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The electrocatalytic nitrogen reduction reaction (NRR) is of significant interest as an environmentally friendly method for NH 3 production for agricultural and clean energy applications. Selectivity of NRR vis-à-vis the hydrogen evolution reaction (HER), however, is thought to adversely impact many potential catalysts, including Earth-abundant transition metal oxynitrides. Relative HER/NRR selectivities are therefore directly compared for two transition metal oxynitrides with different metal oxophilicities—Co and V. Electrocatalytic current–potential measurements, operando fluorescence, absorption, and GC measurements of H 2 and NH 3 production, ex situ X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations are combined to directly compare NRR and HER activities under identical reaction conditions. Results show that cobalt oxynitrides – with Co primarily in the Co( ii ) oxidation state – are NRR active at pH 10, with electrochemical reduction of both lattice nitrogen and dissolved N 2 , the latter occurring without N incorporation into the lattice. Removal of lattice N then yields Co( ii ) oxide, which is still NRR active. These results are complemented by calculations showing that N 2 binding at Co( ii ) sites is energetically favored over binding at Co( iii ) sites. GC analysis demonstrates that H 2 production occurs in concert with ammonia production but at a far greater rate. In contrast, vanadium oxynitride films are HER inactive under the same (pH 10) conditions, as well as at pH 7, but are NRR active at pH 7. DFT calculations indicate that a major difference in the two materials is hindered O–H dissociation of H 2 O adsorbed at O-ligated Co vs. V cation centers. The combined studies indicate significant variation in HER vs. NRR selectivity as a function of employed transition metal oxynitrides, as well as different HER mechanisms in V and Co oxynitrides.
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B-terminated (111) polar surfaces of BP and BAs: promising metal-free electrocatalysts with large reaction regions for nitrogen fixation
The nitrogen electroreduction reaction (NRR) in aqueous solutions under ambient conditions represents an attractive prospect to produce ammonia, but the development of long-term stable and low-cost catalysts with high-efficiency and high-selectivity remains a great challenge. Herein, we investigated the potential of a new class of experimentally available boron-containing materials, i.e. , cubic boron phosphide (BP) and boron arsenide (BAs), as metal-free NRR electrocatalysts by means of density functional theory (DFT) calculations. Our results revealed that gas phase N 2 can be sufficiently activated on the B-terminated (111) polar surfaces of BP and BAs, and effectively reduced to NH 3 via an enzymatic pathway with an extremely low limiting potential (−0.12 V on BP and −0.31 V on BAs, respectively). In particular, the two proposed B-terminated (111) surfaces not only have a large active region for N 2 reduction, but also can significantly inhibit the competitive hydrogen evolution reaction, and thus have rather high efficiency and selectivity for the NRR. Therefore, cubic BP or BAs with mainly exposed (111) facets may serve as promising metal-free NRR catalysts with superior performance.
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- Award ID(s):
- 1736093
- PAR ID:
- 10107001
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 7
- Issue:
- 21
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 13284 to 13292
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1039/c9ta01410a
