Formic-Acid In Situ Hydrogen Enables Programmable Ru–Pd–Pt/C Trimetallic Interfaces for Durable Acidic HER

Proton exchange membrane (PEM) electrolysis demands hydrogen evolution reaction (HER) cathodes that minimize precious-metal usage while remaining durable in strongly acidic media, yet improving Pt utilization and stability under reduced PGM loading remains challenging. Here, we develop a programmable nucleation pathway that uses in situ H* from formic acid (HCOOH) dehydrogenation at a Ru–Pd interface to gate reducing equivalents during nucleation. This enables site-confined Pt nucleation/immobilization and stabilizes a hydrogen-management interface for H* generation, relay, and recombination. The resulting Ru–Pd–Pt/C comprises uniformly dispersed ternary nanodomains (~2.46 nm). In 0.5 M H 2 SO 4 , Ru–Pd–Pt/C delivers η 10 = 12.24 mV, an apparent Tafel slope of 29.9 mV dec −1 from a unified 20–50 mV fitting window, and a PGM mass activity of 6.58 A mg PGM −1 at η = 50 mV. More importantly, membrane electrode assembly tests under identical assembly conditions show lower cell voltages than commercial 40 wt% Pt/C over 1–4 A cm −2 and stable operation at 1 A cm −2 . Density functional theory calculation shows Ru/Pd neighbors tune Pt toward near-thermoneutral H adsorption (ΔG H* = −0.19 eV), consistent with density of states/d-band shifts and the improved apparent HER response. This formation-stage programming offers a transferable framework for durable, low-PGM HER cathodes in acidic water electrolysis.