Hispec 3000

Zinc nitrate hexahydrate (Zn (NO₃)₂·6H₂O, ≥99.99%), 2-methylimidazole (98%), Iron (III) acetylacetonate, Melamine (Fe(acac)₃, 99%) were obtained from Aladdin company. Commercial state-of-the-art 20 wt% Pt/C (Johnson Matthey Company, HiSPEC™ 3000) was used as the benchmark for comparison. 5 wt% Nafion ionomer, Nafion 212 membrane were purchased from DuPont Co. MEthanol (CH₃OH, 99.5%), Ethanol (C₂H₅OH, 99.5%), Hydrochloric acid (HCl, 37%) were provided by Beijing Chemical Works. Ultrapure water (Millipore, 18.25 MΩ cm) was used throughout all experiments.

Electrocatalyst ink preparation—for this study, three different catalysts were used for ink preparation. Two Pt/C references (Hi-spec 3000 with 20 wt% Pt and Hi-spec 4000 with 40 wt% Pt from Johnson Matthey) and an in-house-synthesized de-alloyed PtCu/KB electrocatalyst (26 wt% Pt and 17 wt% Cu; Figure S1a). In all three cases, the ionomer (5 wt% solution, NS-5 QuinTech) content within the ink was kept constant at 30 wt% of dry electrocatalyst mass (to balance between good proton transport to the active sites in dry conditions and proper water removal in wet conditions [53 (link),54 (link)]). This resulted in I/C ratios of 0.8 for Hi-spec 4000, 0.6 for Hi-spec 3000, and 0.8 for the de-alloyed PtCu/KB electrocatalyst. In order to obtain stable inks, a ratio of 0.97:0.03 between 2-propanol (Honeywell, Chromasolv for HPLC, ≥99.9%) and water (Milli-Q) was used for both Pt/C references while the ratio had to be adjusted to 0.85:0.15 for the de-alloyed PtCu/KB. Before preparing the catalyst-coated membranes (CCMs), all catalyst inks were ultrasonicated for 45 min under ice cooling. The list of prepared MEAs can be found in Table 1. Only the CCM marked with * was subjected to a high-humidity stressor 12 h before polarization recording. One commercially available CCM was purchased from QuinTech, Pittsburgh PA, USA (CCM-H25-N212) and used as a reference. All other CCMs were fabricated by using ultrasonic spray coating using an ExactaCoat OP3 from Sono-Tek with the number of deposited layers determining the loading. Previous coating trials with weight monitoring were performed to determine the weight deposited per layer with each ink as described in the Supplementary Information.
CCM fabrication via ultrasonic spray coating (Sono-tek ExactaCoat OP3, Milton, NY, USA) was performed in the same way as in our previous publication [39 (link)]. In summary, the membrane was fixed to a porous PTFE filter by vacuum suction, heated to 80 °C, and the catalyst ink was sprayed upon it in a serpentine pattern until target loading was reached. The number of passes needed to reach the loading was determined by the procedure described in the Supplementary Information. A shim mask was used to define the active area of 25 cm². The finished CCMs were left on the PTFE filter plates to dry for ten minutes, and then used for cell assembly.
Cell assembly and break-in and high humidity stressor—Assembly and break-in of finished CCMs (Figure S1b) into the testing cell (S⁺⁺ Simulation Services) was performed identically to our previously published procedure [39 (link)] and with the same hardware.
In short, the CCMs were placed in between two gas diffusion layers and inserted into the 25 cm² testing cell. (Figure S1c). Reactant gas conditions were set to 100 %RH at 80 °C and 600 mL min⁻¹ air/H₂ at atmospheric pressure. After the cell reached OCV (held for 5 min), cell break-in was performed by switching between 0.4, 0.5, and 0.6 V and holding each point for thirty seconds for a total of three hours. The Potential profile and current density response can be seen in Figure S2b. Only CCM PtCu/KB_0.8* (Table 1) was subjected to a high-humidity stressor directly after the break-in to be later compared to CCM PtCu/KB_0.8, which was subjected to the normal break-in. The stress test was aimed at determining the influence of additional load changes in humid environments, which have been reported to promote Pt dissolution and migration [55 ,56 (link)]. The humidity profile over time compared to normal break-in is depicted in Figure S2a. In total, this leads to 2 h of additional time at 100 %RH, which includes 1 h of additional polarization recording. After the break-in, all cells were held at 0.5 V and the humidification was controlled at 60 %RH overnight (10 h), before polarization curves were recorded using H₂/air at 80 °C, 250 kPa, 100 %RH, and 600 mL min⁻¹.
Hydrogen crossover-test and in-situ cyclic voltammetry—Hydrogen crossover and in situ cyclic voltammogramms were recorded in the same manner as previously published [39 (link)]. In short, this means supplying the cells with H₂/N₂ 500 mL min⁻¹ 100% RH until the OCV is stable at 125 mV and then performing a linear potential sweep with 1 mV sec⁻¹ from OCV to 0.5 V at differential pressures of 0, 50, and 100 mbar. If the crossover current density was below 15 mA cm⁻², then the CCMs were used for further testing.
In situ cyclic voltammorgrams with a scan rate of 50 mV s⁻¹ at 0 mbar differential pressure were recorded between 70 and 600 mV for three cycles.
Single-cell electrochemical testing—All polarization curves were recorded in concordance with our previously published procedures [39 (link)], with operating conditions of H₂/air at 80 °C, 250 kPa, 60 %RH and 600 mL min⁻¹ constant flow to maintain an air stoichiometry of 1.1 at the highest current density recorded. Underlying conditions assure that the influence of anode limitations can be neglected, while mass transport limitations on the cathode side appear more distinctly. Before recording polarization, OCV was held for 5 min. Electrochemical Impedance Spectra (EIS) were recorded from 60 kHz to 0.1 Hz with an amplitude of 0.1 A. The operating points of 0.2, 0.3, and 0.4 A cm⁻² were held for at least five minutes before recording the corresponding spectrum to ensure steady-state conditions. On CCM Pt/Vul_0.6, additional EIS were recorded at atmospheric pressure, 60 %RH, and 100 %RH after finishing the standard testing to verify the influence of the change in the high frequency impedance arc. An overview of the operating conditions is shown in Table 2.