Electrochemical characteristics and transport phenomena in direct methanol fuel cells operating with highly concertrated fuels

Abstract

Direct methanol fuel cells (DMFCs), which promise to be a clean and efficient energy production technology, have been regarded as a promising power source for portable applications, primarily because methanol is a sustainable fuel and possesses many unique physicochemical properties including high energy density and ease of transportation, storage as well as handling. Nevertheless, conventional DMFCs have to operate with excessively diluted methanol solutions due to methanol crossover to limit its detrimental consequences. Operating the cell with diluted methanol solutions significantly reduces the energy density of the power pack and thus prevents it from competing with advanced batteries. In this thesis, the electrochemical and mass transport characteristics under high-concentration operation are investigated, based on which the strategy of addressing methanol crossover is proposed. Firstly, a microporous anode flow field is developed, which enables DMFCs to operate with methanol solutions as concentrated as 22.0 M without sacrificing the performance achieved with diluted fuel. Secondly, a superhydrophobic diffusion layer and a hydrophilic-hydrophobic dual-layer diffusion layer are further developed to improve the water management for the high-concentration operation, respectively, both of which have been proven to be effective in facilitating the water recovery and improving the water starvation on the anode. Thirdly, a thin reaction layer, consisting of PtRu/silica nanocatalysts, is introduced to the cathode architecture, which not only reduces mixed-potential loss, but also alleviates Pt poisoning, solving the detrimental consequences of methanol crossover. In addition, a methanol-tolerant Prussian Blue cathode is developed for a DMFC operating with hydrogen peroxide, which can completely avoid the negative impacts caused by the presence of methanol on the cathode. Furthermore, a monolayer graphene-Nafion sandwiched membrane is developed and it is demonstrated that only one carbon atom increment in thickness can decrease the methanol permeability by 68.6%. Lastly, a nanochannel membrane using inorganic silica nanotubes as the framework and assembled with proton-conducting molecular monolayer is proposed and prepared. Unlike conventional polymer-based membranes, channels in the present membrane are controllable in size and uniform in distribution. Theoretically, the size-controllable nanochannel can separate methanol and protons via size exclusion effect, thus this type of nanochannel membrane might be a final solution of methanol crossover. Keywords: Direct methanol fuel cell; Methanol crossover; Energy density; Concentrated fuel; Mass transport

Date of Award	2016
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology