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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 electricity1. Apropos of these goals is a surge of fundamental research targeting heterogeneous materials as electrocatalysts for the nitrogen reduction reaction (N2RR)2. These systems generally suffer from poor stability and NH3 selectivity; the hydrogen evolution reaction (HER) outcompetes N2RR3. Molecular catalyst systems can be exquisitely tuned and offer an alternative strategy4, 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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Fig. 1: Approaches to N2RR electrocatalysis.
Fig. 2: Electrocatalytic N2RR via tandem catalysis.
Fig. 3: Mechanistic insights into tandem PCET N2RR.
Fig. 4: Electrocatalytic N2RR using reported molecular catalysts.

Data availability

Details on the procedures and the corresponding datasets generated during and/or analysed during the current study are available within the paper and its Supplementary Information files, and from the corresponding author on reasonable request.

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Acknowledgements

We thank the Dow Next Generation Educator Funds and Instrumentation Grants for their support of the NMR facility at Caltech. We also thank the Resnick Water and Environment Laboratory (WEL) and the Molecular Materials Resource Center at Caltech for the use of their instrumentation. We thank the following funding agencies: Department of Energy, Office of Basic Energy Sciences (DOE-0235032), Catalysis Science Program (for the development and applications of CPET mediators); National Institutes of Health (R01 GM-075757) (for studies of Fe-mediated N2RR). P.G.-B. thanks the Ramón Areces Foundation for a postdoctoral fellowship. J.D. thanks the Arnold and Mabel Beckman Foundation for a postdoctoral fellowship. M.J.C. thanks the Resnick Sustainability Institute for a graduate fellowship.

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P.G.-B., M.J.C. and J.C.P. conceptualized the work. P.G.-B. designed and executed the experiments. J.D. assisted with the execution of the catalytic experiments. All authors analysed and interpreted the data and co-wrote the manuscript.

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Correspondence to Jonas C. Peters.

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Supplementary Sections 1–15, Figs. 1–108, Table 1 and references.

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Garrido-Barros, P., Derosa, J., Chalkley, M.J. et al. Tandem electrocatalytic N2 fixation via proton-coupled electron transfer. Nature 609, 71–76 (2022). https://doi.org/10.1038/s41586-022-05011-6

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