Scale-up of a high temperature polymer electrolyte membrane fuel cell based on polybenzimidazole
Introduction
Renewable energy sources and technologies based on hydrogen as an energy carrier, are considered an excellent alternative for generating clean and efficient energy.
From the different types of fuel cells that exist, the most interesting both for stationary and portable applications are polymer electrolyte membrane fuel cells (PEMFCs) due to its high efficiency and zero emission of pollutants [1].
The advantages of polymer electrolyte membrane fuel cells when operating at high temperature (above 100 °C and up to 200 °C) are: (i) increase kinetics for cathode and anode reactions, (ii) greater tolerance of catalyst to impurities and (iii) possibility of using waste heat generated [2]. One of the most promising membranes is polybenzimidazole (PBI) doped with phosphoric acid. Thus, high temperature PEMFC using polybenzimidazole (PBI) doping phosphoric acid membranes as proton conducting electrolyte are available to work at high temperatures and low humidity environment [3], [4], [5], [6].
Stability and performance of the high temperature PEMFC are significantly affected by operating conditions such as temperature, flow rates and pressure. In order to achieve a better performance and reach an integration of these systems in power generation applications, several parameters in a fuel cell must be optimized.
The most significant advances in this field reached by this working group have been: (i) development of membranes based on polybenzimidazole (PBI) doped with phosphoric acid [2], [7], [8], (ii) study of the catalyst ink, optimization of the amount of catalyst and the amount of ionomer in the catalytic layer [9], [10], [11] and its subsequent impregnation with the electrolyte to facilitate proton transport and (iii) optimization of the amount of active carbon and Teflon, both on microporous and gas diffusion layers [12], [13].
The next step was focused on the assembly and operation of a high temperature PEM stack which is shown in this work. Fuel cell stack development is a key technology for the fuel cell commercialization. Characteristics of the fuel cell stack performance are different from combustion engine and batteries. For an optimal design of the high temperature fuel cell system is required to gain experience previously at a lower scale (pilot plant). Thus, it was studied the influence of temperature and reaction stoichiometry (flow rate of reactants) on the overall performance. Also a durability–stability test was performed on the stack. The durability–stability test was carried out at constant and variable current using air as comburent in order to simulate a real engine operation. During these experiments the losses of electrolyte contained in both cathode and anode were analysed due to that the loss of electrolyte seems to be one of the main mechanisms of degradation of the PEM fuel cell based on PBI [14]. The electrochemical characterization was performed using cyclic voltammetry measurements for the overall system and for each cell.
Section snippets
Experimental
On top of a gas diffusion layer (Toray Graphite Paper, TGPH-120, 350 μm thick, 10% wet-proofing, BASF Fuel Cell, Inc.), a microporous layer (MPL) was deposited by N2-spraying, consisting of 2 mg cm−2 Vulcan XC-72R Carbon Black (Cabot Corp.) and 10% PTFE (Teflon™ Emulsion Solution, Electrochem Inc.). Then, a catalyst layer was also deposited by N2-spraying, composed of Pt/C catalyst (40% Pt on Vulcan XC-72R Carbon Black, ETEK-Inc.), PBI ionomer (1.24 wt.% PBI in N,N-dimethylacetamide, DMAc) and DMAc
Operation variables study
Fig. 1 shows performance results of the 150 cm2 stack cell at different oxygen flow rates and 1500 ml min−1 of hydrogen flow rate. It is observed that the cell performance increased significantly from 554 ml min−1 to 1105 ml min−1 O2 due to that at high currents, reagent requirements are much higher, according to Faraday's Law, than at low currents. Therefore, the stack performance drops at current density near 0.6 A cm−2 for an O2 flow rate of 554 ml min−1, due to the absence of reagent available to
Conclusions
From the study of the influence of stoichiometry of the reagent the values of were the optimum under our operation conditions. Moreover, for a better reagent utilization is recommended work at constant stoichiometry.
The back-pressure study showed an improvement of the stack performance being this more beneficial in the case of the cathode. As it was expected, an increase of temperature has a positive effect on the stack performance. Thus, an increase of 30% of the stack voltage
Acknowledgements
The authors want to thank the Ministry of Education and Science of the Spanish Government, the Junta de Comunidades de Castilla-La Mancha and the enterprise CLM-H2 for the financial support through the projects CTM2007-60472 and PBI08-151-2045, respectively.
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