An energy storage system includes: a reformer configured to receive natural gas and steam and to output reformed natural gas; a combustion turbine configured to output heated sweep gas; and a molten carbonate electrolyzer cell (“MCEC”) including: an MCEC anode, and an MCEC cathode configured to receive the heated sweep gas from the combustion turbine. The energy storage system is configured such that: when no excess power is available, the combustion turbine receives the reformed natural gas from the reformer, and when excess power is available, the MCEC operates in a hydrogen-generation mode in which the MCEC anode receives the reformed natural gas from the reformer, and outputs MCEC anode exhaust that contains hydrogen.