Anion exchange membrane (AEM) electrolysis is hampered by two main issues: stability and performance. Focusing on the latter, this work demonstrates a highly active NiMo cathode for hydrogen evolution in AEM electrolysis. We demonstrate an electrolyzer performance of 1 A cm⁻² at 1.9 V (total cell voltage) with a NiMo loading of 5 mg cm⁻² and an iridium black anode in 1 M KOH at 50 °C, that may be compared to 1.8 V for a similar cell with Pt at the cathode. The catalysts developed here will be significant in supporting the pursuit of cheap and environmentally friendly hydrogen fuel.

The development of active hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) catalysts for use in anion exchange membrane fuel cells (AEMFCs), which are free from platinum group metals (PGMs), is expected to bring this technology one step closer to commercial applications. This paper reports our recent progress developing HOR Pt-free and PGM-free catalysts (Pd/CeO₂ and NiCo/C, respectively), and ORR PGM-free Co₃O₄ for AEMFCs. The catalysts were prepared by different synthesis techniques and characterized by both physical-chemical and electrochemical methods. A hydrothermally synthesized Co₃O₄ + C composite ORR catalyst used in combination with Pt/C as HOR catalyst shows good H₂/O₂ AEMFC performance (peak power density of ~388 mW cm⁻²), while the same catalyst coupled with our flame spray pyrolysis synthesised Pd/CeO₂ anode catalysts reaches peak power densities of ~309 mW cm⁻². Changing the anode to nanostructured NiCo/C catalyst, the performance is significantly reduced. This study confirms previous conclusions, that is indeed possible to develop high performing AEMFCs free from Pt; however, the challenge to achieve completely PGM-free AEMFCs still remains.

Tailoring catalyst–ionomer and electrolyte interaction is crucial for the development of anion exchange membrane (AEM) water electrolysis. In this work, the interaction of Ni–MoO₂ nanosheets with ionomers and electrolyte cations was investigated. The activity of Ni–MoO₂ nanosheets for the hydrogen evolution reaction (HER) increased when tested in 1 M NaOH compared to 1 M KOH; however, it decreased when tested in 0.01 M KOH compared to 1 M KOH electrolyte. The capacitance minimum associated with the potential of zero free charge (pzfc) was shifted negatively from 0.5 to 0.4 V versus RHE when KOH concentration increased from 0.1 mM to 1 M KOH, suggesting a softening of the water in the double-layer to facilitate the OH^– transport and faster kinetics of the Volmer step that lead to improved HER activity. The catalyst interaction with cationic moieties in the anion ionomer (or organic electrolytes) can also be rationalized based on the capacitance minimum, because the latter indicates a negatively charged catalyst during the HER, attracting the cationic moieties leading to the blocking of the catalytic sites and lower HER performance. The HER activity of Ni–MoO₂ nanosheets is lower in benzyltrimethylammonium hydroxide (BTMAOH) than in tetramethylammonium hydroxide (TMAOH). Anion fumion ionomer and electrolytes with organic cations with benzyl group adsorption (such as BTMAOH) lead to decreased HER activity in comparison with TMAOH and Nafion. By utilizing Ni–MoO₂ nanosheet electrodes as a cathode in a full non-platinum group metal (PGM) AEM electrolyzer, a current density of 1.15 A/cm² at 2 V cell voltage in 1 M KOH at 50 °C was achieved. The electrolyzer showed exceptional stability in 0.1 M KOH for 65 h at 0.5 A/cm².

A novel sonochemical approach to synthesize platinum (Pt) nanoparticles has been developed to produce smaller, and more monodisperse particles than traditional chemical reduction methods such as the sodium borohydride (NaBH₄) reduction method. It was also found that careful experimental manipulation of the ultrasonic conditions such as the ultrasonic frequency (f), the acoustic power (P_acoustic) and the solution volume (V) may be employed to tailor the sonochemical radical yield to the same value for two different ultrasonic systems (20 kHz provided by an ultrasonic probe or sonifier, and 408 kHz by an ultrasonic plate transducer) leading to a similar reduction rate, and as a consequence, to Pt-nanoparticles with similar sizes. The reduction of Pt(IV) was found to be a first order reaction with a rate constant (k) of 0.062 min⁻¹ for both ultrasonic systems, and the particle sizes from TEM measurements were found to be (2.2 ± 0.5) nm and (2.3 ± 0.4) nm for the 20 kHz and the 408 kHz samples, respectively. In comparison, the chemical reduction method resulted in particle sizes of (3.0 ± 0.5) nm. It was also found that the sonochemical synthesis at 408 kHz was more efficient than at 20 kHz with an initial sonochemical efficiency of (0.20 ± 0.04) μmol kJ⁻¹ at 408 kHz and (0.060 ± 0.004) μmol kJ⁻¹ at 20 kHz. The sonochemical synthesis at 408 kHz also yielded nanoparticles of higher purity. Interestingly, direct sonication into the reaction vessel using the 20 kHz sonication probe produced impurities of Ti, V and Al in the solution, indicating that direct sonication at lower frequencies led to (i) probe material (made of Ti alloy – Ti-6Al-4V) erosion induced by cavitation and (ii) a continuous supply of micro meter sized impurities. It was also observed that the introduction of impurities increased the reduction rate constant of Pt(IV) through heterogeneous nucleation (k = 0.12 min⁻¹) as was demonstrated for the 408 kHz system under controlled addition of Ti/Al/V seed microparticles. Direct sonication using a sonifier should therefore be avoided when high purity catalytic materials are required.