Reaction-Transport Imbalance Generates pH Landscapes Governing Pure-Water-Fed Anion Exchange Membrane Electrolysis

Pure-water-fed anion exchange membrane water electrolysis (AEMWE) offers system-level advantages, yet its efficiency and durability remain constrained by a poorly quantified anode pH landscape under unbuffered operation. Here, an ionomer-free, electrodeposited Co-based anode architecture was employed as a mechanistically diagnostic baseline to isolate the membrane-defined hydroxide boundary condition and quantify how a hydroxide-supply/reaction-demand mismatch establishes spatial pH gradients. By combining half-cell pH–kinetic calibration, membrane–potential sensing, operando spectroscopy, and reaction–transport simulations, a current-dependent and spatially structured pH landscape was resolved. Membrane-delivered OH⁻ was shown to sustain an alkaline microdomain adjacent to the membrane at low current densities despite a neutral bulk feed, whereas strong temperature dependence and an elevated apparent activation energy indicated transport-limited hydroxide delivery. At higher current densities, pH gradients were steepened, alkaline domains were compressed, and interior loss of alkalinity was induced, consistent with Co phase evolution and modeled profiles. Catalyst-layer pore architecture was shown to control gradient amplification, yielding a non-monotonic porosity dependence consistent with confinement- and dilution-limited regimes. Guided by this framework, an ionomer-free Co-based anode achieved 2 A cm⁻² at 1.91 V and >1200 h stability in a pure-water-fed AEMWE cell.