A novel three-dimensional gold catalyst prepared by simple pulse electrodeposition and its high electrochemical performance for hydrogen peroxide reduction

Abstract

Novel Au nanoparticles (NP), Au pinecones (PC) and Au nanodendrites (ND) supported on carbon coated titanium dioxide (C@TiO₂) nanoarrays were successfully obtained through a facile chemical vapor deposition of three-dimensional (3D) C@TiO₂ substrate, followed by potential pulse electrodeposition of Au electrocatalysts. The morphology and structure of the open 3D Au–C@TiO₂ electrodes was characterized by scanning electron microscopy and X-ray diffractometry. The different morphology of electrodeposited Au can be easily controlled by the applied potential (E_o). Electrochemical methods, including cyclic voltammetry, linear sweep voltammetry and chronoamperometry, were used to examine the catalytic activity of the electrode for H₂O₂ electroreduction in H₂SO₄ solution. The Au ND–C@TiO₂ electrode exhibited the largest effective specific surface area among the Au–C@TiO₂ electrodes, owing to its open nanodendritic structure allowing the full utilization of Au surface active sites. A nearly constant reduction current density of 0.655 A cm⁻² was successfully achieved on the Au ND–C@TiO₂ electrode at the potential of 0 V in 2.0 mol L⁻¹ H₂O₂ + 2.0 mol L⁻¹ H₂SO₄ solution, which was significantly higher than the catalytic activity of H₂O₂ electroreduction achieved previously with precious metals as catalysts.

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1. Introduction

Nowadays, the demand for electrical energy has greatly increased with the rapid development of modern society and massive energy needs as well as the depletion of fossil fuels. Fuel cells (FCs) have attracted considerable attention in recent years because they can provide a promising alternative to incumbent electrical power generation technologies, for medium-scale applications such as remote or backup power, as well as small-scale applications.^1–6 Over the years, the investigation of liquid-based FCs employing hydrogen peroxide (H₂O₂) to replace oxygen (O₂) as oxidant (e.g. metal semi-FCs, direct peroxide–peroxide FCs, direct borohydride–hydrogen peroxide FCs and direct methanol–hydrogen peroxide FCs) has been a hot topic.^7–28 Compared with O₂, H₂O₂ has many advantages such as faster reduction kinetics, easier storage and feeding, the device compact and no production with toxicity during the reaction process.^7–28

In addition, fuel cells using H₂O₂ as oxidant in acid solution could achieve higher cell performance than that in alkaline medium.^24–28 For example, the theoretical open circuit voltage (OCP) of direct borohydride FCs (DBFCs) with the electroreduction of H₂O₂ in acid is as high as 3.016 V, which is 0.9 V higher than that in basic medium. Walsh et al.²⁴ exploited a basic-acidic bipolar electrolytes DBFC configuration, with NaBH₄ electrooxidation in NaOH and H₂O₂ electroreduction in HCl. A high cell performance with the OCP of 1.9 V and peak power density of 34 mW cm⁻² was successfully obtained. Direct peroxide–peroxide FCs also use H₂O₂ as oxidant in acid solution to get a high cell performance. Sanli and Aytaç²⁸ reported a two-compartment H₂O₂ fuel cell having the conventional fuel cell configuration. By separating the anode and cathode compartments and operating with basic H₂O₂ as fuel and acidic H₂O₂ as oxidant, the cell demonstrated an open circuit voltage of around 0.9 V and a peak power density of 3.8 mW cm⁻². Thus, in order to further improve the cell performance using H₂O₂ as oxidant, the electrocatalysts with high performance for H₂O₂ electroreduction in acid are imperious demands.

At present, macrocycle complexes of transition metals (e.g. Fe– and Co–porphyrin, Cu–triazine complexes^13,14) and transition metal oxides (e.g. cobalt oxides, ferric oxides^15,16) are widely used for H₂O₂ electroreduction due to their low cost. However, they suffer the drawback of inferior catalytic activity and unstability in acid medium. Noble metals (e.g. platinum, palladium, iridium, gold, silver and their alloys^{8,10–12,17–23}) are the most effective catalysts for the electroreduction of H₂O₂ due to their high activity and superior stability in harsh acid and alkaline solution, but their applied range is greatly limited by their high price. So improving the specific surface area and then reducing the use level of noble metals are very interesting.

Generally, noble metals are loaded on carbon black to form powder catalysts. They are mixed with conducting carbons and polymer binders to form pastes, and then applied to a carbon paper current collector. Such obtained electrodes usually suffer drawbacks of low catalyst utilization because some catalysts are unable to contact with the current collector and electrolyte to form the three-phase reaction zone.^29–31 Furthermore, if gas products were involved (e.g. methanol oxidation, borohydride hydrolysis), they may block the active sites of catalysts causing a reduction of catalytic efficiency due to the slow removal of gas bubbles from compact electrodes. The existence of binder will also greatly decrease the electrical conductivity of the electrode materials, reducing their electrochemical performance. To overcome these problems, one possible solution is preparing the three dimensional (3D) structured electrocatalysts directly supported on an open structural current-collecting substrate without any conductive agent and binder.

Recently, transition metal oxide nanoarrays (e.g. TiO₂, α-Fe₂O₃, ZnO, Co₃O₄, Co₃O₄@NiO, CuO/ZnO nanoarrays)^15,32–37 directly grown on the substrate have been in various applications, such as catalysts, electrochemical capacitors, lithium ion batteries and solar cells, because nanoarrays with the open structure usually possess a larger electrochemical active surface area, a higher utilization efficiency of the active materials, and a superior mass transport property. However, pure transition metal oxide has the disadvantages of poor electrical conductivity and instability in acid media.^38,39 As a consequence, transition metal oxide nanoarrays coated by highly conductive carbon are urgently required as an excellent current collector.

In this work, nanoarrays consisting of carbon coated titanium dioxide (C@TiO₂) by a one-step chemical vapor deposition without any template were used as the high conductive skeleton for Au deposits using potential pulse electrodeposition. Au was selected as the catalyst because it is a non-platinum catalyst with good stability in the harsh acidic H₂O₂ solution. Moreover, the Ti foil substrate and C@TiO₂ nanoarrays are also stable in acid. C@TiO₂ nanoarrays with open structures were used both as the support and the current collector to enable the electrode to have good mass transport property. The electrodes with different structural Au catalysts (nanoparticles, pinecones and nanodendrites) were obtained by varying the electrodeposition potentials. The Au loading and the electrodes for the electrochemical performance of H₂O₂ reduction were systematically investigated. The Au nanodendrites deposited on the surface of C@TiO₂ (Au ND–C@TiO₂) electrode demonstrated the highest activity, Au utilization and catalytic performance for H₂O₂ electroreduction among the three electrodes in H₂SO₄ solution.

2. Experimental

2.1. Reagents

Chloroauric acid tetrahydrate (HAuCl₄·4H₂O) (>99.9%), sulphuric acid (H₂SO₄), hydrogen peroxide (H₂O₂), acetone (CH₃COCH₃), isopropanol ((CH₃)₂CHOH), ethanol (C₂H₅OH), hydrofluoric acid (HF) and nitric acid (HNO₃) were obtained from Enterprise Group Chemicals Reagent Co. Ltd China. Ti foil (thickness: 1 mm) was purchased from Baoji Yiyuan titanium industry Co., Ltd All chemicals are analytical grade and were used as-received without further purification. Ultra-pure water (Millipore, 18 MΩ cm) was used throughout the study.

2.2. Preparation and characterization of Au–C@TiO₂ electrodes

The schematic illustration for the synthesis of Au–C@TiO₂ electrodes is shown in Fig. 1a. The C@TiO₂ nanoarrays were synthesized according to the procedure reported by Huo et al.³⁹ In brief, Ti foil (10 mm × 10 mm × 1 mm) was degreased ultrasonically in acetone, isopropanol and ethanol sequentially, followed by polishing in a solution containing H₂O, HF and HNO₃ with a volume ratio of 5 : 1 : 4 for 5 min to remove the surface native oxide layer. After rinsing with deionized water and drying under flowing N₂ (99.999%) gas, the Ti foil was loaded onto a ceramic substrate placed in the center of an alumina tube inside a horizontal tube furnace. The reactor was purged with argon several times to remove residual oxygen and/or moisture before being heated to 850 °C under Ar (99.9999%) gas. Acetone was subsequently introduced into the chamber by argon at a flow rate of 150 SCCM (SCCM denotes standard cubic centimeter per minute at STP). The reaction proceeded for 1.5 h and then the sample was cooled to room temperature under argon gas.

Fig. 1 Schematic diagram of the preparation process of Au–C@TiO₂ electrodes (a) and the potential pulse electrodeposition (b).

The Au NP–C@TiO₂, Au PC–C@TiO₂ and Au ND–C@TiO₂ electrodes were fabricated by potential pulse electrodeposition of Au nanoparticles, pinecones and nanodendrites directly on the C@TiO₂ nanoarrays (Fig. 1a). The electrodeposition was carried out in a standard three-electrode electrochemical cell controlled by computerized potentiostat (Autolab PGSTAT302, Eco Chemie). A piece of C@TiO₂ substrate was employed as the working electrode, and a platinum foil (10 × 10 mm) was served as the counter electrode. A saturated Ag/AgCl (3 mol L⁻¹ KCl) electrode was used as the reference electrode, and all potentials in this work were referred to this reference electrode. The solution contains 0.5 mmol L⁻¹ HAuCl₄ and 0.5 mol L⁻¹ H₂SO₄. Before electrodeposition, a piece of C@TiO₂ substrate was first immersed in HNO₃ solution for 30 min, and then the Au nanoparticles, pinecones and nanodendrites were achieved by potential pulse electrodeposition method (Fig. 1b). The reduction potential (E_r) was fixed at −0.5 V, and the oxidation potential (E_o) was changed to be 0.7, 0.9 and 1.1 V, respectively. The frequency was 5 Hz and the deposition time was 30 min. The electrolyte was kept stirring under flowing nitrogen during the electrodeposition. After deposition, the electrode was removed from the solution and washed with ultrapure water thoroughly, and air-dried prior.

The Au–C@TiO₂ electrodes were characterized by a scanning electron microscope (SEM, JEOL JSM-6480). The structure was analyzed using an X-ray diffractometer (Rigaku TTR III) with Cu-Kα radiation (λ = 0.1514178 nm). The Au loading was measured using an inductive coupled plasma emission spectrometer (ICP, Xseries II, Thermo Scientific). Au in the 1.0 cm² electrode was first dissolved in aqua regia solution and then diluted to 1 L solution for the ICP measurement.

2.3. Electrochemical measurements

The electrochemical measurements were also performed in a standard three-electrode electrochemical cell with saturated Ag/AgCl (3 mol L⁻¹ KCl) reference electrode and Pt counter electrode. The electrolyte for H₂O₂ electroreduction was H₂O₂ containing H₂SO₄. The electrolyte solutions were deoxygenated by bubbling ultrahigh purity N₂ for 15 min and maintained under N₂ atmosphere during measurements. All measurements were performed at ambient temperature (20 ± 2 °C). Linear sweep voltammetry (LSV), cyclic voltammetry (CV) and chronoamperometry (CA) were conducted using a computerized potentiostat (Autolab PGSTAT302, Eco Chemie) controlled by GPES software. The reported current densities were calculated using the geometrical area of the electrode.

3. Results and discussion

3.1. Characterization of Au–C@TiO₂ electrodes

The Au–C@TiO₂ electrodes were prepared by potential pulse electrodeposition of Au electrocatalysts directly on the C@TiO₂ nanoarrays at different applied potentials (E_o). Fig. 2 shows the SEM images with different magnification of Au NP–C@TiO₂, Au PC–C@TiO₂, Au ND–C@TiO₂ electrodes obtained at E_o of 0.7 V, 0.9 V and 1.1 V, respectively. The C@TiO₂ nanoarrays were also shown for comparison. Ti foil as the substrate for the direct growth of C@TiO₂ nanoarrays can be achieved many advantages including high stability in the harsh acid medium and easy preparation of membrane electrolyte assembly due to the ultrathin properties (the thickness of 1 mm). It can be seen from Fig. 2g that the surface of Ti foil was completely covered by the dense growth of C@TiO₂ nanoarrays. The C@TiO₂ nanoarrays grew almost vertically from the substrate with the length of ∼5 μm and the diameter of ∼150 nm, which provided an open three-dimensional skeleton for the electrodeposition of Au, allowing electrolytes to access the full electrode surface. Besides, the high electrical conductivity and stability of C@TiO₂ nanoarrays make it a desirable support of Au catalysts and the current collector of the electrodes. Fig. 2a and b present the different magnification SEM images of typical Au NP–C@TiO₂ electrode and demonstrate that a series of Au nanoparticles with the size of ∼160 nm uniformly covered the surface of C@TiO₂ nanoarrays. As can be observed from Fig. 2c and d, the homogeneous and compact Au pinecones were equably formed on the surface of C@TiO₂ nanoarrays after Au electrodeposition. The lower magnification SEM image of the Au ND–C@TiO₂ electrode (Fig. 2e) manifested that dendritic Au was deposited on the C@TiO₂ and aligned perpendicularly to the nanoarray surface. The width of these dendritic nanostructures was hundreds of nanometers and the length was up to about 0.8 μm. There were also some particle-like structures on the surface of C@TiO₂ nanoarrays in the high magnification SEM image (Fig. 2f). Higher magnification SEM image (insert of Fig. 2f) revealed the detail of an Au dendrite. The single Au dendrite was composed of a backbone with symmetrical side branches. The diameters of both the trunks and side branches were less than 100 nm. This unique structure of the Au ND–C@TiO₂ electrode ensured the full utilization of Au surfaces because all of the Au surfaces are accessible to electrolytes. The results show that the surface morphology of Au electrocatalysts changes from nanoparticles prepared at E_o of 0.7 V to pinecone-like morphology obtained at E_o of 0.9 V and finally to dendritic morphology deposited at E_o of 1.1 V with the increase of E_o. Consequently, under the condition of potential pulse electrodeposition, the morphologies of Au electrocatalysts can be adjusted simply by changing the applied oxidation potential. Above all, the obtained Au NP, Au PC and Au ND–C@TiO₂ electrodes may be particularly suitable for H₂O₂ electroreduction in acid medium, making electroactive Au with a large bare surface place in direct contact with the electrolyte.

Fig. 2 Low-magnification and high-magnification SEM images of Au NP–C@TiO₂ electrode (a and b), Au PC–C@TiO₂ electrode (c and d), Au ND–C@TiO₂ electrode (e and f) and C@TiO₂ nanoarrays (g).

Based on the results obtained by scanning electron microscopy, X-ray diffraction analysis was employed to further identify the structure of the Au–C@TiO₂ electrodes. Fig. 3 displays the XRD patterns of Au NP–C@TiO₂, Au PC–C@TiO₂ and Au ND–C@TiO₂ electrode, along with that of the bare C@TiO₂ nanoarrays for comparison. The pattern of Au ND–C@TiO₂ electrode exhibited four diffraction peaks at 2θ = 38.2°, 44.4°, 64.5° and 77.5°, which can be indexed to the diffraction from the (111), (200), (220) and (311) plane of the face-centered cubic Au metal, respectively, according to the standard crystallographic spectrum of Au (JCPDS card no. 65-2870), indicating that Au nanodendrites present as the metallic state. XRD patterns of the Au PC–C@TiO₂ and Au NP–C@TiO₂ electrode also displayed the characteristic peak of metallic Au. Differing from the Au ND–C@TiO₂ electrode, the peak intensities of (111), (200), (220) and (311) plane of Au–C@TiO₂ electrodes obviously decreased in the order of Au ND–C@TiO₂, Au PC–C@TiO₂ and Au NP–C@TiO₂ electrode, suggesting that the Au nanodendrites include the best well-crystallized gold nanocrystals among the three electrodes. Furthermore, the intensities of the main diffraction peaks located at 2θ = 27.3° and 39.8° for C@TiO₂ substrate were dramatically diminished after being covered by Au nanoparticles, pinecones and nanodendrites, revealing that Au nanoparticles, pinecones and nanodendrites were uniformly deposited upon the C@TiO₂.

Fig. 3 XRD patterns of the C@TiO₂ nanoarrays (a), Au NP–C@TiO₂ electrode (b), Au PC–C@TiO₂ electrode (c) and Au ND–C@TiO₂ electrode (d).

3.2. Electrocatalytic performance of H₂O₂ electroreduction on the Au–C@TiO₂ electrodes

Fig. 4 exhibits the typical cyclic voltammograms (CVs) of the C@TiO₂, Au NP–C@TiO₂, Au PC–C@TiO₂, Au ND–C@TiO₂ electrodes measured in 1.0 mol L⁻¹ H₂SO₄ at a scan rate of 50 mV s⁻¹. It is clear that no obvious oxidation/reduction peaks were observed on the C@TiO₂ substrate, indicating that C@TiO₂ was stable in acid. Differing from the CV of C@TiO₂ substrate, the hydrogen adsorption/desorption peaks and surface oxide formation/reduction peaks are readily seen in the CVs of Au NP, Au PC and Au ND–C@TiO₂ electrodes. It displays a typical response of polycrystalline Au in H₂SO₄ solution. The anodic peaks starting from 1.04 V is due to the oxidation of surface Au leading to the formation of an Au surface oxide layer. The cathodic peak centered at 0.98 V corresponds to the reduction of Au surface oxides.

Fig. 4 Cyclic voltammograms of the C@TiO₂ substrate, Au NP–C@TiO₂ electrode, Au PC–C@TiO₂ electrode and Au ND–C@TiO₂ electrode in 1.0 mol L⁻¹ H₂SO₄ at a scan rate of 50 mV s⁻¹.

Assuming a monolayer of AuO was formed on Au surface in the positive potential scan, the electrochemically active surface area (EASA) of Au catalyst can be estimated from the charge corresponding to the cathodic reduction peak of surface oxide.^40,41 Based on the monolayer charge of 390 μC cm⁻² for AuO, it can be calculated that the EASA of Au NP–C@TiO₂, Au PC–C@TiO₂ and Au ND–C@TiO₂ electrode per cm² (geometrical area) was 53.3, 76.6 and 91.7 cm², respectively. The EASAs of Au ND and Au PC–C@TiO₂ electrode were ∼1.7 times and 1.4 times larger than that of Au NP–C@TiO₂ electrode, respectively. By contrast, the value of both Au ND and Au PC–C@TiO₂ electrodes mentioned here was much larger than that of dendritic Au and Pd prepared in our previous work.^23,42 ICP measurement shows that the loading of Au in the Au NP, Au PC and Au ND–C@TiO₂ electrode was 0.1342, 0.1486 and 0.1507 mg cm⁻², respectively. So the effective specific surface area (ESSA) of Au reached 39.7, 51.5 and 60.8 m² g⁻¹ for Au nanoparticles, Au pinecones and Au nanodendrites, respectively. Therefore, the effective specific surface area of Au was increased by forming pinecones and nanodendrites. In addition, the hydrogen adsorption/desorption current on the Au ND–C@TiO₂ electrode could be more clearly observed than that on the Au NP–C@TiO₂ and Au PC–C@TiO₂ at −0.2 V. This implies that nanodendritic Au has higher electrocatalytic activity than Au pinecones and Au nanoparticles. Moreover, the ESSA value of nanodendritic Au on the C@TiO₂ was much larger than that of nano-Au and nano-Pd supported on carbon materials (carbon fiber cloth, carbon black, carbon nanotubes) reported in the literatures.^23,42–44 The large surface area of Au ND–C@TiO₂ electrode was most possibly provided by the excellent skeleton of C@TiO₂ nanoarrays and its dendritic structure. Importantly, high ESSA usually means high utilization of Au, which is very important for the reduction of the cost of precious metal catalysts.

In order to further confirm that the Au ND–C@TiO₂ electrode is the superior electrocatalyst to Au NP and Au PC–C@TiO₂ electrodes, the comparative polarization curves for H₂O₂ electroreduction in H₂SO₄ on the different electrodes were investigated in Fig. 5a. The linear sweep voltammograms (LSVs) of C@TiO₂ substrate in 1.0 mol L⁻¹ H₂SO₄ and 1.0 mol L⁻¹ H₂O₂ was also shown for comparison. The C@TiO₂ substrate exhibited no catalytic activity for H₂O₂ electroreduction, revealing that the high performance was contributed to the Au deposits. The current densities at the same potential increased significantly in the sequence of Au NP, Au PC and Au ND–C@TiO₂, indicating that the Au ND–C@TiO₂ electrode had higher electrocatalytic performance than the Au NP and Au PC–C@TiO₂ electrodes for H₂O₂ electroreduction. This result is consistent with the order of ESSA (Au nanodendrites > Au pinecones > Au nanoparticles).

Fig. 5 (a) Comparative polarization curves for H₂O₂ electroreduction on the C@TiO₂ substrate, Au NP–C@TiO₂, Au PC–C@TiO₂ and Au ND–C@TiO₂ electrode in 1.0 mol L⁻¹ H₂SO₄ + 1.0 mol L⁻¹ H₂O₂ solution at a scan rate of 5 mV s⁻¹; (b) chronoamperometric curves for H₂O₂ electroreduction on the Au NP–C@TiO₂, Au PC–C@TiO₂ and Au ND–C@TiO₂ electrode in 1.0 mol L⁻¹ H₂SO₄ + 1.0 mol L⁻¹ H₂O₂ solution at the potential of 0 V.

Chronoamperometry was carried out to further study the optimal Au–C@TiO₂ electrode for catalyzing H₂O₂ electroreduction. Fig. 5b shows the chronoamperometric curves (CAs) for H₂O₂ electroreduction in H₂SO₄ on the Au–C@TiO₂ electrodes. As can be seen, the reduction current density of all the Au–C@TiO₂ electrodes reached to steady state after a few seconds and displayed no sign of decrease within 1000 s test period at the potential of 0 V, indicating that all the electrodes have a superior stability for H₂O₂ electroreduction in acid medium. In addition, no obvious chemical decomposition of H₂O₂ was observed during the tests. It is clear that the current density on the Au ND–C@TiO₂ electrode kept at 0.347 A cm⁻², but that on the Au NP and Au PC–C@TiO₂ electrode was just 0.202 and 0.283 A cm⁻². The current densities of Au ND and Au PC–C@TiO₂ electrode were ∼1.7 times and 1.4 times higher than that of Au NP–C@TiO₂ electrode, respectively. The ratios of the current densities were proportional to the corresponding ratios of the EASAs, indicating that the specific catalytic activity might be the same for the three electrocatalysts. It also demonstrated that the optimum Au–C@TiO₂ electrode was Au nanodendrites and the result was in good accordance with the LSV test (Fig. 5a).

3.3. Effects of H₂SO₄ and H₂O₂ concentration for H₂O₂ electrochemical reduction on the Au ND–C@TiO₂ electrode

Karl J. J. Mayrhofer et al.^45–49 found that the electrolyte anions and concentration had a large impact on the electrochemical reduction of hydrogen peroxide, and the electroactivity of the anions decreased in the order ClO₄⁻ > HSO₄⁻ > Cl⁻ > Br⁻ > I⁻. Considering the instability of ClO₄⁻, H₂SO₄ was selected as the electrolyte in this work. In order to identify an appropriate H₂SO₄ concentration for Au ND–C@TiO₂ electrode, LSV and CA measurements were examined. The concentration of H₂SO₄ was changed with H₂O₂ concentration constantly kept at 2.0 mol L⁻¹. Fig. 6 presents the influence of H₂SO₄ concentration for H₂O₂ electroreduction on the Au ND–C@TiO₂ electrode. It can be observed from Fig. 6a that the onset reduction potentials were ∼0.58 V and the reduction current density similarly increased with the negative scan going in the potential range of 0.6 to −0.2 V. Furthermore, excess electrolyte made no contribution to the enhancement of the activity of H₂O₂ reduction. H₂SO₄ concentration of 2.0 mol L⁻¹ yielded the best performance. CAs for H₂O₂ electroreduction were also studied in detail (Fig. 6b). At the potential of 0 V, a nearly constant reduction current density of all the different concentration H₂SO₄ was achieved on the Au ND–C@TiO₂ electrode within 1000 s test period, suggesting that the electrode has a good stability for H₂O₂ electroreduction. The result revealed that the current density with 2.0 mol L⁻¹ H₂SO₄ kept at 0.655 A cm⁻², while the current density with 1.0 and 3.0 mol L⁻¹ H₂SO₄ was 0.428 and 0.537 A cm⁻². It was also displayed that the best suitable H₂SO₄ concentration was 2.0 mol L⁻¹.

Fig. 6 (a) Linear sweep voltammograms for H₂O₂ electroreduction on the Au ND–C@TiO₂ electrode in x mol L⁻¹ H₂SO₄ + 2.0 mol L⁻¹ H₂O₂ (x = 1.0, 2.0 and 3.0, scan rate: 5 mV s⁻¹); (b) chronoamperometric curves for H₂O₂ electroreduction at the potential of 0 V in 2.0 mol L⁻¹ H₂O₂ with different H₂SO₄ concentrations.

Fig. 7a shows the dependence of H₂O₂ concentration for H₂O₂ reduction on the Au ND–C@TiO₂ electrode. The current density increased dramatically with increasing the H₂O₂ concentration from 0.5 to 1.5 mol L⁻¹ at the same potential, but the further increase of H₂O₂ concentration from 1.5 to 2.0 mol L⁻¹ only leads to a slight increase of current density. It is worth to mention here that the chemical decomposition of H₂O₂ can be discovered as the H₂O₂ concentration was higher than 2.0 mol L⁻¹, which not only causes the waste of oxidant but also the bubbles produced from H₂O₂ decomposition can block the mass transport and active sites of catalyst. The Au ND–C@TiO₂ electrode showed a reduction current density of 369 mA cm⁻² at 0 V in the solution of 2.0 mol L⁻¹ H₂SO₄ + 0.5 mol L⁻¹ H₂O₂, which was much higher than the commercial Pd/C electrode with a Pd loading of 0.31 mg cm⁻² and other precious metal composite electrodes reported in the literatures.^22,23,50,51 The high electrocatalytic performance of the commercial Pd/C, Au or Pd composite electrodes and Au ND–C@TiO₂ electrode was also summarized and compared in the Table 1. By comparison with the commercial Pd/C, Pd/CFC, Au–Pd NPs/CFC, Au@Pd/CFC, Au/CFC, Pd nanofilm electrodes previously reported, nanodendritic Au on the C@TiO₂ nanoarrays exhibited significantly higher catalytic property to H₂O₂ electroreduction in acid, which is owing to the unique open 3D skeleton of C@TiO₂ nanoarrays, enabling the full utilization of Au surfaces and making the electrode have higher electrochemical activity.

Fig. 7 (a) Linear sweep voltammograms for H₂O₂ electroreduction on the Au ND–C@TiO₂ electrode in 2.0 mol L⁻¹ H₂SO₄ + x mol L⁻¹ H₂O₂ (x = 0.5, 1.0, 1.5 and 2.0, scan rate: 5 mV s⁻¹); (b) chronoamperometric curves for H₂O₂ electroreduction at the potential of 0 V in 2.0 mol L⁻¹ H₂SO₄ with different H₂O₂ concentrations.

Table 1 Current density (mA cm⁻²) for H₂O₂ electroreduction on commercial Pd/C, Pd/CFC, Au–Pd NPs/CFC, Au@Pd/CFC, Au/CFC, Pd nanofilm and Au ND–C@TiO₂ electrode in 2.0 mol L⁻¹ H₂SO₄ solution with different H₂O₂ concentrations at 0 V (vs. Ag/AgCl)

H2O2 concentration (mol L^−1)	0.5	1.0	1.5	2.0	Ref.
Commercial Pd/C electrode	95	—	—	112	22 and 23
Pd/CFC electrode	152	294	398	480	23
Au–Pd NPs/CFC electrode	183	326	357	388	50
Au@Pd/CFC electrode	205	—	—	—	51
Au/CFC	246	—	—	—	42
Pd nanofilm electrode	213	329	404	415	22
Au ND–C@TiO2 electrode	369	468	590	655	This work

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Fig. 7b exhibits the CAs with different H₂O₂ concentrations at the potential of 0 V. The reduction current density reached to the steady state after the initial current decay at all the different H₂O₂ concentrations within 1000 s test period, which indicated that the Au ND–C@TiO₂ electrode possesses an excellent stability for H₂O₂ electroreduction in acid media.

The stability of Au ND–C@TiO₂ electrode for H₂O₂ electroreduction at different applied potential was investigated by chronoamperometric experiment. Fig. 8 shows CAs of H₂O₂ electroreduction at different applied potential in 2.0 mol L⁻¹ H₂SO₄ + 2.0 mol L⁻¹ H₂O₂. After a rapid initial wave, the current density reached steady state at the different control potentials (0.2, 0 and −0.2 V) with slightly decreasing in 1000 s test period. In addition, the current density increased with changing the potential to more negative direction, which was in good agreement with the LSV results shown in Fig. 7a. The final current densities after 1000 s reaction at 0.2, 0 and −0.2 V were 0.418, 0.637 and 0.836 A cm⁻², respectively. Notably, all of the above electrochemical tests were carried out by using the same electrode and the total time in the testing solution was about 8 hours and based on the stable CAs, it can be concluded that the nanodendritic Au on the C@TiO₂ has superior electrochemical stability and is promising catalytic materials for fuel cells employing H₂O₂ as oxidant in acid.

Fig. 8 Chronoamperometric curves for H₂O₂ electroreduction at different potentials in 2.0 mol L⁻¹ H₂SO₄ + 2.0 mol L⁻¹ H₂O₂.

4. Conclusions

A novel three-dimensional electrode facilely fabricated by the potential pulse electrodepositing Au nanoparticles, Au pinecones and Au nanodendrites on the surface of C@TiO₂ nanoarrays are employed for H₂O₂ electroreduction and exhibit high catalytic performance and superior stability. The surface morphologies of Au electrocatalysts on the C@TiO₂ plays an important role in determining the electrocatalytic activity for H₂O₂ electroreduction in acid and they are strongly dependent on the applied oxidation potential (E_o) of pulsed electrodeposition. With the increase of E_o, the surface morphology of Au electrocatalysts changes from nanoparticles prepared at E_o of 0.7 V to pinecone-like morphology obtained at E_o of 0.9 V and finally to dendritic morphology deposited at E_o of 1.1 V. The dendritic Au on the C@TiO₂ possesses much larger ESSA for the H₂O₂ electroreduction, followed by the Au with pinecone-like morphology while the smooth Au nanoparticles have the smallest ESSA. The Au ND–C@TiO₂ electrode related to its special morphology and large ESSA exhibited the highest electrocatalytic activity among the Au–C@TiO₂ electrodes, which has great significance to lower the use of noble metal. This unique open structure of the electrode enables the full utilization of Au surfaces and allows the easy transportation of reactants to the catalyst as well as the quick removal of gaseous products from the electrode. In summary, an original Au ND–C@TiO₂ electrode can be candidate for the promising application in fuel cells employed H₂O₂ as oxidant in acid.

Acknowledgements

We gratefully acknowledge the financial support of this research by the National Natural Science Foundation of China (21403044), the Heilongjiang Postdoctoral Fund (LBH-Z13059), the China Postdoctoral Science Foundation (2014M561332) and the Fundamental Research Funds for the Central Universities (HEUCF201403018).

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