A study on the membrane and the cathodic binder incorporating structural bias into proton exchange membrane fuel cells

Abstract

An energy shortage is having occurred due to the rapid exploitation of fossil energy along with the continuous growth of the global economy. In addition, the large amount of fossil energy use causes serious environmental pollution. Developing diverse, renewable, and clean energy sources and reducing environmental pollution have become inevitable trends in future energy development. The fuel cells generally have the advantages such as high energy efficiency, environmental friendliness, and fuel diversity. Therefore, fuel cell technology is currently considered one of the most promising and attractive green and power generation technologies. There are many types of fuel cells according to the electrolyte, reaction mechanism, and operating temperature. Among them, the proton exchange membrane fuel cell (PEMFC) is emerging as a leading energy conversion technology for stationary, transportation, and portable electronic applications. PEMFCs are widely popular as an alternative power source due to their low temperature operation (< 100 °C), quick load change including fast start-up and shut-down cycles, and remarkable efficiency (theoretical efficiency including thermal efficiency of 83%). Nevertheless, the widespread penetration of PEMFCs to the extent that they can replace fossil energy is still hampered by their high costs, environmental risks, and unsatisfactory durability, as they must use large amounts of PFSA and precious metal Pt. The present study is aimed at improving the electrochemical performance of MEA for PEMFC via increased oxygen diffusion in the cathode and partial substitution of fluorine on the cation exchange membrane. Furthermore, for practical application of the fuel cell system, the electrochemical performances of a single cell with the cardo-type cathode binder and polyethylene substrate-based reinforced composite membrane developed in this study were evaluated under PEMFC operation conditions. Chapter 2, it is suggested a method for not only enhancing the performance of the cell but also reducing the cost of the membrane used in PEMFCs. The reinforced composite membrane with a two-layered asymmetric structure was fabricated by impregnating the Nafion on a porous polyethylene (PE) substrate to reduce the cost of the membrane used in PEMFCs as well as to enhance the performance of the cell. The ion conductivity of the reinforced composite membrane annealed at 150 °C increases owing to the high crystallinity of the Nafion ionomer, and the mechanical strength is improved by the strong interaction between the PE substrate and Nafion ionomer. The immiscibility between them was observed to intensify the interfacial resistance in the three-layered symmetric structure. On the other hand, the reduction of the interfacial resistance of the two-layered asymmetric structure membrane leads to similar results to the performance of traditional PEMFC using Nafion XL with a polytetrafluoroethylene (PTFE) substrate similar to a backbone of Nafion ionomer. Its high mechanical strength, low cost, and reliable ion conductivity make it a suitable substitute for PTFE-based membranes used in PEMFCs although the immiscibility with the ionomer in the membrane comprising a PE substrate is higher than that in the membrane with a PTFE substrate. Chapter 3 deals with the study on a sulfonated poly(ether sulfone) having cardo-type fluorenyl groups (FL-SPES) as a cathodic binder to improve fuel cell performance via increased oxygen diffusion in the cathode. The maximum power density was achieved by using the membrane electrode assembly (MEA) prepared with FL-SPES with a low ion exchange capacity (IEC) of 1.31 meq. g–1 was 520 mW cm–2, which is more than twice as high as that of BP-SPES (210 mW cm–2) having typical biphenyl groups with a similar IEC. At a high IEC of 1.55 meq. g–1, the power density obtained by using BP-SPES was improved to 454 mW cm–2 but remained lower than that of FL-SPES. In addition, although the IEC, swelling degree, and specific resistance were similar to each other, the gas permeability of FL-SPES was improved by approximately three times compared to that of BP-SPES. The steric structure of cardo-type FL-SPES increased the free volume between the polymer backbones, leading to an increase in gas transfer. Consequently, oxygen diffusion was promoted at the cathode, resulting in improved fuel cell performance. Chapter 4, it is discussed how to accurately determine the ion transport number, which is one of the characteristics of cation exchange membranes that are crucial for realizing the high performance and durability of electrochemical devices such as fuel cells and water electrolysis. They exhibit properties that can be elucidated by considering accurate ion-transport numbers determined from membrane potential differences. Herein, the accurate sodium-ion transport numbers (t_m^(Na+)) of various cation-exchange membranes were determined in sodium chloride solutions using two silver-silver chloride single-junction electrodes as references. The junction potential differences between these two electrodes were employed for the accurate measurement of membrane potentials in 0.01–1 M sodium chloride solutions and determined for five combinations of low-concentration/high-concentration sodium chloride solutions. They were sufficiently large to affect membrane potentials by 11–19% compared to the measured apparent potentials. It was found that the sodium chloride concentration should be at least 0.1 M, in which the optimal concentration difference was identified as 10 times between high- and low-concentration solutions. In addition, the stirring of the low-concentration solution was essential for minimizing the diffusion boundary layer that distorted the apparent potentials. Consequently, it was proven that a 1/0.1 M concentration difference was most suitable for the determination of t_m^(Na+) using the above setup, as the t_m^(Na+) of a commercial cation-exchange membrane obtained under these conditions (0.995) was close to that reported previously (0.997).