The development of environmentally friendly vehicles such as fuel cell electric vehicles (FCEVs) and battery electric vehicles (BEVs) for effective fuel utilization is one of the major areas upon which efforts are focused to counteract global warming. Among the various types of fuel cells, polymer electrolyte fuel cells (PEFCs) are the most attractive option for development because of their capability to be operated at a low temperature and their highly efficient generation of power without emissions. Meanwhile, energy storages, such as lithium ion batteries (LIBs), are also attractive because of their indispensableness for the application of FCEVs and BEVs. These energy devices are evaluated by electrochemical impedance spectroscopy (EIS) for development of their electrode materials and structures. The electrodes of these energy devices are designed to increase reaction site for higher power density and energy density. Therefore, the electrodes of fuel cells and batteries have a three-dimensional complicated structure. Generally, basic electrochemical systems are evaluated by a simple equivalent circuit, such as Randles-type equivalent circuit, which treats the electrode as a homogeneous system. On the other hand, electrochemical reactions occur inhomogeneously in those practical energy devices due to ionic resistance in the electrode under actual high-load operation. In the present paper, to count the reaction distribution in the electrodes, EIS analysis by using equivalent circuit with transmission line model (TLM) is introduced. For PEFC, a TLM represents only the ionic resistance in the catalyst layer, that is electrode, and the overall distribution of the catalyst. However, because the carbon-supported catalysts form agglomerates in the actual catalyst layers, the inner and outer catalysts of these agglomerates should be considered separately (Figure 1 (a)). Herein, a novel TLM, which counts the distribution of the oxygen reduction reaction (ORR) in primary and secondary pores in a catalyst layer of a MEA, was designed for the evaluation of a degraded cathode catalyst layer in PEFCs (Figure 1 (b)). The equivalent circuit was applicable from low current density region to high current density region, showing excellent match between experimental data and calculated data. The EIS with low current density region enable to understand a degree of degradation of cathode catalyst layer.1 The EIS with high current density region enable to grasp the flooding effect in the microstructured cathode catalyst layer of PEFCs.2 For LIBs, if there is a degradation of the electrodes in LIBs, it is possible that it will change the ionic pass in the electrode layer due to the accumulation of decomposed products inside the electrode layer and due to the collapse of the active materials and the micro pores (Figure 1 (c)). The impedance response of LIBs is composed of overlapped elemental steps such as charge transfer for chemical reaction and ion migration of cathode and anode. Therefore, the LIB impedance response was separated into the cathode and anode impedance response through the use of the LiAl micro reference electrode. The separated impedance responses were analyzed using the equivalent circuit with the TLM to count the ionic resistance in the electrodes (Figure 1 (d)). The equivalent circuit with a TLM was enabled to fit the impedance responses simulated by the equivalent circuit with those measured at various depths of discharge, as well as the Randles type equivalent circuit fit them. The present paper demonstrates the potential of the EIS with an equivalent circuit coupled with a TLM for diagnosis of LIBs in power applications.3 At the presentation, EIS analysis for energy devices by using equivalent circuit with TLM will be discussed in detail. Acknowledgement These work were partly supported by “Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING)” and “R & D Basic research of cell degradation factors and analysis of MEA durability” from New Energy and Industrial Technology Development Organization (NEDO) of Japan, the Grant-in-Aid for Specially Promoted Research “Establishment of Electrochemical Device Engineering”, and the Global COE program “Practical Chemical Wisdom” from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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Jul 1, 2017
Yunfeng Jiang + 5
Open Access
