Characterization of an amorphous iridium water-oxidation catalyst electrodeposited from organometallic precursors

James D. Blakemore, Michael W. Mara, Maxwell N. Kushner-Lenhoff, Nathan D. Schley, Steven J. Konezny, Ivan Rivalta, Christian F.A. Negre, Robert C. Snoeberger, Oleksandr Kokhan, Jier Huang, Andrew Stickrath, Lan Anh Tran, Maria L. Parr, Lin X. Chen, David M. Tiede, Victor S. Batista, Robert H. Crabtree, Gary W. Brudvig

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Abstract

Upon electrochemical oxidation of the precursor complexes [Cp*Ir(H2O)3]SO4 (1) or [(Cp*Ir)2(OH)3]OH (2) (Cp* = pentamethylcyclopentadienyl), a blue layer of amorphous iridium oxide containing a carbon admixture (BL) is deposited onto the anode. The solid-state, amorphous iridium oxide material that is formed from the molecular precursors is significantly more active for water-oxidation catalysis than crystalline IrO2 and functions as a remarkably robust catalyst, capable of catalyzing water oxidation without deactivation or significant corrosion for at least 70 h. Elemental analysis reveals that BL contains carbon that is derived from the Cp* ligand (∼ 3% by mass after prolonged electrolysis). Because the electrodeposition of precursors 1 or 2 gives a highly active catalyst material, and electrochemical oxidation of other iridium complexes seems not to result in immediate conversion to iridium oxide materials, we investigate here the nature of the deposited material. The steps leading to the formation of BL and its structure have been investigated by a combination of spectroscopic and theoretical methods. IR spectroscopy shows that the carbon content of BL, while containing some C-H bonds intact at short times, is composed primarily of components with C=O fragments at longer times. X-ray absorption and X-ray absorption fine structure show that, on average, the six ligands to iridium in BL are likely oxygen atoms, consistent with formation of iridium oxide under the oxidizing conditions. High-energy X-ray scattering (HEXS) and pair distribution function (PDF) analysis (obtained ex situ on powder samples) show that BL is largely free of the molecular precursors and is composed of small, <7 Å, iridium oxide domains. Density functional theory (DFT) modeling of the X-ray data suggests a limited set of final components in BL; ketomalonate has been chosen as a model fragment because it gives a good fit to the HEXS-PDF data and is a potential decomposition product of Cp*.

Original languageEnglish
Pages (from-to)1860-1871
Number of pages12
JournalInorganic Chemistry
Volume52
Issue number4
DOIs
Publication statusPublished - Feb 18 2013

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ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

Cite this

Blakemore, J. D., Mara, M. W., Kushner-Lenhoff, M. N., Schley, N. D., Konezny, S. J., Rivalta, I., Negre, C. F. A., Snoeberger, R. C., Kokhan, O., Huang, J., Stickrath, A., Tran, L. A., Parr, M. L., Chen, L. X., Tiede, D. M., Batista, V. S., Crabtree, R. H., & Brudvig, G. W. (2013). Characterization of an amorphous iridium water-oxidation catalyst electrodeposited from organometallic precursors. Inorganic Chemistry, 52(4), 1860-1871. https://doi.org/10.1021/ic301968j