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Synthesis and demonstration of subnanometric iridium oxide as highly efficient and robust water oxidation catalyst Jingqi Guan, Deng Li, Rui Si, Shu Miao, Fuxiang Zhang, and Can Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02082 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017
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ACS Catalysis
Synthesis and demonstration of subnanometric iridium oxide as highly efficient and robust water oxidation catalyst Jingqi Guan,† Deng Li,† Rui Si,‡ Shu Miao,† Fuxiang Zhang*,† and Can Li*,† †
State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian, 116023, China. ‡
Shanghai Institute of Applied Physics, Chinese Academy Sciences, Shanghai, 201204, China
ABSTRACT: Development of highly efficient and robust water oxidation catalyst (WOC) with reduced usage of noble metals is extremely crucial for water splitting and CO2 reduction by photocatalysis or electrolysis. Herein, we synthesized subnanometric iridium dioxide clusters supported on multiwalled carbon nanotubes (MWCNTs) by a chemical vapor deposition method (nominated as IrO2/CNT). Benefitting from a mild oxidation process in air at 303 K, the deposited iridium clusters can be controlled to have a narrow size distribution from several atoms to 2 nm, having average size of ca. 1.1 nm. The subnanometric iridium-containing sample is demonstrated to be highly efficient and robust for water oxidation. The optimal turnover frequency (TOF) of chemical water oxidation on the asobtained sample can reach 11.2 s-1, and the overpotential of electrochemical water oxidation is 249 mV, 293 mV and 397 mV at 10 mA cm-2 in 1.0 M KOH (pH: 13.6), 0.5 M H2SO4 (pH: 0), and 0.5 M phosphate buffer (pH: 7.0), respectively. Based on the structural characterizations and theory simulation, the extraordinary performances of the ultrasmall iridium dioxide are proposed to mainly originate from enhanced number of unsaturated surface Ir atoms and change of local coordination environment. Our work highlights the importance of subnanometric size of iridium dioxide in water oxidation. KEYWORDS: water oxidation, electrocatalysis, density functional calculations, iridium oxide, subnanometer
INTRODUCTION Water oxidation is considered as the key and bottlenecktype reaction in the photochemical or electrochemical water splitting for hydrogen production due to its sluggish kinetics and four electron–involving transfer.1 Thus, the development of highly active and robust water oxidation catalyst (WOC) with high turnover frequency (TOF) or/and relatively low overpotential is requisite towards efficient H2 generation from water by photocatalysis,2 photoelectrocatalysis,3 and electrolysis.4 For this purpose, many kinds of oxides have been investigated, among which iridium oxide (IrO2) is demonstrated as one of the most active and robust WOCs.5-7 The IrO2 has been understood to own advantages of bearing a broad pH range, possessing low resistivity, and exhibiting high corrosion– resistance property,8-9 and it has been widely employed as cocatalyst to construct enhanced solar fuel conversion systems.10 To date, investigation of the iridium-based WOCs has been predominantly focused on the influence of structure and size on the water oxidation performance by adopting distinct preparative approaches such as atomic layer deposition (ALD),11 chemical vapor deposition (CVD),12 pulsed laser deposition,13 thermal decomposition of an iridium salt,14 and hydrolysis of an iridium salt,15 etc. For example, amorphous iridium oxide is found to be more active than crystalline IrO2, while amorphous IrOx(OH)y nanoparticles exhibit better activity with respect to amorphous iridium oxide.8 Shao-Horn et al. reported that the mass activity of r-IrO2 nanoparticles in 0.1 M HClO4 was higher than that in 0.1 M KOH.5 In addition, more and more attentions have recently been paid on the particle
size effect. Yagi et al. reported that the maximal TOF of electrochemical water oxidation on the citrate-stabilized IrO2 nanoparticles diameter of 50−100 nm is 6.6 s-1 at 581 mV overpotential (η).16 Murray et al. demonstrated that the η value at a current density (J) of 0.5 mA/cm2 is 250 mV for water oxidation on the mesoporous IrOx films with size of ~2 nm.17 Mallouk et al. reported in situ formation of 2~5 nm IrOx•xH2O nanoparticles from [Ir(OH)6]2− solutions, which shows low η (only 200 mV) for oxygen evolution at 1.5 mA cm–2.18 As a whole, decreasing size of IrO2 nanoparticles is favorable for the water oxidation, but the activity of water oxidation on the iridium oxide with particle size below 2 nm is not clear. Additionally, the TOF values of all the iridiumbased oxides (crystalline or amorphous) reported so far for both chemical and photocatalytic water oxidation are less than 0.5 s-1, and the TOF values of iridium oxide for electrochemical water oxidation at overpotential of below 300 mV are also lower than 0.02 s-1.19 Thus, it is highly desirable to achieve controllable synthesis of IrO2 with particle size of below 2 nm and further promote its TOF of water oxidation. Here we report synthesis of subnanometric iridium oxide on the surface of multiwalled carbon nanotubes (MWCNTs) for chemical and electrochemical water oxidation. The MWCNTs are known to own feature of high specific surface area, high conductivity and O2-evolving inertness20. A simple CVD method was adopted for the deposition and decoration of iridium oxide on the surface of MWCNTs. The water oxidation activity of the as-prepared catalyst was evaluated by chemical water oxidation using ammonium cerium nitrate as
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ACS Catalysis the oxidant, or by electrochemical water oxidation in acid and alkaline solutions. RESULTS AND DISCUSSION Typically, the Ir4(CO)12 precursor was sublimated and chemically adsorbed on the surface of MWCNTs, and then decomposed to form Ir/IrOx clusters at 200 oC under Ar atmosphere. In order to completely convert metallic iridium into IrO2, the as-obtained powder is further treated in air at 30 ºC for 24 h. It should be pointed out that the ultrasmall Ir clusters are active enough to be oxidized at low temperature or even room temperature, so the low temperature of 30 ºC is adopted to prevent the aggregation of iridium clusters. The as-obtained sample is denoted as IrO2/CNT for convenience. The content of iridium in IrO2/CNT was measured to be 0.5 wt.% by ICPAES. As a comparison, nano-sized IrO2 colloids were synthesized by thermal hydrolysis of K2IrCl6 without addition of stabilizer ligands,17, 21 which were then simply adsorbed on the surface of MWCNTs to obtain the IrO2/CNT-ref sample. Their detailed experimental illustration is given in the electronic supplementary information.
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even exceed those of Ir-based molecular WOCs (Table S1), demonstrating its good water oxidation behavior. Figure 1b gives the electrochemical water oxidation activity of IrO2/CNT and IrO2/CNT-ref samples in 1.0 M KOH solution. Based on their linear sweep voltammetry (LSV) curves, one can find that the onset overpotential of IrO2/CNT sample is negatively shifted by ca. 40 mV compared with that of IrO2/CNT-ref, and their overpotential (η) at a current density (J) of 10 mA cm-2 are 249 and 306 mV respectively. As given in Figure S2, the measured Tafel slope of IrO2/CNT (32 mV dec−1) is much smaller than that (44 mV dec−1) of IrO2/CNT-ref or those of IrO2 particles or IrOx thin films (Table S2),22 demonstrating its more superior electrochemical water oxidation activity to most of previous heterogeneous Irbased WOCs. Moreover, in 0.5 M H2SO4 solution, the η of IrO2/CNT and IrO2/CNT-ref samples at a J of 10 mA cm-2 are 293 and 432 mV respectively (Figure 1c). The Tafel slope of IrO2/CNT (67 mV dec−1) in 0.5 M H2SO4 solution is also far smaller than that (147 mV dec−1) of IrO2/CNT-ref (Figure S3). In addition, the electrochemical measurement of IrO2/CNT in 1.0 M KOH or 0.5 M H2SO4 as a function of reaction time demonstrates its good electrochemical stability in both acid (pH: 0) and alkaline (pH: 13.6) environments (Figure 1d).
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Figure 1. Chemical (a) and electrochemical (b, c, d) water oxidation properties of IrO2/CNT and IrO2/CNT-ref. (a) Time profile of O2 evolution containing 0.1 mg of catalyst in a 3 mL Ce(NH4)2(NO3)6 (0.15 M) solution. (b) LSV curves of CNT, IrO2/CNT and IrO2/CNT-ref supported on GCE in 1.0 M KOH without iR-compensation. (c) LSV curves of CNT, IrO2/CNT and IrO2/CNT-ref supported on GCE in 0.5 M H2SO4 without iRcompensation. (d) Chronopotentiometry curves of IrO2/CNT in 1 M KOH or 0.5 M H2SO4 solution at a constant current density of 10 mA cm−2.
Water oxidation performances of the as-obtained samples were evaluated both chemical and electrochemical processes. As seen in Figure 1a, the amount of chemical O2 evolution on both samples are linearly increased in the initial reaction stage. Based on the initial constant activity curves, the TOF of water oxidation is thus calculated to be 11.2 and 1.3 s-1 for IrO2/CNT and IrO2/CNT-ref, respectively. In addition, the initial O2 evolution on the representative IrO2/CNT sample is found to be linearly enhanced with concentration of catalyst used in the experimental region (Figure S1). The TOF of IrO2/CNT could
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Figure 2. HAADF-STEM image of IrO2/CNT (a), and its histogram of size distribution (b); and representative TEM image of IrO2/CNT-ref (c) and its histogram of size distribution (d).
In order to get insight into the excellent water oxidation performances, structural characterizations were carried out and previous theory simulations were also introduced. First of all, the morphology and size of them are investigated. As seen in the HAADF-STEM image of Figure 2a, the deposited iridium species on the IrO2/CNT sample are homogeneously dispersed on the surface of MWCNTs, based on which its histogram of size dispersion is given in Figure 2b. It is interesting to see that the size of all the IrO2 clusters is below 2 nm and narrowly distributed with average size calculated to be 1.1 nm. Comparatively, the size of IrO2 in IrO2/CNT-ref is a little larger and broadly distributed, showing average size of about 2.2 nm (Figure 2c and 2d). The possible existence of iridium oxides with bigger size or their aggregation on both samples can be
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ruled out by the no obvious observation of peaks of XRD patterns (Figure S4) and Raman spectra (Figure S5) assigned to the iridium dioxide species. To further demonstrate the structure sensitivity of water oxidation over IrO2, similar catalysts (MWCNTs, chemical vapor deposition of Ir4(CO)12, etc) with a higher weight loading of IrO2 were synthesized. As shown in Figure S6, the particle size of IrO2 is slightly enhanced with increasing the weight loading, and the average size of IrO2 in 2.1% IrO2/CNT (2.1 wt.% Ir) and 4.0% IrO2/CNT (4.0 wt.% Ir) samples is calculated to be 1.3 nm and 1.4 nm, respectively. Based on their activity curves, the initial TOF value of water oxidation over 2.1% IrO2/CNT and 4.0% IrO2/CNT is ca. 6.5 and 4.1 s-1, respectively (Figure S7). These results demonstrate that the decrease of particle size of deposited irridium does make a positive effect on the water oxidation activity.
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Figure 3. Structural characterizations of IrO2/CNT and IrO2/CNT-ref. (a) High-resolution XPS Ir 4f of IrO2/CNT and IrO2/CNT-ref. (b) Normalized XANES spectra at Ir LIII-edge of IrO2/CNT and IrO2/CNT-ref. (c) Ir L-edge FT-EXAFS spectra of IrO2/CNT and IrO2/CNT-ref. (d) FT-EXAFS fitting spectra of IrO2/CNT at the Ir L-edge.
Secondly, the valence state and surface content of the deposited iridium on both samples were examined and analyzed by Ir 4f XPS spectra. According to their similar binding energies (ca. 62.3 eV) of the Ir4f7/2 peak in Figure 3a, the deposited iridium on both samples can be understood to mainly exist as Ir4+ oxidation state.23 The iridium contents on the surface of both samples are analyzed to be 0.1 mol% (Table S3). The similar existed states (Ir4+) of deposited iridium on both samples can be further confirmed by the similar edge position of Ir L-edge XANES spectra (Figure 3b). Differently, the low intensity oscillations directly following the near-edge region indicates the short-range and low coordinate environment of Ir on MWCNTs in the IrO2/CNT sample. In addition, the distances of Ir-O coordination on the IrO2/CNT sample (2.03 Å) are slightly longer than that of IrO2/CNT-ref (2.01 Å) or standard IrO2 sample (1.98 Å),24-26 according to the results of their X-ray absorption fine structure (EXAFS) (Figure 3c, Figure 3d and Table S4). Their distance trend of Ir-O coordination (Figure 3c) is in a good accordance with previous report
that the total disorder, interatomic distance, and the asymmetry in the distribution of distances increased as the metal nanocrystal size decreased.27 Additionally, the very weak amplitude of FT peaks assigned to Ir-Ir shells of IrO2/CNT sample reveals that the Ir metal clusters are unlikely presented, demonstrating their complete oxidization into IrO2 clusters. Thirdly, the good conductivity of MWCNTs favors the electrical conduction efficiency in the water oxidation process.28 The functional groups (-OH, C=O) on the MWCNTs surface could lead to the migration of electron cloud due to their strong electronegativity, which promotes a high valence state for Ir and also reduces the energy consumption for the conversion from Ir4+to Ir5+.29 Finally, density functional theory (DFT) calculations was employed to further correlate the subnanometric-containing iridium with performance of highly efficient and robust water oxidaiton on the IrO2/CNT sample. On one hand, our previous DFT calculation has revealed that the ultrasmall IrmOn (m=1-5, and n=1-2 m) nanoclusters obtained by the successive oxidation of small “core” iridium clusters have relatively high stability at mild environment due to their relatively high binding energy.30 For IrO2, Ir2O4, Ir3O6, Ir4O8, and Ir5O10 clusters, the lowest energy structure is a doublet, singlet, quartet, quintet, and quartet, respectively.30 The O−O bond is not favorable in iridium oxide clusters.30 It might be one of the main reasons for IrO2/CNT-ref with O−O bond (Figure 3c) showing relatively bad stability in electrochemical water oxidation (Figure S8). It is thus reasonable to ascribe the robust water oxidation of IrO2/CNT to its good thermodynamical stability at room temperature. On the other hand, Goddard et al suggested that the unsaturated surface iridium atoms are more easily to react with water to form surface ‒OH sites due to their larger water binding energy with respect to the saturated iridium atoms.31 Compared with commercial IrO2 or IrO2 nanoparticles with size larger than 2 nm, the IrO2/CNT sample with subnanometric size and average size of 1.1 nm is expected to not only provide much more active sites, but also expose much more surface unsaturated surface Ir atoms (Figure 3b), as should be responsible for its highly efficient water oxidation activity. CONCLUSIONS In conclusion, we have prepared ultrasmall iridium oxide clusters on the surface of MWCNTs by a simple CVD approach, whose size can be controlled to be smaller than 2 nm. Subnanometric structure of iridium dioxide are observed and demonstrated for efficient and robust water oxidation for the first time. The optimal TOF of chemical water oxidation can reach 11.2 s-1, which can even surpass those achieved over Irbased molecular WOCs. Together with the observation of its robust electrochemical water oxidation in both acid and alkaline environments, the as-obtained IrO2/CNT sample with both activity and robustness is expected to be widely used for construction of artificial photosynthesis. The finding of ultrasmall iridium oxide as highly efficient and robust WOC will greatly reduce the usage of noble metal and exhibit bright future in the practical application.
ASSOCIATED CONTENT Supporting Information. Experimental details and supporting data. The Supporting Information is available free of charge on the ACS Publications website.
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AUTHOR INFORMATION Corresponding Author *
[email protected] *
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by the Basic Research Program of China (973 Program: 2014CB239403), Natural Science Foundation of China (No. 21633009, 21522306, 21373210) and Key Research Program of Frontier Sciences, CAS, (No. QYZDY-SSWJSC023). F. Zhang thanks the priority support from the “Hundred Talents Program” of Chinese Academy of Sciences.
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