ABSTRACT
Doping with Fe boosts the electrocatalytic performance of NiOOH for the oxygen evolution reaction (OER). To understand this effect, we have employed state-of-the-art electronic structure calculations and thermodynamic modeling. Our study reveals that at low concentrations Fe exists in a low-spin state. Only this spin state explains the large solubility limit of Fe and similarity of Fe-O and Ni-O bond lengths measured in the Fe-doped NiOOH phase. The low-spin state renders the surface Fe sites highly active for the OER. The low-to-high spin transition at the Fe concentration of ~ 25% is consistent with the experimentally determined solubility limit of Fe in NiOOH. The thermodynamic overpotentials computed for doped and pure materials, η = 0.42 V and 0.77 V, agree well with the measured values. Our results indicate a key role of the low-spin state of Fe for the OER activity of Fe-doped NiOOH electrocatalysts.
ABSTRACT
The activity of Pt(111) electrodes for the hydrogen evolution reaction (HER) in 0.5M H2SO4 solution is found to increase with continuous potential cycling in the HER potential region. In addition, the basic cyclic voltammograms obtained in 0.5M H2SO4 saturated with N2 after HER show several characteristic changes: the current waves for hydrogen adsorption in the region of0.2 < E < 0.35 V and for sulfate adsorption at 0.35 < E < 0.5 V decrease and the current spike at 0.44 V for the phase transition of the sulfate adlayer gradually disappears. We suggest that these changes are caused by the absorption of a small amount of hydrogen in the subsurface layer and propose a mechanism by which this enhances hydrogen evolution.
ABSTRACT
Like many branches of science, not to mention culture in general, electrochemistry has a number of recurring topics: Areas of research that are popular for a certain time, then fade away as their possibilities seem to have been exhausted, only to return decades later as progress in experimental or theoretical techniques offer new possibilities for their investigation. A prime example are fuel cells, which have undergone five such cycles, but here we discuss a general concept of kinetics-the pre-exponential factor of a rate constant-which has undergone two such cycles. The first cycle was in the 1950-1980s, when the methods of electrochemical kinetics were developed, and the interpretation was based on transition-state theory. The second was triggered by the re-discovery of Kramers theory for reactions in condensed phases. This Minireview will show that the time has come for a third cycle based on recent progress in electrocatalysis.
ABSTRACT
pH, temperature and H-D kinetic isotope effects (KIEs) on the ORR on Au(100) have been examined systematically using a hanging meniscus rotating disk electrode system. We found that for the cases with pH > 7, the ORR mainly goes through a 4-electron reduction to OH(-) at E > pzc (potential of zero charge) without any pH and H-D KIEs. When the pH at the electrode/electrolyte interface (pH(s)) is below 7, O2 only reduces to H2O2, its activity increases with pH(s), and a H-D KIE of above 2 is observed in 0.1 M HClO4. According to the experimental results in acid solution, a mechanism with O2 + H(+) + e â HO(2,ad) as the rate determining step followed by decoupled electron and proton transfer steps is proposed. The high activation barrier for O-O bond breaking and the fast oxidation of H2O2 or HO2(-) to O2 render the ORR observable only at potentials negative of the equilibrium potential (Eeq) of the redox of H2O2/O2 in acidic media or of HO2(-)/O2 in an alkaline environment. The apparent activation energy (E(a,app)) for O2 reduction to H2O2 is ca. 35 ± 3 kJ mol(-1) and to OH(-) is 60 ± 6 kJ mol(-1), while the pre-exponential factor (A) for the former is ca. 3-6 orders of magnitude smaller than that of the latter. The lower activity for O2 reduction to H2O2 on Au(100) is attributed to the small pre-exponential factor.
