Abstract
The nickel metal hydride (Ni-MH) batteries can be recharged by use of electricity or hydrogen gas. If the negative electrode is recharged with hydrogen gas, it would take only several minutes to complete the charge process. The purpose of this research is to set up a battery system that could be charged with hydrogen gas. The characteristics of the positive electrode were also studied. In the present work, a palladium-silver alloy film was used as the negative electrode that was sputter deposited on three different kinds of substrates, i.e., a porous stainless steel Mott-0.2 with pore size 10~30 um, sintered porous nickel disk, and sintered porous silver disk. Prior to deposition, the surface pores were filled with nano-crystalline Pd70Ag30 particles to reduce the pore size, and then deposition of a Pd77Ag23 alloy film was made. The crystal structure and chemical composition of the membrane were analyzed by X-ray diffraction (XRD). The surface of the sintered substrates was analyzed by electron probe microanalyser (EPMA). The morphologies of the modified surface and coated membrane were examined by scanning electron microscopy (SEM). The discharge characteristics were studied by a battery test apparatus that was charged with hydrogen gas. It was found that a dense and continuous film could be obtained on the modified substrate. For all substrates, the discharge currents followed the Sievert’s law at different hydrogen pressures and temperatures. The amount of discharge efficiency was up to 82.1%. It reveals that most amount of hydrogen was discharged on the exit surface and the others might have leaked from the system. The discharge current was smaller than the theoretically calculated value. The diffusivity and permeability of hydrogen through this membrane are 4.82x10-10 cm2/s and 5.21x10-15 mol/cm • s •Pa1/2, respectively. Both values are about 3 orders lower than those from the literature, which might be due to the weak driving force by using NiOOH as the positive electrode to make all hydrogen atoms discharged spontaneously. An unwary estimation of the working area might be another reason for the low current density and therefore low diffusivity and permeability.