Tandem electrocatalytic N fixation via proton-coupled electron transfer.

New electrochemical ammonia (NH) synthesis technologies are of interest as a complementary route to the Haber-Bosch process for distributed fertilizer generation, and towards exploiting ammonia as a zero-carbon fuel produced via renewably sourced electricity. Apropos of these goals is a surge of fundamental research targeting heterogeneous materials as electrocatalysts for the nitrogen reduction reaction (NRR). These systems generally suffer from poor stability and NH selectivity; the hydrogen evolution reaction (HER) outcompetes NRR. Molecular catalyst systems can be exquisitely tuned and offer an alternative strategy, but progress has been thwarted by the same selectivity issue; HER dominates. Here we describe a tandem catalysis strategy that offers a solution to this puzzle. A molecular complex that can mediate an N reduction cycle is partnered with a co-catalyst that interfaces the electrode and an acid to mediate proton-coupled electron transfer steps, facilitating N-H bond formation at a favourable applied potential (-1.2 V versus Fc) and overall thermodynamic efficiency. Certain intermediates of the NRR cycle would be otherwise unreactive via uncoupled electron transfer or proton transfer steps. Structurally diverse complexes of several metals (W, Mo, Os, Fe) also mediate NRR electrocatalysis at the same potential in the presence of the mediator, pointing to the generality of this tandem approach.