Theoretical and Experimental Investigation of Functionalized Cyanopyridines Yield an Anolyte with an Extremely Low Reduction Potential for Nonaqueous Redox Flow Batteries

Abstract

Cyanopyridines and cyanophenylpyridines were investigated as anolytes for nonaqueous redox flow batteries (RFBs). The three isomers of cyanopyridine are reduced at potentials of −2.2 V or lower vs. ferrocene+/0 (Fc+/0), but the 3-CNPy⋅− radical anion forms a sigma-dimer that is re-oxidized at E≈−1.1 V, which would lead to poor voltaic efficiency in a RFB. Bulk electrochemical charge-discharge cycling of the cyanopyridines in acetonitrile and 0.50 M [NBu4][PF6] shows that 2-CNPy and 4-CNPy lose capacity quickly under these conditions, due to irreversible chemical reaction/decomposition of the radical anions. Density-functional theory (DFT) calculations indicated that adding a phenyl group to the cyanopyridines would, for some isomers, limit dimerization and improve the stability of the radical anions, while shifting their E1/2 only about +0.10 V relative to the parent cyanopyridines. Among the cyanophenylpyridines, 3-CN-6-PhPy and 3-CN-4-PhPy are the most promising as anolytes. They exhibit reversible reductions at E1/2=−2.19 and −2.22 V vs. ferrocene+/0, respectively, and retain about half of their capacity after 30 bulk charge-discharge cycles. An improved version of 3-CN-6-PhPy with three methyl groups (3-cyano-4-methyl-6-(3,5-dimethylphenyl)pyridine) has an extremely low reduction potential of −2.50 V vs. Fc+/0 (the lowest reported for a nonaqueous RFB anolyte) and loses only 0.21 % of capacity per cycle during charge-discharge cycling in acetonitrile.Cyanopyridines are reduced at low potentials in organic solvents and are therefore a promising class of molecules for anolytes for nonaqueous redox-flow batteries (RFBs). Quantum computations suggest that properly placed phenyl and methyl substituents shift the reduction potentials even lower than those of the parent cyanopyridines and prevent dimerization of the radical anions. The optimal molecule has a reduction potential in CH3CN of −2.50 V vs. Fc+/0, the lowest reported for a RFB anolyte.