The bifunctional mechanism that involves adsorbed hydroxide in the alkaline hydrogen oxidation and evolution reactions, important in hydrogen fuel cells and water electrolysers, is hotly debated.... Show moreThe bifunctional mechanism that involves adsorbed hydroxide in the alkaline hydrogen oxidation and evolution reactions, important in hydrogen fuel cells and water electrolysers, is hotly debated. Hydroxide binding has been suggested to impact activity, but the exact role of adsorbed hydroxide in the reaction mechanism is unknown. Here, by selectively decorating steps on a Pt single crystal with other metal atoms, we show that the rate of alkaline hydrogen evolution exhibits a volcano-type relationship with the hydroxide binding strength. We find that Pt decorated with Ru at the step edge is 65 times more active for the hydrogen evolution reaction (HER) than is the bare Pt step. Simulations of electrochemical water dissociation show that the activation energy correlates with the OH* adsorption strength, even when the adsorbed hydroxide is not a product, which leads to a simulated volcano curve that matches the experimental curve. This work not only illustrates the alkaline HER mechanism but also provides a goal for catalyst design in targeting an optimum hydroxide binding strength to yield the highest rate for the alkaline HER. A three-dimensional (H and OH adsorbed species) HER activity volcano and the implications for hydrogen oxidation are discussed.The appropriate descriptors for a catalyst's hydrogen evolution activity in alkaline electrolyte are debated. Combining simulations and single-crystal studies of metal-decorated Pt surfaces, McCrum and Koper show that activity exhibits a volcano-type relationship with the hydroxide binding strength of the catalyst, providing a target for catalyst design. Show less
The electrochemical reduction of carbon dioxide to formate is an appealing carbon utilization method as it can be performed at room temperature and pressure, it only requires two electrons, and it... Show moreThe electrochemical reduction of carbon dioxide to formate is an appealing carbon utilization method as it can be performed at room temperature and pressure, it only requires two electrons, and it has a high atom efficiency. This reaction has been known and studied for decades, but currently there is no commercial process practiced. This Perspective compares over 500 experimental data points from 65 scientific publications and patents and identifies the gas diffusion electrode as the best technology to scale this reaction. We further discuss the complex layers that make up a gas diffusion electrode and view what has been studied with respect to this reaction. Finally, we identify critical areas where more research can provide crucial understandings to allow this technology to become a commercial process. Show less
Pavesi, D.; Poll, R.C.J. van de; Krasovic, J.L.; Figueiredo, M.; Gruter, G.J.M.; Koper, M.T.M.; Schouten, K.J.P. 2020
CO2 electroreduction to formate powered by renewable energy is an attractive strategy to recycle air-based carbon. At the moment, the electrode materials showing high selectivity for formate at... Show moreCO2 electroreduction to formate powered by renewable energy is an attractive strategy to recycle air-based carbon. At the moment, the electrode materials showing high selectivity for formate at high current density are post transition metals such as In, Sn, Bi, and Pb. Scaling up the CO2 electroreduction technology to industrial size requires, among other things, cheap and clean methods to produce cathode materials in the form of particles to fabricate the square meters of the electrode surface area needed for the industrial electrolyzers. We show here that it is possible to easily produce catalytic powders based on Sn and Pb via a process known as cathodic disintegration, driving the reaction with electric power and avoiding the use of organic solvents, stabilizers, and reducing agents. The catalysts produced with this method are highly selective for the reduction of CO2 to formate and show promise for use in industrial electrolyzers. Moreover, the process of cathodic disintegration is quick and clean, it has a high atom efficiency, it uses dilute aqueous electrolytes as solvents, and it has the possibility to be driven by renewable energy. Show less
During hydrogen production for (renewable) energy storage, direct seawater electrolysis offers several notable advantages over freshwater electrolysis. Unfortunately, it is also hindered by... Show moreDuring hydrogen production for (renewable) energy storage, direct seawater electrolysis offers several notable advantages over freshwater electrolysis. Unfortunately, it is also hindered by possible oxidation reactions of chloride and (to a lesser extent) bromide, which can occur in parallel to the evolution of oxygen and form harmful by-products at the anode. Although the respective oxidation reactions of Br- and Cl- have been researched quite well on Pt, not much is known concerning bromide oxidation and its effect on the evolution of chlorine and oxygen for metal oxides, which are the class of electrocatalysts overwhelmingly used in industry. Using glassy carbon-supported iridium oxide (IrOx) as a model system, we investigated the oxidation behaviour of this well-known oxygen evolution catalyst in an acidic Br-/Cl- electrolyte. We first briefly discuss the solution chemistry and oxidation products that may be expected. Model studies were performed of the parallel evolution of Br-2, Cl-2 and O-2 to increase the understanding of the anodic competition problem, with a special focus on the selectivity towards oxygen. Using rotating ring-disk voltammetry and UV-Vis spectroscopy, our results suggest that bromide and chloride competitively absorb on IrOx, but do not alter each other's oxidation reaction mechanisms, which both seem to adhere best to a Volmer-Heyrovsky mechanism. We also find that bromide and chloride adsorption significantly slow down the oxygen evolution reaction, in an additive way. Even a relatively small amount of bromide highly affected the oxygen evolution selectivity. Formation of the interhalogen compound BrCl, which is possible in a mixed Br-/Cl- electrolyte, does not seem to occur. (C) 2020 The Authors. Published by Elsevier Inc. Show less
Many polyols are abundant and cheap molecules highly spread in the biomass. These molecules have an enormous potential to be used in electrochemical devices to generate energy and/or value-added... Show moreMany polyols are abundant and cheap molecules highly spread in the biomass. These molecules have an enormous potential to be used in electrochemical devices to generate energy and/or value-added molecules. The electrooxidation of polyols can produce different substances of interest in the chemical industry concomitantly to high purity hydrogen in electrolyzers. The cost in the production of all these chemicals depends, among other factors, on the develop of more active and selective catalysts. However, in order to search for these materials using computational experiments, it is mandatory to have a better understanding of the fundamental aspect of the reactions, which permit to base the search on the adsorption energies of one or more key reaction intermediates. To contribute to this task, we performed (spectro)-electrochemical and computational experiments to study the electrooxidation of C-4 polyols. We show that the electrooxidation of polyols does not depend on the relative orientation of their OH groups. Besides, using Pt single crystals, we demonstrate that the trend for the oxidation of the primary carbon (relative to the secondary) increases in the order Pt(111) < Pt(100) < Pt(110) and that this result can be extended to polyols with longer carbon chains. Finally, computational experiments permit us to rationalize these trends looking at the relative stability of double dehydrogenated intermediates on the Pt basal planes. Show less
CO(2)electroreduction powered by renewable energy is an attractive strategy to recycle air-based carbon. One of the current challenges for the scale up of the technology is that the catalysts that... Show moreCO(2)electroreduction powered by renewable energy is an attractive strategy to recycle air-based carbon. One of the current challenges for the scale up of the technology is that the catalysts that show high faradaic yield at high current density (post-transitional metals such as In, Sn, Bi, Pb) suffer from very high overpotentials of more than 1 V. On the other hand, Pd can convert CO(2)to formate with almost no overpotential, but is readily poisoned by CO and deactivates when trying to reach industrially relevant currents. In this work we show the effect of the interaction of In and Pd in bimetallic nanoparticles, reaching the conclusion that this interaction causes a loss of selectivity towards formate and at the same time suppresses CO poisoning of Pd sites. The results of the catalyst characterization suggest the formation of intermetallic PdIn compounds that in turn cause the aforementioned behavior. Based on these results, it seems that geometric and electronic effects in Pd based intermetallic compounds can alleviate CO poisoning on Pd sites. In the case of PdIn intermetallics this leads to the loss of CO(2)reduction activity, but this strategy may be useful for other electrochemical reactions that suffer from the same problem of deactivation. It remains to be seen if intermetallic compounds of Pd with other elements can yield viable CO(2)reduction catalysts. Show less
The electric double layer (EDL) formed around charged nanostructures at the liquid-solid interface determines their electrochemical activity and influences their electrical and optical... Show moreThe electric double layer (EDL) formed around charged nanostructures at the liquid-solid interface determines their electrochemical activity and influences their electrical and optical polarizability. We experimentally demonstrate that restructuring of the EDL at the nanoscale can be detected by dark-field scattering microscopy. Temporal and spatial characterization of the scattering signal demonstrates that the potentiodynamic optical contrast is proportional to the accumulated charge of polarizable ions at the interface and that its time derivative represents the nanoscale ionic current. The material specificity of the EDL formation is used in our work as a label-free contrast mechanism to image nanostructures and perform spatially resolved cyclic voltammetry on an ion-current density of a few attoamperes, corresponding to the exchange of only a few hundred ions. Show less
Gold is one of the most selective catalysts for the electrochemical reduction of CO2 (CO2RR) to CO. However, the concomitant hydrogen evolution reaction (HER) remains unavoidable under aqueous... Show moreGold is one of the most selective catalysts for the electrochemical reduction of CO2 (CO2RR) to CO. However, the concomitant hydrogen evolution reaction (HER) remains unavoidable under aqueous conditions. In this work, a rotating ring disk electrode (RRDE) setup has been developed to study quantitatively the role of mass transport in the competition between these two reactions on the Au surface in 0.1 M bicarbonate electrolyte. Interestingly, while the faradaic selectivity for CO formation was found to increase with enhanced mass transport (from 67% to 83%), this effect is not due to an enhancement of the CO2RR rate. Remarkably, the inhibition of the competing HER from water reduction with increasing disk rotation rate is responsible for the enhanced CO2RR selectivity. This can be explained by the observation that, on the Au electrode, water reduction improves with more alkaline pH. As a result, the decrease in the local alkalinity near the electrode surface with enhanced mass transport suppresses HER due to the water reduction. Our study shows that controlling the local pH by mass transport conditions can tune the HER rate, in turn regulating the CO2RR and HER competition in the general operating potential window for CO2RR (-0.4 to -1 V vs RHE). Show less
Hydrogen production from seawater electrolysis is highly promising for the capture and storage of intermittent renewable energy, but is hindered by the possibility of unwanted reactions at the... Show moreHydrogen production from seawater electrolysis is highly promising for the capture and storage of intermittent renewable energy, but is hindered by the possibility of unwanted reactions at the anode. The oxidation reactions of chloride and (to a lesser extent) bromide, which can occur in parallel to the evolution of oxygen, lead to environmentally harmful by-products and thus represent undesirable side-reactions. We present some general considerations of solution chemistry and oxidation products that may be expected in a mixed acidic bromide/chloride electrolyte. We performed electrochemical model studies of the simultaneous oxidation of bromide and chloride and their mutual interaction on a Pt electrocatalyst, with the aim of deepening the general understanding of the anodic competition problem. Using simplified model systems, our findings suggest that the oxidation of bromide is hindered by competing chloride adsorption, in a way that can be quite satisfactorily modelled by a simple Langmuir isotherm describing the competing adsorption and reactivity of all species. The oxidation of chloride was however not properly captured by this same model, and may be substantially different. Furthermore, the formation of the interhalogen compound BrCl seems to occur in-between the oxidation of bromide and chloride. Show less
Chen, X.; Granda Marulanda, L.P.; McCrum, I.T.; Koper, M.T.M. 2020
Specific adsorption of anions is an important aspect in surface electrochemistry for its influence on reaction kinetics in either a promoted or inhibited fashion. Perchloric acid is typically... Show moreSpecific adsorption of anions is an important aspect in surface electrochemistry for its influence on reaction kinetics in either a promoted or inhibited fashion. Perchloric acid is typically considered as an ideal electrolyte for investigating electrocatalytic reactions due to the lack of specific adsorption of the perchlorate anion on several metal electrodes. In this work, cyclic voltammetry and computational methods are combined to investigate the interfacial processes on a Pd monolayer deposited on Pt(111) single crystal electrode in perchloric acid solution. The "hydrogen region" of this PdMLPt(111) surface exhibits two voltammetric peaks: the first "hydrogen peak" at 0.246 V-RHE actually involves the replacement of hydrogen by hydroxyl, and the second "hydrogen peak" H-II at 0.306 V-RHE appears to be the replacement of adsorbed hydroxyl by specific perchlorate adsorption. The two peaks merge into a single peak when a more strongly adsorbed anion, such as sulfate, is involved. Our density functional theory calculations qualitatively support the peak assignment and show that anions generally bind more strongly to the PdMLPt(111) surface than to Pt(111). Show less
Observation of CO formed during electrochemical CO2 reduction using Differential Electrochemical Mass Spectrometry (DEMS) is complicated by the fragmentation of CO2 in the course of the ionization... Show moreObservation of CO formed during electrochemical CO2 reduction using Differential Electrochemical Mass Spectrometry (DEMS) is complicated by the fragmentation of CO2 in the course of the ionization process. Since much more CO2 than CO enters the vacuum of the mass spectrometer, the ion current for mass 28 is dominated by the CO+-fragment of CO2. By reducing the cathode potential of the ion source of the mass spectrometer from-70 V to-27.5 V, fragmentation of CO2 is reduced to a negligible degree. This allows direct observation of electrochemically formed CO by measuring the ionic current for mass 28. We show that this method is superior to matrix calibration in which the ionic current for mass 44 corrected by the CO+/CO2+-intensity ratio is subtracted from the ionic current for mass 28. Using this method, we compare DEMS results for the electrochemical reduction of CO2 at gold electrodes obtained in two different cells, a conventional DEMS cell with the working electrode sputtered onto the membrane in contact with the vacuum and a flow cell where the interface to the vacuum is separated from the working electrode. We show that in the conventional cell at the interface between electrolyte and vacuum, the local CO2 concentration is reduced as the nearby vacuum interferes with the equilibria of reactions involving gases, and the local pH is increased. Therefore, in DEMS cells where the working electrode is positioned in the vicinity of the interface, the onset potential for CO2 reduction and hydrogen evolution are shifted and the observed faradaic efficiency for CO2 reduction are considerably reduced compared to literature values. This can be rectified by using flow cells that allow a spatial separation between vacuum/electrolyte interface and working electrode. We describe how the Dual Thin Layer Cell can be calibrated for detecting CO, thus allowing quantification of evolved amounts of CO from the ionic current for mass 28. (C) 2020 The Authors. Published by Elsevier B.V. Show less
We report, for the first time, the observation of a Gouy–Chapman capacitance minimum at the potential of zero charge of the Pt(111)‐aqueous perchlorate electrolyte interface. The potential of zero... Show moreWe report, for the first time, the observation of a Gouy–Chapman capacitance minimum at the potential of zero charge of the Pt(111)‐aqueous perchlorate electrolyte interface. The potential of zero charge of 0.3 V vs. NHE agrees very well with earlier values obtained by different methods. The observation of the potential of zero charge of this interface requires a specific pH (pH 4) and anomalously low electrolyte concentrations (<10−3 m). By comparison to gold and mercury double‐layer data, we conclude that the diffuse double layer structure at the Pt(111)‐electrolyte interface deviates significantly from the Gouy–Chapman theory in the sense that the electrostatic screening is much better than predicted by purely electrostatic mean‐field Poisson–Boltzmann theory. Show less
Electrode degradation under oxidizing conditions is a major drawback for large-scale applications of platinum electrocatalysts. Subjecting Pt(111) to oxidation-reduction cycles is known to lead to... Show moreElectrode degradation under oxidizing conditions is a major drawback for large-scale applications of platinum electrocatalysts. Subjecting Pt(111) to oxidation-reduction cycles is known to lead to the growth of nanoislands. We study this phenomenon using a combination of simultaneous in situ electrochemical scanning tunneling microscopy and cyclic voltammetry. Here, we present a detailed analysis of the formed islands, deriving the (evolution of the) average island growth shape. From the island shapes, we determine the densities of atomic-scale defect sites, e.g., steps and facets, which show an excellent correlation with the different voltammetric hydrogen adsorption peaks. Based on this combination of electrochemical scanning tunneling microscopy (EC-STM) and CV data, we derive a detailed atomistic picture of the nanoisland evolution during potential cycling, delivering new insights into the initial stages of platinum electrode degradation. Show less
Probing pH gradients during electrochemical reactions is important to better understand reaction mechanisms and to separate the influence of pH and pH gradients from intrinsic electrolyte effects.... Show moreProbing pH gradients during electrochemical reactions is important to better understand reaction mechanisms and to separate the influence of pH and pH gradients from intrinsic electrolyte effects. Here, we develop a pH sensor to measure pH changes in the diffusion layer during hydrogen evolution. The probe was synthesized by functionalizing a gold ultramicroelectrode with a self-assembled monolayer of 4-nitrothiophenol (4-NTP) and further converting it to form a hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) redox couple. The pH sensing is realized by recording the tip cyclic voltammetry and monitoring the Nernstian shift of the midpeak potential. We employ a capacitive approach technique in our home-built Scanning Electrochemical Microscope (SECM) setup in which an AC potential is applied to the sample and the capacitive current generated at the tip is recorded as a function of distance. This method allows for an approach of the tip to the electrode that is electrolyte-free and consequently also mediator-free. Hydrogen evolution on gold in a neutral electrolyte was studied as a model system. The pH was measured with the probe at a constant distance from the electrode (ca. 75 μm), while the electrode potential was varied in time. In the nonbuffered electrolyte used (0.1 M Li2SO4), even at relatively low current densities, a pH difference of three units is measured between the location of the probe and the bulk electrolyte. The time scale of the diffusion layer transient is captured, due to the high time resolution that can be achieved with this probe. The sensor has high sensitivity, measuring differences of more than 8 pH units with a resolution better than 0.1 pH unit. Show less
The development of energy conversion systems depends strongly on our fundamental understanding of the electrochemical interface of the electrocatalyst. Here, we study the changes in the surface... Show moreThe development of energy conversion systems depends strongly on our fundamental understanding of the electrochemical interface of the electrocatalyst. Here, we study the changes in the surface morphology of a platinum polycrystalline electrode during oxidation-reduction cycles in a wide potential window (0.05-2.0 V) in sodium hydroxide by in situ atomic force microscopy. Platinum nanoparticles are observed on the surface after cycling due to the redeposition of dissolved platinum ions. The influence of scan rate on platinum redeposition is studied by applying asymmetric potential sweep programs. The negative-going scan rate is the key factor here, as it controls the reduction of platinum oxide, as well as the redeposition of platinum. Moreover, it is found that chloride ions in the electrolyte impede the redeposition by complexing with platinum. This investigation enables us to reveal the surface roughening processes on platinum electrode in alkaline electrolyte and assists in understanding the fundamentals of the stability of platinum-containing energy conversion systems. Show less
