Anionic Exchange Membrane for Photo-Electrolysis Application

Abstract

Tandem photo-electro-chemical cells composed of an assembly of a solid electrolyte membrane and two low-cost photoelectrodes have been developed to generate green solar fuel from water-splitting. In this regard, an anion-exchange polymer-electrolyte membrane, able to separate H₂ evolved at the photocathode from O₂ at the photoanode, was investigated in terms of ionic conductivity, corrosion mitigation, and light transmission for a tandem photo-electro-chemical configuration. The designed anionic membranes, based on polysulfone polymer, contained positive fixed functionalities on the side chains of the polymeric network, particularly quaternary ammonium species counterbalanced by hydroxide anions. The membrane was first investigated in alkaline solution, KOH or NaOH at different concentrations, to optimize the ion-exchange process. Exchange in 1M KOH solution provided high conversion of the groups, a high ion-exchange capacity (IEC) value of 1.59 meq/g and a hydroxide conductivity of 25 mS/cm at 60 °C for anionic membrane. Another important characteristic, verified for hydroxide membrane, was its transparency above 600 nm, thus making it a good candidate for tandem cell applications in which the illuminated photoanode absorbs the highest-energy photons (< 600 nm), and photocathode absorbs the lowest-energy photons. Furthermore, hydrogen crossover tests showed a permeation of H₂ through the membrane of less than 0.1%. Finally, low-cost tandem photo-electro-chemical cells, formed by titanium-doped hematite and ionomer at the photoanode and cupric oxide and ionomer at the photocathode, separated by a solid membrane in OH form, were assembled to optimize the influence of ionomer-loading dispersion. Maximum enthalpy (1.7%), throughput (2.9%), and Gibbs energy efficiencies (1.3%) were reached by using n-propanol/ethanol (1:1 wt.) as solvent for ionomer dispersion and with a 25 µL cm^-2 ionomer loading for both the photoanode and the photocathode.

Keywords: anion-exchange membrane; ionic conductivity; ionomer; photo-electro-chemical applications; tandem cell.

Figure 1

Sketch of the photo-electro-chemical cell for water-splitting.

Figure 2

Influence of base type and concentration during the ion-exchange process.

Figure 3

UV-Vis–NIR absorption for FAA3-50 in chloride, bromide, or hydroxide form.

Figure 4

(a,b) Anion conductivity of the membrane with different counter-ions for the quaternary ammonium at (a) 30 °C, (b) 60 °C.

Figure 5

Polarization chopped curves for the two different ionomer dispersion at photoanode.

Figure 6

(a) On–off polarization curves by varying ionomer loading at photoanode; (b) efficiency versus photoanode ionomer loading.

Figure 7

(a) On–off polarization curves by varying ionomer loading at photocathode; (b) efficiency versus photocathode ionomer content.