KEYWORDS: CB-PTFE; Monolithic Gas Diffusion Media; Pore Formers; Perforations;
GDL-MPL; Fuel cells.
Renewable energy technologies based on Proton Exchange membrane Fuel Cells (PEMFC) are in the forefront for mobile and stationary applications. In PEMFCs, Gas Diffusion Layer (GDL) and Micro Porous Layer (MPL) play critical roles in the distribution of reactants at the
electrodes. Carbon paper and cloth made of carbon fibres are the most common GDLs in use.
The overall cost of this component contributes significantly to the cost of the fuel cell stack, and the performance of existing designs need improvement to perform well despite fluctuations in reactant humidity. This work focuses on replacing graphitized Carbon fibres with a carbon
black and polymeric Teflon@ composite, to produce a self-standing MPL- GDL layer, referred to as Self Standing Monolithic Diffusion Layer (SMDL). The usual additional step of coating an MPL onto the traditional GDL, and the subsequent high temperature heat treatments required, is circumvented in these SMDLs, resulting in a 51 % reduction in cost of
manufacturing the GDL-MPL duo. The SMDL thus synthesized is compared to a conventional GDL-MPL in terms of electrical conductivity, hydrophobicity, pore size distribution, mechanical integrity and performance in a single cell. The SMDLs are synthesized by a novel process of using axial compression on a die cast mould containing the Carbon black-PTFE composites. The axial forces applied on the composite by the die design results in mechanically strong and integrated layers with a control in thickness from batch to batch and within a batch. This process enables tailoring the thickness of SMDLs
as well as mass production for industrial application.
Based on the results obtained, it was deduced that improvement in the volume of Macro pores was needed to further improve water management at high current densities during the fuel cell operation. This issue was addressed by incorporating pore formers, and evaporating them to introduce macro pores. Glycerol and Ammonium Chloride were utilized and were chosen based on their boiling point or melting point being below or close to the melting point of Teflon©
(MP: 327 oC). Pore distribution was assessed for Macro pore volume % and a significant improvement was observed. The presence of macro sized pores and its effect could be amplified by physically inducing
pores with the help of needle tip, 300 µm - 500µm. The needle tip has a conical structure, and perforations were made in a way that the tapered tip faces the catalyst layer and the wider tip faces flow field. This directionality helped in avoiding flooding at the Catalyst layer and easy
removal of excess water by Capillary effect, from the Catalyst layer towards the flow field. Their I-V performance was evaluated under different conditions of humidity to determine the effect of macro pores under flooding conditions as compared to standard MPLGDL.
The SMDLs fabricated and tested, were mechanically stable to be used by catalyst coated GDL method to make MEAs, and they functioned robustly under a wide range of humidity conditions. The SMDLs performed well under wet conditions, which represents excellent
functioning of the SMDL since these conditions greatly increase the life of the membrane and reduce the membrane ohmic resistance. This study therefore demonstrates an alternative, inexpensive, carbon fibre free layer SMDL, and its performance is found to be on par or better
than standard GDLs.