LIANG Fengbo, XIONG Hongxu, CHEN Zihao, GONG Qiqi, SUN Liangliang, LIU Lili

(Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China)

Extended Abstract:[Background and purpose] Hydrogen energy is regarded as an ideal energy carrier due to its high energy density and large combustion calorific value. Recently, the utilization of green hydrogen has become a hot topic of social concern as it can effectively achieve the sustainable developments known as "carbon neutrality". Among various energy conversion and transforming technologies, proton exchange membrane (PEM) water electrolysis for hydrogen production has several advantages, such as abundant raw materials, high hydrogen production facile operation and so on. Considering reforming reaction of fossil fuels with low output and harsh carbon emissions, water electrolysis via PEM technology meets sustainable goals for the production of green hydrogen. Unfortunately, the slow oxygen evolution reaction (OER) in the PEM electrolyze is the bottleneck of water electrolysis for hydrogen production. OER kinetics rate is of great significance to develop efficient and stable electrocatalysts.[Methods] Ruthenium dioxide, as one of the noble metal oxides, which is insoluble in water and acid, is widely used in chemical catalysts, capacitors and ruthenium resistance paste. In the OER reaction, the lower equilibrium potentials of Ir³⁺/Ir⁴⁺ and Ru³⁺/Ru⁴⁺ (0.93, 0.94 V) make them suitable for commercial oxygen evolution catalysts. The order of OER activity in acidic conditions is Ir~Ru>Pd>Rh>Pt>Au>Nb>Zr~Ti~Ta. RuO₂ and IrO₂ with excellent activity are also different. Specifically, RuO₂ has higher catalytic activity and is cheaper than IrO₂, but IrO₂ is more stable than RuO₂. It should be pointed out that the main reason for the low stability of Ru materials is the deactivation of RuO₂ in the acid reaction. In addition, high applied potential during operation leads to the conversion of RuO₂ into soluble RuO₄. Although high potential and high current density can increase the oxygen evolution, the accelerated dissolution of RuO₂ cannot be ignored. It is believed that this dissolution reaction occurs at voltages of above 1.3 V. Considering the theoretical oxygen evolution potential (1.23 V), the key problems are mainly that RuO₂ catalysts are effectively protected against dissolution reaction with the lowered catalytic overpotential. Recently, the single atom Ru-N-C catalyst and the Co single atom have been reported to be inserted into RuO₂ spheres, which showed excellent oxygen evolution catalytic performance and long-term stability. The result exhibited the potential of RuO₂ materials as oxygen evolution catalysts. To obtain a catalyst with high activity and stability, RuO₂@ IrO₂ composite catalyst was developed for efficient and fast water electrolysis in acid PEMFC. IrO₂ (5–10 nm) is deposited on the surface of nano RuO₂ by using the Adams melting method, while the morphology and catalytic performance of the catalyst were optimized by adjusting the ratio of the two components (8:2, 6:4, 5:5).[Results] Crystal structure, elemental composition and surface morphology of the materials were characterized by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical workstation. Compared with RuO₂ and IrO₂, the order of OER activity at 10 mA·cm⁻² is: RuO₂> RuO₂@IrO_2-1> RuO₂@IrO_2-2> RuO₂@IrO_2-3> IrO₂. However, RuO₂@IrO_2-1 and RuO₂@IrO_2-2 outperform RuO2 at 1.5 V, showing excellent catalytic performance at high current densities. The measured currents of the membrane electrodes prepared with the corresponding powders are 1.17 A and 1.25 A at 2.2 V. A small content of IrO2 (20 wt.%) can optimize the transition oxidation solution of RuO₂ during oxygen evolution. In stability test, RuO₂ has a potential attenuation of 54 mV under continuous 20 h testing, the value of RuO₂@IrO_2-1 is reduced to 29 mV. When the content of IrO₂ was further increased to 50 wt.%, the coating structure will be destroyed and excessive exposure to RuO₂ will accelerate the catalyst deactivation. A sea urchin-structured RuO₂@ IrO₂ catalyst was prepared by using this method. The loading of an appropriate content of IrO₂ can alleviate the excessive oxidation and dissolution of RuO₂ during the OER process, providing strong protection for RuO₂ and thereby enhancing the stability of the catalyst.[Conclusions] In summary, RuO₂ as IrO₂ carrier is in line with the low technology of PEM water electrolysis development direction of cost, high efficiency and long life. Especially after improving the preparation process and the content of precious metals, the material has excellent oxygen evolution performance and high stability, showing potential applications and long-term development in the field of PEM electrolysis cells.

Key words: PEMFC; oxygen evolution reaction; iridium/ruthenium oxide; catalytic activity