Comparative Study of Cobalt Oxide and Cobalt-Molybdenum Oxide Electrodes for Seawater Electrolysis
Authors: Abdel-Galeil Mohamed , Takahashi Soma , Ito Seira , Kato Zenta 
Published: December 2024

INTRODUCTION
The quest for sustainable and efficient hydrogen production has led to the exploration of various catalyst materials for seawater electrolysis, a promising avenue for renewable energy generation. Seawater, abundant and accessible, offers a compelling resource for electrolytic hydrogen production, but its high salinity and corrosive nature pose challenges for catalyst stability and performance. The development of efficient catalysts capable of withstanding the harsh conditions of seawater while achieving high oxygen evolution efficiency (OEE) has captured the attention of researchers in this field.
The development of high activity/high stability/low-cost oxygen evolution reaction (OER) catalysts for large-scale production is an important issue for academia and the industry today [1]. Although RuO2 and IrO2 noble metal oxides are considered the most efficient OER electrocatalysts, their high costs, apparent scarcity, and low stability have greatly impeded their large-scale application as OER electrocatalysts [2]. In recent years, manganese, cobalt, molybdenum and other transition metals have emerged as promising candidates for catalyzing the OER in seawater electrolysis applications.
Cobalt oxide electrodes are well-known for their catalytic activity towards the OER and stability in alkaline medium specially when it modified with other transition metals such as Mo [3]. Some researchers use manganese as anodic catalysts for oxygen evolution from seawater or MnO2 decorated with Co3O4 yields the best performances in alkaline seawater splitting [4]. Also, these electrodes were applied on FTO surface in acid medium [5]. We consider testing cobalt (Co) and Co mixed with molybdenum are important challenge in acidic medium under high current density.
The present work aimed to (i) determine the optimum conditions for production of durable Co/Ti and Co-Mo/Ti anodes using a fast and easy fabricated method. (ii) focus on durability test of these electrodes under high current density without using intermediate conductive layer (iii) comparative analysis involves evaluating various aspects including catalytic activity, stability, structural properties, and OEE.
EXPERIMENTAL
First, the titanium substrate underwent a treatment for surface roughening. Subsequently, the treated titanium substrate was coated with cobalt oxide (Co3O4) through the calcination of cobalt nitrate hexahydrate (Co(NO3)2.6H2O) dissolved in a butanol solution, which was then brushed onto the substrate. Using the same method, a Co-Mo oxide was also prepared by calcinating butanol solutions of Co(NO3)2.6H2O and molybdenum chloride (MoCl5) at 450°C.
The fabricated electrodes were characterized by XRD and EPMA to examine the structural and elemental properties of these electrodes. Moreover, electrochemical testing was conducted to evaluate the catalytic activity and stability of the electrodes during seawater electrolysis. The performance of the electrode was examined by electrolysis of 0.5 M NaCl solution at 1000Am-2. The OEE was estimated by the difference between the total charge passed during electrolysis and the formation charge of chlorine analyzed by iodometric titration. Polarization curves were measured using multi electrochemical measurement system. Correction for iR drop was made with a current-interruption technique. Durability test was measured using DC power supply 30V/5A.
RESULTS AND DISCUSSION
This study compares the performance of cobalt oxide electrodes, a well-established material for seawater electrolysis, with cobalt-molybdenum oxide composite electrodes. SEM images show the mesoporous structure for Ti/Co-Mo electrodes which is suitable for various applications (Fig. 1). X-ray diffraction clarified that all fabricated electrode consisted of free Co3O4 on Ti plate and cobalt molybdenum oxide composite on Ti plate. EPMA indicate that the samples contain Co, Ti, O and/or Mo for the modified electrode.
Polarization curves and OEE analysis indicate that incorporating Mo to Co electrode enhances efficiency and reduces resistance. Additionally, the resulting electrode composition exhibits a porous structure expecting high performance in electrolysis. Mo addition led to a decrease in polarization potential, and the analysis of polarization curves showed that Mo addition increased the specific conductivity of the oxide. The value of the OEE is 18.98% for Co-Mo electrode compared to 13.15% for only Co electrode.
Durability tests were conducted to evaluate the stability of both electrodes at a current density of 1000 A/m² over an extended period. The results indicate that the Co-Mo electrode exhibits inferior stability for two weeks and higher OEE compared to Co electrode under the same operating conditions.

References

[1] Suen, N. et al., Chem. Soc. Rev. 46 (2017). 337.
[2] Liu, Z. et al., J. Mat. Chem. A 7 28 (2019a) 1676.
[3] Yang J., et al., Front. Mater. 9 (2022) 1089695.
[4] Bigiani L., et al., Appl. Catal. B: Environ. 284 (2021) 119684.
[5] Chau Ta X. M., et al., Small (2023) 2304650.

Figure 1