Hydrothermal synthesis of Mo and Fe sulfides for the HER in PEM water electrolysis

Abstract

Hydrothermal synthesis of Mo and Fe sulfides for the HER in PEM water electrolysis Naomi Billieta,b, Dries De Slooverea,b, Mohammadhosein Safarib,c, Marlies K. Van Baela,b, An Hardya,b aUHasselt, Institute for Materials Research (IMO-Imomec) and Imec division Imomec, DESINe, Agoralaan, building D, 3590 Diepenbeek, Belgium. bEnergyVille, Thor Park 8320, 3600 Genk, Belgium. cUHasselt, Institute for Materials Research (IMO-Imomec) and Imec division Imomec, Electrochemical Engineering (EE), Wetenschapspark 1, 3590 Diepenbeek, Belgium.

Hydrogen gas (H2) has the potential to be a sustainable energy carrier, as it has a high energy density, does not release CO2, and is a feedstock chemical in various industries. Unfortunately, H2 is mainly produced from fossil fuels by steam methane reforming, naphtha reforming, and coal gasification.1,2 A promising alternative is water splitting through electrolysis, which produces H2 and O2 via the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively. As a major advantage, this method does not release CO2 or other volatile by-products.3 Commercial water electrolysis methods are alkaline electrolysis and PEM (proton exchange membrane) electrolysis. The advantages of PEM electrolysis are higher energy efficiency, quick response, and scalability.4 However, the used catalyst for the HER in PEM electrolysis is Pt, an expensive and low-abundance material. Therefore, this research aims to find earth-abundant alternatives to catalyze the HER. Molybdenum disulfide (MoS2) is a widely known alternative catalyst due to its high catalytic activity and high stability.5 Other sulfides such as Fe sulfides, were also found to be active as HER catalysts.6 This implies that a whole range of mixed metal sulfides may have a catalytic performance and stability that outperforms the more established materials. In this study, Mo and Fe sulfides were synthesized via similar hydrothermal methods. The variation of parameters such as temperature and reaction time, allowed the synthesis of particles with a wide range of morphologies. However, attempts to synthesize mixed metal (Mo,Fe) sulfides via similar routes resulted in the formation of distinct MoS2 and Fe sulfide phases. Instead, the synthesis of (Mo,Fe) sulfides required the preparation of an amorphous Keplerate-type phase as a synthetic intermediate. The synthesized materials were characterized using X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM) to investigate their morphology and crystal structure. The catalytic performances of the synthesized materials were compared with linear sweep voltammetry (LSV) using a three-electrode cell. Whatever the material and morphology, the quality of the electrocatalytic films was determining for its catalytic activity. Our solution-based synthesis strategies allow the synthesis of (mixed) metal sulfides in a range of different particle morphologies. Therefore, these strategies provide an ideal way to tune the electrocatalytic behavior of these materials as HER catalyst.

1. Agyekum, E. B., et al. (2022). In Membranes (Vol. 12, Issue 2). MDPI. https://doi.org/10.3390/membranes12020173 2. Das, A., et al. (2022). In Sustainability (Switzerland) (Vol. 14, Issue 18). MDPI. https://doi.org/10.3390/su141811206 3. Shiva Kumar, S., et al. (2019). Materials Science for Energy Technologies, March. https://doi.org/10.1016/j.mset.2019.03.002 4. Guo, Y. et al. (2019). IOP Conference series: Earth and Environmental Science. 371, p4. http://dx.doi.org/10.1088/1755-1315/371/4/042022 5. Li, R., et al. (2022). Chemical Communications, 58(14), 2259–2278. https://doi.org/10.1039/d1cc04004a 6. Heift, D. (2019). In Inorganics (Vol. 7, Issue 6). https://doi.org/10.3390/INORGANICS7060075