  Solar water splitting provides a promising path for sustainable hydrogen
production and solar energy storage. One of the greatest challenges towards
large-scale utilization of this technology is reducing the hydrogen production
cost. The conventional electrolyzer architecture, where hydrogen and oxygen are
co-produced in the same cell, gives rise to critical challenges in
photoelectrochemical (PEC) water splitting cells that directly convert solar
energy and water to hydrogen. Here we overcome these challenges by separating
the hydrogen and oxygen cells. The ion exchange in our cells is mediated by
auxiliary electrodes, and the cells are connected to each other only by metal
wires, enabling centralized hydrogen production. We demonstrate hydrogen
generation in separate cells with solar-to-hydrogen conversion efficiency of
7.5%, which can readily surpass 10% using standard commercial components. A
basic cost comparison shows that our approach is competitive with conventional
PEC systems, enabling safe and potentially affordable solar hydrogen
production.
