Low Oxygen Reduction Reaction Activity Research Articles

Platinum-Enriched Ni/Pt(111) Surfaces Prepared by Molecular Beam Epitaxy: Oxygen Reduction Reaction Activity and Stability

Oxygen reduction reaction (ORR) activities and electrochemical stabilities were evaluated for Ni/Pt(111) model electrode-catalysts fabricated by molecular beam epitaxy (MBE). Exposure of clean Pt(111) to 1.0-Langmuir (i.e., fractional coverage, a = 1.0 in the Langmuir isotherm) CO at 300K produced linear-bonded and bridge-bonded CO­Pt IR bands at 2093 and 1858 cm11. In contrast, 3.0-nm-thick Ni deposition onto Pt(111) at 823K (823K-Ni3.0nm/Pt(111)) showed broad IR bands for adsorbed CO at around 2064 cm11; the separation of reflection high-energy electron diffraction (RHEED) streaks of the 823K-Ni3.0nm/Pt(111) is greater than for clean Pt(111). In contrast, 923KNi3.0nm/Pt(111) yielded a single sharp IR band because of linear-bonded CO at 2080 cm11, and the separation of the RHEED streaks is almost the same as that for the Pt(111). The results suggest that a Pt-enriched topmost surface is generated above 923K through surface segregation of the substrate Pt atoms during the Ni deposition. After transferring the sample from ultra-high vacuum to an electrochemical system, without being exposed to air, changes in ORR activities of the MBE-prepared 923K-Ni3.0nm/Pt(111) were evaluated in a 0.1-mol L11 HClO4 aqueous solution under applied potential cycles of between 0.6 and 1.0V vs. RHE. The ORR activity enhancement factor vs. clean Pt(111) for the asprepared 923K-Ni3.0nm/Pt(111) was estimated to be twelve, and this factor was reduced to four after the application of 1000 potential cycles. However, a 943K-Ni3.0nm/Pt(111) sample, which showed a relatively low ORR activity (an enhancement factor of eight), had the activity enhancement factor of six even after the application of 1000 potential cycles. These results reveal that the sub-surface structures and composition of Pt-based alloys determine not only initial ORR activity but also electrochemical durability of the ORR catalysts. [doi:10.2320/matertrans.M2013061]

Palladium selenides as active methanol tolerant cathode materials for direct methanol fuel cell

Palladium selenides, PdSe, Pd 3Se and PdSe 2 have been prepared by the hydrothermal method and investigated for their structural and electrocatalytic properties toward the oxygen reduction reaction (ORR) using SEM/TEM, XRD, cyclic and linear sweep voltammetries. The crystallites of PdSe and PdSe 2 are found to follow tetragonal and orthorhombic crystal structures, respectively. The PdSe electrode in 0.5 M H 2SO 4 exhibits significantly higher electrocatalytic activity than the Pd 3Se or PdSe 2 electrode under similar experimental conditions. Further, a change in the palladium/selenium ratio from unity in the catalyst results in low ORR activity.

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.

Cobalt–polypyrrole–carbon black (Co–PPY–CB) electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells: Composition and kinetic activity

Electrocatalysts consisting of polypyrrole (PPY) and Co deposited on carbon black (CB) at several compositions were prepared and tested for the oxygen reduction reaction (ORR) in a HClO4 buffer (pH=1) using a rotating ring-disk electrode (RRDE). It was determined that the most favorable catalyst composition (prior to calcination) had a CB:PPY weight ratio of 2 and a pyrrole:Co (i.e., PY:Co) molar ratio of 4. This catalyst had an onset potential of 0.785V (vs. RHE) and a mass activity of ca. 1A/gcata at the fuel cell relevant voltage of 0.65V. Furthermore, it was found that the number of electrons exchanged during the ORR with the catalyst was ca. 3.5 and resulted in 28% yield of H2O2 at 0.65V, which hints to an indirect 4e− reduction of O2 to H2O, with H2O2 as an intermediate. From energy dispersive spectroscopy (EDS) and extended X-ray absorption fine structure (EXAFS) analysis, it is proposed that a PY:Co ratio of 4 favors the formation, prior to calcination, in the catalyst precursor of Co–N complexes in which Co is coordinated to 3 or 4 N atoms, resulting in strong Co–N interactions that limit the formation upon calcination of low ORR activity Co nanoparticles. These Co–N complexes give rise upon calcination to CoNx−2 sites in which the coordination of Co could favor the adsorption on them of O2, which would make those sites particularly active and selective. At the same mass activity of 1A/gcata, the voltage yielded by the catalyst was 200mV lower than that for a state-of-the-art Pt (10wt.%) catalyst, whose H2O2 output at 0.85V was 39% and involves the exchange of 3.2e−, overall making our material an attractive substitute to noble metal ORR electrocatalysts.