Development of metal oxides supported speek materials for hydrogen generation and proton exchange membrane (PEM) fuel cell applications

Abstract

This study focuses on the synthesis and characterization of novel alternative non-platinum electrocatalysts for hydrogen generation and fuel cell applications using SPEEK nanocomposite membranes modified with mixed oxide fillers. Three different mixed oxide electrocatalysts were prepared, namely cerium-zirconium (CeO2:ZrO2), cerium-silica (CeO2:SiO2), and sulfated zirconia-silica (SZr:SiO2). The cerium-zirconium oxide electrocatalysts were synthesized using the coprecipitation method and subsequently annealed with varying CeO2:ZrO2 ratios. Among the various compositions evaluated, the CeO2:ZrO2 nanocomposite demonstrated remarkable efficiency at a 1:1 ratio. A current density of -2000 mA/cm² was achieved on a geometrical area of 0.20 cm². The outstanding performance is attributed to the unique mixture of spherical and rod-like morphology and surface area characteristics of the nanocomposites, which have a BET surface area of 63.41 m²/g and electrochemical active surface area (ECSA) of 4.4 cm². In this system, SPEEK binder plays a dual role as both a protective membrane (Ecorr = −0.33 V vs −0.29 V) and a binder for the inorganic materials. When compared to platinum (Pt), these electrocatalysts showed commendable performance, with only a 2.56% deviation in current. The hydrogen generation rate followed first-order kinetics, with a rate of 9.6 × 10−5 A/s and a rate constant of 8.35 × 10−5 s−1. Graphite electrodes (GE) were modified with SZr and varying proportions of SZr:SiO2 metal oxides. The GE/SZr:SiO2 (1:1) electrode demonstrated exceptional performance, exhibited a hydrogen evolution current density of -1700 mA/cm² at -0.55 V vs. RHE based on ECSA dimension. Furthermore, the enhanced electrocatalytic activity and durability of CeO2:SiO2 mixed oxides were also studied. The SPEEK-promoted CeO2: SiO2 (2:1) exhibited the best HER activity, having a current density of -260 mA/cm² at -0.50 V vs. RHE, an electrochemical surface area of 2.0 cm², and a turnover frequency of -1.42 s⁻¹. The above-mentioned synthesized fillers that exhibited the highest HER activity was utilized to modify the SPEEK membrane, enhancing the thermal stability, proton conductivity, and mechanical properties of the composite membranes. The recast method was used to prepare the composite membranes with varying weight% of fillers. The modified membranes with CeO2:SiO2 nanoparticles showed a significant improvement in water uptake, leading to enhanced hydrophilicity when compared to the pristine SPEEK membrane. The presence of SiO2 enhanced proton conductivity, while CeO2 helped reduce the chemical degradation of the membrane by scavenging free radicals. The proton conductivity of the SPEEK-CeO2:SiO2 (5 wt.%) nanocomposite membrane measured 0.037 S/cm, compared to only 0.004 S/cm for the bare SPEEK membrane under the same conditions. Additionally, oxidative stability tests conducted in 3 ppm Fe2+ at 80 °C revealed that the membrane with 5% CeO2:SiO2 exhibited the highest oxidative stability of 81.6% after 250 minutes. Additionally, SPEEK-CeO2:ZrO2 nanocomposite membranes were also studied. SPEEK-CeO2:ZrO2 (10 wt.%) exhibited higher proton conductivity of 0.07 S/cm. Lastly, composite membranes incorporated with varying amounts of sulfated binary metal mixed oxide (SZr:SiO2) were prepared. SPEEK-SZr:SiO2 (1wt%) membrane exhibited the highest antioxidant stability at 82.87%, surpassing the pure polymer matrix membrane, which had a stability of 77.24%. Furthermore, they also demonstrated the highest proton conductivity of 0.087 S/cm. For assessing fuel cell performance, all composite membranes that displayed the highest proton conductivity within their respective groups were selected for testing. The SPEEK-SZr:SiO2 (1%) membrane outperformed the other composite membranes with the highest current density of 1370.77 mA/cm² and a peak power density of 54.9 mW/cm², demonstrating its superiority as a catalyst in fuel cells due to its high conductivity and stability. Overall, the SPEEK-SZr:SiO2 (1wt%) composite membrane shows great potential for polymer electrolyte membrane fuel cell (PEMFC) applications.