(131b) Understanding Activity and Stability of Earth-Abundant Transition Metal Thin-Film Catalysts for Integration with Photoelectrochemical Devices

The economic, large-scale production of green H₂ and other fuels is vital for a sustainable future energy economy. Photoelectrochemistry (PEC) represents an alternative strategy to directly synthesize H₂ and other important chemicals, relying on solar energy as a renewable power source. The main advantages compared to electrocatalysis is the possibility to combine both light absorption and energy conversion into one integrated device, reducing the amount of required auxiliary systems for storage and transport of either electricity, or chemicals. This can decrease the overall system costs, enable H₂ production on-site, on-demand, thereby largely bypassing issues related to H₂ transportation and storage, and offers the possibility for decentralization. For PEC to be implemented on an industrial scale, optimization is still required on several levels, though. The cost of both the device and H₂ produced by PEC need to be reduced, while the efficiency and durability of the system need substantial improvement.^[1][2] A major factor is hereby the catalyst. Highly efficient photoabsorbers have been developed for photovoltaic applications and are already today fabricated at a scale suitable for large-scale deployment. However, these absorbers require additional catalysts to improve the efficiency of the reaction, and protection layers preventing corrosion in contact with an aqueous electrolyte. These additional components need to be capable of a stable long-term operation under varying conditions, since outdoor operation will subject the device to changes in temperature, light intensity, and energy distribution of the light on a daily, if not hourly basis, which equals to changes in the chemical environment and the potential applied to the catalyst.^[3] Additionally, the different layers and materials need to be well matched to allow for efficient operation. To date, no low-cost, stable and efficient catalyst has been developed, that is capable of fulfilling all of these requirements, highlighting the necessity of both catalyst development and its coupling to a photoabsorber in an efficient, economic way. Tackling these challenges is therefore the focus of this presentation, which will focus on two catalyst systems: MoS₂, and Ni-Fe-sulfides, which are both based on earth-abundant elements – a prerequisite for economic large-scale fabrication. Important for a targeted catalyst development is thereby the characterization and correlation of material properties with activity and durability of the catalyst system under real-world conditions. It is crucial to extent ex-situ investigations to in-situ and operando experiments for a thorough understanding of material changes and their impact on activity and stability. This is in turn vital for addressing potential challenges related to these transformations, and for driving continous improvement and progress in the field.

MoS₂ is one of the most widely investigated transition metal chalcogenide catalysts for the hydrogen evolution reaction (HER), especially since the identification of active edge sites.^[4] This earth-abundant catalyst has been successfully integrated into photoelectrodes and demonstrated a dual function as catalyst and protection layer for state-of-the art absorber materials such as Si, and III-V semiconductors, allowing for stable operation over the course of weeks.^[5][6] While these results are promising, they have been achieved under constant illumination in a controlled environment, which does not represent the conditions of real-world application. Our studies show how MoS₂ is corroding under simulated diurnal conditions, and how corrosion is especially problematic at the open circuit potential, which represents the electrochemical conditions during nighttime. We vary the material properties of thin-films obtained via Mo sputtering followed by sulfidation to tune activity and stability, and correlate those metrics also using in-situ and operando techniques. Understanding the driving forces for durability in these thin-film catalysts, we then modify the design also using additional protection layer, to improve the stability in PEC devices.

While MoS₂ is a good HER catalyst in acidic media, it is unstable in alkaline media. Ni-Fe-sulfides on the other hand have shown very high activities in an alkaline environment. They can act as catalysts for both the HER, and the corresponding oxygen evolution reaction (OER), which makes them especially interesting for systems targeting water splitting.^[7][8] In contrast to MoS₂, that is expected to retain its 2D crystal structure, Ni-Fe-sulfides commonly undergo phase transitions.^[7][9] This makes it harder to identify the active species and reliably assess its durability, and may introduce cracks or defects that prevent a homogenous coverage protecting against electrolyte penetration. The activity can be highly promising, though, and stable operation of the active species at least at a constant potential has been demonstrated.^[7] This approach is thus complementary to that using MoS₂ and relies on the addition of protection layer if used in a PEC device. In our studies, we prepare Ni-Fe sulfide thin-film catalysts of varied compositions and demonstrate how crystalline sulfides can be obtained at low sulfidation temperatures (250 °C or less) out of Ni-Fe films, which makes them suitable for integration with photoabsorber materials. The crystal structure (pentlandite vs thiospinel) can be tuned via the sulfidation temperature, and the influence of material properties such as crystal phase, crystallinity, Ni-Fe ratio, and surface roughness are correlated to different activity and durability in these thin-film catalysts.

[1] K. Sivula, R. van de Krol, Nat. Rev. Mater. 2016, 1, 15010.

[2] M. T. Spitler, M. A. Modestino, T. G. Deutsch, C. X. Xiang, J. R. Durrant, D. V. Esposito, S. Haussener, S. Maldonado, I. D. Sharp, B. A. Parkinson, et al., Sustain. Energy Fuels 2020, 4, 985–995.

[3] K. M. K. Yap, S. A. Lee, T. A. Kistler, D. K. Collins, E. L. Warren, H. A. Atwater, T. F. Jaramillo, C. Xiang, A. C. Nielander, 2024, 1–11.

[4] T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science 2007, 317, 100–102.

[5] L. A. King, T. R. Hellstern, J. Park, R. Sinclair, T. F. Jaramillo, ACS Appl. Mater. Interfaces 2017, 9, 36792–36798.

[6] M. Ben-Naim, C. W. Aldridge, M. A. Steiner, R. J. Britto, A. C. Nielander, L. A. King, T. G. Deutsch, J. L. Young, T. F. Jaramillo, ACS Appl. Mater. Interfaces 2021, 14, 1–8.

[7] M. B. Z. Hegazy, J. Zander, M. Weiss, C. Simon, P. Gerschel, S. A. Sanden, M. Smialkowski, D. Tetzlaff, T. Kull, R. Marschall, et al., Small 2024, 2311627, 1–10.

[8] B. Konkena, K. J. Puring, I. Sinev, S. Piontek, O. Khavryuchenko, J. P. Dürholt, R. Schmid, H. Tüysüz, M. Muhler, W. Schuhmann, et al., Nat. Commun. 2016, 7, 12269.

[9] L. Wu, X. Shen, Z. Ji, J. Yuan, S. Yang, G. Zhu, L. Chen, L. Kong, H. Zhou, Adv. Funct. Mater. 2022, 33, 2208170.