PNNL

Abstract

A nickel complex has been synthesized that contains the P4N2 ligand, a tetradendate phosphine ligand with two pendant amines in the ligand backbone. The rigidly square planar Ni(P4N2)2+ complex is an active electrocatalyst for reduction of protons to hydrogen. Using N,N-dimethylformamidium as the acid in wet acetonitrile solution, Ni(P4N2)2+ displays a turnover frequency of 1.6 × 106 s–1, which is among the fastest rates reported for any molecular electrocatalyst. This high catalytic rate comes at the cost of a 1200 mV overpotential at the catalytic half wave potential. The Ni(II) state of the catalyst was found to be stable under strongly acidic conditions, with only trace decomposition observed over two weeks in the presence of 0.1 M trifluoromethanesulfonic acid. The catalyst was less stable in the Ni(0) state due to the inability of the rigid P4N2 ligand to adopt a tetrahedral geometry. Using variable scan rate cyclic voltammetry, a first order rate constant of ~1 s–1 was measured for dissociation of a phosphine from Ni(0), which is proposed to be a key step for determining the lifetime of the catalyst during electrolysis. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for DOE.