24 research outputs found
Locally engineered PEM cells components with optimized operation for improved durability
No full textLa prestazione delle celle a combustibile è nota per non essere omogeneamente distribuita su tutta la superficie attiva del dispositivo stesso, bensì di presentare una distribuzione della densità di corrente localmente eterogenea; tale distribuzione tende inoltre a variare durante l’operazione, generalmente tendendo a rimpicciolire il dominio di maggiore operatività della cella, estremizzando di conseguenza le disuniformità operative. Il fenomeno determina una operatività sempre differente da zona a zona nel corso della vita utile del dispositivo, portando ad un invecchiamento non omogeneo dei suoi componenti: la zona maggiormente sollecitata della cella a combustibile, che tenderà ad invecchiare precocemente, potrebbe risultare limitante nei confronti della vita utile dell’intero dispositivo.
La distribuzione della densità di corrente locale sulla superficie attiva della cella è determinata dall'interazione di una molteplicità di fattori influenzanti la prestazione quali geometria dei distributori, pressione parziale dei reagenti e dei prodotti e loro cross-over, quantità di acqua e conseguente stato di idratazione dei componenti, temperatura locale, che determinano dunque locali disuniformità nelle condizioni operative.
Nonostante queste disuniformità sulle condizioni di operazione locale, i componenti normalmente impiegati in tali dispositivi sono realizzati con proprietà omogenee, ottimizzate sull'intera area operativa della cella, risultando dunque nell'operazione non ottimale di buona parte di tale superficie attiva. Il funzionamento in condizioni non ottimizzate locali, può determinare una accelerazione dei fenomeni di invecchiamento, determinando prestazioni inferiori, una minore stabilità e una non ottimale durata del dispositivo.
L’invenzione in oggetto applicata porta a fornire una cella a combustibile con un gradiente nelle proprietà di uno o più suoi componenti, ad esempio è fornito un profilo parabolico nello spessore dell’elettrodo catalitico, con l’obiettivo di favorire le zone di ingresso e uscita dell’aria, operanti in condizioni non ottimali, a discapito della quantità di catalizzatore caricato nelle zone centrali della cella, operanti già in condizioni favorite, per non incrementare il carico catalitico complessivo.
La distribuzione della densità di corrente è regolabile sulla superficie attiva della cella inoltre tramite l’ottimizzazione delle condizioni operative: l’incremento della densità di corrente prodotta nella zona di ingresso della cella ottenuto con l’aumento del carico catalitico locale, determina un accresciuto consumo di ossigeno localizzato in tale zona. Questo consegue in una diminuzione di prestazione della zona di uscita dell’aria, a questo punto operante mediamente con un reagente più povero di ossigeno e più carico di acqua del caso con componente tradizionale.
La possibilità di incrementare leggermente la stechiometria catodica (agendo sulla portata di aria reagente) permette di regolare quanto la zona di uscita della cella possa contribuire alla produzione di corrente complessiva incrementando sensibilmente la sua prestazione locale. Al contrario, l’accresciuto carico catalitico e di ionomero nella zona di ingresso, che consegue in una maggior produzione di corrente locale e quindi di acqua (favorendo il meccanismo di auto-idratazione), infatti, permette che l’incremento di portata di aria non determini una perdita di prestazione locale dovuta all'incremento dell’effetto deidratante dell’aria stessa
METHOD OF CONTROL OF A PEM FUEL OR ELECTROLYSIS CELL
No full textA method of control of a PEM fuel or electrolysis cell with an extended lifetime, improved performance and uniform and stable operation is disclosed wherein a membrane electrode assembly (2) is provided with a gradient of one or more properties in combination with a modification of one or more control parameters of the cell during its long term operation
Fast and reliable calibration of thermal-physical model of lithium-ion battery: a sensitivity-based method
No full textPhysical simulation of lithium-ion battery is crucial to consolidate the understanding of its operating mechanisms and, potentially, its state of health; nevertheless, a reliable model calibration is complex due to the large number of physical parameters involved. Here, a thorough sensitivity analysis is performed on the simulation of discharge, relaxation and impedance spectroscopy tests, to highlight the response of the Doyle-Fuller-Newman model output, implemented with a thermal model to compute heat transfer effects, to a variation of 28 model parameters as a function of ⁓160 combinations of temperature, battery state of charge and C-rate. The analysis highlights how up to 14 parameters can be regarded as insensitive, reasonably excludable from model calibration, while other parameters show a strongly miscellaneous response, possible to be maximized adopting specific conditions. Therefore, an innovative method is proposed by experimentally exploiting two temperature levels and combining the three techniques, demonstrated to be highly complementary for a fast and reliable model calibration. As a case study, it is applied on a commercial battery sample, enabling a repeatable and physically sound calibration of the model parameters, as successfully demonstrated over a set of full discharges in 12 combinations of temperatures and C-rate. The comparison with a standard discharge-based calibration process highlights the strength of the proposed protocol
Degradation of lithium-ion batteries under automotive-like conditions: aging tests, capacity loss and q-OCP interpretation
No full textBattery electric vehicles are spreading worldwide as a relevant solution for the decarbonization of the transportation sector, ensuring high volume and weight-based energy density, high efficiency and low cost. Nevertheless, batteries are known to age in a rather complex and conditions-dependent way. This work aims at investigating battery aging resulting from close-to-real world conditions, highlighting single stressors role. Hence, aiming at representativeness for automotive application, an extensive literature review is performed, identifying a wide set of representative conditions together with their specific variations to be investigated. Realistic driving schedules like WLTP is identified and continuously applied in cycling on commercial samples, investigating the capacity loss from a q-OCP perspective with an equilibrium model. In general, loss of lithium inventory is detected as the main degradation parameter, likely related to SEI growth. Recharge C-rate and load profile appear as poorly-affecting degradation, while a dominant role is associated with operating temperature. Interestingly, temperature and cycling-related degradation appears to be independent and their effects can be effectively superimposed. Loss of active positive electrode material seems particularly affected by cycling depth of discharge, likely having mechanical origin as particle cracking
In operando measurement of localised cathode potential to mitigate DMFC temporary degradation
No full textAn innovative external reference electrode technique has been applied to the cathode of an operating DMFC in order to identify variations in electrode potential across the active area of the cell. The evolution of cathode potential at two different locations in the cell was monitored during operation, with the primary focus on studying the potential dynamics during the temporary degradation recovery procedure, the so-called refresh cycle. The results highlight for the first time a non-uniform local recovery of temporary degradation at the cathode during refresh cycles, associated with varying rates of platinum oxide reduction across the cell, which could lead to current density redistribution and contribute to an uneven degradation of the components. The technique shows great promise for the improvement of long term DMFC performance via optimisation of refresh cycle protocols
A Comprehensive Physical‐Based Sensitivity Analysis of the Electrochemical Impedance Response of Lithium‐Ion Batteries
No full textThe electrochemical impedance spectroscopy (EIS) characterization technique, although widely adopted in electrochemistry for understanding operational issues and degradation, has a less consolidated physical interpretation in lithium-ion batteries (LIBs), often relying on circuital methods. Herein, the Doyle-Fuller-Newman model is adapted and experimentally validated for the physical simulation of electrochemical impedance; then, it is applied in a comprehensive one-factor-at-time sensitivity analysis on an impedance spectrum from 4 kHz to 0.005 Hz; 28 physical parameters, which represent the kinetic, resistive, diffusive, and geometric characteristics of the battery, are varied within broad literature-based ranges of values, for each of the 20 analyzed battery states, characterized by different state-of-charge and temperature values. The results show a miscellaneous sensitivity of parameters on impedance spectra, which ranges from highly sensitive to negligible, often resulting in a strong dependence on operating conditions and impedance frequency. Such results consolidate the understanding of LIB electrochemical impedance and demonstrate that 40% of the parameters, 12 out of 28, can be considered poorly sensitive or insensitive parameters; therefore, fitting the experimental EIS data, their value can be assumed from the literature without significantly losing accuracy
Diagnosis of lithium-ion batteries degradation with P2D model parameters identification: A case study on low temperature charging
No full textThe estimation of the state of health (SoH) of a lithium-ion battery is still a hot topic in the scientific research. This publication deals with the combined use of optimized tests, also involving impedance spectroscopy, and physical models to investigate lithium-ion batteries degradation. As a case study, this method is firstly applied on a low-temperature charging degradation campaign, in order to expectedly generate a lithium plating-dominated ageing state. Degradation tests, performed under previously selected combinations of operating conditions, are performed down to 75 % SoH on commercial samples, determining severe ageing rate up to 1.5 % capacity loss per equivalent full cycle. The proposed interpretation methodology identifies the ageing to be dominated by the loss of lithium inventory, consistently with the expected degradation mechanism. Large electrolyte consumption is also detected, which induces a strongly anisotropic utilization of the electrodes during discharge, as confirmed by pseudo-two-dimensional (P2D) model simulations. This activity contributes to verify the reliability of the methodology, elucidate the effect of lithium plating on the performance and underline the effect of the operating conditions at low temperature, paving the way to the application on real-world conditions
A locally resolved investigation on direct methanol fuel cell uneven components fading: Local cathode catalyst layer tuning for homogeneous operation and reduced degradation rate
No full textDurability issues of direct methanol fuel cell still hinder technology widespread commercialization; uneven aging of MEA components, generally harsher in air outlet region, is known to exasperate overall performance degradation. In a previous work, the authors selected a stable cathode electrode, demonstrated to fade homogenously: uneven water-related limitations, such as dehydration and flooding, were revealed to locally worsen performance at cathode inlet and outlet regions, leading to current redistribution. Aiming to reduce degradation rate, in this work homogeneous current distribution during operation is pursued by tuning MEA properties to meet local operating conditions. A properly improved 1D+1D physical model is used to support the development of a gradient MEA, featuring 1.6 mg cm−2 and 0.8 mg cm−2 of catalyst and ionomer respectively at inlet/outlet and center regions of cathode electrode. Tests based on custom macro-segmented cell demonstrated 55% more homogeneous current distribution, controllable during operation by means of cathode air stoichiometry. 500 h degradation test revealed 70% decreased degradation rate from uniform MEA (11 μV h−1) with a homogenous fading of performance. An 18% lower Pt nanoparticle growth at cathode outlet and limited ionomer degradation at cathode inlet were identified by ex-situ analyses (TEM and XPS), indicating locally mitigated fading mechanisms
Investigation of vanadium redox flow batteries performance through locally-resolved polarisation curves and impedance spectroscopy: Insight into the effects of electrolyte, flow field geometry and electrode thickness
No full textThe inhomogeneous distribution of electrolyte over the porous electrode is one of the main issues hindering vanadium flow battery performance, limiting the system power density. Moreover, the interplay between fluid dynamics in distribution channels and morphology of porous electrodes is not well understood at local level throughout the cell active area, where local reactant starvation may occur and further limit system performance. Therefore, a systematic local characterization is fundamental to improve the understanding of system operation and to enhance system power density. This work discusses the implementation of a 10 regions macro-segmented flow battery with 25 cm2 active area, that permits the local measurement of polarisation curves and impedance spectra. Both positive and negative symmetric cells, as well as all-vanadium configuration, are adopted, combining carbon paper electrodes of different thickness with single serpentine and interdigitated distributors. The characterization evidences that single serpentine presents poor performance towards cell outlet, while interdigitated geometry is limited in the central region of active area. In most of the investigated operating conditions, single serpentine induces a more heterogeneous operation, especially if coupled with thinner electrode and positive electrolyte. Moreover, the representativeness at local level of symmetric cells compared to all-vanadium one is analysed and discussed
Mitigated Start-Up of PEMFC in Real Automotive Conditions: Local Experimental Investigation and Development of a New Accelerated Stress Test Protocol
No full textThis study combines local electrochemical diagnostics with ex situ analysis to investigate degradation mechanism associated to start-up/shut-down (SU/SD) of PEMFC and mitigation strategies adopted in automotive stacks. Local degradation resulting from repeated SU/SD was analyzed with and without mitigation strategies by means of a macro-segmented cell setup provided with Reference Hydrogen Electrodes (RHEs) at both anode and cathode to measure local electrodes potential and current. Accelerated Stress Test (AST) for start-up with and without mitigation strategies are proposed and validated. A ten-fold acceleration of performance loss due to un-mitigated SU/SD has been calculated with respect to AST for catalyst support. Under mitigated SU/SD, instead, a strong degradation was observed as localized at cathode inlet region (i.e. −38% ECSA loss and −22 mV voltage loss after 200 cycles) due to local potentials transient reaching up to 1.5 V vs RHE. These localized stress conditions were additionally reproduced in a zero-gradient and a new AST protocol (named start-up AST) was proposed to mimic the potential profile observed upon SU/SD cycling. Representativeness of the start-up AST for real world degradation was confirmed up to 200 cycles. Platinum dissolution and diffusion/precipitation within the polymer electrolyte was shown to be the dominant mechanism affecting performance loss
