Kinetic Analysis of Competitive Electrocatalytic Pathways: New Insights into Hydrogen Production with Nickel Electrocatalysts

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• 作者：Eric S. Wiedner ; Houston J. S. Brown ; Monte L. Helm
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：January 20, 2016
• 年：2016
• 卷：138
• 期：2
• 页码：604-616
• 全文大小：710K
• ISSN：1520-5126

文摘

The hydrogen production electrocatalyst Ni(P^Ph₂N^Ph₂)₂²⁺ (1) is capable of traversing multiple electrocatalytic pathways. When using dimethylformamidium, DMF(H)⁺, the mechanism of H₂ formation by 1 changes from an ECEC to an EECC mechanism as the potential approaches the Ni(I/0) couple. Two electrochemical methods, current–potential analysis and foot-of-the-wave analysis (FOWA), were performed on 1 to measure detailed kinetics of the competing ECEC and EECC pathways. A sensitivity analysis was performed on the methods using digital simulations to understand their strengths and limitations. Chemical rate constants were significantly underestimated when not accounting for electron-transfer kinetics, even when electron transfer was fast enough to afford a reversible noncatalytic wave. The EECC pathway of 1 was faster than the ECEC pathway under all conditions studied. Buffered DMF:DMF(H)⁺ mixtures afforded an increase in the catalytic rate constant (k_obs) of the EECC pathway, but k_obs for the ECEC pathway did not change when using buffered acid. Further kinetic analysis of the ECEC path revealed that base increases the rate of isomerization from exo-protonated Ni(0) isomers to the catalytically active endo-isomers, but decreases the rate of protonation of Ni(I). FOWA did not provide accurate rate constants, but FOWA was used to estimate the reduction potential of the previously undetected exo-protonated Ni(I) intermediate. Comparison of catalytic Tafel plots for 1 under different conditions reveals substantial inaccuracies in the turnover frequency at zero overpotential when the kinetic and thermodynamic effects of the conjugate base are not accounted for properly.