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We present an analysis of the mass transport of methanol at the anode of a direct methanol fuel cell ͑DMFC͒ and show that the overall mass-transfer coefficient can be determined by measuring the cell limiting current density. We measured the cell limiting current density of an in-house-fabricated DMFC with different flow fields for various methanol concentrations and flow rates of methanol solution. The experimental data showed that the overall mass-transfer coefficient was nearly independent of current density, although the rate of CO 2 gas bubble liberation changed with current density. We found that the overall methanol-transfer coefficient in the serpentine flow field could be significantly increased with increased methanol flow rate due to the enhanced under-rib convection. We developed the correlation equations predicting the overall methanol transfer coefficient in terms of the methanol flow rate for a given DMFC hardware. Finally, we showed that the polarization curves predicted based on the correlation equation of the overall mass-transfer coefficient in the DMFC with the serpentine flow field for different flow rates were in fairly good agreement with the experimental data.The liquid feed direct methanol fuel cell ͑DMFC͒, based on a solid polymer electrolyte, has received much attention as a leading candidate power source for portable electronic devices, electric vehicles, and other mobile applications because of its unique advantages such as high energy-conversion efficiency, easy delivery and storage of liquid fuel, ambient temperature operation, and simple construction. 1-4 However, the commercialization of the DMFC is still hindered by several technological problems, including a substantial methanol crossover through polymer membranes, low electroactivity of methanol oxidation on the anode, and severe cathode flooding. 4 Over the last decade, different fundamental aspects of the DMFC system have been studied extensively. [2][3][4][5][6][7][8][9][10][11][12][13][14] It has been understood that mass transport of reactants in a fuel cell is one of the crucial issues in improving cell performance. To alleviate the problem of methanol crossover, a DMFC typically has to be operated with diluted methanol solution. However, rather diluted methanol solution results in large mass-transport polarization, particularly at high current densities, leading to lower cell performance. Therefore, it is essential to optimize the mass-transport process of methanol at the DMFC anode such that both the rate of methanol crossover and the mass-transport polarization can be minimized. To this end, it is essential to gain a better understanding of mass-transport phenomena in fuel cells. Compared with other types of fuel cells, relatively few papers have been reported on the study of the methanol transport process at the anode of a DMFC. 10-14 This is mainly because the mass transport of methanol occurs in a liquidgas two-phase flow, consisting of methanol solution and reactionproduced gas CO 2 , under typical DMFC operating c...

We studied the performance and CnormalO2 bubble evolution behavior in an in-house fabricated micro-direct methanol fuel cell (DMFC, 1.0×1.0cm in the active area) with the anode flow field consisting of various-sized microchannels down to 400μm . We found that the flow field was blocked periodically by elongated gas slugs due to the increased capillary force in microchannels. This transient capillary blocking caused CnormalO2 bubbles to be evolved in the flow field and to be removed from the cell periodically. We further found that with a reduction in channel size, both gas slugs and the residence time of gas slugs in the flow field became longer. As a result, the effective mass-transfer area of methanol solution on the diffusion layer became smaller, causing the cell performance to decline. At the same fuel feed rate, a smaller flow channel led to a higher mass-transfer coefficient. The competition between the favorable effect of the increased mass-transfer coefficient and the adverse effect of the reduced effective mass-transfer area results in an optimal channel size that gives the best cell performance.

In a flow battery, the salient impact of the electrolyte velocity on the mass transfer coefficient in carbon felt electrodes is demonstrated and quantified. A lab-scale flow battery, fed with identical electrolyte solutions containing Fe 2+ /Fe 3+ as active substances in both the anode and the cathode, is used to realize stable tests free from side reactions in a broad range of current densities. The electrolyte velocities ranging from 2.5 to 15 mm s −1 are selected in this work, which are typical in flow through electrodes in most flow batteries. By measuring limiting currents at various flow rates, a correlation between the mass transfer coefficient and the velocity in dimensionless form is obtained as Sh = 1.68 Re 0.9 . Meanwhile, a 2-D numerical model incorporating this correlation and the experimentally measured electrolyte conductivity is proposed. Voltage losses of the battery fed with adequate reactants at different velocities are both experimentally measured and numerically simulated. The agreement between simulated results and experimental data verifies the applicability of this correlation under normal operating conditions below limiting currents. Owing to the exclusive advantage of decoupling power generation and energy storage, redox flow batteries (RFBs) have been considered as a critical candidate for large-scale electrical energy storage (EES). [1][2][3][4] Unlike the electrodes of conventional secondary batteries, the electrodes of RFBs do not participate in reactions but only provide electrochemical reaction sites for active ions and support electrons transfer. For numerous kinds of developed RFBs, the porous carbon felts, composed of randomly interlaced carbon fibers, are generally selected as electrodes. [5][6][7] Obtaining high voltage efficiency at high current densities is one of the major challenges in the commercialization of RFBs. Much efforts have been devoted to diminishing the activation loss by modifying the carbon fiber surface. [8][9][10][11][12] In addition, ohmic loss and mass transfer loss also lead to the reduction of the voltage efficiency. It is generally recognized that supplying high flow rate of electrolyte for the RFB is the most feasible method to minimizing mass transfer loss, but more pumping power is inevitably required for increasing flow rates. Tang et al. 6 and Ma et al. 7 carried out special research on optimizing the operating strategy of electrolyte flow rate for kilowatts class allvanadium flow battery (VRFB) systems, in which the significant impact of flow rate on cell voltage was detected during the whole charge/discharge process. In a carbon felt electrode, the mass transfer loss is caused by the species transport between the bulk solution in the pore and the carbon fiber surface, which is usually quantified by the mass transfer coefficient k m . 13 For cells operating under high current densities, the mass transfer loss also contributes a significant proportion to the voltage loss, especially when approaching the end of charge/discharge process. Considerin...

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Gas evolution, principally consisting of carbon dioxide and hydrogen, was observed in the anode flow field of a direct methanol fuel cell ͑DMFC͒ running under open-circuit conditions and at low oxygen flow rates. This finding is contrary to conventional wisdom that electrochemical reactions cease as an external load is removed. The mechanism leading to this peculiar phenomenon is explained theoretically and confirmed experimentally.Direct methanol fuel cells ͑DMFCs͒ have been envisaged as potential power sources for electric vehicles and portable applications because of their high energy density and simpler operating system. 1 Nevertheless, issues such as poor methanol electro-oxidation kinetics and methanol crossover through the polymer electrolyte membranes ͑PEMs͒ from anode to cathode remain as obstacles to deploy DMFCs for their practical applications. It has been found that using the Nafion type membranes nearly 30-40% of methanol undergoes crossover, which causes not only a depolarization effect on the cathode, but also a decrease in the utilization efficiency of fuel. [2][3][4] Under open-circuit conditions, the methanol crossover is mainly caused by diffusion as a result of the concentration difference between anode and cathode.When used as portable power sources, DMFCs are preferred running under air breathing mode without the help of oxygen pumping and air blowing devices to achieve compact and simplified system design and low parasitic power cost. 5 In this case, oxygen shortage and severe flooding problems are usually encountered on the cathode, which reduce not only the open-circuit voltage ͑OCV͒ but also cell performance at high current densities. Qi et al. 6 reported that the OCV of a DMFC declined as the airflow rate decreased in the temperature range of 60 to 80°C.During the process of testing DMFCs, we have repeatedly observed gas evolution in the anode flow field when the cell was maintained under open-circuit conditions and at low oxygen flow rates. This finding is something contrary to conventional wisdom that no gas evolution normally occurs on the anode because electrochemical reaction ceases under open-circuit conditions. We found that the onset of this gas evolution was accompanied by a decline in OCV. In this paper, we report on experimental evidence for this peculiar phenomenon and discuss the underlying mechanism leading to the phenomenon. ExperimentalMembrane-electrode assembly (MEA).-An MEA having an active area of 4.0 ϫ 4.0 cm was fabricated in house employing two single-side ELAT electrodes from E-TEK and a Nafion membrane 115. Both anode and cathode electrodes used carbon cloth ͑E-TEK, Type 'A'͒ as the backing support layer with 30% PTFE wet-proofing treatment. The catalyst loading on the anode side was 4.0 mg cm Ϫ2 with unsupported ͓Pt:Ru͔ Ox ͑1:1 a/o͒, while the catalyst loading on the cathode side was 2.0 mg cm Ϫ2 using 40% Pt on Vulcan XC-72. The final MEA was formed by hot pressing at 135°C and 5 MPa for 3 min.Single cell assembly and flow visualization.-The MEA was inserted ...

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