Research On Proton Exchange Membrane Fuel Cell Start-up At Subzero Temperature

Cold start-up from subzero temperature is one of the key barriers which prevents proton exchange membrane fuel cell (PEMFC) from commercial using. The subfreezing environmental adaptability, freeze durability and self start ability are challenging tasks and need to be settled as soon as possible in the research of PEMFC. Behaviors and mechanisms of cold start-up performance decay, effect of ice formation on in-house materials and components such as proton exchange membrane, catalyst layer, diffusion layer, etc, has been reviewed in this paper. To investigate the effect of sub-zero temperature on the start ability and performance of PEMFC, a single cell was assembled and experienced several times' actual freeze start. Performance decay and its cause are analysed by means of polarization curves, cyclic voltammetry, Surface appearance of components were characterized by SEM and Image Analysis System. The cold start-up stategies for fuel cells at sub-zero temperature are overall introduced, including water removal, such as gas purge and vacuum drainage, cold start-up stategies with assistance and without assistance, such as open-door patents in-country and overseas. Emphasis on the warm Strategy, take the kind of stacks with parallel-channel fluid field configuration as instance, through a simple heat system model, the time of stacks' temperature rise up to the fixed value under different boundary conditions is computed .Sum up above reseach related results and main conclusions are general follows:(1) Operating conditions and design parameters play important influence on performance of PEMFC cold start. It is found that the self start ability can be enhanced by increasing inlet flux and reducing initial current density, the polarization curves of actual cold start show that single cell has a strong start ability and adaptability to the range of load change in the subzero temperature of-5℃, but more sensitive in the subzero temperature of -7℃and -10℃. Conclude the results, PEMFC own a powful cold start-up ability in the subzero temperature of -5℃,but weak in the subzero temperature of -10℃. (2) Cyclic voltammetry and polarization curves show that cold-start failure cases an obvious damnification than FT circles, the single cell suffers a degree of damnification just after two times of cold-start failure in the subzero temperature of -5℃,the damnification goes on and so that the cell can't be used for applying cold start continuativly after only one start-up failure in the subzero temperature of-10℃. It is concluded that the PEMFC performance decay in cold start failure, the lower the subzero temperature, the serious damages suffered. Cyclic voltammetry show that cold start-up failures result in a decreasing area of catalyze active area and oxygen disengage peak area which will effect the performance of PEMFC operating with high current density.(3) SEM and Image Analysis System analysis show that the microcosmic configuration of the single cell components changed after several times'cold start, such as severe desquamation of particulate of PTFE on carbon fiber, change of aperture distribution on surface of carbon paper. The changes will serious effect the hydrophobic-hydrophilic of GDL and may cause water flood in bosom of cells especially operating with high current density.(4) Nowadays, cold-start strategies main focus on water removal, antifreeze prevents liquid water freezeing, keeping internal temperature of cells above zero, warming with assistance. All correlative patents show that Fuel cell can successfully start up and the damnification also can be obviously reduced by adopting cold-start strategy with pertinence. The cold-start strategies should be focused on stacks' configuration design, conformity of the stack system with auxiliary equipments as well as the synthesis of several cold-start strategies.(5) A thermal model for warming stack with parallel-channel fluid field configuration was built up. Computing results show that warming stack by hot gas has a certain effect, but the stack temperature rise slowly, while the effect will be more clearly when warming by hot water, especially increasing the temperature of water will reduce the start time to a large extent.