Investigation of Mesoporous Niobium-Doped TiO2 ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Investigation of Mesoporous Niobium-Doped TiO2 as an Oxygen Evolution Catalyst Support in an SPE Water Electrolyzer Chuanpu Hao,†,‡ Hong Lv,*,†,‡ Cangen Mi,§ Yukun Song,†,‡ and Jianxin Ma†,‡ †

School of Automotive Studies, Tongji University, Shanghai 201804, People’s Republic of China Clean Energy Automotive Engineering Center, Tongji University, Shanghai 201804, People’s Republic of China § College of Materials and Engineering, Hunan University, Changsha, Hunan 410082, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Titania doped by niobium was successfully synthesized via a modified evaporation-induced self-assembly method (EISA) as a support of IrO2 for a solid polymer electrolyte water electrolyzer (SPEWE). The doping amount of niobium (5, 10, 20 at. %) was emphatically investigated to evaluate the effects on nanostructure, morphology, and oxygen evolution reaction (OER) activity of Nb-doped titania supported IrO2. The high-resolution transmission electron microscopy (TEM) results show that IrO2 supported by Nbdoped titania exhibits grain refinement and uniform dispersion. An investigation of the electrocatalytic activity by half-cell electrochemical testing reveals that the Nb-doped titania supported IrO2 catalyst demonstrates significant OER activity. When the Nb content reaches 20 at. % in the support, the Nb-doped titania supported IrO2 possesses the highest OER activity, which is superior to that of pristine titania supported IrO2 and unsupported IrO2. The single-cell tests also prove that 20 at. % is the best Nb doping amount for titania supports of IrO2. It is found that the majority of the OER activity increase is due to the Nb-doping induced enhancement of the specific surface area and surface activity of transferring charge and species. The additional specific surface area and redox couples of Nb(IV)/Nb(V) are also responsible for this performance enhancement. Herein, the as-synthesized Nb-doped titania is considered to be a promising oxygen evolution catalyst support for SPEWE applications. KEYWORDS: SPE water electrolysis, Titania support, IrO2, Nb-Doped titania, Oxygen evolution reaction (OER), Catalyst support

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INTRODUCTION The solid polymer electrolyte water electrolyzer (SPEWE) has been proven to be a potential solution for converting the current large amount of excess electric power generated by renewable energy, such as wind and photovoltaic power, to a storable form due to its several advantages including high efficiency, high product purity, and good adaptability to fluctuant power input.1,2 The efficiency of the SPEWE is mainly determined by the electrochemical processes at the anode, where the oxygen evolution reaction (OER) takes place, and the overpotential causes power losses.3 Among all OER catalysts, IrO2 is considered to be the most suitable for the anode of the SPEWE due to its high electrochemical activity, as well as its considerable stability upon exposure to an acidic polymer electrolyte under a high anode potential.4,5 Unfortunately, the cost and scarcity of iridium limits the expansion of SPEWE application. Therefore, reducing the iridium loading and the improving OER activity are urgently required to further the acceptance of SPEWEs. Utilizing a catalyst support emerges as an effective solution to improve the electrocatalytic performance and reduce the consumption of noble metal by achieving a higher dispersion and greater surface area for the catalyst.6 In fuel cells, carbon© XXXX American Chemical Society

based supports are usually employed because of their excellent electric conductivity and high specific surface area for nano-Pt deposition. However, because they suffer fast corrosion under a relatively high potential, they are unsuitable for use at the SPEWE’s anode.7 As alternatives to carbon, a number of corrosion-resistant ceramic powders have been applied as OER catalyst supports. For instance, SnO2-based supports have been frequently studied due to the achievements of Ir−Sn DSA electrodes.8−11 TiO2 is also a potential option because of its high resistance against anodic corrosion and controllable morphology. However, the drawback of both oxides is their electric inertness, manifesting as low conductivity and low adsorption/desorption capability toward the charge and species in the OER. Its impediment to the performance of IrO2 supported by TiO2 was thoroughly analyzed by P. Mazúr,12 who proposed that an interconnected network of IrO2 particles on the surface of the support has to be formed to mitigate the hindrance of the electric inertness of TiO2. This interconnected network, which is actually a layer of agglomerates, requires a Received: July 6, 2015 Revised: December 2, 2015

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DOI: 10.1021/acssuschemeng.5b00531 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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(Nb(C2H5O)5, Alfa Aesar, Heysham, Lancashire, UK) was first mixed into the ethanolic solution of Ti(C4H9O)4, which was partially replaced with Nb to achieve doping amounts of 5, 10, and 20 at. %. The total molar ratio of the raw materials was kept at (Ti(C4H9O)4 + Nb(C2H5O)5):HCl:H2O:CTAB: ethanol = 1:1.02:12.95:0.15:25. After the final stirring, a homogeneous transparent liquid was obtained. Traditional EISA requires several days of evaporation to achieve complete self-assembly into a highly ordered mesostructure,25 but this is not necessary for the catalyst supports in this work. We applied rotary evaporation at 60 °C and room pressure for 24 h to accelerate the formation of the mesostructure, disregarding the pore ordering. The obtained semitransparent gel was transferred to a muffle furnace and heat-treated at 120 °C for 8 h and then at 200 °C for 2 h to condense the colloidal Ti-based particles into a firm mesoporous matrix.23 The resulting dark and fragile gel was grounded into a powder, followed by final calcination at 350 °C for 4 h for the complete removal of the surfactant. White titania powders were obtained after cooling down. The heating rate was kept as low as 1 °C· min−1 for all heat treatments mentioned above. Preparation of Supported Catalysts. For the IrO2 loading of 40 wt %, 0.196 g of chloroiridic acid (H2IrCl6·6H2O, Ir content is 35%, Hesen, Shanghai, China) was dissolved in 10 mL of isopropyl alcohol (CH3CH(OH)CH3, Sinopharm, Shanghai, China) in which 0.12 g of as-prepared titania support powder and 5 g of ultrafine sodium nitrate (NaNO3, Sinopharm, Shanghai, China) were suspended. After ultrasonic homogenization for 30 min, the mixture was dried at 70 °C overnight. The light yellow powder mixture was treated at 500 °C for 1 h with a heating rate of 5 °C·min−1 for the Adams fusion reaction. Black catalyst powders were obtained after being washed with deionized water and centrifuging several times to eliminate residual salts, followed by drying at 80 °C overnight. Unsupported IrO2 was also prepared for comparison using the same procedure. The supports were denoted as TN-x, where x (0, 5, 10, 20) accorded with the percent content of Nb, and correspondingly the supported catalysts were denoted as 40I/TN-x. Physical Characterization. X-ray diffraction (XRD) analysis was performed on the TN-x supports, 40I/TN-x catalysts and unsupported IrO2. The XRD patterns were collected using a Bruker D8 Advance Xray diffractometer (Bruker, Karlsruhe, Germany) with a Cu Kα radiation source (λ = 0.154 056 nm). The specific surface area and pore size distribution were measured by the N2 absorption/desorption technique using a Micromeritics ASAP 2020 system (Micromeritics, Norcross, Georgia, USA) and calculated by the Brunauer−Emmett−Teller (BET) and Barrett− Joyner−Halenda (BJH) methods, respectively. Transmission electron microscopy (TEM) observations were recorded on a JEOL 2010F microscope (JEOL, Tokyo, Japan) to elucidate the dispersion of IrO2 on the supports. As-prepared catalysts were held on carbon-coated copper grids after ultrasonic dispersion in ethanol. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an AXIS UltraDLD instrument (Shimadzu-Kratos, Hadano, Kanagawa, Japan) using a monochromatic Al Kα X-ray source. A low-energy (∼1 eV) electron neutralizer was used for charge neutralization. Both survey scans (160 eV pass energy, with 0.800 eV/ step) and high-resolution scans (20 eV pass energy, with 0.100 eV/ step) were obtained for the catalysts. The surface ratio of titanium, niobium and iridium was determined by comparison of the Ti 3p/2, Nb 5d/2 and Ir 4f7/2 photoelectron lines based on appropriate sensitivity factors.26 Curve fitting was carried out using Gaussian− Lorentzian type profiles (CasaXPS 2.3). Electrical conductivity measurements were carried out on cylindrical pellets compressed from the powder samples at 30 MPa between two copper electrodes, as shown in the schematic diagram in Figure S1 (see the Supporting Information). The basal area of the cylindrical pellet was restricted by the fixture to 1 cm2, and the thickness was measured by a vernier caliper fastened on the fixture. The resistivity was directly measured by a JG-ST2258A resistivity tester (Jingge Electronic, Suzhou, Jiangsu, China) by inputting the thickness-area ratio as a parameter, followed by conversion to conductivity.

sufficient number of active IrO2 particles covering the support and thus sacrifices part of the dispersivity of the IrO2, resulting in a low usage efficiency. Therefore, the OER performance is seriously limited by the poor electric properties of the TiO2 support when its highly porous surface supports IrO2 with a low loading. According to Mazúr, to achieve the optimal performance of supported IrO2 with a loading of 60 wt %, the specific surface area of the TiO2 should be no higher than 50 m2·g−1. To optimize the electrical properties, some modifications have been reported, such as nonstoichiometric treatment and doping with metallic elements with different valences. Siracusano13 prepared TinO2n−1 (4 ≤ n ≤ 10) by the temperature-programmed reduction of self-made porous titania. As low as 30 wt % IrO2 was loaded on these titania suboxide powders by an incipient wetness procedure, which achieved good MEA performance. SnO2 is often doped with Sb to form a conductive ceramic powder known as ATO, which has been repeatedly reported to be a satisfactory catalyst support for RuO2 and IrO2.14−17 TiO2 doped with Nb and W has been reported to achieve considerable conductivity and high stability, enabling it to be a candidate material for oxygen reduction catalyst supports in fuel cells.18,19 For the SPEWE anode, the use of 5 at. %-Nb doped TiO2 as an IrO2 support prepared by the sol−gel method has recently been reported,20 achieving an optimized IrO2 loading of 26 wt %. The great potential of Nb-doped TiO2 as an IrO2 catalyst support for SPEWE has emerged, but the effect of the Nb doping amount on the OER performance has not yet been evaluated in detail. As proven both experimentally21 and computationally,22 the increased amount of Nb is beneficial to elevating the charge carrier concentration and improving the conductivity of doped titania. Therefore, the influence of different Nb doping amounts on the OER performance of doped titania supported IrO2 would be very valuable to investigate. In this work, doping amounts of Nb up to 20 at. % have been prepared by a modified evaporation-induced self-assembly method (EISA),23 which is effective for the synthesis of porous titania with a high specific surface area and narrow pore size distribution. The Adams fusion method24 was adopted due to its high feasibility and reproducibility for supported IrO2 synthesis. To amplify the performance variation of the Nbdoped titania supports without being concealed by IrO2, an IrO2 interconnected network should be avoided. Therefore, an IrO2 loading of 40 wt % was applied, and the specific surface area of the titania support was made sufficiently high. The structure, morphology, and electrocatalytic activity for OER of the supports and supported catalysts were determined, and single cells based on these catalysts were tested. The influence of the Nb doping amount in the titania support on the OER performance was demonstrated as a supplement to Hu’s research.

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EXPERIMENTAL SECTION

Preparation of Nb-Doped Titania Support. In the typical synthesis of pristine titania, 15 mL of tetrabutyl titanate (Ti(C4H9O)4, Sinopharm, Shanghai, China) was dissolved in 20 mL of ethanol (Sinopharm, Shanghai, China) in which 5 mL of concentrated hydrochloric acid (37.5%, Sinopharm, Shanghai, China) diluted by 10 mL of ethanol was dropped in under vigorous stirring at 50 °C. Then, a solution containing approximately 2.4 g of hexadecyl trimethylammonium bromide (CTAB, Sinopharm, Shanghai, China) in 30 mL of ethanol was added dropwise to the precursor mixture, followed by 10 mL of deionized water 1 h later and continuous vigorous stirring for 1 h. In the synthesis of Nb-doped titania, niobium(V) ethoxide B


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ACS Sustainable Chemistry & Engineering Electrochemical Characterization. For the electrochemical evaluation of the as-prepared catalysts in the half-cell, 11.3 mg of catalyst powder was suspended in 1 mL of methanol/Nafion (DuPont, Wilmington, Delaware, USA) solution (50:1, wt %), followed by ultrasonic dispersion for 1 h to prepare a homogeneous ink. Then, 5 μL of ink was deposited onto a clean glassy carbon disk electrode (GCE, 5.6 mm diameter) twice. The electrochemical OER activity of the IrO2 and 40I/TN-x was measured in a glass cell consisting of a three-electrode system in HClO4 (0.1 M, Sinopharm, Shanghai, China) electrolyte at 25 °C. The solution was bubbled with N2 for 30 min prior to the experiments until the measurements ended. A reversible hydrogen electrode (RHE) was used as the reference electrode, and a platinum wire was used as the counter electrode. A 100 cycle sweep of cyclic voltammetry (CV) was carried out before the data were recorded to obtain a reproducible voltammetric response. The CV was measured at a scan rate of 50 mV·s−1 between 0 and 1.45 V vs RHE. Linear sweep voltammetry (LSV) from 1.25 to 1.75 V vs RHE at a scan rate of 5 mV·s−1 was measured to reveal the OER performance of all the prepared catalysts in the half-cell. To evaluate the stability of the catalytic activity, electrochemical accelerated durability tests (ADT) were conducted by CV between 1.2 and 1.6 V vs RHE for 3000 cycles. All the electrochemical measurements were performed with a Model 760 electrochemical workstation (CH Instruments, Austin, Texas, USA). SPEWE Single Cell Studies. A Nafion 117 (DuPont, Wilmington, Delaware, USA) membrane was used as the solid polymer electrolyte after being sequentially boiled in a 3 wt % H2O2 solution (Sinopharm, Shanghai, China), distilled water, and a 5 wt % H2SO4 solution (Sinopharm, Shanghai, China) for 1 h each as a pretreatment. The asprepared catalysts were used as anodic electrocatalysts (for OER), and a commercial 40 wt % Pt/C (Johnson Matthey, London, UK) catalyst was adopted as the cathodic electrocatalyst. The anodic and cathodic catalysts were directly deposited onto one and the other side of the Nafion 117 membrane, respectively, by a spray coating technique to prepare the membrane electrode assemblies (MEAs). The homogeneous inks were sonicated from mixtures of catalysts, Nafion solution (5 wt %, DuPont, Wilmington, Delaware, USA), isopropyl alcohol (Sinopharm, Shanghai, China), and deionized water. For each MEA, the Nafion loading in the catalyst layer was 25 wt % for both sides, and the noble metal loadings were 0.5 mg cm−2 Pt for the cathode and 2.5 mg cm−2 Ir for the anode. After being sandwiched by a Ti mesh and carbon paper (current collecting layers) over the anode and cathode, respectively, the MEA was clamped between Ti flow field plates for assembly into a single-cell electrolyzer. The single cell (with an effective area of 3.645 cm2) performance was evaluated at 80 °C under 0.8 MPa. Deionized water was preheated to the target temperature and supplied to the anode compartment by a pump at a flow rate of 40 mL min−1. The single cell was charged by a Motech LPS305 programmatic DC power supply (Motech, Tainan, Taiwan, China) for the polarization curve measurement. The EIS for the single cell was measured at 0.1 A·cm−2 via a two-electrode method, with the anode being tested as the working electrode and the cathode as the counter and reference electrodes. It was carried out by a Solartron Analytical 1260 impedance analyzer connected to a Solartron Analytical 1287 potentiostat (Solartron Analytical, Farnborough, Hampshire, UK).

Figure 1. XRD patterns of the supports and catalysts.

coordinated by 6 oxygen ions),27 it is verified that the Nb was totally doped into the titania lattice, forming a substitutional solid solution. Using the Nb dopant, the titania was prepared with different phase compositions. Unlike the pristine TiO2 (TN-0), whose structure is composed of anatase and rutile phases, the doped titania has a single-phase anatase structure. The tendency for Nb-doped titania to form anatase rather than rutile has been repeatedly reported.28−30 According to the systematic study by J. Arbiol on the influence of Nb doping on the anatase-to-rutile phase transition,30 the oxygen vacancies in the anatase lattice, which act as nucleation sites of the anataseto-rutile lattice transformation, would be decreased as a charge compensation by Nb5+ substituting for Ti4+, leading to a better structural stability of the Nb-doped anatase against phase transition. The slight stress introduced by Nb5+ substituting for Ti4+ (slightly larger in radius) may also hinder the growth of the titania grains. It can also be found that for TN-0, the peaks of the rutile phase are slightly narrower than those of anatase, indicating that the prepared rutile is larger in grain size than the anatase. As seen from the patterns of the supported catalysts 40I/TNx (x = 0, 5, 10, 20) in Figure 1, the peaks of both titania phases became slightly narrower and sharper, indicating that the grain sizes of the titania supports slightly increased through the hightemperature Adams fusion treatment. Among these supported catalysts, the phase transformation from anatase to rutile took place in the pristine titania support after the Adams fusion treatment, which is indicated by the obvious decline in the peak intensity ratio of anatase to rutile upon comparing 40I/TN-0 with TN-0. No peaks belonging to the rutile phase are found in the patterns of the catalysts containing Nb, proving the enhanced structural stability of anatase caused by Nb doping. The lattice of IrO2 is quite similar to the tetragonal rutile structure, with typical peaks representing the (110), (101), and (111) planes located at approximately 28°, 35°, and 40°, respectively. In comparison with the pattern of unsupported

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RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the unsupported IrO2 and the synthesized supports TN-x and the catalysts 40I/TN-x (x = 0, 5, 10, 20). From the patterns of the bare titania supports, the typical peaks at approximately 25° and 48°can be easily recognized as the anatase lattice planes (101) and (200), whereas the peaks at approximately 27°, 36°, and 41° represent the (110), (101), and (111) planes of rutile, respectively. Apart from these titania phases, no compound of Nb was detected at any Nb content. Because Nb5+ is very close to Ti4+ in ionic radius (0.064 nm versus 0.0605 nm)21 and ionic coordination (both Nb5+ in Nb2O5 and Ti4+ in anatase are octahedrally C


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solute, the anatase was restrained from both phase transformation and evident grain growth. This improvement in the stability of the phase structure and grain size helped maintain a highly porous surface structure and a large amount of outer defects, which offers sufficient sites for the highly dispersive nucleation of nano-IrO2. This has further effects on the refinement of the nano-IrO2 grains. In Table 1, the unsupported IrO2 is shown to have the greatest crystalline size, and 40I/TN-0 comes in second. The nano-IrO2 supported by Nb-doped titania has achieved significant grain refinement, with an average diameter below 3 nm. Understandably, by nucleating at more sites such as surface defects on more porous Nb-doped supports, IrO2 nanocrystals have been synthesized in larger amounts with smaller dimensions and a higher dispersivity. The grain sizes of IrO2 supported by Nb-doped titania are similar, indicating that the surface properties are the key factors in the nanocrystallization of IrO2 grains with Adams fusion rather than the total composition and grain size of the doped titania. Table 2 gives the specific surface areas of the titania supports calculated via the BET and BJH methods based on the nitrogen adsorption isotherms. It is obvious that niobium doping improves the specific surface area of the synthesized titania by a factor of more than 2. With 5 or 10 at. % niobium in the titania lattice, the specific surface area reaches more than 140 m2·g−1, whereas it decreases by a small amount when the niobium content reaches 20 at. %. According to the theory for EISA synthesis from Soler-Illia,23 the porous titania is formed by the packing of Ti nanobuilding blocks and CTAB micelles, which turn into titania grains and pores, respectively, forming a porous framework after calcination. In our work, the cohydrolysis of Nb and Ti precursors gave rise to singlephase anatase nanobuilding blocks of similar size. With the evaporation of the solvent, these blocks were condensed and packed with micelles of matching size and dispersion, resulting in similar specific surface areas and pore widths among the Nbdoped titania. For TN-0, the additional larger rutile nanobuilding blocks were packed with larger micelles that turned into wider pores with a reduced overall surface area. The pore size distribution calculated by BJH (Figure S3 in the Supporting Information) shows a wide diameter span of mesopores created in the titania samples, which can be divided into two ranges of diameter, 2−3 nm and 5−11 nm. This could be attributed to the elevated rate of solvent evaporation during the titania synthesis, which greatly shortens the time for achieving completely homogeneous packing between the CTAB micelles and nanobuilding blocks. The major contribution to the total pore volume is from the relatively large pores, which are wide enough for the IrO2 deposition (less than 4 nm in average grain size) as well as the accessibility of these IrO2 to reactant and product species. As seen from the diameter distributions of the relatively large pores, the pristine titania is found to be the largest in pore size among the titania supports, for which the relatively large rutile grains could be responsible.

IrO2 in Figure 1, 40I/TN-x shows slightly broader diffraction peaks of IrO2, suggesting that the supported IrO2 grains are smaller in size than the unsupported one. In this work, the widely adopted Debye−Scherrer equation has been applied for the approximation of the average crystalline size from the full-width at half-maximum (fwhm) of the XRD peaks. Notably, the fwhm values of most peaks in Figure 1 cannot be directly measured because there are many overlaps of two or three peaks, which are greatly broadened by the ultrasmall grain sizes of both the active IrO2 and titania supports. To work out the lattice constants and the approximate crystalline sizes, deconvolution was carried out on the overlapped pattern of 40I/TN-x, and then the peaks of IrO2 at approximately 35° and anatase at approximately 48°, which were least affected by overlapping, were refined, and the diffraction angle and fwhm were acquired for the following calculations by Bragg and Debye−Scherrer equations, respectively. The calculated lattice constant and crystalline size results are summarized in Figure S2 (see the Supporting Information) and Table 1, respectively. Table 1. Average Crystalline Size of the anatase and IrO2 in the Supports and Catalysts sample

average crystalline size of IrO2 (nm)

unsupported IrO2 TN-0 TN-5 TN-10 TN-20 40I/TN-0 40I/TN-5 40I/TN-10 40I/TN-20

average crystalline size of anatase (nm)

4.1

4.0 2.9 3.0 2.9

8.5 7.9 8.6 9.0 11.6 8.5 9.2 9.7

The size of the anatase lattice cell in the supported catalyst shows a slight increase with the niobium content, which is attributed to the slightly larger diameter of the introduced Nb5+ compared with that of Ti4+ and also due to the lowered shrinkage caused by the decreased oxygen vacancy.30 On the other hand, the IrO2 lattice cell expands with the niobium content, which is more likely to be induced by the enhanced refinement at the nanometer scale.31,32 As listed in Table 1, the crystalline size of anatase in TN-x slightly increases with the niobium content. After the Adams fusion, all anatase crystals grew a little in accordance with the above observation from the shape of the XRD peaks. It can also be recognized that the grain growth of pristine anatase is more significant than that of the Nb-doped titania. The bulge in grain size and anatase−rutile transition would greatly destroy the mesoporous structure of the pristine titania support, resulting in the surface area plummeting. Nevertheless, with niobium as a Table 2. BET and BJH Results of the Titania Supports sample

BET surface area (m2·g−1)

BJH adsorption surface area (m2·g−1)

BET adsorption average pore diameter (nm)

BJH adsorption average pore diameter (nm)

TN-0 TN-5 TN-10 TN-20

68.10 143.03 146.52 132.42

73.14 160.17 164.25 142.98

9.4 5.6 5.8 6.8

9.4 5.4 5.4 6.5

D


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Figure 2. TEM images of unsupported IrO2 (a), 40ITN-0 (b), 40ITN-5 (c), 40ITN-10 (d), and 40ITN-20 (e).

which is characterized by light gray grains of diameter approximately 8 nm and white piled pores among the particles. This corroborates the previously discussed effect of the Nb solute on the improvement of the anatase’s structural stability, which has maintained the single anatase phase and highly porous morphology of Nb-dope titania throughout the Adams fusion. Its contribution to the dispersion of the IrO2 grains is evident. As can be seen from Figure 2c−e, tiny equiaxed IrO2 nanocrystals cling to the outer surface of the Nb-doped titania in a much better dispersion in comparison with those supported by pristine titania, leading to a more effective exposure of the catalyst’s electrochemically active surface. Figure 2c−e shows a slight increase in the grain size of the titania with the increasing Nb amount, consistent with the discussion in the XRD section. Moreover, the proportion of darker aggregates of nano-IrO2 gradually decreases, suggesting a slight improvement of the IrO2 dispersivity with the increasing Nb amount. An increase in the OER activity with the Nb doping amount could be accordingly expected as more IrO2 particles are exposed rather than mutual covering. To summarize, the major role of Nb doping on the morphology of the supported catalysts is to maintain the highly porous surface structure of the titania supports, which contributes to an improvement in the dispersivity of active IrO2 particles and a consequent elevation in the electrochemically active area of the supported IrO2. XPS is a widely used method in the field of surface chemistry and catalysis, as it examines the elemental composition, chemical state and electronic state of the sample’s outer surface by detecting the photoelectrons excited only from several nanometers beneath the surface. The surface metal elemental

For the Nb-containing supports, the average sizes of the relatively large pores show a sequence of TN-5 ≈ TN-10 < TN20, which nearly accords with the increase in anatase grain size with the Nb amount. Moreover, the distributions of pore sizes for the Nb-doped samples are narrower than that for the pristine titania due to their grains of more uniform size that assembled into the walls of the pores. The morphologies of unsupported IrO2 and 40ITN-x (x = 0, 5, 10, 20) are shown in Figure 2, which reveals the dispersivity distinction between unsupported IrO2 and IrO2 supported by pristine titania and titania with Nb dopant. As seen from Figure 2a, the unsupported IrO2 particles are of a broad size distribution: there are lath-shaped crystals of 10−20 nm in length and equiaxed ones with diameters of approximately 3 nm. Being supported by titania whose grains present as light gray polygons, as shown in Figure 2b−e, the IrO2 particles are totally shaped into tiny equiaxed nanocrystals as darker dots, and those on the Nb-doped titania become even smaller in size. A significant morphological change between the pristine and Nb-doped titania can be observed. In Figure 2b, the huge titania crystals with grain sizes over 20 nm can be easily recognized. According to the discussion above on the XRD patterns, the prepared rutile has a much larger grain size than anatase, suggesting that the huge crystals in Figure 2b are the rutile grains of the pristine titania support. The overgrown grains caused a considerable drop in the surface area and roughness, and as a result, the supported IrO2 nanoparticles were synthesized as agglomerates around these large titania grains. In Figure 2c−e, the situation was ameliorated by the introduction of Nb dopant. Unlike the pristine titania, the Nbdoped titania supports maintain a highly porous morphology, E


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Figure 3. Typical Ti 2p and Nb 3d regions of TN-20 and 40I/TN-20. ●, experimental data of Ti 2p doublets; doublets; solid lines, fitted peaks; dotted line, superposition of the fitted peaks.

composition of unsupported IrO2 and 40I/TN-x (x = 0, 5, 10, 20) (Table S1 in the Supporting Information) were calculated from photoelectric lines in the XPS survey spectra (Figure S4 in the Supporting Information) according to their relative sensitivity factors (RSFs).26 The niobium proportions of the TN-x detected by XPS are significantly larger than the total doping amounts, indicating a Nb enrichment on the titania particle surface. It is noteworthy from the table that Ir is in the majority among all the metal elements, which proves that the IrO2 particles have covered most of the support surfaces and thus have sheltered most of the support atoms from ejecting photoelectrons to the XPS detector. By comparing the XPS lines of titania before and after Adams fusion, some changes in the valence states of Ti and Nb are revealed. Figure 3 shows the high-resolution spectra for the Ti 2p and Nb 3d photoelectron doublets of TN-20 and 40I/TN20, as well as the fitting peaks calculated by the method of deconvolution. The spectrum of TN-20 shows only one pair of Ti 2p doublets and one pair of Nb 3d doublets, similar to those of the other TN-x supports, with binding energies that suggest single valence states as Ti (+4) and Nb (+5), respectively. However, after 40I/TN-20 was synthesized by Adams fusion, lower valence states of Ti and Nb have appeared, which are indicated by the additional doublets at lower binding energies shown in Figure 3. It is interesting to find that the titania supports have been somewhat reduced in the strong oxidative melt NaNO3 during the Adams fusion process. By calculating the areas of the peaks by deconvolution refinement with the same line-shape function, the approximate proportion of these elements in the lower valence states for the 40I/TN-x were obtained, as listed in Table 3. The first two columns are the proportions of the lower-valence elements in respective elements, whereas the last two columns are those of the lower-valence elements in the total surface metal atoms of TNx. It is obvious that the pristine titania could generate only a small amount of lower-valence Ti, 6.8 at. %. When Nb is present in the titania lattice, a larger proportion of Ti could be

▲,

experimental data of Nb 3d

Table 3. Molar Proportions of Ti and Nb in Lower Valence Calculated from the Fitting Peaks of the Ti 2p and Nb 3d Doublets of 40I/TN-x

40I/TN-0 40I/TN-5 40I/TN-10 40I/TN-20

Ti (lower valence) proportion in surface Ti (at. %)

Nb (lower valence) proportion in surface Nb (at. %)

Ti (lower valence) proportion in surface Ti and Nb (at. %)

Nb (lower valence) proportion in surface Ti and Nb (at. %)

6.8 25.7 32.0 27.5

0 16.6 28.1 24.5

6.8 24.4 28.8 22.0

0 0.83 2.81 4.90

reduced together with Nb. The proportion of lower-valence Ti reaches a maximum of 28.8% in TN-10, whereas that of lowervalence Nb rises linearly with the Nb dopant until 4.90% in TN-20. According to Liu’s21 research on conductive Nb-doped titania films, the lower valence state of the metal atoms in their oxide initiates successful optimization in electric conductivity as they behave as donors that introduce additional free electrons to the conduction band of titania. Moreover, an additional redox couple such as Nb(IV)/Nb(V) could be more helpful due to their capability of valence transition toward the catalytic process of OER. The effects of redox couple Nb(IV)/Nb(V) are described in detail in the following discussions of electrochemical characterizations. For the IrO2 and all the supported catalysts, more information can be obtained by the deconvolution of the Ir 4f doublets. The Ir 4f lines of these samples could be fitted with 3 pairs of peaks with the same line-shape functions, standing for Ir in the valence states of +3, + 4 and +6, respectively (Figure S5 in the Supporting Information). In the catalysis of the OER, the alternation between Ir(III) and Ir(IV) plays an important role in the oxidation of the hydroxyl, whereas Ir(VI) is easily corroded through the reaction shown in eq 3.1.33 Therefore, the Ir(VI) proportion will give an approximate estimate of the catalyst durability. According to the proportions of Ir in F


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ACS Sustainable Chemistry & Engineering different valence states out of the deconvolution of Ir 4f doublets (Table S2 in the Supporting Information), unsupported IrO2 contains as much as 19% Ir(VI), whereas when supported by titania, the Ir(VI) content declines by at least 2.8%. With 20 at. % Nb dopant in the titania support, the proportion of Ir(VI) is significantly reduced to 14.1% in 40I/ TN-20. This could be an indication that the titania support plays a considerable role in constraining the generation of unstable Ir oxide during Adams fusion, and the durability of the supported catalyst would be improved with more Nb doping in the support. IrO3 + H 2O → IrO4 2 − + 2H+

(3.1)

The results of the powder conductivity measurements are listed in Table 4. As shown in the table, the conductivities of Figure 4. Cyclic voltammetry of 40I/TN-x and unsupported IrO2 in N2-saturated 0.1 M HClO4 solution. Sweep rate is 50 mV·s−1.

Table 4. Powder Conductivity of Supports TN-x, Catalysts 40I/TN-x, and Unsupported IrO2 samples Unsupported IrO2 TN-0 TN-5 TN-10 TN-20 40I/TN-0 40I/TN-5 40I/TN-10 40I/TN-20

conductivity (S·cm−1) 3.01 4.42 7.30 5.75 9.53 1.13 8.39 4.41 6.36

× × × × × × × × ×

underpotential deposition (UPD) and desorption of H take place. At potentials above 0.4 VRHE, as in the oxygen region, the Ir-based catalysts cause the adsorption−desorption of OH groups and carry out further redox, such as the Ir(III)/Ir(IV) and Ir(IV)/Ir(VI) transition at approximately 0.8 VRHE and 1.15 VRHE, respectively.33 From Figure 4, it is quite clear that when supported by TN-x, the voltammograms of these supported catalysts are dissimilar in shape from that of unsupported IrO2. For 40I/TN-0, the voltammetric charge in the hydrogen region is almost the same as that of the unsupported IrO2, whereas the charge in the oxygen region became lower than that of the unsupported IrO2, with remarkably weaker peaks related to the redox couples Ir(III)/ Ir(IV) and Ir(IV)/Ir(VI). With the increase in Nb doping, the voltammetric charge over the whole potential window gradually grows, and the hydrogen region charge of 40I/TN-20 is raised to nearly twice that of unsupported IrO2. The voltammetric charge could be considered an estimate of the number of surface active sites.34−36 However, it would not be easy to work out the exact electrochemical surface area (ECSA) of IrO2 like that of nano-Pt due to the uncertainty in distinguishing the pseudocapacitance and verifying the state of the adsorbed OH groups on the catalyst surface. Hence, in this work, the charges from 0.4 to 1.4 VRHE were calculated for the qualitative comparison of the numbers of active sites. The results shows a sequence of 40I/TN-20 (71.0 C·g−1) > 40I/TN-10 (64.1 C· g−1) > 40I/TN-5 (55.1 C·g−1) > IrO2 (45.4 C·g−1) > 40I/TN-0 (40.6 C·g−1), indicating the significant effect of Nb doping on the increment in the number of surface active sites. This promotion is mainly attributed to the increase in the accessible surface area of the IrO2 particles and the surface optimization of the Nb-doped supports. As discussed in the physicochemical characterization section above, IrO2 undergoes crystalline refinement and dispersion optimization when supported by the Nb-doped titania, leading to a significant augmentation of the surface area of the IrO2 particles for 40I/TN-x (x > 0). Being connected to the titania particles, part of the IrO2 surface is blocked from interaction with the OH groups, explaining why the supported catalyst voltammetric peaks at higher potential appear to not be as prominent as those of unsupported IrO2. Nevertheless, it could be somewhat compensated for by the additional surface of the ultrafine IrO2 grains, especially for 40I/ TN-x (x > 0). On the other hand, a portion of the Nb in lower valence states on the support surface brought extra Nb(IV)/

101 10−6 10−4 10−3 10−3 10−2 10−2 10−1 10−1

the Nb-doped titania are much higher than that of the pristine titania, and they gradually rise with the Nb amount, indicating a significant effect of the Nb dopant on the improvement of the titania conductivity. After the IrO2 loading, the conductivity of 40I/TN-x is elevated to the order of magnitude of 10−1 S·cm−1. Similarly, with an increase in Nb doping, a gradual improvement in the conductivity is observed. As mentioned above, a considerable amount of low-valence-Nb on the doped titania surface could further elevate the conductivity of titanias supports, and consequently contribute to the overall conductivity enhancement of the catalysts. However, the difference of conductivity among 40I/TN-x is much smaller, and the conductivity of 40I/TN-x is still behind that of unsupported IrO2 by 2 or more orders of magnitude, which indicates that the overall conductivities of catalysts are still dominated by highly conductive IrO2 on the surface. Considering the absence of a highly conductive interconnected network of IrO2, it is unavoidable that the conductivity of the supported catalysts will be depressed by the resistivity of the titania supports. Cyclic voltammetry measurements of TN-x and 40I/TN-x, as well as unsupported IrO2, were taken to determine how the amount of surface active sites varies with the increase in Nb doping. Figure 4 shows the catalyst voltammograms obtained in N2-saturated 0.1 M HClO4 at a sweep rate of 50 mV·s−1. The voltammograms of the inactive bare titania supports are not included in this figure as they did not show an obvious voltammetric charge in the absence of low-valence Nb and Ti. The data of the current density in this figure are normalized to the IrO2 loading. All catalysts show high charge storage capacities, which come from both double-layer capacitance and pseudocapacitance caused by an ion reaction between the catalysts and the electrolyte. To be more specific, in the hydrogen region of the voltammograms below 0.4 VRHE, the G


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Figure 5. Linear sweep voltammetry of 40I/TN-x and unsupported IrO2 before (solid line) and after (dotted line) 3000 cycles’ ADT.

Nb(V) redox couples into this charge−discharge process. As reported by many researchers, the OER reaction catalyzed by IrO2 proceeds mainly along the electrochemical oxide path and the oxide path, as described in eqs 3.2−3.7), where S denotes an active site on the IrO2 surface. As a redox couple, Nb(IV)/ Nb(V) is also capable of forming or breaking additional Nb−O or Nb−OH bonds under some situations. This process could become much easier when the redox spot is close to that of the highly active IrO2 in the form of the transfer of the oxygenated species with the existing Ir−O or Ir−OH bonds. With the removal of the oxygenated species by neighboring Nb(IV)/ Nb(V) couples, the active sites on the IrO2 surface would be quickly renewed and be ready for the next cycle of water splitting. Therefore, the Nb(IV)/Nb(V) couples could play a particular role in transferring the adsorbed charge or species with IrO2, which improves the availability of active sites on the IrO2 surface and thus amplifies the total charge over the potential window. Although a part of the Ti in the lower valence states also exists in all supported catalysts, less significant contribution to the charging process can be recognized in comparison with Nb. Electrochemical oxide path:37 S + H 2O ⇌ S − OHads + H+ + e−

(3.2)

S − OHads ⇌ S − O + H+ + e−

(3.3)

2S − O → 2S + O2 (g)

(3.4)

Oxide path:

Figure 5 shows the LSV results of the catalyst samples before and after 3000 cycles’ ADT. Because the investigated catalyst film is extremely thin, the influence of the ohmic resistivity and mass transfer resistance crossing the film can be ignored. Therefore, this measurement is of great value in estimating the activity toward OER, as well as the catalytic stability. In the figure, the onset potentials for the OER of all samples are almost the same, whereas the difference in the slope in the potential above 1.6 V reflects the catalytic activity toward the macroscopic OER of these samples. A growth in the slope with the increase of x in 40I/TN-x can be recognized, indicating an improvement in the OER activity with the rising Nb-doping amount. The enhancements in the dispersion of IrO2 and the synergy between the catalytic particles and Nb dopant are confirmed to improve the OER activity. The improvement in the surface area and morphology of the porous supports with Nb doping could also assist in the adsorption/desorption of the species involved in OER. It is very encouraging to find that the OER activity of 40I/TN-20 has achieved a significant elevation and has even exceeded that of the unsupported IrO2. The dotted lines in Figure 5 represent the polarization curves after 3000 cycles’ ADT, which show different degrees of decline of the curve slope. It is quite obvious that a remarkable degradation occurs in the unsupported IrO2, as evidenced by the great drop in slope, whereas the supported catalysts are maintained stable with only a little loss in the catalytic activity. This corroborates the effect of the titania supports on the stabilization enhancement of the supported IrO2 particles, coinciding with the finding about the decrease of the easy-tocorrode Ir(VI) in the supported catalysts, as discussed in the XPS section. Among these samples, 40I/TN-20 still exhibits the best catalytic activity toward OER after 3000 cycles’ ADT. All of the half-cell tests above show the excellent catalytic activity and stability possessed by 40I/TN-20. However, the influence of the conductivity of the supported catalysts was not

37

S + H 2O ⇌ S − OHads + H+ + e−

(3.5)

2S − OHads ⇌ S − O + S + H 2O

(3.6)

2S − O → 2S + O2 (g)

(3.7) H


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V) to electrolyze water at 1 A·cm−2. The slope of the polarization curves of 40I/TN-x decreases with the Nb amount, consistent with the measured conductivities of 40I/TN-x. The cell potentials at 1 A·cm−2 are the comprehensive results of the overpotentials on active sites, species transfer and ohmic loss, exhibiting a sequence of 40I/TN-0 (2.128 V) > 40I/TN-5 (2.097 V) > 40I/TN-10 (2.028 V) ≥ 40I/TN-20 (2.027 V). It is noteworthy that the performances of 40I/TN-10 and 40I/ TN-20 are very close, indicating that the top performance has been reached and the best doping amount is approximately 20 at. %. As shown in Figure 7, the EIS results of single cells at 0.1 A· cm−2 coincide with those of the polarization very well. From

considered in the half-cell test on GCE. Therefore, the final OER performance should be evaluated in a single-cell electrolyzer. The polarization curves of single cells equipped with 40I/ TN-x and unsupported IrO2 at 80 °C are shown in Figure 6,

Figure 6. Polarization curves of single cells equipped with 40I/TN-x (solid line) and unsupported IrO2 (dotted line) at 80 °C. The inset shows the magnified curves at 0.01−0.1 A·cm−2.

with magnified curves at 0.01−0.1 A·cm−2 shown in the inset. At the initial current density of 0.01 A·cm−2, where the mass transfer resistance and electric resistance are negligible, the exhibited overpotentials come mostly from the catalytic activity. With the largest grain size of IrO2 among the catalysts, the unsupported IrO2 possesses the lowest active surface area and defect amount, leading to its highest overpotential at 0.01 A· cm−2. The overpotentials of 40I/TN-x are at least 20 mV lower than that of unsupported IrO2 due to the significant dispersion improvement and grain refinement of IrO2 brought about by the titania supports. With the increase in the Nb amount, the overpotential declines slightly. At this very low current density, the coverage of oxygenated species on the surfaces of IrO2 in all samples remains at a very low level,38 which makes the contribution of the Nb dopant negligible. When the current density increases to 0.1 A·cm−2, the difference in overpotentials increases in the sequence of 40I/TN-0 > unsupported IrO2 > 40I/TN-5 > 40I/TN-10 > 40I/TN-20, which coincide well with the anodic charges measured in CV. At this current density, the coverage of oxygenated species has increased to some extent, under which the assistance from Nb dopant is involved. Additional Nb(IV)/Nb(V) redox couples could depress part of the coverage on IrO2 by transferring the oxygenated species and exposing more active sites of IrO2. With the increase in the Nb amount, the portion of uncovered active sites is enlarged, leading to a lower overpotential at 0.1 A·cm−2 and a slower increase of the overpotential with the rising current density. At large current densities over 0.6 A·cm−2, the ohmic loss dominates in the whole cell potential, as reflected by the linear increase of potential versus current. The coverage of oxygenated species is very high on all the catalysts. Thanks to its excellent powder conductivity and its relatively low catalytic layer thickness (the thickness of 40I/TN-x is much higher than that of unsupported IrO2 due to the additional volume from the porous titania), the unsupported IrO2 possesses the lowest ohmic loss, as shown by the smallest slope of the polarization curve, which makes it require the lowest cell potential (2.021

Figure 7. Nyquist diagrams of 40I/TN-x and unsupported IrO2 at 0.1 A·cm−2 and 80 °C.

the ohmic resistance of the cell (the intercept by the real axis from the extension of the high-frequency side of the Nyquist curve), it is easily observed that the single cell containing unsupported IrO2 possesses the lowest ohmic resistance, followed by 40I/TN-20, 40I/TN-10, 40I/TN-5 and 40I/TN0 in sequence, which matches the results of the conductivity and polarization curve sections. With respect to the polarization resistance (the differential value between the intercepts by the real axis from the extension of the high-frequency side and the low-frequency side), the sequence is unsupported IrO2 > 40I/ TN-0 > 40I/TN-5 > 40I/TN-10 ≥ 40I/TN-20. The decrease in the polarization resistance comes from two aspects: due to the TN-x supports, the surface area and defects are increased dramatically by the grain refinement of the IrO2, and with the increase of the Nb doping, more Nb(IV)/Nb(V) redox couples assist in the OER catalysis of IrO2 by charge and species transfer. The EIS results of 40I/TN-10 and 40I/Tn-20 are still close, also indicating that 20 at. % may be the best amount of Nb doping. Although the OER performance of 40I/TN-20 is still lower than that of the unsupported IrO2, the selection of the Nb doping amount was achieved. The performance of IrO2 supported by Nb-doped titania has great potential for further improvement by adjusting the loading of IrO2 to a point that both the formation of an interconnected IrO2 network and the catalytic assistance of the Nb dopant are achieved.

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CONCLUSIONS Significant optimization of the OER performance is achieved when Nb doping is implemented in the titania supports. The introduction of Nb into the titania lattice restricts the crystal I


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(5) Slavcheva, E.; Radev, I.; Bliznakov, S.; Topalov, G.; Andreev, P.; Budevski, E. Sputtered iridium oxide films as electrocatalysts for water splitting via PEM electrolysis. Electrochim. Acta 2007, 52, 3889−3894. (6) Ma, L.; Sui, S.; Zhai, Y. Preparation and characterization of Ir/ TiC catalyst for oxygen evolution. J. Power Sources 2008, 177, 470− 477. (7) Yu, X.; Ye, S. Recent advances in activity and durability enhancement of Pt/C catalytic cathode in PEMFC. J. Power Sources 2007, 172, 133−144. (8) Marshall, A.; Børresen, B.; Hagen, G.; Tsypkin, M.; Tunold, R. Preparation and characterisation of nanocrystalline IrxSn1−xO2 electrocatalytic powders. Mater. Chem. Phys. 2005, 94, 226−232. (9) De Pauli, C. P.; Trasatti, S. Composite materials for electrocatalysis of O2 evolution: IrO2 + SnO2 in acid solution. J. Electroanal. Chem. 2002, 538−539, 145−151. (10) Balko, E. N.; Nguyen, P. H. Iridium-tin mixed oxide anode coatings. J. Appl. Electrochem. 1991, 21, 678−682. (11) Xu, J.; Liu, G.; Li, J.; Wang, X. The electrocatalytic properties of an IrO2/SnO2 catalyst using SnO2 as a support and an assisting reagent for the oxygen evolution reaction. Electrochim. Acta 2012, 59, 105− 112. (12) Mazúr, P.; Polonský, J.; Paidar, M.; Bouzek, K. Non-conductive TiO2 as the anode catalyst support for PEM water electrolysis. Int. J. Hydrogen Energy 2012, 37, 12081−12088. (13) Siracusano, S.; Baglio, V.; D’Urso, C.; Antonucci, V.; Aricò, A. S. Preparation and characterization of titanium suboxides as conductive supports of IrO2 electrocatalysts for application in SPE electrolysers. Electrochim. Acta 2009, 54, 6292−6299. (14) Marshall, A. T.; Haverkamp, R. G. Electrocatalytic activity of IrO2-RuO2 supported on Sb-doped SnO2 nanoparticles. Electrochim. Acta 2010, 55, 1978−1984. (15) Wu, X.; Scott, K. RuO2 supported on Sb-doped SnO2 nanoparticles for polymer electrolyte membrane water electrolysers. Int. J. Hydrogen Energy 2011, 36, 5806−5810. (16) Marshall, A. T.; Haverkamp, R. G. Nanoparticles of IrO2 or Sb− SnO2 increase the performance of iridium oxide DSA electrodes. J. Mater. Sci. 2012, 47, 1135−1141. (17) Á vila-Vázquez, V.; Cruz, J.; Galván-Valencia, M.; LedesmaGarcía, J.; Arriaga, L.; Guzmán, C.; Durón-Torres, S. Electrochemical Study of Sb-Doped SnO2 Supports on the Oxygen Evolution Reaction: Effect of Synthesis Annealing Time. Int. J. Electrochem. Sci. 2013, 8, 10586−10600. (18) Do, T. B.; Cai, M.; Ruthkosky, M. S.; Moylan, T. E. Niobiumdoped titanium oxide for fuel cell application. Electrochim. Acta 2010, 55, 8013−8017. (19) Subban, C. V.; Zhou, Q.; Hu, A.; Moylan, T. E.; Wagner, F. T.; DiSalvo, F. J. Sol-gel synthesis, electrochemical characterization, and stability testing of Ti0.7W0.3O2 nanoparticles for catalyst support applications in proton-exchange membrane fuel cells. J. Am. Chem. Soc. 2010, 132, 17531−6. (20) Hu, W.; Chen, S.; Xia, Q. IrO2/Nb-TiO2 electrocatalyst for oxygen evolution reaction in acidic medium. Int. J. Hydrogen Energy 2014, 39, 6967−6976. (21) Liu, Y.; Szeifert, J. M.; Feckl, J. M.; Mandlmeier, B.; Rathousky, J.; Hayden, O.; Fattakhova-Rohlfing, D.; Bein, T. Niobium-doped titania nanoparticles: synthesis and assembly into mesoporous films and electrical conductivity. ACS Nano 2010, 4, 5373−81. (22) Dy, E.; Hui, R.; Zhang, J.; Liu, Z. S.; Shi, Z. Electronic Conductivity and Stability of Doped Titania (Ti1−XMXO2, M = Nb, Ru, and Ta)A Density Functional Theory-Based Comparison. J. Phys. Chem. C 2010, 114, 13162−13167. (23) Soler-Illia, G. J. A. A.; Louis, A.; Sanchez, C. Synthesis and characterization of mesostructured titania-based materials through evaporation-induced self-assembly. Chem. Mater. 2002, 14, 750−759. (24) Adams, R.; Shriner, R. Platinum oxide as a catalyst in the reduction of organic compounds. III. Preparation and properties of the oxide of platinum obtained by the fusion of chloroplatinic acid with sodium nitrate1. J. Am. Chem. Soc. 1923, 45, 2171−2179.

structure to a single phase of anatase, expands the specific surface area of the doped titania, and introduces the redox couple of Nb(IV)/Nb(V) onto the support surface. All these optimizations have induced improvements in the properties of the supported IrO2 synthesized subsequently by Adams fusion, such as smaller IrO2 particles with a diameter smaller than 3 nm, better dispersion of the IrO2 particles, additional capacitance for anodic charge, and elevated catalytic stability. As a consequence, both the voltammetric capacitance in a halfcell and the OER performance in a single cell have been significantly improved with the increase of the Nb doping amount. Among all the Nb-doped titania supports, TN-20 (doped by 20 at. % Nb) performs the best as an OER catalyst support; it is a solid solution of anatase phase with a high specific surface area of 132.42 m2·g−1. The IrO2 loaded on TN20 is of high dispersion with a refined grain size of 2.9 nm. The portion of lower-valence Nb ions reaches 4.9 at. % of the surface ions, introducing many additional redox couples of Nb(IV)/Nb(V) as species carriers. This has a great effect on the improvement of OER performance, leading to a cell potential as low as 2.027 V at 1 A·cm−2 and 80 °C.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00531. Test fixture for measuring substrate resistivity, lattice constants of the anatase and IrO2 phases, BJH pore size distribution of the titania supports, survey XPS spectra of catalysts, elemental composition data of catalysts, Ir 4f XPS doublets and molar composition of Ir in different valence (PDF).

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AUTHOR INFORMATION

Corresponding Author

*H. Lv. Tel: +86-21-6958-3850. Fax: +86-21-6958-3850. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the National Natural Science Foundation of China (Grant No. 21306141) and the National High-tech R&D Program of China (No. 2012AA053301). We thank Prof. Cunman Zhang for the use of his lab equipment and for his advice and guidance on revising the manuscript and the supporting information.

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REFERENCES

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