Swelling response behavior of palladium during hydrogen absorption and discharge

Swelling characteristics of hydrogen storage alloy (HSA) with hydrogen content change were experimentally determined. Its kinetic behavior is important for an HSA actuator which is driven by the volume change of the HSA in a hydrogen atmosphere. Although hydrogen absorption characteristics of the alloy are expected to be strongly related to the actuator performance, it is difficult to clarify the relationship using the previous HSA actuators which consist of plural materials. In this paper, the authors investigated the swelling ratio and the response time of palladium powder in a hydrogen atmosphere. As a result, the swelling ratio increased with hydrogen absorption amount. The response time increased with hydrogen pressure during pressurization, while it was independent of pressure. It was revealed from reaction kinetics that a rate-limiting step was diffusion and interfacial reaction during the pressurization and the evacuation, respectively. Thus, the swelling property of palladium is determined from its hydrogen absorption behavior, which suggests the HSA actuator can be designed based on hydrogen storage characteristics.