新型节能阳极材料制备技术及电化学性能研究

【作者】 陈步明；

【导师】 杨显万； 郭忠诚；

【作者基本信息】 昆明理工大学 ， 有色金属冶金， 2009， 博士

【摘要】 目前在锌电积中采用的阳极材料一般为铅基合金,这种电极存在的不足之处是：(1)电解时形成的PbO2膜对氧析出反应(OER)有很高的超电位,使电解能耗增加；(2)阴极产品易受铅的污染；(3)机械强度低,易弯曲甚至造成短路。一种新型惰性二氧化铅阳极受到广泛的应用,此电极一般由钛基体、底层、中间层以及表面层组成。底层一般是为了改善二氧化铅镀层与钛基体的结合性能；中间层是为了增强二氧化铅镀层与电极结合的牢固度,以及缓和镀层中的电沉积畸变的产生(一般使用不存在电积畸变的α-PbO2作中间层)；表面层是β-PbO2。与旧式二氧化铅相比较,新型的二氧化铅提高了电极的坚固性、导电性和耐蚀性,但钛价格高；而铝价格便宜,导电性好,质量轻,也是阀型金属。在铝基体上制得的新型二氧化铅电极材料用在有色金属电积中有广阔的应用前景。本文首先通过热力学分析讨论了水溶液中PbO2和MnO2生成的条件；其次,采用阳极电沉积方法,在A1基上电沉积二氧化铅镀层；然后,通过改进实验条件将具有特殊性能的纳米或微米颗粒以及离子掺杂进入PbO2中,分别制备了以α-PbO2-CeO2-TiO2作为中间层,以β-PbO2-WC-ZrO2或β-PbO2-MnO2-WC-ZrO2作为最外层的两种具有高催化活性和耐蚀性的新型节能惰性阳极材料。利用SEM、XRD、EDS、金相显微镜、激光粒度分析、HX-1等手段对镀层的表面形貌、结构物相、成分、金相组织、固体颗粒大小以及显微硬度进行了表征。采用阳极极化曲线、Tafel、EIS、循环伏安等手段在酸性介质中测定了动力学参数。采用快速寿命法考察了复合电极在硫酸溶液中2A/cm2下的预期使用寿命。探讨了固体颗粒在PbO2复合镀层中的作用机制。对Pb-H2O系和Mn-H2O系进行了系统的热力学分析,分析表明：镀液温度对生成MnO2和α-PbO2的影响比生成β-PbO2大,且MnO2和β-PbO2共沉积之前已发生了MnO2的沉积。在氢氧化钠体系、硝酸体系和醋酸体系中分别制备了A1基α-PbO2、Al基α-PbO2/β-PbO2 (NO3-)和A1基α-PbO2/β-PbO2 (CH3COO-)电极。在硝酸体系中得到的电极表面粗糙度最高,催化活性好；外层β-PbO2与中间层α-PbO2之间形成最大固溶体,增加了电极的导电性和耐蚀性能。提出了A1基α-PbO2和A1基α-PbO2/β-PbO2(NO3-)两种电极的结构模型,在析氧反应机理中Tafel斜率b值大于120mV是由于电极在阳极极化过程中表面的活性颗粒数目减少所致。在碱性镀液中研究了A1基上阳极电沉积α-PbO2镀层的工艺。电沉积的电流密度对镀层的表面形貌影响最大,在电流密度小于3mA cm-2所得的镀层致密均匀；进一步提高电流密度则镀层晶体减小,但会产生孔g隙。HPbO2浓度在0.12M和温度为40℃下所得的镀层的晶体分布良好；但在较长的电沉积时间下会产生孔隙。在碱性镀液中得到的α-PbO2镀层具有很高的非计量式,尤其是在电流密度较大时得到的PbO2镀层更甚；镀层中除了α-PbO2外,还含有少量的PbO物相。利用电化学阳极复合电沉积技术,掺杂TiO2和CeO2纳微米颗粒于碱性镀液中,在A1基上制备了α-PbO2-CeO2-TiO2复合镀层。确定了制备复合电极材料的最佳配方及操作条件：氧化铅加入4M氢氧化钠水溶液中至饱和、TiO2浓度(15g/L)、CeO2浓度(10g/L)、温度(40℃)、电流密度(0.5A/dm2)、电沉积时间(3h)。在此条件下获得PbO2-(3.77wt.%)TiO2-(2.13wt.%)CeO2复合镀层。α-PbO2-CeO2-TiO2复合镀层的厚度、显微硬度和致密性都优于未掺杂的α-PbO2镀层。A1基α-PbO2-3.70wt.% TiO2-2.14wt.% CeO2电极在碱性溶液中在相同的电势下所需要的活化能最小,并且在电势为0.35V左右可分为电化学步骤和扩散步骤控制。在硫酸溶液中,α-PbO2-3.69 wt.% TiO2-2.12wt.% CeO2镀层的耐蚀性最强,催化活性最好。初步提出了两种颗粒的复合共沉积模型。通过β-PbO2和纳米Zr02.WC微粒的阳极共沉积,在A1基α-PbO2-CeO2-TiO2上制备了β-PbO2-WC-ZrO2复合镀层,确定了制备复合电极材料的最佳配方及操作条件：Pb(NO3)2 250g/L、HNO3 15g/L、WC 40g/L、ZrO2 50g/L、温度50℃、电流密度3A/dm2、电沉积时间4h。在此条件下,可获得β-PbO2-6.56wt.% WC-3.74wt.% ZrO2综合性能较好的复合电极材料,沉积厚度为408μm,显微硬度为723Hv。β-PbO2-6.56wt.% WC-3.74wt.% ZrO2镀层在硫酸中的催化活性和耐蚀性最好。纯β-PbO2镀层的晶粒呈金红石结构。掺WC的β-PbO2镀层中的晶胞较大,晶胞之间有裂纹,晶胞由许多纳米球形形状的晶粒堆积而成,在高放大倍数下发现WC颗粒呈不规则形状；β-PbO2晶粒形状轮廓模糊。掺纳米ZrO2的β-PbO2镀层中的晶粒细小且均匀,β-PbO2晶粒轮廓清晰,呈八面体结构,ZrO2纳米颗粒均匀地镶嵌在基质β-PbO2中。掺纳米ZrO2和WC的β-PbO2镀层中晶粒更加细小,纳米ZrO2和WC弥散分布于β-PbO2基体中,极大地增加了镀层表面的粗糙度。掺WC的β-PbO2复合镀层中固体微粒WC上发生了二氧化铅的电沉积反应、掺纳米ZrO2的β-PbO2复合镀层中纳米ZrO2上未发生电化学反应,即ZrO2颗粒未进入β-PbO2的晶格中,只掺杂在晶界中,但其具有吸附二氧化铅晶粒的能力。WC和ZrO2能相互产生协同作用。三种掺杂的复合镀层与纯β-PbO2镀层的衍射峰相比：(1)掺WC的β-PbO2复合镀层的衍射峰强度明显减弱了,且晶面间距d值变大了。WC颗粒的衍射峰在掺WC的β-PbO2复合镀层衍射峰中表现突出,尤其是WC的晶面(110)和(101),甚至比最强的p(211)衍射峰还强。(2)纳米ZrO2可抑制α-PbO2晶体的产生,掺杂纳米ZrO2能使β-PbO2镀层的晶面发生择优取向。(3)掺WC和ZrO2的β-PbO2复合镀层发现了新相PbWO4,并且PbWO4在复合镀层中的衍射峰最强。掺杂固体颗粒的β-PbO2复合电极的寿命都比纯β-PbO2电极的长,尤其是A1基α-PbO2-CeO2-TiO2/β-PbO2-6.58wt.% WC-3.78wt.%ZrO2电极在硫酸溶液中高电流密度2A/cm2下的寿命可达441h。通过MnO2、PbO2和WC、ZrO2颗粒的复合电沉积,在A1基α-PbO2-CeO2-TiO2上制备了催化活性好的β-PbO2-MnO2-WC-ZrO2复合镀层。当电流密度和硝酸锰浓度分别控制在1A/dm2和80g/L时,复合镀层的固体颗粒含量达到最高值,分别为6.63%和3.49%左右,此电极用于高电流密度2A/cm2条件下电解时,寿命可达到368h,锌电积的槽电压比Pb-1wt.%Ag合金电极低0.4V。电流密度升高,镀层中的晶粒会长大,镀层中的二氧化锰含量将降低,但对镀层相结构影响不大；增加硝酸锰含量,复合镀层的晶粒得到细化,二氧化锰的含量将呈指数增加；当浓度超过80g/L时复合镀层因内应力作用而产生裂纹,镀层将由晶态转化为混晶态。更多还原

【Abstract】 At present, the lead alloys containing small amounts of silver, tin, calcium or antimony have been widely used as insoluble anodes in zinc electrowinning industry. The anodes can meet the needs of zinc electrowinning, but oxygen overpotential of the anodes is still high. Lead ions dissolved in the electrolyte can be reduced on the cathode and contaminate the cathode zinc. A new type of PbO2-coated metal anode, which has been widely used in electrolysis, is made up of four layers. The base is made of titanium plate and is covered with a conductive undercoating(such an undercoating is necessary for protecting the substrate from passivation) as bottom. The intermediate coating is composed ofα-PbO2, andβ-PbO2 is electrodeposited as the surface layer. Titanium is not, however, a viable substrate for practical electrodes in electrodepositing nonferrous metals for its cost. Aluminum is relatively cheap and has a good conductivity. The electrode material by electrodepositing lead dioxide on Al substrate has a huge market prospects.Firstly, thermodynamic analysis for the feasibility of electrodepositing PbO2 and MnO2 from aqueous solution was discussed. Secondly, lead dioxide on Al substrate was prepared through anodic deposition from aqueous solutions. Foreign ions or fine particles were added to the electrolyte aims at incorporate with the lead dioxide. Therefore, the electrocatalytic activity and the stability of the PbO2 electrodes can be enhanced. For example, the intermediate coating, consisting ofα-PbO2-CeO2-TiO2, was prepared by electrodepositing lead dioxide with doping nano-titanium dioxide and rare-earth oxide. The surface coating, consisting ofβ-PbO2-WC-ZrO2(orβ-PbO2-MnO2-WC-ZrO2), was prepared by doping WC and nano-ZrO2 particles. The morphology, crystal phase, surface composition, microstructure, particles size and microhardness of the electrode were characterized by means of SEM, XRD, EDS and so on. The electrocatalytic properties of electrodes have also been studied by linear sweep voltammetry, A.C.Impedance, Tafel plot and Cyclic voltammetry. The anticipated service lives of electrodes were measured by accelerated life test using in 150g/L H2SO4 solution at a current density of 2A/cm2. The electrocatalytic activity and service life of the composite layers in the oxygen evolution reaction have been investigated with aim of assessing the effect of fine particles. The E-pH diagrams of Pb-H2O and Mn-H2O system were calculated and drawn by a number of thermodynamic data. The results showed that the potential of electrodepositedα-PbO2 andγ-MnO2 were changed more obviously than that ofβ-PbO2 by increasing the temperature. On the basis of standard redox potentials, anodic deposition of MnO2 was easier than deposition of PbO2.α-PbO2、α-PbO2/β-PbO2 (NO3-) andα-PbO2/β-PbO2 (CH3COO-) were obtained from alkaline、nitrate and acetate bath, respectively. The surface roughness ofα-PbO2/β-PbO2 (NO3-) on A1 substrate was the largest, resulting in a considerable electrocatalytic activity. The solid solubility ofα-PbO2 inβ-PbO2 (NO3-) presented the maximal value, providing a way to improve the conductivity and corrosion resistance. The structural models of A1/α-PbO2 and A1/α-PbO2/β-PbO2 (NO3-) electrodes were proposed. In the reaction mechanisms for oxygen evolution, the relatively high values of Tafel slope were interpreted by the decreasing of the number of activity particles covered on electrode surface.The electrodeposition process ofα-PbO2 coating on the A1 substrate was studied. Applied current density during the deposition of PbO2 had a strong influence on the morphology of the prepared film. A compact and uniform layer of lead dioxide was obtained at the current density≤3mA/cm2. A further increase in current density resulted in a smaller particle with high porosity. The morphology and particle size distribution could be improved when the HPbO2- concentration was 0.12M or higher and the bath temperature was 40℃. However, theα-PbO2 coating with a few porosities was obtained at a very long plating time. The PbO2 deposited in alkaline conditions was highly non stoichiometric. The PbO impurities were formed on the surface of the electrode besides theα-PbO2 phase.Theα-PbO2-CeO2-TiO2 composite electrodes prepared by electrochemical anodic codeposition technique by doping CeO2 and TiO2 in alkaline bath were studied. The best formula and process conditions were got as follows:4M NaOH with litharge PbO, current density 0.5A/dm2, TiO2 15g/L, temperature 40℃, electrodeposition time 3h, CeO2 10g/L. On this condition, theα-PbO2-(3.77 wt.%)TiO2-(2.13 wt.%)CeO2 codeposited electrode material could be prepared. The thickness, hardness and film density of composite coating was better than that of the pureα-PbO2 coating. The apparent energies of activation of theα-PbO2-(3.77wt.%)TiO2-(2.13wt.%)CeO2 electrode was the lowest in alkaline bath. In the potential of 0.35V, the lead dioxide deposition was influenced by the kinetic control and diffusion control. In H2SO4 solution, the Al substrateα-PbO2-(3.77wt.%)TiO2-(2.13wt.%) CeO2 electrode showed the best catalytic activity and corrosion resistance. The Guglielmi model for CeO2 and TiO2 codeposition withα-PbO2 was proposed.WC-ZrO2/β-PbO2 composite coatings were prepared on the Al/α-PbO2-CeO2-TiO2 surface by anodic codeposition ofβ-PbO2, nano-ZrO2 and WC particles. The best formula and process conditions were got as follows:Pb(NO3)2 250g/L, HNO3 15g/L, WC 40g/L, ZrO2 50g/L, temperature 50℃, current density 3.0A/dm2, electrodeposition time 4h. On this condition, theβ-PbO2-(6.56wt.%)WC-(3.74wt.%) ZrO2 codeposited electrode material with a thickness of 408μm and a microhardness of Hv 723 could be prepared. Theβ-PbO2-(6.56wt.%)WC-(3.74wt.%) ZrO2 showed the best electrocatalytic activity and good corrosion resistance.A typical rutile structure was observed in undoped-β-PbO2 coating. The doped-WC-β-PbO2 coating was composed of a lot of bigger crystal cell, and there was a larger crack between the crystal cells. The crystal cell was made up of the clusters of nano-grains. The WC particles had the random structure on the composite coating. In addition, the composite coating possessed an unclear outline of the rutile crystallite ofβ-PbO2. The doped nano-ZrO2-β-PbO2 coating had a fine grain and uniform distribution besides a clear outline ofβ-PbO2 structure. The composite coating possessed a uniform and dispersive distribution of nano-ZrO2 particles. The doped nano-ZrO2-WC-β-PbO2 electrode possessed more fine grain, and a uniform, dispersive distribution of nano-ZrO2 and WC particles within theβ-PbO2 matrix oxide. The codeposition of WC and nano-ZrO2 particles induced a markedly increase in the roughness of PbO2 deposits. The new crystallites were easily nucleated on the WC particle. The new crystallites could not be nucleated on nano-ZrO2 particles. However, they could absorbe the nano-ZrO2 particles. There was a synergic effect of the nano-ZrO2 and WC particles.Compared with undoped-β-PbO2, the intensities of the undoped-WC-β-PbO2 peaks became less intense, and the halfwidth increased as the degree of crystallinity decrease. The peak intensity of WC phase was high in the doped-WC-β-PbO2 composite coating, especially the WC(110) and WC(101) planes, which were higher than theβ(200) andβ(211). Compared with undoped-β-PbO2, the doped-nano-ZrO2 particle could decrease the content of a-phase. The phase obtained in the doped-ZrO2-β-PbO2 composite coating had a strong preferential orientation of the crystallites. The presence of peak(2θ=27.43,32.75,44.738°) was regarded as the attributive indicator of the tetragonal PbWO4, and the PbWO4(112)’s peak was the highest intensity. The service life of the composite coating was longer than the undoped-β-PbO2 in H2SO4 solution, especially the anticipated service life of Al/α-PbO2-CeO2-TiO2/β-PbO2-(6.58wt.%)WC-(3.78wt.%) ZrO2 which could reach 20.1y in an industrial current density(1000A/m2).WC-ZrO2/β-PbO2-MnO2 composite coatings were prepared on the Al/α-PbO2-CeO2-TiO2 surface by anodic codeposition of PbO2、MnO2、nano-ZrO2 and WC particles. When the current density and Mn(NO3)2 concentration in electrolyte were controlled at 1.0A/dm2 and 80g/L, the contents of the nano-ZrO2 and WC particles of the composite coatings could reach 6.63% and 3.49%, respectively, and the service life of the composite electrode could reach 368h in a current density 2A/cm2. The increase of the current density leaded to the upgrowth of the grain structure and decreased the content of MnO2 in the composite coating. However, the increase of the current density had a little effect on the phase. The increase of the Mn(NO3)2’s concentration made the grain structure refine, and the content of MnO2 of the composite coating increased exponentially. When Mn(NO3)2 concentration >80g/L, the crack of the composite coating appeared and the structure of deposit changed from crystalline to amorphous step by step.更多还原