A direct process can use a catalyst that reduces protons to molecular hydrogen upon electrons from an excited photosensitizer. Several such catalysts have been developed as proof of concept, but not yet scaled up for commercial use; nevertheless, their relative simplicity gives the advantage of potential lower cost and increased energy conversion efficiency.[1][8] One such proof of concept is the "artificial leaf" developed by Nocera and coworkers: a combination of metal oxide-based catalysts and a semiconductor solar cell produces hydrogen upon illumination, with oxygen as the only byproduct.[9]