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Abstract One approach to creating physics-based reduced-order models (ROMs) of battery-cell dynamics requires first generating linearized Laplace-domain transfer functions of all cell internal electrochemical variables of interest. Then, the resulting infinite-dimensional transfer functions can be reduced by various means in order to find an approximate low-dimensional model. These methods include Pade approximation or the Discrete-Time Realization algorithm. In a previous article, Lee and colleagues developed a transfer function of the electrolyte concentration for a porous-electrode pseudo-two-dimensional lithium-ion cell model. Their approach used separation of variables and Sturm–Liouville theory to compute an infinite-series solution to the transfer function, which they then truncated to a finite number of terms for reasons of practicality. Here, we instead use a variation-of-parameters approach to arrive at a different representation of the identical solution that does not require a series expansion. The primary benefits of the new approach are speed of computation of the transfer function and the removal of the requirement to approximate the transfer function by truncating the number of terms evaluated. Results show that the speedup of the new method can be more than 3800.

The influence of microporous layer (MPL) design parameters for gas diffusion layers (GDLs) on the performance of polymer electrolyte fuel cells (PEFCs) was clarified. Appropriate MPL design parameters vary depending on the humidification of the supplied gas. Under low humidification, decreasing both the MPL pore diameter and the content of polytetrafluoroethylene (PTFE) in the MPL is effective to prevent drying-up of the membrane electrode assembly (MEA) and enhance PEFC performance. Increasing the MPL thickness is also effective for maintaining the humidity of the MEA. However, when the MPL thickness becomes too large, oxygen transport to the electrode through the MPL is reduced, which lowers PEFC performance. Under high humidification, decreasing the MPL mean flow pore diameter to 3 μm is effective for the prevention of flooding and enhancement of PEFC performance. However, when the pore diameter becomes too small, the PEFC performance tends to decrease. Both reduction of the MPL thickness penetrated into the substrate and increase in the PTFE content to 20 mass% enhance the ability of the MPL to prevent flooding.

Abstract Differential scanning calorimetry (DSC) has been used to measure the thermal interactions between several binder materials and representative anode carbons both in the presence of cell electrolyte (EC:DEC/1M LiPF 6 +2 wt .% vinylene carbonate) and after washing/drying. Binders consisting of homo- or copolymers of vinylidene fluoride (VDF) were examined as well as other fluorinated and non-fluorinated binder materials. The heat evolved by the reactions of these materials was compared to that arising from other exothermic phenomena occurring in charged anodes at elevated temperatures. A matrix of anode material combinations was designed to investigate the role of carbon structure, carbon surface area, state of charge, binder level and presence of electrolyte. The temperature and magnitude of the exothermic reactions were measured up to 375 °C and average enthalpy values were obtained over several duplicate samples to allow good quantitative comparison of the material reactions. The exothermic anode reactions were sensitive to the state of charge and presence of electrolyte. The magnitude of the reactions increased with increasing surface area of the carbon particles. However, similar reaction enthalpies were seen for all binder materials and binder levels.

Abstract Conventional polymer electrolyte fuel cells (PEFCs) require a means of placing the series of laminar components that make up cells under mechanical compression so as to ensure effective electrical conduction, mass transport and gas-tight operation. This review describes the effect of mechanical compression and dimensional change on the components of PEFCs and reviews the range of methods used to achieve desired stack compression. The case is made for improved understanding of the mechanisms of fuel cell component compression and greater attention to the development of technological approaches for stack compression.

Abstract Aromatic isocyanate, 4-fluorophenyl isocyanate and phenyl isocyanate, were first used to reduce the initial irreversible capacities during the formation of the solid electrolyte interface (SEI) on a graphite surface. Results showed that the addition of 1–5 wt.% isocyanate to propylene carbonate-containing electrolytes could effectively reduce the initial irreversible capacities in the SEI formation and increase the cycleability of Li-ion batteries. The improvement is attributed to the high reactivity of isocyanate with chemisorbed oxygen groups, such as carboxyl and phenol, which are inevitably present in the prismatic (edge) sites of graphite and are known among the sources to cause the initial irreversible capacities of a graphite anode. It is proposed that the isocyanate reacts with carboxyl and phenol groups to form more stable products, and that the resulting products have a better affinity to the subsequently formed SEI. In addition, the presence of isocyanate assists in scavenging water and acidic HF impurities from the electrolyte.

Abstract Lithium ion conducting polymer electrolytes were prepared by mixing insoluble lithium tetrakis(pentafluorobenzenethiolato) borate (LiTPSB) with poly(vinylidene fluoride) (PVDF) or poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). Their films were prepared by hot pressing and are investigated for ionic conductivity and thermal properties. LiTPSB is insoluble in PVDF. Ionic conductivity was largely dependent on the salt content for LiTPSB–PVDF composite polymer electrolytes, and exhibited higher ionic conductivity than homogeneous LiTFSI–PVDF based polymer electrolytes. Melting point and crystallinity of PVDF were independent on LiTPSB content, resulting in no difference for melting point and crystallinity between pure PVDF and LiTPSB–PVDF. Ionic conductivity was effectively improved by incorporation of 18-crown-6 or kryptofix222 for LiTPSB–PVDF based polymer electrolytes.

Abstract Residual stresses in the electrolytes of segmented-in-series solid oxide fuel cells (SIS-SOFCs) and anode-supported cells (ASCs) were estimated at room temperature by X-ray diffraction. In the SIS-SOFCs, the residual stresses in the electrolyte were smaller than in the ASCs and did not change significantly after redox cycling. For both designs, numerically calculated values of the residual stresses in the electrolyte were found to be comparable to the experimental results. Next, in order to simulate the reoxidation reaction, the anode was subjected to forced expansion, and the residual stresses were estimated at high temperatures. It was found that in the SIS-SOFC, the dimensional changes and residual stresses were smaller than those in the ASC. The high redox tolerance of the SIS-SOFC is considered to stem from the fact that the electrically insulated substrate prevents the expansion and deformation of the positive electrode–electrolyte–negative electrode structure.

Several years ago, Li/Na carbonate (Li2CO3/Na2CO3) was developed as the electrolyte of molten carbonate fuel cells (MCFCs) in place of the usual Li/K carbonate (Li2CO3/K2CO3) to the advantage of a higher ionic conductivity and lower rate of cathode NiO dissolution. To estimate the potential of Li/Na carbonate as the MCFC electrolyte, the dependence of the cell performance on the operating conditions and the behavior during long-term performance was investigated in several bench-scale cell operations. The obtained data on the performance of Li/Na cells was analyzed to estimate the impact of voltage losses by using a performance model and discussed in comparison with the data of conventional Li/K cell performance.

A brief discussion on transport measurements for lithium ion conducting polymer electrolytes is given. An engineering approach to obtain a complete set of transport and thermodynamic properties for a binary salt dissolved in a polymer electrolyte solvent is described. The technique is based on concentrated solution theory and requires a minimal amount of experimentation. Results from measurements on a representative polymer electrolyte system are given. The measured transport and thermodynamic properties of the polymer electrolyte are used to simulate the performance of symmetric Li/polymer/Li cells and compare to experimental data.