Electrochemical Detection of Transient Cobalt Hydride Intermediates of Electrocatalytic Hydrogen Production
2016
Wiedner, Eric S. | Bullock, R Morris
A large variety of molecular cobalt complexes are used as electrocatalysts for H₂ production, but the key cobalt hydride intermediates are frequently difficult to detect and characterize due to their high reactivity. We report that a combination of variable scan rate cyclic voltammetry and foot-of-the-wave analysis (FOWA) can be used to detect transient CoᴵᴵᴵH and CoᴵᴵH intermediates of electrocatalytic H₂ production by [Coᴵᴵ(Pᵗᴮᵘ₂Nᴾʰ₂)(CH₃CN)₃]²⁺ and Coᴵᴵ(dmgBF₂)₂(CH₃CN)₂. In both cases, reduction of a transient catalytic intermediate occurs at a potential that coincides with the Coᴵᴵ/ᴵ couple. Each reduction displays quasireversible electron-transfer kinetics, consistent with reduction of a CoᴵᴵᴵH intermediate to CoᴵᴵH, which is then protonated by acid to generate H₂. A bridge-protonated Coᴵ species was ruled out as a catalytic intermediate for Coᴵᴵ(dmgBF₂)₂(CH₃CN)₂ from voltammograms recorded at 1000 psi of H₂. Density functional theory was used to calculate Coᴵᴵᴵ–H and Coᴵᴵ–H bond strengths for both catalysts. Despite having very different ligands, the cobalt hydrides of both catalysts possess nearly identical heterolytic and homolytic Co–H bond strengths for the CoᴵᴵᴵH and CoᴵᴵH intermediates.
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