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 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 (SEIRAS), we will 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.Electrooxidation of formic acid (HCOOH) to CO2, the reaction taking place at the anode of direct formic acid fuel cells, is one of the most fundamental model electrocatalytic reactions and has been investigated intensively over the last four decades mostly in acidic media. [1][2][3][4] It is generally accepted that HCOOH is oxidized via a dual pathway mechanism; 1 a main pathway via a reactive intermediate and a pathway involving adsorbed CO (COads), a catalytic poison. COads is oxidized to CO2 at high potentials. The pathway involving COads has well been established in the 1980s, while the main pathway, non-CO pathway, is still matter of strong debate. Samjeské et al. 5 and others 6,7 proposed, on the basis of surface-enhanced infrared spectroscopy in an ATR geometry (ATR-SEIRAS) 8 and electrochemical measurements, that a formate species adsorbed on the electrode surface though its two O atoms (bridge-bonded formate) is the intermediate in the non-CO pathway and its decomposition to CO2 is the rate-determining step (bridge-bonded formate mechanism), while Chen et al. 9 argued that the adsorbed formate is a site-blocking spectator and that HCOOH is directly oxidized via a weakly adsorbed HCOOH precursor (direct HCOOH mechanism). A consensus has not been reached yet, either in theoretical studies of the reaction. [10][11][12] The aim of the present Communication is to make clear the real reaction mechanism through a systematic investigation of the reaction over a wide range of pH (0-12).Since HCOOH is a weak acid with a pKa of 3.75, 13 if the direct HCOOH pathway were the main reaction route, the oxidation current should decrease with increasing pH due to the decrease of HCOOH concentration. However, several earlier studies have reported that the oxidation current increases with pH. 14 Figure 1a shows representative cyclic voltammograms (CVs) for a rotating Pt disc electrode in 0.2 M

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.

We develop a computationally efficient scheme to determine the potentials of zero charge (PZC) of metal-water interfaces with respect to the standard hydrogen electrode. We calculate the PZC of Pt(111), Au(111), Pd(111) and Ag(111) at a good accuracy using this scheme. Moreover, we find that the interface dipole potentials are almost entirely caused by charge transfer from water to the surfaces, the magnitude of which depends on the bonding strength between water and the metals, while water orientation hardly contributes at the PZC conditions. DOI: 10.1103/PhysRevLett.119.016801 Metal-water interfaces are of great technological importance in many energy storage and conversion devices such as fuel cells and batteries. Fundamentally, they are the primary subjects for studying electrochemical processes (i.e., electrocatalysis and corrosion) in electrochemistry and play a crucial role in the development of electric double layer (EDL) theories (i.e., Gouy-Chapman-Stern model). Direct probing of structures and dynamics of the interfaces at a molecular level is extremely challenging for experiment. First principles simulations, on the other hand, can offer detailed microscopic information on the interfaces. However, due to high computational costs, it was not long ago that ab initio modeling of metal-water interfaces became affordable [1,2].Potential of zero charge (PZC) is a fundamental concept in the EDL theories, defined as the potential at which no excess charge exists on metal surfaces, and deviation from the PZC will lead to attraction of counterions to the surfaces, building up the EDL [3]. Because of its significance to the microscopic understanding of an EDL and interfacial potentials, numerous experimental techniques have been developed to determine the PZC of metal electrodes, e.g., surface tension methods, capacitance measurement methods, CO charge-displacement methods, etc [4]. Despite repeated efforts, many measurements are still subject to uncertainties because of difficulties in preparing single crystal electrodes and excluding specific adsorption of electrolyte ions [5][6][7]. In the presence of specific adsorption, electrochemists distinguish the subtlety between the potential of zero total charge (PZTC) and the potential of zero free charge (PZFC), and only the latter is an intrinsic property of metal electrodes [8].First principles calculation of well-defined metal surfaces is ideal for determining the PZFC. There are two issues in computational methods. First, how is the solvent treated in the simulation models? In the literature, water has often been treated with either an implicit dielectric continuum [9][10][11] or some representations of static water structures for efficiency [12]. It, however, has been reported that the dynamics of water on surfaces has significant effects on interface potentials [13]. As yet, very few studies have modeled full metal-water interfaces and accounted for water dynamics using density functional theory based molecular dynamics (DFTMD) [14,15]. Second, how are th...

The nature of the electrolyte cation is known to affect the Faradaic efficiency and selectivity of CO electroreduction. Singh et al. (J. Am. Chem. Soc. 2016, 138, 13006-13012) recently attributed this effect to the buffering ability of cation hydrolysis at the electrical double layer. According to them, the pK of hydrolysis decreases close to the cathode due to the polarization of the solvation water molecules sandwiched between the cation's positive charge and the negative charge on the electrode surface. We have tested this hypothesis experimentally, by probing the pH at the gold-electrolyte interface in situ using ATR-SEIRAS. The ratio between the integrated intensity of the CO and HCO bands, which has to be inversely proportional to the concentration of H, provided a means to determining the pH change at the electrode-electrolyte interface in situ during the electroreduction of CO. Our results confirm that the magnitude of the pH increase at the interface follows the trend Li > Na > K > Cs, adding strong experimental support to Singh's et al.'s hypothesis. We show, however, that the pH buffering effect was overestimated by Singh et al., their overestimation being larger the larger the cation. Moreover, our results show that the activity trend of the alkali-metal cations can be inverted in the presence of impurities that alter the buffering effect of the electrolyte, although the electrolyte with maximum activity is always that for which the increase in the interfacial pH is smaller.

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