In this work we show how the contamination of gold electrodes with alumina particles by electrode polishing leads to an enhancement in activity for hydrogen evolution (HER). Polishing is one of the... Show moreIn this work we show how the contamination of gold electrodes with alumina particles by electrode polishing leads to an enhancement in activity for hydrogen evolution (HER). Polishing is one of the most used electrode treatments, however, we show that particles from the polishing media cannot be easily removed from the electrode surface by standard cleaning procedures. Comparing the HER activity of gold disc electrodes polished with either diamond suspension or alumina paste, we show that the latter leads to higher current densities, specifically for the water reduction to hydrogen. A similar enhancement in HER activity was observed by the addition of Al3+ cations to the electrolyte, demonstrating that the particles are not catalytically active, but that the Al3+ species released in solution due to corrosion promote the water reduction reaction. Due to an increase in the local OH− concentration during HER, the contaminating Al3+ cations precipitate and may deposit at the electrode surface as Al(OH)3. In the presence of a high enough Al3+ concentration, layered Al(OH)3 plates cover the whole electrode surface. The plates are composed of Al(OH)3 sheets intercalated by sulphate anions. Surprisingly, the Al(OH)3 sheets do not affect the gold blank voltammetry, and therefore remain undetected by simple electrochemical characterization methods. Show less
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... Show moreIn 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. Show less
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... Show moreElectrode 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. Show less
Use of a rotating ring-disk electrode during gas-evolving reactions has been shown liable to errors under higher current densities, since product collection on the ring is vulnerable to the... Show moreUse of a rotating ring-disk electrode during gas-evolving reactions has been shown liable to errors under higher current densities, since product collection on the ring is vulnerable to the formation of gas bubbles at the disk-ring interspace. In this study, we explored methods of reducing such bubble-related errors and improving the reliability of the collection factor under high-intensity gas evolution. We attempted the mounting of a thin wire close to the surface, to dislodge bubbles that formed specifically on the interface between the disk and the disk-ring spacer. This approach was tested for the detection of chlorine during parallel chlorine and oxygen evolution, and resulted in a notable alteration of the collection efficiency; its value became lower than theoretical expectations and also quite stable, even under higher current densities. We also coated the RRDE tip in a hydrophilic polymer, to reduce the tendency of bubble formation; this was tested for the collection of hydrogen and oxygen gas, and led to a mild increase in overall performance. The coating allowed for approximately 50% higher hydrogen evolution current density without ring failure, and for oxygen collection led to an overall improvement in behaviour. Show less
The electrochemical chlorine evolution reaction (CER) and oxygen evolution reaction (OER) represent core processes in the production of chlorine, relevant to bulk chemical manufacturing, and water... Show moreThe electrochemical chlorine evolution reaction (CER) and oxygen evolution reaction (OER) represent core processes in the production of chlorine, relevant to bulk chemical manufacturing, and water splitting, the most promising technology for renewable energy storage. Unfortunately, because of an apparent coupling between their key binding intermediates, the two reactions can easily occur simultaneously, which is never an attractive outcome. In this work, using a series of iridium-based double perovskites and rotating ring-disk voltammetry to deconvolute parallel OER and CER currents, we explored the interdependence of CER and OER in dilute acidic chloride solutions of up to 120 mM, where both reactions may occur in parallel with similar current densities. We also employed online inductively coupled plasma-mass spectrometry (ICP-MS) measurements to probe the material stability and its dependence on chloride concentration. For all studied materials, we found a strong linear correlation between CER and OER activity as well as a comparable selectivity, strengthening the suggestion that OER and CER follow a scaling relationship. It was also found that chloride selectively enhances the dissolution of the noble metal component. A reaction order analysis was performed to gain insight into the CER mechanism, the effect of surface area changes due to adventitious leaching, and the observed suppressing effect of chloride on OER. Show less
Granda Marulanda, L.P.; Builes, S.; Koper, M.T.M.; Calle-Vallejo, F. 2019
Solvation can significantly modify the adsorption energy of species at surfaces, thereby influencing the performance of electrocatalysts and liquid‐phase catalysts. Thus, it is important to... Show moreSolvation can significantly modify the adsorption energy of species at surfaces, thereby influencing the performance of electrocatalysts and liquid‐phase catalysts. Thus, it is important to understand adsorbate solvation at the nanoscale. Here we evaluate the effect of van der Waals (vdW) interactions described by different approaches on the solvation energy of *OH adsorbed on near‐surface alloys (NSAs) of Pt. Our results show that the studied functionals can be divided into two groups, each with rather similar average *OH solvation energies: (1) PBE and PW91; and (2) vdW functionals, RPBE, PBE‐D3 and RPBE‐D3. On average, *OH solvation energies are less negative by ∼0.14 eV in group (2) compared to (1), and the values for a given alloy can be extrapolated from one functional to another within the same group. Depending on the desired level of accuracy, these concrete observations and our tabulated values can be used to rapidly incorporate solvation into models for electrocatalysis and liquid‐phase catalysis. Show less
This paper studies the electrochemical hydrogenation of the carbonyl functional group of acetophenone and 4-acetylpyridine at platinum single-crystal electrodes. Comparison with results obtained... Show moreThis paper studies the electrochemical hydrogenation of the carbonyl functional group of acetophenone and 4-acetylpyridine at platinum single-crystal electrodes. Comparison with results obtained for the hydrogenation of acetone featuring an isolated carbonyl functional group reveals the influence of the phenyl ring and the pyridine ring, respectively. Lack of acetone adsorption at Pt(111) and Pt(100) due to a weak interaction between surface and carbonyl functional group renders these surfaces inactive for the hydrogenation of acetone. Adsorption through a strong interaction with the phenyl ring of acetophenone activates the Pt(111) and Pt(100) surfaces for hydrogenation of the acetyl substituent. In agreement with previous results for acetone reduction, the Pt(100) surface is specifically active for the hydrogenolysis reaction, breaking the C–O bond, whereas the other surfaces only hydrogenate the carbonyl functionality. In contrast to the phenyl ring, the pyridine ring has a very different effect: due to the dominant interaction of the N atom of the pyridine ring with the platinum electrode, a vertical adsorption mode is realized. The resulting large physical distance between the carbonyl functional group and the electrode surface inhibits the hydrogenation at all platinum surfaces. This also holds for the Pt(110) electrode, which is otherwise active for the electrochemical hydrogenation of the isolated carbonyl functional group of aliphatic ketones. Our results show how the combination of molecular structure of the reactant and surface structure of the catalyst determine the selective electroreduction of functionalized ketones. Show less
In this work, we have studied tin electrodeposition on polycrystalline gold electrodes from two different supporting electrolytes: sulfuric acid (SA) and methanesulfonic acid (MSA), both of them... Show moreIn this work, we have studied tin electrodeposition on polycrystalline gold electrodes from two different supporting electrolytes: sulfuric acid (SA) and methanesulfonic acid (MSA), both of them commonly used in the industry. This work aims to understand the effect of the different electrolyte anions on the deposition process. We show at least three different tin deposition mechanisms on gold: irreversible adsorption, underpotential deposition, and overpotential (bulk) deposition. Underpotential deposition leads to the formation of a layer of tin in SA and MSA with a coverage around θSn(H2SO4)=0.45θSn(H2SO4)=0.45 ML (monolayer) and θSn(CH3SO3H)=0.42θSn(CH3SO3H)=0.42 ML, respectively. The UPD Sn layer is however somewhat uncharacteristic as it is associated with island formation and surface alloying. Cyclic voltammograms in an extended potential range showed five distinct peaks: two cathodic peaks associated with tin underpotential and overpotential deposition, and three main anodic peaks, corresponding to the oxidation of the bulk Sn, of the AuSn intermetallic layer, and of the adsorbed Sn(II) to Sn(IV). Both voltammetric and rotating disk electrode measurements show that the kinetics of tin electrodeposition in MSA is slower than in SA, which we ascribe to Sn-MSA complex formation in solution. Slow Sn deposition in MSA promotes AuSn formation, in contrast to SA in which bulk tin deposition is more prominent. Complete Levich-type mass transport control of tin deposition in SA and MSA was only reached at low scan rate due to concurrent HER on the uncovered gold surface during the deposition process at higher scan rates. An unexpected surface-confined passivation process is observed in both electrolytes. Show less
Bondü, C.J.; Calle Vallejo, F.; Costa Figueiredo, M.; Koper, M.T.M. 2019
Due to a general feedstock shift, the chemical industry is charged with the task of finding ways to transform renewable ketones into value-added products. A viable route to do so is the... Show moreDue to a general feedstock shift, the chemical industry is charged with the task of finding ways to transform renewable ketones into value-added products. A viable route to do so is the electrochemical hydrogenation of the carbonyl functional group. Here we report a study on acetone reduction at platinum single-crystal electrodes using online electrochemical mass spectroscopy, in situ Fourier transform infrared spectroscopy and density functional theory calculations. Acetone reduction at platinum displays a remarkable structural sensitivity: not only the activity, but also the product distribution depends on the surface crystallographic orientation. At Pt(111) neither adsorption nor hydrogenation occur. A decomposition reaction that deactivates the electrode happens at Pt(100). Acetone reduction proceeds at the (110) steps: Pt[(n – 1)(111) × (110)] electrodes produce 2-propanol and Pt[(n + 1)(100) × (110)] electrodes produce propane. Using density functional theory calculations, we built a selectivity map to explain the intricacies of the acetone reduction on platinum. Finally, we extend our conclusions to the reduction of higher aliphatic ketones. Show less
Govindarajan, N.; Koper, M.T.M.; Meijer, E.J.; Calle-Vallejo, F. 2019
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... Show moreElectrode 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. Show less
