Pt Single‐Atom Catalysts on FTO for Optically Invisible pH‐Universal Hydrogen Evolution Electrodes

Transparent hydrogen‐evolution electrodes are essential for photoelectrochemical and optoelectronic energy‐conversion devices, yet typically suffer from a trade‐off between catalytic activity and optical transmission. Here, we demonstrate that an ultra‐low loading of Pt single atoms (SA, 0.03 at%) directly anchored on fluorine‐doped tin oxide (FTO) via a reactive deposition delivers hydrogen evolution reaction (HER) kinetics approaching bulk Pt, while remaining optically invisible. At this low SA loading, HAADF‐STEM directly resolves atomically isolated Pt sites, while x‐ray photoelectron and absorption spectroscopy confirm exclusively cationic Pt δ + (δ ≈ 2) in Pt–O 4 coordination. The Pt SA/FTO electrodes exhibit pH‐universal HER activity, alongside turnover frequencies exceeding 500 s −1 at moderate overpotentials. Density Functional Theory calculations support the high stability of Pt SA/FTO and explain exceptional HER activity by optimized energetics of intermediate formation. Achieving comparable kinetics with conventional sputtered Pt films requires >1000‐fold higher Pt loading and leads to substantial optical losses. Owing to its optical neutrality, the Pt SA/FTO electrode enables 2–3 × higher photocurrents than activity‐matched Pt films when used as a counter electrode in sandwich‐type photoelectrochemical cells. These results on Pt SA‐decorated FTO thus describe a general strategy to unite noble‐metal utilization, high catalytic efficiency, and optical transparency in electro‐ and photoelectrochemical devices.