reactions, thus playing an important role in catalyst performance. Show less
The homogeneity of molecular Co-based wateroxidation catalysts (WOCs) has been a subject of debate over thelast 10 years as assumed various homogeneous Co-based WOCswere found to actually form... Show moreThe homogeneity of molecular Co-based wateroxidation catalysts (WOCs) has been a subject of debate over thelast 10 years as assumed various homogeneous Co-based WOCswere found to actually form CoOxunder operating conditions. Thehomogeneity of the Co(HL)(HL=N,N-bis(2,2 '-bipyrid-6-yl)amine) system was investigated with cyclic voltammetry,electrochemical quartz crystal microbalance, and X-ray photo-electron spectroscopy. The obtained experimental results werecompared with heterogeneous CoOx. Although it is shown thatCo(HL) interacts with the electrode during electrocatalysis, theformation of CoOxwas not observed. Instead, a molecular depositof Co(HL) was found to be formed on the electrode surface. Thisstudy shows that deposition of catalytic material is not necessarilylinked to the decomposition of homogeneous cobalt-based water oxidation catalysts. Show less
Jiang, L.; Dijk, B. van; Wu, L.; Clément, M.; Hofmann, J.P.; Tudor, V.; ... ; Schneider, G.F. 2022
In this work, we study how the cation identity and concentration alter the kinetics of the hydrogen evolution reaction (HER) on platinum and gold electrodes. A previous work suggested an inverted... Show moreIn this work, we study how the cation identity and concentration alter the kinetics of the hydrogen evolution reaction (HER) on platinum and gold electrodes. A previous work suggested an inverted activity trend as a function of alkali metal cation when comparing the performance of platinum and gold catalysts in alkaline media. We show that weakly hydrated cations (K+) favor HER on gold only at low overpotentials (or lower alkalinity), whereas in more alkaline pH (or high overpotentials), a higher activity is observed using electrolytes containing strongly hydrated cations (Li+). We find a similar trend for platinum; however, the inhibition of HER by weakly hydrated cations on platinum is observed already at lower alkalinity and lower cation concentrations, suggesting that platinum interacts more strongly with metal cations than gold. We propose that weakly hydrated cations stabilize the transition state of the water dissociation step more favorably due to their higher near-surface concentration in comparison to a strongly hydrated cation such as Li+. However, at high pH and consequently higher near-surface cation concentrations, the accumulation of these species at the outer Helmholtz plane inhibits HER. This is especially pronounced on platinum, where a change in the rate-determining step is observed at pH 13 when using a Li+- or K+-containing electrolyte. Show less
The production of hydrogen via water electrolysis using renewable electricity is a promising carbon-neutral technology. In this contribution, we report insights into the hydrogen evolution reaction... Show moreThe production of hydrogen via water electrolysis using renewable electricity is a promising carbon-neutral technology. In this contribution, we report insights into the hydrogen evolution reaction (HER) in H2SO4 on Pt(111) and graphene-covered Pt(111), in addition to the electrochemical properties of graphene overlayers. As-prepared graphene overlayers are selectively permeable to H+ ions in the electrolyte, allowing H+ ions into the confined layer between graphene and Pt(111) while excluding SO42- and other anions. We demonstrate that defects in these as-prepared graphene overlayers can be generated from oxidation at high overpotentials or reduction from the production of H-2 bubbles and postulate that HER occurs locally at only Pt(111) in the proximity of defects in graphene overlayers on asprepared G/Pt(111) electrodes, and as defects in graphene increases, more of the Pt(111) surface becomes utilized for HER. Kinetically, the addition of defective graphene overlayers can increase the geometric HER rate by up to 200%, while Tafel slopes and [H+] reaction orders remain unchanged. These results shed kinetic insight into the nature of graphene overlayers and their effect on HER catalysis and also demonstrate the promise of confinement modifications in designing catalysts with properties closer to achieving optimum rates. Show less
In the pursuit of a hydrogen economy, extensive research has been directed at developing acidic and alkaline hydrogen fuel cells. Such fuel cells often utilize platinum-based catalysts. These... Show moreIn the pursuit of a hydrogen economy, extensive research has been directed at developing acidic and alkaline hydrogen fuel cells. Such fuel cells often utilize platinum-based catalysts. These materials have been studied extensively in acidic conditions but not in alkaline ones. This focus on acidic systems creates a marked knowledge gap, since recent studies indicate that carbon-supported platinum (Pt/C) electrocatalysts degrade more rapidly in bases than in acids. Addressing this gap, the present work investigates Pt/C degradation at pH 2 and pH 12 using electrochemistry, transmission electron microscopy (TEM), and in situ X-ray absorption spectroscopy (XAS). TEM and XAS reveal accelerated Pt/C degradation at high pH levels, which results in increased Ostwald ripening, Smoluchowski agglomeration, and nanoparticle detachment. These processes are driven by platinum-catalyzed carbon corrosion and the dissolution and redeposition of platinum nanoparticles. Although these processes take place at both low and high pH levels, basic conditions accelerate the degradation. Base-enhanced Pt dissolution and redeposition was assessed in further detail, revealing an oxidation onset reduction of 100 mV in the base; however, there were no significant differences between undissolved Pt oxidation in acid and in base. The results suggest that soluble Pt oxidation products are stabilized in the base instead. These conclusions are important for translating acidbased literature to alkaline conditions. Show less
The electrochemical reduction of CO2 aims to be a central technology to store excess electricity generated by wind and solar energy. However, the reaction is hindered by the competition with the... Show moreThe electrochemical reduction of CO2 aims to be a central technology to store excess electricity generated by wind and solar energy. However, the reaction is hindered by the competition with the hydrogen evolution reaction. In this paper, we present a detailed quantitative study of the Faradaic efficiency (FE) to CO on a gold electrode under well-defined mass-transport conditions using rotating ring-disk electrode voltammetry. Varying the concentration of the bicarbonate and the electrolyte cation employing different rotation rates, we map out how these parameters affect the FE(CO). We identify two different potential regimes for the electrolyte effects, characterized by a different dependence on the cation and bicarbonate concentrations. For hydrogen evolution, we analyze the nature of the proton donor for an increasingly negative potential, showing how it changes from carbonic acid to bicarbonate and to water. Our study gives detailed insights into the role of electrolyte composition and mass transport, and helps defining optimized electrolyte conditions for a high FE(CO). Show less
The complex alpha-[Fe(mcp)(OTf)(2)] (mcp = N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)-cyclohexane-1,2-diamine and OTf = trifluoromethanesulfonate anion) was reported in 2011 by some of us as an... Show moreThe complex alpha-[Fe(mcp)(OTf)(2)] (mcp = N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)-cyclohexane-1,2-diamine and OTf = trifluoromethanesulfonate anion) was reported in 2011 by some of us as an active water oxidation (WO) catalyst in the presence of sacrificial oxidants. However, because chemical oxidants are likely to take part in the reaction mechanism, mechanistic electrochemical studies are critical in establishing to what extent previous studies with sacrificial reagents have actually been meaningful. In this study, the complex alpha-[Fe(mcp)(OTf)(2)] and its analogues were investigated electrochemically under both acidic and neutral conditions. All the systems under investigation proved to be electrochemically active toward the WO reaction, with no major differences in activity despite the structural changes. Our findings show that WO-catalyzed by mcp-iron complexes proceeds via homogeneous species, whereas the analogous manganese complex forms a heterogeneous deposit on the electrode surface. Mechanistic studies show that the reaction proceeds with a different rate-determining step (rds) than what was previously proposed in the presence of chemical oxidants. Moreover, the different kinetic isotope effect (KIE) values obtained electrochemically at pH 7 (KIE similar to 10) and at pH 1 (KIE = 1) show that the reaction conditions have a remarkable effect on the rds and on the mechanism. We suggest a proton-coupled electron transfer (PCET) as the rds under neutral conditions, whereas at pH 1 the rds is most likely an electron transfer (ET). Show less
Seawater electrolysis is an attractive approach for producing clean hydrogen fuel in scenarios where freshwater is scarce and renewable electricity is abundant. However, chloride ions (Cl-) in... Show moreSeawater electrolysis is an attractive approach for producing clean hydrogen fuel in scenarios where freshwater is scarce and renewable electricity is abundant. However, chloride ions (Cl-) in seawater can accelerate electrode corrosion and participate in the undesirable chlorine evolution reaction (CER). This problem is especially acute in acidic conditions that naturally arise at the anode as a result of the desired oxygen evolution reaction (OER). Herein, we demonstrate that ultrathin silicon oxide (SiOx) overlayers on model platinum anodes are highly effective at suppressing the CER in the presence of 0.6 M Cl- in both acidic and unbuffered pH-neutral electrolytes by blocking the transport of Cl- to the catalytically active buried interface while allowing the desired oxygen evolution reaction (OER) to occur there. The permeability of Cl- in SiOx overlayers is 3 orders of magnitude less than that of Cl- in a conventional salt-selective membrane used in reverse osmosis desalination. The overlayers also exhibit robust stability over 12 h in chronoamperometry tests at moderate overpotentials. SiOx overlayers demonstrate a promising step toward achieving selective and stable seawater electrolysis without the need to adjust the pH of the electrolyte. Show less
Hersbach, T.J.P.; Ye, C.; Garcia, A.C.; Koper, M.T.M. 2020
Accurately identifying the active species of catalytic materials and understanding how they catalyze the oxygen evolution reaction (OER) are critical for the development of energy storage... Show moreAccurately identifying the active species of catalytic materials and understanding how they catalyze the oxygen evolution reaction (OER) are critical for the development of energy storage technologies. In this contribution, we identify two pH-dependent active oxides by mapping the reduction behavior of gold oxide and by in situ surface-enhanced Raman spectroscopy. It was found that α-oxide is preferentially formed in an acidic solution, whereas β-oxide, Au(OH)3, is preferably formed in an alkaline solution. In line with the presence of two different surface structures on gold, there are two OER mechanisms: one mechanism wherein water splitting occurs via proton-coupled electron-transfer steps mediated by α-oxide and the other mechanism wherein electron transfer and proton transfer are decoupled and mediated by a deprotonated form of Au(OH)3. This identification of pH-dependent oxides offers a different perspective in our understanding of the OER mechanism on metal oxides in a full pH scale range. Show less
Granda Marulanda L.P., Rendón-Calle A., Builes S., Illas F., Koper M.T.M., Calle-Vallejo F. 2020
Computational models of adsorption at metal surfaces are often based on DFT and make use of the generalized gradient approximation. This likely implies the presence of sizable errors in the gas... Show moreComputational models of adsorption at metal surfaces are often based on DFT and make use of the generalized gradient approximation. This likely implies the presence of sizable errors in the gas-phase energetics. Here, we take a step closer toward chemical accuracy with a semiempirical method to correct the gas-phase energetics of PBE, PW91, RPBE, and BEEF-vdW exchange-correlation functionals. The proposed two-step method is tested on a data set of 27 gas-phase molecules belonging to the carbon cycle: first, the errors are pinpointed based on formation energies, and second, the respective corrections are sequentially applied to ensure the progressive lowering of the data set's mean and maximum errors. We illustrate the benefits of the method in electrocatalysis by a substantial improvement of the calculated equilibrium and onset potentials for CO2 reduction to CO on Au, Ag, and Cu electrodes. This suggests that fast and systematic gas-phase corrections can be devised to augment the predictive power of computational catalysis models. Show less
