Palladium dispersed in three-dimensional polyaniline networks as the catalyst for hydrogen peroxide electro-reduction in an acidic medium

Abstract

A novel Pd/polyaniline/CFC electrode is prepared by electroless deposition of palladium (Pd) onto three-dimensional polyaniline networks. The polyaniline matrix on the carbon fiber cloth (CFC) in the reduction state is electro-synthesized by cyclic voltammetry and has a lower vertex potential of −0.4 V vs. Ag/AgCl. The particle size of the Pd coated on the polyaniline chains is gradiently distributed. The as-prepared Pd/polyaniline/CFC electrode is characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FITR) and X-ray diffraction (XRD). The hydrogen peroxide (H₂O₂) electro-reduction reaction in H₂SO₄ solutions on the Pd/polyaniline/CFC electrode is investigated using cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). The results reveal that the electrode exhibited high catalytic activity and excellent stability in the strong oxidizing solution of H₂O₂ and H₂SO₄. Polyaniline itself shows electro-catalytic activity towards H₂O₂ to some extent involving the chemical–electrochemical (C–E) coupling mechanism.

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1. Introduction

A fuel cell (FC), distinguished from other rechargeable secondary power devices such as Li ion batteries and supercapacitors,^1–3 is a kind of power source that can constantly generate electricity.^4–7 FCs convert chemical energy efficiently into electrical energy without the limitation of the Carnot cycle. Hydrogen, methane and alcohols are the three main fuel sources for FCs. Compared with the abundant resources for the anode fuel, there is only one kind of universal oxidant, that is, oxygen. Oxygen can be extracted directly from air, however, its electro-reduction activity is closely dependent on the platinum catalyst. The gas needs extra humidifying apparatus and high-strength pumping to achieve good FC performance. For the past few years, hydrogen peroxide (H₂O₂) was used as the oxidant for the metal semi-fuel cell,⁸ as well as direct borohydride,⁹ direct hydrogen peroxide¹⁰ and hydrazine¹¹ fuel cells. Using H₂O₂ instead of O₂ will make FC systems more compact, and especially help FCs work in oxygen-free environments such as underwater or in outer space.

Catalysts with a high specific surface area supply more active sites for H₂O₂ electro-reduction.^12,13 Commercial Pd/C and Pt/C catalysts are obtained by chemically reducing Pd or Pt on carbon black to maintain their nano-particle morphology. Others have favored dispersing noble metal particles on graphene, which has a thickness of a few nanometers, for catalytic applications. Polyaniline, synthesized by a simple process, generally has various nanoscale structures, such as particles, fibers, tubes and spheres.^14–17 Lamy et al.¹⁸ prepared polyaniline on a glassy carbon stationary electrode by the cyclic voltammetric technique, afterwards, they potentiostatically electrodeposited platinum onto the polyaniline film to design a Pt/polyaniline electrode for the oxygen reduction reaction. The supply of electrodeposition potential for depositing the Pt would change the redox states and impair the electrical conductivity of the polyaniline, invalidating the composite electrode. Stejskal et al.¹⁹ adopted silver nitrate as the oxidant to oxidize aniline to polyaniline, and in the meantime silver ions were reduced to form a silver coating on the polyaniline. Li and co-workers²⁰ synthesized Au/polyaniline through the mutual redox of aniline vapor and HAuCl₄ aqueous solution for the oxidation and sensing of ascorbic acid. The ways of using noble metal compounds as the oxidant to chemically make noble metal/polyaniline electrodes need extra processes, like mixing noble metal/polyaniline powder with a binder, coating on a support for testing, etc. The binder would lower the conductivity of the electrode and, for long duration tests, the electrode powders may fall off the support in the aqueous electrolyte.^21–23

Electro-synthesized polyaniline assisted by the CV technique has different redox states and conductivity depending on the selected potential range of the CV.^24,25 Fig. 1 illustrates the common three redox states of polyaniline. Emeraldine has the highest conductivity while lencomeraldine and pernigraniline are electrically isolated.²⁶ If as-prepared polyaniline on a support is in the reduction state, and can thermodynamically reduce noble metal complex ions, a binder-free noble metal/polyaniline electrode would be successfully achieved. Moreover, the oxidation of polyaniline cannot go too far and thus, the mutual redox formed between polyaniline and the noble metal compounds has less severe influence on the electrode conductivity.

Fig. 1 Schematic illustration of the three redox states of polyaniline and the mutual redox between Pd(II) and polyaniline.

In this work, Pd was electrolessly deposited on polyaniline fibrils by the reduction-state polyaniline itself. The Pd/polyaniline/CFC electrode was introduced to catalyze H₂O₂ electro-reduction because of the synergistic properties of high-surface-area polyaniline doping–dedoping in H₂SO₄ and Pd electro-reduction towards H₂O₂. The obtained electrode possesses a unique three-dimensional network structure, which enables easy access of the fuel and product into or out of the catalytic active sites. Results indicated that the Pd/polyaniline/CFC electrode for H₂O₂ electro-reduction exhibited high catalytic activity and excellent stability.

2. Experimental

2.1. Preparation of the polyaniline/CFC and Pd/polyaniline/CFC electrodes

Aniline monomers were in situ electro-polymerized on the CFC by cyclic voltammetry in 1.0 mol L⁻¹ sulfuric acid and 0.1 mol L⁻¹ aniline (obtained from Aladdin Industrial Inc.) with a volume of 40 mL. Prior to use, the CFC (purchased from Shanghai Hesen Electric Inc.) was soaked in acetone for 20 minutes, washed copiously and preserved in Milli-Q water (18.2 MΩ cm) successively. The CFC was fixed between a pair of home-made titanium frames with an area of 1.0 × 1.0 cm and exposed to the electrolyte. A platinum sheet (2.0 × 1.0 cm) and a saturated Ag/AgCl electrode (0.1981 V vs. SHE) were used as the counter electrode and reference electrode, respectively. All potentials in this work were referred to this reference electrode except where noted. The electro-deposition solution was deoxygenated by bubbling ultrahigh purity N₂ for 10 min and the N₂ atmosphere was maintained during the polymerization. The CV polymerization was performed for 16 cycles with a potential range of −0.4 to 1 V at 50 mV s⁻¹.

To obtain the electrolessly precipitated Pd/polyaniline/CFC modified electrode, the as-prepared polyaniline/CFC electrode was firstly washed by Milli-Q water for several times in order to remove the aniline monomer and oligomer, and then immediately transferred to the Pd complex ions (Pd(II)) solution under an open circuit condition within the shortest possible lapse (typically 10 s). The Pd(II) solution is composed of 1.0 mmol L⁻¹ PdCl₂ (Sinopharm Chemical Reagent Co., Ltd) and 20 mmol L⁻¹ HClO₄. The mutual redox between the Pd(II) and polyaniline lasted for 2 hours. All experiments were carried out at ambient temperature (20 °C ± 1 °C).

2.2. Characterization of the polyaniline/CFC and Pd/polyaniline/CFC electrodes

The morphologies of the polyaniline/CFC and Pd/polyaniline/CFC electrodes were examined by a scanning electron microscope (SEM, JEOL JSM-6480). The structure was analyzed using an X-ray diffractometer (XRD, Rigaku TTR III) with Cu Kα radiation (λ = 0.1514178 nm). The relevant groups of polyaniline before and after the mutual redox with Pd(II) were investigated with Fourier transform infrared spectroscopy (FTIR, Equinos55, Bruker) using the potassium bromide pellet technique. The active materials of polyaniline and Pd, together with CFC were ground into powder for the FTIR characterizations.

2.3. Electrochemical measurements

Electrochemical measurements were performed in a conventional three-electrode or two-electrode electrochemical cell using a computerized potentiostat (Autolab PGSTAT302, Eco Chemie) controlled by GPES software. The open circuit potentials (E_oc) were monitored in a two-electrode system of Pd(II) solution with a saturated Ag/AgCl electrode as the reference electrode. For comparison, the E_oc of the polyaniline electrode immersed in PdCl₂-free solution was also recorded. The equilibrium potentials of the polyaniline/CFC electrode in 20 mmol L⁻¹ HClO₄ and the CFC electrode in Pd(II) solution were measured by potentiodynamic polarization at a scan rate of 1 mV s⁻¹. Electrochemical tests of cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) for H₂O₂ electro-reduction were performed in a typical three-electrode electrochemical cell. The obtained polyaniline/CFC or Pd/polyaniline/CFC electrode, platinum foil and a saturated Ag/AgCl electrode were employed as the working electrode, counter electrode and reference electrode, respectively. The EIS tests were operated after an equilibrium time of 600 s at a fixed potential. The frequency region was 100 kHz to 10 mHz with a 5 mV potential amplitude.

3. Results and discussion

Fig. 2 shows the cyclic voltammograms for the polymerization of polyaniline on the CFC. The first cycle shows an onset oxidation potential of aniline at approximately 0.8 V. The anodic and cathodic current responses continuously increased, indicating the regular growth of polyaniline.^27,28 The peaks during the potential range referred to the various redox states of polyaniline, representing simultaneous electro-polymerization and the doping/dedoping process.^29,30 In the previous literature,^{18,24–26,28,30,31} the lower vertex potential of CV for electro-polymerization was mostly −0.16 V vs. Ag/AgCl (converted from the SCE scale in the references) or higher. For the sake of the reduction state polyaniline (labeled as PAN_re), a more negative vertex potential of −0.4 V was set in this study. During the CV polymerization from −0.4 V to the higher vertex of 1.0 V, the color of the polyaniline turned darker and darker from light green to black. The resulting polyaniline coated on the CFC appeared a light green color. Before modification with Pd, the reaction feasibility of PAN_re oxidation and Pd(II) reduction was confirmed by their own equilibrium potentials. The two reactions are listed below:

PAN_re − xe⁻ ⇄ PAN_ox − xClO₄⁻ (1)

x/2Pd(II) + xe⁻ ⇄ x/2Pd(0) (2)

Fig. 2 Cyclic voltammograms for the electro-polymerization of polyaniline on CFC in a solution containing 0.1 mol L⁻¹ aniline and 1.0 mol L⁻¹ sulfuric acid.

In eqn (1), PAN_ox refers to the oxidation state of PAN_re after losing x electrons. Fig. 3a shows the Tafel plots derived from the potentiodynamic polarization curves. The polyaniline/CFC electrode was immersed in perchloric acid solution, involving the proton doping/dedoping and simultaneous redox reaction in the potential sweep (eqn (1)). The CFC electrode was tested in perchloric acid and PdCl₂ solution, involving the Pd(II)/Pd(0) redox reaction (eqn (2)). Based on the linear fittings of the experimental data in Fig. 3a, the equilibrium potentials were 0.03 V for eqn (1) and 0.53 V for eqn (2). The oxidation reaction had a more negative equilibrium potential than the reduction reaction, proving that Pd can be electrolessly deposited on the polyaniline, synthesized in this work, thermodynamically (as explained in the right of Fig. 1). The E_oc–t curves of the polyaniline/CFC electrode in 20 mmol L⁻¹ HClO₄ with and without 1.0 mmol L⁻¹ PdCl₂ were demonstrated in Fig. 3b. The open circuit potential (E_oc) in the PdCl₂-free solution shows a slightly upward tendency from 0.17 V at 0 s to 0.18 V at 7200 s, due to the weak oxidizability of the dilute HClO₄ at room temperature. E_oc in PdCl₂-free solution was nearly unchanged, while that in PdCl₂ solution gradually rose up from 0.15 V at 0 s to 0.35 V at 2000 s. After that, the open circuit potential with time kept parallel with that of the PdCl₂-free solution. This platform illustrated that after 2000 s the mutual redox of Pd(II) and PAN_re is finished. The long duration time of 2000 s, and the slow rise between 0 s and 2000 s, for the chemical redox reaction suggested that there was probably a gradient diffusion of Pd(II) ions inside the polyaniline matrix where more active sites were available for the reduction of Pd(II) to Pd(0).³²

Fig. 3 (a) Tafel plots of CFC in 1.0 mmol L⁻¹ PdCl₂ and 20 mmol L⁻¹ HClO₄, and of the polyaniline/CFC in 20 mmol L⁻¹ HClO₄; (b) open circuit potential (E_oc) vs. time (t) plots of polyaniline/CFC electrode in 20 mmol L⁻¹ HClO₄ with and without 1.0 mmol L⁻¹ PdCl₂.

Fig. 4a–c are the SEM micrographs of the polyaniline and the Pd/polyaniline composite supported on CFC. In Fig. 4a, the polyaniline synthesized by CV showed typically loose and fibrillar structure.^25,28,33,34 The fibril diameter was ∼150 nm on average. Many monofilament fibers cross-linked with each other to form networks. Fig. 4b and c demonstrate the SEM images of the Pd/polyaniline composite in low and high magnification. The much longer fibers in Fig. 4b with a diameter of ∼9 μm were carbon fibers. Based on previous reports,^33,34 the white spots, distinguished from the darker CFC and polyaniline, were Pd centers. Zooming in on the Pd/polyaniline/CFC, as shown in Fig. 4c, the Pd dispersion in the polyaniline networks was inhomogeneous. Furthermore, the size of the Pd particles was not uniform and even appeared to be in a gradient distribution. The A, B and C spots were where polyaniline faced the Pd(II) solution from outside to inside. In the A spot, the Pd deposits nearly in situ wrapped the polyaniline fibrils. In the B spot, the agglomeration of Pd occurred with clusters of particles on the polyaniline fibrils. Pd particles at the B spot had a size of 120–200 nm in diameter. Penetrating in the polyaniline networks at the C spot, the Pd particles dotted sporadically on the polyaniline fibrils had a much smaller diameter of 50–80 nm. As Pd(II) ions diffused into the polyaniline matrix for reduction, the concentration gradient would form from the A to the C spot, which undoubtedly made the size of the Pd particles gradiently distributed.

Fig. 4 SEM images of polyaniline before (a) and after (b and c) modification with Pd; (d) FTIR spectra of the polyaniline/CFC and Pd/polyaniline/CFC.

Fig. 4d shows the FTIR spectra of polyaniline before and after modification with Pd. The band at 3418 cm⁻¹ represented the N–H stretching mode.³⁵ The bands at 1383, 1301 and 1241 cm⁻¹ were attributed to the C–N stretching vibrations in QBQ, QBB, BBQ and BBB (Q denotes quinoid ring, B denotes benzenoid ring).^36,37 In the region of 1138 cm⁻¹, the –NH⁺= stretching mode was observed. Out-of-plane deformations of C–H on the 1,4-disubstituted benzenoid rings were located in the region of 833 cm⁻¹.³⁸ The two main bands of 1619 and 1498 cm⁻¹ were respectively assigned to C=C stretching vibrations in the quinoid and benzenoid rings. It can be seen that the peak at 1619 cm⁻¹ in the Pd/polyaniline/CFC spectrum became stronger while that at 1498 cm⁻¹ was weaker compared with the peaks for the polyaniline/CFC. It was elucidated that after modification with Pd, the polyaniline was oxidized from PAN_re to PAN_ox with more quinoid and less benzenoid units.

XRD was utilised to confirm the existence of crystalline Pd. In Fig. 5a, the CFC displayed three broad peaks centered at about 23°, 43° and 80°, which can be attributed to carbon. The characteristic peaks of polyaniline probably overlapped with those of the CFC and thus, polyaniline cannot be identified in the XRD profiles. After the polyaniline/CFC was modified with Pd, there were five diffraction peaks at 40°, 47°, 68°, 82° and 87°, corresponding well to the (111), (200), (220), (311) and (222) planes of Pd, respectively, according to the standard crystallographic spectrum of Pd (JCPDS card no. 46-1043).

Fig. 5 (a) XRD patterns of CFC, polyaniline/CFC and the Pd/polyaniline/CFC electrode; (b) cyclic voltammograms of polyaniline/CFC and the Pd/polyaniline/CFC electrode in 1.0 mol L⁻¹ H₂SO₄ at 10 mV s⁻¹.

These peaks indicated that the Pd modified polyaniline had a face-centered cubic (fcc) structure and presented as the metallic state. The FTIR and XRD patterns indicated that a binder-free and self-catalytically reductive Pd/polyaniline/CFC electrode was successfully fabricated. Fig. 5b shows the cyclic voltammograms (CV) of the polyaniline/CFC and the Pd/polyaniline/CFC electrode measured in 1.0 mol L⁻¹ H₂SO₄ at a scan rate of 10 mV s⁻¹. The CV curve of the polyaniline/CFC exhibited two pairs of redox peaks between −0.4 V and 1.0 V, due to the conversions of emeraldine/lencomeraldine and lencomeraldine/pernigraniline. The CV of the Pd/polyaniline/CFC displayed the absorption and/or evolution of hydrogen region from −0.1 V to −0.2 V and the hydrogen desorption region at −0.08 V, which were the typical features of Pd.³⁹ Due to the metallic Pd oxidation and reduction reactions in H₂SO₄,⁴⁰ compared with the polyaniline/CFC electrode, the current density of the Pd/polyaniline/CFC between the potential ranges of 0.6–1.0 V and 0–0.8 V was larger. The CV of Pd/polyaniline/CFC showed more redox peak couples than that of polyaniline/CFC, implying that Pd modification could facilitate the successive conversions of more redox states of polyaniline.

Fig. 6 demonstrates the linear sweep voltammograms (LSVs) of the Pd/polyaniline/CFC electrode in 0.5 mol L⁻¹ H₂O₂ and x mol L⁻¹ H₂SO₄ (x = 0.5, 1.0, 2.0). The potential applied to the Pd/polyaniline electrode was swept from the open circuit potential (OCP) to −0.2 V at a scan rate of 10 mV s⁻¹. With the concentration of H₂SO₄ increasing from 0.5 mol L⁻¹ to 2.0 mol L⁻¹, the OCP moved positively from 0.558 V to 0.608 V. The OCP shift trend was in accordance with the previous literatures on Pd electrodes for H₂O₂ electro-reduction.^40,41 The Pd/polyaniline/CFC revealed the best performance in 1.0 mol L⁻¹ H₂SO₄ and 0.5 mol L⁻¹ H₂O₂ according to the current density. The current density at −0.2 V was 189 mA cm⁻² with an OCP of 0.580 V. As is known, the function of H₂SO₄ is not just as the supporting electrolyte, it is a reactant that reacts with H₂O₂ at the chemical ingredient ratio of 2 : 1 (H₂O₂ + 2H⁺ + 2e⁻ ⇄ 2H₂O). As a result, inadequate (0.5 mol L⁻¹, 1 : 1 stoichiometry) and excess (2.0 mol L⁻¹, 4 : 1 stoichiometry) H₂SO₄ at a fixed concentration of H₂O₂ would both suppress the H₂O₂ electro-reduction reaction at the Pd/polyaniline/CFC electrode.

Fig. 6 Linear sweep voltammograms of the Pd/polyaniline/CFC electrode in different concentrations of H₂SO₄ in the presence of 0.5 mol L⁻¹ H₂O₂ at a scan rate of 10 mV s⁻¹.

Fig. 7 shows the effects of H₂O₂ concentration on the catalytic behavior of the Pd/polyaniline/CFC electrode. The concentration of H₂SO₄ was fixed at 1.0 mol L⁻¹. As seen, the OCP increased from 0 to 0.2 mol L⁻¹ H₂O₂ and then remained unchanged even at a high concentration of 1.0 mol L⁻¹. Cao et al.⁴¹ analyzed the influence of H₂O₂ concentration on the OCPs at a variety of electrodes in 1.0 mol L⁻¹ H₂SO₄. Each electrode had its own OCP trend. The OCP at the Pd electrode stepped up from 0.02 to 0.1 mol L⁻¹ and then stayed relatively constant from 0.1 to 1.0 mol L⁻¹. The Pd/polyaniline/CFC had the same OCP tendency as the Pd electrode in different concentrations of H₂SO₄ and H₂O₂, meaning that the equilibrium state of H₂O₂ electro-reduction was mostly established on the modified Pd, and so proceeded via the sequent electro-reduction process. Without any addition of H₂O₂, the LSV of Pd/polyaniline/CFC showed the reduction current of polyaniline and hydrogen absorption in H₂SO₄. The reduction peak of H₂O₂ diminished gradually and disappeared from 0.1 to 0.5 mol L⁻¹, since more H₂O₂ content reduced the concentration polarization. When the concentration of H₂O₂ was further increased to 1.0 mol L⁻¹, the current density at −0.2 V was 195 mA cm⁻², only 6 mA cm⁻² larger than that of 0.5 mol L⁻¹. Taking into account the fuel costs, 0.5 mol L⁻¹ was considered the optimal concentration of H₂O₂.

Fig. 7 Linear sweep voltammograms of the Pd/polyaniline/CFC electrode in different concentrations of H₂O₂ with the presence of 1.0 mol L⁻¹ H₂SO₄ at a scan rate of 10 mV s⁻¹.

The stability of polyaniline/CFC and the Pd/polyaniline/CFC electrode in 1.0 mol L⁻¹ H₂SO₄ and 0.5 mol L⁻¹ H₂O₂ was tested by applying different polarization potentials. The current–time curves are depicted in Fig. 8. Starting from 0.4 V, the polarization potential was decreased to −0.2 V by three steps and held for 30 minutes at each potential. As seen, the lower the potential, the larger the reduction current density. The current densities were almost kept constant at 0.4 V (14 mA cm⁻²) and 0.1 V (75 mA cm⁻²). The current density at −0.2 V on the Pd/polyaniline/CFC electrode delivered a slight decrease from 166 mA cm⁻² at 3721 s to 157 mA cm⁻² at 5400 s during the 1800 s testing time, which likely resulted from the consumption of H₂O₂ near the electrode surface. The polyaniline/CFC electrode was also tested in 1.0 mol L⁻¹ H₂SO₄ and 0.5 mol L⁻¹ H₂O₂ at −0.2 V. It demonstrated a low current density of ∼3.0 mA cm⁻², negligible compared with the Pd/polyaniline/CFC electrode. The low current density probably resulted from the reduction of polyaniline. The inset (left) of Fig. 8 is the SEM image of the Pd/polyaniline/CFC electrode after the choronoamperometric test. The morphologies of the polyaniline fibrils and the Pd particles were well retained. In conclusion, the Pd/polyaniline/CFC electrode exhibited excellent stability for the H₂O₂ electro-reduction reaction because of the synergistic properties of high-surface-area, polyaniline doping–dedoping in H₂SO₄ and Pd electro-reduction towards H₂O₂.

Fig. 8 Chronoamperometric curves of polyaniline/CFC and the Pd/polyaniline/CFC electrode for H₂O₂ electro-reduction at different potentials in 1.0 mol L⁻¹ H₂SO₄ and 0.5 mol L⁻¹ H₂O₂. Inset (left) is the SEM image of the Pd/polyaniline/CFC electrode after the chronoamperometric test; inset (right) is the enlarged i–t curve of the polyaniline/CFC electrode at −0.2 V in 1.0 mol L⁻¹ H₂SO₄ and 0.5 mol L⁻¹ H₂O₂.

Fig. 9a illustrates the Nyquist plots of the Pd/polyaniline/CFC electrode at 0.1 V containing 1.0 mol L⁻¹ H₂SO₄ with and without 0.5 mol L⁻¹ H₂O₂, respectively. The electrochemical system was first polarized for 600 s at 0.1 V to achieve a quasi-stable state. In the absence of H₂O₂, when only polyaniline was electro-reduced, one depressed semi-circle in the higher frequency region and a spike in the lower frequency region appeared. After adding H₂O₂, there were two depressed semi-circles. One in the higher frequency region was ascribed to the electro-reduction reaction of polyaniline, since the frequency of first circle, 8.23 kHz, is very close to that without H₂O₂, 5.28 kHz.^10,42 The other circle with a frequency of 96.81 mHz was attributed to the H₂O₂ electro-reduction reaction on Pd with a diameter of 0.876 Ω (R_ct2). The inset of Fig. 9a is the equivalent circuit of both polyaniline electro-reduction and H₂O₂ electro-reduction on Pd. H₂O₂ electro-reduction (R_ct2) happened in parallel with the polyaniline reduction (R_ct1). In comparison with the semi-circle without H₂O₂, the ohmic resistance (R_u) of that with H₂O₂ was smaller, this could be because of the increased solution and/or polyaniline conductivity after adding H₂O₂. The diameter of the semi-circle was the charge transfer resistance (R_ct), which could be used to evaluate the electrochemical reaction rate. The R_ct1 of polyaniline (1.372 Ω) electro-reduction in H₂SO₄ and H₂O₂ was smaller than that (2.342 Ω) only in H₂SO₄, implying the addition of H₂O₂ facilitated the polyaniline electro-reduction reaction. To gain further understanding of the interaction between polyaniline and H₂O₂, the CV synthesized polyaniline was solely tested in 1.0 mol L⁻¹ H₂SO₄ with and without 0.5 mol L⁻¹ H₂O₂. As shown in Fig. 9b, the OCP of the polyaniline electrode changed from 0.31 V to 0.51 V. The positive shift of OCP implied that polyaniline was oxidized after adding the H₂O₂. The amplitude of reduction current density was increased after adding H₂O₂. It was demonstrated that polyaniline itself showed electro-catalytic activity towards H₂O₂ to some extent. It could be speculated that H₂O₂ electrode-reduction on polyaniline proceeded following a chemical–electrochemical process (C–E process).

Fig. 9 (a) Nyquist plots of the Pd/polyaniline/CFC electrode at 0.1 V in 1.0 mol L⁻¹ H₂SO₄ in the absence and presence of 0.5 mol L⁻¹ H₂O₂, inset is the equivalent circuit of polyaniline and H₂O₂ electro-reduction on Pd/polyaniline/CFC electrode; (b) linear sweep voltammograms of the polyaniline/CFC electrode in 1.0 mol L⁻¹ H₂SO₄ with and without 0.5 mol L⁻¹ H₂O₂ at 10 mV s⁻¹.

Chemical process:

PAN_re + H₂O₂ + 2H₂SO₄ ⇄ PAN_ox⁺·2HSO₄⁻ + 2H₂O (3)

Electrochemical process:

PAN_ox⁺·2HSO₄⁻ + 2e⁻ ⇄ PAN_re + 2HSO₄⁻ (4)

Overall reaction

H₂O₂ + 2H⁺ + 2e⁻ ⇄ 2H₂O (5)

Polyaniline was first chemically-oxidized to its oxidation state by H₂O₂ (chemical process), and then electrochemically-reduced to its reduction state by an external potential driving force (electrochemical process). As a result, in the overall reaction H₂O₂ was electro-reduced to H₂O on the polyaniline electrode. The C–E coupling mechanism was commonly seen on the Cu₂O and Pt electrodes for H₂O₂ electro-reduction or electro-oxidation^43–45 as well.

4. Conclusions

In conclusion, the binder-free Pd/polyaniline/CFC electrode was successfully prepared based on the equilibrium potential difference between Pd complex ions and the CV synthesized polyaniline. The size of the Pd particles coated on the polyaniline networks were gradiently distributed from the outside to inside. The modified Pd/polyaniline/CFC electrode demonstrated high catalytic activity and remarkable stability towards H₂O₂ electro-reduction in sulfuric acid even at relatively negative potentials. Polyaniline exhibited electro-catalytic activity for H₂O₂ electro-reduction by way of a chemical–electrochemical coupling process.

Acknowledgements

We gratefully acknowledge the financial support of this research by the National Natural Science Foundation of China (21403044), the Natural Science Foundation of Heilongjiang Province of China (LC2015004), the China Postdoctoral Science Special Foundation (2015T80329), the Heilongjiang Postdoctoral Fund (LBH-Z14211), 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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