Three-dimensional porous electrodes for direct formate fuel cells

The dual-layer electrode for fuel cells is typically prepared by binding discrete catalyst nanoparticles onto a diffusion layer. Such a random packing forms a dense catalyst layer and thus creates a barrier for mass/ion transport, particularly for direct liquid fuel cells. Three-dimensional porous electrodes, a thin nano-porous catalyst layer uniformly distributed on the matrix surface of a foam-like structure, are typically employed to improve the mass/ion transport. Such a three-dimensional porous structure brings two critical advantages:（i） reduced mass/ion transport resistance for the delivery of the reactants via shortening the transport distance and（ii） enlarged electrochemical surface area, via reducing the dead pores, isolated particles and severe aggregations,for interfacial reactions. Moreover, the three-dimensional design is capable of fabricating binder-free electrodes, thereby eliminating the use of ionomers/binders and simplifying the fabrication process. In this work, three types of three-dimensional porous electrode are fabricated, via different preparation methods, for direct formate fuel cells:（i） Pd/C nanoparticles coating on the nickel foam matrix surface（Pd-C/NF） via a dip-coating method,（ii） Pd nanoparticles depositing on the nickel foam matrix surface（Pd/NF） via reduction reaction deposition, and（iii） Pd nanoparticles embedding in the nickel foam matrix(Pd/（in）NF)via replacement reaction deposition. The latter two are binder-free three-dimensional porous electrodes. As a comparison, a conventional dual-layer design, Pd/C nanoparticles painting on the nickel foam layer（Pd-C//NF）, is also prepared via direct painting method. It is shown that the use of the three-dimensional Pd-C/NF electrode as the anode in a direct formate fuel cell results in a peak power density of 45.0 mW cm-2 at 60°C, which is two times of that achieved by using a conventional dual-layer design(19.5 mW cm-2). This performance improvement is mainly attributed to the unique three-dimensional structure design,which effectively enhances the mass/ion transport through the porous electrode and enlarges the electrochemical surface area（accessible active area） for interfacial reactions. In addition, the delivery of the fuel solution is still sufficient even when the flow rate is as low as 2.0 mL min-1. It is also demonstrated that direct formate fuel cells using two binder-free electrodes yield the peak power densities of 13.5 mW cm-2(Pd/（in）NF) and 14.0 mW cm-2（Pd/NF） at 60°C, respectively, both of which are much lower than the power density achieved by using the Pd-C/NF electrode. This is because the electrochemical surface areas of two binderfree electrodes are much smaller than the Pd/C-based electrodes, since the specific area of Pd/C nanoparticles is much larger.