Regulating surface state of WO 3 nanosheets by gamma irradiation for suppressing hydrogen evolution reaction in electroc
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TRACT Realizing the reduction of N2 to NH3 at low temperature and pressure is always the unremitting pursuit of scientists and then electrochemical nitrogen reduction reaction offers an intriguing alternative. Here, we develop a feasible way, gamma irradiation, for constructing defective structure on the surface of WO3 nanosheets, which is clearly observed at the atomic scale by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The abundant oxygen vacancies ensure WO3 nanosheets with a Faradaic efficiency of 23% at −0.3 V vs. RHE. Moreover, we start from the regulation of the surface state to suppress proton availability towards hydrogen evolution reaction (HER) on the active site and thus boost the selectivity of nitrogen reduction.
KEYWORDS nitrogen reduction reaction, ammonia, WO3, oxygen vacancies, surface state
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Introduction
Ammonia (NH3) is not only an indispensable raw material in agriculture, chemical industry and other fields, but also a new type of “carbon-free” energy carrier [1–3]. Although there are nearly 80% N2 in the atmosphere [4, 5], it is not easy to convert these free nitrogens into high-value ammonia. Since the beginning of the last century, industrial ammonia production has mainly relied on the Haber-Bosch process [6, 7], which needs to be carried out under harsh conditions [8, 9], and accounts for about 1%–2% of the world’s total energy expenditure [10, 11]. The feedstock of Haber-Bosch process, H2, is acquired from steam methane reforming and coal gasification [12], inevitably increasing CO2 emissions. Comparatively, the electrochemical nitrogen reduction reaction (NRR) offers a new benign for ammonia production, which can be carried out under room conditions and meanwhile directly use water as hydrogen source. More importantly, renewable solar and wind energy can be integrated to power the electroreduction of N2 to NH3 [13]. At present, one of the main challenges faced by electrochemical nitrogen fixation is the low efficiency. This is mainly attributed to the chemical inertness of N2 at normal temperature and pressure. Besides, since the potential for hydrogen evolution reaction (HER) and NRR are so close [14], the HER as a strong competitors will seriously restrict the efficiency of NH3 synthesis from N2 [15]. Therefore, it is predictable that the efficiency of N2 reduction to NH3 can be significantly improved by suppressing hydrogen evolution. However, the problem is how to mitigate proton availability towards HER without affecting the activity of NRR as both NRR and HER invole proton-coupled
electron transfer processes [16]. In proton-containing solutions, protons are undoubtedly easier to activate than N≡N bonds. In other words, a catalyst does not activate protons, let alone activates N2. Therefore, it is especially important for the careful design to balance the competition between nitrogen activation and hydrogen evolution. A large amount of research work on electrocatalysts for N2 reduction to NH3 in recent years has shown that the ammonia pr
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