The development of efficient catalyst materials for water oxidation is bottleneck to construct artificial photosynthesis that is a promising future energy-providing system. We review the recent ...progress on development of IrOx electrocatalyst films for water oxidation based on our latest research works. A controlled fabrication approach of an efficient IrOx electrocatalyst films is introduced by a facile spin-coating of a homogenous solution of Ir precursor complexes followed by simple annealing at different temperature. The composition and crystallinity of the IrOx film are tunable by a simple annealing treatment. The electrocatalytic properties of the IrOx film dramatically depended on annealing temperature are characterized to give the noticeably high performance at 300 °C annealing. A soft-template-assisted technique using different polymer surfactants as structure directing agents (SDAs) is illustrated in the spin-coating procedure to reveal impacts of SDAs on nanostructures and electrocatalytic water oxidation performances of the IrOx films. The characterization of the first accessible channel-like open pore architecture of the ordered 2D-hexagonal mesoporous IrOx film is described by a facile one-pot strategy using Pluronic F127 as SDA. The novel controlled nanostructures including mesoporous architecture of IrOx improves electrocatalytic performance for water oxidation relative to a conventional nanoparticulate structure of IrOx coating.
•Recent development of IrOx electrocatalyst films for water oxidation is reviewed.•Controlled fabrications of efficient IrOx electrocatalyst films are introduced.•Controlled nanostructures improve electrocatalytic performance for water oxidation.
Synthetic models of oxygen evolving complex (OEC) are used not only to gain better understanding of the mechanism and the roles of cofactors for water oxidation in photosynthesis, but also as water ...oxidation catalysts to realize artificial photosynthesis, which is anticipated as a promising solar fuel production system. However, although much attention has been paid to the composition and structure of active sites for development of heterogeneous OEC models, the cofactors, which are essential for water oxidation by the photosynthetic OEC, remain little studied. The high activity of CoO(OH) nanoparticles for electrocatalytic water oxidation is shown to be induced by a CO32− cofactor. The possibility of CO32− ions acting as proton acceptors for O−O bond formation based on the proton‐concerted oxygen atom transfer mechanism is proposed. The O−O bond formation is supposed to be accelerated due to effective proton acceptance by adjacent CO32− ions coordinated on the CoIV center in the intermediate, which is consistent with Michaelis–Menten‐type kinetics and the significant H/D isotope effect observed in electrocatalysis.
Carbonated catalysis: Cofactors are essential for water oxidation by the photosynthetic oxygen evolving complex. The high activity of CoO(OH) nanoparticles for electrocatalytic water oxidation is shown to be induced by a CO32− cofactor. The possibility of CO32− ions acting as proton acceptors for O−O bond formation based on the proton‐concerted oxygen atom transfer mechanism is proposed.
With a view to developing a photoanode for visible light‐driven water oxidation in solar water splitting cells, pure‐monoclinic WO3 nanorod crystals with N2 intercalated into the lattice were ...synthesized by using hydrazine with a dual functional role—as an N atom source for the in situ N2 intercalation and as a structure‐directing agent for the nanorod architecture—to gain higher incident photon‐to‐current conversion efficiency at 420 nm than with most previously reported WO3 electrodes.
The N of the rod for WO3: Pure‐monoclinic WO3 nanorod crystals with N2 intercalated into the lattice were synthesized as a photoanode for visible light‐driven water oxidation by using hydrazine as both an N atom source for N2 intercalation and a structure‐directing agent for the nanorod architecture, thus enhancing the incident photon‐to‐current conversion efficiency.
N2‐Intercalated crystalline mesoporous tungsten trioxide (WO3) was synthesized by a thermal decomposition technique with dodecylamine (DDA) as a surfactant template with a dual role as an N‐atom ...source for N2 intercalation, alongside its conventional structure‐directing role (by micelle formation) to induce a mesoporous structure. N2 physisorption analysis showed that the specific surface area (57.3 m2 g−1) of WO3 templated with DDA (WO3‐DDA) is 2.3 times higher than that of 24.5 m2 g−1 for WO3 prepared without DDA (WO3‐bulk), due to the mesoporous structure of WO3‐DDA. The Raman and X‐ray photoelectron spectra of WO3‐DDA indicated intercalation of N2 into the WO3 lattice above 450 °C. The UV/Vis diffuse‐reflectance spectra exhibited a significant shift of the absorption edge by 28 nm, from 459 nm (2.70 eV) to 487 nm (2.54 eV), due to N2 intercalation. This could be explained by the bandgap narrowing of WO3‐DDA by formation of a new intermediate N 2p orbital between the conduction and valance bands of WO3. A WO3‐DDA‐coated indium tin oxide (ITO) electrode calcined at 450 °C generated a photoanodic current under visible‐light irradiation below 490 nm due to photoelectrochemical water oxidation, as opposed to below 470 nm for ITO/WO3‐bulk. The incident photon‐to‐current conversion efficiency (IPCE=24.5 %) at 420 nm and 0.5 V versus Ag/AgCl was higher than that of 2.5 % for ITO/WO3‐bulk by one order of magnitude due to N2 intercalation and the mesoporous structure of WO3‐DDA.
N‐riched electrode: N2‐intercalated crystalline mesoporous WO3 was synthesized by thermal decomposition of H2WO4/dodecylamine (DDA) precursors. The mesoporous structure of WO3‐DDA shows a narrower distribution of smaller pores compared to the interparticle spaces of bulk WO3. An ITO/WO3‐DDA electrode can utilize visible light below 490 nm for photoelectrochemical water oxidation, as opposed to below 470 nm for an ITO/WO3‐bulk electrode (see figure).
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•β-Co(OH)2 were electrophoretically deposited on an electrode to yield a stably adherent and uniform layer.•β-Co(OH)2 did not work as a water oxidation catalyst in a K2SO4 solution at ...pH 7.0.•β-Co(OH)2 was oxidized to the CoO(OH) layer with the hexagonal-disc-shape remained.•Formed CoO(OH) works efficiently for electrocatalytic water oxidation.
The hexagonal-shaped β-Co(OH)2 discs were electrophoretically deposited on an electrode to yield a stably adherent and uniform β-Co(OH)2 layer. The electrocatalytic water oxidation at the β-Co(OH)2 layer-deposited electrode was investigated to seek efficient water oxidation catalyst for artificial photosynthesis as promising energy-providing systems for the future. The β-Co(OH)2 layer did not work as a water oxidation catalyst in a K2SO4 solution at pH 7.0 when applying 1.70V vs. reversible hydrogen electrode (RHE), although the layer was oxidized to the CoO(OH) layer. In a phosphate buffer solution (pH 7.0), the β-Co(OH)2 layer was dissolved and oxidatively re-deposited as the Co-Pi catalyst (Science, 321 (2008) 1072–1075.) when applying the same potential. While in a Na2B4O7 solution at pH 9.4, the β-Co(OH)2 layer was oxidized to the CoO(OH) layer with the hexagonal-disc-shape remained when applying 1.7V vs. RHE, and the formed CoO(OH) layer works efficiently for electrocatalytic water oxidation in contrast to no catalytic activity in a K2SO4 solution at pH 9.4. The catalytic performance of the formed CoO(OH) layer was evaluated; the overpotential (η) of 0.42V for current generation of 100μAcm−2 and the Tafel slope of 0.21∼0.29V dec−1 were provided for water oxidation at pH 9.4, which are comparable with efficient cobalt-based catalyst films reported so far.
A heterogeneous catalyst incorporating an inorganic ion cofactor for electrochemical water oxidation was exploited using a CoO(OH) nanoparticle layer-deposited electrode. The significant catalytic ...current for water oxidation was generated in a Na2B4O7 solution at pH 9.4 when applying 0.94 V versus Ag/AgCl in contrast to no catalytic current generation in the K2SO4 solution at the same pH. HB4O7 – and B4O7 2– ions were indicated to act as key cofactors for the induced catalytic activity of the CoO(OH) layer. The Na2B4O7 concentration dependence of the catalytic current was analyzed based on a Michaelis–Menten-type kinetics to provide an affinity constant of cofactors to the active sites, K m = 28 ± 3.6 mM, and the maximum catalytic current density, I max = 2.3 ± 0.13 mA cm–2. The I max value of HB4O7 – and B4O7 2– ions was 1.4 times higher than that (1.3 mA cm–2) for the previously reported case of CO3 2– ions. This could be explained by the shorter-range proton transfer from the active site to the proton-accepting cofactor because of the larger size and more flexible conformation of HB4O7 – and B4O7 2– ions compared with that of CO3 2– ions.
The Cover Feature shows N2‐intercalated pure‐monoclinic WO3 nanorod crystals as a photoanode for visible light‐driven water oxidation. The nanorod crystals with N2 intercalated into the lattice were ...synthesized to gain higher incident photon‐to‐current conversion efficiency at 420 nm than with most previously reported WO3 electrodes. More information can be found in the Communication by Li et al. on page 1151 in Issue 7, 2018 (DOI: 10.1002/cssc.201702439).
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•Transparent IrOx catalyst films for water oxidation are fabricated.•Nanostructues of IrOx are tunable by surfactant structure directing agents.•Brij 58 results in the layer of very ...small IrOx nanoparticles of ca. ≤2nm.•Pluronic F127 results in the organized mesoporous structures of IrOx.•Brij 58 leads to the higher activity of IrOx for water oxidation than untemplated.
Transparent IrOx catalyst films for water oxidation were fabricated on fluorine-doped tin oxide electrodes from Ir precursor complex solutions containing Pluronic P123, Brij 58 or Pluronic F127 as surfactant structure directing agents (SDAs). The thin layers composed of highly dispersed and very small nanoparticles of ca. ≤2nm were formed for IrOx films derived by Pluronic P123 and Brij 58, both of which act as a ‘spacer’ between the Ir-building-blocks to inhibit further agglomeration of the nanoparticles. Pluronic F127 with relatively longer hydrophilic poly(ethylene oxide) blocks stabilizes Ir-complexes in the precursor solution due to stronger interaction between its longer EO-units and Ir-complexes, affording the organized mesoporous structures of ca. 8nm pores templated by micelles formed in the IrOx film derived by Pluronic F127. The cyclic voltammetry revealed the catalytic currents for water oxidation for the SDAs-derived IrOx films were 1.3–1.8 times higher than the IrOx film without SDA. The higher water oxidation activity observed is attributed to the higher intrinsic catalytic activity of Ir-sites on very small nanoparticles in the Pluronic P123 and Brij 58-derived IrOx films, whereas it is attributed to the increased number of active Ir-sites due to the organized mesoporous structures of the Pluronic F127-derived IrOx film.
N
-Intercalated crystalline mesoporous tungsten trioxide (WO
) was synthesized by a thermal decomposition technique with dodecylamine (DDA) as a surfactant template with a dual role as an N-atom ...source for N
intercalation, alongside its conventional structure-directing role (by micelle formation) to induce a mesoporous structure. N
physisorption analysis showed that the specific surface area (57.3 m
g
) of WO
templated with DDA (WO
-DDA) is 2.3 times higher than that of 24.5 m
g
for WO
prepared without DDA (WO
-bulk), due to the mesoporous structure of WO
-DDA. The Raman and X-ray photoelectron spectra of WO
-DDA indicated intercalation of N
into the WO
lattice above 450 °C. The UV/Vis diffuse-reflectance spectra exhibited a significant shift of the absorption edge by 28 nm, from 459 nm (2.70 eV) to 487 nm (2.54 eV), due to N
intercalation. This could be explained by the bandgap narrowing of WO
-DDA by formation of a new intermediate N 2p orbital between the conduction and valance bands of WO
. A WO
-DDA-coated indium tin oxide (ITO) electrode calcined at 450 °C generated a photoanodic current under visible-light irradiation below 490 nm due to photoelectrochemical water oxidation, as opposed to below 470 nm for ITO/WO
-bulk. The incident photon-to-current conversion efficiency (IPCE=24.5 %) at 420 nm and 0.5 V versus Ag/AgCl was higher than that of 2.5 % for ITO/WO
-bulk by one order of magnitude due to N
intercalation and the mesoporous structure of WO
-DDA.
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