Hydrogen adsorption on doped MoS2 nanostructures

Abstract

Electrochemical devices for efficient production of hydrogen as energy carrier rely still largely on rare platinum group metal catalysts. Chemically and structurally modified metal dichalcogenide MoS2 is a promising substitute for these critical raw materials at the cathode side where the hydrogen evolution reaction takes place. For precise understanding of structure and hydrogen adsorption characteristics in chemically modified MoS2 nanostructures, we perform comprehensive density functional theory calculations on transition metal (Fe, Co, Ni, Cu) doping at the experimentally relevant MoS2 surfaces at substitutional Mo-sites. Clear benefits of doping the basal plane are found, whereas at the Mo- and S-edges complex modifications at the whole edge are observed. New insight into doping-enhanced activity is obtained and guidance is given for further experiments. We study a machine learning model to facilitate the screening of suitable structures and find a promising level of prediction accuracy with minimal structural input.

Introduction

The concept of hydrogen economy comprises the idea to produce, store, distribute and use hydrogen as renewable fuel1. In this technology hydrogen can be cleanly produced by electrolytic splitting of water to hydrogen and oxygen if the process is powered by renewable energy sources1,2. However, the water-splitting process relies currently on catalysts comprised of platinum group metals (PGMs), which are considered as critical raw materials in terms of supply3. The metal dichalcogenide MoS2 has been suggested experimentally and theoretically as a promising candidate to replace the PGMs for the hydrogen evolution reaction (HER) at the cathode side2,4. The recent steps in the development (see, for example5,6,7,8,9,10,) have been to modify it structurally, e.g., by synthesizing various types of nanostructures and chemically, e.g., by doping, which are both procedures to maximize the area of the active surface/edge configurations and sites to obtain optimal HER performance. For guiding and supporting the experimental search of replacement materials, detailed theoretical information on the chemically and structurally modified nanostructures is essential. The Gibbs free energy of adsorption ΔGH for the reaction intermediate, i.e., hydrogen at the electrode surface, has been a widely used descriptor for predicting catalytic performance based on experimental correlations and mathematical models (Refs11,12 and references therein). It has been used for various transition metal dichalcogenides and doped MoS2previously7,13,14.

Synthesized MoS2 nanostructures have differently S-covered edges at various proportions, lengths and distributions depending on the preparation method6,15,16. The structures can also contain less regular parts such as defects and terrasses. Importantly, each geometrically and chemically different part may correspond to specific HER efficiency. The undoped, pristine basal plane of 2H-MoS2 is understood to be inactive4,13,17. Several theoretical studies have been devoted to the pristine Mo and S-edges of MoS2 in terms of ΔGH. Especially the Mo-edges are considered as active: the 100% S-covered Mo-edge of nanoclusters6 and the 50% S-covered Mo-edge in industrial-style catalysts18. Regarding modification with doping, Kibsgaard et al.6studied Fe, Co, Ni and Cu and obtained truncated triangle-shaped nanoclusters, finding Ni the best and Co the second best for promoting HER activity. In their clusters doping itself changes the morphology of the cluster (the relative linear lengths of the Mo- and S-edges) and thereby the activity. Šarić et al.19 studied by density functional theory (DFT) calculations the corresponding Co-doped nanoclusters. Escalera-Lopez et al.20 reported Ni-MoS2 hybrid nanoclusters which showed a roughly 3-fold increase in exchange current density compared with undoped nanoclusters. They associated the findings to Ni-doped Mo-edge and S-edge sites. Deng et al.14 performed experiments on the doped basal plane of MoS2 and found the trend for HER activity as Pt (highest) > Co > Ni as dopants. They found a similar trend in their DFT calculations for various dopants. Li et al.21 studied single Pt atomic structure and dynamics in monolayer MoS2 experimentally and by DFT calculations. Dai et al.22 reported enhanced electrocatalytic properties for Co-doped MoS2 nanosheets and attributed the finding to doping at the Mo and S edges. Wang et al.7 reported DFT calculations for ΔGH of Mo- and S-edges for pristine and TM-doped (Fe, Co, Ni, Cu) MoS2. They also synthesized and characterized doped vertically aligned nanofilms which expose alternatingly infinite Mo- and S-edges. Their results for the doped S-edge suggested enhanced catalytic activity as close to optimal (ΔGH = 0 eV) values of hydrogen adsorption were found compared to the undoped edge. Finally, doped (Fe, Co, Ni) amorphous MoS2 was studied by Morales et al.5.

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