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Activating proton exchange membrane fuel cells is crucial to systematically build proton conductive pathways, remove contaminants from the electrode surface and obtain the highest performances, with air interruption emerging as one of the most powerful methods. While this approach enables fast activation and considerable recovery after extended periods of shutdown or operation, the impact of relative humidity on this process has not been explored. Herein, the impact of the relative humidity on the air interruption is investigated using an array of operando methods such as polarization, electrochemical impedance spectroscopy, and high-resolution localized current and mapping. It is found that while air interruptions in humidified conditions can improve performance, they rapidly reduce them in dry conditions. Specifically, air interruptions in fully humidified conditions improve ions, charges, and gas diffusion to the active sites, which may be attributed to the removal of platinum oxides increasing active site availability. However, air interruptions under low relative humidity rapidly reduce the cell voltage by increasing charge transfer and mass transport limitations, while causing extensive overheating as the cell is starved of oxygen and hydrogen might be generated. Overall, this work reveals the potential, yet limitations of air interruptions in operando hydrogen fuel cells. [Display omitted] • Air interruptions can either improve or degrade PEMFC performance. • Advanced characteriation with EIS with DRT and current and heat cartography. • Air interruptions in dry conditions lower performance via hydrogen pumping. • Air interruptions in humidified conditions improve performance by oxide removal. • Potentiostatic air interruptions are less impactful than galvanostatic ones. [ABSTRACT FROM AUTHOR]

The electrochemical reactions in proton exchange membrane fuel cells take place at the interface between the proton conductive electrolyte, gases, and catalyst active site, or the so-called triple-phase boundary. The triple-phase boundary area can increase through catalyst layer designs and hydrogen fuel cell operations. While catalyst layers are commonly characterized using physical methods, the impact of hydrogen fuel cell operations on triple-phase boundaries is significantly more challenging to analyze, with operando electrochemical characterizations currently lacking the necessary accuracy and sensitivity to capture these evolutions. Herein, we introduce an innovative approach to monitoring triple-phase boundary evolutions by simultaneously capturing the ohmic resistance and double-layer capacitance in transient operations using high-frequency zero-phase impedance spectroscopy. The operando catalyst layer utilization quickly reduces through-out dehydration, flooding, excessive load increase, and cell degradations. This approach precisely pinpoints how proton transport, electron transport and oxygen diffusion can altogether reduce the triple-phase boundary area. This new analysis, conducted using commercial materials, unravels the evolutions of the active site and electrolyte interactions throughout the hydrogen fuel cell operations and lifecycle, evidencing a rapid evolution in the triple-phase boundary area throughout the hydrogen fuel cell operations. [Display omitted] • The operando catalyst layer utilization changes with PEMFC operations and health. • Accurate frequency of zero-phase measurement captures double-layer capacitance. • Zero-phase impedance increases ohmic accuracy by 10% in transient conditions. • Cell hydration and health impacts frequency of zero-phase, capacitance and ohmic. [ABSTRACT FROM AUTHOR]

Selecting the ideal operating point for a fuel cell depends on the application and consequent trade-off between efficiency, power density and various operating considerations. A systematic methodology for determining the optimal operating point for fuel cells is lacking; there is also the need for a single-value metric to describe and compare fuel cell performance. This work shows how the ‘current of lowest resistance’ can be accurately measured using electrochemical impedance spectroscopy and used as a useful metric of fuel cell performance. This, along with other measures, is then used to generate an ‘electro-thermal performance map’ of fuel cell operation. A commercial air-cooled open-cathode fuel cell is used to demonstrate how the approach can be used; in this case leading to the identification of the optimum operating temperature of ∼45 °C. [ABSTRACT FROM AUTHOR]

Low-cost, high-performances and durable hydrogen fuel cells are crucial for the success of the hydrogen economy. While Fe–N–C structures containing Fe-N x active sites are amongst the most promising platinum (Pt)-group metal (PGM)-free catalysts for the oxygen reduction reaction, their highest performances-to-date are still inferior to commercial Pt in real proton exchange membrane fuel cells. Herein, we shed light on this performance gap by using the distribution of relaxation times to quantify the proton transport and oxygen reduction reaction kinetics of a high-performance Fe–N–C catalyst (1.08 W cm−2) and a commercial Pt catalyst (1.7 W cm−2) in hydrogen fuel cell. This study unveils that the Fe–N–C catalyst has slower proton transport and oxygen reduction reaction kinetics than Pt as the Fe–N–C nanoporous carbon matrix limits active site accessibility. Furthermore, while increasing the Fe–N–C catalytic mass loading (from 1 to 3 mg Fe-N-C cm−2) enhances the power density in hydrogen fuel cells, it also slows down proton transport and oxygen reduction reaction kinetics by lengthening the gas, electron, and proton pathways to the active sites. This finding will drive the development of PGM-free catalysts for hydrogen fuel cells and of single-atom catalysts for electrochemical applications. [Display omitted] • The distribution of relaxation times captures ORR kinetics of PGM-free catalysts. • PGM-free catalysts nanoporous structure reduces ORR and proton transport kinetics. • Using high loadings of PGM-free catalysts increases the pathway to active sites. [ABSTRACT FROM AUTHOR]

Abstract: Dead-ended anode operation, with intermittent purge, is increasingly being used in polymer electrolyte fuel cells as it simplifies the mass flow control of feed and improves fuel efficiency. However, performance is affected through a reduction in voltage during dead-ended operation, particularly at high current density. This study uses electrochemical impedance spectroscopy (EIS), off-gas analysis and high resolution thermal imaging to examine the source of performance decay during dead-ended operation. A novel, ‘reconstructed impedance’ technique is applied to acquire complete EIS spectra with a temporal resolution that allows the dynamics of cell processes to be studied. The results provide evidence that upon entering dead-ended operation, there is an initial increase in performance associated with an increase in anode compartment pressure and improved hydration of the membrane electrolyte. Subsequent reduction in performance is associated with an increase in mass transport losses due to a combination of water management issues and build-up of N2 in the anode. The purge process rapidly recovers performance. Understanding of the processes involved in the dead-end/purge cycle provides a rationale for determining the optimum cycle frequency and duration as a function of current density. [Copyright &y& Elsevier]

Proton exchange membrane fuel cells involve complex electrochemical processes. Electrochemical impedance spectroscopy is a suitable technique to study these reactions occurring with different kinetics with the ohmic, charge transfer and mass transport processes routinely observed from 1 kHz to 1 Hz. However, a low-frequency capacitive/inductive component (<1 Hz) has been discovered in standard and extreme conditions, while its origin is yet to know. Here, we demonstrate that imposing air perturbations of the frequency and amplitude ratio of the current density perturbations can induce a low-frequency inductive semi-circle. Such air perturbation enables the accurate detection of the polarization resistance with a low-frequency impedance and can reach steady-state conditions during the low-frequency electrochemical impedance spectroscopy measurements. Controlling the air perturbation amplitude and current density (0.2–1 A cm−2) dominates the state of low-frequency arc, and the presence of a threshold for the air perturbation amplitude is crucial for its formation. Our findings elucidate why the low-frequency component has not been systematically detected before, due to variations of the oxygen concentration at the electrode. Finally, these results propose an easy-to-implement and widely applicable strategy to observe the low-frequency arc, investigate the cell diffusion properties and monitor the oxygen concentration at the electrode surface. Image 1 • Imposing air perturbations during galvanostatic EIS induces a low-frequency semi-circle. • Increasing the air perturbation increases the size of the low-frequency arc. • Same air and current density amplitude capture steady-state low-frequency limit. • Similar low-frequency arc for low oxygen concentrations and high air amplitudes. [ABSTRACT FROM AUTHOR]

Proton exchange membrane fuel cells (PEMFCs) are considered a significant player in the hydrogen economy. However, before mass production is possible, significant improvements in durability are necessary. Monitoring the changes in the electrode structure is challenging without a complex measurement apparatus. Precisely, the changes in electrode properties during carbon corrosion (increase in the porosity and electrode collapse) cannot be quantified using conventional electrochemical methods. Here, we report capturing the oxygen diffusivity in the PEMFC cathode catalyst layer using low-frequency electrochemical impedance spectroscopy (0.3-0.01 Hz). The low-frequency arc is fitted with resistance, inductance, and capacitance in parallel to represent the resistance to oxygen supply, inertia to oxygen diffusion, and oxygen storage capacity in the catalyst layer, respectively. Over 600 cycles of accelerated stress test (ASTs) of carbon corrosion, the capacitance increases by 25–45% (0–150 ASTs), indicating an increase in oxygen storage capacity and electrode porosity. Then, (150–600 ASTs) the resistance and inductance increase while the capacitance decreases by 80%, highlighting a decrease of the oxygen diffusivity and storage in the catalyst layer as the electrode collapses, which causes oxygen starvation. Altogether, this low-frequency approach correlates electrochemical impedance measurements with the changes in electrode structure during carbon corrosion. • A low-frequency inductive impedance arc appears during PEMFC's carbon corrosion. • Oxygen starvation is shown by fitting the low-frequency arc with a resonant loop. • Capturing the oxygen diffusivity links structure and electrochemical performance. [ABSTRACT FROM AUTHOR]

Wide–scale commercialisation of proton exchange membrane fuel cell (PEMFC) is hindered by the degradation of cathode catalyst and carbon support, making it crucial to understand these mechanisms. This study compares the cathode catalyst and carbon support degradations mechanisms using commercial gas diffusion electrode in operating PEMFCs. The impact of the accelerated stress tests (ASTs) is monitored by recording the polarisation, electrochemical surface area (ECSA) and mass activity. Furthermore, the electrochemical impedance spectroscopy (EIS) is monitored at seven operating points from 0.05 to 1 A cm−2. For both phenomena, the charge transfer resistance increases at high current densities, which is attributed to the losses in activity observed with the ECSA and the mass activity degradations. In the carbon–corroded PEMFC, the mass transport resistance first reduces by 45% (0–100 cycles), suggesting an increase in electrode porosity due to carbon corrosion. At a later stage (200–5000 cycles), the mass transport resistance increases by 225% as the electrode collapses. On the other hand, in the catalyst–degraded PEMFC, the mass transport resistance uniformly reduces while the charge transfer resistance increases by 30%. Altogether, EIS provides additional sensitivity to differentiate catalyst and carbon support degradation within PEMFCs. • EIS differentiates PEMFC cathode catalyst and carbon support degradations. • The cathode catalyst and carbon support degradations are monitored in PEMFC ASTs. • The charge transfer resistance increases during the ASTs due to loss of activity. • The transport resistance first reduces, then increases during carbon corrosion. [ABSTRACT FROM AUTHOR]

Advances in bespoke diagnostic techniques for polymer electrolyte fuel cells continue to provide unique insight into the internal operation of these devices and lead to improved performance and durability. Localised measurements of current density have proven to be extremely useful in designing better fuel cells and identifying optimal operating strategies, with electrochemical impedance spectroscopy (EIS) now routinely used to deconvolute the various losses in fuel cells. Combining the two techniques provides another dimension of understanding, but until now each localised EIS has been based on 2-electrode measurements, composed of both the anode and cathode responses. This work shows that a reference electrode array can be used to give individual electrode-specific EIS responses, in this case the cathode is focused on to demonstrate the approach. In addition, membrane hydration dynamics are studied under current load steps from open circuit voltage. A three-stage process is identified associated with an initial rapid reduction in membrane resistance after 10 s of applying a current step, followed by a slower ramp to approximately steady state, which was achieved after ∼250 s. These results support previously published work that has looked at membrane swelling dynamics and reveal that membrane hydration/membrane resistance is highly heterogeneous. [ABSTRACT FROM AUTHOR]

High temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are used for stationary to mobile applications and show increased tolerances to fuel impurities like CO compared to their low temperature counterpart. However, conventional HT-PEMFC membranes, based on polybenzimidazole (PBI), suffer from creeping during long-term operation and thus a significant increase in surface resistance. In this study, inorganic fillers in terms of silicon carbide (SiC) are incorporated into the standard PBI-based Celtec®-P membrane by a well-established large-scale polyphosphoric acid process. Operations over 1000 h of load cycling between 0.6 and 1.0 A cm−2 of SiC and for conventional HT-PEMFCs reveal differences in long-term durability. The ohmic resistance of the SiC-based single cell is 1/3 lower, while the membrane has a higher thickness retention. Cell performances are improved with lower degradation rates (<65 μV h−1 for both SiC and >100 μV h−1 for both conventional HT-PEMFCs) after 1000 h. However, SiC particle mobility and increased hydrogen permeability are observed after testing. • Incorporating SiC into the industrial polybenzimidazole membrane fabrication. • Long-term testing by HT-PEM single cell operations for 1000 h of load cycling. • Lower HT-PEMFC ohmic resistance with SiC compared to conventional cells. • Identified degradation in terms of SiC particle mobility and hydrogen permeability. [ABSTRACT FROM AUTHOR]

The performance of the polymer electrolyte membrane (PEM) fuel cell is governed by a complex interaction of the structure of the membrane electrode assembly (MEA), cell compression, and operating parameters. Adequate cell compression for improved current collection and gas sealing can structurally deform MEA with adverse consequences. Non-uniform MEA compression exerted by the flow-field design and arrangement induces heterogeneous transport properties. Hence, understanding morphological evolution and effective transport properties as an effect of MEA compression is an important factor for improving fuel cell performance and durability. In this paper, an X-ray computed tomography study of the entire MEA compression is presented, comprising of gas diffusion and microporous layers, catalyst layers, and the electrolyte membrane, subjected to non-uniform compression under two distinct flow-field arrangements. This study presents a comprehensive dataset of the heterogeneous effective properties required for robust computational modelling; including porosity, permeability, tortuosity, and diffusivity, along with the extent of blocking of the flow channel due to cell compression and effect of compression on the structural properties of the membrane. Image 1 • X-ray CT characterisation of the membrane electrode assembly used in PEMFC. • Compression of commonly used flow-field arrangements on the entire MEA. • Effect of channel/land arrangement on the morphological properties of the MEA. • Effective parameter derived for inputs to the PEMFC computational models. [ABSTRACT FROM AUTHOR]