In situ measurements of water adsorption on a platinum electrode by an electrochemical quartz crystal microbalance

Effects of Sulfate on the Oxygen Reduction Reaction Activity on Stabilized Pt Skin/PtCo Alloy Catalysts from 30 to 80 °C

The effects of the concentration of H₂SO₄ ([H₂SO₄]), which is the major decomposition product of polymer electrolyte membranes during the operation of fuel cells, on the performance of stabilized Pt skin/PtCo alloy nanocatalysts supported on high-surface-area carbon (Pt_xAL-PtCo/C) were investigated. Kinetically controlled activities for the oxygen reduction reaction (ORR) and the H₂O₂ yields ( P(H₂O₂)) on the Pt_xAL-PtCo/C were examined based on hydrodynamic voltammograms in O₂-saturated 0.1 M HClO₄ + X M H₂SO₄ ( X = 0 to 5 × 10^-2) by use of the channel flow double electrode method at temperatures between 30 and 80 °C. At X ≤ 10^-6 (1 μM) and all temperatures examined, the apparent ORR rate constants k_app@0.85 V (per unit electrochemically active surface area) on Pt_xAL-PtCo/C at 0.85 V vs the reversible hydrogen electrode (RHE) were nearly identical with those in sulfate-free 0.1 M HClO₄ and were at least twice as high as those on a commercial Pt/C catalyst (c-Pt/C). The values of k_app@0.85 V on both Pt_xAL-PtCo/C and c-Pt/C decreased linearly with log[H₂SO₄] in the concentration range 10^-6 < X ≤ 5 × 10^-2. The detrimental effect by H₂SO₄ was less pronounced on Pt_xAL-PtCo/C than on c-Pt/C at high temperatures; the k_app@0.85 V value at X = 5 × 10^-2 on the former at 80 °C was maintained as high as 87%, whereas that of the latter was 66% (34% loss). The values of peroxide production percentage P(H₂O₂) on Pt_xAL-PtCo/C at 80 °C were nearly constant (ca. 0.22% at 0.76 V vs RHE) up to X = 5 × 10^-2. These superior characteristics are ascribed to weakened adsorption of sulfate on the Pt skin surface, supported by DFT calculations, which provides the great advantage of robustness in the presence of impurities, maintaining active sites for the ORR during the PEFC operation.

Interfacial structure of atomically flat polycrystalline Pt electrodes and modified Sauerbrey equation

The electrochemical quartz-crystal nanobalance (EQCN) measures in situ mass changes associated with interfacial electrode processes. Real electrodes are not atomically flat, thus their surface roughness affects the conversion of frequency variations (Δf) to mass changes (Δm) associated with electrochemical processes. Here, we analyze Δm associated with the electrochemical H adsorption/desorption and surface oxide formation/reduction on Pt electrodes of gradually increasing surface roughness using the EQCN and cyclic-voltammetry in an aqueous H₂SO₄ solution. These two interfacial processes are ideal to probe changes in the electrochemically active surface area. The surface roughness of Pt-coated resonators is fine-tuned through Pt electrodeposition and examined using atomic force microscopy. The results acquired using Pt electrodes of increasing roughness factor (1.61 ≤ R ≤ 13.0) reveal a linear relationship between Δm and R. Extrapolation of this relationship to R = 1.00 leads to the determination of Δm associated with H adsorption/desorption and oxide formation/reduction on an atomically flat polycrystalline Pt electrode. The values of Δm associated with these processes are analyzed in terms of the number of H, O, water, and ionic species interacting with each Pt atom of the electrode surface. We find that the charge densities associated with these electrochemical processes and mass variations do not scale up by the same factor. This leads to a modified version of the Sauerbrey equation for Pt electrodes, which takes into account the intrinsic surface roughness.

Theoretical Study on Solubility from Pt Electrocatalyst and Reactivity in Electrolyte Environment of Pt Complex in PEFC

We theoretically analyzed the formation energy and solvation free energy of Pt(II) and Pt(IV) complexes with three types of ligands(H2O,OH−,and CF3SO3 −)in electrolyte environment under the low- and high-humidity conditions to study the Pt electrocatalyst degradation and dissolution mechanisms for polymer electrolyte fuel cell. To represent the low- and high-humidity conditions in perfluorosulfonic acid (PFSA) polymer electrolyte membrane, we controlled the dielectric constant based on the experimental result. We observed general tendencies that the formation energy becomes larger while the solvation free energy becomes smaller under the low-humidity condition. The degradation of Pt complex from Pt surface is indicated to be accelerated by the adsorption of the end group of PFSA polymer side chain, on the Pt surface by comparing the desorption energies of [Pt(H2O)2(OH)3(CF3SO3)] and [Pt(H2O)2(OH)4]. The [Pt(H2O)4]2+is not formed by the proton addition reaction between Pt complexes under the low-humidity condition of PFSA environment. From the analysis of possible reaction pathways of Pt complexes, we found the influence of humidity on the reactivity of Pt complex.

Electrochemical quartz crystal microbalance analysis of the oxygen reduction reaction on Pt-based electrodes. Part 1: Effect of adsorbed anions on the oxygen reduction activities of Pt in HF, HClO4, and H2SO4 solutions

The effects of anion adsorption on the activities for the oxygen reduction reaction (ORR) at a Pt film electrode in electrolyte solutions (HClO(4) and HF at various concentrations) were analyzed using an electrochemical quartz crystal microbalance (EQCM) and a rotating disk electrode (RDE). With an increasing HClO(4) concentration [HClO(4)], the onset potential for the Pt oxide formation in the voltammogram shifted in the positive direction, accompanied by a compression of the hydrogen adsorption/oxidation wave to less positive potentials. This is ascribed to a specific adsorption of the ClO(4)(-) anion, because the [HClO(4)] dependence of the mass change Δm detected by EQCM in the double-layer region was found to be fitted well by a Frumkin-Temkin adsorption isotherm. The potential dependencies of Δm in both 0.1 and 0.5 M HClO(4) solutions accord well with those of the ν(Cl-O) intensities observed by in situ Fourier transform infrared (FTIR) spectroscopy in the potential range from 0.3 to 0.6 V. The kinetically controlled current densities j(k) for the ORR at the Pt-RDE were found to decrease with increasing [HClO(4)], because of the blocking of the active sites by specifically adsorbed ClO(4)(-). The values of j(k) in the non-adsorbing 0.1 M HF electrolyte solution, however, were smaller than those in 0.1 M HClO(4). It was found that the low ORR activity could be ascribed to the low H(+) activity in the weak acid solution of HF. We, for the first time, detected a reversible mass change for one or more adsorbed oxygen species on the Pt-EQCM in O(2)-saturated and He-purged HF and HClO(4) solutions. The coverages of oxygen species θ(O) on Pt were found to increase in the O(2)-saturated solution. High values of θ(O) in O(2)-saturated 7 mM HF suggest that the ORR rate was limited by the very low H(+) activity in the solution, and the adsorbed oxygen species remained on the surface because of a slow consumption rate.

First observation of luminescence from tris(2,2'-bipyridine)ruthenium(II) triggered by anodic oxidation of oligodeoxyribonucleotide

Anodic oxidation of oligodeoxyribonucleotide in an alkaline aqueous medium containing tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)3(2+)) was shown to cause luminescence around +1.3 V (vs. Ag/AgCl) with a maximal intensity at approximately 600 nm, possibly originating from Ru(bpy)3(2+) in the d-pi* triplet state. A pivotal initial stage in the light production path was postulated to be the anodic oxidation of 2-deoxyribose residue. This reaction seems to be available for the determination of sub-micromol dm(-3) levels of oligodeoxyribonucleotide.