Unveiling a Binder-Free Approach through Plasma Treatment of NiCoO2 Anode Multi-Layer Coatings for Alkaline Water Electrolysis
Authors: Chatwani Mohit , Marcks Christian , Jain Adarsh , Wagner Timo , Wöhrl Nicolas , Mechler Anna K. , Vinayakumar Vineetha , Segets Doris 
Published: December 2024

Over the past decades, the challenge of sustainable energy production at low cost has persisted. Hydrogen emerges as a viable alternative fuel for various applications, yet its utilization faces hurdles. This particularly concerns the sluggish kinetics of the oxygen evolution reaction (OER) at the anode, necessitating higher input energy than theoretically optimal. Apart from an active electrocatalyst, the structure of the anode coating significantly influences performance [1-2-3]. Typically, fluorinated polymer compounds serve as a binder/ionomer, enhancing ionic transport during OER [4]. However, its exorbitant price and compliance with European Union regulations banning per- and polyfluoro substances (PFAS) poses challenges. One solution is to develop binder-free anodes. In this study, we explore the effectiveness of a nitrogen plasma treatment (PT) on catalyst-coated nickel plates with varying mass loadings to achieve binder-free and efficient OER performance.
As a starting point, we utilized commercial nickel-cobalt-oxide (NiCoO2, Merck group), featuring processable primary particle sizes < 150 nm. Characterization of the powder involved various techniques including X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Next, Hansen solubility parameters were recorded to select the suitable dispersant and an ink was prepared using an ethanol-water mixture. Ink stability was demonstrated through analytical centrifugation measurements analyzed by transmittograms [5]. Stable inks passing this quality gate were spray coated onto Ni metal plates with varying layer numbers of 40 (anode 40L), 80 (anode 80L), and 120 (anode 120 L), corresponding to mass loadings of 375 µg cm-2, 750 ug cm-2 and 1125 ug cm-2, respectively. Noteworthily, anode 40L exhibited incomplete catalyst coverage with visible substrate whereas anode 80L and anode 120L displayed a fully covered substrate. As these layers suffered from mechanical instability due to delamination, we added post-treatment by nitrogen plasma. All anodes were analyzed before and after plasma treatment in terms of their morphology, physico-chemical characteristics, and adhesion properties. The mechanical properties of these coatings were evaluated using an adhesion analyser, revealing enhanced adhesion strength for plasma treated samples compared to their as-prepared counterparts. Scanning electron microscopy (SEM) analysis exhibited notable morphological alterations in the plasma-treated samples as illustrated in Figure 1a-c. Two distinct morphologies were observed. In case of anode 40L with plasma treatment (40L PT), the sample displayed incomplete coverage and exhibited spherical clusters of agglomerates. In contrast, the fully coated anodes 80L PT and 120L PT showcased enhanced coverage and distinct agglomerates. Besides these morphological observations, energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS) showcased a notable increase in the oxygen to metal ratio in anode 40L PT compared to anodes 80L PT and 120L PT. We attribute this to the synergistic interaction between the N2 plasma and the Ni plate substrate. This holds particularly for anode 40L PT, due to its low catalyst density and exposed Ni surface from the substrate which might have potentially induced the presence of oxygen molecules on the electrode surface. Finally, electrochemical measurements showed similar activity for the fully covered anodes 80L PT and 120L PT as shown in Figure 1d. This was expected due to very similar morphologies and surface properties evidenced by the chemical analysis. This indicates that higher mass loading does not increase the OER performance after PT. In contrast, anode 40L PT exhibited clearly superior performance with an overpotential value of 373 ± 3 mV. This can be potentially attributed to the higher abundance of oxides at the surface, which might have increased the amount of active sites in the anode.
Our study highlights the potential of binder-free anodes for enhancing the efficiency of the OER in hydrogen production. By employing commercial NiCoO2 powder as an electrocatalyst, coated on nickel substrates and finally utilizing a post-nitrogen plasma treatment, we achieved optimization of properties among the different layers, resulting in improved OER performance. These findings provide valuable insights into advancing the development of efficient and sustainable energy production technologies, thereby facilitating progress towards binder-free innovations in the field of electrocatalysis.

References:

[1] M.-I. Jamesh and M. Harb, Journal of Energy Chemistry, 56, 299–342 (2021)
[2] V. Vinayakumar et al., ECS Meeting Abstracts, MA2023-02, 2846–2846 (2023)
[3] A. Jain et al., ChemCatChem, 16 (2024)
[4] G. Li, D. Yang, and P. A. Chuang, ACS catalysis, 11688–11698 (2018)
[5] S. Bapat and D. Segets, ACS Applied Nano Materials, 3, 7384–7391 (2020)

Figure 1