Design and Fabrication Three-Dimensional Structure Electrode and Its Application in NaBH4 Electrooxidation and H2O2 Electroreduction

The direct liquid feed borohydride–hydrogen peroxide fuel cell (DBHPFC), comprised of borohydride oxidation at the anode and H₂O₂ reduction at the cathode, has attracted increasing attentions recently due to its high energy density, compactness, capable of operating in air-free environment (space and underwater), and the easy storage and distribution of both the fuel and the oxidant^[1-2]. The main issues existing in DBHPFC are the hydrolysis and incomplete electrooxidation of borohydride at the anode resulting in evolution of H₂ and the chemical decomposition of H₂O₂ at the cathode leading to the formation of O₂. The releases of H₂ and O₂ not only reduce the energy density of DBHPFC, but also cause problems in the design and safety management of fuel cell systems^[3-4]. Another important issue is how to reduce the use of noble metals in both borohydride oxidation and H₂O₂reduction. Nowadays, nano-materials have attracted much attention due to their unique physical and chemistry properties. The unique properties of nano-materials in facilitating the mass transport, ion diffusion and electron transfer, thus dramatically boosting the electrochemical performance, suggests that the main challenge of DBHPFC could be resolved or greatly ameliorated. In general, self-supported nano-materials growing directly on a current-collecting substrate represent an amazing architecture. Such structures have been demonstrated to possess larger electrochemical active surface area, higher utilization efficiency of the active materials, and superior mass transport property than conventional electrodes fabricated by mixing and pressing powder of active material with conducting materials (e.g., carbon black) and polymer binders (e.g., polytetrafluorethylene). State of the art, Solid template and structure-directing agents are commonly used to fabricate materials with hierarchical structures. However, some problems and challenges still remain, since complete template removal is needed, which means a much more complicated process including the selection of appropriate solvent or calcination at elevated temperature. Furthermore, impurities can be introduced from the templates or agents that affect the properties adversely. Therefore, it is necessary to find a simple and feasible method for the preparation of electrodes with three-dimensional structure .

We reported a newly designed and fabricated electrode with three-dimensional structure^[5]. The electrode consists of a conducting nanoarray substrate and Pd nanoparticles. The substrate is an array of TiO₂ nanowires with a carbon coating layer prepared via a thermal evaporation method. Pd nanoparticles were electro-deposited on the substrate surfaces by the potentiostatic pulse method. The electrode exhibited excellent catalytic performance for NaBH₄ electro-oxidation. The current density for NaBH₄ electro-oxidation at the electrode (524 mA mg^-1) is about 5 times of that at conventional Pd/C (100 mA mg^-1). This enhanced performance is likely to be due to the improved mass transport of NaBH₄, good electronic conductivity and high Pd utilization of the electrode.

We alos reported a porous (Co, Mn)₃O₄ nanowires freely standing on Ni foam electrode, which are synthesized via a template-free growth method, followed by a thermal treatment in the air^[6]. Results show that thermal treatment leads to the conversion of solid nanowires of MnCO₃+CoCO₃ to porous nanowires of (Co, Mn)₃O₄ via decomposition and reconfiguration, which is identified to be the catalytic active component for H₂O₂ electroreduction. The nanowires calcined at 300 °C exhibit the highest activity for the H₂O₂ reduction and a current density of 329 mA cm^-2 is obtained in 3.0 mol dm^-3 KOH + 0.6 mol dm^-3 H₂O₂ at -0.4 V (vs. Ag/AgCl, KCl). The catalytic activity of (Co, Mn)₃O₄ nanowires is almost twice than that of Co₃O₄nanowires. The enhanced catalytic activity is explained by the decrease in the energy barrier of O-O bond cleavage caused by the introduction of Mn.

In conclusion, compared to conventional fuel cell electrodes fabricated by mixing active materials with conducting agents and polymer binders, this three-dimensional structure electrode directly grown on substrate has superior mass transport property, which combining with its low-cost and facile preparation, make it a promising electrode for DBHPFC.

References:

[1] D. Cao, D. Chen, J. Lan, G. Wang, J. Power Sources 190,346 (2009).

[2] D. Cao, Y. Gao, G. Wang, R. Miao, Y. Liu, Int. J. Hydrogen Energy 35,807 (2010).

[3] K. Cheng, F. Yang, Y. Xu, L. Cheng, Y. Bao, D. Cao, G. Wang, J. Power Sources 240,442 (2013).

[4] K. Cheng, D. Cao, F. Yang, D. Zhang, P. Yan, J. Yin, G. Wang, J. Power Sources 242,141 (2013).

[5] K. Cheng, D. Cao, F. Yang, L. Zhang, Y. Xu, G. Wang, J. Mater. Chem. , 22,850 (2012).

[6] K. Cheng, F. Yang, G. Wang, J. Yin, D. Cao, J. Mate. Chem. A, 1, 1669 (2013).