Impedance Analysis on Transient Behavior of DMFC Anodes during Preconditioning Process

Abstract

Transient behavior of direct methanol fuel cells ͑DMFCs͒ during the preconditioning process has been investigated with a focus on changes manifested in the anode by using an electrochemical impedance spectroscopic analysis. Preconditioning of membrane electrode assembly ͑MEA͒ is essential to achieve maximum cell performance, and the time required to accomplish the conditioning may vary depending on the methods that are adopted. Preconditioning temperature substantially influences the behaviors of MEAs, and room-temperature treatment has been found to be more favorable than high-temperature treatment at 90°C. Application of electric load during the preconditioning process accelerates the hydration of electrolyte and thus reduces the time required to reach a maximum performance limit. The anode impedance data of DMFC has been deconvoluted by using an equivalent circuit to investigate three kinds of resistances in the MEA, such as resistance of charge transfer ͑R ct ͒, electrolytes ͑R e ͒, and pore electrolytes ͑R p ͒. The underpinning mechanism which causes the change in performance during the preconditioning process is discussed in terms of the changes in the resistance elements of the anode. In recent times, intense research interest has been channeled in improving the performance of direct methanol fuel cells ͑DMFCs͒ based on polymer electrolyte membranes. Performance of a DMFC is strongly influenced not only by the fabrication procedure of membrane electrode assembly ͑MEA͒ but also by the preconditioning method adopted. Hence, to improve the performance of DMFCs, it is important to establish an appropriate conditioning method for MEAs. Several methods have been proposed for conditioning of MEAs to achieve a stable and improved performance of DMFCs. [1][2][3][4] In DMFCs as in the case of polymer electrolyte membrane fuel cell ͑PEMFCs͒, sufficient hydration of polymer electrolyte membrane and recast ionomer in the catalyst layer is necessary to obtain stable proton conductivity, and hence the hydration process might require about 24-48 h. For instance, Aricó et al.1 supplied water to both the anode and cathode compartments of a DMFC, and the cell was warmed up stepwise to a maximum operating temperature of 95°C, followed by an operation with the supply of methanol ͑0.5 M͒ and oxygen at a high current for a period of 24 h. Shukla et al. 2 collected the polarization curves of DMFCs after hydrating the MEAs for 24 h by circulating a 2 M methanol solution through the anode compartment at 80°C. Scott et al. 3 adopted both the above methods.They 4 suggested that hydrogen evolution on electrodes could influence the performance of DMFCs by reducing the overpotential of both oxygen reduction as well as methanol oxidation reactions. Though various routes have been employed for conditioning MEAs, to the best of our knowledge, no definite explanation has been offered in the literature on the mechanism of performance changes that occur during the conditioning period, except a brief mention in our previous study. 5Our presen...

Zeolites in Microsystems for Chemical Synthesis and Energy Generation

“…The Nafion-MEA displays a low cell voltage (i.e., 0.33 V) due to a high activation polarization. Preconditioning the Nafion-MEA overnight at 340 K in recirculating 1.8 ml.min -1 2 M methanol solution and 10 sccm dry O 2 resulted in a higher cell voltage of 0.54 V. The improved cell performance after preconditioning was attributed to catalyst activation [62][63][64] and changes in the cell's electronic properties [64][65][66]. It is evident from Fig.…”

An activation procedure applied to fluorinated and non-fluorinated proton exchange membranes

“…Previous studies [2,3] indicate that the temperature plays a crucial role on the in situ activation procedure, namely affecting the development of the triple phase boundary. From the impedance experiments it was shown that using different proton exchange membranes results in different double layer capacitances after the activation procedure.…”

“…Beyond these challenges, the development of quick and effective procedures to ensure maximum performance of the membrane electrode assembly (MEA) during the start-up or after resting periods is also very important. These procedures have received scarce attention from the scientific community and there is a clear lack of available information in the open literature [2,3]. It is known that a MEA performs below the nominal power whenever started from fresh or after a resting period [4].…”

“…The pre-treatment includes all actions carried on the proton exchange membrane (PEM) and electrodes that are made over a fresh MEA, while the in situ activation procedure includes all the actions used to improve the performance of a MEA when the fuel cell is on a working state. Along the activation procedure, the proton exchange membrane and the diffusion and catalyst layer experiment strong changes in their properties [2,3]. Thus, it is expectable that the activation procedure leads to different responses from MEA to MEA depending on the selected membrane and catalyst.…”

Performance degradation study of a direct methanol fuel cell by electrochemical impedance spectroscopy

“…Further studies showed that EIS results could provide information of individual fuel cell component [12][13][14][15][16]. Müller and Urban [8] suggested a method to separate anode and cathode impedance by measuring anode and full cell impedance data, respectively.…”