MnO₂ nanosheets as a high-efficiency electrocatalyst for H₂O₂ reduction in alkaline medium

Abstract

Considering the good ability of MnO₂ for the breakage of the HO–OH bond in H₂O₂, we employed C@TiO₂ nanowire supported MnO₂ as a novel catalyst for H₂O₂ electroreduction. The morphology and phase structure of the MnO₂/C@TiO₂ electrode are characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffractometry. The catalytic activity of the MnO₂/C@TiO₂ electrode for H₂O₂ electroreduction is investigated by means of cyclic voltammetry and chronoamperometry. The catalyst exhibits a high catalytic activity and good stability in the electrochemical reaction process. The oxidation current density is higher than 200 mA cm⁻² at −0.7 V in 1.6 mol dm⁻³ H₂O₂. The popular price, abundant reserves, and promising electrocatalytic performance make it a viable catalyst for H₂O₂ electroreduction.

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1. Introduction

Fuel cells (FCs), devices directly converting the chemical energy of a fuel into electrical energy, are highly desirable with the depletion of fossil fuels and the ever increasing demands for clean energy.^1–3 Compared with lithium batteries, FCs have intrinsic advantages in power density and energy supply, which make them promising for use in spaceships,⁴ underwater,⁵ hybrid vehicles⁶ energy supplying devices and so on. Hydrogen peroxide (H₂O₂) has been seen as a viable oxidizer for liquid-based FCs, such as metal semi-FCs,^7,8 direct borohydride–hydrogen peroxide FCs,^9–14 direct peroxide–peroxide FCs,^15,16 direct methanol–hydrogen peroxide FCs and so on.^17,18 The electroreduction towards H₂O₂ in the cathode is a two-electron transfer process involving the breakage of single dioxygen bonds (eqn (1))^7–32 with a standard electrode potential of 0.878 V, leading a lower reaction active-energy compared with the O₂ electroreduction of the double of single dioxygen bonds (eqn (2))^33–39 and higher cathode potential (0.401 V). Besides, the H₂O₂ is liquid, which establish a simple solid–liquid two-phase reaction and is easy to construct a more stable, compact and convenient FC system.^7–18 So FCs employing H₂O₂ as oxidant with high performance may be potential electrochemical devices to replace O₂ in some operational places without oxygen, such as space station, submarine and so on.

H₂O₂ + 2e⁻ → 2OH⁻, E⁰ = 0.878 (1)

O₂ + 2H₂O + 4e⁻ → 4OH⁻, E⁰ = 0.401 (2)

The design of cathode catalysts with excellent electrocatalytic activity for H₂O₂ reduction is one of the decisive factors that determine the performance of FCs. In general, precious metals, such as Pt,^19,20 Pd,²¹ Au²² and their alloys, are considered as the best catalysts for various electrocatalytic reactions due to their superior ability to trap electrons. However, the use of noble metals is limited by their scarcity and high cost, which give impetus to the attention on the inexpensive choices to supersede noble metals.

Currently, macrocycle complexes of transition metals, such as Fe- and Co-porphyrin,^23–25 transition metal and their oxides, such as cobalt,²¹ copper oxides,²⁶ cobalt oxides,^27–31 ferric oxides,³² perovskite-type oxides¹⁴ are studied as low cost catalysts and alternative to noble metals for H₂O₂ electroreduction. Lei et al.³¹ reported an aligned Co₃O₄ nano-walls electrode as the H₂O₂ sensor based on the electrocatalytic oxidation and reduction of H₂O₂. A fast response and high sensitivity were successfully obtained at applied potentials of +0.8 V and −0.2 V (vs. Ag/AgCl). Our team²⁶ previously prepared a Cu foil based CuO nanosheets through a simple chemical oxidation process. The low-cost, abundant resource and easy preparation of CuO/Cu made it a promising electrode for FCs using H₂O₂ as oxidants.

MnO₂, as a cost-effective and important electrochemical material, has been widely used in lithium batteries,^40,41 supercapacitors,^42,43 FCs,^44,45 electrochemical sensors^46,47 and so on. For example, owning to its high specific capacitance, low cost, natural abundance, and environmental benignity, MnO₂ is regarded as the most practical stuff in the field of electrochemical energy storage. Moreover, MnO₂ can efficaciously break O–O bond in HO₂⁻ and catalytic HO₂⁻ disproportionation (eqn (3) and (4))^33–35 and receives intensively interests on the FCs (employing O₂ as oxidizer), metal/air (O₂) batteries, electrochemical sensors (H₂O₂).^{33–35,45,47} However, to the best of our knowledge, few of report applied MnO₂ for H₂O₂ electrochemical reduction in fuel cells.

HO₂⁻ + 2H₂O + 2e⁻ → 3OH⁻ (3)

2HO₂⁻ → 2OH⁻ + O₂ (4)

In this paper, we use a C@TiO₂ nanowire based MnO₂ nanosheets⁴² as an active and stable catalyst for H₂O₂ electroreduction. The C@TiO₂ nanowire were prepared by a simple chemical vaporous deposition (CVD) using Ti foil as substrate and acetone as carbon source and following a electrodeposition process to deposit MnO₂. This MnO₂/C@TiO₂ electrode owns a unique three dimensional (3D) porous structure, which is facile for the diffusion of H₂O₂ and the release of O₂ during the reaction process (eqn (4)). The MnO₂/C@TiO₂ achieved an open circle potential of −0.2 V and a reduction current density of 200 mA cm⁻² at −0.7 V in a solution containing 3 mol dm⁻³ NaOH and 1.6 mol dm⁻³ H₂O₂.

2. Experimental

2.1. Reagents

Acetone (CH₃COCH₃), isopropanol ((CH₃)₂CHOH), ethanol (C₂H₅OH), hydrofluoric acid (HF), nitric acid (HNO₃), manganese acetate (MnAc₂), ammonium acetate (NH₄Ac), dimethyl sulfoxide (DMSO), sodium hydroxide (NaOH), hydrogen peroxide (H₂O₂) were obtained from Enterprise Group Chemicals Reagent Co. Ltd. China. Ar gas was gotten from Liming Gas Co. Ltd. Ti foil 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 MU cm) was used throughout the study.

2.2. Preparation and characterization of MnO₂/C@TiO₂

The synthesis of MnO₂/C@TiO₂ is shown by Fig. 1. The C@TiO₂ substrate was first prepared by a Huo's method.⁴⁸ Briefly, Ti foil (10 × 10 × 1 mm³ sizes, 1 cm² planar area) were degreased ultrasonically in acetone, isopropanol and ethanol sequentially for 15 minutes, then polished with a solution containing H₂O, HNO₃ and HF with a volume ratio of 5 : 4 : 1 for 5 minutes. Following, the Ti foil were loaded onto a ceramic substrate and put into the center of a horizontal tube furnace after rinsing with deionized water. Before being heated to 850 °C under argon, the tube furnace was purged with argon several times. Acetone was introduced into the tube furnace by argon at a flow rate of 150 SCCM (SCCM represents standard cubic centimeter per minute at STP) for 1.5 h under 850 °C. C/TiO₂ nanowires were obtained after they were cooled to room temperature under argon. During this process, carbon-containing and oxygen-containing species including ·CH₃ radicals and CO were produced first by the thermal decomposition of CH₃COCH₃. Then, Ti atoms on the surface of Ti foil will adsorb and reacts with the as-generated CO to form TiO₂ and C (Ti + 2CO → TiO₂ + C). Notably, TiO₂ nanowires shaped with the diffusion of Ti atoms upwards along the formed TiO₂ and ·CH₃ also took part in the formation of C, as described by Huo's work.⁴⁸

Fig. 1 Fabrication process of MnO₂/C@TiO₂ electrode.

Then, MnO₂ nanosheets were coated on the as-prepared C@TiO₂ through one step electrodeposition process as our previous report.⁴² The C/TiO₂ nanowires were used as working electrode, a carbon rod (3 mm in diameter) was served as counter electrode, and a saturated Ag/AgCl, KCl electrode was used as reference electrode. Anodic electrodeposition of MnO₂ nanoplates were performed at a constant current of 1 mA cm⁻² for 5 minutes in a solution contained 0.01 mol dm⁻³ manganese acetate (MnAc₂) and 0.02 mol dm⁻³ ammonium acetate (NH₄Ac) and 10% dimethyl sulfoxide (DMSO) at 70 °C in a typical three-electrode electrochemistry cell with glass cell with an electrochemical station (CHI660D).

H₂O₂ electroreduction was also performed in the same three-electrode electrochemical cell using the 1 cm² MnO₂/C@TiO₂ electrode. All potentials were referred to the saturated Ag/AgCl, KCl reference electrode. The morphology of the electrodes was determined using a scanning electron microscope (SEM, JEOL JSM-6480) and transmission electron microscope (TEM, FEI TeccaiG2S-Twin, Philips). The structure was analyzed by a powder X-ray diffractometer (XRD, Rigaku TTR-III) equipped with Cu Kα radiation (λ = 0.15406 nm).

3. Results and discussion

MnO₂ nanoplates were deposited on the C@TiO₂ via an electrodeposition process, in which the electrodeposition time was used to play an important role in both surface morphology and electrocatalytic performance. Fig. 2 shows the SEM images of C@TiO₂ substrate and MnO₂/C@TiO₂ (Fig. 2b–f) with different electrodeposition time (10–120 min), insert in Fig. 2d is the TEM image of the single MnO₂/C@TiO₂ nanowire. After the chemical vapor deposition (CVD), aligned nano-needles with smooth face and about 500–600 nm length were observed uniformly distributed on the Ti foil surface (Fig. 2a) and formed a three dimensional (3D) open porous structure, which is favorable for the electrolyte diffusion⁴² and MnO₂ deposition. The subsequent electrodeposition process make MnO₂ equably coated around the C@TiO₂ nanowires. Interestingly, the MnO₂ exhibits nanoplate-like structure rather than nanoparticles, leading more catalytic sites and ensures closely contact between reactant and catalyst. The loading of MnO₂ increases with increase the electrodeposition time (Fig. 2b–f). When the time is 10, 30 and 60 min, we can see both of the C@TiO₂ wires and MnO₂ plates. The C@TiO₂ wires were completely covered by MnO₂ and disappeared in our vision field with the time reached to 90 and 120 min. TEM image was employed to gain further insights of the MnO₂/C@TiO₂ (insert in Fig. 2d). Clearly, the length and width of C@TiO₂ are 600 and 200 nm, respectively, with a 30 nm thickness of carbon shell. MnO₂ nanoplates were observed in the carbon surface, which is consistent with the SEM result. The crystallographic property was analyzed by X-ray diffraction and shown in Fig. 3. There are three kinds of characteristic peaks assigned to Ti (JCPDS card: 44-1294), TiO₂ (JCPDS card: 21-1276) and MnO₂ with unknown structure (12-0141),^42,43 respectively, during scan range of 15° to 80°.

Fig. 2 SEM images of C@TiO₂ substrate (a) and MnO₂/C@TiO₂ (b–f) with different electrodeposition time (10–120 min), insert in (d) is TEM image of the single MnO₂/C@TiO₂ nanowire.

Fig. 3 XRD patterns of MnO₂/C@TiO₂ electrode.

XPS measurement was investigated to further analyze the surface composition of MnO₂/C@TiO₂ and the result was shown in Fig. 4. The survey scan indicated the presence of Mn, O, C and Ti in the electrode. Insert in Fig. 4 is the high resolution XPS data for Mn 2p, presenting two peaks at 641.6 and 653.3 eV owning to the binding energy of Mn 2p3/2 and Mn 2p1/2, respectively, from the Mn⁴⁺ (MnO₂).

Fig. 4 XPS survey scan spectra of MnO₂/C@TiO₂ (insert is the high resolution XPS data for Mn 2p).

Cyclic voltammetry (CV) curve is engaged to assess the electrocatalytic performance of MnO₂/C@TiO₂ electrode in H₂O₂ solution. We first investigate the effect of electrodeposition time on the electrocatalytic activity of MnO₂/C@TiO₂ for H₂O₂ reduction and the results are shown by Fig. 5. We can see that the reduction current densities seriously depend on the MnO₂ loading and the C@TiO₂ substrate almost exhibits no electrocatalytic performance for H₂O₂ reduction, demonstrating that the C coated TiO₂ only serves as supporter for the loading of MnO₂, which will play the role of an efficient catalyst for H₂O₂ electroreduction. The current density first increased remarkably with the increase of electrodeposition time and reached to 180 mA cm⁻² at −0.7 V when the time rises to 60 min, demonstrating that extending electrodeposition can obviously improve the catalytic properties of MnO₂/C@TiO₂. However, when the time is longer than 60 min, the current density decreased from 180 to 60 mA cm⁻², respectively, in response to 90 and 120 min. Clearly, the MnO₂ prepared with 60 min shows the best performance among these samples. Besides, a weak reduction peak, involved to the electroreduction of H₂O₂ (eqn (1)),^7–32 centered at around −0.62 V, when the electrodeposition times are fixed at 10, 30 and 120 min and this phenomenon only occurred at the electrode with low efficiency. As shown in Fig. 2, the MnO₂ loading is insufficient in short electrodeposition time (10 and 30 min) and superfluous in long electrodeposition time (90 and 120 min), the first situation will not provide competent active sites to contact with oxidant and the other may block up the 3D porous structure and the oxygen can't be released soon, both of which will lead a damping for H₂O₂ reduction. Besides, manganese oxides are semiconducting materials, increasing the amount of MnO₂ will weaken the electric conductivity and further reduce the catalytic activity.

Fig. 5 Cyclic voltammetry (CV, scan rate: 5 mV s⁻¹) curve of MnO₂/C@TiO₂ with different electrodeposition time (0–120 min) in 3 mol dm⁻³ NaOH and 1.2 mol dm⁻³ H₂O₂.

OH⁻ plays an important role in the whole reaction process (eqn (1)–(4))^7–39 and it is significative to discuss the effect of NaOH concentration on the electrochemical performance. Fig. 6a presents the CVs of MnO₂/C@TiO₂ electrode in different concentrations of NaOH (1–4 mol dm⁻³) with H₂O₂ concentration constantly kept at 1.2 mol dm⁻³. It can be observed that the reduction current density of MnO₂/C@TiO₂ increased with the increase of NaOH concentration from 1 to 3 mol dm⁻³ and slightly decreased with the further increase to 4 mol dm⁻³. So 3 mol dm⁻³ NaOH gives the best performance when the H₂O₂ concentration is 1.2 mol dm⁻³ and demonstrate that the optimal electrochemical performance only occurs within suitable ration of [OH⁻/H₂O₂] (around 2) rather than immensely raise or reduce alkaline concentration.

Fig. 6 CVs (scan rate: 5 mV s⁻¹) of MnO₂/C@TiO₂ (60 min) in 1.2 mol dm⁻³ H₂O₂ + x (x = 1, 2, 2.5, 3, 4) mol dm⁻³ NaOH.

The effects of H₂O₂ concentration on the catalytic behavior of MnO₂/C@TiO₂ were investigated and the results are shown in Fig. 7. Similar as the Fig. 5, a weak reduction peak emerged in the CV curves (Fig. 7) with low catalytic performance and the current density increased with the increase of H₂O₂ concentration, manifesting the reaction was controlled by diffusion. The increasement tendency of reduction current density between the H₂O₂ concentration of 1.2 and 1.6 mol dm⁻³ decreased abruptly demonstrated that the current density isn't in proportion to the fuel concentration. The conclusion is in point to the NaOH concentration at the same time. Besides, it must be pointed that the bare C@TiO₂ substrate almost has no catalytic activity for H₂O₂ reduction. Although the catalytic activity of MnO₂/C@TiO₂ for H₂O₂ reduction can't be comparable to the previous noble metals and some our prior Co₃O₄, Co_xMn_yO, NiCo₂O₄ electrodes,^27–30 it is still a potential electrode due to its low cost and rich deposits.

Fig. 7 CVs (scan rate: 5 mV s⁻¹) of MnO₂/C@TiO₂ (60 min) in 3 mol dm⁻³ NaOH + x (x = 0, 0.4, 0.8, 1.2, 1.6) mol dm⁻³ H₂O₂.

The catalytic activity of MnO₂/C@TiO₂ for H₂O₂ electroreduction was further tested by changing the reaction temperature in a solution containing 3 mol dm⁻³ NaOH and 1.2 mol dm⁻³ H₂O₂, and the result was recorded in Fig. 8. Higher temperature will lead faster electrode kinetics²¹ and accelerate H₂O₂ reaction speed on MnO₂ surface. Under this circumstance, more HO–OH bonds were broken, resulting in the increasing of reduction current density (from 303.15 to 333.15 K). Unfortunately, over high temperature cause a critical hydrolysis of H₂O₂,^21,49 which will reduce the utilization of fuel and may destruct the electrode structure. As a consequence, the catalytic performances rapidly fall off when the reaction was conducted at 343.15 and 353.15 K.

Fig. 8 CVs (scan rate: 5 mV s⁻¹) of MnO₂/C@TiO₂ (60 min) in 3 NaOH mol dm⁻³ + 1.2 mol dm⁻³ H₂O₂ at different temperature (303.15–353.15 K).

The highest current density reached up to 175 mA cm⁻² at −0.7 V in 1.2 mol dm⁻³ H₂O₂ at 303.15 K (Fig. 7a), which is much higher than our previous Cu based CuO electrode.²⁶ The good electrocatalytic activity can be attributed to the following three reasons: first, the MnO₂/C@TiO₂ owns a unique 3D porous structure and provide an excellent electronic conductivity; second, the MnO₂ nanoplates uniformly coated on the C/TiO₂ surface, instead agglomeration as some other electrode prepared through slurry and coating with binders; last and the most important, according to the previous reports,^33–35 MnO₂ has fine ability to break the HO–OH bond during the reaction, which is the essential factor to drive the occurrence of H₂O₂ electroreduction. As seen from Fig. 9, the atomic oxygen may be firstly adsorbed on the MnO₂ (Mn⁴⁺) surface (Fig. 9A → B) due to its weak electronegativity in H₂O₂. Then MnO₂ absorbs some electrons, transported from external circuit, and transformed to Mn³⁺ (Fig. 9C → D). Mn ion has a high electron affinity, so the Mn³⁺ will release the electron, mentioned in (Fig. 9C → D), to the adsorbed H₂O₂ molecule (Fig. 9E). At last, the HO–OH bond was broken after obtain electrons and formed OH– (Fig. 9C → D). Synchronously, Mn³⁺ returns to Mn⁴⁺ (MnO₂).

Fig. 9 Schematic diagram depicting the mechanism for the H₂O₂ electroreduction on the MnO₂/C@TiO₂.

The stability of MnO₂/C@TiO₂ for H₂O₂ electroreduction at different applied potential was performed by CA test. The potential ranges from −0.6 to −0.3 V chosen from the Fig. 7 in 3 mol dm⁻³ NaOH and 1.2 mol dm⁻³ H₂O₂. As seen from Fig. 10, the reduction densities steady at around −140, −110, −75 and −30 mA cm⁻², respectively, when the potentials are fixed at −0.6, −0.5, −0.4 and −0.3 V. Super current density will be achieved high applied potential, which can be ascribed to the drive force and fast kinetics at noble potential, similar results can be seen from some previous reports.^21,50 Although some oxygen gas was produced from the hydrolysis of H₂O₂ on the MnO₂/C@TiO₂ surface,⁵¹ there isn't anything falling from the electrode during the whole reaction process, demonstrating that MnO₂/C@TiO₂ is stable and equal to fuel cell system.

Fig. 10 CAs of MnO₂/C@TiO₂ (60 min) in 3 NaOH mol dm⁻³ + 1.2 mol dm⁻³ H₂O₂ at different potential (−0.6 to −0.3 V).

4. Conclusions

In this paper, MnO₂ was demonstrated to be an effectively catalyst for H₂O₂ electroreduction that atomic oxygen in H₂O₂ is reduced on the MnO₂ surface accompanied with the transformation of Mn³⁺ and Mn⁴⁺, and the MnO₂/C@TiO₂ owns high electrocatalytic activity and super stability in a H₂O₂ contained alkaline solution. In consideration of the advantages, MnO₂/C@TiO₂ can be appreciable to reduce the cost and goes into service for FCs.

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), the Major Project of Science and Technology of Heilongjiang Province (GA14A101), the Project of Research and Development of Applied Technology of Harbin (2014DB4AG016) and the Fundamental Research Funds for the Central Universities (HEUCF20151004).

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