Preparation and characterisation of porous electrodes from nickel powder for fuel cells

Porous gas diffusion electrodes are used in fuel cells. The essential requirement of the porous electrodes is to establi sh an internal gas/liquid interface as long as the current is drawn from the cells. INCO nickel-255 powder is used for making porous electrodes by powder compaction and sintering techniques. The porosity .and pore-size di stribution in the electrodes are correlated with the compaction pressure, sintering temperature, arnount of pore formers and particle sizes of catalysts. A method for the fabrication of two layer and three layer electrodes with graded porosity is outlined . Metal powders of nickel, iron especially carbonyl variety have been widely used in battery industry. INCO nickel-255 and 287 have been employed for the fabrication of electrodes in alkaline nickel-cadmium, nickel-iron and nickel-zinc batteries ' - 3 and in fuel cells 4 • The high compressibility of these metal powders during compaction associated with good mechanical strength during sintering is the essential criteria for the long-term stability of these battery electrodes. In fuel cells, especiall y in low temperature hydrogen-oxygen fuel cell s, which operate with aqueous KOH as electrolyte at 60-80·C, the electrodes are made up of INCO nickel-255 powder. The hydrogen or oxygen gas, which is supplied to the anode or cathode compartment is allowed to establish a contact with the KOH electrolyte with in the porous electrode structure (Fig. 1). The generation of current is thus dependent upon the wetted area of th e three­ phase reaction zone. This requires a narrow pore size di stribution of the electrode structure to avoid either complete flooding of the pores with the electrolyte or bubbling of gas through the pores. Hence, the establishment of mixed pores with macro, micro and interconnected pores is the choice. This can be easily achieved by proper control of the: particle size of the nickel powders, catalysts, pore formers and the amount of pore formers . Usually the catalyst powders which essentially catalyse the electrochemical reactions (called electrocatalysts) are mixed with nickel powder (support) in the fabrication procedures. More frequently, electrodes with graded porosity structure have been prepared 5 • A double layer or triple layer structure is usually envisaged (Fig. I). The layer exposed to the electrolyte consists of micro pores and is completely filled with electrolyte . This is called fine pore layer (FPL). The adjoining layer contains the catalysts and usually has slightly bigger (medium) pores, where the three-phase react ion zone exists. This layer is called the catalyst layer (CL) or medium pore layer. This layer is further supported with another layer containing macro pores called coarse pore layer (CPL). This is essentially a tripl e laye r mode l. In certain cases, both CL and CPL are merged to form a single layer such that the e lectrode is essentially a double layer model. This paper describes the fab ricati on procedures for making double layer and tripl e layer electrodes by powder compaction and sintering techniques. The effects of different amoun ts of pore-form ers on th e pore size distribut ion of the various layers are presented. The ratios of thickness of FPL to CL/CPL are varied with a view to achieve mechanically stable and highly perfo rming porous gas diffusion electrodes.