Hydrocarbon Proton and Anion Exchange Polymers for Water Electrolysis Applications
Authors: Viviani Marco , Rusitov Dennis , Sulaberidze Rusudan , Khutsishvili Lasha , Münchinger Andreas , Klose Carolin , Titvinidze Giorgi 
Published: December 2024

Renewable energy and water electrolysis (WE) are deemed to be the pillars over which the incoming green hydrogen era and energy transition will stand. Water electrolysis can undergo in both acidic and alkaline conditions. In modern times, solid polymer electrolyte WE has attracted particular interest, due to the advantage of a compact design combined with the possibility to operate at higher pressure and temperatures even using pure water.

Still to date, the most used membranes in PEMWE are poly(perfluorosulfonic) acids (PFSAs) (e.g., Nafion™) due to their mechanical strength, strong chemical resistance, and high proton conductivity. However, recent concerns regarding their cost, environmental impact due to fluorine chemistry, mechanical instability above 80 °C, and low gas barrier (especially H2) require urgent development of suitable hydrocarbon alternatives. Sulfonated poly(phenylene sulfones) (sPPS) represent a valuable hydrocarbon alternative to PFSAs as they exhibit lower gas crossover, higher conductivity, better thermal stability and low production cost.[1] [2] This class of polymer is able to outperform Nafion™ in PEMWE thanks to their higher proton conductivity (Fig. 1 b)).[1] To improve long term-stability of these polymers in operative PEMWE conditions (T = 80°C, > 1 Acm-2 ), fluorine-free reinforcement and Ce-based “damage-reparation”[3] strategies have been successfully implemented extending their stability in the range of thousands of hours.

A major drawback of PEMWE technology is the reliance on platinum group metals (PGMs) (i.e. Pt and Ir). This stimulated the research of alternative strategies to respond to the demand of green H2 necessary to mitigate global warming reduction. AEMWE is the ideal alternative to PEMWE since it can be performed efficiently with non-PGMs, i.e., iron and nickel. The main shortcoming in AEMWE comes from the need of an alkaline electrolyte, which drastically limited the development of AEM technology in past decades. The harsh alkaline and electrochemical stress present in AEMWE have only recently been overcome by a new class of polyphenylene piperidinium and polynorbornene-based polymers which are also commercially available as Piperion™ and Pention™ respectively. The first are obtained by super-acidic Friedel-Craft addition[4] while the polynorbornene chemistry requires special catalysts.[5]

Inspired by these results, in our lab, we have dedicated particular efforts to develop hydrocarbon AEM materials implementing phosphorous superbases as anion conducting functionalities to eliminate the presence of constant charges close to the polymer chain. AEMs based on polynorbornenes are proposed, including stable amino-linkage inspired by previous work.[6] Additionally, anion exchange ionomers (AEI) for catalyst layers with enhanced gas permeability were synthesized based on “canal” polynorbornene and novel crosslinked polyarylpiperidinium. The main synthetic strategies and properties (e.g., IEC, WU, alkaline resistance) of the resulting polymers are presented.