Benefit Analysis Of Critical Raw Materials' Retrieval Strategies From Used Hydrogen Production Membranes

Abstract

With the advent of hydrogen and its derivatives being the emerging green fuel sources in aviation, the question of its sustainable production arises. Out of the various production methods, anion exchange membrane water electrolysis (AEMWE) stands out because of the less corrosive environment and the use of base metal electrocatalysts. Even so, for better performance, the expensive and exhaustible Platinum Group Metals - Platinum, Iridium, Ruthenium, etc. are the preferred electrocatalyst choices. The rarity of these metals has earned them a place in the European Union’s Critical Raw Materials (CRM) list. Furthermore, the commonly chosen Nickel and Cobalt electrocatalysts are also CRMs. For the purpose of a circular economy and to ensure the uninterrupted supply of these CRMs, the current work estimates the amount of Nickel, Cobalt and Platinum exhausted in AEMWE electrocatalysts per year from 2035-2050.
To convey the issue of membrane degradation in alkaline media and the importance of catalysts for electrolysis, lab-scale electrolyser cell runs and material characterisation were carried out using the best-performing commercial anion exchange membrane, Sustainion X37-50 Grade RT. Thermogravimetric analysis, Fourier Transform InfraRed spectroscopy, and chronopotentiometry were performed. Even for a lower current density of 0.3 A/cm2, 1.890 V was required. The unstable nature of the chronopotentiometry curves and the higher potential needed to maintain a small current emphasized the need for catalysts. The shift in the degradation temperature and the FTIR spectra variation confirmed the degradation of the membrane.
Following this, the amount of CRMs exhausted from 2035-2050 was calculated by assessing the global hydrogen aviation demand evolution and AEMWE’s contribution. LDS, MDS, and HDS of 10%, 30%, and 60% denoting the hydrogen demand met by AEMWE production were fixed. Using Enapter’s 1 MW capacity AEM Nexus 1000 electrolyser model specifications and state-of-the art catalyst loading trends, Nickel, Cobalt, and Platinum exhaustion trends were determined.
The final step was analysing the retrieval strategies available for CRM recovery. New developments in hydrometallurgy, pyrometallurgy, and bio metallurgy were researched and weighed against current industry practices. The weighing was based on time, temperature, pH, and recovery efficiency. Microwave-assisted hydrometallurgy using aqua regia was the optimal route for Platinum. For Nickel and Cobalt, ultrasound-assisted hydrometallurgy with lemon-juice/H2O2 leachant system was the viable technique. Employing these techniques, 98.3% Platinum, 100% Nickel, and 100% Cobalt can be recovered. It was also determined that for nickel and cobalt, the analysed bio metallurgical methods could be employed for retrieval, if they could be modified for time optimization.