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

ABSTRACT 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.