Ammonia Tolerant Oxygen Reduction Reaction Studies on Metal Oxide Catalysts in Alkaline Media Using the Gas Diffusion Electrode Half Cell

The oxygen reduction reaction (ORR) is of crucial importance in fuel cells and metal-air batteries. However, ORR remains sluggish and is easily poisoned due to anolyte crossover in alkaline fuel cells using fuels such as methanol, urea, and ammonia 1, 2. Spinels ABxO4 (A, B - metals) are interesting replacements for platinum/carbon because of their relatively high ORR activity and tolerance to anolyte fuel crossover 2, 3. In this work, we investigated mixed transition metal catalysts (A, B - Ni, Co, Fe, Mn) for the ORR. The catalysts were electrodeposited on carbon cloth with a microporous layer on one side, using nitrate precursor salts in the molar ratios of 2:1 and 1:2, for NiCo, FeCo, MnCo and 0.5: 0.5: 2.0 for NiMnCo. For comparison, Co3O4 was prepared using a 0.1 M nitrate precursor solution. X-ray fluorescence (XRF) measurements confirm the thickness of the electrodeposited catalysts as ranging between 100 - 175 nm for MnCo, CoFe and MnCoNi, and 180 - 205 nm for NiCo. We used a Fumasep FAA-3-50 anion exchange membrane to test the ORR activity of the catalysts, in a gas diffusion electrode half-cell to avoid mass transport limitations 4. The polarization curves showed that the significant increase in geometric current densities of MnCoO (1:2) and MnCoNiO proceeded at 0.79 V and 0.85 V vs. RHE at room temperature (25 °C) respectively in 1.0 M potassium hydroxide (KOH). We achieved a geometric current density of 100 mA/cm2 for MnCoO (1:2) and NiMnCoO at 0.65 V and 0.64 V vs. RHE, respectively. Tafel slopes at low overpotential are in the range 45 - 90 mV/dec, 75 - 120 mV/dec, 128 mV/dec, 60 - 80 mV/dec, and 100 mV/dec for CoFeO, NiCoO, and Co3O4, MnCoO, and MnCoNiO, respectively, suggesting differences in reaction mechanisms. The degree of reduction of the overpotential and thus enhancement of the ORR catalytic activity in KOH compared to Co3O4, is higher for the addition of Mn than Ni, while Fe has the opposite effect. Addition of 0.5 M ammonia to the electrolyte, increases the overpotential compared to plain KOH by 40 mV, 120 mV, 50 mV, 110 mV and only 4 mV for MnNiCoO (0.5:0.5:2.0), CoNiO (2:1), CoNiO (1:2), MnCoO (2:1) and MnCoO (1:2), respectively. Table 1 presents the potential (E−iR) vs RHE at the acquired ORR current density, j = 100 mA/cm² for the examined catalysts along with their corresponding Tafel slopes. The relative metallic elemental concentrations of the more active catalysts, MnCoO (1:2) and MnNiCoO (0.5:0.5:2.0) catalysts were determined using XRF as, Mn9Co91Ox and Mn4Co88Ni8Ox. The enhanced concentration of cobalt can be attributed to the favorable standard reduction potential of cobalt ions (−0.28 V), leading to preferential deposition. These catalysts showed 72 mV higher overpotential at 100 mA/cm2 than commercial Pt/C catalysts. Thus, the catalyst with the best combination of high ORR activity and NH3 tolerance was Mn9Co91Ox. This catalyst also exhibited consistent and stable performance over 8 h hours after exposure to accelerated testing conditions cycling between 25 mA/cm2 and 50 mA/cm2 in 1.0 M KOH containing 0.5 M NH3 at room temperature. In the next steps, we shall investigate, the dependence of the Tafel slope and stability of catalytic activity, on temperature of this material. References: Lee, et al., ACS Appl. Mater. Interfaces 2020, 12, 44588-44596. Hu, et al., ACS Appl. Mater. Interfaces 2021, 13, 53945−53954. Si, et al., Nano Energy 2016, 23, 105-113. Ehelebe, et al., J Electrochem Soc 2019, 166, F1259. Acknowledgements: We acknowledge the support from the German Federal Ministry of Education and Research (BMBF) in the framework of the project CatLab (03EW0015A) for the work related to the ORR. The work related to ammonia tolerance was done under the Telegram project. This project has received funding from the European Union’s Horizon 2020 Research and Innovation programme under grant agreement No 101006941. The project started on the 1st of November 2020 with a duration of 42 months. Figure 1