Objective: The wide application of proton exchange membrane (PEM) electrolyzers is limited by their high capital costs, which are primarily due to the extensive use of precious metal iridium. However, reducing iridium loading still poses challenges related to the efficiency and stability of PEM electrolyzers. Although the catalysts prepared using manganese dioxide loaded with iridium have achieved a high iridium mass activity, the design and preparation of iridium-manganese oxide catalyst electrodes for PEM electrolysis continue to face other challenges, including catalyst layer (CL) dead zones, reduced three-phase interfaces, and poor stability. This study reports an oxygen evolution reaction (OER) electrode with an ultralow iridium load dispersed by manganese dioxide carriers. The highly dispersed iridium is anchored on the surface of manganese dioxide with a high electrochemical surface area and low ohmic resistance, thereby achieving high performance and stability. Methods: Manganese dioxide electrodes prepared by pulsed electrodeposition (PED), continuous electrodeposition (CED), and pulse-gradient electrodeposition (PGED) were fabricated on titanium felt, followed by reaction with iridium precursor solutions for iridium loading to yield PED-IrMnO2/porous transport layer (PTL), CED-IrMnO2/PTL, and PGED-IrMnO2/PTL, respectively. The physical properties of the samples were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of the samples was evaluated in a three-electrode system and a PEM electrolyzer. Linear sweep voltammetry, cyclic voltammetry, polarization curves, and electrochemical impedance spectroscopy (EIS) were also analyzed. Results: SEM results showed that PED increased the MnO2 particle size and decreased the number of particles per unit area. The BET values were the lowest in the PED sample, followed by the PGED sample, and the highest in the CED sample at 44.78 m2·g-1. SEM analysis of the PED sample revealed that a small duty cycle resulted in uniform MnO2 growth, indicating the presence of many contact sites with the PTL. Thus, the PED sample had the lowest resistance. Compared with the CED and PED samples, the PGED sample had reduced resistance and increased specific surface area, respectively. XRD and TEM results revealed the (131) crystal plane corresponding to γ-MnO2. XPS results showed that PGED-Ir MnO2/PTL had the highest Ir valence state of +5.73, indicating its high OER activity. In the three-electrode system, PGED-Ir MnO2/PTL exhibited the highest iridium mass activity, reaching 112.32 A·g-1 at 1.53 V versus reversible hydrogen electrode (RHE), which was 4.41 times that of IrO2/PTL. The stability test results showed that the potential change of PGED-IrMnO2/PTL was negligible throughout a 100 h duration test in 0.5 mol·L-1 H2SO4. In the PEM electrolyzer, the PGED sample exhibited the best performance with a voltage of only 1.859 V at 2 A·cm-2 and the lowest Tafel slope of 74.3 mV·dec-1. EIS analysis indicated that the improved OER performance of the PGED sample was due to a balance between ohmic losses and charge transfer losses. Conclusions: This study develops a preparation technique for 3D gradient-ordered OER electrodes for PEM electrolyzers, reducing iridium usage to 1% of commercial levels while maintaining high performance. The prepared gradient-ordered OER electrodes demonstrates enhanced electron conduction sites between the CL and the PTL. The gradient design increases the specific surface area, allowing PGED-IrMnO2/PTL to achieve an iridium mass activity of up to 1.1 × 105 A·g-1 in the PEM electrolyzer. These findings contribute to the development of low-cost, high-performance, and stable ultralow-iridium-loading OER electrodes, thereby advancing the terawatt-scale application of PEM electrolyzers.
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