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

Journal Article · · Journal of the American Chemical Society
DOI:https://doi.org/10.1021/jacs.5b10853· OSTI ID:1387532
 [1];  [1];  [1]
  1. Energy Frontier Research Centers (EFRC) (United States). Center for Molecular Electrocatalysis (CME); Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
The hydrogen production electrocatalyst Ni(PPh2NPh2)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. In this work 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 (kobs) of the EECC pathway, but kobs 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.
Research Organization:
Energy Frontier Research Centers (EFRC) (United States). Center for Molecular Electrocatalysis (CME); Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES)
Grant/Contract Number:
AC05-76RL01830
OSTI ID:
1387532
Alternate ID(s):
OSTI ID: 1236309
Journal Information:
Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 2 Vol. 138; ISSN 0002-7863
Publisher:
American Chemical Society (ACS)Copyright Statement
Country of Publication:
United States
Language:
English

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Figures / Tables (17)