Activity and Stability of Iridium-Based Alloy Catalysts for PEM Electrolyzer

For large-scale production of green hydrogen via renewable energies, large capacities of highly performing, cost-effective water electrolyzers are needed. Polymer electrolyte electrolysis (PEMEL) operates at high current and pressure and is very compact compared to alkaline technology. However, it relies on very expensive noble catalysts such as Pt and Ir for H 2 and O 2 electrode, respectively. Meanwhile, unsupported Iridium black is considered as state-of-the-art in terms of corrosion stability and activity, whereas a lot of efforts aim at reducing loading by alloying or/and developing supported materials [1,2]. In this work, Ir black and bi-metal Ir-M (M = Pt, Pd, Cr, Co) black catalysts were synthesized by co-reduction of iridium chloride and M-ion containing precursor in 0.1 M sodium borohydride at 180°C for 3 h under hydrothermal condition. The nominal mass ratio of Ir:M was fixed to 3:1. The structural characterization of the as-prepared catalysts was performed by means of XRD, TEM, Raman, and BET. Electrochemical activity of Ir-M black catalyst for OER was studied in deaerated 0.5 M H 2 SO 4 electrolyte at room temperature by using a rotating disk electrode (RDE) cell. The total catalyst loading was about 200 μg Ir cm -2 . Additional cyclic voltammetry measurements over the potential range of -0.1–1.4 V vs. RHE at 40 mV s -1 were performed to identify redox behavior of the catalysts (fig. 1a). The accelerated degradation tests (ADT) were performed between 1.1 and 1.6 V at dE/dt=0.5 V s -1 with RDE at 1600 rpm for 1000 cycles. A commercial Ir-black catalyst purchased from Alfa Aesar was used as reference. Based on half-wave potential value at 30 mA cm -2 (fig. 1b), the following trend in lower overpotential for OER was obtained: Ir-Cr > Ir-Pt > Ir-Pd >> Ir-Co > Ir-black. PEMEL full cell experiments were performed in single cell with 5 cm² geometrical area in a balticFuelCells fixture at 60°C. The catalyst coated membrane (CCM) consists of 2 mg Ir cm -2 anode and 1 mg Ir cm -2 cathode coated on Nafion 117. Titanium felt and carbon paper were used as porous transport layers in the anode and cathode side, respectively. Accelerated stress tests (AST) were performed by using rectangular polarization signal to reflect dynamic electrolyzer operation. Thereby, the current density was varied between 0.05 and 3 A cm -2 in 10 s for at least 1000 h. After test, the cell components were post analyzed by SEM/EDX, XRD and XPS technique and anolyte/catholyte were a