Scalability and feasibility of photoelectrochemical H₂ evolution: the ultimate limit of Pt nanoparticle as an HER catalyst

The recent surge in investigating electrocatalysts for the H₂ evolution reaction is based on finding a cheap alternative to Pt. However platinum's excellent catalytic activity means very little catalyst needs to be used. The present study combines model experiments with numerical modeling to determine exactly how little catalyst is needed. Specifically we investigate ultra-low Pt loadings for use in photoelectrochemical H₂ evolution using TiO₂-Ti-pn⁺Si photocathodes. At a current density of 10 mA cm^-2, we photocathodically evolve H₂ at +465, +450, +350 and +270 mV vs., RHE at Pt loadings of 1000, 200, 50, and 10 ng cm^-2 corresponding to HER overpotentials of η_1000ng = 32 mV, η_200ng = 46 mV, η_50ng = 142 mV, and η_10ng = 231 mV. To put this in perspective, if 30% of the world's current annual Pt production was used for H₂ evolution catalysis, using a loading of 100 ng cm^-2 and a current of 10 mA cm^-2 would produce 1 TW_average of H₂. The photoelectrochemical data matched the modeling calculations implying that we were near the fundamental maximum in performance for our system. Furthermore modeling indicated that the overpotentials were dominated by mass transfer effects, rather than catalysis unless catalyst loadings were less than 1000 ng cm^-2.