Difference between Metal-S and Metal-O Bond Orders: A Descriptor of Oxygen Evolution Activity for Isolated Metal Atom-Doped MoS2 Nanosheets

摘要

class="head no_bottom_margin" id="sec1title">IntroductionThe design of high-performance catalysts for energy conversion and storage is a high priority in light of the popular pursuit of sustainable energy (, , , , ). As a critical process, oxygen evolution reaction (OER) is usually the rate-limiting step for such energy conversions since the kinetics of OER are often unsatisfactory due to the four-electron transfer process, which usually requires a large overpotential to drive the reaction (, , , , , , , ). As a result, the large overpotential ineluctably results in low efficiency. Over the past few years, reports show that different noble-metal-based catalysts have been found to significantly reduce the overpotential of OER. However, these reported noble-metal based catalysts have been limited in their practical applicability owing to their high cost, which hinders the development of energy conversion devices (, , href="#bib38" rid="bib38" class=" bibr popnode">Yu et al., 2018, href="#bib12" rid="bib12" class=" bibr popnode">Guo et al., 2018).An ideal catalyst should possess both high activity and low cost. For this purpose, a great deal of research has been performed to seek highly active materials based on earth-abundant elements. Recently, supported monatomic catalysts have drawn increasing attention because of their high utilization of active sites and the resulting high performance of the OER (href="#bib21" rid="bib21" class=" bibr popnode">Li et al., 2018, href="#bib7" rid="bib7" class=" bibr popnode">Fei et al., 2018, href="#bib37" rid="bib37" class=" bibr popnode">Yan et al., 2018, href="#bib3" rid="bib3" class=" bibr popnode">Chen et al., 2017) and have been considered as promising alternative electrocatalysts for the OER. However, how to rapidly obtain these desired monatomic OER catalysts without tedious “trial and error” remains a problem. An easily obtainable descriptor is urgently required for the rapid screening of desirable high-performance materials (href="#bib16" rid="bib16" class=" bibr popnode">Huang et al., 2017, href="#bib17" rid="bib17" class=" bibr popnode">Jacobs et al., 2018, href="#bib34" rid="bib34" class=" bibr popnode">Wang et al., 2018a, href="#bib35" rid="bib35" class=" bibr popnode">Wang et al., 2018b, href="#bib22" rid="bib22" class=" bibr popnode">Lin et al., 2017, href="#bib6" rid="bib6" class=" bibr popnode">Fang et al., 2017, href="#bib11" rid="bib11" class=" bibr popnode">Gu et al., 2018). By means of a descriptor, we can rapidly evaluate OER activity since obtaining the descriptor value is usually much faster than calculating the Gibbs free energy via frequency calculations. This method would significantly shorten the calculation time and thus reduce the calculation cost. Before experimentally preparing the catalysts, we can first screen out the desired materials quickly based on the descriptor, avoiding tedious trial and error and improve research efficiency. Hence, we seek to better understand the correlation between electrochemical performance and chemical structure by means of high-throughput density functional theory (DFT) calculations owing to their successful applications in material screening and then obtain a credible descriptor of OER activity for surface monatomic materials to predict and design highly active monatomic catalysts, that is, the metal atoms-doped ultrathin MoS2 nanosheets (M-UMONs) detailed in this work. Fortunately, previously reported research provides us with a good reference. For examples, surface oxygen binding energy (href="#bib28" rid="bib28" class=" bibr popnode">Rossmeisl et al., 2007, href="#bib26" rid="bib26" class=" bibr popnode">Man et al., 2011), the enthalpy of a lower to higher oxide transition (href="#bib33" rid="bib33" class=" bibr popnode">Trasatti, 1980), and the 3d electron number (href="#bib2" rid="bib2" class=" bibr popnode">Bockris and Otagawa, 1984) are reported as successful OER activity descriptors. Although these proposed descriptors are not straightforward in their practical application (href="#bib31" rid="bib31" class=" bibr popnode">Suntivich et al., 2011a, href="#bib32" rid="bib32" class=" bibr popnode">Suntivich et al., 2011b), their exploration method and processes offer us much inspiration.In this work, the credibility of our calculations of theoretical overpotentials was first validated when we explored the descriptor based on the DFT calculations. For the validation purpose, more than ten kinds of M-UMONs were prepared and systematically characterized, and the electrochemical experimental results confirmed our calculations. Based on this, different kinds of M-UMONs were theoretically investigated by means of using high-throughput DFT calculations. Our calculation results demonstrated a volcanic-shaped correlation between the activity and metal-sulfur (M-S) bond orders. As the differences between the M-S bond orders and corresponding metal-oxygen (M-O) bond orders increased, the OER performance would decline, meaning that peak activity is predicted to occur when the bond order difference is minimal. More importantly, this successful exploration of the descriptor of OER performance for M-UMONs systems implies this calculation-based method for the exploring of descriptors would provide new promising avenues of research for the development of high-performance OER catalysts.