Effect of Micro-porous Layer on PEMFC Performance

Abstract

Polymer electrolyte membrane fuel cell (PEMFC) is one of the types of fuel cells operating at low temperature used in household appliances and vehicles due to its high-power density and fast start / stop operating characteristics. Generally, the components of the PEMFC can be classified into gas diffusion layer (GDL), bipolar plate and membrane electrode assembly (MEA). Among them, GDL facilitates the diffusion of reactant gas and the discharge of the generated water. Additionally, it performs various functions such as the role of the conductor of heat and electrons generated by the electro-chemical reaction and the mechanical support of the catalyst layer (CL). Therefore, it is certain that research on the GDL is very important.

In general, the GDL is composed of the substrate and the micro-porous layer (MPL) and is treated with the PTFE to promote the water discharge from the GDL. Additionally, the MPL mainly consists of carbon black and PTFE, and the design of these elements affects the PEMFC performance. Also, when laminating the MPL onto the substrate, the MPL penetrates the substrate, and this MPL penetration thickness also has substantial affects the performance. In this study, the PEMFC performance tendency was investigated with varying the MPL PTFE content and MPL carbon loading per unit area. Regarding the MPL PTFE content, the performance improved as the cathode MPL PTFE content increased at low RH, but at high RH, the PEMFC performance deteriorated as the cathode MPL PTFE increased. In the case of the MPL carbon loading, the MPL carbon loading of 2 mg/cm2 demonstrated the best performance based on the experimental results in this study, and this suggests that there exists an optimal MPL carbon loading for the optimum performance of the PEMFC. In addition, the effects of ratio variation in the substrate and MPL penetration on the PEMFC performance were investigated with the reaction area of 25 cm2 for each driving condition. And the reasons for these performance tendencies are explained by various experiments, physical properties measurement results and in terms of the capillary pressure gradient. In summary, based on the experimental results in this study, the MPL penetration ratio within 15% to 20% of the total GDL thickness and the combined ratio of the MPL and MPL penetration within 35% to 40% is the best for overall PEMFC performance. In addition, when the substrate ratio to the total GDL thickness is too low, such as 150T (~20%), water flooding occurs severely in the substrate, and this accumulated water acts as a back pressure, thereby causing severe capillary condensation in the MPL penetration region, which prevents the supply of the reactant gas. Further, if the substrate ratio is too low, the discontinuity in the porosity and surface properties between the MPL and the substrate are weakened; thus, the ability to discharge water from high capillary pressure to low capillary pressure is deteriorated, thereby resulting in poor performance.

고분자 전해질형 연료전지 (Polymer Electrode Membrane Fuel Cell, PEMFC)는 전해지막-전극 접합체 (Membrane Electrode Assembly, MEA)를 전해질로 사용하는 연료전지로서, 100℃이하의 낮은 작동 온도에서 구동되며, 출력 전력 밀도가 뛰어나고 빠른 시동 및 정지의 장점으로 인하여 자동차용 혹은 가정용 발전 시스템으로 주목받고 있다. 이러한 고분자 전해질형 연료전지는 전해지막-전극 접합체, 기체확산층 (Gas diffusion layer, GDL), 반응 가스의 유로 역할을 하는 분리판 (Bipolar plate, BP)으로 구성되어 있다. 이 증 기체확산층은 촉매층으로의 반응 가스 확산, 전기 화학 반응 후 생성된 물 배출을 용이하게 하며, 전자 및 열 전도체 역할 그리고 기계적 강성 지지체 등의 다양한 기능을 수행하기에, 기체확산층에 관한 연구가 중요하다고 볼 수 있다.

일반적으로 기체확산층은 기판 (Substrate)과 미세기공층 (Micro porous layer, MPL)으로 구성되어 있으며, 미세기공층은 카본 블랙 (Carbon black)과 PTFE로 구성되어 있다. 또한, 미세기공층을 기판에 적재할 때 미세기공층이 기판을 침투하며 해당 미세기공층의 관통 두께는 PEMFC 성능에 상당한 영향을 미친다. 본 연구에서는 미세기공층의 PTFE 함량 및 단위 면적당 미세기공층 탄소 부하량을 달리하여 PEMFC 성능 경향성을 확인하였다. 또한 미세기공층 침투 두께는 같지만 기판 두께가 다른 기체확산층과 기체확산층의 두께는 같지만 미세기공층 침투 두께가 다른 기체확산층이 PEMFC 성능에 미치는 영향을 확인하였다. 실험 변수에 따른 성능 경향의 이유를 다양한 실험, 물리적 특성 측정 결과 및 기체확산층 내의 모세관 압력 구배 측면에서 설명하였다.