Carbon-based fossil fuels have been the main source of global energy. The combustion of fossil fuels has led to serious environment problems, such as ecological destruction, air pollution, and global warming.[1,2] These problems stimulate prevailing effort on generating electricity from sustainable energy such as solar, wind, and hydropower. The challenge of utilizing renewable energy is the intermittency, which calls for energy storage and conversion techniques to provide energy on-demand.3] A promising technique is the use of H2 as energy carrier.