Publications by authors named "Marc T M Koper"

153 Publications

Double-layer structure of the Pt(111)-aqueous electrolyte interface.

Proc Natl Acad Sci U S A 2022 Jan;119(3)

Leiden Institute of Chemistry, Leiden University, 2300 RA Leiden, The Netherlands

We present detailed measurements of the double-layer capacitance of the Pt(111)-electrolyte interface close to the potential of zero charge (PZC) in the presence of several different electrolytes consisting of anions and cations that are considered to be nonspecifically adsorbed. For low electrolyte concentrations, we show strong deviations from traditional Gouy-Chapman-Stern (GCS) behavior that appear to be independent of the nature of the electrolyte ions. Focusing on the capacitance further away from PZC and the trends for increasing ion concentration, we observe ion-specific capacitance effects that appear to be related to the size or hydration strength of the ions. We formulate a model for the structure of the electric double layer of the Pt(111)-electrolyte interface that goes significantly beyond the GCS theory. By combining two existing models, namely, one capturing the water reorganization on Pt close to the PZC and one accounting for an attractive ion-surface interaction not included in the GCS model, we can reproduce and interpret the main features the experimental capacitance of the Pt(111)-electrolyte interface. The model suggests a picture of the double layer with an increased ion concentration close to the interface as a consequence of a weak attractive ion-surface interaction, and a changing polarizability of the Pt(111)-water interface due to the potential-dependent water adsorption and orientation.
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http://dx.doi.org/10.1073/pnas.2116016119DOI Listing
January 2022

Predoped Oxygenated Defects Activate Nitrogen-Doped Graphene for the Oxygen Reduction Reaction.

ACS Catal 2022 Jan 14;12(1):173-182. Epub 2021 Dec 14.

Leiden Institute of Chemistry, Leiden University, 2333CC Leiden, The Netherlands.

The presence of defects and chemical dopants in metal-free carbon materials plays an important role in the electrocatalysis of the oxygen reduction reaction (ORR). The precise control and design of defects and dopants in carbon electrodes will allow the fundamental understanding of activity-structure correlations for tailoring catalytic performance of carbon-based, most particularly graphene-based, electrode materials. Herein, we adopted monolayer graphene - a model carbon-based electrode - for systematical introduction of nitrogen and oxygen dopants, together with vacancy defects, and studied their roles in catalyzing ORR. Compared to pristine graphene, nitrogen doping exhibited a limited effect on ORR activity. In contrast, nitrogen doping in graphene predoped with vacancy defects or oxygen enhanced the activities at 0.4 V vs the reversible hydrogen electrode (RHE) by 1.2 and 2.0 times, respectively. The optimal activity was achieved for nitrogen doping in graphene functionalized with oxygenated defects, 12.8 times more than nitrogen-doped and 7.7 times more than pristine graphene. More importantly, oxygenated defects are highly related to the 4e pathway instead of nitrogen dopants. This work indicates a non-negligible contribution of oxygen and especially oxygenated vacancy defects for the catalytic activity of nitrogen-doped graphene.
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http://dx.doi.org/10.1021/acscatal.1c03662DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8749962PMC
January 2022

How palladium inhibits CO poisoning during electrocatalytic formic acid oxidation and carbon dioxide reduction.

Nat Commun 2022 Jan 10;13(1):38. Epub 2022 Jan 10.

Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300, RA, Leiden, the Netherlands.

Development of reversible and stable catalysts for the electrochemical reduction of CO is of great interest. Here, we elucidate the atomistic details of how a palladium electrocatalyst inhibits CO poisoning during both formic acid oxidation to carbon dioxide and carbon dioxide reduction to formic acid. We compare results obtained with a platinum single-crystal electrode modified with and without a single monolayer of palladium. We combine (high-scan-rate) cyclic voltammetry with density functional theory to explain the absence of CO poisoning on the palladium-modified electrode. We show how the high formate coverage on the palladium-modified electrode protects the surface from poisoning during formic acid oxidation, and how the adsorption of CO precursor dictates the delayed poisoning during CO reduction. The nature of the hydrogen adsorbed on the palladium-modified electrode is considerably different from platinum, supporting a model to explain the reversibility of this reaction. Our results help in designing catalysts for which CO poisoning needs to be avoided.
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http://dx.doi.org/10.1038/s41467-021-27793-5DOI Listing
January 2022

Probing the local activity of CO reduction on gold gas diffusion electrodes: effect of the catalyst loading and CO pressure.

Chem Sci 2021 Dec 9;12(47):15682-15690. Epub 2021 Nov 9.

Analytical Chemistry-Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr-Universität Bochum Universitätsstr. 150 D-44780 Bochum Germany

Large scale CO electrolysis can be achieved using gas diffusion electrodes (GDEs), and is an essential step towards broader implementation of carbon capture and utilization strategies. Different variables are known to affect the performance of GDEs. Especially regarding the catalyst loading, there are diverging trends reported in terms of activity and selectivity, for CO reduction to CO. We have used shear-force based Au nanoelectrode positioning and scanning electrochemical microscopy (SECM) in the surface-generation tip collection mode to evaluate the activity of Au GDEs for CO reduction as a function of catalyst loading and CO back pressure. Using a Au nanoelectrode, we have locally measured the amount of CO produced along a catalyst loading gradient under conditions. We observed that an optimum local loading of catalyst is necessary to achieve high activities. However, this optimum is directly dependent on the CO back pressure. Our work does not only present a tool to evaluate the activity of GDEs locally, it also allows drawing a more precise picture regarding the effect of catalyst loading and CO back pressure on their performance.
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http://dx.doi.org/10.1039/d1sc05519dDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8654039PMC
December 2021

The Role of Cation Acidity on the Competition between Hydrogen Evolution and CO Reduction on Gold Electrodes.

J Am Chem Soc 2021 Dec 28. Epub 2021 Dec 28.

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

CO electroreduction (CORR) is a sustainable alternative for producing fuels and chemicals. Metal cations in the electrolyte have a strong impact on the reaction, but mainly alkali species have been studied in detail. In this work, we elucidate how multivalent cations (Li, Cs, Be, Mg, Ca, Ba, Al, Nd, and Ce) affect CORR and the competing hydrogen evolution by studying these reactions on polycrystalline gold at pH = 3. We observe that cations have no effect on proton reduction at low overpotentials, but at alkaline surface pH acidic cations undergo hydrolysis, generating a second proton reduction regime. The activity and onset for the water reduction reaction correlate with cation acidity, with weakly hydrated trivalent species leading to the highest activity. Acidic cations only favor CORR at low overpotentials and in acidic media. At high overpotentials, the activity for CO increases in the order Ca < Li < Ba < Cs. To favor this reaction there must be an interplay between cation stabilization of the *CO intermediate, cation accumulation at the outer Helmholtz plane (OHP), and activity for water reduction. molecular dynamics simulations with explicit electric field show that nonacidic cations show lower repulsion at the interface, accumulating more at the OHP, thus triggering local promoting effects. Water dissociation kinetics is increasingly promoted by strongly acidic cations (Nd, Al), in agreement with experimental evidence. Cs, Ba, and Nd coordinate to adsorbed CO steadily; thus they enable *CO stabilization and barrierless protonation to COOH and further reduction products.
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http://dx.doi.org/10.1021/jacs.1c10171DOI Listing
December 2021

Electrolyte buffering species as oxygen donor shuttles in CO electrooxidation.

Phys Chem Chem Phys 2021 Dec 15. Epub 2021 Dec 15.

Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands.

Electrolyte buffering species have been shown to act as proton donors in the hydrogen evolution reaction (HER). Analogously, we study here whether these electrolyte species may participate in other reactions by investigating CO electrooxidation (COOR) on a gold rotating disk electrode. This model system, characterized by fast kinetics, exhibits a diffusion-limited regime, which helps in the identification of the species dictating the diffusion-limited current. Through a systematic concentration dependence study in a variety of buffers, we show that electrolyte buffering species act as oxygen donor shuttles in COOR, lowering the reaction overpotential. A similar correlation between electrolyte and electrocatalytic activity was observed for COOR on a different electrode material (Pt). Probing the electrode-electrolyte interface by attenuated total reflection infrared spectroscopy (ATR-FTIR) and modelling the surface speciation to include the effect of the solution reactions, we propose that the buffer conjugated base generates the oxygen donor ( OH) through its acid-base reaction with water.
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http://dx.doi.org/10.1039/d1cp05030cDOI Listing
December 2021

Understanding Cation Trends for Hydrogen Evolution on Platinum and Gold Electrodes in Alkaline Media.

ACS Catal 2021 Dec 12;11(23):14328-14335. Epub 2021 Nov 12.

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

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 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.
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http://dx.doi.org/10.1021/acscatal.1c04268DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8650008PMC
December 2021

Time-Resolved Local pH Measurements during CO Reduction Using Scanning Electrochemical Microscopy: Buffering and Tip Effects.

JACS Au 2021 Nov 13;1(11):1915-1924. Epub 2021 Oct 13.

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

The electrochemical reduction of CO is widely studied as a sustainable alternative for the production of fuels and chemicals. The electrolyte's bulk pH and composition play an important role in the reaction activity and selectivity and can affect the extent of the buildup of pH gradients between the electrode surface and the bulk of the electrolyte. Quantifying the local pH and how it is affected by the solution species is desirable to gain a better understanding of the CO reduction reaction. Local pH measurements can be realized using Scanning Electrochemical Microscopy (SECM); however, finding a pH probe that is stable and selective under CO reduction reaction conditions is challenging. Here, we have used our recently developed voltammetric pH sensor to perform pH measurements in the diffusion layer during CO reduction using SECM, with high time resolution. Using a 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) functionalized gold ultramicroelectrode, we compare the local pH developed above a gold substrate in an argon atmosphere, when only hydrogen evolution is taking place, to the pH developed in a CO atmosphere. The pH is monitored at a fixed distance from the surface, and the sample potential is varied in time. In argon, we observe a gradual increase of pH, while a plateau region is present in CO atmosphere due to the formation of HCO buffering the reaction interface. By analyzing the diffusion layer dynamics once the sample reaction is turned "off", we gain insightful information on the time scale of the homogeneous reactions happening in solution and on the time required for the diffusion layer to fully recover to the initial bulk concentration of species. In order to account for the effect of the presence of the SECM tip on the measured pH, we performed finite element method simulations of the fluid and reaction dynamics. The results show the significant localized diffusion hindrance caused by the tip, so that in its absence, the pH values are more acidic than when the tip is present. Nonetheless, through the simulation, we can account for this effect and estimate the real local pH values across the diffusion layer.
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http://dx.doi.org/10.1021/jacsau.1c00289DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8611793PMC
November 2021

Understanding the role of mass transport in tuning the hydrogen evolution kinetics on gold in alkaline media.

J Chem Phys 2021 Oct;155(13):134705

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

In this work, we present an in-depth study of the role of mass transport conditions in tuning the hydrogen evolution kinetics on gold by means of rotation rate control. Interestingly, we find that the hydrogen evolution reaction (HER) activity decreases with the increasing rotation rate of the electrode. As we increase the rotation (mass transport) rate, the locally generated hydroxyl ions (2HO +2e → H + 2OH) are transported away from the electrode surface at an accelerated rate. This results in decreasing local pH and, because of the need to satisfy local electroneutrality, decreasing near-surface cation concentration. This decrease in the near-surface cation concentration results in the suppression of HER. This is because the cations near the surface play a central role in stabilizing the transition state for the rate determining Volmer step (*H-OH-cat). Furthermore, we present a detailed analytical model that qualitatively captures the observed mass transport dependence of HER solely based on the principle of electroneutrality. Finally, we also correlate the cation identity dependence of HER on gold (Li < Na < K) to the changes in the effective concentration of the cations in the double layer with the changes in their solvation energy.
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http://dx.doi.org/10.1063/5.0064330DOI Listing
October 2021

Morphological Stability of Copper Surfaces under Reducing Conditions.

ACS Appl Mater Interfaces 2021 Oct 6;13(41):48730-48744. Epub 2021 Oct 6.

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

Though copper is a capable electrocatalyst for the CO reduction reaction (CO2RR), it rapidly deactivates to produce mostly hydrogen. A current hypothesis as to why this occurs is that potential-induced morphological restructuring takes place, leading to a redistribution of the facets at the interface resulting in a shift in the catalytic activity to favor the hydrogen evolution reaction over CO2RR. Here, we investigate the veracity of this hypothesis by studying the changes in the voltammetry of various copper surfaces, specifically the three principal orientations and a polycrystalline surface, after being subjected to strongly cathodic conditions. The basal planes were chosen as model catalysts, while polycrystalline copper was included as a means of investigating the overall behavior of defect-rich facets with many low coordination steps and kink sites. We found that all surfaces exhibited (perhaps surprisingly) high stability when subjected to strongly cathodic potentials in a concentrated alkaline electrolyte (10 M NaOH). Proof for morphological stability under CO2RR-representative conditions (60 min at -0.75 V in 0.5 M KHCO) was obtained from identical location scanning electron microscopy, where the mesoscopic morphology for a nanoparticle-covered copper surface was found unchanged to within the instrument accuracy. Observed changes in voltammetry under such conditions, we found, were not indicative of a redistribution of surface sites but of electrode fouling. Besides impurities, we show that (brief) exposure to oxygen or oxidizing conditions (i.e., 1 min) leads to copper exhibiting changing morphology upon cathodic treatment which, we posit, is ultimately the reason why many groups report the evolution of copper morphology during CO2RR: accidental oxidation/reduction cycles.
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http://dx.doi.org/10.1021/acsami.1c13989DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8532114PMC
October 2021

Efficiency and selectivity of CO reduction to CO on gold gas diffusion electrodes in acidic media.

Nat Commun 2021 Aug 16;12(1):4943. Epub 2021 Aug 16.

Leiden Institute of Chemistry, Leiden University, Leiden, The Netherlands.

The electrochemical reduction of CO to CO is a promising technology for replacing production processes employing fossil fuels. Still, low energy efficiencies hinder the production of CO at commercial scale. CO electrolysis has mainly been performed in neutral or alkaline media, but recent fundamental work shows that high selectivities for CO can also be achieved in acidic media. Therefore, we investigate the feasibility of CO electrolysis at pH 2-4 at indrustrially relevant conditions, using 10 cm gold gas diffusion electrodes. Operating at current densities up to 200 mA cm, we obtain CO faradaic efficiencies between 80-90% in sulfate electrolyte, with a 30% improvement of the overall process energy efficiency, in comparison with neutral media. Additionally, we find that weakly hydrated cations are crucial for accomplishing high reaction rates and enabling CO electrolysis in acidic media. This study represents a step towards the application of acidic electrolyzers for CO electroreduction.
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http://dx.doi.org/10.1038/s41467-021-24936-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8368099PMC
August 2021

High-Pressure CO Electroreduction at Silver Produces Ethanol and Propanol.

Angew Chem Int Ed Engl 2021 Sep 25;60(40):21732-21736. Epub 2021 Aug 25.

Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300, RA, Leiden, The Netherlands.

Reducing CO to long-chain carbon products is attractive considering such products are typically more valuable than shorter ones. However, the best electrocatalyst for making such products from CO , copper, lacks selectivity. By studying alternate C producing catalysts we can increase our mechanistic understanding, which is beneficial for improving catalyst performance. Therefore, we investigate CO reduction on silver, as density functional theory (DFT) results predict it to be good at forming ethanol. To address the current disagreement between DFT and experimental results (ethanol vs. no ethanol), we investigated CO reduction at higher surface coverage (by increasing pressure) to ascertain if desorption effects can explain the discrepancy. In terms of product trends, our results agree with the DFT-proposed acetaldehyde-like intermediate, yielding ethanol and propanol as C products-making the CO electrochemistry of silver very similar to that of copper at sufficiently high coverage.
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http://dx.doi.org/10.1002/anie.202108902DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8518692PMC
September 2021

Electrolyte Effects on the Faradaic Efficiency of CO Reduction to CO on a Gold Electrode.

ACS Catal 2021 May 8;11(9):4936-4945. Epub 2021 Apr 8.

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

The electrochemical reduction of CO 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).
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http://dx.doi.org/10.1021/acscatal.1c00272DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8154322PMC
May 2021

The Importance of Acid-Base Equilibria in Bicarbonate Electrolytes for CO Electrochemical Reduction and CO Reoxidation Studied on Au() Electrodes.

Langmuir 2021 May 29;37(18):5707-5716. Epub 2021 Apr 29.

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

Among heterogeneous electrocatalysts, gold comes closest to the ideal reversible electrocatalysis of CO electrochemical reduction (CO2RR) to CO and, vice versa, of CO electroxidation to CO (COOR). The nature of the electrolyte has proven to crucially affect the electrocatalytic behavior of gold. Herein, we expand the understanding of the effect of the widely employed bicarbonate electrolytes on CO2RR using gold monocrystalline electrodes, detecting the CO evolved during CO2RR by selective anodic oxidation. First, we show that CO2RR to CO is facet dependent and that Au(110) is the most active surface. Additionally, we detect by FTIR measurements the presence of adsorbed CO only on the Au(110) surface. Second, we highlight the importance of acid-base equilibria for both CO2RR and COOR by varying the electrolyte (partial pressure of CO and the concentration of the bicarbonate) and voltammetric parameters. In this way, we identify different regimes of surface pH and bicarbonate speciation, as a function of the current and electrolyte conditions. We reveal the importance of the acid-base bicarbonate/carbonate couple, not only as a buffering equilibrium but also as species involved in the electrochemical reactions under study.
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http://dx.doi.org/10.1021/acs.langmuir.1c00703DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8154874PMC
May 2021

Dissociative Adsorption of Acetone on Platinum Single-Crystal Electrodes.

J Phys Chem C Nanomater Interfaces 2021 Apr 18;125(12):6643-6649. Epub 2021 Mar 18.

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

In this article, we investigate the poisoning reaction that occurs at platinum electrodes during the electrocatalytic hydrogenation of acetone. A better understanding of this poisoning reaction is important to develop electrocatalysts that are both active for the hydrogenation of carbonyl compounds and resilient against poisoning side reactions. We adsorb acetone to Pt(331), Pt(911), Pt(510), and Pt(533) (i.e., Pt[2(111) × (110)], Pt[5(100) × (111)], [5(100) × (110)], and Pt[4(111) × (100), respectively])) as well as Pt(100) single-crystal electrodes and perform reductive and oxidative stripping experiments after electrolyte exchange. We found that acetone adsorbs molecularly intact on all sites apart from Pt(100) terrace sites and can be stripped reductively from the electrode surface at a potential positive of hydrogen evolution. However, at Pt(100) terraces, acetone adsorbs dissociatively as carbon monoxide, which remains attached to the electrode surface and leads to its poisoning. Strikingly, dissociative adsorption does not occur on step sites with (100) geometry, which suggests that the dissociative adsorption of acetone is limited to Pt(100) terraces featuring a certain minimum "ensemble" number of freely available Pt atoms.
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http://dx.doi.org/10.1021/acs.jpcc.0c11360DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8042992PMC
April 2021

The Interrelated Effect of Cations and Electrolyte pH on the Hydrogen Evolution Reaction on Gold Electrodes in Alkaline Media.

Angew Chem Int Ed Engl 2021 Jun 7;60(24):13452-13462. Epub 2021 May 7.

Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300, RA, Leiden, The Netherlands.

In this work we study the role of alkali metal cation concentration and electrolyte pH in altering the kinetics of the hydrogen evolution reaction (HER) at gold (Au) electrodes. We show that at moderately alkaline pH (pH 11), increasing the cation concentration significantly enhances the HER activity on Au electrodes (with a reaction order ≈0.5). Based on these results we suggest that cations play a central role in stabilizing the transition state of the rate-determining Volmer step by favorably interacting with the dissociating water molecule (*H-OH -cat ). Moreover, we show that increasing electrolyte pH (pH 10 to pH 13) tunes the local field strength, which in turn indirectly enhances the activity of HER by tuning the near-surface cation concentration. Interestingly, a too high near-surface cation concentration (at high pH and high cation concentration) leads to a lowering of the HER activity, which we ascribe to a blockage of the surface by near-surface cations.
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http://dx.doi.org/10.1002/anie.202102803DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8252582PMC
June 2021

A simple method to calculate solution-phase free energies of charged species in computational electrocatalysis.

J Phys Condens Matter 2021 Apr 27;33(20). Epub 2021 Apr 27.

Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands.

Determining the adsorption potential of adsorbed ions in the field of computational electrocatalysis is of great interest to study their interaction with the electrode material and the solvent, and to map out surface phase diagrams and reaction pathways. Calculating the adsorption potentials of ions with density functional theory and comparing across various ions requires an accurate reference energy of the ion in solution and electrons at the same electrochemical scale. Here we highlight a previously used method for determining the reference free energy of solution phase ions using a simple electrochemical thermodynamic cycle, which allows this free energy to be calculated from that of a neutral gas-phase or solid species and an experimentally measured equilibrium potential, avoiding the need to model solvent around the solution phase ion in the electronic structure calculations. While this method is not new, we describe its use and utility in detail and show that this same method can be used to find the free energy of any ion from any reaction, as long as the half-cell equilibrium potential is known, even for reactions that do not transfer the same number of protons and electrons. To illustrate its usability, we compare the adsorption potentials obtained with DFT of I, Br, Cl, and SOon Pt(111) and Au(111) and OHand Agon Pt(111) with those measured experimentally and find that this simple and computationally affordable method reproduces the experimental trends.
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http://dx.doi.org/10.1088/1361-648X/abf19dDOI Listing
April 2021

Emergence of Potential-Controlled Cu-Nanocuboids and Graphene-Covered Cu-Nanocuboids under CO Electroreduction.

Nano Lett 2021 Mar 22;21(5):2059-2065. Epub 2021 Feb 22.

Max Planck-EPFL Laboratory for Molecular Nanoscience and Technology and IPHYS, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland.

The electroreduction of CO (CORR) is a promising strategy toward sustainable fuels. Cu is the only Earth-abundant and pure metal capable of catalyzing CO-to-hydrocarbons conversion with significant Faradaic efficiencies; yet, its dynamic structure under CORR conditions remains unknown. Here, we track the Cu structure by electrochemical scanning tunneling microscopy and Raman spectroscopy. Surprisingly, polycrystalline Cu surfaces reconstruct forming Cu nanocuboids whose size can be controlled by the polarization potential and the time employed in their synthesis, without the assistance of organic surfactants and/or halide anions. If the Cu surface is covered by a graphene monolayer, smaller features with enhanced catalytic activity for CORR can be prepared. The graphene-protecting layer softens the 3D morphological changes that Cu-based catalysts suffer when exposed to aggressive electrochemical environments and allows us to track the kinetic roughening process. This novel strategy is promising for improving Cu long-term stability, and consequently, it could be used as a platform to ultimately control product selectivity.
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http://dx.doi.org/10.1021/acs.nanolett.0c04703DOI Listing
March 2021

Direct and Broadband Plasmonic Charge Transfer to Enhance Water Oxidation on a Gold Electrode.

ACS Nano 2021 Feb 26;15(2):3188-3200. Epub 2021 Jan 26.

Leiden Institute of Chemistry, Leiden University, 2333 CD Leiden, The Netherlands.

Plasmonic photocatalysis hot charge carriers suffers from their short lifetime compared with the sluggish kinetics of most reactions. To increase lifetime, adsorbates on the surface of a plasmonic metal may create preferential states for electrons to be excited from. We demonstrate this effect with O adsorbates on a nanoporous gold electrode. Nanoporous gold is used to obtain a broadband optical response, to increase the obtained photocurrent, and to provide a SERS-active substrate. Only with adsorbates present, we observe significant photocurrents. Illumination also increases the adsorbate coverage above its dark potential-dependent equilibrium, as derived from a two-laser SERS approach. Density functional theory calculations confirm the appearance of excitable states below the Fermi level. The photocurrent enhancement and broadband characteristics reveal the potential of the plasmonic approach to improve the efficiency of photoelectrochemical water splitting.
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http://dx.doi.org/10.1021/acsnano.0c09776DOI Listing
February 2021

Suppression of Hydrogen Evolution in Acidic Electrolytes by Electrochemical CO Reduction.

J Am Chem Soc 2021 Jan 24;143(1):279-285. Epub 2020 Dec 24.

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

In this article we investigate the electrochemical reduction of CO at gold electrodes under mildly acidic conditions. Differential electrochemical mass spectroscopy (DEMS) is used to quantify the amounts of formed hydrogen and carbon monoxide as well as the consumed amount of CO. We investigate how the Faradaic efficiency of CO formation is affected by the CO partial pressure (0.1-0.5 bar) and the proton concentration (1-0.25 mM). Increasing the former enhances the rate of CO reduction and suppresses hydrogen evolution from proton reduction, leading to Faradaic efficiencies close to 100%. Hydrogen evolution is suppressed by CO reduction as all protons at the electrode surfaces are used to support the formation of water (CO + 2H + 2e → CO + HO). Under conditions of slow mass transport, this leaves no protons to support hydrogen evolution. On the basis of our results, we derive a general design principle for acid CO electrolyzers to suppress hydrogen evolution from proton reduction: the rate of CO/OH formation must be high enough to match/compensate the mass transfer of protons to the electrode surface.
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http://dx.doi.org/10.1021/jacs.0c10397DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7809687PMC
January 2021

Nanoscale morphological evolution of monocrystalline Pt surfaces during cathodic corrosion.

Proc Natl Acad Sci U S A 2020 12 7;117(51):32267-32277. Epub 2020 Dec 7.

Leiden Institute of Chemistry, Leiden University, 2300 RA Leiden, The Netherlands

This paper studies the cathodic corrosion of a spherical single crystal of platinum in an aqueous alkaline electrolyte, to map out the detailed facet dependence of the corrosion structures forming during this still largely unexplored electrochemical phenomenon. We find that anisotropic corrosion of the platinum electrode takes place in different stages. Initially, corrosion etch pits are formed, which reflect the local symmetry of the surface: square pits on (100) facets, triangular pits on (111) facets, and rectangular pits on (110) facets. We hypothesize that these etch pits are formed through a ternary metal hydride corrosion intermediate. In contrast to anodic corrosion, the (111) facet corrodes the fastest, and the (110) facet corrodes the slowest. For cathodic corrosion on the (100) facet and on higher-index surfaces close to the (100) plane, the etch pit destabilizes in a second growth stage, by etching faster in the (111) direction, leading to arms in the etch pit, yielding a concave octagon-shaped pit. In a third growth stage, these arms develop side arms, leading to a structure that strongly resembles a self-similar diffusion-limited growth pattern, with strongly preferred growth directions.
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http://dx.doi.org/10.1073/pnas.2017086117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7768681PMC
December 2020

Understanding the Voltammetry of Bulk CO Electrooxidation in Neutral Media through Combined SECM Measurements.

J Phys Chem Lett 2020 Nov 2;11(22):9708-9713. Epub 2020 Nov 2.

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

Recently, the bulk electrooxidation of CO on gold or platinum has been used to detect CO produced during CO reduction in neutral media. The CO bulk oxidation voltammetry may show two distinct peaks depending on the reaction conditions, which up to now have not been understood. We have used scanning electrochemical microscopy (SECM) to probe CO oxidation and pH in the diffusion layer during CO reduction. Our results show that the two different peaks are due to diffusion limitation by two different species, namely, CO and OH. We find that between pH 7 and 11, CO oxidation by water and OH gives rise to the first and second peak observed in the voltammetry, respectively. Additional rotating disc experiments showed that specifically in this pH range the current of the second peak is diffusion limited by the OH concentration, since it is lower than the CO concentration.
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http://dx.doi.org/10.1021/acs.jpclett.0c02779DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7681782PMC
November 2020

Competition between CO Reduction and Hydrogen Evolution on a Gold Electrode under Well-Defined Mass Transport Conditions.

J Am Chem Soc 2020 03 19;142(9):4154-4161. Epub 2020 Feb 19.

Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands.

Gold is one of the most selective catalysts for the electrochemical reduction of CO (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).
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http://dx.doi.org/10.1021/jacs.9b10061DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7059182PMC
March 2020

Adsorption processes on a Pd monolayer-modified Pt(111) electrode.

Chem Sci 2020 Jan 7;11(6):1703-1713. Epub 2020 Jan 7.

Leiden Institute of Chemistry, Leiden University PO Box 9502 Leiden 2300 RA The Netherlands

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 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 PdPt(111) surface exhibits two voltammetric peaks: the first "hydrogen peak" at 0.246 V actually involves the replacement of hydrogen by hydroxyl, and the second "hydrogen peak" H at 0.306 V 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 PdPt(111) surface than to Pt(111).
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http://dx.doi.org/10.1039/c9sc05307gDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8148025PMC
January 2020

Atomic-Scale Identification of the Electrochemical Roughening of Platinum.

ACS Cent Sci 2019 Dec 15;5(12):1920-1928. Epub 2019 Nov 15.

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

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 the growth of nanoislands. We study this phenomenon using a combination of simultaneous 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.
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http://dx.doi.org/10.1021/acscentsci.9b00782DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6935890PMC
December 2019

Mediator-Free SECM for Probing the Diffusion Layer pH with Functionalized Gold Ultramicroelectrodes.

Anal Chem 2020 01 8;92(2):2237-2243. Epub 2020 Jan 8.

Leiden Institute of Chemistry , Leiden University , P.O. Box 9502, 2300 RA , Leiden , The Netherlands.

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. 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 LiSO), 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.
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http://dx.doi.org/10.1021/acs.analchem.9b04952DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6977089PMC
January 2020

The dualism between adatom- and vacancy-based single crystal growth models.

Nat Commun 2019 11 20;10(1):5233. Epub 2019 Nov 20.

Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, 2333 CC, Leiden, The Netherlands.

In homoepitaxial crystal growth, four basic growth morphologies (idealized growth modes) have been established that describe the deposition of atoms on single crystal surfaces: step-flow, layer-by-layer, mound formation, and random/self-affine growth. Mound formation leads to nano-scale surface patterning. However, the formation of (nano)-islands, patterns, and roughness occurs also during ion bombardment, electrochemical etching and oxidation/reduction cycling. Here we show, in analogy to many particle/anti-particle formalisms in physics, the existence of the dualism between individual adatom and single vacancy growth modes. We predict that all standard adatom growth modes do exist also in their counter, vacancy version. For the particular case of mound formation, we derive the theoretical equations and show the inverse similarity of the solution. We furthermore treat simultaneous growth by adatoms and vacancies, and derive the analytical solution of the growth shape evolution of the mounds. Finally, we present an experimental verification, in which both adatom and vacancy mound formation are active. The theoretically predicted mound shape nicely fits the experimental observation.
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http://dx.doi.org/10.1038/s41467-019-13188-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6868172PMC
November 2019

Thermodynamics of the formation of surface PtO stripes on Pt(111) in the absence of subsurface oxygen.

Phys Chem Chem Phys 2020 May 8;22(19):10634-10640. Epub 2019 Nov 8.

Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

This paper examines the thermodynamics of PtO stripes formed as intermediates of Pt(111) surface oxidation as a function of the degree of dilation parallel to the stripes, using density functional theory and atomistic thermodynamics. Internal energy calculations predict 7/8 and 8/9 stripe structures to dominate at standard temperature and pressure, which contain 7 or 8 elevated PtO units per 8 or 9 supporting surface Pt atoms, respectively. Moreover, we found a thermodynamic optimum with respect to mean in-stripe Pt-Pt spacing close to that of α-PtO. Vibrational zero point energies, including bulk layer contributions, make a small but significant contribution to the stripe free energies, leading to the 6/7 stripe being most stable, although the 7/8 structure is still close in free energy. These findings correspond closely to experimental observations, providing insight into the driving force for oxide stripe formation and structure as the initial intermediate of platinum surface oxidation, and aiding our understanding of platinum catalysts and surface roughening under oxidative conditions.
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http://dx.doi.org/10.1039/c9cp05107dDOI Listing
May 2020

Double Layer at the Pt(111)-Aqueous Electrolyte Interface: Potential of Zero Charge and Anomalous Gouy-Chapman Screening.

Angew Chem Int Ed Engl 2020 Jan 26;59(2):711-715. Epub 2019 Nov 26.

Leiden Institute of Chemistry, Leiden University, 2300, RA, Leiden, The Netherlands.

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 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  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.
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http://dx.doi.org/10.1002/anie.201911929DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6973170PMC
January 2020

Hydrogen-Induced Step-Edge Roughening of Platinum Electrode Surfaces.

J Phys Chem Lett 2019 Nov 23;10(21):6842-6849. Epub 2019 Oct 23.

Leiden Institute of Chemistry , Leiden University , P.O. Box 9502, 2300 RA Leiden , The Netherlands.

Electrode surfaces may change their surface structure as a result of the adsorption of chemical species, impacting their catalytic activity. Using density functional theory, we find that the strong adsorption of hydrogen at low electrode potentials promotes the thermodynamics and kinetics of a unique type of roughening of 110-type Pt step edges. This change in surface structure causes the appearance of the so-called "third hydrogen peak" in voltammograms measured on Pt electrodes, an observation that has eluded explanation for over 50 years. Understanding this roughening process is important for structure-sensitive (electro)catalysis and the development of active and stable catalysts.
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http://dx.doi.org/10.1021/acs.jpclett.9b02544DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6844181PMC
November 2019
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