The effect of temperature on the output characteristics of micro direct methanol fuel cell

doi:10.1016/j.jpowsour.2015.03.094

Journal of Power Sources

Volume 285, 1 July 2015, Pages 318-324

https://doi.org/10.1016/j.jpowsour.2015.03.094 Get rights and content

Highlights

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A novel two-dimensional, multi-physics model is established.
•
A 0.64 cm² metal-based μDMFC is fabricated by micro-stamping technology.
•
The experimental validation with high power density is conducted.
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The experimental results are in good agreement with the simulation.

Abstract

In this paper, the effects of operating temperature on mass transport and micro direct methanol fuel cell (μDMFC) performance are presented. Furthermore, a whole two-dimensional model coupled with mass/momentum transports and temperature characteristic is established. Simulation results show that the temperature has significant effects on methanol concentration/CO₂ distributions, crossover current density, and the polarization curve. The metal-based μDMFC with the effective area of 0.64 cm² is fabricated using micro-stamping technology, and the detailed experimental validation is conducted. The results reveal that when the cell is supplied with a relatively low aqueous methanol flow rate, the peak power density exhibits a trend of initially going up, reaching the peak value of 85.3 mW cm⁻² at 60 °C, and then dropping off. At the higher flow rate, however, a proportional relationship between the power density and temperature is obtained. The experimental results are in good agreement with the simulation.

Introduction

Conventional batteries have the disadvantage of serious environmental impact. Therefore, it is urgent to find a clean and high-energy power source for portable electronics [1], [2], [3], [4]. Meanwhile, the micro direct methanol fuel cell (μDMFC) has been considered as a prime candidate due to the advantages of high-efficiency, low-emission, silent-operation and simplicity [5], [6].

Presently, one of the most challenges for μDMFC application is the low power density. To improve the power density of μDMFC, many studies have been conducted to explore the mechanisms of methanol oxidation kinetics, methanol crossover and the mass transport inside the cell [7], [8], [9], [10], [11]. It is widely acknowledged that temperature has dramatic effects upon above aspects and the performance of μDMFC [12], [13]. Therefore, significant attentions have been devoted to the effect of operating temperature on μDMFC performance recently. Alizadeh et al. [14] analyzed the performances of the direct methanol single cell at various cell temperatures. The results indicated that the cell performance improved with an increase of temperature in a certain range because the conductivity of the membrane and the reaction kinetics at both the anode and cathode were increased. Chen et al. [15] also investigated the effects of methanol concentration, methanol flow rate, oxygen flow rate and cell temperature on DMFC performance, and concluded that the DMFC performance increased significantly with an increase in cell temperature. In previous studies, however, only experimental investigations on the temperature effect were engaged briefly without in-depth theoretical analysis. Given the importance of temperature on cell performance, it is essential to conduct a comprehensive study on both simulation and experiment to fully understand the relationship between the temperature and the inner transport characteristics and cell performance.

Based on this understanding, a novel two-dimensional cell model coupled with mass/momentum transports and temperature effect was established. In this model, the methanol solution transport, CO₂ distribution and crossover current density of different operating temperatures were numerically defined. In addition, a 0.64 cm² stainless-based μDMFC was fabricated using micro-stamping technology, at which the effects of operating temperature on cell performance were experimentally investigated. The results from a series of experiments including polarization curve and Electrochemical Impedance Spectroscopy (EIS) showed that the μDMFC behaviors are influenced by the temperature in a complex manner. At low methanol flow rate, the power density of the cell exhibits a non-monotonic relationship with the temperature; while at higher methanol flow rate, the cell performance monotonously increased with the temperature.

Section snippets

The model analysis

A two-dimensional model was established to investigate the methanol/CO₂ transports and the temperature effect. Fig. 1 shows the calculation domain of the model. The μDMFC is assumed to be under steady-state conditions, and the diffusion layer is defined as homogeneous porous electrode.

The mass transport can be described using the Convection–Diffusion equation as follows: $\nabla \cdot (- D_{i, l}^{e f f} \nabla C_{i, l} + C_{i, l} u_{l}) = S_{i, l}$ $\nabla \cdot (- D_{i, g}^{e f f} \nabla C_{i, g} + C_{i, g} u_{g}) = S_{i, g}$ where subscripts l/g represent liquid/gas substance,C_i is the

Simulation results and discussion

The above equations were couple-solved by the finite element method. The parameters used during the calculation are listed in Table 1. The structure of the fabricated μDMFC was identical to the calculated domain, and the boundary setting was also consistent with experimental condition.

The simulation results were compared with the experimental polarization curves to verify the accuracy of this model. As shown in Fig. 2, the simulation results demonstrate the typical μDMFC polarization trend and

The design and fabrication of μDMFC

A metal-based μDMFC was designed and fabricated with the basic structure shown in Fig. 4(a). The μDMFC is constructed by current collectors, end plates, assembly gaskets and a membrane electrode assembly with the effective area of 0.64 cm².

The current collectors were fabricated from 304 L stainless steel using micro-stamping technology, as shown in Fig. 4(c). The local topography of the current collector, in which the widths of the flow channel and the rib are 1000 μm and 400 μm, respectively.

Conclusions

In this paper, the effects of temperature on mass transport and output characteristics were conducted by simulation and experimental investigations. Firstly, a multi-physics model coupled with mass/momentum transports and temperature effect was established, the simulation results were well fitted with the experimental data. The methanol concentration, crossover current density, polarization curve and the distribution of CO₂ were positively correlated to the temperature. Secondly, a metal-based

Acknowledgements

The work described in this paper was supported by the National Natural Science Foundation of China (No. 61372015), Research Fund for the Doctoral Program of Higher Education (No. 20130042120023) and Fundamental Research Funds for the Central Universities in China (No. N140403001 and N120204001).

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View full text

The effect of temperature on the output characteristics of micro direct methanol fuel cell

Highlights

Abstract

Introduction

Section snippets

The model analysis

Simulation results and discussion

The design and fabrication of μDMFC

Conclusions

Acknowledgements

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

J. Power Sources

J. Power Sources

Electrochim. Acta

J. Power Sources

J. Power Sources