Selective Silicon Oxide Overlayers for Solar Hydrogen Production By Seawater Electrolysis

Abstract

In PV-electrolysis, solar energy can be used to electrolyze water and produce energy-rich hydrogen (H2), a robust energy storage medium. This hydrogen can be fed into a fuel cell for dispatchable electricity to mitigate the daily intermittency of solar photovoltaic generation, or stored and transported over longer timescales. When used as a precursor for traditional fuels, H2 can bring solar energy to difficult-to-electrify sectors such as transportation. However, solar hydrogen production consumes water, and many of the world’s most sun-drenched regions, such as arid coastal areas, have a scarcity of freshwater and an abundance of seawater. It would be preferable to perform PV-electrolysis with seawater instead of freshwater, but oxidation of the chloride ions from seawater causes the undesired chlorine evolution reaction (CER). This reaction competes directly with the desired oxygen evolution reaction (OER) at the water-splitting anode. CER possesses a strong kinetic advantage over OER, largely because CER involves the transfer of only 2 electrons, while OER is a 4-electron reaction. As such, CER can outcompete OER in saline electrolyte, greatly reducing the faradaic efficiency of the electrolyzer and producing toxic Cl2 gas that is difficult to dispose of. This research aims to develop an OER-selective catalyst for seawater electrolysis by encapsulation with overlayers of silicon oxide (SiOx)—an amorphous film 2-20 nm thick that is fully stable in the acidic anodic environment. These overlayers were fabricated on planar thin-film platinum electrocatalysts by spin-coating an overlayer of PDMS precursor and converting the PDMS to SiOx in a UV-ozone treatment. The coated electrocatalysts were tested through electrochemical characterization in a three-electrode cell in an acidic aqueous electrolyte with the salinity of seawater. Through electrochemical tests including linear sweep voltammetry, we have shown SiOx-encapsulated Pt electrocatalysts to achieve greatly reduced CER activity compared to bare Pt, leading to faradaic efficiencies of up to 98%, compared to 2.7% for bare Pt. We see that SiOx selectively rejects transport of Cl- through to the buried catalyst while only slightly reducing maximum OER activity. In addition, SiOx has maintained its OER-selectivity throughout long-term operation in chronoamperometry tests. This research makes possible offshore ‘solar fuels rigs’ that use solar electricity to electrolyze seawater. With a SiOx-coated anode catalyst, the electrolyzer could suppress the competing chlorine evolution reaction and produce sustainable hydrogen at high faradaic efficiency.