Abstract

The future of energy supply depends on innovative breakthroughs regarding the design of cheap, sustainable and efficient systems for the conversion and storage of renewable energy sources. The production of hydrogen through water splitting seems a promising and appealing solution. We found that a robust nanoparticulate electrocatalytic material, H₂–CoCat, can be electrochemically prepared from cobalt salts in a phosphate buffer. This material consists of metallic cobalt coated with a cobalt-oxo/hydroxo-phosphate layer in contact with the electrolyte and mediates H₂ evolution from neutral aqueous buffer at modest overpotentials. Remarkably, it can be converted on anodic equilibration into the previously described amorphous cobalt oxide film (O₂–CoCat or CoPi) catalysing O₂ evolution. The switch between the two catalytic forms is fully reversible and corresponds to a local interconversion between two morphologies and compositions at the surface of the electrode. After deposition, the noble-metal-free coating thus functions as a robust, bifunctional and switchable catalyst.

Figures at a glance

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Figure 1: Linear voltammetry experiments.

Black: current densities (solid line) recorded at a FTO electrode (1 cm²) in phosphate buffer (KPi, 0.5 mol l⁻¹, pH 7) containing Co(NO₃)₂·6H₂O (0.5 mmol l⁻¹) at a low scan rate (0.05 mV s⁻¹) with simultaneous GC monitoring of H₂ evolution (dotted line): N₂ was continuously bubbled through the electrolyte at a constant flow (5 ml min⁻¹) during the experiment and the concentration of H₂ in the output gas was determined every 2 min by gas chromatography. This allowed the determination of the specific rate of H₂ evolution, which is reported in the right scale. With the right and left scales used, superimposition of both current density and H₂ evolution rate traces indicates 100% faradaic yield. Red: current densities (solid line) and specific rate of H₂ evolution (dotted line) recorded at a FTO electrode previously modified by controlled potential electrolysis for 3 h at −385 mV versus RHE in the Co-containing solution described above and then transferred into a cobalt-free buffer. Inset: blow-up of the low overpotential region showing evolution of H₂ with 50 mV onset overpotential.

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Figure 2: Scanning electrochemical micrographs.

a,b, SEM images of electrodes modified by electrolysis at −0.9 V versus Ag/AgCl for 1 h ([Co(DO)(DOH)pnCl₂], 0.5 mmol l⁻¹, ITO, 0.1 C cm⁻²_geometric) (a) and −1.0 V versus Ag/AgCl for 3 h (Co(NO₃)₂·6H₂O or [Co(DO)(DOH)pnCl₂], 0.5 mmol l⁻¹, FTO, 6.5 C cm⁻²_geometric) versus RHE in phosphate buffer (KPi, 0.5 mol l⁻¹, pH 7) (b). c, SEM image of an H₂–CoCat film on a FTO electrode equilibrated at 1.16 V versus Ag/AgCl for 90 min in a cobalt-free 0.5 mol l⁻¹ KPi, pH 7 electrolyte. d, SEM image of an H₂–CoCat film on a FTO electrode initially equilibrated at −1.0 V versus Ag/AgCl and taken out of the solution just after a potential switch to 1.16 V versus Ag/AgCl before equilibration of the current.

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Figure 3: Compared HER properties of H₂–CoCat and metallic cobalt.

Linear voltammetry experiments recorded at a FTO electrode (1 cm²) in phosphate buffer (KPi, 0.5 mol l⁻¹, pH 7) at a low scan rate (0.05 mV s⁻¹) for a FTO electrode modified with H₂–CoCat (filled triangles) and at a Co foil electrode (open squares). Potentials have been converted to RHE. The corresponding Tafel plots are shown in the inset.

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Figure 4: XPS characterization.

a–c, XPS survey (a), Co_2p (b), O_1s (c, top) and P_2p (c, bottom) core levels spectra of H₂–CoCat deposited on FTO substrates (red) and commercial Co₃(PO₄)₂·xH₂O (black).

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Figure 5: Fourier-transformed EXAFS spectra collected at the Co K edge.

Red trace: H₂–CoCat equilibrated at −1.0 V versus Ag/AgCl. Blue trace: H₂–CoCat equilibrated at +1.16 V versus Ag/AgCl for 4 min. Black trace: Co metal foil (hexagonal close-packed state). The XANES spectra of the film formed at −1.0 V versus Ag/AgCl and of metallic cobalt are shown in the inset. The blue arrows mark features that are assignable to the contribution (~50%, Supplementary Fig. S6) of a phase of edge-sharing CoO₆ octahedra to the spectrum of the film equilibrated at +1.16 V versus Ag/AgCl (blue line).

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Figure 6: Electrocatalytic H₂ and O₂ evolution catalysed by the CoCat film.

a, Charge passed through a FTO electrode (1 cm²) during controlled potential coulometry initially at −1.0 V versus Ag/AgCl (3 h, H₂–CoCat deposition, black curve) in 0.5 mol l⁻¹ KPi, pH 7 electrolyte containing Co(NO₃)₂·6H₂O (0.5 mmol l⁻¹) and after transfer to a cobalt-free 0.5 mol l⁻¹ KPi, pH 7 electrolyte, with potential switching between oxidative (blue, 1.16 V versus Ag/AgCl) and reductive conditions (red, −1 V versus Ag/AgCl). b, Specific rates of hydrogen (red) and oxygen (blue) evolution quantified through gas chromatography measurements during the same experiment. N₂ was continuously bubbled through the electrolyte at a constant flow (5 ml min⁻¹) during the experiment and the concentration of O₂ and H₂ in the output gas was determined every 2 min by gas chromatography.

ftp://mail.ihim.uran.ru/localfiles/NM/nmat3385.pdf

A Janus cobalt-based catalytic material for electro-splitting of water

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Abstract