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.

Water Splitting with Enhanced Efficiency Using a Nickel-Based Co-Catalyst at a Cupric Oxide Photocathode

“…As shown in Figure 1 and in our previous papers [18,19], an n-type Titanium-doped hematite photoanode is characterized by a dark orange color corresponding to a UV/vis absorption edge below 600 nm. The support, herein represented by a drilled transparent conductive oxide (TCO), is designed to permit both the water flow inside the scalable photoelectrochemical cell and the escape of the oxygen produced by water splitting.…”

“…The efficiencies of photoelectrochemical cells were calculated as reported in Equations ( 1)-( 3) and in previous papers [17][18][19]36,37].…”

“…Moreover, a gas diffusion layer (GDL) carbonaceous substrate, previously hydrophobized to avoid water flooding, was used to guarantee selective hydrogen escape from the electrode. The working principle of a tandem cell relates to the generation of electron-hole pairs at the anode and the cathode, respectively, through the interaction with incoming light, as described in our previous studies [17][18][19]. In the Fe2O3-based photoanode, band edge bending drives photogenerated holes towards the electrolyte; the edge of its valence band (VB) is at a sufficiently high potential for triggering the evolution of oxygen from water.…”

“…Herein, we present a tandem PEC cell based on low-cost and non-critical raw materials such as iron oxide and copper oxide. A solid polymeric membrane is employed as the electrolyte and gas separator with very low hydrogen and oxygen crossover [19,[27][28][29].…”

“…This study concerns a novel tandem cell architecture with a 0.25 cm 2 exposed area composed as reported in Figure 1 and focuses on the co-catalyst effect. To form a suitable ntype hematite semiconductor, Ti-doping [19] was adopted to increase the photo-response of the bare α-Fe 2 O 3 /TCO. The GDL-based-CuO photocathode was optimized in our previous study [18], achieving greater efficiency than that obtained with TCO-based electrodes (conductive glass).…”