Fuel cells are amongst the most mature technologies for green electricity production, specifically the proton exchanged membrane fuel cells (PEMFC); unfortunately, PEMFC extensively rely on the use of expensive platinum group metals (PGMs) [1]. Recently, anion exchange membrane fuel cells (AEMFC) have received major attention, because they can employ low-cost (usually non-platinum) catalysts. However, these membrane-based fuel cells are less robust than liquid alkaline fuel cells (AFC), that can be operated more reliably in extreme operating conditions. GenCell Energy, Israel, is one of the most advanced actors worldwide for membraneless liquid-electrolyte flow AFC for small stationary applications. Although there are efficient non-PGM catalyst for alkaline oxygen reduction reaction (ORR), Pt/C still has the highest performance for alkaline Hydrogen Oxidation Reaction (HOR) [2]. Besides their cost, the biggest disadvantage of Pt/C and other monometallic PGMs/C catalysts is their drastic degradation at high pH (Pt assists the local carbon corrosion, leading to CO₂/carbonates formation, and particles detachment [3,4]). In contrast, bimetallic or bifunctional catalysts showed better durability compared to monometallic ones, such as Pd-CeO₂/C [5], Pd-Pt [6] and Pd-Ni [7] alloy nanoparticles. Bifunctional catalyst can also enhance the sluggish kinetics of monometallic PGM in alkaline HOR, according to the metal-metal oxide theory of Markovic et al. [8].

Following this insight, GenCell developed Pd-Pt and Pd-Ni on proprietary C to be used in caustic solution (up to 6.6 M KOH @ 70°C in a flow AFC). In the work of Doan and Morais et al., they studied these catalysts in model conditions (0.1 M KOH @ 25°C) before and after Ar-accelerated stress test (AST), consisting of 150 cycles of voltamperometry in supporting electrolyte (0.1 – 1.23 V vs RHE), both in a rotating disk electrode (RDE) and identical location transmission electron microscopy (ILTEM) set-up. [9] As expected, both Pd-Pt/HSAC and Pd-Ni/HSAC showed better stability compared to commercial 20% Pt/Vulcan (ETEK) and 20% Pd/Vulcan (Premetek). Herein, the same catalysts were evaluated for a much harsher AST in H₂ environment at low potential to compare to the AFC anode operation. Electrodes that have operated in Gencell’s membraneless flow AFC system have also been expertized pre and post-tests. The electrodes, employing Pd-Pt/C or Pd-Ni/C catalysts on the anode and Co-porphyrin-graphite on the cathode, were analyzed by complementary techniques to evaluate their behavior upon real operation. These analysis techniques, including optical microscopy, scanning electron microscopy (SEM, both surface and cross-section), Raman spectroscopy and X-ray diffraction (XRD) on top of electrochemistry, enabled to unveil potential degradation phenomena undergone by their constitutive materials in close-to-real operation.

References

1. Vielstich, W.; Gasteiger, H. A.; H. Yokokawa Eds. Wiley, 2009.
2. Dekel, D. R. J. Power Sources 2018, 375, 158–169.
3. Wang, J. et al. Nat. Energy 2019, 4 (5), 392–398.
4. Zadick, A., Dubau, L., Sergent, N., Berthome, G., Chatenet, M. Acs Catalysis, 2015, 5(8), 4819-4824.
5. Miller, H. A. et al. Angew. Chemie - Int. Ed. 2016, 55 (20), 6004–6007.
6. Kiros, Y.; Schwartz, S. J. Power Sources 2000, 87 (1), 101–105.
7. Sankar, S.; Anilkumar, G. M.; Tamaki, T.; Yamaguchi, T. Chem.Cat.Chem. 2019, 11 (19), 4731–4737.
8. Danilovic, N., Subbaraman, R., Strmcnik, D., Chang, K. C., Paulikas, A. P., Stamenkovic, V. R., Markovic, N. M. Angew. Chemie - Int. Ed. 2012, 51 (50), 12495–12498.
9. Doan, H., Morais, T., Borchtchoukova, N., Wijsboom, Y., Sharabi, R., Chatenet, M. and Finkelshtain, G., 2022. Applied Catalysis B: Environmental, 301, p.120811.