Analysis of Homogeneous Water Oxidation Catalysis with Collector-Generator Cells

A collector-generator (C-G) technique has been applied to determine the Faradaic efficiencies for electrocatalytic O2 production by the homogeneous water oxidation catalysts Ru(bda)(isoq)2 (1; bda = 2,2'-bipyridine and isoq = isoquinoline) and [Ru(tpy)(bpz)(OH2)](2+) (2; tpy = 2,2':6',2″-terpyridine and bpz = 2,2'-bipyrazine). This technique uses a custom-fabricated cell consisting of two fluorine-doped tin oxide (FTO) working electrodes separated by 1 mm with the conductive sides facing each other. With a catalyst in solution, water oxidation occurs at one FTO electrode under a sufficient bias to drive O2 formation by the catalyst; the O2 formed then diffuses to the second FTO electrode poised at a potential sufficiently negative to drive O2 reduction. A comparison of the current versus time response at each electrode enables determination of the Faradaic efficiency for O2 production with high concentrations of supporting electrolyte important for avoiding capacitance effects between the electrodes. The C-G technique was applied to electrocatalytic water oxidation by 1 in the presence of the electron-transfer mediator Ru(bpy)3(2+) in both unbuffered aqueous solutions and with the added buffer bases HCO3(-), HPO4(2-), imidazole, 1-methylimidazole, and 4-methoxypyridine. HCO3(-) and HPO4(2-) facilitate water oxidation by atom-proton transfer (APT), which gave Faradaic yields of 100%. With imidazole as the buffer base, coordination to the catalyst inhibited water oxidation. 1-Methylimidazole and 4-methoxypyridine gave O2 yields of 55% and 76%, respectively, with the lower Faradaic efficiencies possibly due to competitive C-H oxidation of the bases. O2 evolution by catalyst 2 was evaluated at pH 12 with 0.1 M PO4(3-) and at pH 7 in a 0.1 M H2PO4(-)/HPO4(2-) buffer. At pH 12, at an applied potential of 0.8 V vs SCE, water oxidation by the Ru(IV)(O)(2+) form of the catalyst gave O2 in 73% yield. In a pH 7 solution, water oxidation at 1.4 V vs SCE, which is dominated by Ru(V)(O)(3+), gave O2 with an efficiency of 100%. The lower efficiency for Ru(IV)(O)(2+) at pH 12 may be due to competitive oxidation of a polypyridyl ligand.