Electrocatalytic Water Oxidation Catalysis Beginning with Cobalt Polyoxometalates: Searching for the True Catalyst
Renewable Hydrogen is an attractive alternative to replace carbon based fuels. Efficiently generating H2 in a carbon neutral process requires electrochemical splitting of water into its H2 and O2 components, thereby storing 285.5 kJ/mol of H2 produced. However, the kinetic bottleneck, and therefore efficiency-limiting reaction of H2 generation, is the 4 e–, 4H+ oxidation of water to O2. To improve the efficiency of this half reaction, a significant effort has been made worldwide to make both homogeneous and heterogeneous water oxidation catalysts (WOCatalysts). Cobalt polyoxometalates (Co-POMs) are a class of entirely inorganic, soluble metal oxide clusters that have attracted special interest as WOCatalysts because they are molecularly defined, mechanistically understandable, and in principle rationally fine-tunable. Moreover, they have been repeatedly claimed to be stable homogeneous WOCs,1,2 a point that, if true, makes them truly important as WOCs. However, a 2011 report, demonstrated that Co4P2W18O6810− (an archetypal Co-POM used in WOCatalysis) leaches a small amount of Co(II)aq which forms heterogeneous CoOx under electrochemically driven WOC conditions.3 This electrode bound CoOx accounts for 100 ± 12 % of the observed WOC activity under electrochemical WOC conditions.3 Herein, we explore seven prototype Co-POMs that are important in the field of WOC, including Co4P2W18O6810−, its structural analog Co4P4W30O11216− and the recently reported Co4V2W18O6810− among others.4 We use multiple techniques, including Co(II)aq induced 31P NMR line broadening and cathodic stripping, to quantify the stability and in particular the amount of Co(II)leached from each Co-POMs under solution WOCatalysis conditions. We demonstrate that the Co(II)leached from the Co-POMs serves as a precursor to heterogeneous CoOx under electrochemically driven WOC conditions—that is, none of the Co-POMs are completely stable as the literature has generally implied. We then characterize the electrode bound CoOx using electrochemistry, SEM, EDX and XPS, and demonstrate that the deposited film can quantitatively account for the observed WOCatalysis. The seminar will conclude with the interesting case of the V-congner,5 Co4V2W18O6810−, a Co-POM claimed to be both 200-fold more active than its P-analog, Co4P2W18O6810−, as well as completely stable, for reasons that were not provided in the literature and have no firm precedent.
(1) Yin, Q.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L. Science 2010, 328 (5976), 342.
(2) Lv, H.; Song, J.; Geletii, Y. V.; Vickers, J. W.; Sumliner, J. M.; Musaev, D. G.; Kögerler, P.; Zhuk, P. F.; Bacsa, J.; Zhu, G.; Hill, C. L. J. Am. Chem. Soc. 2014, 136 (26), 9268.
(3) Stracke, J. J.; Finke, R. G. J. Am. Chem. Soc. 2011, 133 (38), 14872.
(4) Folkman, S. J.; Finke, R. G. ACS Catal. 2016, 7.
(5) Folkman, S. J.; Kirner, J. T.; Finke, R. G. Inorg. Chem. 2016, 55 (11), 5343.
Speaker: Scott Folkman
Speaker Institution: Colorado State University
Event Date: 03-10-2017
Event Time: 4:00 PM
Event Location: Chemistry A101
Mixer Time: 3:45 PM
Mixer Location: Chemistry B101E
Host: R. Finke