A Cobalt@Cucurbit[5]uril Complex as a Highly Efficient Supramolecular Catalyst for Electrochemical and Photoelectrochemical Water Splitting

Abstract

Ah ost-guest complex self-assembled through Co 2+ and cucurbit[5]uril (Co@CB[5]) is used as as upramolecular catalyst on the surface of metal oxides including porous indium tin oxide (ITO) and porous BiVO 4 for efficient electrochemical and photoelectrochemical water oxidation. When immobilized on ITO, Co@CB[5] exhibited at urnover frequency (TOF) of 9.9 s À1 at overpotential h = 550 mV in ap H9.2 borate buffer. Meanwhile,w hen Co@CB[5] complex was immobilized onto the surface of BiVO 4 semiconductor,t he assembled Co@CB-[5]/BiVO 4 photoanode exhibited al ow onset potential of 0.15 V(vs.R HE) and ah igh photocurrent of 4.8 mA cm À2 at 1.23 V(vs.R HE) under 100 mW cm À2 (AM 1.5) light illumination. Kinetic studies confirmed that Co@CB[5] acts as asupramolecular water oxidation catalyst, and can effectively accelerate interfacial charge transfer between BiVO 4 and electrolyte.S urface charge recombination of BiVO 4 can be also significantly suppressed by Co@CB[5].

Remarkable synergy of borate and interfacial hole transporter on BiVO4 photoanodes for photoelectrochemical water oxidation

“…Open Generally, loading an OEC catalyst on a photoanode should result in an increase in water oxidation rates. 16,20,43 However, k trans of bare BiVO 4 appeared higher than that of NiFeV/B-BiVO 4 to some degree at most potentials (Fig. 4d).…”

“…3g) composed of a series resistance (R s ), an interfacial charge transfer resistance (R ct ) and a constant phase angle element (CPE). 20,65,66 All modified photoanodes featured noticeably smaller semicircles in EIS plots compared to bare BiVO 4 . In particular, R ct of NiFeV/B-BiVO 4 was only 289.2 Ω (Table S2 †) at 0.6 V RHE , implying the fastest charge transfer at the electrode/electrolyte interface by borate and NiFeV comodification, which was demonstrated in the trends of η surface in Fig.…”

“…Intensity Modulated Photocurrent Spectroscopy (IMPS) were performed on a Zahner IMPS electrochemical workstation setup with the scanning frequency range from 0.1 Hz to 10 kHz (amplitude: 5 mV). 20,47 The modulated light source (10 mW cm -2 ) for IMPS characterization was a calibrated light-emitting diode (LED). H/D kinetic isotope effect (KIE) measurements were employed according to the previous report 20 by converting the charge passed to µmol gas according to Faraday's Law.…”

“…20,47 The modulated light source (10 mW cm -2 ) for IMPS characterization was a calibrated light-emitting diode (LED). H/D kinetic isotope effect (KIE) measurements were employed according to the previous report 20 by converting the charge passed to µmol gas according to Faraday's Law.…”

“…9 Unfortunately, a pure BiVO 4 photoelectrode still suffers from excessive electron-hole recombination (carrier mobility of ~ 4×10 -2 cm 2 V -1 s -1 ), poor charge transport properties (hole diffusion length of ~ 70 nm), and sluggish water oxidation kinetics, mostly resulting in disappointing photocurrent densities. 10 To alleviate these limitations, numerous approaches have been proposed that usually combine effective synthesis methods with multiple modification strategies, including crystal facet engineering, [11][12][13] construction of heterojunctions, [14][15][16] substitutional doping, [17][18][19] oxygen evolution catalyst (OEC) loading, [20][21][22][23][24] postsynthetic treatments, [25][26][27][28] etc. For example, a landmark work reported by Choi and co-workers displayed a nanoporous BiVO 4 film photoanode (mean particle size ~ 76 nm) via a two-step method of electrodeposition of BiOI and ensuing thermal conversion of BiOI to BiVO 4 by introducing a proper vanadium source.…”