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| Fig. 1: Schematic of the reaction for water to split to H2 and O2. (Source: Wikimedia Commons) |
In a previous report, I described a device for energy conversion called a Photoelectrochemical (PEC) cell. [1] A typical PEC cell consists of both anode, where oxidation reaction occurs and cathode, where reduction reaction occurs. [2] Here I will talk chiefly about the anode side of this device.
For the anode side of a PEC cell or electrochemical cell, the anode side is positively biased, and there are extra holes (h+) generated here. These holes are very oxidative and will oxidize something that they meet in the electrolyte. [3] The most common electrolyte is water. It can be oxidized to three different products through three different pathways - one hole to oxidize water to OH radical (*OH), two holes to oxidize water to H2O2, and four holes to oxidize water to O2 (Fig. 1):
| H2O + h+ | → | *OH + H+ |
| 2 H2O + 2 h+ | → | H2O2 + 2 H+ |
| 2 H2O + 4 h+ | → | O2 + 4 H+ |
For all three reactions, there will be protons (H+) generated, which will decrease the local pH environment of the cell. As for the products, O2 is a common species that can be used for medical field or combustion. The water oxidation to O2 has been studied for decades. [4,5] The abundance of O2 molecules in the air lowers the significance of the generated O2 from this electrochemical approach. Previous research on water oxidation mainly has focused on the benefit from it to promote H2 evolution at the cathode side. [6]
Compared to the four-hole pathways for O2 generation, the other approaches seem more attractive. The product from the two-hole pathway, H2O2, is a mild oxidant widely used in industry and medicine. Typical uses include paper bleaching, disinfection, and formation of precursors for other chemical synthesis. [7] The product from the one-hole pathway, OH radical, is also a good oxidant, but too much so. It is extremely unstable and has a short lifetime. It mainly appears as an intermediate in many reactions. [8]
These three pathways constitute competing reactions. The total number of holes at the anode sides are fixed, so if we want to get more H2O2 from water oxidation we need to enhance the two-hole pathway while depressing the other two. To this end, people are researching the catalysts which selectively oxidize water to H2O2, as opposed to the other two products. In recent years, many metal oxides have been studied as possible electrochemical water oxidation catalysts. The principle of the two-hole pathway has been investigated both through density functional theory (DFT) and through experiment. [9] With the continuation of these studies, it is hoped that better catalysts for for water oxidation will be developed in the future.
© Xinjian Shi. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] X. Shi, "Photoelectdrochemical Cell for Energy Conversion," Physics 240, Stanford University, Fall 2018.
[2] M. Grätzel, "Photoelectrochemical Cells," Nature 414, 338 (2001).
[3] S. R. Pendlebury et al., "Dynamics of Photogenerated Holes in Nanocrystalline α-Fe2O3 Electrodes for Water Oxidation Probed by Transient Absorption Spectroscopy," Chem. Commun. 47, 716 (2011).
[4] D. K. Zhong et al., "Solar Water Oxidation by Composite Catalyst/α-Fe2O3 Photoanodes," J. Am. Chem. Soc. 131, 6086 (2009).
[5] D. K. Zhong et al., "Photo-Assisted Electrodeposition of Cobalt-Phosphate (CoPi) Catalyst on Hematite Photoanodes for Solar Water Oxidation," Energy Environ. Sci. 4, 1759 (2011).
[6] O. Khaselev and J. A. Turner, "A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting," Science 280, 425 (1998).
[7] M. L. Bianchi, R. Crisol, and U. Schuchardt, "Bleaching of Commercial Pulps with H2O2 Catalyzed by Heteropolyacids," Bioresource Technol. 68, 17 (1999).
[8] C. Comninellis, "Electrocatalysis in the Electrochemical Conversion/Combustion of Organic Pollutants for Waste Water Treatment," Electrochim. Acta 39, 1857 (1994).
[9] X. Shi et al., "Understanding Activity Trends in Electrochemical Water Oxidation to Form Hydrogen Peroxide," Nat. Commun. 8, 701 (2017).
