(4gj) Electrolyte-Modulated Electrodeposition of Co-Mo Catalysts for Enhanced Alkaline Hydrogen Evolution for Anion Exchange Membrane Water Electrolyzers

Introduction

Sustainable future covets innovative solutions for renewable energy conversion and storage. Hydrogen based technologies are pioneers in this endeavor owing to their lightweight, high combustion capacity, pollution-free, and cyclic utilization. Water electrolysis is the cleanest and most promising route to hydrogen production, with anion-exchange membrane alkaline water electrolysis (AEMWE) leading the charge. AEMWE's potential for significant cost reduction through non-precious metal catalysts makes it a frontrunner in sustainable hydrogen generation[1-2]. However, its widespread adoption is limited by cost effective active catalysts with good stability. A major challenge lies in the low activity of currently employed non-noble metal catalysts for the hydrogen evolution reaction (HER) in alkaline environments compared to their precious metal counterparts (e.g., Pt, Ru), these non-noble metal catalysts exhibit significantly lower HER activity[3]. This diminished activity acts as a bottleneck, hindering the overall efficiency of the water electrolysis process. To address this hurdle by fabricating highly active, non-precious metal HER catalysts through electrodeposition has been carried out. This method offers a simpler, more scalable, and potentially more cost-effective alternative to conventional fabrication techniques. The hydrogen evolution catalyst development has recently focused on the cobalt (Co) and molybdenum (Mo) based non-noble electrocatalysts owing to their favorable hydrogen adsorption energy[3]. Nevertheless, conventional fabrication methods such as solvothermal methods, or heating through the furnace, require long reaction times and harsh conditions[4]. The current study proposes a facile approach for the fabrication of active non-precious cobalt and molybdenum-based catalysts through a simple and scalable electrodeposition route and the effect of electrolytes on the deposition and activity.

Method of Fabrication

Cobalt -Molybdenum thin film layer was electrodeposited on acid-cleaned and pressed nickel foam (NF) substrate using a three-electrode setup with ammonium molybdate tetrahydrate and cobalt chloride hexahydrate as precursors. Co-electrolyte influence was studied using electrolytes containing alkali/alkaline earth metal halides (A: Li, Na, K, Be, Mg, Ca). The deposited CoMo thin film CoMo was characterized by XPS, Raman spectroscopy, ICP-AES, and SEM-EDX. The electrochemical hydrogen evolution activity of the deposited catalyst was analyzed via linear sweep voltammetry (LSV) after the Cyclic voltammetric activation. Charge transfer resistance and double-layer capacitance were measured from impedance spectroscopy and scan rate-dependent end non-faradaic CV cycles. Overall water splitting (OWS) and AEM-AWE were also performed by fabricating CoMo/NF as cathode and anode.

Result and Discussion

The study explores the electrodeposition from the metal precursor solution on the nickel foam substrate under ambient conditions. We investigated the influence of various electrodeposition parameters and their effect on the electrochemical activity and chemical properties. The composition ratio and deposition time play an important role in determining the activity of the catalyst. The introduction of molybdenum during electrodeposition demonstrated enhanced surface activity. The Co: Mo precursor ratio of 1:4 showed enhanced catalytic activity. Additionally, extended electrodeposition duration resulted increase in catalytically active surface microstructure area which led to improved catalytic activity. The effect of the various electrolytes on the precursor solution significantly impacted the electrodeposition potential as well as the hydrogen evolution activity of the catalyst. The electrolyte effect was studied in detail through various alkali and alkaline earth metal halide electrolytes. Notably among other co-electrolytes, the calcium chloride during the electrodeposition promoted the formation of the uniform electrodeposition and resulted in a superior HER catalytic activity of 96 mV at a current density of -10 mA/cm² (Figure 1). Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) clearly show a thin layer of uniform microstructure of the electrodeposited surface. Improved molybdenum inclusion was observed upon Calcium chloride co-electrolyte assisted electrodeposition as seen from ICP-AES. Mo: Co ratio increases from 0.16 to 0.24 upon calcium chloride-assisted electrodeposition. Cross-sectional SEM-EDX clearly shows the uniform thin film consisting of both cobalt and molybdenum. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy revealed an increase in Co²⁺ content (compared to Co³⁺) and the presence of Mo in a more electron-rich state within the catalyst. This rational design and synergistic functionality arise from the oxophilic Co(OH)â‚‚ component, which excels in water molecule adsorption and dissociation, and the Mo moiety, which promotes hydrogen generation. The overall water splitting setup constructed with CoMo/NF as cathode and anode showed a cell voltage as low as 1.58V and 1.79V for 10 and 100mA/cm² respectively.

Conclusion

In conclusion, we have successfully fabricated a thin film of cobalt and molybdenum over nickel foam through a simple electrodeposition method. Through meticulous optimization of the electrodeposition conditions, especially through the electrolyte variation we were able to alter the deposition mechanism, leading to tailored deposited structures and surface characteristics. This approach offers a promising avenue for the development of low-cost and scalable electrolyzer systems for future sustainable hydrogen production.

References

1. Gopinathan M Anilkumar, Takeo Yamaguchi, et al., J. Chem. Eng. Jpn., 56,1,2210195 (2023)
2. Gopinathan M Anilkumar, Takeo Yamaguchi et al., ACS Sustainable Chem. Eng., 11,25,9295 (2023)
3. Nørskov, J. K. et al., J. Phys. Chem. C., 114,42,18182 (2010)
4. Gopinathan M Anilkumar, Takeo Yamaguchi et al., ACS Sustainable Chem. Eng., 11,3,854 (2023)