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.

In this work we provide a systematic study of the hydrogen adsorption structures and energetics for Fe, Co, Ni and Cu-doped 2 H basal plane and Mo- and S-edges at low H coverage conditions to clarify the precise effect of chemical modification of MoS2. For comparison with earlier work, additional calculations are performed for Pd and Pt dopants and for higher H coverages. All the systems are calculated using the same level of description and without structural constraints, which provides a unique set of data. For Mo we study the 0%, 50% and 100% sulfidized edges and for S the 50%, 75% and 100% ones. The edge structures are illustrated in Fig. 1, denoted hereafter Mo-X or S-X, where X indicates the degree of sulfur coverage in percents. We calculate relative substitutional energies (RSEs) to assess the affinities of doping at different edges and analyze the local structural changes. By using the calculated ΔGH as a descriptor and comparing the results with experiments, we discuss the suitability of doping in the various cases for improving the HER activity. Since the detailed structure-property relationship, directly or indirectly via ΔGH, i.e., [atomic and electronic structure of the surface] → ΔGH → [i0, exchange current density], is far from trivial, our results offer new interpretations, suggestions and trends for experimental synthesis and for further theoretical work. For facilitating fast prediction of ΔGH values (i.e., bypassing the DFT step), we illustrate a supervised machine learning (ML) model, which also informs about the importance of the structural features that determine the strength of H adsorption. Low H coverage is obtained in supercell calculations by adsorbing single H atoms on target areas. Studying this regime for the edge structures is consistent with the finding by Wang et al.7 for the Tafel slopes in their doped nanofilm experiments, which indicated that the rate-limiting HER step is the Volmer reaction, which corresponds to low H surface coverage. We will monitor ΔGH not only in reference to the optimal condition (ΔGH = 0 eV), but for description and classification consider also ΔGH’s that are found within a range of values (such as −0.5 eV < ΔGH < 0.5 eV). In this work we perform the calculations in electrically neutral supercells, but to assess the possible effects of non-neutral charge states, we also analyze explicitly two cases for an illustrative example: the doped basal plane and the undoped and Fe-doped S-100 edge in charge states +1, −1 and −2 of the supercell.

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