Towards Selective Partial Water Oxidation to Form H2O2: An Attractive Substitute for O2 Evolution in Water Splitting

Abstract

Efficient and financially attractive (photo)electrochemical water splitting for the production of hydrogen (H2) is essential to drive a green hydrogen economy. However, techno-economic analysis predicts that the price of H2 obtained through (P)EC water splitting is over $10 000 ton-1 at solar-to-hydrogen (STH) efficiencies of ca. 10 %, whereas the current market value of H2 produced by conventional steam methane reforming is only $1400 ton-1.[1, 2] Co-production of high-value chemicals at the anode is an appealing strategy to increase the financial attractiveness of the production of 'green hydrogen.In ‘classic’ (P)EC water splitting, oxygen is the main product at the anode, formed at a potential of E0 (O2/H2O) = +1.23 V vs RHE. Though thermodynamically less favorable (E0 (H2O2/H2O) = +1.78 V vs RHE), the selective partial oxidation of water to hydrogen peroxide (H2O2) is of industrial interest.[2] With a market price of $500-1200 ton-1 for H2O2 and given the low market value of O2 (only $35 ton-1), the financial incentive is clearly demonstrated. Considering that the demand for H2O2 is constantly increasing, hydrogen production through a process with concomitant H2O2 has a high potential of becoming competitive with steam methane reforming.Here, we discuss the techno-economics of a PEC water splitting system producing H2O2 and H2. We will clearly show that an electrochemical process holds great financial promise for industrial development. Not only do we demonstrate at which prices H2 and H2O2 can be sold to make a profit, we also discuss which parameters need to be optimized to decrease the hydrogen price. Interestingly, when the H2O2 price is $0.85 kg-1, an STH of only 9.39% is required to compete with hydrogen production by steam methane reforming.As the anodic formation of H2O2 is not yet well explored, it will also be shown that continuous electrochemically production of H2O2 at a rate of 0.092 µmol min-1 cm-2 @ 7.08 mA/cm2 through partial water oxidation can easily be achieved in an non-optimized electrochemical cell. Finally, the importance of electrolyte, electrode and reactor engineering to increase the Faradaic efficiency and the rate of H2O2 production will be revealed.