Understanding the “Anti-Catalyst” Effect with Added CoO_x Water Oxidation Catalyst in Dye-Sensitized Photoelectrolysis Cells: Carbon Impurities in Nanostructured SnO₂ Are the Culprit

In 2017, we reported a dye-sensitized, photoelectrolysis cell consisting of fluorine-doped tin oxide (FTO)-coated glass covered by SnO2 nanoparticles coated with N,N′-bis(phosphonomethyl)-3,4,9,10-perylenediimide (PMPDI) dye and then a photoelectrochemically deposited CoOx water oxidation catalyst (WOCatalyst), FTO/nano-SnO2/PMPDI/CoOx. This system employed nanostructured SnO2 stabilized by a polyethyleneglycol bisphenol A epichlorohydrin (PEG-BAE) copolymer and other C-containing additives based on a literature synthesis to achieve a higher surface area and thus greater PMPDI dye absorption and resultant light collection. Surprisingly, the addition of the well-established WOCatalyst CoOx resulted in a decrease in the photocurrent, an unexpected “anti-catalyst” effect. Two primary questions addressed in the present study are (1) what is the source of this “anti-catalyst” effect? and (2) are the findings of broader interest? Reflection on the synthesis of nano-SnO2 stabilized by PEG-BAE, and the large, ca. 10:1 ratio of C to Sn in synthesis, led to the hypothesis that even the annealing step at 450 °C in of the FTO/SnO2 anode precursors was unlikely to remove all the carbon initially present. Indeed, residual carbon impurities are shown to be the culprit in the presently observed “anti-catalyst” effect. The implication and anticipated broader impact of the results of answering the two abovementioned questions are also presented and discussed along with a section entitled “Perspective and Suggestions for the Field Going Forward.”