CoSex and CoSex/Reduced GO Composites as Electrocatalysts for Water Splitting
Authors: Collins Gillian 
Published: August 2024

Hydrogen stands as an attractive energy source, offering a way to meet increasing energy demands without the reliance on fossil fuels and thus harming the environment. Renewable energy sources such as wind, wave and solar power can be used to generate inexpensive electricity, which we can then use to power water electrolysis.1 The generated hydrogen can then be converted back into electricity by using fuel cells, leaving only water as a by-product, rendering this technology clean and devoid of harmful emissions. The electrolysis cell can be operated at normal room temperatures without the need for high pressure rectors. At present, the practical application of hydrogen production using electrochemical water splitting is hindered by electrocatalyst limitations. Currently, the best electrocatalytic materials for water splitting are Platinum-based for the hydrogen evolution reaction (HER) and Ruthenium-based for the oxygen evolution reaction (OER). These materials are expensive and have limited reserves, 2 and therefore cost-effective alternatives must be developed. Transition metal dichalcogenides (TMDs) are of particular interest due to their tuneable structures. 3,4

Herein we have chosen to investigate CoSe2 as a potential electrocatalyst for overall water splitting (OWS). We describe both the electrochemical and solvothermal synthesis of CoSex. The CoSex was then combined with a carbon substrate to create a total of four electrocatalysts. Reduced graphene oxide (rGO) was employed as the carbon substrate and was synthesised via modified Hummer’s method, drop casted on to a GCE and electrochemically reduced to create a rGO modified GCE. The electrodeposited CoSex/rGO composite (ED-CoSex/rGO) was prepared by immersing a rGO modified GCE in an electrodeposition solution and deposited by holding the electrode at a fixed potential for a period of time (Figure 1). The solvothermal/rGO composite ST-CoSex/rGO was prepared by drop casting the solvothermally prepared CoSex ink on top of a rGO modified GCE. While most of the investigation on these electrocatalysts was done using simple glassy carbon electrode (GCE), the best performing material was then screened on a more porous electrode such as Nickel Foam (NF).
Materials were characterised using a combination of HR-SEM, XRD, Raman and XPS, while cyclic voltammetry, linear sweep voltammetry and chronoamperometry were employed to examine catalytic activity of the materials. SEM unveiled distinct morphologies, with the electrochemically formed CoSex appearing as flower-like deposits, while sheet-like morphology was seen with the solvothermal synthesis. The solvothermally produced CoSex was more crystalline, as evidenced from XRD. Despite the morphological differences, both CoSex variants exhibited commendable electrochemical attributes.
Materials also showed good stability over a period of 18 hours with the ED-CoSex material showing the best performance over 18 hours. Interestingly, all materials showed improved HER performance when subsequently tested after 18 hours via polarisation curves. All materials exhibited high electrochemical active surface areas (ECSA), stability, and favourable low overpotentials for both the hydrogen evolution reaction (HER) and the Oxygen evolution (OER) in alkaline media, however. It is worth noting that while the introduction of rGO significantly lowers the overpotential of electrodeposited and solvothermally synthesised materials, following chronoamperometry, it appears the beneficial effect of the rGO addition is less evident. These results underscore the potential efficacy of CoSex and CoSex composites in facilitating efficient water splitting for hydrogen production, with the potential to rival the performance of platinum-based electrocatalysts. The significance of this research lies in offering sustainable alternatives to platinum in electrolysers, a crucial step toward establishing a more accessible and eco-friendly hydrogen economy.
References
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2015, 8 (7). https://doi.org/10.1039/c5ee01283j.
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(3) Minadakis, M. P.; Tagmatarchis, N. Exfoliated Transition Metal Dichalcogenide-Based Electrocatalysts for Oxygen Evolution Reaction. Advanced Sustainable Systems. 2023. https://doi.org/10.1002/adsu.202300193.
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Figure 1