Percolation-driven iridium nanotube network linking utilization and durability for oxygen evolution electrode in proton exchange membrane water electrolysis

Abstract

The high cost and limited availability of iridium restrict large-scale deployment of proton-exchange-membrane water electrolyzers (PEMWEs). Addressing this challenge requires maintaining electronic connectivity and catalytic activity at low Ir loadings. This study introduces an ionomer-free, iridium nanotube (IrNT) network that forms a contiguous one-dimensional electronic scaffold coated onto a titanium porous transport layer. IrNTs reach electrical percolation at an order-of-magnitude lower loading than IrO2 nanoparticles (Lc = 31.3 vs. 350.1 µg cm⁻²), boosting current distribution and Ir utilization at ultralow loadings. This enables mass activity to maximize at 50 µg cm−2, reaching 79.03 A mg−1. Durability testing up to 960 h identifies a two-step degradation pathway: (i) nanotube-to-nanoparticle morphological deformation that primarily elevates mass-transport resistance, followed by (ii) rapid failure via catalyst detachment once the entangled network collapses. Importantly, a distinct mechanical percolation threshold (≥125 µg cm−2) is required to suppress detachment. A practical loading window near 300 µg cm−2 balances utilization and longevity (45.5 µV h⁻¹ over 720 h at 1 A cm⁻²). These results establish electronic and mechanical percolation thresholds as quantitative descriptors for PEMWE catalyst layers and reveal design principles—junction-rich 1D networks, sufficient aspect ratio, and controlled wall thickness—to achieve Ir-lean, durable PEMWE anodes.