Novel phosphorus-doped PbO₂–MnO₂ bicontinuous electrodes for oxygen evolution reaction

Abstract

We report a facile electrochemical approach for the fabrication of phosphorus-doped (P-doped) PbO₂–MnO₂ composite electrodes with a microporous bicontinuous structure. Modification of such structures was achieved by controlling the MnO₂ incorporation during an anodic co-deposition process. The results indicate the anodic co-oxidation of Pb²⁺ and Mn²⁺ yielded a P–(PbO₂–MnO₂) deposit with a flat, compact and smooth surface. Meanwhile, the anodic composite deposition of Pb²⁺ and MnO₂ particles resulted in the bicontinuous (P–PbO₂)–MnO₂ composite with a well-defined microporous morphology. Tafel and EIS were used to characterize their electrocatalytic performances for the oxygen evolution reaction in a typical anodic water-splitting process. The results indicate that such a novel bicontinuous (P–PbO₂)–MnO₂ composite anode exhibits significantly improved electrocatalytic activity as compared to the P–PbO₂ and P–(PbO₂–MnO₂) anodes. The oxygen evolution kinetics and possible reaction mechanism are further described.

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1. Introduction

Enhancing ion- and electron-transfer kinetics has been emphasized as an important approach in improving the efficiency of electrochemical reactions.^1–3 This enhancement can be achieved by designing electrode materials with high ion diffusion constants^4,5 and further coating the electrocatalytically active material on a conductive layer.^6,7 However, such approaches were considered to be difficult due to the inefficiency of uniformly decorating active materials in an electrode with a high-volume fraction for ion and electron transport.¹ It was proposed that an ideal electrode for providing efficient ion and electron transport should consist of a three-dimensional interpenetrating network of electron and ion pathways.^4,8 Accordingly, Braun^1,9 and Stein¹⁰ reported a bicontinuous bulk electrode concept consisting of an electrolytically active material sandwiched in a microporous network matrix with highly conductive ion- and electron-transport pathways. Such bicontinuous electrodes were reported of particular interest since they provide (1) an interconnected electrolyte-filled pore network that enables rapid ion transport, (2) a short solid-phase ion diffusion length that minimizes the effect of sluggish solid-state ion transport, (3) a large electrode surface area and (4) high electron conductivity in the electrode assembly.¹ However, so far no efforts have been expended to apply the concept of bicontinuous electrodes in the study and development of metal oxide electrodes for electrocatalysis. Moreover, these aforementioned bicontinuous electrodes were exclusively prepared though a template-based multi-step process involving a complex colloid,^1,11,12 and achieving the fabrication through a cost-effective single-step process remains a big challenge.

Lead dioxide has been considered as one of the interesting metal–oxide electrode materials for oxygen evolution reactions in applications such as fuel cells, hydrometallurgy, metal–air batteries, and solar-driven water splitting.^13–15 This is due to several unique properties of PbO₂ such as high electrical conductivity (10⁴ S cm⁻¹), good chemical stability and cost-effective production.^16–18 The structure of PbO₂-based electrodes can be altered, and the electrocatalytic performance of these electrodes can be improved, by chemical doping or incorporation of foreign atoms or particles.^19,20 For instance, introducing a well-dispersed phase in the PbO₂ matrix was found to be of great use in modifying its physical structure and improving its electrocatalytic activity.^21,22 In general, two co-deposition strategies can be summarized for the preparation of such PbO₂-based composite materials: “anodic co-oxidation” (ACO) and “anodic composite deposition” (ACD). In the ACO method, a complex deposition solution containing Pb²⁺ as well as other metal ions such as Co²⁺, Ru²⁺, Sn²⁺ and Mn²⁺ was used,^23–26 and the composite deposits were formed by simultaneous oxidation of these ions on the anode. Meanwhile, the ACD method was achieved by introducing well-dispersed insoluble micro/nanoparticles (such as Co₃O₄, RuO₂, PTFE, TiO₂) into the normal deposition solution.^27–30 The co-deposition was typically completed in a three-step process comprising electrophoretic migration, physical adsorption, and chemical adsorption.³¹ Note that the incorporation of external particles in the PbO₂ matrix effectively modulated the nucleation and growth kinetics of PbO₂, resulting in significant modification of the texture and morphology of the deposits.^21,23 Accordingly, the electrocatalytic activity of such a composite anode was largely improved due to the existence of active particles and the enlargement of specific surface area.^21,28

MnO₂ has been demonstrated as a useful catalytic material for the oxygen evolution reaction,^32–34 but this oxide's poor electrical conductivity (∼10⁻⁵–10⁻⁶ S cm⁻¹) and poor ductility make it very difficult for it to be used as the sole component of an electrode.³⁵ Combining the high catalytic activity of MnO₂ particles with a conducive matrix is therefore of great potential and interest. The incorporation of MnO₂ particles in a traditional lead matrix was in fact found not only to improve the electrocatalytic activity, but also to protect the anode from intensive corrosion.^33,36 In this paper, we propose a cost-effective and facile approach to achieving the chemical and structural modifications of a PbO₂ electrode by incorporating the MnO₂ catalyst into the PbO₂ matrix. These modifications were attempted by carrying out a single-step anodic co-deposition in a lead pyrophosphate electrodeposition bath containing Mn²⁺ or MnO₂ particles. An industrial lead electrode was used as the initial substrate. As a result, a phosphorus-doped (P-doped) PbO₂–MnO₂ composite electrode with a well-defined bicontinuous structure was obtained. The influence of Mn²⁺ or MnO₂ particles on the electrodeposition mechanism and the structure/composition of the composite deposits was studied in detail. The electrocatalytic performance of such a composite electrode for the oxygen evolution reaction in a typical water-splitting process was also studied.

2. Experimental details

2.1. Preparation of P-doped PbO₂–MnO₂ deposit

The preparation of a P-doped PbO₂ deposit on lead substrate (1 × 1 cm²) was achieved by anodic oxidation.³⁷ The deposition bath consisted of 35 g L⁻¹ Na₄P₂O₇, 16 g L⁻¹ Pb(NO₃)₂, 2 g L⁻¹ Cu(NO₃)₂, 1 g L⁻¹ NaF and 0.2 g L⁻¹ peptone (pH ∼11). The aforementioned ACO approach was used to prepare a P-doped (PbO₂–MnO₂) deposit (referred as P–(PbO₂–MnO₂) below) by adding Mn²⁺ (in the form of Mn(NO₃)₂) into this basic (i.e., high pH) deposition bath. The ACD approach was used to prepare a (P-doped PbO₂)–MnO₂ deposit (referred as (P–PbO₂)–MnO₂ below) by dispersing MnO₂ particles in the basic deposition bath. (Magnetic stirring was applied throughout the electrodeposition to achieve the uniform dispersion of particles.) As shown in the schematic of Fig. 1a, for both approaches the electrodeposition was carried out in a 250 mL beaker, where a 4 cm² platinum plate was used as the cathode. The deposition temperature and agitation rate were controlled at 70 ± 0.5 °C and 400 ± 20 rpm, respectively, by a DF-101S constant-temperature magnetic agitator. Unless specified, the constant anodic current and the deposition time were 10 mA cm⁻² and 60 min, respectively. γ-MnO₂ particles (CMD) with sizes of 1–2 μm and purity > 95% were purchased from Huitong Co. Ltd. (Changsha, China).

Fig. 1 (a) Schematic showing the anodic electrodeposition of P–(PbO₂–MnO₂) and (P–PbO₂)–MnO₂ composites in the presence of Mn²⁺ (b) and MnO₂ particles (c) in the pyrophosphate-based solution, respectively. Note: (1) dispersing and charging of MnO₂ particles; (2) charged particles transfer to the electrode surface; (3) particles incorporate into the P–PbO₂ matrix.

2.2. Morphology and composition characterization

The microscopic morphology of these composite deposits was obtained using a JSM-6360 scanning electron microscope (SEM). The chemical composition analysis was conducted on a GENESIS 60S energy dispersive spectrometer (EDS). The mass fraction (wt%) of P and MnO₂ in the deposits was estimated using eqn (1) and (2), respectively.where M_PbO2, M_MnO2 and M_P represent the relative molecular masses of PbO₂, MnO₂ and P, respectively, and N_Pb, N_Mn and N_P are the atomic percentages of Pb, Mn and P detected by EDS, respectively.

2.3. Electrochemical characterization for the co-deposition

The electrochemical study for the co-deposition process was carried out on an EG&G Princeton Applied Research model 2273 potentiostat/galvanostat controlled by PowerSuite software. These experiments were conducted in the aforementioned pyrophosphate deposition solution in the absence or presence of Mn²⁺/MnO₂ particles. The standard three-electrode system was used, where the lead substrate or the obtained deposits were used as the working electrode, a platinum plate of 4 cm² was used as the counter electrode and a 217 double salt bridge saturated calomel electrode (SCE) (YUECI Co. Ltd., Shanghai, China) was used as the reference electrode. All potentials shown in the figures are referred to the SCE. The cyclic voltammetry tests were carried out after electrodeposition for 40 min to evaluate the influence of additives (Mn²⁺ and MnO₂) on the deposition process. The scanning process was as follows: 0.5 V → 2.1 V → −1.0 V → 0.5 V (scanning was started at 0.5 V rather than −1.0 V to eliminate the heavy reduction of the oxide deposit in the negative region). The scanning rate was 10 mV s⁻¹.

2.4. Electrocatalytic performance for OER

Galvanostatic polarization, Tafel analysis and electrochemical impedance spectroscopy (EIS) were used to study the oxygen evolution performance of these composite electrodes in 1.63 M H₂SO₄ solution at a constant temperature of 35 ± 0.5 °C. Three kinds of deposits were used: P–PbO₂, P–(PbO₂–MnO₂) obtained by the ACO approach and (P–PbO₂)–MnO₂ obtained by the ACD approach. The MnO₂ content in the latter two composites is 0.60 wt% and 8.17 wt%, respectively. The three-electrode system that was used was the same as described above. The characterization process was as follows: (1) Preconditioning of the anodes at a constant current density of 50 mA cm⁻² for 90 min to reach the steady oxygen evolution state. (2a) Tafel polarization was carried out in the range of 1.7 to 1.9 V at the scanning rate of 0.2 mV s⁻¹; (2b) The impedance spectra were recorded from 100 kHz to 10 mHz with an AC amplitude of 5 mV rms. Note that the Tafel and EIS experiments were conducted on a newly preconditioned electrode and the tests were started immediately after the preconditioning of the as-prepared samples.

3. Results and discussion

3.1. Morphology and composition of composites

The preparation of P-doped PbO₂ electrodes in lead pyrophosphate solution has been described previously by our group.³⁶ The surface morphology of the as-prepared P–PbO₂ deposit is shown in Fig. 2. Overall, the deposit was bright, compact and tightly combined with the substrate. Well-ordered crystalline assemblies of ∼10–30 micron-diameter grains were observed, which have been regarded as the typical morphology of α-PbO₂ obtained from basic solutions.¹⁵ EDS analysis shows the phosphorus content in the deposit was ∼1.81%. Such a P–PbO₂ electrode was found to show improved oxygen evolution activity as compared to the traditional Pb/PbO₂ electrode.³⁶

Fig. 2 Surface morphology of P–PbO₂ deposit: (a) ×1000, (b) ×100.

With the presence of Mn²⁺ in this deposition solution, a P–(PbO₂–MnO₂) composite deposit was obtained through the simultaneous oxidation of Pb²⁺ and Mn²⁺ on the lead substrate (Fig. 1b). The influence of Mn²⁺ concentration on the surface morphology is shown in Fig. 3a–c. Corresponding low-resolution SEM images are shown in Fig. S1 (ESI†). In general, the deposit surface became more compact and smooth with increasing Mn²⁺ concentration. Note, however, that at high concentrations of Mn²⁺ (>0.5 g L⁻¹) the deposit, although locally very smooth (Fig. 3c), appears non-uniform over the whole substrate. As shown in Fig. S1c (ESI†), the deposit was not formed on some regions of the substrate. This is probably because the competitive deposition of MnO₂ prevented the growth of PbO₂.²³ Accordingly, one can summarize that a P–(PbO₂–MnO₂) deposit with smooth, compact and uniform morphology can be obtained only with the addition of a proper amount of Mn²⁺ (as shown in Fig. 3b and Fig. S2b, ESI†). The content of MnO₂ and of phosphorus in the composite deposit was further calculated using eqn (1) and (2), respectively, and the changes in these contents as a function of Mn²⁺ concentration are shown graphically in Fig. 3g. MnO₂ is a minor component in the composition as compared to PbO₂, and the content of MnO₂ increases with the increase of Mn²⁺ concentration in the deposition solution. Meanwhile, the presence of Mn²⁺ has very little influence on the phosphorus content, which keeps a constant value of ∼1.2 wt% for all Mn²⁺ concentrations tested.

Fig. 3 (a–c) Surface morphology of the P–(PbO₂–MnO₂) deposits obtained by adding (a) 0.01 g L⁻¹, (b) 0.03 g L⁻¹, (c) 0.05 g L⁻¹ Mn²⁺ (in the form of Mn(NO₃)₂) in the deposition solution. (d–f) Surface morphologies of (P–PbO₂)–MnO₂ deposits obtained by adding (d) 1 g L⁻¹ and (e–f) 20 g L⁻¹ MnO₂ particles in the deposition solution. (g–h) Influence of Mn²⁺ concentration (g) and MnO₂ content (h) in the deposition solution on the P and MnO₂ content in the deposits.

To incorporate the dispersed phase into the PbO₂ matrix as the electrocatalytically active sites for the oxygen evolution reaction and further modify the surface morphology of the deposit, the anodic composite deposition (ACD) approach was used to prepare the (P–PbO₂)–MnO₂ composite electrode. γ-MnO₂ particles with sizes of 1–2 μm were uniformly dispersed in the P–PbO₂ deposition solution under constant magnetic stirring. As illustrated in Fig. 1c, this composite electrodeposition process can be described by the following steps.^38,39 (1) Negative ions (such as P₂O₇⁴⁻ and F⁻) and organic surfactant (peptone) were reversibly adsorbed on the particle surface, forming a charged complex suspension under the magnetic stirring. (2) These charged particles were constantly transported to the anode surface by means of convective mass transfer and electrophoretic migration. (3) After the well-defined weak physical adsorption and strong chemical adsorption steps,³⁸ the particles were embedded into the P–PbO₂ matrix.

The concentration of MnO₂ in the solution was varied to control its content in the deposit. Fig. 3d–f and S2 (ESI†) show the surface morphology of the (P–PbO₂)–MnO₂ composite deposits with MnO₂ contents of 1.78 wt% and 8.17 wt%. The adsorbed external particles on the electrode surface were reported to act as the heterogeneous sites for further nucleation and growth of PbO₂, and as a result cause the surface roughness to be significantly increased.²¹ As shown in Fig. 3d, with an addition of 1 g L⁻¹ of MnO₂ particles, the deposit becomes coarse and non-uniform as compared with the P–PbO₂ deposit in Fig. 2. Also, newly formed small crystalline grains were observed on the deposit surface. With the addition of 20 g L⁻¹ of MnO₂ particles, a (P–PbO₂)–MnO₂ deposit with a MnO₂ content of 8.17 wt% was obtained, as shown in Fig. 3e–f. In this case, a well-defined bicontinuous microporous structure^1,2 with a pore size between 1 and 3 μm was observed. MnO₂ particles (arrows in Fig. S3, ESI†) were observed uniformly dispersed on the electrode surface. Such a MnO₂-decorated interpenetrating network will largely enhance the ion and electron transport^1,9 and provide numerous electrocatalytic sites. Thus, we expect this electrode to present significantly improved activity for the oxygen evolution reaction.⁴⁰

The variation of MnO₂ and P content in the composite deposits as a function of the concentration of the MnO₂ particles in suspension is further shown in Fig. 3h. As the particle concentration was increased from 1 to 20 g L⁻¹, MnO₂ content in the deposit was significantly increased as well. However, further increase in the MnO₂ concentration showed a minimal effect on the MnO₂ content in the deposit, indicating that the adsorption of MnO₂ on the substrate had reached its maximum and plateaued. The co-deposition was limited by the formation of PbO₂.³⁸ In addition, varying the MnO₂ concentration in the suspension had little effect on the content of phosphorus in the deposit.

XRD was further used to demonstrate the phase composite of the PbO₂, P–(PbO₂–MnO₂), and (P–PbO₂)–MnO₂ deposits. As shown in Fig. S4 (ESI†), the PbO₂ phase for all the three samples presents well-defined broad peaks due to the formation of amorphous structure during the electrodeposition.^36,41 Two major crystal planes were observed at ∼29° and ∼49°, which have been identified as the characteristic planes of scrutinyite (α-PbO₂).⁴² No obvious MnO₂ peak was observed on the P–(PbO₂–MnO₂) deposit due to the low MnO₂ content (0.60 wt%), while several peaks for γ-MnO₂ were observed on the (P–PbO₂)–MnO₂ deposit, which further confirms the formation of such bicontinuous composite structure.

3.2. Anodic electrodeposition characterization

The electrodeposition was carried out under constant current throughout the study. The influence of Mn²⁺ or MnO₂ particles on the anodic potential is shown in Fig. 4a. Two potential steps at ∼−0.5 V and ∼−0.3 V were in every case observed at the very beginning of the electrodeposition and was attributed to the formation of preliminary intermediates for the growth of PbO₂.⁴³ Such intermediates prevented the dissolution of the lead substrate in the deposition solution and were considered to be important for the formation of the initial PbO₂. Thus the actual electrodeposition of PbO₂ was thought to begin after ∼5 min, where the potential curve became stable and level.

Fig. 4 (a) Variation of anodic potential as a function of time during the constant-current electrodeposition. (b) Cyclic voltammograms obtained for the deposits after electrodeposition for 40 min in the deposition solution in the absence and presence of Mn²⁺ or MnO₂. In this figure, anodic branch (1) indicates oxygen evolution and PbO₂ formation, cathodic peak (2) indicates PbO₂ reduction, and (3) and (4) indicate the reduction of MnO₂ and γ-MnO₂ particles, respectively. Note: These experiments were conducted in the basic electrodeposition bath for P–PbO₂ and in the absence/presence of Mn²⁺/MnO₂ particles.

The stable potential was increased by ∼80 mV when Mn²⁺ was present in the deposition solution. This increase is probably due to the adsorption of intermediates of the Mn²⁺ oxidation reaction, which increases the activation energy for the oxidation of Pb²⁺, and thus impedes the growth of PbO₂.²³ Since this impeding process was well defined to take a function of effective grain refinement,⁴⁴ one can understand that the deposit becomes flatter and smoother after the addition of Mn²⁺ (Fig. 3a–c). In addition, the co-oxidation of Pb²⁺ and Mn²⁺ resulted in uniform dispersion of MnO₂ in the PbO₂ matrix. Thus, the increase of electrodeposition potential can also be attributed to the increase of electrical resistance of the P–(PbO₂–MnO₂) deposit.

For the ACD of (P–PbO₂)–MnO₂, MnO₂ particles were gently added into the P–PbO₂ deposition solution after electrodeposition for 10 min (marked as the τ period in Fig. 4a). We observed that the addition of MnO₂ particles had very little influence on the anodic potential during the first 10 to 20 minutes of the co-deposition. Afterwards, however, the anodic potential slowly increased, and was ∼40 mV larger at ∼30 min following addition of the particles as compared with the P–PbO₂ system. This potential difference can be attributed to the increase of electrode resistance due to the incorporation of MnO₂ particles and further confirmed the effective incorporation of MnO₂ in the PbO₂ matrix.

A cyclic voltammetry study was carried out for P–PbO₂, P–(PbO₂–MnO₂), and (P–PbO₂)–MnO₂ deposition systems after electrodeposition for 40 min (Fig. 4b). One anodic branch (1) was observed for all three electrodes in the positive scan. This branch was well-defined^27,45 and attributed to a combination of the PbO₂ formation reaction and the oxygen evolution reaction. The branch for the P–(PbO₂–MnO₂) electrode was first shifted positively as compared with the P–PbO₂ electrode, which is consistent with the observed increase of electrodeposition potential in Fig. 4a. In the high-potential region (>1.5 V), the oxygen evolution reaction was more pronounced than the PbO₂ formation reaction.⁴⁴ Since electrocatalytic activity of the P–(PbO₂–MnO₂) electrode was increased due to the incorporation of MnO₂, the anodic branch (1) became more negative than that of P–PbO₂ electrode. For the (P–PbO₂)–MnO₂ bicontinuous electrode, the incorporation of MnO₂ particles and the enlargement of specific surface area resulted in significantly improved electrocatalytic activity. As a consequence, the overall branch (1) was obviously shifted to the negative region as compared with the other two electrodes.

In the negative scan, one cathodic peak (2) corresponding to the reduction of PbO₂ was observed on all three electrodes.²⁷ This peak on the (P–PbO₂)–MnO₂ electrode was enlarged, indicating more PbO₂ was reduced due to the increase of specific surface area. The appearance of cathodic peaks (3) and (4) on the P–(PbO₂–MnO₂) and (P–PbO₂)–MnO₂ electrodes in the negative scan can be attributed to the reduction of MnO₂ and γ-MnO₂ particles, respectively, which was considered as a further confirmation of the existence of MnO₂ in the composite deposits.^46,47

3.3. Electrocatalytic performances for OER

The evaluation of electrocatalytic properties of the P-doped PbO₂–MnO₂ composite electrodes for the oxygen evolution reaction was conducted using Tafel and EIS techniques. Before the characterization, the electrodes were preconditioned at constant current for 90 min to reach the steady state of the oxygen evolution reaction. The preconditioning electrolysis conditions were the same as those used for industrial zinc electrowinning.⁴⁸ The variation of anodic potential as a function of electrolysis time is shown in Fig. 5a. The anodic potential of P–PbO₂ and of the P–(PbO₂–MnO₂) anode (MnO₂ content 0.60 wt%, corresponding to Fig. 3b) gradually decreased, and the stable potential of the latter was ∼40 mV lower than that of the former, indicating the electrocatalytic activity was improved by the incorporation of MnO₂. Electrolysis on the (P–PbO₂)–MnO₂ anode (MnO₂ content 8.17 wt%, corresponding to Fig. 3e) reached the stable state in a very short period and the anodic potential, which was ∼150 mV lower than that of the P–PbO₂ anode, remained constant during the preconditioning. This improvement in electrocatalytic activity can be attributed to the incorporation of MnO₂ particles³² as well as the enlargement of the specific surface area.³⁹

Fig. 5 (a) Variation of anodic potential as a function of time obtained on the composite electrodes during preconditioning for 90 min. (b) Experimental Tafel plots obtained on the composite anodes after preconditioning for 90 min. Note: These experiments were conducted in a 1.63 M H₂SO₄ solution.

Tafel polarization was carried out immediately after the preconditioning in the same electrolysis bath. The corresponding anodic potential was corrected by E_c = E_appl − iR_s,^49–51 where E_c and E_appl represent the corrected potential and the applied potential, i is the faradaic current and R_s is the electrolyte resistance, which was obtained from the EIS test below. The corrected Tafel plots for the three anodes are shown in Fig. 5b. In general, all three plots show two distinct linear segments, one in the low-potential region and the other in the high-potential region, which has been reported as the double-slope behavior for the oxygen evolution reaction on such metal–oxide electrodes.^52,53 The increase of the Tafel slope in the high-potential region was considered to result from partial evolution of O₃.^49,54 The corresponding values for the slopes and the intercepts were fitted via OriginLab and the results are shown in Table 1. The overpotential, η_a, for the oxygen evolution reaction at a specific current density of 50 mA cm⁻¹ was calculated using the Tafel equation η_a = (a₀ − φ) + b log i, in which a₀ represents the Tafel intercept obtained from OriginLab and φ represents the equilibrium electrode potential corresponding to the following oxygen evolution half-reaction of a typical water-splitting process,³² namely:

2H₂O − 4e⁻ → 4H⁺ + O₂ (3)

Table 1 Kinetic parameters for oxygen evolution reaction obtained from the Tafel plots shown in Fig. 5b^a

Anodes	b1 (mV dec^−1)	b2 (mV dec^−1)	ηa (mV)	J0 (A cm^−2)
a Note: b2 was used to calculate ηa for P–PbO2 and P–(PbO2–MnO2) anodes and b1 was used for (P–PbO2)–MnO2 anode according to the anodic potential corresponding to the current density in industrial electrolysis.
P–PbO2	143	250	615	1.74 × 10^−4
P–(PbO2–MnO2)	170	260	602	2.42 × 10^−4
(P–PbO2)–MnO2	203	377	476	2.26 × 10^−4

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Comparing the above Tafel equation with the Butler–Volmer equation in the high anodic polarization region (η_a ≥ 0.116 V),⁵⁵ the exchange current density J₀ can be described as log J₀ = −(a₀ − φ)/b. As shown in Table 1, the calculated J₀ for all the three anodes was very small, indicating that the oxygen evolution reaction on such metal oxide electrodes is a highly irreversible process.⁵⁶ In addition, these small J₀ values were considered to be meaningless in evaluating the electrocatalytic activity of anode materials.⁵² As a result, the overpotential η_a was frequently used as the only important criterion. In general, the calculated overpotential shown in Table 1 is consistent with the trend of stable anodic potential shown in Fig. 5a. The overpotential of the P–(PbO₂–MnO₂) anode was slightly lower than that of the P–PbO₂ anode, while that of the (P–PbO₂)–MnO₂ anode was decreased by ∼139 mV.

The oxygen evolution reaction achieved by water splitting (eqn (3)) on the PbO₂-based electrode can be divided into several substeps,^51,52 as shown in Table 2. The corresponding Tafel slope values are widely used to determine the rate determination step (rds).^49,57 Since the Tafel slopes shown in Table 1 are larger than 120 mV for all these three anodes, it is reasonable to predict that the oxygen evolution reaction on such P-doped PbO₂–MnO₂ anodes was controlled by step 1 – the formation and adsorption of the first intermediate (S–OH_ads), and thus steps 2 and 3 can be considered as the fast steps.⁴⁸ Therefore, apart from the applied potential, only the first intermediate, S–OH_ads, makes a significant contribution to the faradaic impedance of the oxygen evolution reaction.⁵⁸ Based on the expression reported by Hu⁵⁸ and Cao,⁵⁹ the faradaic impedance, Z_F, for the oxygen evolution reaction on such irreversible electrodes can be written as

Z_F = R_t + R_a/(1 + jωR_aC_a) (4)

where R_t is the charge-transfer resistance and always has a positive value, and R_a and C_a are equivalent resistance and pseudo-capacitance associated with the adsorption of intermediate, respectively. According to eqn (4), the equivalent electrical circuit (EEC), shown in Fig. 6a, can be used to simulate the impedance spectra for the oxygen evolution reaction on such metal–oxide electrodes.^48,58 R_s in the EEC stands for the uncompensated electrolyte resistance (R_s was used above for correcting the Tafel plot) and C_dl represents the double-layer capacitance.

Table 2 Sub-steps and the corresponding Tafel slope for oxygen evolution reaction when the sub-reaction is the rate determine step^a

Steps	Sub-reactions	Tafel slope/mV dec^−1
a Note: S stands for the reaction active sites on the electrode surface.
1	S + H2O → S–OHads + H^+ + e^−	≥120
2a	S–OHads → S–Oads + H^+ + e^−	40
2b	2S–OHads → S–Oads + S + H2O	40
3	S–Oads → S + 1/2O2	15

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Fig. 6 (a) Electrical equivalent circuit (EEC) used to simulate the impedance spectra of oxygen evolution reactions on the metal oxide anodes. (b) Nyquist spectra obtained on stabilized composite anodes at various anodic potentials. (c) Calculated Tafel plots based on the R_a obtained from the Nyquist spectra in (b). Note: These experiments were conducted in a 1.63 M H₂SO₄ solution.

EIS spectra of P–PbO₂, P–(PbO₂–MnO₂), and (P–PbO₂)–MnO₂ anodes were collected at various potentials immediately after the preconditioning. As shown in Fig. 6b (scatters), the spectra were mainly composed of one small capacitive arc (emphasized in Fig. S5, ESI†) in the high-frequency region and one large capacitive arc in the low-frequency region. The small capacitive arc distributed in the high-frequency region was observed to be independent of potential and considered as a description of the resistive/capacitive behavior associated with the charge-transfer processes.^60,61 The large capacitive arc in the low-frequency region decreased exponentially with the increase of applied potential, and was related to the adsorption of intermediates.^49,62

The experimental EIS data were further simulated using the EEC shown in Fig. 6a. Constant-phase elements (CPE) were used to replace the capacitors (C) in the simulation. The capacity of the introduction of CPE to provide a good match to the surface roughness, physical non-uniformity or the non-uniform distribution of the surface reaction site has been well described.^31,49 The impedance of CPE can be written as Z_CPE = 1/Q(jω)ⁿ, where j = (−1)^1/2 and n represents the deviation from the ideal behavior, n being 1 for perfect capacitors. The simulated spectra at various potentials for all three electrodes are also shown in Fig. 6b and S5† (solid lines). The simulated data (solid lines) and the experimental data (scatters) show a very good agreement. The EEC parameters at 1.80 V for the three anodes are further shown in Table 3. As mentioned before, R_s stands for the uncompensated ohmic resistance, composed of the electrolyte resistance and the electrode resistance. For all three anodes, R_s shows an equivalent value of about 0.6 to 0.9 Ω cm². The charge-transfer resistance (R_t) generated from the small capacitive arc (Fig. S5, ESI†) in the high-frequency region was relatively small and thus the total resistance was mainly dominated by the adsorption resistance (R_a). This observation is consistent with that predicted from Tafel analysis. The adsorption resistance of P–(PbO₂–MnO₂) was slightly decreased as compared with that of P–PbO₂. R_a for the (P–PbO₂)–MnO₂ anode showed the smallest value (0.25 Ω cm²) as compared with the former two, indicating the formation and adsorption of S–OH_ads intermediate on the surface of this electrode is much faster.³¹ This can be attributed to the catalytic effect of MnO₂ particles as well as the enhanced ion and electron transport resulting from the bicontinuous structure.

Table 3 EEC parameters for the oxygen evolution reaction simulated from the impedance spectra at 1.80 V in Fig. 6b

Anodes	Rs (Ω cm^2)	Qdl (S s^n cm^−2)	n	Rt (Ω cm^2)	Qa (S s^n cm^−2)	n	Ra (Ω cm^2)
P–PbO2	0.748	0.106	0.84	0.080	0.168	0.94	2.433
P–(PbO2–MnO2)	0.860	0.200	0.79	0.098	0.123	0.99	2.303
(P–PbO2)–MnO2	0.678	0.530	0.97	0.098	0.225	0.77	0.205

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To further confirm the consistency of Tafel and EIS analyses, the Tafel slope, b, was theoretically generated using an experimental profile E versus (R_a⁻¹) according to the definition b = (∂E/∂log R_a⁻¹)_T.^18,41 As shown in Fig. 6c, the calculated Tafel slope was 257 mV dec⁻¹, 260 mV dec⁻¹, and 196 mV dec⁻¹ for the P–PbO₂, P–(PbO₂–MnO₂), and (P–PbO₂)–MnO₂ anodes, respectively. These values are well matched with the experimental Tafel slopes obtained at the corresponding oxygen evolution region in Fig. 5b. In addition, Q_a can be used to describe the variation of adsorption pseudo capacitance (C_a), which was considered as a direct reflection of the variation of intermediate coverage on the anode surface.^63,64 The calculated Q_a for (P–PbO₂)–MnO₂ anode was much larger than that of the other two anodes, as shown in Table 3. This is understandable since the microporous bicontinuous structure of such a (P–PbO₂)–MnO₂ anode provides a significantly increased contacting surface area and thus is able to adsorb greater amounts of intermediates (S–OH_ads).⁶³

The electrocatalytic principle that relates such a bicontinuous (P–PbO₂)–MnO₂ anode to the oxygen evolution reaction is proposed in Fig. 7. In general, the PbO₂ matrix acts as the conductive media for the transfer of electrons,³² while the porous structure provides sufficient channels for the diffusion of electrolyte.² As a result, the ion and electron transport through the electrode is significantly facilitated, and MnO₂ particles exposed on the electrode surface, as well as those incorporated into the matrix, are able to act as the catalytic sites for oxygen evolution. It is reasonable to predict that the intersections of the PbO₂ matrix, MnO₂ particles, and the electrolyte are the most active sites for the oxygen evolution reaction (Fig. 7b). The first intermediate, S–OH_ads, is much easier to form and adsorb on these MnO₂ catalysts due to the low adsorption resistance. At the same time, the microporous structure significantly enlarges the specific surface area and allows the adsorption of a large amount of intermediate (indicated by the increase of adsorption pseudo capacitance). Thus, the speed of the rate determining step is effectively increased. This can be considered as the electrochemical reason for the highly improved electrocatalytic activity of the (P–PbO₂)–MnO₂ anode.

Fig. 7 Schematic showing the transfer of electrolyte (black arrows) and electrons (red arrows) in the porous (P–PbO₂)–MnO₂ anode during the oxygen evolution process. The blue boxes marked in (b) indicate possible active sites for the oxygen evolution reaction, and are enlarged as (c).

Finally, it is worth mentioning that such a bicontinuous microporous electrode may allow the direct contact of electrolyte with the lead substrate and cause its unexpected corrosion. Thus, in further practical applications, the pre-deposition of the flat, compact P–(PbO₂–MnO₂) protective underlayer for the bicontinuous (P–PbO₂)–MnO₂ electrode will be of great use and importance.

4. Conclusions

We have demonstrated a facile and cost-effective approach for the fabrication of a (P–PbO₂)–MnO₂ bicontinuous electrode on a lead substrate. This was achieved through the anodic composite electrodeposition of PbO₂ in a pyrophosphate solution containing MnO₂ particles. The anodic co-oxidation of the P–(PbO₂–MnO₂) deposit, which was achieved by adding Mn²⁺ in the pyrophosphate solution, was also reported for comparison. The ACO and ACD strategies used to incorporate MnO₂ in the PbO₂ matrix allowed for structural modifications and enhancement of the catalytic content of the composite electrodes, and thus significantly increased their electrocatalytic activity for the water-splitting oxygen evolution reaction. The P–(PbO₂–MnO₂) anode with a MnO₂ content of 0.60 wt% shows slightly improved activity as compared with the P–PbO₂ anode. With respect to the (P–PbO₂)–MnO₂ bicontinuous electrode with a MnO₂ content of 8.17 wt%, the calculated oxygen evolution overpotential is ∼139 mV lower than that of the P–PbO₂ electrode. The improvement in the electrocatalytic activity can be attributed to the incorporation of MnO₂ catalysts (which significantly lowers the adsorption resistance of S–OH_ads) as well as the well-defined bicontinuous structure (which facilitates the ion and electron transport and contacts).

Acknowledgements

This research was supported by the National Science & Technology pillar program of China under grant no. 2012BAA03B04, the Chinese National Natural Science Foundation under grant no. 51204208 and China Postdoctoral Science Foundation under grant no. 2013M540638. The authors thank Dr Wenhua Hu and Dr Kuldeep Kumar for the proofreading of the manuscript.

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Footnote

† Electronic supplementary information (ESI) available: Low resolution SEM images for the P-doped PbO₂–MnO₂ deposits with different MnO₂ content. SEM back-scattered electron image of (P–PbO₂)–MnO₂ deposit. XRD data, EIS spectra. See DOI: 10.1039/c4ra01831a

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