Novel phosphorus-doped lead oxide electrode for oxygen evolution reaction

Abstract

A facile and cost-effective electrochemical approach was proposed for the first time to prepare a phosphorus-doped lead dioxide (P-PbO₂) electrode on a lead substrate. This was achieved by introducing a pyrophosphate solution containing Cu²⁺, F⁻ and peptone as essential additives. Such a novel P-PbO₂ electrode exhibits a significantly improved electrocatalytic activity for the oxygen evolution reaction as compared to a traditional Pb/PbO₂ electrode.

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Improving the electrocatalytic activity of an oxygen evolving electrode has been considered as one of the most facile and efficient approaches in dealing with the increasing energy crisis and environmental problems in the electrolysis industries such as hydrometallurgy and water splitting.¹ Traditional lead-based alloys have been widely used as initial anode materials where during long-stage electrolysis a PbO₂ layer forming on the electrode surface acts as the actual reaction media.² Notably this oxide layer normally exhibits a porous structure and poorly adheres to the substrate, providing insufficient protection to the substrate from further corrosion, which heavily reduces the effective service life. Moreover, due to the lack of electrocatalytic activity, electrolysis with such a Pb/PbO₂ anode is considered as the most energy-consuming part of the whole production process.³ Therefore, the preparation and application of a novel electrocatalytically active and cost-effective anode material will be of great interest and promise.

Electrodeposited PbO₂ has been widely reported as a promising substitute to conventional electrocatalytic electrode materials such as Au, Pt, Pb, and glass carbon due to its high electrochemical activity, low electrical resistivity, good chemical stability and cost-effective production.⁴ One can summarize the past-reported studies on the electrochemical preparation of PbO₂ materials as mainly focused on the selection of plating solutions,⁵ substrate materials,⁶ doping ions,⁷ and other deposition parameters⁸ due to their dominant influence on surface morphology and phase composition, as well as electrochemical activity. In order to dope foreign atoms into PbO₂, the general approach is to add an additional dopant-containing salt in the deposition solutions. Doping is achieved in a co-deposition process. As compared with pristine PbO₂, the introduction of foreign dopants, such as F⁻, Bi³⁺, Co²⁺, and Fe³⁺ enhanced the reaction activity and chemical stability of the product.⁹ Phosphorous, another widely used dopant, was reported to be of significant use in enhancing the chemical and physical properties of matrix materials.¹⁰ For instance, phosphorus doped nickel, cobalt and zinc alloys have been widely studied due to their satisfactory corrosion resistance and excellent electrocatalytic activity for hydrogen evolution and organic compound oxidation. It was also reported that the presence of phosphorous induces the formation of hypophosphite, phosphates, phosphides or a stable amorphous phase, which act as a barrier layer for reducing the dissolution of the matrix metals.¹¹ However, doping of phosphorus into a PbO₂ matrix has not yet been reported. Moreover, the deposition of PbO₂ is normally conducted on substrates such as Pt, Au, glass carbon, etc.,⁶ achieving electrodeposition on an industrial lead substrate is still a big challenge due to the inevitable lead dissolution in the conventional deposition solutions.

In this letter, we propose a novel and cost-effective approach to prepare an electrocatalytically active P-PbO₂ electrode in a facile lead pyrophosphate solution. Industrial lead (Yuguang Gold & Lead Co. LTD, Henan, China) was used as the initial substrate. The obtained Pb/P-PbO₂ electrode was characterized with SEM, EDS, and XRD. The co-deposition mechanism was experimentally demonstrated. The electrocatalytic activity for the oxygen evolution reaction (OER) in a sulfuric acid solution was characterized using galvanostatic polarization and impedance spectroscopy. Experimental details can be found in part I of the ESI.†

The basic solution for the electrodeposition of P-PbO₂ on a lead substrate was prepared by adding lead nitrate to a sodium pyrophosphate solution, then a complex, Na₂[Pb(P₂O₇)], was formed.¹² The interaction of the lead substrate with the deposition solution at the first stage of electrodeposition is discussed in detail in part II of the ESI.† This initial stage was considered to be of great importance. The following growth principle can be proposed for the formation of preliminary P-PbO₂.

2Pb + P₂O₇⁴⁻ − 4e⁻ → Pb₂P₂O₇ (1)

Pb₂P₂O₇ + P₂O₇⁴⁻ − 4e⁻ → 2PbP₂O₇ (2)

Pb²⁺ + P₂O₇⁴⁻ − 2e⁻ → PbP₂O₇ (3)

Pb₂P₂O₇ + 8OH⁻ − 4e⁻ → 2PbO₂ + P₂O₇⁴⁻ + 4H₂O (4)

As mentioned before, the electrodeposition of PbO₂ on an industrial lead substrate is difficult in normal nitrate, acetate and fluoborate solutions due to the inevitable dissolution of the substrate in the deposition solution. However, in this study the dissolution was alleviated due to the formation of an intermediate Pb₂P₂O₇ layer, and then the happening of reaction (4) makes the formation of PbO₂ become possible. In the following continuous growth, Pb ions were oxidized on the anode surface and able to directly deposit onto the preliminary PbO₂.⁵ Furthermore, it is also reasonable to predict that phosphorous was doped in the PbO₂ deposit in the form of PbP₂O₇ due to the happening of reaction (2) and (3).

To optimize the electrodeposition process and harvest a high quality P-PbO₂ deposit, a detailed parametric study was carried out, as discussed in part III in the ESI.† We are able to summarize that the optimized conditions are as follows: additives 2 g L⁻¹ Cu(NO₃)₂ + 1 g L⁻¹ NaF + 0.4 g L⁻¹ peptone, temperature 70 °C, current density 10 mA cm⁻², and agitation 500 rpm. The corresponding anodic potential curve during the constant-current deposition for 40 min is shown in Fig. 1a. The same procedure was carried out on a Pt substrate (1 × 1 cm²) for comparison. At the initial stage, the potential of the Pb substrate was in the negative region. The first two potential steps at ∼−0.55 V and ∼−0.20 V were considered to be due to the formation of intermediate products corresponding to peak (a) and (b) in Fig. S1,† respectively. Thus, the real deposition of P-PbO₂ occurred after 5 min when the potential curve becomes level and stable. With respect to the Pt substrate, the initial intermediate stage was not observed, indicating that PbO₂ can be directly deposited onto its fresh surface.¹⁵ Cyclic voltammetry was carried out immediately after electrodeposition for 40 min in the same bath. At this time, we believe a stable P-PbO₂ deposit was formed on the substrate. As shown in Fig. 1b, the anodic peaks (a) and (b) disappear since the formation of the PbO₂ layer prevents direct contact of initial substrate and solution. The anodic branches (d) represent the PbO₂ formation and oxygen evolution, shifted to a negative potential as compared with the initial stage. In the low potential region, oxygen evolution decreased, which can enable the deposition of PbO₂. Meanwhile, the appearance of cathodic peak (e) in the negative scanning is due to the reduction of the PbO₂ layer, which was considered as proof of the presence of PbO₂.¹³

Fig. 1 (a) Variation of anodic potential during constant-current electrodeposition. (b) CV of the P-PbO₂ deposits formed after deposition for 40 min.

Fig. 2a and b show the surface morphology of the P-PbO₂ deposit obtained at the above-mentioned optimized conditions, the deposition time was 90 min. The deposit is bright, flat, ordered, and tightly bound to the lead substrate. The elemental analysis is shown in Fig. 2c. Besides the major peaks of elemental lead and oxygen, a sharp peak of phosphorus can also be observed. The calculated mass fraction of phosphorus (equation (1), ESI†) is 1.54%. The phosphorus content in PbO₂ is influenced by the different deposition conditions. It is worth mentioning that only after the optimization of additives did the deposition solution become stable and the deposit came out uniform and smooth. Then the electrodeposition of P-PbO₂ is practically feasible. The influence of current density and agitation on phosphorus content is shown in Table 1. The phosphorus content increased with an increase in current density. This is probably because the adsorption and incorporation of phosphorous become easier with increased electrode potential.¹⁰ The influence of agitation on phosphorus content is relatively small. The slight increase is due to the enhancement of ion migration. XRD was further used for the phase composition and crystal structure analysis. As shown in Fig. 2d, the P-PbO₂ deposit obtained in this study shows the co-existence of α-PbO₂ and β-PbO₂. However, different from the typical sharp and high-intensity peaks reported in the literature,⁵ the XRD pattern for P-PbO₂ shows broad peaks, which is probably due to the presence of amorphous PbO₂ resulting from the electrodeposition.^14,15

Fig. 2 (a and b) Surface morphology of the P-PbO₂ deposit, (c) EDS spectrum and (d) XRD pattern of the P-PbO₂ deposit on lead substrate.

Table 1 Variation of phosphorus content in the P-PbO₂ deposits according to varied current densities and agitation^a

Current density/mA cm^−2	10	15	20
P content/%	1.04	1.46	1.65
Agitation rate/rpm	300	400	500
P content/%	0.98	1.04	1.18
a Note: the agitation rate was 400 rpm for different current densities and the current density was 10 mA cm^−2 for different agitation rates.

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The electrochemical performances of the Pb/P-PbO₂ electrode in the oxygen evolution reaction were characterized through galvanostatic polarization preconditioning and EIS. The Pb/PbO₂ electrode (control sample) was made by oxidizing an industrial lead anode in the H₂SO₄ solution for 72 h (see Part IV of the ESI for details). The surface morphology and chemical composition of this control sample are shown in Fig. S7.† The PbO₂ layer exhibited a porous and loose structure, and was poorly adhered to the substrate. Some PbO₂ powder was observed dropping into the bath during the electrolysis. Fig. 3a shows the potential–time curve of the Pb/P-PbO₂ and Pb/PbO₂ anodes during the preconditioning. The Pb/PbO₂ anode exhibited a level potential curve as it has reached a stable state after 72 h of electrolysis during the preparation process. The potential of the Pb/P-PbO₂ anode largely decreased at the beginning of polarization and then reached a steady state. Significantly improved electrocatalytic activity was observed on the Pb/P-PbO₂ anode since its stable potential was ∼100 mV lower than that of the Pb/PbO₂ anode. EIS measurements were immediately carried out after the preconditioning and the corresponding spectra at various potentials are shown in Fig. 3b. Similar to that reported before,¹⁶ these spectra are mainly composed of one large capacitive arc of which the resolution depends on the potential applied in the whole frequency region. Higher impedance values were observed at less positive potentials. The polarization resistance for OER, R_p, calculated from the diameter of the capacitive arc, decreased exponentially with applied potential according to a Buttler–Volmer trend.¹⁷ It is well-defined^18,19 that the Tafel behavior of OER on a metal oxide electrode can be described with an experimental profile E vs. (R_P⁻¹), and thus it is possible to estimate the Tafel slope, b, according to the definition,

b = (∂E/∂ log R_P)_T. (5)

Fig. 3 (a) Variation of anodic potential on Pb/P-PbO₂ and Pb/PbO₂ electrodes during preconditioning for 90 min. (b) Nyquist spectra obtained on stabilized Pb/P-PbO₂ and Pb/PbO₂ anodes at various applied potentials. (c) Tafel plots calculated from the impedance data in (b). (d) Experimental Tafel plots. (E) Schematic showing the crystal structure of P-PbO₂ (plattnerite).

As shown in Fig. 3c, the calculated Tafel slope was 142 mV dec⁻¹ for the Pb/PbO₂ anode and 256 mV dec⁻¹ for Pb/P-PbO₂, which shows a good match with the experimental values shown in Fig. 3d. The increase in Tafel slope has been observed before when a high anodic potential was applied and is attributed to the partial evolution of O₃.^20,21 Since ozone evolution normally takes place at a high potential, one can predict that achieving oxygen evolution at a low current density is possible on Pb/P-PbO₂. Further, at a given anodic potential, the polarization resistance of Pb/P-PbO₂ is much smaller than that of Pb/PbO₂. This difference is more obvious with a decrease in potential. The improvement in electrocatalytic activity can be attributed to crystal defects and lattice distortions^22,23 resulting from the introduction of external phosphorus atoms. According to the aforementioned deposition mechanism, we believe that phosphorus exists in the PbO₂ deposit in the form of PbO₂·P₂O₅ and thus the following defect equation can be derived²⁴ (as schematically shown in Fig. 3d),

Accordingly, we propose that the introduction of phosphorus substitutes () and oxygen interstitials () created extra electrons and holes, as well as lattice distortion, which were able to serve as the active sites for the absorption of oxygen-containing species and transfer of electrons^7,23 during the OER process, rendering a significantly improved reaction activity for the Pb/P-PbO₂ anode.

In conclusion, for the first time we achieved the doping of PbO₂ with phosphorus atoms and a P-PbO₂ electrode was prepared on a lead substrate. A facile and cost-effective electrodeposition approach was proposed based on the introduction of a pyrophosphate-containing deposition solution. The obtained P-PbO₂ deposit is compact, smooth, uniform and tightly combined to the substrate. Phase analysis shows the P-PbO₂ deposit was mainly composed of α-PbO₂ and β-PbO₂. The electrodeposition process largely benefited from the addition of Cu²⁺, F⁻ and peptone. Cu²⁺ was partially pre-coated on the substrate and made the formation of PbO₂ become feasible. F⁻ and peptone can effectively improve the quality of the deposit (to be flat, compact and uniform), while peptone, in addition, significantly increases the chemical stability of the pyrophosphate solution, making the long-time electrodeposition possible.

With respect to the application for the oxygen evolution reaction, the anodic potential of the Pb/P-PbO₂ anode is ∼100 mV lower than that of the traditional Pb/PbO₂ anode. The polarization resistance is much smaller than that of the Pb/PbO₂ anode at specific potentials. The enhancement in electrocatalytic activity is considered to be due to the crystal defects and lattice distortions resulting from the introduction of external phosphorus atoms.

Acknowledgements

This research was supported by the National Science & Technology pillar program of China (No. 2012BAA03B04), the Chinese National Natural Science Foundation (No. 51204208 and No. 51374240) and China Postdoctoral Science Foundation (No. 2013M540638). The authors thank Dr. Wenhua Hu for proof reading the manuscript.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, electrodeposition mechanism, parameter study, and characterization for Pb/PbO₂ electrode. See DOI: 10.1039/c3ra45760e

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