g-C 3 N 4 encapsulated ZrO 2 nanofibrous membrane decorated with CdS quantum dots: A hierarchically structured, self-sup
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Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 17 July 2020 / Revised: 22 October 2020 / Accepted: 22 October 2020
ABSTRACT The advancement of electrocatalytic N2 reduction reaction (NRR) toward ambient NH3 synthesis lies in the development of more affordable electrocatalysts than noble metals. Recently, various nanostructures of transition metal compounds have been proposed as effective electrocatalysts; however, they exist in the form of loose powders, which have to be immobilized on a matrix before serving as the electrode for electrolysis. The matrix, being it carbon paper, carbon cloth or metal foam, is electrocatalytically inactive, whose introduction inevitably raises the invalid weight while sacrificing the active sites of the electrode. Herein, we report on the fabrication of a flexible ZrO2 nanofibrous membrane as a novel, self-supported electrocatalyst. The heteroatom doping can not only endow the nanofibrous membrane with excellent flexibility, but also induce oxygen vacancies which are responsible for easier adsorption of N2 on the ZrO2 surface. To improve the electrocatalytic activity, a facile SILAR approach is employed to decorate it with CdS quantum dots (QDs), thereby tuning its Fermi level. To improve the conductivity, a g-C3N4 nanolayer is further deposited which is both conductive and active. The resulting hierarchically structured, self-supported electrocatalyst, consisting of g-C3N4 encapsulated ZrO2 nanofibrous membrane decorated with CdS QDs, integrates the merits of the three components, and exhibits a remarkable synergy toward NRR. Excellent NH3 yield of 6.32 × 10–10 mol·s–1·cm–2 (–0.6 V vs. RHE) and Faradaic efficiency of 12.9% (–0.4 V vs. RHE) are attained in 0.1 M Na2SO4.
KEYWORDS ZrO2 nanofibrous membrane, CdS quantum dots, g-C3N4 nanolayer, self-supported electrocatalyst, NH3 synthesis
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Introduction
The importance of artificial NH3 synthesis through the N2 reduction reaction (NRR) has long been established since it lays the basis for modern fertilizer industries. Currently, NH3 is synthesized by the Haber–Bosch process under high-temperature and pressure conditions, which is energy-intensive and environment-unfriendly. To go out of this dilemma, some pioneering work turned to the electrocatalytic NRR to realize ambient NH3 synthesis [1–4]. The earliest electrocatalysts investigated were noble metals, such as Au [5–7], Pd [8], Ru [9–11], or their combinations (bi-metallic electrocatalysts) [12]. Recently, researchers’ focus shifted to more affordable electrocatalysts than noble metals, such as various nanostructures of transition metal compounds including carbide [13–15], nitride [16–18], oxide [19–21], and sulfide [22–24]. It was proposed that the empty d-orbital of these transition metals could withdraw electrons from the N2 molecules, favoring their adsorption and activation. Despite much progress in this aspect, it should
be noted that these transition metal electrocatalysts existed in the form of loose powders, which had to
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