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Replacing Pt-based catalysts with cost-effective, highly efficient, and durable platinum group metal-free catalysts for the oxygen reduction reaction (ORR) is crucial for commercializing hydrogen fuel cells. Herein, we present a highly active Fe–N–C electrocatalyst that contains both Fe nanoparticles and FeNx active sites derived from an Fe-doped carbonized zeolitic imidazolate framework (ZIF-8). It is found that adjusting the doping amount of Fe in the Fe-doped ZIF-8 precursor alters the morphology of the catalyst after heat treatment. The Fe–N–C-300 composite catalyst with the optimized Fe doping amount exhibits excellent activity, good stability, and remarkable methanol tolerance in the challenging acid environment. This study reveals that a suitable amount of Fe nanoparticles in the catalyst can alter the structure of the FeNx active moieties and increase three-phase boundaries to boost the mass transport, thus leading to improved fuel cell performance. This will have implications for using Fe–N–C catalysts in real applications, as the formation of Fe NPs during the synthesis and reaction is almost inevitable. [ABSTRACT FROM AUTHOR]

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]

Abstract Flow-fields are key factors in determining the operation of fuel cells. While extensive work has been conducted to develop and optimise the reactant flow and current collection performance of polymer electrolyte membrane fuel cell (PEMFC) components, there is a factor that remains largely unaccounted for. Depending on how a membrane electrode assembly (MEA) is incorporated into a cell, there will often be a small gap between the edge of the gas diffusion layer (GDL) and the seal or bipolar plate. This gap acts as a 'secondary flow-field' (SFF) that can bypass or affect/augment the conventional or 'primary flow-field'. Understanding how this affects performance (either positively or adversely) is essential for holistic flow-field design. This paper describes the issues associated with the SFF, examines how cell compression affects its width due to lateral expansion of the GDL and discusses the results of a 3-D computational model that investigates the effect of the SFF during dead-ended anode (DEA) operation for a fuel cell without a macroscopic (conventional) anode flow-field. Highlights • The gap around GDL and seal represents a secondary flow-field. • Effect of GDL compression on its lateral expansion quantified. • Secondary flow-field improves dead-ended operation. [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]

The effect of nitrogen content within the hydrogen fuel supplied to a polymer electrolyte fuel cell (PEFC) operating in dead-ended anode mode is examined, with a view to using an ammonia decomposition product gas mix (containing 75H 2 :25N 2 ) as the hydrogen-containing fuel. The impact of this impure hydrogen stream, supplied to the anode, was evaluated in terms of mean cell voltage and in relation to actual operating conditions (purge interval, dead-ended interval and fuel cell load). Design of Experiments (DoE) methodology, using multi-linear models, assessed hydrogen utilisation in terms of stack efficiency and identified an effective and viable dead-ended anode purge strategy for this nitrogen-containing hydrogen fuel. [ABSTRACT FROM AUTHOR]

In-operando diagnostic techniques provide a means of understanding the internal workings of fuel cells under normal operating conditions so that improved designs and operating regimes can be identified. Here, an approach is used which combines ex‐situ characterisation of two anode gas diffusion/microporous layers (GDL-A and GDL-B) with X-ray computed tomography (CT) and in-operando analysis using neutron imaging of operating fuel cells. The combination of TGA, SEM and X-ray CT reveals that GDL-A has a thin microporous layer with ∼26% PTFE covering a thick diffusion layer composed of ‘spaghetti’-shaped fibres. GDL-B is covered by two microporous media (29% and 6.6% PTFE) penetrating deep within the linear fibre network. The neutron imaging reveals two pathways for water transport underneath the cooling channel, either diffusing through the cathode GDL to the active channels, or diffusing through the membrane and towards the anode. The behaviour of water in the GDL is directly affected by the anode gas diffusion, PTFE content and porosity. [ABSTRACT FROM AUTHOR]

Dead-ended anode operation has a number of practical advantages that simplify system complexity and lower cost for polymer electrolyte fuel cells. However, dead-ended mode leads to performance loss over time which can only be reversed by performing intermittent purge events. This work applies a combined hydro-electro-thermal analysis to an air-cooled open-cathode fuel cell, presenting experimental functional maps of water distribution, current density and temperature. This approach has allowed the identification of a ‘nitrogen blanketing’ effect due to nitrogen cross-over from the cathode and a ‘bypass’ effect where a peripheral gap between the gasket and the GDL offers a hydrogen flow ‘short circuit’ to the border of the electrode. A consequence of high local current density at the margin of the electrode, and resulting high temperatures, may impact the lifetime of the cell in dead-end mode. [ABSTRACT FROM AUTHOR]

Open-cathode polymer electrolyte fuel cells have the advantage of simplified construction and the potential for cost reduction. However, effective water and temperature management calls for a different approach to conventional fuel cells. Improved understanding of the link between current density, temperature, water formation and hydration of the membrane is required to optimise cell design. This work uses thermal imaging, optical visualisation, gravimetric analysis and electrochemical impedance spectroscopy to study the links between these factors and also examines the effect of cell orientation on performance. The results reinforce the importance of cell temperature for water management, which is a function of current density. Regions of the cell with higher temperature avoid flooding by promoting vapour phase water; however, this can lead to higher membrane resistance due to dehydration. The results reveal that transition in the water balance regime from continuous hydration and flooding to drying with increasing current density occurs between 34 °C–40 °C (at 250 mA cm −2 ) and water generation is balanced with evaporation at ∼50 °C (550 mA cm −2 ). Finally, fuel cell orientation affects performance after extended operation due to the effect of water accumulation and evaporation, with the cathode in a ‘flat upwards’ orientation found to be most resistant to flooding and cathode in a flat downwards orientation the most likely to flood. [ABSTRACT FROM AUTHOR]

Overall fuel cell system efficiency must include consideration of the parasitic loads associated with operating the stack. For an air-cooled open-cathode polymer electrolyte fuel cell (PEFC) the air blowers form the largest parasitic load and have a direct influence of the performance of the stack and its temperature; increasing air flow often leading to improved performance at the expense of increasing parasitic load. Here, electro-thermal performance maps are used to characterise stack operation with varying air flow rate (for both cooling and cathode oxygen supply). The non-linear power requirement of air blowers is used to modify stack-level electro-thermal maps to give system-level performance maps capable of identifying the optimum air flow rate to maximise net power output and consequently efficiency at a given operating point. The electro-thermal map concept is invoked in the consideration of dynamics of fuel cell system operation with issues pertaining to speed of response, durability and system self-sustainable operation. [ABSTRACT FROM AUTHOR]

In situ diagnostic techniques provide a means of understanding the internal workings of fuel cells so that improved designs and operating regimes can be identified. Here, a novel metrology approach is reported that combines current and temperature mapping with water visualisation using neutron radiography. The approach enables a hydro-electro-thermal performance map to be generated that is applied to an air-cooled, open-cathode polymer electrolyte fuel cell. This type of fuel cell exhibits a particularly interesting coupled relationship between water, current and heat, as the air supply has the due role of cooling the stack as well as providing the cathode reactant feed via a single source. It is found that water predominantly accumulates under the cooling channels (thickness of 70-100 μm under the cooling channels and 5-25 μm in the active channels at 0.5 A cm −2 ), in a similar fashion to the lands in a closed-cathode design, but contrary to passive open-cathode systems. The relationship between current, temperature and water accumulation is complex and highly dependent on location within the cell. However, there is a general trend that higher currents and cooling limitations, especially above 0.7 A cm −2 and below 3.9 × 10 −3 m 3 s −1 , leads to temperatures above 60 °C, which dehydrate the membrane (water thickness of 10-25 um) and the cell operates below 0.5 V. [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]

• Proton exchange membrane fuel cell materials 3D structure are captured by micro-CT. • A symmetrical U-ResNet convolutional neural network model is used for segmentation. • CNN segmentation outperforms the manual segmentation, especially for carbon fibre and binder. • CNN-based segmentation captures realistic fuel cell structure parameters. This study demonstrates the benefit of convolutional neural networks to accurately classify the different materials of proton exchange membrane fuel cells using X-ray micro-computed tomography. Nineteen 2D orthoslices from a 3D tomography dataset were segmented with high quality and used to train a novel U-ResNet convolutional neural network (CNN) to segment the complete volume. The results were compared with a 3D manual segmentation performed under time constraints. The CNN segmented all phases with equal or greater accuracy in comparison to the manual segmentation. In particular, the CNN excelled in separating the carbon fibres and binder phase in the gas diffusion layer, which is usually completely avoided due to difficulty. Further, permeability calculations were performed on the binder void space for both segmentations, with the CNN displaying realistic results. Therefore, CNNs have been shown to be a viable and valuable method in segmenting such fuel cells with increased efficiency and accuracy. [ABSTRACT FROM AUTHOR]

Open-cathode air-breathing fuel cells have the advantage of reduced system complexity and simplified operation, as oxygen is taken directly from ambient air without the need for blowers/compressors. In this study, printed circuit boards (PCBs) are used as flow-field plates. The use of PCBs offers the potential for significant cost reduction due to their well-established manufacturing processing and low materials cost. This study investigates the effect of varying the cathode geometry (parallel and circular) and opening ratios (43%, 53% and 63%) on fuel cell performance using polarisation curves, electrochemical impedance spectroscopy (EIS) and thermal imaging. The results obtained indicate that circular openings afford lower Ohmic resistance than parallel flow-field designs, which helps improve contact between the gas diffusion layer and flow-field plate. However, flow-field plates with circular openings suffer from greater mass transport limitation effects. Likewise, greater opening ratios offer better mass transport but increased Ohmic resistance as a result of the reduced area of lands/ribs. The thermal imaging results reveal lower temperature in the middle of the fuel cell due to “bowing” of the printed circuit board flow field plates which reduces the local current density. A trade-off between these factors results in a design with a maximum area specific power density of 250 mW cm −2 . [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]

Aqueous proton batteries are promising candidates for the harvest and utilization of renewable yet intermittent energies. The redox couple of MnO2/Mn2+ is one of the most competitive cathodes to enable proton batteries with high voltage. However, electrolytic products of the MnO2/Mn2+ reactions tend to disperse into acidic electrolytes, and the composition of the electrolytic products as well as their influences on the counter electrode and the overall batteries are still unclear. Herein, the behaviors of the manganese electrolysis are studied with electrolytes of different proton concentrations and under variant current densities. The electrolytic products are disclosed to be ɛ‐MnO2 regardless of the acidities of electrolytes and report, for the first time, the dispersed MnO2 can chemically oxidize or dissolve the anode materials and subsequently induce self‐discharging. A membrane‐assisted protection strategy is proposed to prevent the free‐shuttle of MnO2 particles and mitigate the battery self‐discharging. Accordingly, a much‐enhanced shelf‐life performance (61.3% capacity retention over a one‐week rest) is achieved for pyrene‐4,5,9,10‐tetraone//MnO2 full‐cell. Furthermore, a customized device is developed with Nafion membrane, reaching excellent cycling stability (3000 cycles, 54 days) and a low self‐discharging rate. The findings and strategies for mitigating the self‐discharging issues are anticipated to advance the MnO2/Mn2+‐based aqueous batteries and beyond. [ABSTRACT FROM AUTHOR]

Platinum metal (PtM, M=Ni, Fe, Co) alloys catalysts show high oxygen reduction reaction (ORR) activity due to their well‐known strain and ligand effects. However, these PtM alloys usually suffer from a deficient ORR durability in acidic environment as the alloyed metal is prone to be dissolved due to its high electronegativity. Herein, we report a new class of PtMn alloy nanodendrite catalyst with low‐electronegativity Mn‐contraction for boosting the oxygen reduction durability of fuel cells. The moderate strain in PtMn, induced by Mn contraction, yields optimal oxygen reduction activity at 0.53 A mg−1 at 0.9 V versus reversible hydrogen electrode (RHE). Most importantly, we show that relative to well‐known high‐electronegativity Ni‐based Pt alloy counterpart, the PtMn nanodendrite catalyst experiences less transition metals' dissolution in acidic solution and achieves an outstanding mass activity retention of 96 % after 10,000 degradation cycles. Density functional theory calculation reveals that PtMn alloys are thermodynamically more stable than PtNi alloys in terms of formation enthalpy and cohesive energy. The PtMn nanodendrite‐based membrane electrode assembly delivers an outstanding peak power density of 1.36 W cm−2 at a low Pt loading and high‐performance retention over 50 h operations at 0.6 V in H2‐O2 hydrogen fuel cells. [ABSTRACT FROM AUTHOR]

Platinum metal (PtM, M=Ni, Fe, Co) alloys catalysts show high oxygen reduction reaction (ORR) activity due to their well‐known strain and ligand effects. However, these PtM alloys usually suffer from a deficient ORR durability in acidic environment as the alloyed metal is prone to be dissolved due to its high electronegativity. Herein, we report a new class of PtMn alloy nanodendrite catalyst with low‐electronegativity Mn‐contraction for boosting the oxygen reduction durability of fuel cells. The moderate strain in PtMn, induced by Mn contraction, yields optimal oxygen reduction activity at 0.53 A mg−1 at 0.9 V versus reversible hydrogen electrode (RHE). Most importantly, we show that relative to well‐known high‐electronegativity Ni‐based Pt alloy counterpart, the PtMn nanodendrite catalyst experiences less transition metals' dissolution in acidic solution and achieves an outstanding mass activity retention of 96 % after 10,000 degradation cycles. Density functional theory calculation reveals that PtMn alloys are thermodynamically more stable than PtNi alloys in terms of formation enthalpy and cohesive energy. The PtMn nanodendrite‐based membrane electrode assembly delivers an outstanding peak power density of 1.36 W cm−2 at a low Pt loading and high‐performance retention over 50 h operations at 0.6 V in H2‐O2 hydrogen fuel cells. [ABSTRACT FROM AUTHOR]

Fe-N-C catalysts are an attractive low-cost alternative to Pt for the oxygen reduction reaction (ORR) in high-temperature proton exchange membrane fuel cells (HT-PEMFCs) as they are less sensitive to phosphate poisoning, but they suffer from poor stability and unclear degradation mechanisms. In this study, the activation and degradation mechanisms of HT-PEMFCs with several classes of Fe-N-Cs operating for 90 h are identified using a combination of operando electrochemical and post-mortem physical analysis. While simultaneous activation in terms of accelerated ORR processes takes place by acid redistribution, the degradation of the catalyst leads to drastic performance loss (– 27%) within the first 24 h. Carbon corrosion has a lower impact and the strongest degradation is attributed to active site deactivation e.g. by transformation to inactive nanoparticles or by site loss. This study helps to understand the degradation of Fe-N-Cs in HT-PEMFC and points to the necessary and inevitable modification of single-atom catalysts. [Display omitted] • Voltage decay within first 24 h of HT-PEMFC operation independent of Fe-N-C type. • Stability of Fe-N-C in HT-PEMFC independent of electrochemical characterization in single-cell. • Voltage loss attributed to reactive oxygen species formation and Fe-N x deactivation. • No positive effect from acid distribution due to degradation of catalyst in beginning of operation. [ABSTRACT FROM AUTHOR]

Electrocatalysts are crucial to drive the electrochemical carbon dioxide reduction reaction (CO2RR) which lower the energy barrier, tune the intricate reaction pathways and suppress competitive side‐reaction. Beyond the efficient active sites and advantageous local environment, a rapid mass transfer ability is also crucial for the catalyst design. However, it is rare that research has been done to investigate in detail the mass transfer process in CO2RR, and expose the underlying relationship between mass transfer and final performance. Here, a single‐atom Fe‐N‐C catalyst is shown with a highly ordered porous substrate containing hierarchical micropores, mesopores, and macropores. Such a delicate porous structure significantly facilitates the mass transfer process toward the isolated Fe sites, achieving excellent CO2RR performance, especially in the limited mass transfer region in a H‐cell with a maximum CO partial current density of ‐19 mA cm−2. Operando electrochemical impedance spectroscopy and relevant distributed relaxation times analysis reveal the rapidly decreased mass transfer resistance with the increase of reduction potential. The Lattice Boltzmann method with Discrete Element method (LBM‐DEM) simulations are further performed to exhibit the origin of enhanced CO2RR performance from the facilitated gas diffusion process. [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]

NiFe-based and NiCo-based catalysts are promising alternatives to noble-metal catalysts for the oxygen evolution reaction (OER) due to their excellent catalytic activity, low cost and abundant resources. However, the active sites nature, along with the pathway to achieve the highest electroactivity, remains unclear. Herein, we use nickel foam (NF) as carrier and prepared NiFe/NF and NiCo/NF catalysts with excellent OER catalytic activity and stability by electrodeposition and calcine them either in Air or H 2 /Ar after deposition. In 1 M KOH media, the overpotential of NiFe/NF at 50 mA cm−2 is only 182 mV, which one of the lowest for OER reported to date. By analyzing the influence of the morphology, structure and surface chemical states caused by calcination on performances, we found that the ratio of hydroxide/hydroxyl oxide to lattice oxygen heavily impacts the OER with higher hydroxide/hydroxyl oxide to lattice oxygen ratio achieving the highest catalytic activity. This work significantly furthers the understanding of the OER mechanism on the surface of NiFe and NiCo catalysts and provides insights for regulating NiFe and NiCo catalysts to achieve higher OER performance. [Display omitted] [ABSTRACT FROM AUTHOR]

Catalytic synergy is an unresolved activity descriptor in energy conversion reactions. Here, this study reports the synergistically enhanced intrinsic oxygen evolution reaction activity of a FeNi model catalyst modified with molybdate oxo‐anions via concertedly boosted surface/interface interactions such as interfacial charge transfer, surface intermediate adsorption, in situ phase transformation, and gas bubble evolution. Capturing such cosynergistic impact reveals the accelerated transition of oxygen deprotonation to oxyhydroxide state, ready‐to‐function Fe and Ni active sites, the instant transformation of Fe and Ni local microenvironments to γ ‐FeOOH and β ‐NiOOH phases, and ultrafast gas bubble growth and release. The accelerated oxygen bubble evolution dynamic is monitored in operando with a decreased dynamic variation of the interfacial Faradaic resistance. Such cosynergistic molybdate modification results in a tenfold increase in oxygen evolution turnover frequency as well as a 55 mV decrease in the overpotentials to deliver 10 and 1000 mA cm−2 with respect to the FeNi‐model catalyst. [ABSTRACT FROM AUTHOR]

Abstract: The temperature of operation is a key parameter in determining the performance and durability of a polymer electrolyte fuel cell (PEFC). Controlling temperature and understanding its distribution and dynamic response is vital for effective operation and design of better systems. The sensitivity to temperature means that uncertainty in this parameter leads to variable response and can mask other factors affecting performance. It is important to be able to determine the impact of temperature uncertainly and quantify how much PEFC operation is influenced under different operating conditions. Here, a simple lumped mathematical model is used to describe PEFC performance under temperature uncertainty. An analytical approach gives a measure of the sensitivity of performance to temperature at different nominal operating temperatures and electrical loadings. Whereas a statistical approach, using Monte Carlo stochastic sampling, provides a ‘probability map’ of PEFC polarisation behaviour. As such, a polarisation ‘area’ or ‘band’ is considered as opposed to a polarisation ‘curve’. Results show that temperature variation has the greatest effect at higher currents and lower nominal operating temperatures. Thermal imaging of a commercial air-cooled stack is included to illustrate the temporal and spatial temperature variation experienced in real systems. [Copyright &y& Elsevier]

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]

Operating proton exchange membrane fuel cells in dead-ended anode mode results in fewer subsystem components and a lower cost and less complex system. However, dead-ended operation results in a gradual accumulation of water and impurities within the anode compartment, which leads to performance degradation. Therefore, anode purging is required to partially remove impurities and water from the anode and to recover performance. In the present study, a mathematical model, incorporating nitrogen crossover from the cathode to the anode and water build-up in the anode is developed. This model simulates the dead-ended anode proton exchange membrane fuel cell performance during the purge and the subsequent performance recovery. The model is in good agreement with experimental results. By using this model and introducing the concept of the 'total wasted energy', the purge parameters (purge interval and purge duration) can be optimized. The predicted optimum purge duration and purge interval for a sample single cell are 25 ms and 260 s, respectively. The effect of operating condition parameters on this optimization are investigated, showing that the hydrogen purity has a strong effect on the total wasted energy. By increasing the hydrogen purity from 99.5% to 99.99%, the efficiency increases by 2.4%. • Mathematical model developed for Dead-Ended Anode PEM Fuel Cells (DEA PEMFC). • The effects of purge valve characteristics on wasted energy are analyzed. • Hydrogen purity has a strong effect on wasted energy of DEA PEMFCs. • The wasted energy is minimized by optimizing the purge valve timing. [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]

Electrochemical impedance spectroscopy (EIS) is widely used to characterize electrochemical systems. The distribution of relaxation times (DRT) has emerged as a powerful, non-parametric alternative to circumvent the inherent challenges of EIS analysis through equivalent circuits or physical models. Recently, deep neural networks have been developed to estimate the DRT. However, this line of research is still in its infancy, and several issues remain unresolved, including the long training time and unknown accuracy of this method. Furthermore, deep neural networks have not been used for deconvolving DRTs with negative peaks. This work addresses these challenges. A pretraining step is included to decrease the computation time; error analysis allows error estimation and the development of error reduction strategies. Furthermore, the training loss function is modified to handle DRTs with negative peaks. For most cases tested, this new framework outperforms ridge regression. Moreover, these advances are validated with an array of synthetic and real EIS spectra from various applications, including lithium-metal batteries, solid oxide fuel cells, and proton exchange membrane fuel cells. Overall, this research opens new avenues for the development and application of the deep-neural-network-based analysis of EIS data. [ABSTRACT FROM AUTHOR]

Fe–N–Cs are an alternative to Pt/C catalysts as they show promising activity towards the oxygen reduction reaction. While several studies are based on thin-film analysis in diluted electrolytes, only few reports show the application of Fe–N–Cs in gas diffusion electrodes (GDE) and their tests under high temperature proton exchange membrane fuel cell (HT-PEMFC) conditions. Here, a series of Fe–N–Cs with different physical properties are analyzed. GDEs are fabricated using ultrasonic spray and doctor blade coating to investigate the effect of coating method on the morphology and performance. Morphological analysis identified thinner catalyst layers for spray coating and inhomogeneous PTFE distribution for doctor blade coated GDEs. Electrochemical characterization in the HT-PEM half-cell displays significantly lower performance for ultrasonic spray-coated GDEs, possibly due to more pronounced phosphoric acid penetration. On the contrary, implementation of GDEs as cathodes in the HT-PEMFC yields comparable performances for both coating methods but differences in ORR and proton transport resistances determined by distribution of relaxation times analysis. Moreover, it was shown that inhomogeneous PTFE distribution caused by high amounts of hydrophilic O-/N-functionalities negatively affects HT-PEMFC performance. This study helps to understand the impact of GDE fabrication and Fe–N–C catalyst properties in HT-PEM half- and single-cell application. • Comparison of two coating processes for Fe–N–C-based GDEs. • Morphological analysis of GDEs using μ-CT and confocal microscopy. • Implementation of Fe–N–Cs in HT-PEM half-cell setup and HT-PEMFC. • Advanced characterization of MEAs using CV and DRT analysis. [ABSTRACT FROM AUTHOR]