In Situ Electroplating of Ir@Carbon Cloth as High-Performance Selective Oxygen Evolution Reaction Catalyst for Direct Electrolytic Recovery of Lead

Abstract

The hydrometallurgical technology provides an efficient and sustainable green lead recovery process from lead acid batteries. Methanesulfonic acid has been widely considered as a green solvent for lead electrolytic recovery. However, the competitive precipitation of PbO₂ at anode and higher overpotential for OER limit the lead recovery efficiency. In this work, an anodic oxygen evolution reaction (OER) catalyst with a low Ir mass fraction of 7.2% is obtained by electroplating iridium on carbon cloth (CC), exhibiting a lower overpotential of 256 mV, longer lifetime of 10 h, and better stability in the 0.5 M MSA solution. When CC-Ir is used as an anodic catalyst for lead recovery in the lead methanesulfonate electrolyte, only a lesser Pb precipitation product with Pb atom mass fraction of 1.42% is found after electrolysis of 10 h, demonstrating the suppression effect of CC-Ir for a PbO₂ side reaction. This work proves that the anodic catalyst plays an important role in the lead electrolytic recovery process, which can inhibit the side reaction, reduce the energy consumption, and increase recovery efficiency.

1. Introduction

Lead acid batteries are widely used in automobiles and electric bicycles because of their strong stability, high reliability, and low price. China is a large country of lead consumption, in which lead acid batteries consume more than 3.3 million tons of Pb every year, making up approximately 70% of the total lead production. The secondary lead produced in the recovery process of the lead acid battery has gradually become the main source of lead in many regions of the world. More than 2/3 of the world’s refined Pb comes from the recovered Pb, and waste lead acid batteries are the main source of secondary lead, accounting for 85% of the total secondary lead [1]. Therefore, the energy-saving and green recycling process of scrap lead acid batteries has attracted much attention.

A lead acid battery is made of lead paste, lead alloy grid, plastic shell, diaphragm, and sulfuric acid electrolyte [2]. The recovery process is mainly to recover the metallic lead from the lead paste, which consists of lead sulfate, lead oxide, lead dioxide, and lead powder [3]. Until now, the lead paste recovery process mainly included traditional pyrometallurgy, hydrometallurgy, and new reported technologies [2]. Among those, the traditional pyrometallurgical process is the primary lead recovery method, but it produces a large number of heavy metals and acid gases with high energy consumption [1,3], which pollute the environment and damage human brain function [4,5,6]. A hydrometallurgy of electrodeposited lead has been considered as a clean and efficient lead recovery process [1,7], which can be carried out in the alkaline [8,9,10,11] or acidic solution, such as sulphuric acid [12], perchloric acid [13,14], nitric acid [15,16], acetic acid [2], fluoroboric acid [17], etc. However, the electrodeposition process has some problems, such as low metal solubility, massive wasted water, and severe equipment corrosion. Recently, methanesulfonic acid (CH₃SO₃H, MSA) as a green solvent with low toxicity, low volatility, and high metal solubility and conductivity [18,19] has been considered as a green electrolyte and solvent for the leaching of leady paste to form soluble Pb(CH₃SO₃)₂. However, the existence of MSA in the electrodeposition solution leads to only a quantitative difference in the PbO₂ electrodeposition process, but will not obviously change the reaction mechanism [20]. In this electrochemical system, the main cathodic reaction is the reduction of Pb²⁺ to obtain electrons to form metallic lead during the electrodeposition progress, and the primary anodic reaction is the anodic oxidation of water, accompanied by the side reaction of the oxidation of lead ions into PbO₂ which not only greatly decreases the recovery rate of Pb, but also increases the additional energy consumption. In order to overcome this problem, it is very important to study a new OER catalyst in the Pb recovery progress in the acidic MSA solution, thus reducing the precipitation overpotential of oxygen and greatly inhibiting the side reaction.

The existing OER catalysts include metal-free (heteroatom-doped carbon), non-precious metal, and precious metal matrix materials [21]. To effectively prevent the dissolution and corrosion of the catalyst under acidic conditions, the catalyst usually uses precious metal matrix materials, among which Ru- and Ir-based catalysts are the best. As rare and precious metals, their annual output is far lower than other metals, which greatly limits their application [22]. Therefore, optimizing the design of acidic OER precious metal catalysts, minimizing the content of precious metals, and maintaining the high activity and stability of OER have become the main problems at present [23]. The structure of noble metal particles was designed to improve the surface-active sites of the catalyst for performing good catalytic activity. Huang et al. have synthesized 3D Ir superstructures composed of ultra-thin Ir nanosheets as subunits by wet chemical methods, exposing more catalytic active sites of precious metals [24]. Yang et al. have made a class of precious metal nanowires with an ultrathin wave structure and numbers of defects, exhibiting an attractive OER performance [25]. Recently, the researchers loaded precious metals on carbon materials, titanium oxide, or other carrier materials at low content to maintain an excellent OER catalytic performance [24,25,26], such as Ir/g-C₃N₄/NG [27], Pd@Ir Core-shell structure [28], and non-precious metal doped Ir nanocrystals [29]. Among these materials, the utilization efficiency and activity of precious metals are excellent. Tackett et al. have reported a unique core-shell structure with iridium/metal nitride, which is better than commercial IrO₂ [30]. Marjan Bele et al. have fixed iridium nanoparticles on a titanium oxide nanotube film with a high surface area by nitriding and the electrochemical growth method [31]. Chen et al. have prepared RuO₂ nanoparticles rich in vacancies on carbon paper by immersion coating, annealing, and acid etching methods [32]. Xing et al. synthesized nano porous IrO₂ with an ultra-high specific surface area, showing excellent catalytic activity [33]. Though various highly active catalysts have been successfully prepared, the synthesis methods are complex and time consuming. It is necessary to develop a simple effective method to prepare a low content and high active previous metal catalyst.

Herein, a simple electrodeposition method was used to fabricate the Ir catalyst on the pre-treated CC surface. By tuning the electrodeposition condition, the CC-Ir catalyst with Ir nanoparticles of uniform size, coverage, and a low mass fraction of 7.2% was obtained. The optimized catalyst exhibited a low overpotential of 256 mV for OER in the MSA solution with a longer lifetime and better catalytic stability than Ir/C. Furthermore, CC-Ir was used as the anodic catalyst for recovery Pb from lead methanesulfonate electrolyte, which can suppress the PbO₂ precipitation side reaction and lower the overpotential of the anode, saving energy consumption of the electrolytic progress. Therefore, the catalyst with good catalytic activity, stability, and high selectivity is necessary for the efficient recovery of Pb from lead acid batteries.

2. Results

In the oxygen evolution reaction for the recovery of lead, the primary anodic reaction is the anodic oxidation of water, accompanied by the side reaction of the anodic oxidation of lead ions into PbO₂. Therefore, it is necessary to develop an anode catalyst to lower the overpotential of oxygen evolution and decrease the precipitation of PbO₂. The schematic of the selective OER mechanism for the anodic catalyst is shown in Figure 1. Due to the low cost, high mechanical flexibility, and conductivity, CC has been used as the catalyst support. The CC was pre-treated by chemical degreasing, etching, cathodic electrolytic activation, and pre-nickel plating to improve hydrophilicity and expose more active sites. After that, the CC was coated with iridium by electroplating for fabricating the Ir-based OER catalyst, and as the catalyst for the anodic oxygen evolution of the methyl sulfonic acid solution.

The effects of the electroplating conditions, such as temperature, time, and current density, on the content and distribution of Ir on the surface of CC are demonstrated in Figure 2. The electroplating temperature was studied at 2 mA cm⁻² for 2 h (Figure 2a–d). The products obtained at the electroplating temperatures of 25 °C, 75 °C, 85 °C, and 95 °C were called CC-1, CC-2, CC-3, and CC-4. A higher temperature is beneficial to decrease the deposition overpotential and accelerate the diffusion rate of ions. As the temperature increased, the amount of Ir nanoparticles on the CC surface increased, reaching a maximum at 85 °C. However, as temperatures further increased to 95 °C, severe electrodeposition solution evaporation increased the viscosity of the solution and slowed ion transfer rate, resulting in less deposition of Ir nanoparticles. Then, the electroplating time was studied at 2 mA cm⁻² and 85 °C. The Ir content on the surface of CC increased with the increase in time from 1 to 6 h (Figure 2e–h). In a short time of 1 h, only a small amount of iridium particles was formed. At 2 h, iridium particles with uniform size were distributed on the CC surface. As the time further increased to 6 h, Ir particles were clumped together, which was not beneficial to the catalytic performance. Hence, the time of 2 h is optimal. Finally, the current density was further improved under optimal electroplating temperature and time. Figure 2i–k shows that the variation of Ir particle distribution was similar to that of the time effect, and the optimal current density was 2 mA cm⁻². Therefore, the optimal electroplating condition for obtaining the uniform size and distribution of Ir nanoparticles was an electroplating temperature of 85 °C, electroplating time of 2 h, and current density of 2 mA cm⁻². Figure 2l shows the distribution of elements of C, O, and Ir of the catalyst obtained under optimal conditions were nearly uniform.

The XRD patterns of Ir-based catalysts obtained at different electroplating temperatures are given in Figure 3a. The characteristic peaks of all samples from different electroplating temperatures were well matched with XRD standard cards, PDF#41-1487 and PDF#06-0598, corresponding to C and Ir, respectively. The peak at 26.38° and 44.39° corresponded to C (002) and (101) [34]. Furthermore, the peaks at 40.66°, 47.31°, and 69.14° were (111), (200), and (220) of metallic Ir [35], which were more evident for CC-3, consisting with SEM images.

The types and valence state of CC-3 were analyzed by XPS. Three elements of C, O, and Ir were detected, as shown in Figure 3b. The XPS peak of C1s in Figure 3c can be decomposed into four peaks 284.6 eV, 285.4 eV, 286.5 eV, and 288.6 eV, corresponding to C-C/C=C, C-N/C-O, C=N/C=O, and C-C=O, respectively [36,37,38]. The functional groups of C-O and C=O can improve the wettability of carbon material, which is conducive to the adsorption of electrolyte ions. In Figure 3d, the peaks at 64.0 eV and 60.9 eV, 65.1 eV and 62.0 eV, respectively, corresponded to 4f_5/2 and 4f_7/2 of Ir⁰ and Ir⁴⁺ [39,40]. The small amount of Ir⁴⁺ may be from the residue of iridium tetrachloride in electrolyte. As shown in Figure 3e, the lower energy peaks of 532.6 eV could be attributed to the C=O, and the peaks at 535.2 eV were usually related to the O species in the C-O bonds on the surface of the CC. Due to the presence of Ir⁴⁺, the higher energy peaks of 536.6 eV could be attributed to the metal=O group (Ir=O) [41]. The Ir content was roughly estimated by EDS in Figure 3f, and the mass fraction was as low as 7.16%.

The OER catalytic properties of all samples from different electroplating temperatures were tested in the 0.5 M MSA solution. As shown in Figure 4a, CC-3 exhibited the lowest overpotential of 256 mV at the current density of 10 mA cm⁻², far lower than 304 mV of Ir/C, which was lower than the overpotential of the catalyst reported for OER in the acid solution (Figure 4f [42,43,44,45,46]). The Tafel slopes of all samples from different electroplating temperatures, and Ir/C in Figure 4b, were 84.4, 80.1, 73.5, 103.0, and 87.0 mV dec⁻¹, respectively. The Tafel slope of CC-3 had the smallest value among these samples, indicating the fastest reaction rate and highest catalytic activity. In addition, the EIS measurement was performed to evaluate the involved reaction kinetics. We have studied the equivalent circuit and conducted an impedance analysis [47], as shown in Figure 4c. The equivalent electrical circuit (Figure 4c) consisted of a solution resistance (Rs), a charge-transfer resistance (R_ct), a constant phase element (CPE), and a Warburg impedance (Z_w). The CPE accounted for the non-ideality in the electrode that caused a frequency dispersion in the capacitance response. In Figure 4c, the high-frequency zone reflected the effective charge-transfer impedance and Ohmic resistance of the catalyst layer. The Nyquist plots exhibited that the charge-transfer resistance of CC-3 (Rct, 1.47 Ω) was obviously smaller than the CC, CC-1, CC-2, CC-4, and Ir/C as evidenced by a smaller semicircle. Meanwhile, CC-3 revealed a smaller series resistance (Rs = 0.72 Ω). A higher slope of Warburg impedance (Z_w) of CC-Ir than that of CC, CC-1, CC-2, CC-4, and Ir/C, means that the catalyst has better conductivity to accelerate the electron transmission rate [48,49,50]. The catalytic kinetics between the evolution of species on the electrode surface was analyzed by EIS, which proved that the CC-Ir catalyst had a better performance. In addition, the low-frequency zone reflected the double-layer capacitance. To further study the electrochemically active area of Ir-based catalysts, CV tests were carried out at different scan rates in the voltage ranges 1.044–1.144 V (vs. RHE) (Figure S1a–e). The C_dl can be obtained from the curves of current density vs. scan rate in Figure 4d. The C_dl value of CC-3 was largest among Ir-based catalysts, up to 51.1 mF cm⁻², indicating a larger electrochemical active area and better catalytic performance. Furthermore, the constant current charging test of 10 h had been performed to check the stability of CC-3 and Ir/C in Figure 4e. It noteworthily mentioned that the voltage retention of CC-3 was 98.5%, while the potential for Ir/C increased 23%, up to 1.9 V, indicating the better stability of CC-3. Hence, CC-3 has the best OER catalytic activity and stability, called CC-Ir.

Slower reaction kinetics, and higher overpotential for OER led to the competitive precipitation of PbO₂ in the recovery of lead by lead methanesulfonate electrolysis. In this work, CC-Ir was employed as an anode catalyst for lead methanesulfonate electrolysis, in order to improve the efficiency of the electrolytic process. The effects of lead ion concentration, MSA concentration, and the temperature of the electrolyte on OER and PbO₂ precipitation were studied.

CC-Ir was firstly run 50 cycles at the rate of 50 mV s⁻¹ in the voltage range of 0.2–2.2 V (vs. RHE) in 1 M MSA electrolyte before lead methanesulfonate electrolysis. Figure 5a shows the influence of Pb²⁺ concentration in 1 M MSA electrolytes. With the increase of Pb²⁺ concentration from 0.01 to 0.6 M, the oxygen release rate first increased and then decreased, reaching the maximum at a Pb²⁺ concentration of 0.2 M. Furthermore, at this Pb²⁺ concentration, the reduction peak area of PbO₂ was quite small, indicating less PbO₂ precipitation. Therefore, the lead ion concentration of 0.2 M was the best. Next, the concentration of MSA was studied at the optimal Pb²⁺ concentration. The regularities of the oxygen release rate and the amount of PbO₂ precipitation were not obvious with the increase of MSA concentration (Figure 5b). After comprehensive consideration of the above factors, the optimal concentration of MAS was 1 M. Finally, the electrolytic temperature was further improved under the optimum Pb²⁺ concentration and MSA concentration. With the temperature increasing, the oxygen evolution rate increased and the amount of PbO₂ precipitation decreased in Figure S2. High temperature caused a higher degree of crystallinity as well as higher roughened surfaces. In order to conserve energy consumption in the electrolytic process, 40 °C was selected. In order to clarify the role of CC-Ir in inhibiting the PbO₂ precipitation, the CV curve and constant current charging test of 10 h of blank CC and CC-Ir have been compared. For blank CC, the oxygen-releasing rate started to increase rapidly with the formation of PbO₂ at about 2.1 V (vs. RHE) in the oxidation stage, and a large reduction peak occurred during the reverse scan stage in Figure 5c. By comparison, the CC-Ir carried anode showed the rapidly increased oxygen evolution rate at a lower potential and the oxidation of PbO₂ were not obvious, which implies the suppression of PbO₂ formation. Furthermore, a constant current charging test of 10 h has been performed to check the stability and energy consumption of CC-Ir and CC in the electrolytic process and the properties of the product in Figure 5d. The CC-Ir anode demonstrated the lower and stable electrolytic voltage of 2.2–2.3 V. After electrolysis of 10 h, the potential of the CC-Ir anode increased 6.6%. However, the blank CC electrode displayed a higher electrolytic voltage of 2.7 V, then increased rapidly to 3.1 V, indicating the excellent OER catalytic performance and durability of the CC-Ir.

The anode products after lead methanesulfonate electrolysis with CC and CC-Ir also were analyzed. Much PbO₂ with a columnar structure after electrolysis with CC can be found by SEM and mapping images in Figure 6a,b. With CC-Ir, less-spherical PbO₂ nanoparticles were presented on the surface of CC in Figure 6c, and the mass fraction of the Pb atom was about 1.42% from the EDS analysis (Figure S3). The above results can be further verified by XRD and XPS. The peaks at 28.47° and 55.86° corresponded to (111) and (113) of PbO₂ from the XRD spectrum of CC [51], indicating the formation of PbO₂. For the CC-Ir electrolysis, PbO₂ nanoparticles can be identified from the peak of (202) at 49.43° in Figure 6d. Next, the XPS analysis of anode products after CC-Ir electrolysis is shown in Figure 6e,f. The peak at 529.85 eV in O 1s was unstable oxygen [52], and the small peak at 528.15 eV and 531.30 eV corresponded to O-Pb and O-C [53]. Combining Pb 4f_7/2 and Pb 4f_5/2 peaks at 137.0 eV and 141.9 eV in Figure 6f [54], the formation PbO₂ in the product can be verified. Compared to the peak areas of -OH with O-Pb in Figure 6e, the amount of PbO₂ was less. These results all demonstrate that CC-Ir can better suppress the formation of the PbO₂ byproduct and is conducive to the efficient recovery of Pb from the lead methanesulfonate electrolyte.

3. Materials and Methods

3.1. Materials

Iridium tetrachloride (IrCl₄), ethyl alcohol (EtOH), hydrochloric acid (HCl), sulphuric acid(H₂SO₄), boric acid(H₃BO₃), and methanesulfonic acid (CH₃SO₃H) were received from the Beijing chemical factory, China. Lead dioxide (PbO₂) was provided by Shanghai Aladdin, China. Nickel chloride (NiCl₂), sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), and sodium phosphate (Na₃PO₄) were supplied from Tianjin Fuchen Industry, China. Carbon cloth (CC) was provided from Cyber.

3.2. Pre-Treatment of CC

The catalyst support was treated by four steps of chemical degreasing, etching, cathodic electrolytic activation, and pre-nickel plating. Specifically, CC was cut to 1 cm × 1.2 cm rectangular pieces, then soaked into a mixed solution with 30 g L⁻¹ NaOH, 40 g L⁻¹ Na₂CO₃, and 40 g L⁻¹ Na₃PO₄ at 80–90 °C for 30 min and with 250 g L⁻¹ H₂SO₄ and 120 g L⁻¹ HCl at 50 °C for 30 min. After being washed with water, the CC was activated by cathodic electrolysis in the H₂SO₄ solution at 30 mA cm⁻² for 1 min. Finally, the CC was immersed into a mixed solution with 200 g L⁻¹ NiCl₂ and 130 g L⁻¹ HCl for 10 min and electroplated at 100 mA cm⁻² for 6 min at room temperature.

3.3. Preparation of Ir-Based Catalysts

The mixed solution of 7 mmol L⁻¹ IrCl₄ and 20 g L⁻¹ H₃BO₃ was stirred at 85 °C for 1 h as an electroplating solution. With a graphite rod as the anode and carbon cloth as the cathode, constant current charging was carried out for Ir-based catalyst fabrication. The current density, time, and temperature in the electroplating process were tuned. During the optimization of the electroplating temperature, Ir-based catalysts synthesized at the different temperature at 2 mA cm⁻² for 2 h were called CC-n (n = 1, 2, 3, and 4).

3.4. Characterization

The surface morphology and elemental analysis of Ir-based catalysts were obtained by the SUPRA 55 Scanning Electron Microscope (Carl Zeiss AG, Oberkochen, Germany) with energy dispersive spectrometer (EDS). X-ray photoelectron spectroscopy (XPS) was analyzed by using Thermo VG Scientific ESCALAB 250 (Thermo Fisher Scientific, Waltham, MA, USA). X-ray diffraction patterns (XRD) were collected from D/max2500VB2+/PC X-ray diffraction with a Cu Kα anticathode (40 kV, 200 mA) in the 2 θ range of 5–90° (Rigaku Corporation, Tokyo, Japan).

3.5. Electrochemical Measurement

All electrochemical properties were measured by CHI 760D (Shanghai Chenhua Instrument, Shanghai, China) by a three-electrode system. The prepared catalysts were applied as working electrodes, Pt plate as a counter electrode, and Hg/Hg₂Cl₂ as reference electrode. The applied potential vs. Hg/Hg₂Cl₂ was converted into that vs. reversible hydrogen electrode (RHE) by the following formula:

$${\mathrm{E}}_{\mathrm{vs}. \mathrm{RHE}}={ \mathrm{E}}_{\mathrm{vs}.\mathrm{Hg}/{\mathrm{Hg}}_2{\mathrm{Cl}}_2}+{ \mathrm{E}}_{\mathrm{vs}.\mathrm{Hg}/{\mathrm{Hg}}_2{\mathrm{Cl}}_2}^{\mathsf{\theta }}+0.0591*\mathrm{pH}$$

Linear sweep voltammetry (LSV) was recorded at the potential range of 1.044–1.844 V (vs. RHE) at the scan rate of 5 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was recorded with a frequency range from 0.01 Hz to 100,000 Hz with 5 mV amplitude. The potential range of cyclic voltammetry (CV) is 1.044–1.844 (vs. RHE) at a scan rate of 50 mV s⁻¹. The galvanostatic test was worked out at a fixed current density of 10 mA cm⁻² for 10 h. Electric double-layer capacitance (C_dl) can be calculated by the following equation:

$${\mathrm{i}}_{\mathrm{c}}={\mathrm{C}}_{\mathrm{dl}}\frac{\mathrm{d}\mathsf{\psi }}{\mathrm{dt}}$$

where $\frac{\mathrm{d}\mathsf{\psi }}{\mathrm{dt}}$ is the unit scan speed.

4. Conclusions

In summary, a simple electroplating method was used to fabricate an Ir-based catalyst on the surface of CC as an anode catalyst for the electrolysis of lead. By the investigation of the electroplating conditions, the optimal conditions are as follows to obtain a uniform size and coverage of Ir nanoparticles with a low Ir load of 7.2%: current density of 2 mA cm⁻² under electroplating temperature of 85°C in 7 mmol L⁻¹ IrCl₄ and 20 g L⁻¹ H₃BO₃ solution for 2 h. The optimized CC-Ir catalyst demonstrated a lower overpotential of 256 mV at the current density of 10 mA cm⁻², showing a more increased electrochemical performance than that of the commercial Ir/C catalyst in the 0.5 M MSA solution. The Nyquist plots exhibited that the CC-Ir has higher conductivity to accelerate the electron transmission rate. The assembled electrolysis bath with CC-Ir anodic catalyst in lead methanesulfonate electrolyte presented a lower electrolytic voltage of 2.2 V, much lower than that of the blank CC anode, suggesting higher selectivity of CC-Ir and lower energy consumption. Therefore, the proposed CC-Ir will be an efficient OER catalyst for the electrolytic recovery of lead from spent lead acid batteries.