Promoting fuel utilization and power performance of microfluidic methanol fuel cell : from 3D anode to solar fuel processor

Abstract

Fuel cell is a promising clean energy technology to supply electricity continuously with a variety of fuel. The by-product of fuel cell operation could be controlled by suitable selection of fuels advantageous to eliminate toxic pollutant emission to the atmosphere. Methanol is particularly among all the organic fuels, thanks to its high energy density (17.9 MJ/m3 and better oxidation kinetics. The direct methanol fuel cell (DMFC) was well established and commercialized in the market. However, those DMFCs suffered from their bulky design, high cost and low efficiency. The research on miniature DMFC, which remediates those problems and facilitates the micro devices development, drew attention to researchers in recent years. Microfluidic fuel cell (MFC) utilizes the laminar flow characteristic of fluid in a microchannel separating the electrolyte streams, such that the cell can be operated without the expensive membrane. The structure is very simple because there are no ancillary components (cooling and humidifying parts). Methanol can still be directly fed into the MFC. However, the methanol MFC encounters similar problems as the commercial DMFC, which is high cost due to the noble metal based anode catalyst for methanol oxidation reaction (MOR) and poor electrode stability due to catalyst poisoning and oxidation of the carbon support under high voltage. The trade-off problem between high power output and fuel utilization is yet to be solved. In this research, lowering the noble metal content in the anode for MOR while maintaining the performance of MFC is one of the targets. Ultrafine Pt nanoparticles are produced which had superior large electroactive surface area for MOR. Furthermore, extraordinary catalytic stability is obtained by growing the Pt nanoparticles onto graphene nanosheets. The replacement of carbon paper based 2D anode with a graphene based 3D flow-through anode has a 358% increment in specific power (normalized by the mass of platinum). Ru@Pt core shell nanoparticles are synthesised to further improve the MFC performance. Noticing that the electrode conductivity plays an important role, Carbon nanotube (CNT) reinforced graphene aerogel composite is produced, which outperforms the reported liquid feed methanol MFC in the literature. To further improve the performance and fuel utilization, the liquid feed system is replaced with a gaseous feed MFC. As expected, the 3D flow-through anode has a nearly doubled peak power output of the 2D counterpart. Inspired by the gaseous feed system and traditional methanol reforming in fuel cell, methanol reforming by solar light is proposed. A fuel tank carrying photocatalyst is adopted to convert the methanol fuel to hydrogen gas under solar light illumination. It is shown that hydrogen and other intermediate products are generated and consumed together with methanol by the MFC, producing 10 times more power improvement. Finally, an optimized plate shaped fuel tank is developed to facilitate the catalysts collection and dehydrogenation of methanol under one sun’s illuminating intensity. The methanol utilization is increased from 50% to 78% operated at peak power voltage under simulated solar light illumination. Such a solar fuel processor diminishes the trade-off between power and fuel utilization.