(Invited) Atomic Layer Deposited Sulfides As (Pre)Catalysts for Electrochemical Water Splitting
Authors: Mattinen Miika 
Published: December 2024

Electrocatalysis is projected to play a key role in building a sustainable world based on renewable energy. Electrochemical water splitting produces H2 on the cathode (hydrogen evolution reaction, HER), which can be used as a fuel and reactant in diverse industries. The oxygen evolution reaction (OER) producing O2 at the anode is a complex reaction that limits the overall efficiency of water electrolyzers. Active, stable, and affordable catalysts are required for both reactions. The range of potential catalysts has expanded from metals and oxides to others, sulfides having become one of the most actively studied material groups. Our work focuses on understanding inherent activity and structural changes under HER and OER conditions.
Different water electrolyzer technologies subject the catalysts to different operating conditions, which limits the available catalyst materials. For proton exchange membrane electrolyzers featuring an acidic environment, platinum group metals are currently used. However, several earth abundant catalysts have been identified for HER in acid. Among these, molybdenum sulfides in both amorphous (a-MoSx) and crystalline (c-MoS2) forms have attracted attention.1 We developed a plasma-enhanced ALD (PEALD) process that can deposit both amorphous MoSx with adjustable S/Mo ratio (2.8–4.7) and crystalline MoS2 with different crystallinity, morphology, and electrical properties.2 Electrochemical measurements in 0.5 M H2SO4 combined with ex situ and quasi in situ XPS revealed a-MoSx films to transform into highly active a-MoS2. C-MoS2 films were less active and did not show structural changes under HER conditions.3

Alkaline electrolyzers and anion exchange membrane electrolyzers operate under alkaline conditions and use earth abundant Fe/Co/Ni based catalysts. Most if not all materials based on these metals convert to oxyhydroxides under the highly oxidizing OER conditions and readily incorporate iron present in electrolytes.4,5 Thus, the as-deposited OER catalysts are best described as precatalysts. We developed new ALD processes to deposit NiSx and Co9S8,6,7 while oxide, metal, and hydroxide thin films were deposited for comparison using different methods. Extensive electrochemical measurements in 0.1 M KOH electrolyte with controlled Fe impurities combined with ex situ XPS revealed that in absence of iron all the Ni-based precatalysts convert to an OER-active oxyhydroxide, yet at substantially different rates (NiSx ≈ Ni(OH)2 >> Ni > NiO).8 However, good OER activity and stability was only achieved in the presence of Fe impurities. The highest amount of iron incorporation and the best OER performance was found for the NiSx and Ni(OH)2 precatalysts, which convert the fastest into the active oxyhydroxide form. Studies on Co-based OER precatalysts are currently ongoing, including use of operando characterization methods.
Our studies on ALD sulfides as both HER and OER catalysts showcase the importance of understanding catalyst structure and its changes under catalytic conditions. Armed with this knowledge and unique capabilities of ALD, catalysts with higher performance in electrolyzer applications can be designed and engineered.
References
[1] Mattinen, Leskelä, Ritala, Adv. Mater. Interfaces, 8, 2001677 (2021). [2] Mattinen...Kessels, Bol et al., Chem. Mater. 34, 7280 (2022). [3] Mattinen, Chen...Hensen, Kessels, Bol et al., in revision for ACS Catal. [4] Burke et al., Chem. Mater., 2015, 27, 7549. [5] Kawashima et al., Chem. Rev., 2023, 123, 12795. [6] Mattinen...Ritala et al., Dalton Trans. 50, 13264 (2021). [7] Mattinen...Ritala et al., manuscript. [8] Mattinen, Schröder...Jaramillo, Burke Stevens, Bent et al. manuscript.