Tree-like hierarchical porous anode catalyst layer for efficient proton exchange membrane water electrolyzer by optimization of transportation

The microstructure of the catalyst layer (CL) is a key factor in constructing an effective triple-phase boundaries for the activity presentation of membrane electrode assembly for proton exchange membrane (PEM) water electrolysis. In this work, with TiO₂ nanotube arrays and leached Ni as a template, an IrNiO_x catalyst layer with tree-like hierarchical pores (TLHP-CL) is developed to achieve full utilization of Iridium. For TLHP-CL structure, the vertically oriented mesopores provide facile mass transport pathways, while the ionomer-free nanopores in the IrNiO_x sphere provide abundant active sites and hydrophilicity that facilitate active site accessibility. TLHP-CL23 shows 1.63 times enhancement compared to IrO_x nanoarrays (NAs) CL with similar intrinsic activity in the half-cell study. A prominent single-cell voltage of 1.666 V at 1 A cm⁻² and 1.787 V at 2 A cm⁻² for TLHP-CL23 is achieved. Polarization breakdown shows that with a current density under 2 A cm⁻², the mass transfer overpotential of TLHP-CL23 is negligible, but it increases rapidly when the applied current density is higher than 2.5 A cm⁻², showing the mass transfer limitation of nanopores. Also, prominent stability is presented with no significant degradation after 700 h operation under 1.5 A cm⁻².