Solar Hydrogen Generation: Photocatalytic and Photoelectrochemical Methods - Hydrogen Generation, Storage, and Utilization - page 27

3

Solar Hydrogen Generation:
Photocatalytic and
Photoelectrochemical Methods

3.1 BASICS ABOUT SOLAR WATER SPLITTING

Water splitting is a chemical process that converts water into hydrogen and
oxygen. It represents one of the most important reaction for hydrogen fuel
production, as water is the most abundant hydrogen source on the Earth.
However, water splitting is a thermodynamically uphill reaction:

1
2

H O H → + O ( ∆ G ~ 237 2 . kJ mol ⋅

− 1

; 1 23 . V vs NHE . ).

2 2 2

Therefore, a minimum potential of 1.23 V is required to overcome this
thermodynamic barrier. In practice, there is an additional kinetic barrier
(overpotential) for charge carrier transfer for water oxidation and proton
reduction. The overpotentials for these reactions are material dependent.
Thus, practically the minimum energy required for water splitting is higher
than 1.23 V. The required energy can be obtained from a nonrenewable or
renewable energy sources. For instance, electrolysis is the most direct method
to split water by applying a potential higher than the minimum required
energy. The common industrial electrolyzer with platinum as catalyst can
achieve a hydrogen production efficiency of around 70% [1]. Nevertheless,