1,721,008 research outputs found
Experimental investigation of DMFC degradation in real operation representative conditions and during accelerated stress tests (AST)
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In operando investigation of anode overpotential dynamics in direct methanol fuel cells
No full textThis work illustrates the application to direct methanol fuel cell technology of an innovative reference electrode setup with a through-plate configuration, which enables localised measurement of electrode potential in an operating cell. The utility of the technique is demonstrated by monitoring the evolution of anode overpotential at two different locations in the cell over different time scales, ranging from minutes to hundreds of hours. The measurements provide valuable insight into critical degradation phenomena, identifying localised hydrogen evolution on the anode during short term operation and highlighting the contribution of anode temporary degradation to the overall performance decay during long term operation. This novel approach can be used as a diagnostic tool to improve operational protocols, such as refresh cycles, for direct methanol fuel cells
On the effect of gas diffusion layers hydrophobicity on direct methanol fuel cell performance and degradation
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A Novel Accelerated Stress Test for a Representative Enhancement of Cathode Degradation in Direct Methanol Fuel Cells
Full text linkPerformance decay of direct methanol fuel cells hinders technology competitiveness. The cathode electrochemical surface area loss is known to be a major reason for performance loss and it is mainly affected by cathode potential and dynamics, locally influenced by water and methanol crossover. To mitigate such phenomenon, novel materials and components need to be developed and intensively tested in relevant operating conditions. Thus, the development of representative accelerated stress tests is crucial to reduce the necessary testing time to assess material stability. In the literature, the most diffused accelerated stress tests commonly enhance a specific degradation mechanism, each resulting in limited representativeness of the complex combination and interaction of mechanisms involved during real-life operation. This work proposes a novel accelerated stress test procedure permitting a quantifiable and predictable acceleration of cathode degradation, with the goal of being representative of the real device operation. The results obtained with a 200 h accelerated stress test are validated by comparing both in situ and post mortem measurements with those performed during a 1100 h operational test, demonstrating an acceleration factor equal to 6.25x and confirming the development of consistent cathode degradation
Novel macro-Segmented Fuel Cell approach to investigation of localized degradation in PEMFCs
No full textHere we present a novel concept of a segmented fuel cell, developed to fully and independently characterize four macro- segments of a single cell without introducing any modification to the MEA structure itself. This permits complete characterization of local electrochemical performance (using polarization curves, impedance spectra, cyclic and linear voltammetry) and mass composition (via gas-chromatography) during durability tests. Moreover, each of the four segments is fitted with an innovative reference electrode using an external, through-plane array configuration, enabling separation of anode and cathode contributions and their spatial evolution. This setup has been firstly applied to generate unprecedented insights into the evolution of local performance during DMFC operation. The variation of anode and cathode potential as a function of location within the cell and the correlation with heterogeneous current density distribution are discussed
On the actual cathode mixed potential in direct methanol fuel cells
No full textMethanol crossover is one of the most critical issues hindering commercialization of direct methanol fuel cells since it leads to waste of fuel and significantly affects cathode potential, forming a so-called mixed potential. Unfortunately, due to the sluggish anode kinetics, it is not possible to obtain a reliable estimation of cathode potential by simply measuring the cell voltage. In this work we address this limitation, quantifying the mixed potential by means of innovative open circuit voltage (OCV) tests with a methanol-hydrogen mixture fed to the anode. Over a wide range of operating conditions, the resulting cathode overpotential is between 250 and 430 mV and is strongly influenced by methanol crossover. We show using combined experimental and modelling analysis of cathode impedance that the methanol oxidation at the cathode mainly follows an electrochemical pathway. Finally, reference electrode measurements at both cathode inlet and outlet provide a local measurement of cathode potential, confirming the reliability of the innovative OCV tests and permitting the evaluation of cathode potential up to typical operating current. At 0.25 A cm−2 the operating cathode potential is around 0.85 V and the Ohmic drop through the catalyst layer is almost 50 mV, which is comparable to that in the membrane
Investigation of calendar ageing of lithium-ion battery through physical models with ex-situ validation
Full text linkThe estimation of the state of health of a lithium-ion battery is a topic of interest with the spread of battery electric vehicles. According to the desired long lifetime, calendar ageing is a matter of concern due to the known deterioration effect of mid-high environmental temperatures, even during parking periods. In this work, the combined use of an optimised sequence of tests and physical models is applied to investigate degradation due to calendar ageing on a commercial NMC + LMO|Graphite cell type. A calendar ageing campaign on 12 samples is carried out with periodic interruptions for characterisation. This method, already applied to a low-temperature charging campaign, proves its suitability in identifying ageing mechanisms and reproducing the performance of aged cells. SEI layer growth with significant consumption of lithium and electrolyte material is the dominant phenomenon, but mass-transport limitations arise from the positive electrode, too. Physical model parameter identification is challenged with verification measurements like tests on coin cell configuration and microscopies, resulting in compatibility. This work further verifies the suitability of such a methodology for the degradation of lithium-ion batteries
Physical Modeling of Catalyst Degradation in Low Temperature Fuel Cells: Platinum Oxidation, Dissolution, Particle Growth and Platinum Band Formation
Full text linkThe loss of electrochemical active surface area (ECSA) at the cathode is one of the main causes of performance degradation in Polymer Electrolyte Membrane Fuel Cells (PEMFCs). In order to investigate the catalyst degradation and the influence of the operating conditions we develop a multiscale degradation model which includes the formation and reduction of platinum oxides, platinum dissolution, particle growth due to Ostwald ripening, platinum ion transport through the ionomer and platinum band formationinthe membrane.Thisdegradationmodeliscoupled with a2DPEMFC performancemodel andpredictionsregardingion concentration, ECSA evolution and particle growth are validated with dedicated experiments and literature data. Degradation under several AST protocols and under steady state operation are compared and discussed. The importance of a spatially resolved catalyst degradation model is conveyed by the occurrence of a depletion zone in the catalyst layer close to the membrane due to the platinum migration into the membrane. By comparing the correlation between platinum mass loss in the catalyst layer and the ECSA loss we conclude that catalyst degradation under AST conditions with nitrogen is not representative for the degradation under normal operation
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