Mass and charge transport characteristics of direct formate fuel cells

Abstract

Direct formate fuel cells (DFFCs), as a promising, clean and efficient power generation technology, can directly produce electricity by converting the chemical energy stored in formate, which offer many advantageous characteristics: i) facile formate oxidation reaction, ii) convenient and cost-effective storage, transportation and handling of solid formate, and iii) feasibility of formate to serve as the energy storage medium for utilizing renewable energy, e.g. wind and solar. However, the widespread commercialization of DFFCs is hindered by the low performance and the high cost. Achieving high-performance and low-cost DFFCs requires highly active and cost-effective catalysts to accelerate the reaction and lower the system cost, highly conductive and selective membranes to boost ion transport and alleviate fuel crossover, as well as rational structural designs at electrode/cell levels and operating conditions to enhance the mass and charge transport. The primary objective of this thesis is aimed at understanding the mass and charge transport characteristics of DFFCs and improve the fuel cell performance by achieving the optimal structural designs and operating conditions. This thesis begins with the development and characterization of a passive DFFC and an active DFFC that use dual-layer electrodes, respectively, showing peak power densities of 16.6 mW cm-2 and 21.8 mW cm-2 at 60°C. To further improve the performance, therefore, a mathematical model is developed and validated, which provides comprehensive insights into mass/charge transport coupled with electrochemical reactions in DFFCs with a deeper understanding of how various structural design parameters and operating conditions affect the fuel cell performance. The modeling results reveal that the performance bottleneck of the DFFC results from the anode, followed by the cathode and membrane. This performance limitation is primarily because of the dual-layer electrode structural design, which is just borrowed from the electrode design of proton exchange membrane fuel cells typically running on gaseous hydrogen. Such a design is made of a dense catalyst layer coated on a porous diffusion layer, creating a large barrier for the transport of reactants to active sites. To address this issue, three types of three-dimensional porous electrodes are designed and fabricated as follows: 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, and iii) Pd nanoparticles embedding in the nickel foam matrix (Pd/(in)NF) via replacement reaction. Such three-dimensional electrode structural designs can shorten the transport distance of reactants from the flow channel to catalytic surface. It is experimentally demonstrated that the application of the three-dimensional structure design (Pd-C/NF) in DFFCs increases the peak power density from 21.8 mW cm-2 to 45.0 mW cm-2 at 60°C. This performance improvement is mainly attributed to the fact that this three-dimensional structure design not only promotes the mass/charge transport, but also enlarges the electrochemical surface area. It is also found that other two three-dimensional binder-free electrodes exhibit peak power densities of around 13.5 mW cm-2, which is about 30% lower than the dual-layer electrode does. The lower performance is mainly attributed to the design of an ultrahigh porosity structure, which causes seriously limited electrochemical surface area and aggravated fuel crossover phenomenon.