An intramolecular cobalt-peptoid complex as an efficient electrocatalyst for water oxidation at low overpotential

Abstract

Water electrolysis is the simplest way to produce hydrogen, as a clean renewable fuel. However, the high overpotential and slow kinetics hamper its applicability. Designing efficient and stable electrocatalysts for water oxidation (WO), which is the first and limiting step of the water splitting process, can overcome this limitation. However, the development of such catalysts based on non-precious metal ions is still challenging. Herein we describe a bio-inspired Co(iii)-based complex i.e., a stable and efficient molecular electrocatalyst for WO, constructed from a peptidomimetic oligomer called peptoid - N-substituted glycine oligomer - bearing two binding ligands, terpyridine and bipyridine, and one ethanolic group as a proton shuttler. Upon binding of a cobalt ion, this peptoid forms an intramolecular Co(iii) complex, that acts as an efficient electrocatalyst for homogeneous WO in aqueous phosphate buffer at pH 7 with a high faradaic efficiency of up to 92% at an overpotential of about 430 mV, which is the lowest reported for Co-based homogeneous WO electrocatalysts to date. We demonstrated the high stability of the complex during electrocatalytic WO and that the ethanolic side chain plays a key role in the stability and activity of the complex and also in facilitating water binding, thus mimicking an enzymatic second coordination sphere.

Fig. 1. (a) Peptoid ligand TBE and (b) the Co-peptoid complex CoTBE.

Fig. 2. (a) UV-Vis spectra for the titration of peptoid TBE with Co ions (20 μM) in 0.1 M PBS at pH 7.0 and a metal-to-peptoid ratio plot for Co binding with TBE (inset). (b) Job-plot of TBE with Co measured in methanol (33 μM total concentration).

Fig. 3. (a) CVs of 0.5 mM CoTBE and blank at scan rate 100 mV s⁻¹ (b) CVs of 0.5 mM CoTBE at scan rates 100 and 50 mV s⁻¹; all the CVs are obtained in 0.1 M phosphate buffer at pH 7, using Ag/AgCl as the reference electrode with glassy carbon as the working electrode (0.07 cm²) and a Pt wire as the counter electrode.

Fig. 4. (a) The evolution of O₂ and (b) total accumulated charge during CPE in 0.1 M phosphate buffer at pH 7.0 containing 0.5 mM catalyst CoTBE and the buffer only using porous glassy carbon as the working electrode at +1.25 V vs. NHE for 10 hours. Evolution of O₂ was measured with a fluorescent probe.

Fig. 5. CVs of 0.5 mM CoTBE at different scan rates in 0.1 M phosphate buffer pH 7 in a (a) broad scanning range and (b) narrow scanning range. (c) Linear regression of i_dversus ν^1/2. (d) Linear regression of i_cat/i_dversus ν^−1/2.

Scheme 1. Plausible mechanistic cycle of CoTBE for water oxidation, where X is H₂O, H₂PO₄, ClO₄ or their combination.

Fig. 6. (a) Peptoid ligand TE and (b) peptoid ligand BE.

Fig. 7. (a) UV-Vis titration spectra of the dried sample after CPE with Ni and (b) titration of the 25 μM complex CoTBE with 1 μL aliquots of 2.5% H₂O₂ followed by UV-Vis spectroscopy. All the experiments were performed in 0.1 M PBS pH 7 at room temperature.

Fig. 8. CV scans of 0.1 M phosphate buffer solution at pH 7 in the absence of a catalyst and in the presence and of 0.5 mM CoTBE, CoTB-OCH₃, CoTB-CH₃, CoTB-BZ and CoTB (scan rates = 100 mV s⁻¹).

Fig. 9. (a) Pourbaix diagram of CoTBE in 0.1 M PBS in a pH range between 6.5 and 9. (b) FTIR spectra of the CoTBE (1 mM, 5 mL) complex during electrolysis at 1.25 V vs. NHE in 0.1 M PBS. At pH 7, (c) FTIR spectra of the CoTBE (1 mM, 1 mL) complex after addition of few drops of 2% H₂O₂ in 0.1 M PBS at pH 7.