Hanslin Sander Ø, Jónsson Hannes, Akola Jaakko
Department of Physics, Norwegian University of Science and Technology, No-7491, Trondheim, Norway.
Science Institute and Faculty of Physical Sciences, University of Iceland, IS-107 Reykjavík, Iceland.
Phys Chem Chem Phys. 2023 Dec 6;25(47):32541-32548. doi: 10.1039/d3cp04198k.
A grand-canonical approach is employed to calculate the voltage-dependent activation energy and estimate the kinetics of the hydrogen evolution reaction (HER) on intrinsic sites of MoS, including edges of varying S-coverage as well as S-vacancies on the basal plane. Certain edge configurations are found to be vastly more active than others, namely S-deficient edges on the Mo-termination where, in the fully S-depleted case, HER can proceed with activation energy below 0.5 eV at an electrode potential of 0 V SHE. There is a clear distinction between the performance of Mo-rich and S-rich adsorption sites, as HER at the latter sites is characterized by large (generally above 1.5 eV) Heyrovsky and Tafel energy barriers despite near-thermoneutral hydrogen adsorption energy. Thus, exposing Mo-atoms on the edges to which hydrogen can directly bind is crucial for efficient hydrogen evolution. While S-vacancies on the basal plane do expose Mo-rich sites, the energy barriers are still significant due to high coordination of the Mo atoms. Kinetic modelling based on the voltage-dependent reaction energetics gives a theoretical overpotential of 0.25 V and 1.09 V for the Mo-edge with no S atoms and the weakly sulfur-deficient (2% S-vacancies) basal plane, respectively, with Volmer-Heyrovsky being the dominant pathway. These values coincide well with reported experimentally measured values of the overpotential for the edges and basal plane. For the partly Mo-exposed edges, the calculated overpotential is 0.6-0.7 V while edges with only S-sites give overpotential exceeding that of the basal plane. These results show that the overpotential systematically decreases with increased sulfur-deficiency and reduced Mo-coordination. The fundamental difference between Mo- and S-rich sites suggests that catalyst design of transition metal dichalcogenides should be focused on facilitating and modifying the metal sites, rather than activating the chalcogen sites.
采用巨正则系综方法来计算与电压相关的活化能，并估算在MoS本征位点上析氢反应（HER）的动力学，这些本征位点包括具有不同S覆盖率的边缘以及基面的S空位。发现某些边缘构型比其他构型活性高得多，即在Mo端的S缺陷边缘，在完全S耗尽的情况下，析氢反应在0 V（标准氢电极）的电极电位下，活化能低于0.5 eV时即可进行。富Mo和富S吸附位点的性能存在明显差异，因为后者位点的析氢反应具有较大的（通常高于1.5 eV）海洛夫斯基和塔菲尔能垒，尽管氢吸附能接近热中性。因此，使边缘上的Mo原子暴露以便氢能够直接结合对于高效析氢至关重要。虽然基面的S空位确实暴露了富Mo位点，但由于Mo原子的高配位，能垒仍然很大。基于与电压相关的反应能量学的动力学模型分别给出了无S原子的Mo边缘和弱硫缺陷（2% S空位）基面的理论过电位为0.25 V和1.09 V，其中Volmer - 海洛夫斯基反应是主要途径。这些值与报道的边缘和基面过电位的实验测量值非常吻合。对于部分Mo暴露的边缘，计算得到的过电位为0.6 - 0.7 V，而仅含S位点的边缘给出的过电位超过基面。这些结果表明，过电位随着硫缺陷增加和Mo配位减少而系统地降低。富Mo和富S位点之间的根本差异表明，过渡金属二硫属化物的催化剂设计应侧重于促进和修饰金属位点，而不是激活硫属元素位点。
