Lead-based Anodes Research Articles

Synergistically achieving particulate matter reduction and production performance optimization for zinc electrolysis by ultrasonication coupling anode-coated MnO2

Abating particulate matter (PM) from electrolysis processes is significant because this PM poses occupational threats to workers and has a negative impact on air quality. However, it remains a challenge to synergize the reduction of PM production and the improvement of electrolysis performance. This study developed a green method, termed as the coupling of ultrasonication and pre-coating MnO2 anode treatment (UMT). The effects on PM generation rate and electrolysis performance indicators were estimated using the bench-scale zinc electrolysis device, and the synergistic mechanism of UMT was expanded from the perspective of electrochemical reactions and bubble characteristics using analysis of reaction products, camera technique, and PM generation prediction models. The results showed UMT not only overcame the degradation of deposited zinc quality caused by the ultrasonication treatment but also solved the increased PM generation by the pre-coating MnO2 film treatment. The UMT simultaneously reduced PM (33.9 %-57.5 %), decreased zinc impurity content by (11.2 %-54.3 %), improved current efficiency of zinc deposition (0.19 %-1.71 %), and conserved electrolysis energy (0.27 %-1.01 %). The optimal performance of UMT occurred at 80 kHz. The UMT suppressed the gas evolution reactions and prematurely bursting bubbles, in favor of reducing the number and size of bubbles, to reduce PM generation. Meanwhile, the UMT improved the electrolysis performance by inhibiting the corrosion of lead-based anodes, promoting the mass transfer rate of Zn2+, providing more active surfaces, and decreasing the overpotential of reactions. The findings may provide references for the green development of the metal electrolysis industry.

Kinetic Study of Manganese Oxidative Precipitation Reaction by Using SO2/Air Gas Mixture

Removing manganese from zinc electrolytes is necessary to pave the way for replacing lead-based anodes with mixed metal oxide (MMO) anodes. MMO anodes offer significantly lower overpotential towards oxygen evolution reactions, thus are attractive from an energy consumption viewpoint. Previous studies had shown that, thanks to the catalytic effect of cobalt, manganese can be removed successfully from the zinc purification solution through the oxidative precipitation method using a simulated roasting off-gas plant. This study focuses on understanding the primary mechanism behind manganese oxidation precipitation and investigating the influence of various operating parameters such as temperature, dissolved oxygen (DO), and solution potential on the reaction kinetics. The results revealed that the kinetics of the reaction was highly dependent on the temperature and catalyst activity rather than on the reactant concentration. Additives, with radical scavenging effects, were added to identify the radicals responsible for the oxidation of Mn. The manganese oxidation reaction was dramatically suppressed when methanol was added. However, in the presence of tert-butyl alcohol (TBA), a sensible reduction in manganese removal was not observed, suggesting sulfate radical as the predominant species for oxidizing manganese. The physical and chemical characteristics of the sediments were also presented.

Asymmetric structural tuning of industrial MnO2 arrays on a hierarchical lead-based anode for manganese metallurgy

We provide a hierarchical structural tuning method for producing asymmetrical MnO2 with stable catalysis by preventing Pb toxicity as a long-lasting oxygen evolution selective layer on a commercial lead-based anode for manganese electrowinning.

Ti/Ti4O7 Anodes for Efficient Electrodeposition of Manganese Metal and Anode Slime Generation Reduction.

Preventing lead-based anodes from causing high-energy consumption, lead pollution, and harmful anode slime emission is a major challenge for the current electrolytic manganese metal industry. In this work, a Ti4O7-coated titanium electrode was used as anode material (Ti/Ti4O7 anode) in manganese electrowinning process for the first time and compared with a lead-based anode (Pb anode). The Ti/Ti4O7 anode was used for galvanostatic electrolysis; the cathodic current efficiency improved by 3.22% and energy consumption decreased by 7.82%. During 8 h of electrolysis, it reduced 90.42% solution anode slime and 72.80% plate anode slime formation. Anode product characterization and electrochemical tests indicated that the Ti/Ti4O7 anode possesses good oxygen evolution activity, and γ-MnO2 has a positive catalytic effect on oxygen evolution reaction (OER), which inhibited anode Mn2+ oxidation reaction and reduced the formation of anode slime. In addition, the low charge-transfer resistance, high diffusion resistance, and dense MnO2 layer of the anode blocked the diffusion path of Mn3+ in the system and inhibited the formation of anode slime. The Ti/Ti4O7 anode exhibits excellent electrochemical performance, which provides a new idea for the selection of novel anodes, energy savings and emission reduction, and the establishment of a new mode of clean production in the electrolytic manganese metal industry.

Reaction selectivity-regulation via interfacial reconstruction for preventing hazardous slime generation: Driving mechanism of Pb-based anode with oxygen vacancy-rich MnO2

Limited selectivity in coexisted anodic reactions on lead-based anodes leads to severe lead-dissolution, anodic slime-generation and high energy-consumption. Herein, anodic reaction selectivity was regulated by phase-controlled MnO2 (MnO2-PC) on lead-based anodes which was prepared under a lower voltage and a higher Mn2+ concentration. Compared with α-MnO2 formed under normal conditions, composite phase of γ-MnO2 and ε-MnO2 was obtained from MnO2-PC with specific crystal planes exposure and rich oxygen vacancy defects enabling higher oxygen evolution reaction selectivity (OER, overpotential reduced by ∼99 mV). Meanwhile, reaction of lead corrosion and manganese oxidation, as the main reasons for generation of hazardous lead-containing slime, were synergistically suppressed by 86.8% and 91.7%, respectively. Density functional theory (DFT) calculations further clarified that the change in coordinative environment of Mn and O brought lower energy barrier of rate-determining step of OER and stronger electron delocalization ability of MnO2, beneficial for promoting the catalysis of OER. MnO2-PC also displayed excellent inhibition effects on the generation and accumulation of Mn3+, an important precursor of MnO2 slime, and changed the selectivity in the oxidative path of Mn2+. The long-term electrolysis tests proved the stable performance of Pb–Ag/MnO2-PC. This study provides an efficient, low-cost, high economic benefits and environmental-friendly strategy for designing target functional materials towards reaction selectivity regulation via phase-control and microstructure reconstruction.

A novel lead-based pseudo dimensional stable anode toward efficient and clean extraction of metallic manganese

Although the application of the dimensionally stable anode (DSA) is of great value to meet the environmental demands for sustainable development of electrometallurgy, the unsatisfactory stability and high-costs hinder its industrialization in Mn metallurgy. Hereby, a novel lead-based pseudo dimensional stable anode (LBP-DSA) was proposed firstly to overcome the intrinsic defects of the lead-based anodes and inherit the advanced characteristics of the DSA. The unique advantage for the hierarchical skeleton of LBP-DSA (lead-based/TiB2/γ-MnO2) was discussed in detail. To begin with, a sufficiently robust lead-based/TiB2 substrate was constructed to eliminate the lead pollution in electrolysis system. The key role of TiB2 interlayer was devoted to solidify the lead substrate and provide suitable anchors for subsequent layer adhesion. Subsequently, the as-synthesized γ-MnO2 on different substrates were comparatively analyzed to highlight that the tantalizing oxygen evolution selectivity relied on screening the lead pollution in γ-MnO2 lattice. Beneficial from the synergistic effects of TiB2 with γ-MnO2, our LBP-DSA exhibited a great potential in energy-saving and hazardous anode slime reduction. The satisfactory long-term stability and corrosion resistance made the LBP-DSA as a promising candidate for cleaner production of metallic manganese.

Facile preparation of PbSe@C nanoflowers as anode materials for Li-ion batteries

The PbSe@C nanoflowers were synthesized by solvothermal and high-temperature selenization method and applied to lithium-ion batteries. PbSe@C anode materials have remarkable cyclic performance (the reversible specific capacity reaches 495.4 mAh/g after 100 cycles at a current density of 100 mA g−1.) and satisfactory rate performance (The average specific discharge capacities of PbSe@C nanoflowers at the current densities of 100, 200, 500, 1000, and 2000 mA g−1 are 753.8, 587.5, 465.8, 398.3 and 340.9 mAh/g, respectively. When the current density is restored to 100 mA g−1, the specific discharge capacity of the PbSe@C nanoflowers is 600.1 mAh/g). Not only the carbon component in the nanocomposite can enhance the electrical conductivity of the material and prevent PbSe agglomeration, but also the pores in the nanocomposite provide room for volume expansion during charge and discharge. Therefore, the research of nanoflower structure provides a new direction for improving the performance of lead-based anode materials.

Insights into the Na-Alloying Mechanism and Zintl Phase Transition of Lead-Based Anodes

Sodium-ion batteries (SIBs) has been received growing attention in the electrical energy storage fields due to their low cost and earth-abundant sodium. [1] However, there has been a lack of new discoveries, growth directions, and real advancement with respect to Na-storage anodes. Despite the chemical similarities between sodium and lithium as alkali elements, the larger Na ion than Li ions (ionic radii of 0.98 Å and 0.68 Å, respectively) is limited to insertion into host materials and results in different phase transition behavior. [2] Among the available anode candidate materials for SIBs, lead (Pb), which has a large atomic size than other elements (e.g. Si, Sn), provides a big interstitial space to accommodate large Na ions by fast ionic diffusion, enabling reversible Na alloying/dealloying and exhibiting high volumetric capacity. [3] Furthermore, when Pb is used as anode with layered sodium transition metal oxide as cathode, the energy density of the pouch-type cell is estimated to be 549 Wh/L and the cost is lower than 63.5 USD/kWh according to the Argonne BatPac model. [4] Therefore, Pb-based materials have competitive potential as promising anodes and it is crucial to understand the electrochemical process from a fundamental perspective.Here, we investigate a unique Na storage mechanism using a novel Pb-based carbon nanocomposite anode synthesized by a simple high-energy milling method. The electrochemical data show a decent cycle performance with a reversible capacity of 381 mAh/g. Nevertheless, the Na-storage performance of the Pb-based anode was not attractive compared to Li cells. In-situ X-ray diffraction and ex-situ X-ray absorption spectroscopy reveal the reaction mechanism and Zintl-phase formation that limits the Na storage, unlike the Li reaction. We expect these findings provide fundamental knowledge of Na-alloying reaction and guidance for designing anode materials for high-performance SIBs.[1] K. Kubota and S. Komaba Journal of The Electrochemical Society, 2015, 162 (14) A2538-A2550[2] M. Lao, Y. Zhang, W. Luo, Q. Yan, W. Sun, and S. X. Dou Adv. Mater. 2017, 29 , 1700622[3] Chia-Yun Chou, Myungsuk Lee, and Gyeong S. Hwang, J. Phys. Chem. C 2015, 119, 27, 14843–14850[4] P. Nelson, K. Gallagher, I. Bloom, Dennis Dees, and Shabbir Ahmed, BatPaC, Argonne National Laboratory. http://www.cse.anl.gov/batpac.

Research on microstructure of membrane-slime layer on lead-based anode surface in zinc hydrometallurgy by combining μ-XRF with mm-XRF

In this paper, the synchrotron radiation μ-XRF and the mm-XRF method were used to emphatically study the composition, content and distribution of lead and manganese in anode slime on lead-based surface. The paper mainly explored the chemical features and the content evolution law of lead and manganese in the anode micro-zone of the anode surface, analyzed the corresponding relationship between the variation trend of lead and manganese contents and the thickness of micro-zone of the anode surface, scientifically determined the parameters of membrane preservation and slime removal by studying the quantitative layered structure of membrane-slime layer on the lead-based anode surface, and further verified the accuracy of the layered structure of membrane-slime layer through field slime removal experiment. The research results show that: (1) By using the respective advantages of μ-XRF and mm-XRF, and through in-situ study on the microstructure of anode slime by combining the two methods, it is revealed that there exists quantitative and fine layered structure of membrane-slime layer on the lead-based anode surface.(2) Through the research and analysis based on synchrotron radiation μ-XRF microprobe, the anode slime from the lead-based interface can be divided into membrane layer (high lead and dense), membrane-slime layer (intertwined high lead and low manganese and intertwined medium lead and high manganese) and slime layer (low lead and high manganese.(3) Further mm-XRF analysis of a large amount of data shows that the change rates of lead and manganese contents in anode slime from the lead-based interface vary significantly within the range of unit thickness; when the thickness is ＜1.0 mm, the lead content decreases most rapidly from 99% to 35%, and the lead content change value ΔPb is 64%/1 mm; when the thickness is 1.0–2.6 mm, the lead content change tends to be gentle, with a decrease from 35% to 10%, and ΔPb is 16%/1 mm; when the thickness is ＞2.6 mm, the lead content has no obvious change and fluctuates in a small range of about 10%. (4) Based on the above research, 2.6 mm on the lead-based surface can be determined as the limit thickness of anode membrane preservation and slime removal; the results of the membrane preservation and slime removal equipment developed on this basis show that compared with the manual slime removal results, the lead content of the anode surface after removing the slime by using membrane preservation and slime removal equipment drops from 80% to less than 10%.

Carbon-coated FePO4 nanoparticles as stable cathode for Na-ion batteries: A promising full cell with a Na15Pb4 anode

The cheap and non-toxic FePO4, which can be obtained from LiFePO4 recovered from spent Li-ion batteries, provides a reversible capacity of 103 mAh/g in coin cells, corresponding to 76% of theoretical capacity, with very good rate capability and stability. The study of the electrochemical mechanism during sodiation carried out by in situ X-ray diffraction (XRD) reveals a pure biphasic transition during Na+ (de)insertion. The chemical diffusion coefficient (DNa) determined from galvanostatic intermittent titration technique and electrochemical impedance spectroscopy varies in the range 10−16–10−10 cm2/s at room temperature depending on the degree of sodiation. As a proof of concept, a full cell fabricated using a carbon-coated FePO4@C cathode and a Na15Pb4 anode obtained by electrochemical sodiation of recyclable Pb, maintained 76% of the initial capacity over 100 cycles at constant C/10 rate. This outstanding electrochemical performance, based on the combination of the low-cost FePO4@C cathode with a recyclable lead-based anode, makes this new technology a promising real-world alternative for future commercial Na-ion systems.

Analysis of lead pollution control in anode slime micromorphology evolution induced by Mn2+ ions for cleaner production of zinc electrolysis

To explore cleaner production technologies to control lead (Pb) pollution caused by anode slime is a challenging issue for zinc electrolysis industry. The influence of manganese ions (Mn2+) on anode slime microtopography evolution and its relationship with Pb release of lead-based anodes was studied in detail. Scaling analysis was first introduced to quantitatively evaluate anode slime micromorphology evolution, and reveal its intrinsic growth mechanism induced by Mn2+ ions through parameterizing the morphological information of atomic force microscopy (AFM). Low Mn2+ concentrations (1–3 g/L) were found to induce the initial anode slime micromorphology to be compact, of which growth was revealed to be controlled by surface diffusion alone corresponding the exponent α = 0.95. Results of Scanning electron microscopy (SEM) and inductively coupled plasma mass spectrometry (ICP-MS) extended to 240 h of electrolysis supported that anode slime micromorphology evolution in this case could contribute to inhibiting lead pollution, and the release of Pb from lead-based anodes into electrolytes was controlled at 0.5 mg/L. As the Mn2+ concentration increased to 10 g/L, the initial anode slime micromorphology was found that the evolution trend from smooth to rough. And the changes in the growth mechanism from the surface diffusion control to volume or bulk diffusion control were found, which appeared as two distinct hierarchical features corresponding to static exponents α = 0.92 and α’ = 0.55. In this case, excess Mn2+ ions inducing deteriorated anode slime micromorphology evolution was revealed, and large fluctuations of Pb2+ concentrations (1.2–1.5 mg/L) in electrolytes with prolonged electrolysis suggested that it was inadequate in inhibiting Pb2+ release. Scaling analysis for in-depth analysing and predicting the evolution trend of anode slime microtopography induced by Mn2+ ions has been established, thus new insights gained herein can contribute to exploring approaches to control lead pollution by cleaner production technologies during zinc electrolysis.

Polyacrylonitrile-Based Carbon Fiber as Anode for Manganese Electrowinning: Anode Slime Emission Reduction and Metal Dendrite Control

The traditional lead-based anodes used in industrial electrolysis process trigger the high energy consumption, lead pollution and hazardous anode slime discharge. In this work, an original undecorated Polyacrylonitrile (PAN)-based carbon fiber was employed as anode material (CF anode) to evaluate its electrochemical performance and feasibility, aiming at fundamentally reducing the energy consumption and pollution emissions for the manganese electrolysis. The results shown that the CF anode exhibited excellent electrocatalytic activity and delivered a current density of 350 A m–2 at an overpotential decreased by 112 mV for oxygen evolution reaction (OER) compared with commercial lead-based alloy anode (Pb anode). We also found CF anode exhibited favorable performance with average current efficiency increased by 4.30%, energy consumption decreased by 8.36%, and a noteworthy abatement of anode slime by 80% compared with Pb anode in MnSO4 electrolyte. Additionally, the growth of manganese dendrites on the cathode edge which directly affected the electrolytic efficiency has also been effectively controlled and the possible mechanisms were also discussed. This work displayed the excellent electrocatalytic effect of CF anode serviced as a promising candidate for green and efficient electrolysis process.

Lead release kinetics and film transformation of Pb-MnO2 pre-coated anode in long-term zinc electrowinning

Lead pollution precaution caused by lead-based anode corrosion is a hot and challenging issue for zinc electrowinning. A novel functional lead-based anode (MnO2 pre-coated anode-MPA) was precisely fabricated and its long-term performances were studied compared with typical Pb−1%Ag anode (TPA). Results indicated that MPA posed excellent effects on synergistic inhibiting lead dissolution and reducing hazardous pollutants generation, and decreasing the lead content of zinc products by 81%. Further, the underlying mechanism of film growth and transformation in structure, composition and crystal phase, the migration and distribution of lead and anode slime during electrolytic, were clarified in-depth. Dynamic material flow analysis confirmed that MPA reduced the entire lead migration amount by over 92% compared with TPA. The compact multilayer structure of the MPA film and self-reparation effects of local structure provided better and persistent protection for the lead matrix, which greatly retarded the high-speed corrosion of lead anode. Compared with α-MnO2 in TPA, the formation and maintenance of γ-MnO2 in MPA accelerated the oxygen evolution reaction and inhibited the anode slime generation. This finding provides new insights in pollution precaution and control by designing and tuning new functional anode in hydrometallurgy process.

A novel functional lead-based anode for efficient lead dissolution inhibition and slime generation reduction in zinc electrowinning

Lead-based alloy is the most widely used anode material in zinc electrowinning and other metallurgical processes. Lead dissolution during electrowinning not only results in resource loss and pollution generation, but also leads to deteriorations in product quality and adversely effects economic benefits. In this study, we propose a new functional anode preparation process that was precisely and completely fabricated by rapid electrodeposition of an active γ-MnO2 layer on bare lead-based anode. The prepared anode (MnO2 pre-coated anode [MPA]) exhibited an excellent performance in terms of both pollutant reduction and product promotion as compared with the typical bare Pb–1%Ag anode (TPA). Property characterizations and electrochemical analysis indicated that the active γ-MnO2 layer showed complete and uniform coverage on the lead-based anode, which effectively blocked the contact and reaction of lead with electrolytes. This was beneficial for reducing the amounts of dissolved lead and significantly promoting the purity of zinc. Besides, the needle-like structure and positive electro-catalytical effects of γ-MnO2 on the oxygen evolution reaction (OER) lowered the anode overpotential, reversely inhibited the Mn2+ oxidation reaction, and reduced 94% of anode slime formation and 94% of lead migration. The MnO2 pre-coating process shows bright prospects for industrial applications in zinc electrowinning and other hydrometallurgy processes.

Insight into electrochemical performance of porous FexSiy intermetallic anode for zinc electrowinning

Pb pollution, which is provoked by lead-based anodes in large-scale industrial production, has seriously endangered human health and destroyed the ecological environment in the past decades. In this work, environmentally friendly porous FexSiy intermetallics are prepared by elemental powder reactive synthesis as anodes in zinc electrowinning for the first time. The as-prepared porous FexSiy anodes (Fe3Si, Fe5Si3, and FeSi) have been investigated in terms of electrochemical performance. Among the three anodes, porous Fe5Si3 anode shows relatively better comprehensive properties in 160 g·L−1 H2SO4. Porous Fe5Si3 anode exhibits electrocatalytic activity and long-term stability under the condition of zinc electrowinning, which guarantees a service life up to 400 h. The activity for OER shows that porous Fe5Si3 anode is delivering an overpotential of 1.072 V and tafel slope of 326.6 mV·dec at current density of 50 mA·cm−2. The electrocatalytic activity is positively correlated with the open porosity. The oxides formed in the anode surface are mainly composed of Fe2O3 and amorphous SiO2, which protect the anode against corrosion and help to the activity for OER. This result demonstrated that Fe5Si3 as alternative anode material has potential application value in industry.

Lead-Carbon Nanofiber Based ANODE for High Performance LI-ION Battery

Lead-carbon (Pb-C) nanofibrous anodes for lithium-ion batteries were prepared via the electrospinning method. The use of lead-based anodes with high amounts of carbon has been shown to improve the cycling performance of batteries through increased capacity retention and electrical conductivity. This is attributed to the Pb-C accommodation to volume change during lithiation/de-lithiation upon cycling as well as suppressed formation of the unstable solid-electrolyte interphase layer. By preparing the anode via electrospinning, an increase in specific area and active material content can be achieved with significant improvement to the battery performance. In this report, significant enhancement of battery performance in terms of specific capacity and cyclability is made using the prepared Pb-C nanofiber anodes. The Pb-C nanofiber anode exhibits a high specific capacity of 637 mAh g-1 with excellent cycling performance.

Lead Oxide Microparticles Coated by Ethylenediamine-Cross-Linked Graphene Oxide for Lithium Ion Battery Anodes

Lead-based anodes in Li-ion batteries have potential in stationary storage applications, due to their high theoretical volumetric energy density. In this study, the cycle performance of PbO was improved by coating it with a network of graphene oxide sheets, cross-linked by ethylenediamine. The carbon coating reduced electrode capacity degradation from 0.42% to 0.22% per cycle over 200 cycles, increased Coulombic efficiency from 96% to 99%, and increased the accessible capacity of the electrodes through improved electrical connectivity. This low-temperature carbon coating technique is potentially advantageous to other metastable and low-melting electroactive materials.

Effect of the Sn-Ag addition on the internal stress change and electrochemical properties of lead-based anodes

The corrosion resistance is importantly influenced by the internal stress of the oxide film on lead-alloy anodes. In this article, pure lead anodes and Pb-Sn-Ag alloy anodes are taken as raw materials to study the effect of Sn-Ag addition on the internal stress change and electrochemical properties of oxide film in a chromic acid solution. The structural properties are researched by means of a metallurgical microscope, a Raman spectrometer and an X-ray diffractometer. The electrochemical properties are studied by electrochemical testing methods. During the oxidation process, both anodes behave as the compressive stress, while the internal stress of the Pb-Sn-Ag alloy anode exhibits fewer changes. PbO, which contributes to a compressive stress, is generated first and then turns into PbCrO4 that contributes to a tensile stress. Compared to pure lead anodes, Pb-Sn-Ag alloy anodes possess a better corrosion resistance, which can be maintained for a long time. PbCrO4 tends to weaken the corrosion resistance. The analysis shows that the more internal stresses accumulates, the poorer the corrosion resistance possessed by anodes. Moreover, an equation of σOF=A⋅exp(t/B)+σL is proposed to estimate the internal stress.

An Electrochemical Quartz Crystal Microbalance Investigation of Manganese Oxide Deposition and Dissolution in Sulfuric Acid Relevant for Zinc Electrowinning

Manganese(II)-ions are present in varying amounts in the electrolyte used for zinc electrowinning, based on the manganese content in the ore. The manganese ions are oxidized at the anode during electrowinning to form manganese dioxide alongside the oxygen evolution reaction. This has both positive and negative consequences for the lead-based anodes used in the industry today. The manganese dioxide prevents corrosion of the lead and hence lowers the amount of lead as an impurity in the produced zinc. Manganese dioxide also increases the selectivity of the oxygen evolution reaction to the chlorine evolution reaction. On the other hand, the manganese dioxide layer grows in thickness over time leading to an increase in cell resistance in addition to a possibility of flaking off and short circuiting the cell. The anodes therefore have to be removed from the cells at regular intervals to physically remove the MnO2 layer, which is a labor intensive task. Lead-based electrodes are not ideal as anode materials for several reasons. They are not very active for oxygen evolution and they introduce lead impurities to the produced metal. Lead is also unwanted for environmental reasons. It is desirable to replace them with electrochemically active dimensionally stable anodes (DSAs). The aim of this study is to investigate manganese oxide deposition and dissolution on noble metals by using conventional electrochemical methods and electrochemical quartz crystal microbalance (EQCM). In this work EQCM was used to provide additional information about the manganese oxide deposition and dissolution from a sulfuric acid solution. The electrode materials studied were platinum and gold. The electrolyte consisted of 0.1 M H2SO4 with 5 mM Mn2+-ions and the experiments were conducted at room temperature. Cyclic voltammograms were recorded for a number of different upper reversal potentials and sweep rates. In addition, sweep-hold and step-hold experiments were conducted. In these experiments the potential was swept or stepped to various potentials at which manganese oxide deposition occurred and held for a given time. These sequences were followed by a negative-going sweep from the hold potential. To complement the EQCM experiments, manganese oxide was deposited on a platinum disc at two different potentials (1.45 V and 1.55 V) and studied by scanning electron microscopy (SEM) as a function of deposition time. The cyclic voltammograms showed an oxidation peak commencing at about 1.42 V and peaking around 1.46 V in the positive-going scan (Fig 1), as observed elsewhere [1]. The oxidation peak coincided with an increase in mass. Clear indications of a nucleation, growth and collision mechanism were observed when the upper reversal potential was chosen from the rising part of the oxidation peak, i.e. below 1.46 V. This peak occurs below the onset of oxygen evolution and is thus related to the onset and continuous formation of manganese oxide. The shape and size of the oxidation peak were similar on gold and platinum. Subsequently, a cathodic peak (reduction of manganese oxide) was observed in the negative-going scan at potentials just positive of 1.3 V, which also appeared similarly on both electrode materials. Interestingly, a second reduction peak at around 1.2 V was observed at both electrodes, but only when the upper limit was sufficiently positive (above 1.5 V). Voltammograms recorded at different sweep rates showed no clear trend in the peak current for the oxidation peak. However, the mass deposited did show a very clear dependence and was inversely proportional to the sweep rate. The SEM images showed that the deposit formed at the higher potential had more dendrites than the deposit formed at the lower potential although almost the same amount of charge was passed in the deposition experiments. The change from one to two reduction peaks when reversing the sweep at potentials above 1.55 V indicates a change in the reduction and deposition mechanism. The lack of sweep-rate dependence of the oxidation peak shows that it is at least not entirely governed by bulk diffusion of Mn2+. To be able to implement DSAs in zinc electrowinning it is important to understand the mechanism of manganese oxide deposition and dissolution. W.H. Kao and V.J. Weibel, Electrochemical oxidation of manganese(II) at a platinum electrode, J. Appl. Electrochem., 22 (1992) 21-27. Figure 1

Influence of Mn2+ ions on the corrosion mechanism of lead-based anodes and the generation of heavy metal anode slime in zinc sulfate electrolyte

The influence of Mn2+ ions on the generation of heavy metal anode slime during zinc electrolysis industry was extensively investigated using several electrochemical methods, electron microscope technologies, and particle size analysis. Results showed that the Mn2+ could obviously promote oxygen evolution reaction (OER) and thereby weaken oxidation efficiency of Mn2+ (ηMnO2) and dissolution of Pb2+. The significant improvement in kinetic parameters for OER was found in electrolytes of 1 and 3g/L Mn2+, but became unstable as the Mn2+ concentration increased to 10g/L. This result was correlated with much different properties of oxide layers that its changes of microstructure are involved in, since it confirmed that the positive role of compact oxide layers in contributing to high corrosion resistance and activity for OER, but excessive Mn2+, resulted in its micromorphology of overthickness and instability. Such differences resulted from the effect of the Mn2+ concentration fluctuation on kinetic rates of the nucleation growth process. The formation and adsorption of intermediate MnO2-OHads identified as the controlled step for Mn2+ catalyzing OER was also recommended. The generation mechanism of anode slime was found to be changed in essence due to varying Mn2+ concentrations. In electrolyte of 1g/L Mn2+, results revealed that the root cause of excessive small suspended anode slime (around 20μm) was the change of the initial pathway of Mn2+ electro-oxidation, whereas, it showed great improvement in the settling performance as the Mn2+ concentration was increased to 10g/L. Considering the potential of optimizing Mn2+ concentrations as a cleaner approach to control anode slime, deepening the understanding of the impact mechanism of Mn2+ can provide new insights into intervention in the generation of anode slime.