Tandem electrocatalytic N₂ fixation via proton-coupled electron transfer

Abstract

New electrochemical ammonia (NH₃) 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 (N₂RR). These systems generally suffer from poor stability and NH₃ selectivity; the hydrogen evolution reaction (HER) outcompetes N₂RR. 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 N₂ 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⁺/⁰) and overall thermodynamic efficiency. Certain intermediates of the N₂RR 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 N₂RR electrocatalysis at the same potential in the presence of the mediator, pointing to the generality of this tandem approach.

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