Transition Metal Sulfide-Based Electrocatalysts for Hydrogen Evolution Reaction in Alkaline Environment
Authors: Mendonça Inocêncio Carlos Victor Mendonça , Morais Claudia , Kokoh Boniface 
Published: July 2021

Hydrogen already plays a key role in many industrial processes and the interest in this compound has recently increased since it is pointed-out as a promising future energy carrier. Besides to present a high energy yield, no greenhouse gas is produced during its oxidation. However, most of current hydrogen production still comes from fossil feedstock and, due to environmental concerns, methods aiming the renewable, cleaner and economically suitable production of hydrogen are being developed.
Hydrogen production from water, in a process called “water electrolysis”, has been receiving great attention because if it is coupled to a renewable source of energy, it results in zero carbon emission. In this process, water is splitted to originate two gases, hydrogen at the cathode and oxygen at the anode, both containing high purity. The reactions responsible for their production in each half-cell are called hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively.
The use of catalysts is compulsory in order to minimize the overpotential of aforementioned reactions and, consequently, increase process efficiency. Noble metal-based materials, mainly Pt, Ru and Ir, remain as benchmark to catalyze these reactions, but their high-cost and scarcity are responsible for limiting their application in large scale. In order to overcome these barriers, new candidate catalysts must be searched and they need to be active, stable, cheap and sustainable.
Nørskov et al. [1] presented one important, but not unique, indicator for highly active HER catalysts, the free energy of H adsorption (∆GH
0) on the sites; the closer to zero, the better. They also brought attention to the similar sulfur coordination to metals in both MoS2 edges and active sites of nitrogenase, an efficient hydrogen evolution enzyme. Thus, authors stated that edge sites of MoS2 materials are active since they attend thermoneutrality criterion, although their basal planes are inert. Later, it was experimentally confirmed by Jaramillo et al. [2]. As consequence, further works have also shown good activity of other transition metal sulfides (TMS) towards HER and, therefore, it has become a promising class of materials to replace the expensive Pt-based electrocatalysts.
Noteworthy, Nørskov et al. [1] performed calculations that take into account an acidic environment and, for this reason, only ∆GH
0 descriptor was used to estimate HER activity. However, HER in alkaline medium brings an additional step of water dissociation, responsible to generate the proton which is further reduced to adsorbed hydrogen specie; such step may increase energy consumption of reaction. Furthermore, lower binding energy of water compared to hydronium and the interaction of hydroxyl groups with the catalytic sites, abundant in high pH’s, may play a key role in catalysts’ activity. As result, the mechanism of HER in alkaline electrolyte is more complex. Despite that, the alkaline environment allows the application of wider range of materials, including non-noble metal-based ones, in HER and OER. This, summed up to new developments in alkaline exchange membranes, make it worth to search for materials which may be applied in alkaline exchange membrane water electrolyzers (AEMWE).
In the present work, by applying the synthesis reported by Li et al. [3] with modifications, several TMS were produced by hydrothermal method employing sodium diethyldithiocarbamate (C5H10NS2Na), non-conventional precursor, as sulfur source. Adjusts in synthesis parameters were performed aiming the development of earth-abundant HER electrocatalysts to be applied in AEMWE devices. As preliminary results, 225 and 50 mV overpotentials were measured at -10 and -100 mA cm-2, respectively on HER polarization curves obtained in 1 M KOH electrolyte on MoSx-based cathodes, compared to a commercial 40% Pt/C material as reference. Doping strategies with M-M’ TMS synthesis are being implemented for improving HER efficiency.

Acknowledgements: This work was mainly conducted within the framework of a PhD thesis financially supported by the European Union (ERDF), GrandAngoulême and Région Nouvelle-Aquitaine. References

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F. Jaramillo, K.P. Jørgensen, J. Bonde, J.H. Nielsen, S. Horch, I. Chorkendorff, Science 317 (2007) 100-102.

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