Title: Molecular Electrocatalysts for Oxidation of Hydrogen Using Earth-Abundant Metals: Shoving Protons Around with Proton Relays

Abstract

Sustainable, carbon-neutral energy is needed to supplant the worldwide reliance on fossil fuels, to address the persistent problem of increasing emissions of CO₂. Solar and wind energy are intermittent, highlighting the need to develop energy storage on a huge scale. Electrocatalysts provide a way to convert between electrical energy generated by renewable energy sources and chemical energy in the form of chemical bonds. Here, the oxidation of hydrogen to give two electrons and two protons is carried out in fuel cells, but the typical catalyst is platinum, a precious metal of low earth-abundance and high cost. In nature, hydrogenases based on iron or iron/nickel reversibly oxidize hydrogen with remarkable efficiency and rates. Functional models of these enzymes are synthesized with the goal of achieving electrocatalytic H₂ oxidation using inexpensive, earth-abundant metals along with a key feature identified in the [FeFe]-hydrogenase: an amine base positioned near the metal. The diphosphine ligands P^R₂N^R'₂ (1,5-diaza-3,7-diphosphacyclooctane with alkyl or aryl groups on the P and N) are used as ligands in Ni, Fe and Mn complexes. The pendant amines promote binding and heterolytic cleavage of H₂, placing the hydride on the metal and the proton on the amine. The pendant amines also serve asmore » proton relays, accelerating intramolecular and intermolecular proton transfers. Electrochemical oxidations and deprotonations by an exogeneous amine base lead to catalytic cycles for oxidation of H₂ (1 atm) at room temperature for catalysts derived from [Ni(P^Cy₂N^R'₂)₂]²⁺, Cp^C6F5Fe(P^tBu₂N^Bn₂)H and MnH(P^Ph₂N^Bn₂)(bppm)(CO) [bppm = (PAr^F₂)₂CH₂)]. The Mn cation [Mn(P^Ph₂N^Bn₂)(bppm)(CO)]⁺ mediates the rapid (>10⁴ s^-1 at -95 °C), reversible heterolytic cleavage of H₂. Obtaining the optimal benefit of pendant amines incorporated into the ligand requires that the pendant amine be properly positioned to interact with a M-H or M(H₂) bond. In addition, ligands are ideally selected such that the hydride acceptor ability of the metal and the basicity of a pendant are tuned to give low barriers for heterolytic cleavage of the H-H bond and for subsequent proton transfer reactions. Using these principles allows the rational design of electrocatalysts for H₂ oxidation using earth-abundant metals.« less

Authors:

Bullock, Ronald Morris  ^[1];

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Helm, Monte L. ^[1]

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+ Show Author Affiliations

1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)

Publication Date:

Tue Jun 16 00:00:00 EDT 2015

Research Org.:

Pacific Northwest National Laboratory (PNNL), Richland, WA (United States); Energy Frontier Research Centers (EFRC) (United States). Center for Molecular Electrocatalysis (CME)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1582563

Report Number(s):

PNNL-SA-108095
Journal ID: ISSN 0001-4842

Grant/Contract Number:  

AC05-76RL01830

Resource Type:

Accepted Manuscript

Journal Name:

Accounts of Chemical Research

Additional Journal Information:

Journal Volume: 48; Journal Issue: 7; Journal ID: ISSN 0001-4842

Publisher:

American Chemical Society

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; hydrogen; oxidation; electrocatalysis