Titanium Anodes with an Active Coating Based on Iridium Oxides: The Effect of the Coating's Thickness,Porosity, and Morphology on Its Stability,Selectivity, and Catalytic Activity

The corrosion resistance and the electrochemical behavior of oxide iridium-ruthenium titanium anode (OIRTA) containing 4 mol % RuO₂ + 26 mol % IrO₂ + 70 mol % TiO₂, with a relatively thick active coating (iridium load 15 g m^?2) are studied in conditions of chlorine electrolysis. It is established that polarization curves for the chlorine evolution at low currents exhibit an extended “Tafel” segment with a “Nernstian” slope equal to 0.036 V and that the process rate is limited by the chlorine diffusion away from the electrode surface. In the region of high currents, which is precisely where the chlorine evolution reaction is realized in the industry, polarization curves start displaying an extended, practically horizontal, segment of low polarizability (SOLP) and the chlorine evolution occurs out of the entire depth of the coating. It is shown that the rate of iridium dissolution out of these anodes is in excess of the rate of its dissolution out of OIRTA with a thin coating and the larger the iridium load in the coating, the larger the excess. This phenomenon is attributed to a higher porosity of OIRTA with a thick coating and to the occurrence of the process of iridium dissolution out of the coating throughout the entire depth of the coating. As a result, such an increase in the coating's thickness is likely to lead to a decrease in the lifetime of the anodes. It is discovered that a prolonged polarization of OIRTA in the region of the SOLP leads to an increase in the overvoltage and to a practically complete disappearance of the SOLP from the polarization curves. All this served as the grounds for our drawing the conclusion that it would make no sense to enlarge the thickness and increase the porosity of the active coating of the OIRTA anodes in order to enhance their catalytic activity. It proved manageable to produce substantially more efficient anodes by depositing a thin active coating onto rough titanium out of a diluted covering solution. In so doing, the OIRTA anodes possessed a higher corrosion resistance and a better selectivity at a small iridium load in the coating.     

添加碳纳米管对钌铱锡氧化物阳极性能的影响

本文采用热分解法制备Ti/Ru-Ir-Sn氧化物阳极,运用TGA、SEM、EDS、循环伏安、电化学交流阻抗、极化曲线以及强化电解寿命测试方法,探讨添加不同含量碳纳米管（CNTs）对Ti/Ru-Ir-Sn氧化物阳极的影响规律. 热重分析和阳极能谱数据表明,在烧结温度470 ^oC条件下CNTs未发生高温氧化分解,仍以单质形式存在阳极涂层中. 与对比样品相比,涂层表面裂纹增多,但仍保持典型形貌. 添加CNTs显著改善了Ti/Ru-Ir-Sn氧化物阳极的电化学性能,增大了阳极的活性表面积,减小了涂层内部电阻,提高了阳极析氯电催化活性,析氯电位增加缓慢,阳极强化电解寿命也得到明显延长. 其中,添加0.1 g·L^-1 CNTs的Ti/Ru-Ir-Sn氧化物阳极电催化活性和稳定性最优.     

Kinetics of chlorine evolution on dimensionally stable anodes at high currents: Extending the concept of a self-accelerating electrode process

The chlorine evolution kinetics are studied by taking steady-state polarization measurements on rotating disc electrodes (dimensionally stable ones and those made of RuO₂) in solutions with various chloride concentration, at various acidity and temperatures, at current densities of up to 4 A/cm². At high temperatures and chloride concentrations, linear parts with an abnormally low slope of about 20 mV are observed in anodic polarization curves. Their appearance is attributed to the chlorine reaction self-acceleration due to a change in the chlorine mass transport mechanism in the anode’s porous coating.     

Gd0.2Ce0.8O1.9纳米颗粒对固体氧化物电解池La0.6Sr0.4Co0.2Fe0.8O3-δ阳极 析氧反应的影响

化石燃料的使用排放了大量CO2,对气候和环境造成了日益严重的危害.固体氧化物电解池(SOEC)能够利用可再生能源产生的电能将CO2高效转化成CO,降低CO2排放的同时,又能减少化石燃料的使用,近年来受到研究者的广泛关注.相比于低温液相CO2电还原,SOEC高的运行温度保证了其较高的反应速率,即较高的电流密度.典型的SOEC单电池由多孔阴极、致密电解质和多孔阳极以三明治的方式组装而成.CO2分子在阴极得到两个电子解离成CO和一个O2–;生成的O2–通过致密电解质传导至阳极,在阳极失去四个电子发生析氧反应(OER)生成一个O2.相比于两电子的阴极反应,阳极四电子的析氧反应更难进行,可能是整个电极过程的速控步,因此开发高性能的阳极材料有望显著提高SOEC的CO2电还原性能.La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF)因具有较高的混合离子-电子导电性而被用作SOEC阳极材料,但受LSCF-气体两相界面的限制,其OER性能较低.研究表明,LSCF-掺杂的CeO2-气体所构成的三相界面相比于LSCF-气体两相界面具有更高的电化学反应活性,即OER反应更易在三相界面进行.因此,本文将Gd0.2Ce0.8O1.9(GDC)纳米颗粒浸渍到SOEC LSCF阳极来提高其OER活性,考察了纳米颗粒浸渍量(3,5,10和20 wt%)对SOEC电化学性能的影响.结果表明,SOEC的电化学性能随浸渍量的增加而逐渐升高,当GDC纳米颗粒浸渍量为10 wt%时(10GDC/LSCF),SOEC的电化学性能达到最高,在800 oC和1.6 V的电流密度为0.555 A cm–2,是LSCF阳极SOEC性能的1.32倍.继续增加浸渍量到20 wt%,电化学性能反而开始下降.电化学阻抗谱测试结果表明,GDC纳米颗粒的加入减小了SOEC的极化电阻.对应的弛豫时间分布函数解析结果表明10GDC/LSCF阳极上的OER由四个基元反应构成.电镜和O2-程序升温脱附结果表明,GDC纳米颗粒的加入显著增加了10GDC/LSCF阳极三相界面和表面氧空位的数量以及体相氧的流动性,从而促进了OER四个基元反应的反应速率,降低了这几个过程的极化电阻,因而降低了OER反应的极化电阻,提高了SOEC电还原CO2的电化学性能.     

Kinetics for the Oxygen Evolution Reaction and Application of the Ti/SnO<Subscript>2</Subscript> + RuO<Subscript>2</Subscript> + MnO<Subscript>2</Subscript> Electrode

A Ti/SnO₂ + RuO₂ + MnO₂ electrode was prepared by thermal decomposition of their salts. Results from SEM and XPS analyses, respectively, indicate that the coating layer exhibits a compact structure and the oxidation state of Mn in the coating layer is +IV. The experimental activation energy for the oxygen evolution reaction, which increased linearly with increasing overpotential, is about 8 kJ⋅mol⁻¹ at the equilibrium potential (η=0). The electrocatalytic characteristics of the anode are discussed in terms of ligand substitution reaction mechanisms (Sn1 and Sn2). It was found that the transition state for oxygen evolution at the anode in acidic solution follows a dissociative mechanism (Sn1 reaction). The Ti/SnO₂ + RuO₂ + MnO₂ anode in conjunction with UV illumination was used to degrade phenol solutions, where the concentration of phenol remaining was determined by high-performance liquid chromatography (HPLC). The results indicate that the degradation efficiency of phenol on the anode can reach 96.3% after photoelectrocatalytic oxidation for 3 h.     

Comparison of morphology, stability and electrocatalytic properties of Ru0.3Ti0.7O2 and Ru0.3Ti0.4Ir0.3O2 coated titanium anodes

This paper presents the comparative study of two dimensionally stable anodes (DSA?) of nominal composition Ti/Ru_0.3Ti_0.7O₂ and Ti/Ru_0.3Ti_0.4Ir_0.3O₂, prepared by thermal decomposition of the chloride precursor mixtures at 450°C. The materials were studied by scanning electron microscopy (SEM), cyclic voltammetry and Tafel measurements to obtain information about their surface and electrocatalytic properties towards Cl₂ evolution reaction. The stability of the prepared anodes was investigated under accelerated conditions. It has been observed that the coating surface with 30% mole IrO₂ possesses more compact structure with less cracks. Furthermore, it had excellent electrocatalytic activity for the chlorine evolution. Accelerated stability tests showed long lifetime for Ti/Ru_0.3Ti_0.4Ir_0.3O₂ electrode. On the other hand, besides the excellent improvement of catalytic activity, the stability of the ternary oxide electrode increases compared to the binary oxide one. The deactivation rate appeared to be a consequence of the different morphology and synergetic effects of the prepared coatings.     

低温海水用钛基金属氧化物阳极的制备与性能

将Ti N纳米粉体与Ta Cl5正丁醇饱和溶液混合制得中间层涂覆液,通过热分解法在不同焙烧温度下得到了含有中间层的Ti/(Ti-Tax)O2/Ir O2电极,并与相同工艺下得到的传统Ti/Ir O2电极进行对比分析.采用X射线衍射和扫描电子显微镜对制备的电极进行了表征,通过循环伏安曲线、极化曲线和恒流加速寿命测试等电化学手段对电极性能进行了分析.结果表明,引入中间层可以使Ti/(Ti-Tax)O2/Ir O2电极400℃低温焙烧样品表层Ir O2结晶发育更好,得到了通常高温下才具有的典型龟裂纹形貌;相对于500℃高温焙烧样品,其在海水中的电化学活性表面积提高近6倍,在4℃和100 m A/cm2电流密度下的电极电位(1.37 V)降低100 m V,催化性能得到显著提高;同时加速寿命相对于传统的Ti/Ir O2电极提高10倍以上,是一种适用于低温海水环境的、具有较高活性和耐久性的新型阳极.     

Tantalum oxide effect on the surface structure and morphology of the IrO<Subscript>2</Subscript> and IrO<Subscript>2</Subscript> + RuO<Subscript>2</Subscript> + TiO<Subscript>2</Subscript> coatings and on the corrosion and electrochemical properties of anodes prepared from these

Alterations in the phase composition, porosity, and surface morphology of coatings are examined following the insertion of a quantity of Ta₂O₅ into active coatings prepared from IrO₂ or IrO₂ + RuO₂ + TiO₂ (OIRTA). It is shown that even an insignificant concentration of Ta₂O₅ in a coating renders it substantially amorphous and leads to the appearance of a large number of wide protracted cracks in the coating. The latter extends the surface of anodes and boosts their apparent catalytic activity in the chlorine evolution reaction. In addition, this accelerates the diffusion of chloride ions toward the front surface of anodes, which noticeably reduces the overvoltage of the chlorine evolution reaction when manufacturing sodium chlorate. The coatings’ amorphization and the development of their surface substantially reduce the corrosion resistance of these anodes as compared with OIRTA.     

An alternative approach to selective sea water oxidation for hydrogen production

Sea water electrolysis is one of the promising ways to produce hydrogen since it is available in plentiful supply on the earth. However, in sea water electrolysis toxic chlorine evolution is the preferred reaction over oxygen evolution at the anode. In this work, research has been focused on the development of electrode materials with a high selectivity for oxygen evolution over chlorine evolution. Selective oxidation in sea water electrolysis has been demonstrated by using a cation-selective polymer. We have used a perm-selective membrane (Nafion^®), which electrostatically repels chloride ions (Cl⁻) to the electrode surface and thereby enhances oxygen evolution at the anode. The efficiency and behaviour of the electrode have been characterized by means of anode current efficiency and polarization studies. The surface morphology of the electrode has been characterized by using a scanning electron microscope (SEM). The results suggest that nearly 100% oxygen evolution efficiency could be achieved when using an IrO₂/Ti electrode surface-modified by a perm-selective polymer.     

Titanium Anodes with Active Coatings Based on Iridium Oxides: An Attempt to Elucidate the Possibility of Enhancing Catalytic Activity and Corrosion Resistance of Anodes at the Expense of Variations in the Formation Regime of the Coatings

On the basis of the polarization, corrosion, and radiotracer measurements it is established that the optimum conditions for the deposition of active coatings consisting of IrO₂ and IrO₂ + TiO₂ onto titanium anodes are the performing of the pyrolysis in air at T = 350°C for 15 min with a final anneal in the same environment at T = 450°C for 1 h. Removing the final anneal or reducing its temperature enhances the catalytic activity of the anodes but at the same time reduce their corrosion resistance. Raising the anneal temperature above 450°C makes no sense, as the catalytic activity of the anodes toward the chlorine evolution reaction substantially diminishes and the titanium support undergoes oxidation starting with 500°C.     

合金元素Nd对Pb-Ag阳极在H2SO4溶液中电化学行为的影响

采用循环伏安、线性扫描、电化学阻抗和环境扫描电镜对比研究了Pb-Ag和Pb-Ag-Nd阳极的阳极膜和析氧反应. 结果表明,合金元素Nd促进了Pb/PbO_n/PbSO₄ (1≤n<2)膜层的生长. 在高极化电位区间(高于1.20V (vs Hg/Hg₂SO₄/饱和K₂SO₄溶液)),Nd有利于低价铅的化合物(PbO_n,PbSO₄)向α-PbO₂和β-PbO₂转变. 此外,环境扫描电镜形貌和线性扫描分析证明Pb-Ag-Nd表面生成的阳极膜较Pb-Ag的阳极膜更厚且更致密. 因此,Pb-Ag-Nd阳极表面的阳极膜可以给合金基底提供更好的保护. 另一方面,电化学阻抗测试揭示了两种阳极的析氧反应均受中间产物的形成和吸附控制. Nd可以降低阳极膜/电解液界面处中间产物的吸附阻抗且增加中间产物的覆盖率,从而提高析氧反应活性. 综上所述,合金元素Nd可提高Pb-Ag阳极的耐腐蚀性,降低阳极电位进而起到节能降耗的作用.     

Oxidation of phenol on RuO2–TiO2/Ti anodes

The oxidation of phenol on the RuO₂–TiO₂/Ti electrode has been studied by cyclic voltammetry, polarization measurements, electrochemical impedance spectroscopy and potentiostatic transients in H₂SO₄ and NaCl aqueous solutions. A reaction path with polymerization as the main reaction and side reactions after the initial step, similar to the reaction path on other electrode materials, is suggested. The formation of a phenoxy radical in a diffusion-controlled irreversible process is the initial step. The polymerization of phenoxy radicals leads to the formation of porous polyoxyphenylene film, strongly adherent to the electrode surface. The cyclic voltammetry measurements indicate side products, which could be, according to the literature, of quinone-like structure. Polyoxyphenylene film inhibits further oxidation of phenol, although complete electrode passivation was not observed. The presence of polyoxyphenylene film does not influence the pseudocapacitive behaviour of the electrode to a great extent, since the polyoxyphenylene film covers dominantly the coating surface, while active sites placed within coating cracks remain uncovered. The film seems to be permeable for hydrogen ions and water molecules.     

Corrosion and electrochemical behavior of metal-oxide anodes during electrolysis with an ion-exchange membrane

The reasons for a specific behavior of anodes in chlorine electrolysis with an ion-exchange membrane are considered. The corrosion rate in modeling conditions and the predicted lifetime of anodes of different compositions are studied. The anodes’ coatings contain mixed oxides of Ir, Ru, Sn, or Ti. The effect of the anode’s position relative to the membrane and a relative stability of the anode coatings are examined in conditions simulating a heavy alkalization of the anolyte caused by a membrane rupture. A considerable advantage of using the anode with a coating containing 15, 15, and 70 mol % of RuO₂, IrO₂, and TiO₂ is demonstrated     

Electrocatalytic activity and stability of Ti/IrO2 + MnO2 anode in 0.5 M NaCl solution

Ti/IrO₂(x) + MnO₂(1-x) anodes have been fabricated by thermal decomposition of a mixed H₂IrCl₆ and Mn(NO₃)₂ hydrosolvent. Cyclic voltammetry (CV) and polarization curve have been utilized to investigate the electrochemical behavior and electrocatalytic activity of Ti/IrO₂(x) + MnO₂(1-x) anodes in 0.5 M NaCl solution (pH = 2). Ti/IrO₂+MnO₂ anode with 70 mol% IrO₂ content has the maximum value of q*, indicating that Ti/IrO₂(0.7) + MnO₂(0.3) anode has the most excellent electrocatalytic activity for the synchronal evolution of Cl₂ and O₂ in dilute NaCl solution. Tafel lines displayed two distinct linear regions with one of the slope close to 62 mV dec⁻¹ in the low potential region and the other close to 295 mV dec⁻¹ in the high potential region. Electrochemical impedance spectroscopic is employed to investigate the impedance behavior of Ti/IrO₂(x) + MnO₂(1-x) anodes in 0.5 M NaCl solution. It is observed that as the R _ct, R _s and R _f values for Ti/IrO₂(0.7) + MnO₂(0.3) anode become smaller, electrocatalytic activity of Ti/IrO₂(0.7) + MnO₂(0.3) anode becomes better than that of other Ti/IrO₂ + MnO₂ anodes with different compositions. Ti/IrO₂(0.7) + MnO₂(0.3) anode fabricated at 400 °C has been observed to possess the highest service life of 225 h, whereas the accelerated life test is carried out under the anodic current of 2 A cm⁻² at the temperature of 50 °C in 0.5 M NaCl solution (pH = 2).     

LiNbO3不同晶面的光催化产氢性能

研究了LiNbO₃（001）、（100）和（110）晶面的光催化产氢性能。（001）、（100）和（110）3个晶面光催化产氢性能之比为7.8：1.3：1.0。LiNbO₃[001]晶向存在电偶极矩和自发极化,有利于增加光生电子和空穴的分离效率,减少光生电子和空穴的复合,提高LiNbO₃（001）面的光催化活性。LiNbO₃（001）面的空穴有效质量最小,有利于光生空穴的迁移,从而减少光生电子和空穴的复合,也有利于光催化性能的提高。     

A novel method to fabricate lithium-ion polymer batteries based on LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>/NG electrodes

A novel method to fabricate lithium-ion polymer batteries (LiPBs) has been developed. The LiPBs was fabricated without microporous polyolefin separators, taking spinel lithium manganese oxide (LiMn₂O₄) and natural graphite (NG) as the electrodes. The thicknesses of the cathodes and the anodes are 190 and 110 μm, respectively. The NG anode was coated with a microporous composite polymer film (20 μm thick) which composed of polymer and ultrafine particles. The coating process was effective and simple to be used in practical application, and ensured the composite polymer film to act as a good separator in the LiPB. The LiPBs assembled with the coated NG anodes and pristine LiMn₂O₄ cathodes presented better electrochemical performances than liquid lithium-ion battery counterparts, proving that the microporous composite polymer film can improve the performance of the coated NG anode. In this paper, the spinel LiMn₂O₄/(coated)NG-based LiPBs exhibited high rate capability, compliant temperature reliability, and significantly, excellent cycling performance under the elevated temperature (55°C).     

LiNbO_3不同晶面的光催化产氢性能

研究了LiNbO3(001)、(100)和(110)晶面的光催化产氢性能。(001)、(100)和(110)3个晶面光催化产氢性能之比为7.8∶1.3∶1.0。LiNbO3[001]晶向存在电偶极矩和自发极化,有利于增加光生电子和空穴的分离效率,减少光生电子和空穴的复合,提高LiNbO3(001)面的光催化活性。LiNbO3(001)面的空穴有效质量最小,有利于光生空穴的迁移,从而减少光生电子和空穴的复合,也有利于光催化性能的提高。     

Ni-Co/RuO2复合电极的制备及其电催化析氢性能

采用快速凝固结合去合金化的方法制备纳米多孔Ni-Co合金,利用RuO_2对Ni-Co合金进行表面修饰,通过X射线衍射(XRD)、X射线光电子能谱(XPS)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)对多孔材料进行物相分析和形貌表征,并通过线性扫描伏安法、多电位阶跃法、交流阻抗法和恒电流电解法测试多孔电极的电催化析氢性能。结果表明,Ni-Co/RuO_2复合电极材料在50 mA·cm~(-2)电流密度下析氢过电位为180 mV,析氢过程由Volmer-Heyrovsky步骤控制,交换电流密度为4.42 mA·cm~(-2),经10 h恒电流电解后电位仅增加20 mV,表现出良好的析氢稳定性。     

RuO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> mixed oxide composite coating for improvement of Al-alloy sacrificial anodes

It has been recently proved that RuO₂ can act as an effective surface activator of aluminum alloy sacrificial anodes. TiO₂ has the property of stabilizing RuO₂ coating and resisting biofouling on metal surfaces. Hence, a mixed oxide catalytic coating of TiO₂ and RuO₂ can enhance the galvanic performance of aluminum alloy sacrificial anodes and resists biofouling on the anode surface. In the present work RuO₂–TiO₂ mixed oxide was coated on aluminum alloy sacrificial anodes. The large and uniform porous nature of the coating was found to facilitate efficient ion diffusion. The coating was found to persist on the anode even after 3 months of galvanic exposure. The anode having an optimum combination of the mixed oxide had 70% TiO₂ as the major component in the coating. The catalytic coating significantly improved the performance of the anodes to a large extent.     

Self-accelerating chlorine evolution on porous anodes of finite thickness

Operation of a finite-thickness porous electrode under the chlorine evolution conditions is analyzed in the framework of the convective diffusion model. According to calculations, the gas evolution in the anode’s porous coating is a necessary but insufficient condition for the self-acceleration of the electrode process. Despite the gas evolution process in the anode pores, the anodic process on the low-activity electrodes decelerates, which is externally manifested in an increased Tafel slope of the polarization curve as compared with that for a smooth electrode. Self-acceleration of the anodic chlorine evolution takes place only on electrodes with true exchange currents in excess of 10^-4 A cm^-2. Externally, the self-acceleration effect manifests itself in the emergence of a low-polarizability portion in the high-current region of the polarization curve. Such a different effect of the gas evolution process on the chlorine reaction kinetics at porous electrodes of different catalytic activity is due to an altered balance between the diffusive and convective current constituents in the overall process rate following a change in the exchange current for either electrode.