Currently, the world's energy demand mainly depends on fossil fuels being a limited resource. The consumption of fossil fuels leads to emission of carbon dioxide (CO2) that is one of the major greenhouse gases contributing to global warming and climate changes. We need to urgently establish sustainable energy societies based on renewable energy sources. On the other hand, nature has perfected an ultimate solar energy conversion system, photosynthesis, where high-energy molecules such as carbohydrates are efficiently produced from CO2 and water using solar energy. Inspired by the natural photosynthesis, scientists have been developing artificial systems capable of converting solar energy into valuable fuels. In artificial photosynthetic systems, water oxidation is an essential process to acquire electrons for reduction reactions such as hydrogen production and CO2 fixation, but is considered to be a bottleneck due to its complexity and sluggish kinetics.
