Effect of cobalt additive on the NiOOH/Ni(OH)2phase boundary movement has been investigated by analysing build-up and decay current transients under the application of various voltage steps(70 to 400 mV). The occurrence of current plateau upon hydrogen extraction from the completely hydrogen-injected films indicated that the hydrogen transport through the film proceeds by the movement of NiOOH/Ni(OH)2phase boundary. The velocity and mobility of the phase boundary movement upon hydrogen extraction were determined to be orders of 10−6cm s−1and 10−5cm s−1V−1in magnitude, respectively. It is concluded that β-Ni(OH)2phase is stabilised by the Co(OH)2incorporation into the Ni(OH)2film and hence both the velocity and mobility of the NiOOH/Ni(OH)2phase boundary movement is raised by the Co(OH)2incorporation.