Angel Cuesta

Enhanced electrocatalysis of the oxygen reduction reaction based on patterning of platinum surfaces with cyanide

The slow rate of the oxygen reduction reaction in the phosphoric acid fuel cell is the main factor limiting its wide application. Here, we present an approach that can be used for the rational design of cathode catalysts with potential use in phosphoric acid fuel cells, or in any environments containing strongly adsorbing tetrahedral anions. This approach is based on molecular patterning of platinum surfaces with cyanide adsorbates that can efficiently block the sites for adsorption of spectator anions while the oxygen reduction reaction proceeds unhindered. We also demonstrate that, depending on the supporting electrolyte anions and cations, on the same CN-covered Pt(111) surface, the oxygen reduction reaction activities can range from a 25-fold increase to a 50-fold decrease. This behaviour is discussed in the light of the role of covalent and non-covalent interactions in controlling the ensemble of platinum active sites required for high turn over rates of the oxygen reduction reaction.

In-situ STM characterisation of the surface morphology of platinum single crystal electrodes as a function of their preparation

Abstract A systematic study of the surface morphology of Pt(111) and Pt(100) electrodes as prepared by flame-annealing and cooling in different atmospheres (air, N 2 , H 2 +N 2 and CO+N 2 ) is presented. The electrodes were characterised by cyclic voltammetry and in-situ STM in 0.1 M H 2 SO 4 . Preliminary voltammetric results for Pt(110) are also shown. In this case, Cu upd served as a structure sensitive probe. It was observed that the presence of oxygen during cooling induces surface defects and leads to rough surfaces. Cooling in pure N 2 preserves the reconstructed Pt(100) and Pt(110) surfaces, while cooling in H 2 +N 2 or CO+N 2 lifts the reconstruction. The use of CO as the cooling gas turned out to be advantageous for the preparation of clean and well-ordered (1×1)-surfaces. The stability of reconstructed Pt surfaces in an electrochemical environment is discussed. For H 2 +N 2 -cooled Pt electrodes, a clear influence of the H 2 -concentration on the surface morphology was observed.

The Role of Bridge‐Bonded Adsorbed Formate in the Electrocatalytic Oxidation of Formic Acid on Platinum

The oxidation of formic acid (HCOOH) on platinum electrodes has been extensively investigated as a model electrocatalytic reaction. It is generally accepted that HCOOH is oxidized to CO2 through a dual-pathway mechanism: one pathway (the main pathway) involves a fast reaction via a reactive intermediate and the second pathway includes a step in which a poisoning species is formed. This species, which is oxidized to CO2 at high potentials, has been identified as adsorbed CO, which is formed by dehydration of HCOOH. Adsorbed hydroxycarbonyl (COOHads) has long been assumed to be the reactive intermediate in the main pathway, but the spectroscopic detection of this species has not been reported to date. By using surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATRSEIRAS), Miki et al. observed that formate is adsorbed in a bridge-bonded configuration on Pt electrodes during HCOOH oxidation. On the basis of systematic time-resolved ATR-SEIRAS analysis of the oxidation dynamics, Samjesk et al. suggested that adsorbed formate (HCOOads) is a reactive intermediate in the main pathway and its decomposition to CO2 is the rate-determining step (rds). The adsorbed formate is in equilibrium with HCOOH in the bulk solution and the reaction pathway (formate pathway) can be represented by Equation (1)

Importance of Acid–Base Equilibrium in Electrocatalytic Oxidation of Formic Acid on Platinum

Electro-oxidation of formic acid on Pt in acid is one of the most fundamental model reactions in electrocatalysis. However, its reaction mechanism is still a matter of strong debate. Two different mechanisms, bridge-bonded adsorbed formate mechanism and direct HCOOH oxidation mechanism, have been proposed by assuming a priori that formic acid is the major reactant. Through systematic examination of the reaction over a wide pH range (0-12) by cyclic voltammetry and surface-enhanced infrared spectroscopy, we show that the formate ion is the major reactant over the whole pH range examined, even in strong acid. The performance of the reaction is maximal at a pH close to the pKa of formic acid. The experimental results are reasonably explained by a new mechanism in which formate ion is directly oxidized via a weakly adsorbed formate precursor. The reaction serves as a generic example illustrating the importance of pH variation in catalytic proton-coupled electron-transfer reactions.

Cyclic Voltammetry, FTIRS, and DEMS Study of the Electrooxidation of Carbon Monoxide, Formic Acid, and Methanol on Cyanide-Modified Pt(111) Electrodes

We have used cyanide-modified Pt(111) electrodes, in combination with cyclic voltammetry (CV), Fourier transform infrared spectroscopy (FTIRS), and differential electrochemical mass spectrometry (DEMS), to investigate the oxidation of formic acid and methanol on Pt electrodes. Since CO is the poison intermediate formed during the oxidation of both formic acid and methanol, we have previously characterized the CO adlayer on cyanide-modified Pt(111) electrodes. Poison formation on cyanide-modified Pt(111) is nearly completely inhibited in the case of formic acid and methanol, the corresponding electro-oxidation reaction proceeding, hence, exclusively through the reactive intermediate pathway. These results suggest that, in the oxidation of formic acid and methanol, the formation of adsorbed CO would require the presence of at least three contiguous Pt atoms.

The adsorption of sulfate and phosphate on Au(111) and Au(100) electrodes: an in situ STM study

We have studied by in situ STM the adsorption of sulfate and phosphate species on Au(111) and Au(100) single crystal electrodes in acid (0.1 M H2SO4 and 0.1 M H3PO4) and neutral (0.1 M Na2SO4 and 0.1 M KH2PO4 + 0.1 M K2HPO4) solutions. The well-known (bi)sulfate structures on Au(111) and Au(100) in 0.1 M H2SO4 were compared with a new one found for phosphate species on Au(100) in 0.1 M H3PO4. The presence of non-uniform anion–anion distances in all these cases indicates that this is a characteristic feature common to oxoanion adlayers, due to their ability to form hydrogen-bridge bonds through the lone pairs of their oxygen atoms. No ordered adsorption was observed in neutral solutions, which indicates that the coadsorption of hydronium ions is necessary to stabilise the ordered oxoanion adlattices. The absence of current spikes in the cyclic voltammograms of Au(100) in 0.1 M H2SO4 and 0.1 M H3PO4, indicative of the formation of ordered adlayers, was explained by the fact that in these cases the adlayers are composed of many small domains.

Adsorbed formate: the key intermediate in the oxidation of formic acid on platinum electrodes.

The electrooxidation of formic acid on Pt and other noble metal electrodes proceeds through a dual-path mechanism, composed of a direct path and an indirect path through adsorbed carbon monoxide, a poisoning intermediate. Adsorbed formate had been identified as the reactive intermediate in the direct path. Here we show that actually it is also the intermediate in the indirect path and is, hence, the key reaction intermediate, common to both the direct and indirect paths. Furthermore, it is confirmed that the dehydration of formic acid on Pt electrodes requires adjacent empty sites, and it is demonstrated that the reaction follows an apparently paradoxical electrochemical mechanism, in which an oxidation is immediately followed by a reduction.

A method to prepare single crystal electrodes of reactive metals : Application to Pd(hkl)

A method has been developed to prepare single crystal electrodes of reactive metals by resistive heating. Instead of using a flame, the crystals are annealed in a controlled atmosphere by passing a high electric current through them. This method was tested with Au(111) and Pt(111) electrodes, for which the well-known cyclic voltammograms characteristic of a well-ordered surface were obtained. As a second step, the preparation of Pd(hkl) electrodes was addressed. The cyclic voltammograms for Cu upd on Pd(111), Pd(100) and Pd(110) electrodes, prepared by this method, are shown and compared with literature data.

The impact of spectator species on the interaction of H2O2 with platinum – implications for the oxygen reduction reaction pathways

The impact of electrolyte constituents on the interaction of hydrogen peroxide with polycrystalline platinum is decisive for the understanding of the selectivity of the oxygen reduction reaction (ORR). Hydrodynamic voltammetry measurements show that while the hydrogen peroxide reduction (PRR) is diffusion-limited in perchlorate- or fluoride-containing solutions, kinetic limitations are introduced by the addition of more strongly adsorbing anions. The strength of the inhibition of the PRR increases in the order ClO4(-)≈ F(-) < HSO4(-) < Cl(-) < Br(-) < I(-) as well as with the increase of the concentration of the strongly adsorbing anions. Electronic structure calculations indicate that the dissociation of H2O2 on Pt(111) is always possible, regardless of the coverage of spectator species. However, the adsorption of H2O2 becomes strongly endothermic at high coverage with adsorbing anions. A comparison of our observations on the inhibition of the PRR by spectators with previous studies on the selectivity of the ORR shows that oxygen is reduced to H2O2 only under conditions at which the PRR kinetics is significantly limited, while the ORR proceeds with a complete four-electron reduction only when the PRR is sufficiently fast. Therefore, only a H2O2-mediated pathway that includes a competition between the dissociation and the spectator coverage-dependent desorption of the H2O2 intermediate is enough to explain and unify all the observations that have been made so far on the selectivity of the ORR.

Atomic Ensemble Effects in Electrocatalysis: The Site‐Knockout Strategy

During an electrocatalytic reaction bonds are broken and formed, and this requires that the reactants, the intermediates formed at the elementary reaction steps, and the products interact with a given number of surface atoms of the catalyst. Modifying the number of groups with an adequate number of surface atoms in a suitable geometric arrangement for a determined reaction step to proceed may affect the activity and/or selectivity of the catalyst. Although separating purely geometric atomic ensemble effects from electronic effects is not straightforward, the insights extracted from a detailed investigation of atomic ensemble effects can have a profound impact in the determination of electrocatalytic reaction mechanisms and in the design of more active and more selective electrocatalysts. This Minireview illustrates, using cyanide-modified Pt(111) electrodes as an archetype, how eliminating only one kind of site from the surface (the site-knockout strategy) by means of a regular array of inert adsorbates can be used to successfully study atomic ensemble effects in electrocatalysis. The possible consequences for the design of more efficient and more selective electrocatalysts are also commented on.