Mechanistic Insights of Molecular Metal Polyselenides for Catalytic Hydrogen Generation

Molecular metal chalcogenides have attracted great attention as electrocatalysts for the hydrogen evolution reaction (HER). However, efficient utilisation of the active sites and catalytic performance modulation has been challenging. Here we explore the design of immobilized molecular molybdenum polyselenides [Mo 2 O 2 S 2 (Se 2 )(Se x )] 2– that exhibit efficient hydrogen evolution at low overpotential and stability over 1000 cycles. Density functional calculations provide evidence of a unimolecular mechanism in the HER process via the exploration of viable reaction pathways. The discussed findings are of a broad interest in the development of efficient molecular electrocatalytic materials.

Development of catalysts for the hydrogen evolution reaction (HER) is one of the key challenges in the realisation of a hydrogen economy based on electrocatalytic water splitting. The best performing HER catalyst is platinum, however its high cost and low natural abundance makes the development of noble-metal free catalysts an important area of research. 1 A variety of transition metal chalcogenides (ME x ) (M = Co, Fe, Ni, V, Nb, Ta, Mo, W; E = S, Se, Te) have been explored [2][3][4][5][6][7][8][9] in a quest for discovery of low production cost catalysts with high catalytic activity, and stability under aqueous corrosive conditions. Despite the extensive research conducted in this area, even the best in class chalcogenide, MoS 2 9-11 significantly underperforms as electrocatalyst for HER compared to Pt. This is due to the fact that most of the activity in MoS 2 comes from the edge sites which represent only a small fraction of the overall number of active sites in solid state materials like MoS 2 . Interestingly, molecular oxochalcogenides can incorporate by design a significant number of functional groups (which mimic very effectively the edge of solid-state chalcogenides) and have been shown to be at least as stable and effective as solid-state analogues. For example, the [Mo V 2 (S 2 ) 6 ] 2cluster contains three disulphides per molybdenum, 12 while the hybrid family of LMo VI O(S 2 ) 2 (where L = bidentate ligand) type catalysts exhibited an indicative cooperative electronic effect between the inorganic and organic components. [13][14][15] Therefore, when immobilised on an appropriate surface, they can effectively act as heterogeneous catalysts. Additionally, the design of the catalyst's key components (e.g. metal centres, type and number of chalcogens) can be pre-determined using a precise molecular chemistry approach. For example, the all-inorganic molybdenum-and tungsten-based sulphide catalysts has shown overpotentials as low as -114 ± 3 mV at 10 mA cm -2 . 16 The presence of well-defined molecular structures rendered the computational modelling more informative and reinforced our understanding of the HER mechanism. Theoretical calculations demonstrated that the reduction occurs on the disulphide ligand, while the terminal oxo ligand stabilise the negative charge developed during the catalytic process. Interestingly, investigation of other molecular chalcoxide equivalents [17][18][19][20] for the HER and exploration of the edge based catalytic sites has not previously been attempted.  In this work, we investigated the effects of stoichiometric and structural control resulting from the appropriate combination of chalcogen elements and their ratio (e.g. O 2-/Se 2 2-) within the coordination sphere of earth abundant transition metals and compare their mechanistic differences with their sulfur analogues.
The facile two-step synthetic procedure yields orange-red crystals and their single crystal X-ray diffraction (XRD) analysis shows that [(CH 3 ) 4 N] 2 [Mo 2 O 2 (µ-Se) 2 (Se 2 ) 2 ] 1 crystallises in the orthorhombic system space group Pca2 1 with unit cell dimensions of a = 18.614(10), b = 11.422(6), c = 11.422(6) Å. The powder X-ray diffraction study conducted on the freshly synthesised material 1 (Fig. S5) further confirm the purity of the synthesised material in the hexa-selenide form. Alternatively, the more challenging high temperature and pressure synthesis gives crystals containing a mixture of seleno-containing species with the formula [(CH 3 ) 4   In an effort to further characterize the isolated compounds, we employed high resolution electrospray ionization mass spectrometry (ESI-MS) [21][22][23] to determine the composition of the oxoselenometalate species in solution. The ESI-MS studies were performed in CH 3 CN. A series of singly charged distribution envelopes were observed, all in the region of 500 -1000 m/z (Fig. S4). More specifically, the distribution envelopes centred at 699.18 m/z value can be assigned to the intact compound [Mo 2 O 2 (µ-Se) 2 (Se 2 ) 2 ] 2-. Additionally, 77 Se NMR spectra of 1 recorded in DMF. The spectrum shows two distinct peaks corresponding to the two unique environments in the dimer located at shifts of 134 and 711 ppm relative to Me 2 Se (Fig. S6).
To investigate the HER activity of the molecular catalyst we immobilised the catalyst on a glassy carbon electrode (GCE) support. This allowed us to drop-cast a range of loadings on the electrode's surface and investigate the effects of the catalyst's concentration on the observed HER activity. The reported results have been normalised to the geometric area coverage of the material on the electrode, and all potentials are quoted versus the reversible hydrogen electrode (RHE). We investigated the electrocatalytic HER activity of our catalyst in 1 M H 2 SO 4 using a standard three electrode set-up (see SI). The scan rate for all linear sweep voltammetry (LSV) tests was 5 mV s -1 , and the obtained polarisation curves were iR corrected to compensate for any potential loss arising from external resistance of the electrochemical system. The amount of catalyst on the electrode surface was controlled by adjusting the loading concentration and volume. The performance of Please do not adjust margins Please do not adjust margins the catalyst improves as a function of the catalyst loading up to the value of 3.38 μmol cm -2 . Beyond this value the quality and mechanical stability of the produced electrocatalytic film degrades which leads either to similar or inferior performance due to detachment of the film from the electrode's surface. The results reported in Fig. 2 indicate that the cluster is highly active towards HER revealing a very small onset (40 mV) and low overpotential of - 165 3 mV (at j = 10 mA cm -2 ) as a function of the catalyst loading. ± Consequently, further increase of the applied overpotential results in a rapid rise of the cathodic current, which is indicative of HER activity, in marked contrast to the bare GCE with carbon powder which is relatively inert towards HER. An important prerequisite of functional components used for large scale applications eg. electrochemical water-splitting devices is the durability of the materials used. To assess this, we employed potential cycling as a simulation of the start-up / shutdown conditions found in an electrolyser to provide us with information on the catalysts' long-term stability. Using catalyst 1 (3.38 µmol cm -2 ), we applied an accelerated scan rate of 100 mV s -1 between a potential range of 0.10 to 0.25 V (V vs RHE) for 1000 cycles, Fig. 2b. At the end of the cycling, the catalyst showed negligible loss in cathodic current density, reaching an impressive density of j > 100 mA cm -2 below 250 mV. These results attest to the electrochemical stability of [Mo 2 O 2 (µ-Se) 2 (Se 2 ) 2 ] 2-1 and its potential use in electrochemical HER devices. Finally, to confirm that the reaction taking place on the working electrode corresponds to the hydrogen evolution reaction (HER), the evolved gas was analysed using gas chromatography (Fig. S8). As such, the faradaic efficiency of the molecular catalyst in 1 M H 2 SO 4 was determined to be 102.3 % ± 3.0 with a TOF of 0.019 s 1 at -190 mV demonstrating the evolution of hydrogen at full faradaic efficiency.
To investigate the underlying processes and reaction pathways, the electronic structure of these molecules and their reduced analogues were studied on the basis of density functional theoretical (DFT). The geometries of both anions were optimised starting from the single crystal X-ray diffraction data (see SI). The electronic structure of these anions is similar and according to orbital analysis and charge distribution, both ligands -bidentate κ 1,4 -Se 4 2and side on η 2 -Se 2 2--formally contribute -2 charges each. Mo centres present +5 formal oxidation state with a sigma Mo-Mo bond (2.855 Å) as corroborated by the bond valence sum calculations analyses and the crystal structure. Each perselenide moiety has a doubly filled π* 2 π* 2 group molecular orbital (MO) pair.
Examination of the HOMO-LUMO gap provides a preliminary good estimation of the chemical potential of the species and indication of orbitals populated by the incoming electrons. 0 e-and 1 e-reduction of catalytic species are depicted in Fig. S9. In case of 0e-, the energy gap is higher in [Mo 2 O 2 (µ-Se) 2 (Se 2 ) 2 ] 2than [Mo 2 O 2 (µ-Se) 2 (Se 2 )(Se 4 )] 2and follows the same trend as sulphur analogues of these compounds. However, the HOMO-LUMO gap and LUMO energy are smaller in selenium analogues compared to their respective sulphur anions (Figs. S9-S10). The first unoccupied MOs involve d metallic orbitals and an antibonding sigma Se-Se orbital. Interestingly, in case of 1 e-reduced species, the energy needed to insert the second electron is higher for Se compounds than S analogues (Figs. S9-S10). The half occupied orbital present sigma Se-Se antibonding character. In order to investigate the electrochemical steps that are taking place it is important to identify the different pathways following the two-fold reduction of the molecular catalysts leading to protonation equilibria. Therefore, the stability of all possible isomeric forms was evaluated. Even though those unprotonated and short-lived species may not be detected and characterised in solution, evaluating their relevant stability is relevant as it can serve as a reference state for subsequent steps. Considering the symmetric structure of the [Mo 2 O 2 (µ-Se) 2 (Se 2 ) 2 ] 2complex, we explored a series of isomers that can be formed during the two-variable proton/electron addition leading to the global panorama of possible HER equilibria (Fig. 3). The cleavage of perselenide bond upon 1eand 2ereduction was observed. It is expected based on molecular orbital analysis, because these anions feature antibonding sigma Se-Se orbitals which might in principle be occupied by incoming electrons inducing the breaking of Se-Se bond. This behaviour was also observed in the sulphur analogues of these anions. The complete nomenclature of the obtained isomers and proton affinities are listed in the Figs. S12-S19. For compound 1, the most stable doubly reduced and single-protonated species is B1 since it stabilises the mitigation of charge developed in the newly formed diselenol species. Consequently, the second protonation step which derives from B1 is BB1, which exhibits a free energy value of -40.7 kcal mol -1 (see Fig. S12). Taking into consideration the proximity of the neighbouring hydrogen atoms in this structure, that raised the possibility of the existence of a Tafel-type re-combination step. Further examination of this step for both anions revealed activation energies of 62 kcal mol -1 from double-protonated BB1 (Fig. S21 and S26) which are too high to be operative at room temperature. To this extend, departing from BA1 whereas one sulphur and one oxygen atom are protonated, Taffel reaction activation energy is more favourable with an activation energy of 37.45 kcal mol -1 (Fig. S22). In a similar manner, we aimed to examine the Heyrovsky step departing from single-and double-protonated [H 2 Mo 2 O 2 (µ-Se) 2 (Se 2 )(Se x )] 2-(x = 2, 4) structures. This step is thermodynamically favourable across the range of molecular systems with calculated released energies within -46.12 to -4.51 kcal mol -1 (Fig. 4, S20, S23-S25). The activation energy for this step is in range of +2.54 to +18.62 kcal mol -1 for both anions and all reactant complexes taken into consideration. These E a values indicate that activation can happen at room temperature conditions. For compound 1, BB1 intermediate [H 2 Mo 2 O 2 (µ-Se) 2 (Se 2 ) 2 ] 2-, the obtained transition state has a relative free energy value ΔG ‡ =+18.62 kcal ‧ mol -1 (Fig. S20) which is comparable to the activation energy value obtained for compound 2 when the protons interact with the side on η 2 -Se 2 2terminal group (BB1, Fig. S24, ΔG ‡ =+16.52 kcal ‧ mol -1 ). Interestingly, when the interaction takes place on the bidentate κ 1,4 -Se 4 2terminal site the calculated activation energy and transition state for [H 2 Mo 2 O 2 (µ-Se) 2 (Se 2 )(Se 4 )] 2-(BB3) was found to be lower (ΔG ‡ =+14.4 kcal‧mol -1 (Fig. S25). It is worth noting that single-protonated forms have significantly lower activation energies than the double-protonated ones. The activation barriers for compounds 1 (B1) and 2 are only 2.54 (Fig. 4) and 5.18 kcal‧mol -1 (Fig. S23), respectively. Thus, the model of the transition state for single protonated forms is the most probable candidate. For the non-reduced, single-and double-reduced species, their double-protonated forms are the most thermodynamically stable. Considering electrochemical routes, two electrons addition to double-protonated non-reduced species is non-favourable. In contrast, reduction reactions from single protonated and nonprotonated forms are more feasible (Fig. 3). Thus, our results clearly point towards the Heyrovsky mechanism based on thermodynamic favourability and the high activation energies observed for the alternative Taffel step.
In conclusion, a more accessible and high-yield synthesis for the selenium analogue to the well-known [Mo 2 O 2 S 2 (S 2 ) 2 ] 2structure makes it possible to explore the electronic and chemical properties of the molecule, which have previously remained unexplored. The performance of the molecular catalyst for the hydrogen evolution reaction was evaluated while we propose viable mechanistic pathways in relation to the interaction between catalysts and substrate while we identified intriguing differences with their sulfur analogues. The initial 1ereduction is easier compared to the sulphur containing analogue however the second reduction is more energetically demanding which is reflected to the observed slightly elevated Tafel slope. This indicates the significant electronic changes of the selenium containing structure relative to the sulphur. Notably, these results represent an attractive strategy for preparing next generation HER electrocatalysts. Understanding the electronic underpinnings of trends in activity as a function of the metal or chalcogen incorporated in the molecular species, can help inform the design of new catalysts and exploitation of cooperative and structural effects. Moreover, the development of facile low-cost synthesis of a molecular catalyst is highly desirable for the easier scale up in large scale applications.

Conflicts of interest
There are no conflicts to declare.