Discrete modelling of the electrochemical performance of SOFC electrodes

The composite anode and cathode of solid oxide fuel cells (SOFC) are modelled as sintered mixtures of electrolyte and electrocatalyst particles. A particle packing is first created numerically by the discrete element method (DEM) from a loose packing of 40 000 spherical, monosized, homogeneously mixed, and randomly positioned particles. Once the microstructure is sintered numerically, the effective electrode conductivity is determined by discretization of the particle packing into a resistance network. Each particle contact is characteristic of a bond resistance that depends on contact geometry and particle properties. The network, which typically consists of 120 000 bond resistances in total, is solved using Kirchhoff's current law. Distributions of local current densities and particle potentials are then performed. We investigate how electrode performance depends on parameters such as electrode composition, thickness, density and intrinsic material conductivities that are temperature dependent. The simulations show that the best electrode performance is obtained for compositions close to the percolation threshold of the electronic conductor. Depending on particle conductivities, the electrode performance is a function of its thickness. Additionally, DEM simulations generate useful microstructural information such as: coordination numbers, triple phase boundary length and percolation thresholds.     

Fabrication and characterization of LATP/PAN composite fiber-based lithium-ion battery separators

Lithium aluminum titanium phosphate (LATP)/polyacrylonitrile (PAN) composite fiber-based membranes were prepared by electrospinning dispersions of LATP particles in PAN solutions. The electrolyte uptakes of the electrospun LATP/PAN composite fiber-based membranes were measured and the results showed that the electrolyte uptake increased as the LATP content increased. The lithium ion conductivity, the electrochemical oxidation limit and the interface resistance of liquid electrolyte-soaked electrospun LATP/PAN composite fiber-based membranes were also measured and it was found that as the LATP content increased, the electrospun LATP/PAN composite fiber-based membranes had higher lithium ion conductivity, better electrochemical stability, and lower interfacial resistance with lithium electrode. Additionally, lithium//1 M LiPF₆/EC/EMC//lithium iron phosphate cells using LATP/PAN composite fiber-based membranes as the separator demonstrated high charge/discharge capacity and good cycle performance.     

Investigation of the active thickness of solid oxide fuel cell electrodes using a 3D microstructure model

A 3D microstructure model is used to investigate the effect of the thickness of the solid oxide fuel cell (SOFC) electrode on its performance. The 3D microstructure model, which is based on 3D Monte Carlo packing of spherical particles of different types, can be used to handle different particle sizes and generate a heterogeneous network of the composite materials from which a range of microstructural properties can be calculated, including phase volume fraction, percolation and three phase boundary (TPB) length. The electrode model can also be used to perform transport and electrochemical modelling such that the performance of the synthetic electrode can be predicted. The dependence of the active electrode thickness, i.e. the region of the anode, which is electrochemically active, on operating over-potential, electrode composition and particle size is observed. Operating the electrode at an over-potential of above 200 mV results in a decrease in the active thickness with increasing over-potential. Reducing the particle size dramatically enhances the percolating TPB density and thus the performance of the electrode at smaller thicknesses; a smaller active thickness is found with electrodes made of smaller particles. Distributions of local current generation throughout the electrode reveal the heterogeneity of the 3D microstructure at the electrode/electrolyte interface and the dominant current generation in the vicinity of this interface. The active electrode thickness predicted using the model ranges from 5 μm to 15 μm, which corresponds well to many experimental observations, supporting the use of our 3D microstructure model for the investigation of SOFC electrode related phenomena.     

有机电解液在锂离子电池中的充放电机理

讨论了锂离子电池充放电过程中有机电解液的电化学行为,研究发现,有机电解液会在电极活性材料表面发生电化学反应而形成聚合物钝化层(SEI膜),其厚度和疏密性与电解液的组成及充放电制度有关;其组成和电化学性能还将直接影响锂离子电池的充放电容量和循环寿命。通过改变电解液的导电锂盐成分、有机溶剂组成和加入极性添加剂等方法可优化电解液的电化学特性,从而可有效控制该钝化层的成膜过程、膜组成与膜结构,提高锂离子电池的充放电及循环性能。     

Shaping of advanced ceramics: The case of composite electrodes for lithium batteries

Composite electrodes for electrochemical energy storage systems, such as lithium batteries, are mixtures of a ceramic powder, an electronic conducting agent, often carbon black, and a polymeric binder. Such a complex medium must provide efficient transport of electrons and ions from the current collector/electrode and electrolyte/electrode interfaces, respectively, to the grain surface of the ceramic. It seems quite obvious that the morphology within the composite electrode should have an influence on the electrode performance. However, such an issue has never been studied yet. Here we study composite electrodes prepared by the tape casting method, with the purpose to gain better understanding of the relationships between: the suspension properties, the morphology within the dried composite electrode, and the resulting electrochemical properties.     

α-PbO2-CeO2-TiO2复合电极材料的耐蚀性研究

将TiO2和CeO2颗粒添加到由PbO和NaOH组成的镀液中,通过阳极电沉积的方法在铝基体上制备了α-PbO2-TiO2-CeO2复合镀层,用作铝基二氧化铅电极的中间层。研究了CeO2及TiO2纳米微粒质量浓度及工艺条件对制备的复合电极耐蚀性能的影响。确定了最佳电解液组成为:将PbO加入到140g/LNaOH溶液中至饱和,15 g/LTiO2,10 g/LCeO2。温度40℃,电流密度0.5 A/dm2,电沉积时间3 h。在此条件下所得的复合镀层综合性能较好,复合镀层中w(TiO2)为3.74%,w(CeO2)为2.15%。     

Nano-structured composite cathodes for intermediate-temperature solid oxide fuel cells via an infiltration/impregnation technique

Solid oxide fuel cells (SOFCs) are high temperature energy conversion devices working efficiently and environmental friendly. SOFC requires a functional cathode with high electrocatalytic activity for the electrochemical reduction of oxygen. The electrode is often fabricated at high temperature to achieve good bonding between the electrode and electrolyte. The high temperature not only limits material choice but also results in coarse particles with low electrocatalytic activity. Nano-structured electrodes fabricated at low temperature by an infiltration/impregnation technique have shown many advantages including superior activity and wider range of material choices. The impregnation technique involves depositing nanoparticle catalysts into a pre-sintered electrode backbone. Two basic types of nano-structures are developed since the electrode is usually a composite consists of an electrolyte and an electrocatalyst. One is infiltrating electronically conducting nano-catalyst into a single phase ionic conducting backbone, while the other is infiltrating ionically conducting nanoparticles into a single phase electronically conducting backbone. In addition, nanoparticles of the electrocatalyst, electrolyte and other oxides have also been infiltrated into mixed conducting backbones. These nano-structured cathodes are reviewed here regarding the preparation methods, their electrochemical performance, and stability upon thermal cycling.     

Electrochemical supercapacitor properties of polyaniline thin films in organic salt added electrolytes

We demonstrate optimized supercapacitive characteristics of electrodeposited polyaniline by adding organic salt into electrolyte. The optimum amount of the organic salt is found to be 2 wt % which provides better ionic conductivity of the electrolyte, leading to the improved specific capacitance of 259 Fg^?1. This capacitance remains at up to 208 Fg^?1 (80% capacity retention) after 1000 charge–discharge cycles. The optimized organic salt added electrolyte causes better rate performance and higher cyclability. Significantly reduced electrochemical charge transfer resistance at the electrode/electrolyte interface results in the increased ionic conductivity, which can be useful in electrochemically preferred power devices for better applicability. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40306.     

中间相炭微球在锂离子电池负极材料的应用进展

中间相炭微球(MCMB)具有良好锂离子扩散性、导电性和机械稳定性等优势,是目前应用广泛、综合性能优异的锂离子电池负极材料,但较低理论比容量是制约其发展的关键因素。为了获得性能优良的MCMB基锂离子电池负极材料,改性修饰和复合材料已然成为目前研发重点。笔者论述了碳结构、表界面和复合材料等微观结构设计对MCMB负极材料电化学性能的影响。从碳堆积结构类型、有序性、层间距以及球体粒径大小等方面,论述了碳结构微观设计对MCMB电化学性能的影响。发现具有乱层结构的MCMB在充放电过程中内部产生应力较小,且碳结构较稳定,具有优异循环稳定性;内部具有大量微孔或碳层间距较大的MCMB,在充放电过程中可提高锂离子在电极中的迁移速率,并提供更多的储锂空间,一般具有优良的充放电比容量和倍率性能;小粒径MCMB具有较短的锂离子迁移路径和随之增加的比表面积,通常具有较好倍率性能,伴随着可逆比容量和充放电效率的衰减。从表界面碳层改性、包覆和掺杂改性等方面,论述了表界面改性对MCMB电化学性能的影响。表面碳层修饰可增加MCMB与电解液的相容性及其比表面积,提高了与电解液的接触面积及贮锂容量,改善了锂离子电池负极材料的电化学性能;另外,MCMB表面包覆一层无定型碳,可避免其表面与电解液直接接触,减少电化学副反应的产生,提升其可逆比容量。从碳活性物质复合材料、非碳活性物质复合材料等方面,论述了复合材料微观结构设计对MCMB电化学性能的影响。碳活性物质可降低MCMB内部碳层结构的有序性,减少锂离子嵌入过程中的内部应力,提升MCMB循环稳定性。非碳活性物质诱导MCMB生成更加有序的碳层结构,提高MCMB的比表面积,从而改善MCMB表面与电解液分子的接触能力及其嵌锂性能,有利于提升MCMB负极材料可逆比容量、循环性能和倍率性能。MCMB具有高碳层间距和多缺陷位点等结构特征,有利于钠离子自由脱嵌,应用于钠离子电池时具有良好的可逆比容量、循环稳定性和倍率性能。MCMB的不规则定向层状结构经活化等处理具有较高比表面积,可应用于超级电容器电极材料。最后提出在高性能锂离子电池电极材料快速发展的需求下,从微观结构角度设计MCMB纳米复合材料将是MCMB负极材料的研究重点。     

Fabrication and performance of advanced multi-layer SOFC cathodes

Multilayered cathodes for solid oxide fuel cells are presented. The cathodes are composed of strontium doped lanthanum manganate and yttria stabilised zirconia, the ratio of which is increased with increasing distance from a supporting zirconia electrolyte. Some cathodes additionally carry a number of layers with a graded transition from manganite to cobaltite to add an electronically highly efficient current collector. The cathodes were prepared by spray painting and low temperature sintering. Electrochemical measurements revealed a performance up to 0.2 Ω cm² at 750°C for the first generation of these cathodes. The electrochemical performance was found to be influenced further by the microstructure at the interface close to the solid electrolyte and the quality of electrical contact thorough the sintered electrode.     

Micro‐Scale Modeling of a Functionally Graded Ni‐YSZ Anode

A mathematical model was developed to investigate the coupled transport and electrochemical reactions in a nickel‐yttrial‐stablized zirconia (Ni‐YSZ) anode for use in solid oxide fuel cells (SOFCs). The modeling results were consistent with experimental data from the literature. Comparison between conventional non‐graded (uniform random composites) and two types of functionally graded electrodes (FGE), namely particle size graded and porosity graded SOFC anodes were conducted to evaluate the potential of FGE for SOFC. Improved performance of both types of FGE was observed due to reduced mass transport resistance and increased volumetric reactive surface area close to the electrode‐electrolyte interface. It was found that the particle size graded SOFC anode showed the best performance. This paper demonstrates that the SOFC performance could be enhanced by modifying the microstructures of the electrodes. The results presented in this paper provide a better understanding of the working mechanisms of SOFC electrodes and could serve as an important reference for design optimizations.     

Electrochemical investigations of bare and Pd-coated LaNi4.25Al0.75 electrodes in alkaline solution

Electrochemical investigations were carried out on electrodes made from both bare and Pd-coated LaNi4.25Al0.75 particles. Experimental results showed that the Pd-coating significantly decreases the electrode resistance and increases the electrode discharge capacity. The electrochemical impedance spectroscopy technique was used to determine various resistive components in the electrodes and electrolyte by fitting an equivalent circuit to the experimental data. The results indicated that an electrode made from the Pd-coated alloy has much less ohmic resistance (particle-to-particle contact resistance and current collector to electrode pellet contact resistance) compared to the electrodes made from bare alloy.     

Ni(OH)2/活性炭复合材料在超级电容器中的应用

用化学沉淀法在活性炭(AC)表面和微孔内掺杂不同量的氢氧化镍,制备了氢氧化镍-活性炭[Ni(OH)2-AC]复合材料. 用X射线衍射(XRD)和氮气吸附等温线等对活性炭和复合材料进行表征,结果表明,所制材料为b-Ni(OH)2-AC复合材料. 对不同掺杂量的b-Ni(OH)2-AC复合材料的电化学性能进行了研究,循环伏安、恒流充放电实验表明,少量氢氧化镍掺入活性炭表面和微孔中,所得材料的比电容较活性炭有所提高,并具有良好的充放电性能;当氢氧化镍的掺入量为6%(w)时,所制备的超级电容器单电极表现出优良的电化学性能. 以活性炭电极作负极,复合材料作正极制成复合型超级电容器,循环性能测试发现,掺入6%(w)氢氧化镍的复合材料制成的Ni(OH)2-AC/AC复合型超级电容器比电容高达330.7 F/g,比活性炭(AC/AC)超级电容器比电容(245.6 F/g)提高了34.6%,且Ni(OH)2-AC/AC复合型超级电容器具有更好的循环充放电性能.     

Modelling the porous cathode of a SOFC: oxygen reduction mechanism effect

A composite electrode comprising a porous mixture of solid electrolyte (typically yttria stabilized zirconia, YSZ) and electronically conducting, electrocatalytic material (typically strontium doped lanthanum manganite, LSM) is generally used to improve the cathodic performance of the solid oxide fuel cell (SOFC). The advantage of the composite electrode is that the reaction zone is spread from the electrode/electrolyte interface into the electrode, effectively resulting in a functionally diffuse interface where the charge transfer reaction occurs. The present study proposes a one-dimensional dc and ac model that takes into account mass and charge conservation, transport of species and reaction kinetics. It considers the porous electrode to be a homogeneous medium characterized by a number of parameters, and in particular ionic conductivity and the diffusion coefficient. The influence of kinetic and transport parameters as well as that of the microstructure on the shape of both polarization curves and impedance diagrams is discussed.     

LiFePO4锂离子电池的数值模拟：正极材料颗粒粒径的影响

LiFePO₄（LFP）作为正极材料时,锂离子电池安全性高且循环寿命长,是目前应用最广泛的正极材料,但其电池倍率性能较差。提升倍率性能的有效手段之一是将LFP材料颗粒纳米化,但材料纳米化过程中颗粒粒径减小对于锂离子电池充放电过程中锂在固液相的扩散及表面电化学反应的影响机制仍缺乏清晰的认识。采用锂离子电池的准二维模型,模拟锂离子电池的放电过程,定量研究了正极材料颗粒粒径对锂离子电池倍率性能的影响,分析了固液相扩散速率与电化学反应速率受LFP材料颗粒粒径的影响程度。研究结果表明：电极材料中固相扩散阻力是锂离子电池电化学性能的主要限制因素。小粒径的LFP作为正极材料时,电极材料内的金属锂的迁移路径较短,同时颗粒与电解液的接触面积增加,界面的电化学反应速率较快,放电倍率对于锂离子电池性能影响较小;大粒径的LFP作为正极材料时,电极材料内的金属锂扩散路径的增加和较高的固相扩散阻力限制了界面的电化学反应速率,导致锂离子电池的倍率性能显著降低。LFP材料的纳米化可以有效减小固相扩散阻力,提升锂离子电池的倍率性能。     

木质素基活性炭氮掺杂改性及其电化学性能

以磷酸法木质素基活性炭为原料,三聚氰胺为氮源、KOH为活化剂,采用同步掺杂方式制备了氮掺杂活性炭(NAC)。通过BET、XRD、拉曼光谱和XPS表征手段测试了改性后活性炭的结构及其组分,并通过电化学表征手段,测试了其作为超级电容器电极材料在几种不同性质电解液中的性能,初步探究了电解液对电极材料电化学性能的影响机制。实验结果表明：改性后的活性炭具有丰富的孔结构,比表面积达到2 332 m²/g,微孔孔容为1.37 cm³/g,中孔孔容为0.74 cm³/g,平均孔径为2.79 nm,含氮元素7.5%,其中类石墨型氮(N-Q)结构达到34.6%。丰富的孔结构和氮含量大幅提升了活性炭的电化学性能,其在水系电解液中展现出了高比电容,在1 A/g的电流密度下比电容最高可达424 F/g;在有机系电解液中,尽管其在1 A/g的电流密度下比电容最高仅为87 F/g,由于其工作电压窗口更宽(0～2.5 V),因此具备了更高的能量密度。对结果进行分析,发现：活性炭电极材料在水系电解液中的性能主要受电解液水合离子半径影响,而在有机系电解液中...     

Kinetic investigation of sulfurized polyacrylonitrile cathode material by electrochemical impedance spectroscopy

To understand more about the sulfur composite prepared by sulfurized polyacrylonitrile at 300 °C, electrochemical impedance spectroscopy (EIS) is employed to investigate the electrochemical properties of the sulfur composite cathode during discharge process. The impact of discharge depth on the performance of sulfur composite materials is investigated. The electrolyte solution resistance, the charge transfer resistance and the interface impedance are evaluated from the EIS analysis. The charge transfer resistance and the interface impedance increase during delithiation and decrease during lithiation, while the concentration of Li⁺ in the composite decreases and increases, respectively. Meanwhile the electrolyte solution resistance is likely to keep stable. The interface resistance and charge transfer resistance of the sulfur composite cathode decrease rapidly after initial lithiation, while Li⁺ diffusion coefficient and exchange current density increase rapidly. After the initial lithiation, they are likely to keep stable. This study reveals more characteristics of the sulfur composite, which is considered to be a promising candidate for large capacity cathode material.     

Calculation of ohmic resistance effects in rectangular electrodes of finite thickness

The current distribution in electrochemical cells consisting of parallel rectangular plates is determined. The calculations involve the evaluation of the appropriate analytical solution of Laplace's equation within the electrodes and electrolyte, with boundary conditions corresponding to potential continuity (primary current distribution) or linear electrode kinetics (secondary current distribution) at the electrode-electrolyte interface, and do not make the usual assumption that current flow in the resistive electrode is one-dimensional. The current distributions are given in the form of Fourier series that allow the effects of electrode resistance and electrical contact geometry to be determined.     

复合添加剂对钒电池正极电解液性能的影响

为了提高钒电池电解液的性能，选取了3种复合添加剂，研究了复合添加剂对钒电池正极电解液稳定性和电化学性能的影响。利用电化学方法制备了2 mol/L的全钒液流电池正极5价钒离子电解液，采用临界胶束浓度法得到复合添加剂的配比为：1% KHSO₄+3 mmol/L SDBS（十二烷基苯磺酸钠）、1% KHSO₄+2 mmol/L D-山梨醇、1% KHSO₄+2 mmol/L CTAB（十六烷基三甲基溴化铵），并考察添加剂加入电解液后的稳定性与电化学性能。通过XRD分析手段，对电解液沉淀物的成分进行了表征。研究表明：添加剂的加入，并不会引起钒离子价态的变化，1% KHSO₄+2 mmol/L CTAB加入后，电解液峰电位差减小12 mV，峰电流增加9.8 mA，说明CTAB与KHSO₄在合适配比下，能够有效提高正极电解液的稳定性及可逆性，添加剂的引入并未引起电解液沉淀物的物相组成变化，电解液性能显著提高。     

Three-dimensional random resistor-network model for solid oxide fuel cell composite electrodes

A three-dimensional reconstruction of solid oxide fuel cell (SOFC) composite electrodes was developed to evaluate the performance and further investigate the effect of microstructure on the performance of SOFC electrodes. Porosity of the electrode is controlled by adding pore former particles (spheres) to the electrode and ignoring them in analysis step. To enhance connectivity between particles and increase the length of triple-phase boundary (TPB), sintering process is mimicked by enlarging particles to certain degree after settling them inside the packing. Geometrical characteristics such as length of TBP and active contact area as well as porosity can easily be calculated using the current model. Electrochemical process is simulated using resistor-network model and complete Butler-Volmer equation is used to deal with charge transfer process on TBP. The model shows that TPBs are not uniformly distributed across the electrode and location of TPBs as well as amount of electrochemical reaction is not uniform. Effects of electrode thickness, particle size ratio, electron and ion conductor conductivities and rate of electrochemical reaction on overall electrochemical performance of electrode are investigated.