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Abstract Catalyst poisoning from the impurities in the industrial grade methanol is a major challenge for the large-scale application of direct methanol fuel cells in low cost. In this work, we systematically investigate the impurities influencing on cell performance, and confirm that the adsorption of carbonyl containing intermediate species derived from the partial electro-oxidation of the impurities is a crucial factor leading to performance degradation. Hence, an on-line alleviation strategy by intermittently applying pulse potential (reduction potential on the anode or oxidation potential on the cathode) is proposed. The applied potential will bring a reductive condition on the anode, which releases the active sites via the reduction of carbonyl containing species adsorbed on platinum-ruthenium electro-catalysts. Based on this strategy, the adsorption of carbonyl containing intermediate species are effectively suppressed, and the decay rate declines by nearly two orders of magnitude than that of a single cell under traditional operation, which paves a way for the practical application of direct methanol fuel cells with industrial grade methanol feed. Graphical abstract Image 1 Highlights • Performance/durability of DMFCs with low-cost IG methanol worsen due to impurities. • The effects of impurities on cell performance is systematically investigated. • An electro-reduction route by pulse potential is proposed to recover performance. • Based on the strategy, the decay rate decreased by nearly two orders of magnitude. [ABSTRACT FROM AUTHOR]

Measurement of flow distribution in the fuel cell stack is a challenging task. This study proposes a new method to measure air flow distribution in high-temperature proton exchange membrane fuel cell stacks. The characteristic frequency related to channel impedance is extracted using the distribution of relaxation times analysis. A simple correlation between the air stoichiometry and the characteristic frequency is developed. The influence of various factors on the correlation, including cell operation conditions, fluid physical properties and channel dimensions, are investigated. The experimental results show a good agreement with the correlation. Error estimation indicates that the temperature distribution of the stack, gas humidity and pressure distribution in the inlet manifold have no significant effect on the proposed method under normal conditions. The accuracy and practicability are verified using a three-cell stack with separate mass flow controller (MFC) systems. The results show less than 3.11% of relative error between the reference air stoichiometry given by the MFC and those measured from our proposed method for air stoichiometry below 3.0 at 0.1 A cm−1. The present method provides powerful support for designing high consistency stacks and the real-time monitoring cells state-of-health in stacks. [Display omitted] • Distribution of relaxation times analysis of the channel impedance in fuel cells. • Correlating the characteristic frequency of mass transport and air stoichiometry. • Measuring the air flow distribution in the fuel cell stack through the correlation. • Understanding the impact of the stack parameters' consistency on the measurement. [ABSTRACT FROM AUTHOR]

Abstract: A novel method for measuring effective diffusion coefficient of porous materials is developed. The oxygen concentration gradient is established by an air-breathing proton exchange membrane fuel cell (PEMFC). The porous sample is set in a sample holder located in the cathode plate of the PEMFC. At a given oxygen flux, the effective diffusion coefficients are related to the difference of oxygen concentration across the samples, which can be correlated with the differences of the output voltage of the PEMFC with and without inserting the sample in the cathode plate. Compared to the conventional electrical conductivity method, this method is more reliable for measuring non-wetting samples. [Copyright &y& Elsevier]

Abstract: In this paper, the reversible and the irreversible loss in performance of direct methanol fuel cell (DMFC) single cells induced by freeze/thaw cycling are determined and discussed. The reversible loss in performance is attributed to the decreased activity of electrocatalysts due to the strong adsorption of the intermediates from methanol electro-oxidation at subzero temperatures, while the irreversible loss in performance is due to the damage of membrane electrode assembly (MEA) structure. It is found that the reversible degradation of performance is dominant when the cell operated at low current densities with O2; the irreversible degradation is more obvious when the cell discharged at high current densities or operated with air. A method, applying a reverse current on the single cell, is put forward to recover the reversible loss in performance. By this method, the voltage of the single cell operated with O2 at 100 mA cm−2 drops only about 16 mV after experiencing 100 freeze/thaw cycles between−10 and 60 °C. [Copyright &y& Elsevier]

In vanadium flow batteries, electrolyte transfer across the membrane can lead to a volumetric imbalance between the two half-cell electrolytes and a subsequent loss of available capacity. However, the transfer mechanism has not been comprehensively understood and this lack of knowledge has significantly limited long-term discharge capacity and stability of the vanadium flow battery. To overcome this issue, the electrolyte transfer mechanism is systematically developed in this study by analyzing the pressure drop across the membrane in accordance with Darcy's law and further validated by experiments. The experimental results show that the viscosity difference between the two half-cell electrolytes contributes greatly to the electrolyte transfer from negative half-cell to positive half-cell, while a large flow rate applied to both half-cells may also exacerbate the electrolyte transfer. Moreover, further experiments also demonstrate that the electrolyte transfer in continuous charge-discharge operation can be effectively suppressed by optimizing the flow rates based on viscosity measurements, which subsequently yields a notable improvement in discharge capacity. Revealing the electrolyte transfer mechanism is not only beneficial to enhancing long-term performance and stability of the vanadium flow battery, but also highly valued for understanding the transport phenomena in other flow battery systems. Image 1 • Electrolyte transfer mechanism across a Nafion membrane is revealed for the VFB. • Pressure drop across the membrane arises from difference in electrolyte viscosity. • Appling a large flow rate may also accelerate the electrolyte crossover. • A Flow rate optimization strategy derived from Darcy's law is proposed. • Electrolyte transfer can be effectively mitigated by flow rate optimization. [ABSTRACT FROM AUTHOR]