Carbonate Ions Induce Highly Efficient ... - Wiley Online Library

DOI: 10.1002/cssc.201601494

Communications

Carbonate Ions Induce Highly Efficient Electrocatalytic Water Oxidation by Cobalt Oxyhydroxide Nanoparticles Kaoru Aiso, Ryouchi Takeuchi, Takeshi Masaki, Debraj Chandra, Kenji Saito, Tatsuto Yui, and Masayuki Yagi*[a] 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.

acceptors that help to transport the protons produced by water oxidation at the OEC inside photosystem II out into the chloroplast’s lumen, resulting in light-driven O2 production.[6] Synthetic models of the OEC are required not only to gain better understanding of the mechanism and the roles of cofactors for water oxidation in photosynthesis, but also to act as water oxidation catalysts to realize artificial photosynthesis, which is anticipated to be a promising solar fuel production system in the context of contemporary energy and environment issues.[7, 8] In the last decade, many efforts have been devoted to develop water oxidation catalysts as OEC models, especially efficient, earth-abundant catalysts with low overpotential and sufficient robustness to fabricate artificial photosynthetic devices of practical use.[8–13] Such laudable studies have yielded various types of complex and oxide catalysts based on earth-abundant metals, such as cobalt,[14–29] manganese,[30–32] copper,[33–35] nickel,[36] and iron.[28, 37, 38] However, attention in most of these studies has been paid only to the composition and structure of active sites in the OEC models, even though the cofactors are also essential for water oxidation at the photosynthetic OEC. There have been a few reports on OEC models including roles of cofactors in homogeneous solution systems. Meyer and co-workers reported that water oxidation catalysis by a mononuclear ruthenium complex in solution is enhanced by HPO42@ ions, which act as proton acceptors due to accelerated O@O bond formation by nucleophilic attack of water at the highly oxidized Ru=O, based on the proton-concerted oxygen atom transfer mechanism.[39] A similar enhancement by HPO42@ ions was reported by Wang and Groves in catalysis by a cobalt porphyrin derivative complex in solution.[16] In the context of the important role of HPO42@ ions, Nocera and co-workers reported that these ions are required for formation of a heterogeneous water oxidation catalyst based on Co/phosphate composites (so called Co-Pi catalyst) on the electrode from a phosphate buffer solution containing cobalt(II) nitrate by electrodeposition.[26, 29] However, the effect of HPO42@ confined in the Co-Pi catalyst on water oxidation catalysis cannot be examined in principle without changing the catalyst composite. By association, the enhancement effect on photocatalytic water oxidation by a CO32@ cofactor was reported in a Pt@TiO2 powder system.[40, 41] In this case, CO32@ ions were proposed to be oxidized to CO3@C radicals by photogenerated holes on the photocatalyst surface, producing O2 via a peroxocarbonate (C2O62@) intermediate formed by the coupling of CO3@C radicals. However, no evidence of CO3@C or C2O62@ formation was detected. To date, the roles of cofactors have thus not been clarified for the heterogeneous OEC models that are

Much scientific and technological interest has been attracted to solar fuel production by photosynthesis in which carbohydrate of a high energy compound (fuel) is produced by solar light from water and carbon dioxide. Water is oxidized to O2 to be used as an electron source for the solar fuel production of carbohydrate in photosynthesis. This reaction is catalyzed at the oxygen evolving complex (OEC) in photosystem II, which is known to consist of a cubane-like Mn4CaO5 cluster, as characterized by recently advanced X-ray structure analysis.[1, 2] There has been much interest in the mechanism and the roles of cofactors for water oxidation at the OEC.[3, 4] The possible role of inorganic carbons (CO2, H2CO3, HCO3@ , CO32@) at the OEC has long been the source of debate, including the possibility of photolysis of H2CO3 (not of water) since findings on O2 evolution by photosystem II depending on CO2.[5] Messinger and coworkers demonstrated that HCO3@ ions act as mobile proton [a] K. Aiso, R. Takeuchi, T. Masaki, Dr. D. Chandra, Prof. K. Saito, Prof. T. Yui, Prof. M. Yagi Department of Materials Science and Technology Faculty of Engineering, Niigata University 8050 Ikarashi-2, Niigata 950–2181 (Japan) E-mail: [email protected] Supporting Information for this article, including experimental details, XRD patterns, Raman and EDS spectra, microscopy images, and electrochemical data, can be found under: http://dx.doi.org/10.1002/cssc.201601494.

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Communications desired for the fabrication of artificial photosynthetic devices of practical use. Herein we report a unique heterogeneous OEC model incorporating CoO(OH) nanoparticles with a CO32@ cofactor in which the high-performance electrocatalytic water oxidation by CoO(OH) nanoparticles is induced by CO32@ ions dissolved in solution. The possibility of CO32@ ions acting as proton acceptors for O@O bond formation, based on the proton-concerted oxygen atom transfer mechanism,[39] is investigated by kinetic analysis and a significant H/D isotope effect on electrocatalytic water oxidation. CoO(OH) nanoparticles were synthesized according to a previously reported method.[42] A CoO(OH) layer was formed on the ITO electrode from a CoO(OH) suspension in acetone containing I2 by an electrophoretic deposition technique.[43] The formed CoO(OH) layer was identified by X-ray diffraction (XRD; see the Supporting Information, Figure S1; in agreement with PDF No. 1-072-2280) and Raman spectroscopy with characteristic sharp and broad peaks at n˜ = 502 and 600 cm@1 for CoO(OH) assigned to Eg and A1g, respectively[44] (see the Supporting Information, Figure S2). The field emission scanning electron microscopy (FE-SEM) image shows that the layer surface is composed of CoO(OH) particles approximately 50 nm in diameter, which are well interconnected (see the Supporting Information, Figure S3 A). The layer thickness is approximately 5 mm, as observed by the cross-sectional SEM image (Figure S3 B). The amount of Co ions in the deposited CoO(OH) layer was measured to be 18.8 : 2.0 mmol under the typical preparation conditions by the inductively coupled plasma mass spectrometry (ICP-MS). For the cyclic voltammogram (CV) of the CoO(OH)-coated ITO (CoO(OH)/ITO) electrode in the NaHCO3 solution at pH 10.0 (red line in inset of Figure 1), the anodic current was generated above 0.36 V vs. Ag/AgCl (1.15 V vs. reversible hydrogen electrode (RHE), calculated as described in Figure 1 caption). During the further oxidative scan, the anodic current density remarkably increased and reached at 3.3 mA cm@2 at 0.91 V vs. Ag/AgCl (1.7 V vs. RHE) which is 2.2 or 2.7 times higher than those observed in a K2SO4 (black) or NaNO3 (blue) solution at the same pH 10.0, respectively. K2HPO4 are unavailable as electrolytes because CoO(OH) coated on ITO dissolves in aqueous solutions containing phosphoric salts. Current density vs. time profiles were obtained during the potentiostatic electrocatalysis at 0.91 V vs. Ag/AgCl (Figure 1). 0.9 mA cm@2 of the steady anodic current density was generated with bubbles of O2 evolved (confirmed by O2 detected by gas chromatography) in the NaHCO3 solution, in contrast to the disappearance of the anodic current for several minutes in the K2SO4 or NaNO3 solution. This result shows that electrocatalytic water oxidation at the CoO(OH)/ITO electrode is significantly induced in the NaHCO3 solution, in contrast to being catalytically inactive in the K2SO4 or NaNO3 solution. Electrochemical oxidation of electrolyte species of CO3@ or HCO32@ is thermodynamically impossible at low applied potential of 0.91 V vs. Ag/AgCl. The time course of the amount of O2 evolved (nO2 [mol]) during the electrocatalysis in the NaHCO3 solution was monitored (see the Supporting Information, Figure S4). The slope (1.2 nmol s@1) of the nO2 vs. time curve ChemSusChem 2017, 10, 687 – 692

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Figure 1. Current density–time curves in electrocatalytic water oxidation at 0.91 V vs. Ag/AgCl (1.7 V vs. RHE) in 0.1 m NaHCO3 (red), 0.1 m K2SO4 (green), and 0.1 m NaNO3 (blue) aqueous solutions (pH 10.0) with a CoO(OH)/ITO electrode. The potentials vs. RHE (ERHE) were calculated by ERHE = EAg/ 0 0 AgCl + (0.059pH) + E Ag/AgCl, where EAg/AgCl and E Ag/AgCl are the potentials measured vs. Ag/AgCl and the standard potential (0.199 V vs. SHE) of an Ag/ AgCl reference electrode, respectively. Inset shows cyclic voltammograms (CVs) of a CoO(OH)/ITO electrode in the corresponding electrolyte solutions at a 50 mV s@1 scan rate. The black line is the CV of an ITO electrode as a blank in 0.1 m NaHCO3 at pH 10.0.

at the initial stage (ca. 10 min) was lower (51 %) than that (2.3 nmol s@1) calculated from the charge amount passed during the electrocatalysis. This is explained by the charge accumulation due to partial oxidation of the CoO(OH) layer to the CoIV state prior to water oxidation. The charge accumulation is supported by the cathodic wave at 0.4 V vs. Ag/AgCl in the reductive reverse potential scan on the CV data (inset of Figure 1). However, the slope (1.6 nmol s@1) of the nO2 vs. time curve over 2 h (after the sufficient charge ambulation) is in good agreement with that calculated from the charge amount (1.6 nmol s@1), indicating that the Faraday efficiency for O2 evolution is nearly 100 % for long-term electrocatalysis (over 2 h). The XRD pattern and Raman spectra of the CoO(OH)/ITO electrode did not change after 3 h of electrocatalysis in NaHCO3 solution (see the Supporting Information, Figures S1 and S2), showing that the CoO(OH)/ITO electrode works stably for electrocatalytic water oxidation. No changes in the FE-SEM image, energy-dispersive X-ray spectra, or X-ray photoelectron spectra (see the Supporting Information, Figures S3, S5, and S6) excludes the possibility of the deposited active species[45] on the electrode surface during the electrocatalysis. Linear sweep voltammetry was carried out to reveal the mechanism of catalytic water oxidation at the CoO(OH)/ITO electrode (Figure 2 A). The current peak was detected at 0.48 V vs. Ag/AgCl and pH 10.0, being assigned to oxidation of CoIII centers to CoIV based on reported data.[46] The anodic peak potential (Eap) was pH dependent, with a slope of @0.103 V per pH unit (blue circles in Figure 2 B), showing that the initial oxidation of CoIII centers to CoIV is a proton-coupled redox pro688


Communications .

@E @pH

-

. i

¼@

@E @logi

- . pH

@logi @pH

E

ð1Þ

The common logarithm values of the catalytic current densities at 1.0 V vs. Ag/AgCl are plotted vs. pH in Figure 2 B (red triangles). The plots gave a linear relationship with a slope of @0.58 per pH unit for ½ð@logiÞ=ð@pHÞAE . Tafel plots (see the Supporting Information, Figure S7) provided Tafel slopes of 75– 103 mV dec@1 for ½ð@E Þ=ð@logiÞApH : By using Equation (1), the value of ½ð@E Þ=ð@pHÞAi was calculated from both of the aforementioned slopes to be @0.044 to @0.060 V per pH unit, which is fairly consistent with the experimental value (@0.070 V per pH unit). This result provides strong evidence for the proton-coupled water oxidation process. Potentiostatic electrocatalysis for water oxidation under the same overpotential (1.7 V vs. RHE) was examined with the NaHCO3 concentration changed under the same ion strength conditions ([CO32@] + [SO42@] = 100 mm) at a different pH range of 9.0–11.0. The catalytic current density (Icat [mA cm@2]) was taken from the constant current density after 20 min of electrocatalysis and plotted vs. the total concentration ([CO32@]T) of CO32@ (NaHCO3 added) in the solution (Figure 3 A). Icat increased by a factor of 180 (from 0.005 to 0.89 mA cm@2) with a [CO32@]T increase from 0 to 100 mm at pH 10.0, and Icat underwent a 3.5-fold increase (from 0.27 to 0.93 mA cm@2) with a pH increase from 9.0 to 11.0 at 100 mm NaHCO3 (Figure 3 A). The pH-dependent Icat correlated with distribution of CO32@ dissolved in solution as a function of pH (see the Supporting Information, Figure S8), implying that CO32@ is a key species for the induced electrocatalysis by the CoO(OH) layer. The Icat values given in Figure 3 A were re-plotted against the concentration of CO32@ species dissolved in the electrolyte solution ([CO32@]), which was calculated by using Equation (2), with pKa1 = 6.37 and pKa2 = 10.25 for H2CO3 (Figure 3 B). ½CO2@ 3 A ¼

Figure 2. A) Linear sweep voltammograms of a CoO(OH)/ITO electrode in a 0.1 m NaHCO3 electrolyte solution at different pH as measured at a 0.5 mV s@1 scan rate. Inset shows the linear sweep voltammograms magnified at low potential range of 0.2–0.8 V vs. Ag/AgCl. B) Plots of Eap (blue circles), Eonset (black circles), and log(i [A cm@2]) (red triangles) vs. pH given at the linear sweep voltammogram measurements.

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½CO2@ 3 AT

10@2pH

1 þ 10@pK a2 þ 10@pK a1 > 10@pK a2

ð2Þ

These plots (Figure 3 B) gave a single smooth curve with a saturation tendency above 90 mm, suggesting that Icat is analyzable based on [CO32@]. The saturation tendency is unique because the catalytic current increases linearly with the concentration of proton acceptors (HPO42@ ions) in homogeneous electrocatalysis for water oxidation.[16, 39] It also excludes the possibility of the Icat increase due to a simple decrease of the proton-diffusion-controlled overpotential by proton-accepting CO32@ ions. The saturation tendency might be interpreted by a likely pre-equilibrium between the catalytic sites (Scat@H2O) and CO32@ ions for formation of the Scat@H2O···CO32@ intermediate prior to water oxidation [Eq. (3)]:

cess. The catalytic current was observed at 0.78 V vs. Ag/AgCl (pH 10.0, 1.57 V vs. RHE) of the onset potential (Eonset), which is defined as the potential for 100 mA cm@2 current generation. The overpotential (h = 0.34 V) is not as high as the minimum onset overpotential (h = 0.18 V for FeCoOx at pH 13)[27] of earth-abundant catalysts for electrocatalytic water oxidation, but is comparable with those of highly active cobalt-based catalysts (h = 0.18–0.37 V).[13, 18, 23–28] Eonset are also pH-dependent with a slope of @0.070 V per pH unit (black circles in Figure 2 B), suggesting that water oxidation involves a protoncoupled process subsequently to initial oxidation of CoIII centers to CoIV. The pH dependence of Eonset is described by Equation (1) with potential E [V] and catalytic current density I [A cm@2] in proton-coupled electrocatalysis.[47–49] ChemSusChem 2017, 10, 687 – 692

10@pH

p:e:

Scat @H2 O þ CO3 2@ $ Scat @H2 O ? ? ? CO3 2@ ði:m:Þ ! ðScat Þred þ CO3 2@ þ 1=2 O2 þ 2 Hþ

ð3Þ

where (Scat)red expresses the reduced form of Scat after an O2 release (p.e. = pre-equilibrium; i.m. = intermediat). The curvature 689


Communications the Icat data in Figure 3 B, supporting the pre-equilibrium for intermediate formation prior to water oxidation. The best fitting of Equation (4) to the Icat data gave Imax = 1.3 : 0.07 mA cm@2 and Km = 25 : 2.9 mm. To explore the possible role of CO32@ in inducing catalysis by the CoO(OH) layer, including the possibility of CO32@ acting as a proton acceptor for O@O bond formation based on the proton-concerted oxygen atom transfer mechanism,[16, 39] we evaluated the H/D isotope effect on the electrocatalytic water oxidation in 0.1 m NaHCO3 solutions of H2O and D2O by using the CoO(OH)/ITO electrode. The catalytic current in H2O [(Icat)H = 0.63 : 0.046 mA cm@2 as an average value for three trials] was higher than that in D2O [(Icat)D = 0.090 : 0.005 mA cm@2] by a factor of 7.1 : 0.19 (Figure 4 and Table 1). The value of (Icat)H/(Icat)D = 7.1 is reveals a more significant isotope effect of H/D on the electrocatalysis than electrocatalysis in K2SO4 solutions of H2O and D2O media [(Icat)H/ (Icat)D = 1.6 : 0.17; Table 1 and Figure S9 in the Supporting Information]. Icat could be controlled by a chemical process for O2 production, such as O@O bond formation, rather than an oxidation process at CoIII centers at sufficiently higher applied potential (1.7 V vs. RHE) relative to the oxidation potential (1.25 V vs. RHE) of the CoIII centers.

Figure 3. Plots of the catalytic current densities (Icat [mA cm@2]) at various pH versus (A) total concentration ([CO32@]T) of CO32@ (NaHCO3 added) and (B) concentration ([CO32@]) of CO32@ ions dissolved in solution, calculated by Equation (2) with pKa1 = 6.37 and pKa2 = 10.25 for NaHCO3. Electrocatalysis was conducted by using a CoO(OH)/ITO electrode under the conditions: ion strength, 100 mm ([CO32@] + [SO42@] = 100 mm); applied potential, 1.7 V vs. RHE (0.85, 0.91 0.95 and 0.97 V vs. Ag/AgCl at pH 11.0, 10.0, 9.4 and 9.0, respectively). The red line is a simulated curve based on Equation (4) in Figure 2 B.

Figure 4. Current density–time curve recorded during electrocatalytic water oxidation at 0.91 V vs. Ag/AgCl (1.7 V vs. RHE) in 0.1 m NaHCO3 solutions in H2O (black line) and D2O (red line) media (pH or pD = 10.0) by using a CoO(OH)/ITO electrode.

plots of Icat vs. [CO32@] were analyzed using the Michaelis– Menten type equation [Eq. (4)] on assuming the pre-equilibrium shown by Equation (3): Icat ¼

Imax > ½CO2@ 3 A K m þ ½CO2@ 3 A

Table 1. Summary of Icat data in electrocatalytic water oxidation in electrolyte solutions (pH or pD = 10.0) in H2O [(Icat)H] or D2O [(Icat)D] media by using a CoO(OH)/ITO electrode.

ð4Þ

where Imax [mA cm@2] and Km [mm] are the maximum catalytic current density for water oxidation (signifying the expected maximum catalytic activity) and an affinity constant of CO32@ to Scat@H2O for the intermediate formation (signifying the CO32@ concentration for the half Imax). Equation (4) fitted well to ChemSusChem 2017, 10, 687 – 692

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Electrolyte

(Icat)H [mA cm@2]

(Icat)D [mA cm@2]

(Icat)H/(Icat)D

Applied potential [V vs. RHE]

0.1 m NaHCO3 0.1 m K2SO4

0.63 : 0.046 0.36 : 0.023

0.090 : 0.005 0.22 : 0.010

7.1 : 0.19 1.6 : 0.17

1.7 2.1[a]

[a] The higher applied potential was employed for electrocatalysis in 0.1 m K2SO4 to confirm O2 evolution.

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Scheme 1. Proposed mechanism of electrocatalytic water oxidation induced by CO32@ on CoO(OH) nanoparticles.

Acknowledgements

The proposed mechanism of electrocatalytic water oxidation induced by CO32@ on the CoO(OH)/ITO electrode is shown in Scheme 1. The CoIII centers of CoO(OH) are electrochemically oxidized to CoIV by a proton-coupled redox process (Figure 2). The hydroxo groups on CoIII and/or CoIV centers could be partially exchanged with CO32@ ions at higher CO32@ concentrations, being concerned with the pre-equilibrium prior to water oxidation [Eq. (3)]. On further proton-coupled oxidation, the hydroxo on the CoIV center nucleophilically attacks the oxo on another CoIV center in close proximity, forming a peroxo O@O bond on CoIV centers. The O@O bond formation should be accelerated due to effective proton acceptance by adjacent CO32@ ions coordinated on the CoIV center in the intermediate based on the proton-concerted oxygen atom transfer mechanism, which is consistent with Michaelis–Menten-type kinetics (Figure 3 B) and the significant H/D isotope effect (Figure 4). The O@O peroxo on CoIV centers enables spontaneous O2 release with re-reduction to CoIII centers, and the original CoO(OH) is regenerated with coordination of two water molecules. In conclusion, the CoO(OH) nanoparticle layer proved a highly active electrocatalyst for water oxidation with the help of a CO32@ cofactor. This is a unique heterogeneous OEC model, providing an important proof of concept to develop innovative cofactor-incorporating catalysts for water oxidation. The CoO(OH) nanoparticle catalyst with a CO32@ cofactor is a promising water oxidation catalyst for artificial photosynthesis, owing to its simple and facile synthesis, earth-abundant component elements and good performance in electrocatalytic water oxidation. The presented results shed new light on the role of inorganic carbons for O2 production at the photosynthetic OEC and in artificial OEC models.

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This work was supported by the JST PRESTO program, JSPS KAKENHI Grant Number 24107003, 24350028. D.C. thanks the JSPS for providing a postdoctoral fellowship. Keywords: cobalt · cofactors · electrocatalysis · oxygen evolution · water oxidation

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