Mechanochemical Activation of Nickel Oxide Enabling Binder-Free Anodes for Alkaline Water Electrolysis
Authors: Chatwani Mohit , Marcks Christian , Jain Adarsh , Mechler Anna K. , Vinayakumar Vineetha , Segets Doris 
Published: December 2024

In recent decades, the imperative for clean energy generation has grown significantly. Water electrolysis stands out as a sustainable method for hydrogen production due to its zero carbon emissions. However, high energy requirements and costs hinder its widespread adoption for large-scale applications [1]. Non-noble metal-based materials have emerged as cost-effective and efficient anodes for the oxygen evolution reaction (OER). However, the stability and performance of these materials heavily rely on the integration of ionomers or binders into the electrode coatings. For example, Nafion, renowned for its high ionic conductivity and ability to stabilize coating structures [2], has been a preferred choice [3]. Yet, their high cost and impending regulations concerning per- and polyfluoro alkyl substances (PFAS) by European agencies necessitate the exploration of alternative approaches. Here, we propose a solution involving binder-free anode coatings, achieved through mechano-chemical activation via planetary ball milling of commercially available catalyst materials. This innovative method offers a promising avenue for developing environmentally sustainable anodes with effective performance, circumventing the limitations associated with conventional binder materials.
In this study, nickel oxide (NiO, Merck group) was processed without binders through a modification of the powder material via planetary ball milling where parameters as bead size, rotation speed, and process time were varied. The initial visual contrast transformation from the green hue of the NiO feed to a dark grey or black shade appearance after ball milling which might indicate the presence of Ni2O3. Additionally, certain samples exhibited a needle-like morphology, with particle enlargement occurring selectively in instances involving larger beads during the milling process. These samples were discarded due to missing processability. Powder characterizations were conducted on both the as-purchased NiO feed (as reference) and the ball milled samples. Transmission electron microscopy (TEM) analysis confirmed reduced primary particle size across all ball milled samples as shown in the inset of Figure 1a-b. Energy dispersive X-ray (EDX) analysis indicated a decrease in the Ni metal content with a simultaneous increase in the oxygen content, while – in line with our expectations [4] – the reduced crystallinity of all NiO phases after ball milling was confirmed by X-ray diffraction (XRD) analysis. With regard to the surface species, Fourier-transform infrared (FTIR) spectroscopy evidenced a significant increase of hydroxide (OH) in the ball milled samples. At the same time X-ray photoelectron spectroscopy (XPS) data revealed an increase in the Ni3+ content, followed by a decrease in Ni2+ as analyzed from the Ni 2p spectrum. Combining both findings, we expect an improved anode performance due to the formation of Ni(OH)2 or NiOOH species [5]. Preliminary dispersion experiments were realized in different probe liquids [6] and subsequently the Hansen solubility parameters (HSP) were determined to achieve high-quality dispersions efficiently [7]. Because of the large particle size, all dispersions based on commercial NiO sedimented quickly, whereas the ball milled samples with significantly reduced primary particle size were stable in liquids like ethanol and isopropanol.This finding was quantified by transmittograms, where the sedimentation time, radial position and transmission are plotted together as shown in Figure 1a-b. Due to the inadequate dispersion stability of commercial NiO and thus missing processability, no electrodes were realized with this material. Finally, the ball milled samples were spray-coated onto Ni plate substrates without using binders. Scanning electron microscopy (SEM) images displayed diverse distributions of catalyst particles implying different coating structures and a range of morphologies. Electrochemical measurements at 100 mA cm-2 evidenced an improved activity of ~100 mV for the electrodes that were fabricated from the ball milled samples (Sample 1: 400 rpm, 4 h, Sample 2: 250 rpm, 2 h, constant bead size of 0.3 mm) during oxygen evolution compared to the bare Ni plate substrate as shown in Figure 1c. Within 2 h of testing, no delamination of the material was monitored evidencing its chemical and mechanical stability as a binder-free catalyst layer.
In conclusion, this study presents a promising approach to achieve binder-free anodes enabled by mechanochemical activation. The versatility of this method offers opportunities to optimize non-noble metal-based materials for enhanced OER performance, thereby advancing the prospects of sustainable energy generation.

References:

[1]H. Niu et al., Applied sciences (Basel), 2177 (2023).
[2] A. Jain et al., ChemCatChem, 16 (2024)
[3] G. Li, D. Yang, and P. A. Chuang, ACS catalysis, 11688–11698 (2018)
[4] X. Zhou et al., Advanced materials, 29 (2016)
[5] N. Akbari and M. Najafpour, Inorganic Chemistry, 62, 19107–19114 (2023)
[6] A. S. Amin et al., ChemCatChem, 16 (2024)
[7] S. Bapat and D. Segets, ACS Applied Nano Materials, 3, 7384–7391 (2020)

Figure 1