Molecular-level insight into interfacial water at a buried electrode interface is essential in electrochemistry, but spectroscopic probing of the interface remains challenging. Here, using surface... Show moreMolecular-level insight into interfacial water at a buried electrode interface is essential in electrochemistry, but spectroscopic probing of the interface remains challenging. Here, using surface-specific heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy, we directly access the interfacial water in contact with the graphene electrode supported on calcium fluoride (CaF2). We find phase transition-like variations of the HD-SFG spectra vs. applied potentials, which arises not from the charging/discharging of graphene but from the charging/discharging of the CaF2 substrate through the pseudocapacitive process. The potential-dependent spectra are nearly identical to the pH-dependent spectra, evidencing that the pseudocapacitive behavior is associated with a substantial local pH change induced by water dissociation between the CaF2 and graphene. Our work evidences the local molecular-level effects of pseudocapacitive charging at an electrode/aqueous electrolyte interface. Show less
The Angstrom-scale space between graphene and its substrate provides an attractive playground for scientific exploration and can lead to breakthrough applications. Here, we report the energetics... Show moreThe Angstrom-scale space between graphene and its substrate provides an attractive playground for scientific exploration and can lead to breakthrough applications. Here, we report the energetics and kinetics of hydrogen electrosorption on a graphene-covered Pt(111) electrode using electrochemical experiments, in situ spectroscopy, and density functional theory calculations. The graphene overlayer influences the hydrogen adsorption on Pt(111) by shielding the ions from the interface and weakening the Pt-H bond energy. Analysis of the proton permeation resistance with controlled graphene defect density proves that the domain boundary defects and point defects are the pathways for proton permeation in the graphene layer, in agreement with density functional theory (DFT) calculations of the lowest energy proton permeation pathways. Although graphene blocks the interaction of anions with the Pt(111) surfaces, anions do adsorb near the defects: the rate constant for hydrogen permeation is sensitively dependent on anion identity and concentration. Show less
Liu, X.; Cecilio de Oliveira Monteiro, M.; Koper, M.T.M. 2023
Insights into how to control the activity and selectivity of the electrochemical CO2 reduction reaction are still limited because of insufficient knowledge of the reaction mechanism and kinetics,... Show moreInsights into how to control the activity and selectivity of the electrochemical CO2 reduction reaction are still limited because of insufficient knowledge of the reaction mechanism and kinetics, which is partially due to the lack of information on the interfacial pH, an important parameter for proton-coupled reactions like CO2 reduction. Here, we used a reliable and sensitive pH sensor combined with the rotating ring-disk electrode technique, in which a functionalized Au ring electrode works as a real-time detector of the OH- generated during the CO2 reduction reaction at a gold disk electrode. Variations of the interfacial pH due to both electrochemical and homogeneous reactions are mapped and the correlation of the interfacial pH with these reactions is inferred. The interfacial pH near the disk electrode increases from 7 to 12 with increasing current density, with a sharp increase at around -0.5 V vs. RHE, which indicates a change of the dominant buffering species. Through scan rate-dependent voltammetry and chronopotentiometry experiments, the homogenous reactions are shown to reach equilibrium within the time scale of the pH measurements, so that the interfacial concentrations of different carbonaceous species can be calculated using equilibrium constants. Furthermore, pH measurements were also performed under different conditions to disentangle the relationship between the interfacial pH and other electrolyte effects. The buffer effect of alkali metal cations is confirmed, showing that weakly hydrated cations lead to less pronounced pH gradients. Finally, we probe to which extent increasing mass transport and the electrolyte buffer capacity can aid in suppressing the increase of the interfacial pH, showing that the buffer capacity is the dominant factor in suppressing interfacial pH variations. Show less
Solvent-adsorbate interactions have a great impact on catalytic processes in aqueous systems. Implicit solvent calculations are inexpensive but inaccurate toward hydrogen bonds, while a full... Show moreSolvent-adsorbate interactions have a great impact on catalytic processes in aqueous systems. Implicit solvent calculations are inexpensive but inaccurate toward hydrogen bonds, while a full incorporation of explicit solvation is computationally demanding. Micro-solvation attempts to break this dilemma by including only those solvent molecules directly interacting with the solute and any nearby interfaces, thereby providing a compromise between accuracy and computational expenses. Here, we show that micro-solvation of *OH and its relation to adsorption sites is largely transferable across late transition metal nanoparticles. Solvation energies for *OH on nanoparticles of Ir, Pd, and Pt range from -0.63 +/- 0.04 eV to -0.67 +/- 0.12 eV, while those on Au and Ag are -0.75 +/- 0.07 eV and -1.01 +/- 0.05 eV, respectively. These results enable the use of average solvation corrections for *OH on late transition metal nanostructures. Show less
Fröhlich, N.; Fernández Vidal, J.; Mascaró, F.V.; Shih, A. J.; Luo, M.; Koper, M.T.M. 2023
Seawater electrolysis offers significant logistical advantages over freshwater electrolysis but suffers from a fundamental selectivity problem at the anode. To prevent the evolution of toxic... Show moreSeawater electrolysis offers significant logistical advantages over freshwater electrolysis but suffers from a fundamental selectivity problem at the anode. To prevent the evolution of toxic chlorine alongside the evolution of oxygen, a promising approach is the use of electrochemically inert overlayers. Such thin films can exert a perm-selective effect, allowing the transport of water and oxygen between the bulk electrolyte and the electrocatalytic buried interface while suppressing the transport of chloride ions. In this work, we investigate thin (5-20 nm) overlayer films composed of amorphous silicon oxide (SiOx) and their application to suppressing the chlorine evolution reaction (CER) in favor of the oxygen evolution reaction (OER) during acidic saltwater electrolysis on three different types of electrodes. While SiOx overlayers are seen to be an effective barrier against the CER on well-defined, smooth Pt thin films, decreasing the CER activity roughly 20-fold, this ability has not been previously explored on Ir-based catalysts with a higher surface area relevant to industrial applications. On amorphous iridium oxide electrodes, the selectivity toward the CER versus the OER was marginally reduced from similar to 98 to similar to 94%, which was attributed to the higher abundance of defects in overlayers deposited on the rougher electrode. On the other hand, Ir-based anodes consisting of thick mixed metal oxide films supported on Ti showed a significant decrease in CER selectivity, from similar to 100 to similar to 50%, although this came at the cost of reduced activity toward the OER. These results show that the morphology and composition of the underlying electrode play important roles in the effectiveness of the selective overlayers and provide guidance for further development of high-surface-area OER-selective anodes. Show less
Platinum is the model catalyst in fuel cells because of its high activity toward oxygen reduction and hydrogen oxidation. However, its applicability is limited due to the degradation of the... Show morePlatinum is the model catalyst in fuel cells because of its high activity toward oxygen reduction and hydrogen oxidation. However, its applicability is limited due to the degradation of the catalyst under operating conditions. This degradation process has been extensively studied by repeatedly oxidizing and reducing the electrode, which leads to the roughening of the surface due to the nucleation and growth of platinum nano-islands. Although the general picture of this surface roughening is well known, the atomic details concerning the nucleation and early growth of the islands are still under debate. In this work, we use Density Functional Theory (DFT) to calculate formation energies and diffusion barriers of an adatom, in both the unoxidized and the oxidized state, with the aim to provide further insight into the nucleation phenomena. Moreover, we analyze from STM images obtained experimentally the shape of the nano-islands during the first stages of growth. Our results show not only that the islands form during the reduction of the surface, but also that they grow with a dendritic island shape, similarly to the platinum islands formed in vacuum by Molecular Beam Epitaxy (MBE). Show less
Shih, A.J.; Cecílio de Oliveira Monteiro, M.; Dattila, F.; Pavesi, D.; Philips, M.; Marques da Silva, A.H..; ... ; Koper, M.T.M. 2022
Electrochemical CO2 reduction (CO2R) is an attractive option for storing renewable electricity and for the sustainable production of valuable chemicals and fuels. In this roadmap, we review recent... Show moreElectrochemical CO2 reduction (CO2R) is an attractive option for storing renewable electricity and for the sustainable production of valuable chemicals and fuels. In this roadmap, we review recent progress in fundamental understanding, catalyst development, and in engineering and scale-up. We discuss the outstanding challenges towards commercialization of electrochemical CO2R technology: energy efficiencies, selectivities, low current densities, and stability. We highlight the opportunities in establishing rigorous standards for benchmarking performance, advances in in operando characterization, the discovery of new materials towards high value products, the investigation of phenomena across multiple-length scales and the application of data science towards doing so. We hope that this collective perspective sparks new research activities that ultimately bring us a step closer towards establishing a low- or zero-emission carbon cycle. Show less
Ye, C.; Raaijman, S.J.; Chen, X.; Koper, M.T.M. 2022
Developing active and selective catalysts that convert CO2 into valuable products remains a critical challenge for further application of the electrochemical CO2 reduction reaction (CO2RR).... Show moreDeveloping active and selective catalysts that convert CO2 into valuable products remains a critical challenge for further application of the electrochemical CO2 reduction reaction (CO2RR). Catalytic tuning with organic additives/films has emerged as a promising strategy to tune CO2RR activity and selectivity. Herein, we report a facile method to significantly change CO2RR selectivity and activity of copper and gold electrodes. We found improved selectivity toward HCOOH at low overpotentials on both polycrystalline Cu and Au electrodes after chemical modification with a poly(4-vinylpyridine) (P4VP) layer. In situ attenuated total reflection surface-enhanced infrared reflection-adsorption spectroscopy and contact angle measurements indicate that the hydrophobic nature of the P4VP layer limits mass transport of HCO3- and H2O, whereas it has little influence on CO2 mass transport. Moreover, the early onset of HCOOH formation and the enhanced formation of HCOOH over CO suggest that P4VP modification promotes a surface hydride mechanism for HCOOH formation on both electrodes. Show less
Proton-exchange membrane fuel cells demand efficient electrode-electrolyte interfaces to catalyse the oxygen reduction reaction (ORR), the kinetics of which depends on the energetics of surface... Show moreProton-exchange membrane fuel cells demand efficient electrode-electrolyte interfaces to catalyse the oxygen reduction reaction (ORR), the kinetics of which depends on the energetics of surface adsorption and on electrolyte environment. Here we show an unanticipated effect of non-specifically adsorbed anions on the ORR kinetics on a Pt(111) electrode; these trends do not follow the usual ORR descriptor, that is *OH binding energy. We propose a voltammetry-accessible descriptor, namely reversibility of the *O <-> *OH transition. This descriptor tracks the dependence of ORR rates on electrolyte, including the concentration/identity of anions in acidic media, cations in alkaline media and the effect of ionomers. We propose a model that relates the ORR rate on Pt(111) to the rate of the *O to *OH transition, in addition to the thermodynamic *OH binding energy descriptor. Our model also rationalizes different trends for the ORR rate on stepped Pt surfaces in acidic versus alkaline media. Show less
Marcandalli, G.; Cecílio de Oliveira Monteiro, M.; Goyal, A.; Koper, M.T.M. 2022
The electrochemical reduction of CO2 (CO2RR) constitutes an alternative to fossil fuel-based technologies for the production of fuels and commodity chemicals. Yet the application of CO2RR... Show moreThe electrochemical reduction of CO2 (CO2RR) constitutes an alternative to fossil fuel-based technologies for the production of fuels and commodity chemicals. Yet the application of CO2RR electrolyzers is hampered by low energy and Faradaic efficiencies. Concomitant electrochemical reactions, like hydrogen evolution (HER), lower the selectivity, while the conversion of CO2 into (bi)carbonate through solution acid−base reactions induces an additional concentration overpotential. During CO2RR in aqueous media, the local pH becomes more alkaline than the bulk causing an additional consumption of CO2 by the homogeneous reactions. The latter effect, in combination with the low solubility of CO2 in aqueous electrolytes (33 mM), leads to a significant depletion in CO2 concentration at the electrode surface. The nature of the electrolyte, in terms of pH and cation identity, has recently emerged as an important factor to tune both the energy and Faradaic efficiency. In this Account, we summarize the recent advances in understanding electrolyte effects on CO2RR to CO in aqueous solutions, which is the first, and crucial, step to further reduced products. To compare literature findings in a meaningful way, we focus on results reported under well-defined mass transport conditions and using online analytical techniques. The discussion covers the molecular-level understanding of the effects of the proton donor, in terms of the suppression of the CO2 gradient vs enhancement of HER at a given mass transport rate and of the cation, which is crucial in enabling both CO2RR and HER. These mechanistic insights are then translated into possible implications for industrially relevant cell geometries and current densities. Show less
Gas diffusion electrodes (GDEs) allow electrochemical reactions to occur at higher rates by enhancing the mass transport of gaseous reactants to the catalyst. These electrodes are made of two... Show moreGas diffusion electrodes (GDEs) allow electrochemical reactions to occur at higher rates by enhancing the mass transport of gaseous reactants to the catalyst. These electrodes are made of two layers: the catalyst layer and the gas diffusion layer (GDL). The catalyst layer is frequently studied for gas diffusion electrodes, and the GDL is rarely a focus. Consequently, no studies investigate interaction effects that may be present between these two layers. To study such interactions, it must be possible to obtain GDLs with various characteristics. This study uses a design of experiments to understand how multiple factors in the production method for GDLs can be adjusted to tune the characteristics of the GDL. These GDLs are particularly intended for the electrochemical reduction of CO2. The conductance through the GDL, surface conductivity, thickness, elasticity, hydrophobicity, and porosity are measured for the 26 synthesized electrodes, and the top influential production factors are identified for each characteristic. Show less
Renewable fuel generation is essential for a low carbon footprint economy. Thus, over the last five decades, a significant effort has been dedicated towards increasing the performance of solar... Show moreRenewable fuel generation is essential for a low carbon footprint economy. Thus, over the last five decades, a significant effort has been dedicated towards increasing the performance of solar fuels generating devices. Specifically, the solar to hydrogen efficiency of photoelectrochemical cells has progressed steadily towards its fundamental limit, and the faradaic efficiency towards valuable products in CO2 reduction systems has increased dramatically. However, there are still numerous scientific and engineering challenges that must be overcame in order to turn solar fuels into a viable technology. At the electrode and device level, the conversion efficiency, stability and products selectivity must be increased significantly. Meanwhile, these performance metrics must be maintained when scaling up devices and systems while maintaining an acceptable cost and carbon footprint. This roadmap surveys different aspects of this endeavor: system benchmarking, device scaling, various approaches for photoelectrodes design, materials discovery, and catalysis. Each of the sections in the roadmap focuses on a single topic, discussing the state of the art, the key challenges and advancements required to meet them. The roadmap can be used as a guide for researchers and funding agencies highlighting the most pressing needs of the field. Show less
Koper, M.T.M.; Weber, A.Z.; Chan, K.; Cheng, J. 2022
Abstract Electrochemical reactions in general, and the CO2 reduction reaction (CO2RR) in particular, are commonly studied at room temperature. However, practical electrolysers may use elevated... Show moreAbstract Electrochemical reactions in general, and the CO2 reduction reaction (CO2RR) in particular, are commonly studied at room temperature. However, practical electrolysers may use elevated temperatures. There is currently a lack of fundamental understanding of the effect of temperature on the CO2RR. Here, we performed temperature-dependent studies on the CO2RR on a relatively simple electrode material, namely gold, to obtain insights into how temperature influences this reaction and the competing hydrogen evolution reaction. A rotating ring disk electrode setup was used to show that the CO2RR activity and selectivity increased with temperature, and to obtain kinetic parameters such as the apparent activation energy and transfer coefficient. The magnitude of the temperature effect and the activation energy was affected by both the cation identity and concentration in the electrolyte. Moreover, the positive effect of temperature on the kinetics of the CO2RR was counteracted by the lower CO2 solubility, making efficient mass transport even more important at higher temperatures. Show less
In this paper, we study the electrocatalytic reduction of methyl vinyl ketone on Pt(111) and Pd-modified Pt(111) electrodes, as well as some of its expected hydrogenation derivatives and isolated... Show moreIn this paper, we study the electrocatalytic reduction of methyl vinyl ketone on Pt(111) and Pd-modified Pt(111) electrodes, as well as some of its expected hydrogenation derivatives and isolated functional moieties. The selectivity and Faraday efficiency have been calculated via sampling the catholyte solution after two hours of electrolysis. Furthermore, the adsorbates involved in the deactivation process on both surfaces were studied by means of potential opening experiments and in situ infrared spectroscopy. The Pd-modified Pt(111) electrode is very active (even mass transport limited) for the selective electrochemical hydrogenation of the C=C to 2-butanone, in the hydrogen underpotential deposition potential window (between 0 and 0.2 VRHE), with limited poisoning. Pt(111) is much less active, and poisons rapidly with adsorbed CO. The poison is formed from the C=C bond, not from the C=O moiety, as evidenced by the same poisoning occurring for ethylene. Further hydrogenation to the saturated alcohol happens at more negative potentials, but with 2-butenol as intermediate, not 2butanone, as the latter species interacts too weakly with the (111) surface. Show less
