A new approach to stabilize direct methanol fuel cells (DMFCs) that use high-concentration methanol as their fuel supply has been developed by a research team at the Institute of Process Engineering (IPE), part of the Chinese Academy of Sciences. The method not only boosts fuel cell performance but might also shed light on the design of clean and affordable alternative energy sources for portable electric devices.
When methanol crosses from the anode to the cathode through the cell's proton exchange membrane (PEM), there is an inherent degradation of fuel cell performance. This has been a significant obstacle in the development and the commercialization of DMFCs. "In general, dilute methanol solutions (< 4 molar) are often used as fuel for DMFCs in order to inhibit methanol crossover. However, to compete with lithium-based rechargeable batteries that currently dominate the portable power market, the use of high-concentration methanol (9 molar or higher) as fuel is highly demanded to capitalize the high energy density of DMFCs," Yang told Materials Today. Scientists have investigated several ways to improve DMFC performance when high concentrations of methanol are being used, such as improving the fuel-feed system, developing more robust membranes, modification of the electrode materials, and approaches that improve water management.
"These conventional strategies do not fundamentally overcome the key obstacle, but inevitably complicate the design of DMFCs and hence increase their cost," explains YANG Jun, an IPE professor. Working with postgraduate student FENG Yan and assistant professor LIU Hui, Yang used selective electrocatalysts that would allow them to run a DMFC with a methanol concentration of up to 15 molar. This novel approach successfully solves the methanol crossover problem seen with conventional DMFCs.
The anode and cathode catalysts in DMFCs are usually based on platinum metal. However, platinum is not selective for the methanol oxidation reaction (MOR) at the anode nor the oxygen reduction reaction (ORR) at the cathode. The team had gained several insights into the mechanisms of the electrochemical reactions, and the principles underlying these phenomena were thus guided in their design of a new type of noble metal-based heterogeneous electrocatalyst. These materials have enhanced catalytic activity but perhaps more importantly are highly selective for MOR and ORR.
In tests, the team found that their modified DMFCs operated extremely well even at high concentrations of methanol. In principle, the unique structure and electronic coupling effects among the different domains of the noble metal-based heterogeneous electrocatalysts facilitated this improvement. [Yang et al., Sci Adv (2017) 3(6), e1700580; DOI: 10.1126/sciadv.1700580]
The team's ternary gold-silver sulfide-platinum (Au@Ag2S@Pt) nanocomposites have a core-shell-shell structure that endows it with superior anode selectivity due to the electronic coupling among their different domains, while core-shell gold-palladium (Au@Pd) nanoparticles with thin Pd shells exhibit excellent cathode selectivity because of the synergistic effects between their Au core and the thin Pd shell.
The team explains that their DMFC fabricated this way has a maximum power density of 89.7 milliwatts per square centimeter at a methanol concentration of 10 molar. It continues to work well even up to a methanol concentration of 15 molar.
"We are at the interface of a number of forefront research areas in this period of technological development. We hope this research effort may influence a rethinking of the current technologies for the commercialization of DMFCs," Yang added. "Next, we are going to optimize the overall size of the catalysts, e.g., using gold nanoclusters with fine diameters as starting materials to further enhance the activity and selectivity for DMFC reactions."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".