A unitized reversible fuel cell (URFC) is essentially an electrochemical device that can operate both as a water electrolyzer and a hydrogen fuel cell. If a reversible fuel cell is combined with some kind of hydrogen storage system, it can compete with secondary batteries. The advantage with URFC-based hydrogen systems is that power production capabilities is decoupled from energy storage capabilities, and that large quantities of energy can be stored in the form of hydrogen. This is not the case with secondary batteries, where there is very close correlation between power and energy capacities. Batteries also have self-discharge (voltage losses) and oxygen as side-products. A hydrogen storage demonstration system in Japan involving R&D on URFC has led to the development of a 1 kW-class URFC stack consists of 10 cells. This system has been operated successfully including the switching between electrolyzer and fuel cell operation mode. The results show that the round-trip efficiency is comparable to systems with decoupled electrolyzers and fuel cells. A new modeling study was recently initiated with the aim to develop a URFC-system model. This model is based on the current-voltage characteristics of PEM fuel cells and electrolyzers and the resistance change during electrolyzer/fuel cell switching procedures. The basic equations in the model has been verified using experimental data from various laboratory set-ups, and operational data from the 1 kW demonstration system. The modeling study concludes with a few system simulations, and to find other suitable applications for URFC-systems.