Prospects of spectroscopic neutron imaging: optimizing experimental setups in battery electrolyte research.

Spectral neutron imaging methods provide valuable insights into the characterization of hydrogenous materials, including battery electrolytes. However, their application is constrained by sample geometry, setup parameters, and material chemistries, especially when studying physico-chemical changes in battery electrolytes. To address these limitations, we present a framework for simulating and optimizing the investigation of hydrogenous materials. Our approach combines quantitative modeling with experimental data to predict and optimize the contrast achievable in wavelength-resolved neutron imaging methods, thereby maximizing the information obtained in specific neutron imaging setups. While initially demonstrated at the BOA beamline of the Paul Scherrer Institute, this framework is applicable to any continuous source with spectral neutron imaging capabilities with a chopper disk. This work establishes a pathway for accurate studies of hydrogenous materials and their physico-chemical behavior, paving the way for advancements in the field of material characterization with wavelength-resolved neutron imaging.

Impact of substitution on reactions and stability of one-electron oxidised phenyl sulfonates in aqueous solution.

Highly reactive aromatic cation radicals have been invoked lately in synthetic routes and in the degradation pathways of hydrocarbon-based polymers. Changes in the electron density of aromatic compounds are expected to alter the reaction pathway following one electron oxidation through altering the pKa of the formed intermediate cation radical. Electron-donating groups increase its stability, however, little experimental data are known. While, in theory, the cation radical can be repaired by simple electron transfer, electron transfer to or from its deprotonated form, the hydroxycyclohexadienyl radical, will cause permanent modification or degradation. Time-resolved absorption spectroscopy indicates a pKa ≈ 2-3 for the 4-(tert-butyl)-2-methoxyphenylsulfonate (BMPS) radical cation, while its parent compound 4-(tert-butyl) phenylsulfonate (BPS) is much more acidic. The stability of both compounds towards oxidation by HO˙ was evaluated under air at pH 5 and pH 0. At pH 5, both BMPS and BPS are unstable, and superstoichiometric degradation was observed. Degradation was slightly reduced for BPS at pH 0. In contrast, the more electron rich BMPS showed 80% lower degradation. We unambigously showed that in the presence of Ce(III) and H2O2 at pH 0 both BMPS and BPS could be catalytically repaired via one electron reduction, resulting in further damage moderation.

Correction: Duburg et al. Composite Polybenzimidazole Membrane with High Capacity Retention for Vanadium Redox Flow Batteries. Molecules 2021, 26, 1679.

The authors wish to make the following changes to their paper [...].

Composite Polybenzimidazole Membrane with High Capacity Retention for Vanadium Redox Flow Batteries.

Currently, energy storage technologies are becoming essential in the transition of replacing fossil fuels with more renewable electricity production means. Among storage technologies, redox flow batteries (RFBs) can represent a valid option due to their unique characteristic of decoupling energy storage from power output. To push RFBs further into the market, it is essential to include low-cost materials such as new generation membranes with low ohmic resistance, high transport selectivity, and long durability. This work proposes a composite membrane for vanadium RFBs and a method of preparation. The membrane was prepared starting from two polymers, meta-polybenzimidazole (6 µm) and porous polypropylene (30 µm), through a gluing approach by hot-pressing. In a vanadium RFB, the composite membrane exhibited a high energy efficiency (~84%) and discharge capacity (~90%) with a 99% capacity retention over 90 cycles at 120 mA·cm-2, exceeding commercial Nafion® NR212 (~82% efficiency, capacity drop from 90% to 40%) and Fumasep® FAP-450 (~76% efficiency, capacity drop from 80 to 65%).

Attack of hydroxyl radicals to α-methyl-styrene sulfonate polymers and cerium-mediated repair via radical cations.

Both synthetic polymers (membranes, coatings, packaging) and natural polymers (DNA, proteins) are subject to radical-initiated degradation. In order to mitigate the deterioration of the polymer properties, antioxidant strategies need to be devised. We studied the reactions of poly(α-methylstyrene sulfonate), a model compound for fuel cell membrane materials, with different degrees of polymerization with OH˙ radicals as well as subsequent reactions. We observed the resulting OH˙-adducts to react with oxygen and eliminate H2O, the relative likelihood of which is determined by pH and molecular weight. The resulting radical cations can be reduced back to the parent molecule by cerium(iii). This 'repair' reaction is also dependent on molecular weight likely because of intramolecular stabilization. The results from this study provide a starting point for the development of new hydrocarbon-based ionomer materials for fuel cells that are more resistant to radical induced degradation through the detoxification of intermediates via damage transfer and repair pathways. Furthermore, a more fundamental understanding of the mechanisms behind conventional antioxidants in medicine, such as ceria nanoparticles, is achieved.

Thermochemical unification of molecular descriptors to predict radical hydrogen abstraction with low computational cost.

Chemistry describes transformation of matter with reaction equations and corresponding rate constants. However, accurate rate constants are not always easy to get. Here we focus on radical oxidation reactions. Analysis of over 500 published rate constants of hydroxyl radicals led us to hypothesize that a modified linear free-energy relationship (LFER) could be used to predict rate constants speedily, reliably and accurately. LFERs correlate the Gibbs activation-energy with the Gibbs energy of reaction. We calculated the latter as the sum of one-electron transfer and, if appropriate, proton transfer. We parametrized specific transition state effects to orbital delocalizability and the polarity of the reactant. The calculation time for 500 reactions is less than 8 hours on a standard desktop-PC. Rate constants were also calculated for hydrogen and methyl radicals; these controls show that the predictions are applicable to a broader set of oxidizing radicals. An accuracy of 30-40% (standard deviation) with reference to reported experimental values was found suitable for the screening of complex chemical systems for possibly relevant reactions. In particular, potentially relevant reactions can be singled out and scrutinized in detail when prioritizing chemicals for environmental risk assessment.

Revealing the role of phosphoric acid in all-vanadium redox flow batteries with DFT calculations and in situ analysis.

The present work suggests the use of a mixed water-based electrolyte containing sulfuric and phosphoric acid for both negative and positive electrolytes of a vanadium redox flow battery. Computational and experimental investigations reveal insights on the possible interactions between the vanadium ions in all oxidation states and sulphate, bisulphate, dihydrogen phosphate ions and phosphoric acid. In situ cycling experiments and ion-specific electrochemical impedance measurements confirmed a significant lowering of the charge-transfer resistance for the reduction of V(iii) ions and, consequently, an increase of the voltaic efficiency associated with the negative side of the battery. This increase of performance is attributable to the complexation of this oxidation state by phosphoric acid. So far, mixed acids have mainly been discussed with the focus on V(v) solubility. In this work we rationalize the impact of the mixed acids on the electrochemical efficiency opening new strategies on how to improve the cycling performance with ionic additives.

Cathode Catalyst Layer Design in PEM Water Electrolysis toward Reduced Pt Loading and Hydrogen Crossover.

Reducing the use of platinum group metals is crucial for the large-scale deployment of proton exchange membrane (PEM) water electrolysis systems. The optimization of the cathode catalyst layer and decrease of the cathode Pt loading are usually overlooked due to the predominant focus of research on the anode. However, given the close relationship between the rate of hydrogen permeation through the membrane in an operating cell and the local hydrogen concentration near the membrane-cathode interface, the structural design of the cathode catalyst layer is considered to be of pivotal importance for reducing H2 crossover, particularly in combination with the use of thin (≲50 µm) membranes. In this study, we have conducted a detailed investigation on the cathode structural parameters, covering the Pt wt % of the Pt/C electrocatalyst, the type of carbon support (Vulcan and high surface area carbon, HSAC), and the ionomer content, with a goal to reduce Pt loading to 0.025 mgPt/cm2 while minimizing the rate of cell hydrogen crossover. We found that the electrochemical performance is mainly influenced by the changes in the interfacial contact resistance due to variations in the cathode thickness. Both the Pt wt % in Pt/C and the ionomer content showed a positive correlation with the measured H2 in O2% in the anode outlet, whereas the Pt loading exhibited an opposite trend. The rate of hydrogen crossover was analyzed in relation to the calculated local volumetric current density within the cathode catalyst layer. Based on the obtained hydrogen mass transfer coefficient, a cathode catalyst layer comprising 40 wt % Pt on HSAC support with an ionomer-to-carbon (I/C) ratio of 0.35 was found to be an optimum configuration for achieving a low Pt loading of 0.025 mgPt/cm2 and a reduced rate of hydrogen crossover.

Operando Scanning Small-/Wide-Angle X-ray Scattering for Polymer Electrolyte Fuel Cells: Investigation of Catalyst Layer Saturation and Membrane Hydration- Capabilities and Challenges.

Polymer electrolyte fuel cells are an essential technology for future local emission-free mobility. One of the critical challenges for thriving commercialization is water management in the cells. We propose small- and wide-angle X-ray scattering as a suitable diagnostic tool to quantify the liquid saturation in the catalyst layer and determine the hydration of the ion-conducting membrane in real operating conditions. The challenges that may occur in operando data collection are described in detail─separation of the anode and cathode, cell alignment to the beam, X-ray radiation damage, and the possibility of membrane swelling. A synergistic development of experimental setup, data acquisition, and data interpretation circumvents the major challenges and leads to practical and reliable insights.

Reactions of the tetraoxidosulfate(˙-) and hydroxyl radicals with poly(sodium α-methylstyrene sulfonate).

Poly(α-methylstyrene sulfonic acid) (PAMS) represents a class of polymers that can form the protogenic constituent in electrolyte membranes for fuel cells. Oxidative stress is thought to play an important role in the degradation of the fuel cell membranes. Having previously established that damage may be mediated via abstraction of a benzylic hydrogen, we examined model compounds similar to those used before, but with a methyl group at the α-position. We studied the reaction of HO˙ and SO4(˙-), generated by pulse radiolysis, with model compounds in aqueous solution, and measured k = (2 ± 0.5) × 10(10) M(-1) s(-1) and (2 - 3) × 10(10) M(-1) s(-1) for the reaction of HO˙ with PAMS with average molecular weights of 2640 Da (PAMS-2640) and 6440 Da (PAMS-6440), respectively, at room temperature. At low pH, the decay of the hydroxycyclohexadienyl radical thus formed is accompanied by the formation of an absorption band in the visible region of the spectrum, which we tentatively assign to the radical cation of PAMS-2640 and -6440. The radical cation of PAMS-2640, formed by the reaction of SO4(˙-) with k = (6 ± 1) × 10(8) M(-1) s(-1), has a local absorption maximum at 560 nm, with ε560 ≥ 1400 M(-1) cm(-1). For the reaction of HO˙ and SO4(˙-) with the model compound benzenesulfonate, we measured k = (4-5) × 10(9) M(-1) s(-1) and (1.0 ± 0.3) × 10(8) M(-1) s(-1), respectively, while the reaction of SO4(˙-) with PAMS-6440 proceeds with (0.8-1) × 10(9) M(-1) s(-1). The 4-sulfophenoxyl radical was generated via the reaction of N3˙ with 4-hydroxybenzenesulfonate; ε410 ≥ 2300 M(-1) cm(-1). Not unexpectedly, the radical cation of PAMS is longer-lived than that of polystyrene sulfonic acid. Furthermore, fragmentation may result in desulfonation.

How the Porous Transport Layer Interface Affects Catalyst Utilization and Performance in Polymer Electrolyte Water Electrolysis.

Cost reduction and fast scale-up of electrolyzer technologies are essential for decarbonizing several crucial branches of industry. For polymer electrolyte water electrolysis, this requires a dramatic reduction of the expensive and scarce iridium-based catalyst, making its efficient utilization a key factor. The interfacial properties between the porous transport layer (PTL) and the catalyst layer (CL) are crucial for optimal catalyst utilization. Therefore, it is essential to understand the relationship between this interface and electrochemical performance. In this study, we fabricated a matrix of two-dimensional interface layers with a well-known model structure, integrating them as an additional layer between the PTL and the CL. By characterizing the performance and conducting an in-depth analysis of the overpotentials, we were able to estimate the catalyst utilization at different current densities, correlating them to the geometric properties of the model PTLs. We found that large areas of the CL become inactive at increasing current density either due to dry-out, oxygen saturation (under the PTL), or the high resistance of the CL away from the pore edges. We experimentally estimated the water penetration in the CL under the PTL to be ≈20 µm. Experimental results were corroborated using a 3D-multiphysics model to calculate the current distribution in the CL and estimate the impact of membrane dry-out. Finally, we observed a strong pressure dependency on performance and high-frequency resistance, which indicates that with the employed model PTLs, a significant gas phase accumulates in the CL under the lands, hindering the distribution of liquid water. The findings of this work can be extrapolated to improve and engineer PTLs with advanced interface properties, helping to reach the required target goals in cost and iridium loadings.

Revealing the impact of temperature in battery electrolytes via wavelength-resolved neutron imaging.

Understanding the limitations of electrolyte mixtures under extreme conditions is key to ensure reliable and safe battery performance. Among advanced characterization methods, time-of-flight neutron imaging (ToF-NI) is unique for its capability to map physicochemical changes of H-containing materials inside metallic casings and battery packs. The technique, however, requires long exposures in pulsed sources, which limits its applicability, particularly for analysis at low temperatures. To overcome these limitations, we use high-duty cycle ToF-NI at a continuous source, demonstrating its capability to expose physical and chemical changes of electrolytes due to variations in the overall molecular diffusion. The strategy described in this work reduces the exposure required and provides the baseline to study the thermal stability of electrolyte mixtures, from the proofing of state-of-the-art electrolyte mixtures up to their performance in batteries. This analysis and methodology apply to hydrogenous materials well beyond electrolytes for a wide range of applications.

A Nature-Inspired Antioxidant Strategy based on Porphyrin for Aromatic Hydrocarbon Containing Fuel Cell Membranes.

The use of hydrocarbon-based proton conducting membranes in fuel cells is currently hampered by the insufficient durability of the material in the device. Membrane aging is triggered by the presence of reactive intermediates, such as HO⋅, which attack the polymer and eventually lead to chain breakdown and membrane failure. An adequate antioxidant strategy tailored towards hydrocarbon-based ionomers is therefore imperative to improve membrane lifetime. In this work, we perform studies on reaction kinetics using pulse radiolysis and γ-radiolysis as well as fuel cell experiments to demonstrate the feasibility of increasing the stability of hydrocarbon-based membranes against oxidative attack by implementing a Nature-inspired antioxidant strategy. We found that metalated-porphyrins are suitable for damage transfer and can be used in the fuel cell membrane to reduce membrane aging with a low impact on fuel cell performance.

Renewable Energy from Livestock Waste Valorization: Amyloid-Based Feather Keratin Fuel Cells.

Increasing carbon emissions have accelerated climate change, resulting in devastating effects that are now tangible on an everyday basis. This is mirrored by a projected increase in global energy demand of approximately 50% within a single generation, urging a shift from fossil-fuel-derived materials toward greener materials and more sustainable manufacturing processes. Biobased industrial byproducts, such as side streams from the food industry, are attractive alternatives with strong potential for valorization due to their large volume, low cost, renewability, biodegradability, and intrinsic material properties. Here, we demonstrate the reutilization of industrial chicken feather waste into proton-conductive membranes for fuel cells, protonic transistors, and water-splitting devices. Keratin was isolated from chicken feathers via a fast and economical process, converted into amyloid fibrils through heat treatment, and further processed into membranes with an imparted proton conductivity of 6.3 mS cm-1 using a simple oxidative method. The functionality of the membranes is demonstrated by assembling them into a hydrogen fuel cell capable of generating 25 mW cm-2 of power density to operate various types of devices using hydrogen and air as fuel. Additionally, these membranes were used to generate hydrogen through water splitting and in protonic field-effect transistors as thin-film modulators of protonic conductivity via the electrostatic gating effect. We believe that by converting industrial waste into renewable energy materials at low cost and high scalability, our green manufacturing process can contribute to a fully circular economy with a neutral carbon footprint.

Quantification of PEFC Catalyst Layer Saturation via In Silico, Ex Situ, and In Situ Small-Angle X-ray Scattering.

The complex nature of liquid water saturation of polymer electrolyte fuel cell (PEFC) catalyst layers (CLs) greatly affects the device performance. To investigate this problem, we present a method to quantify the presence of liquid water in a PEFC CL using small-angle X-ray scattering (SAXS). This method leverages the differences in electron densities between the solid catalyst matrix and the liquid water filled pores of the CL under both dry and wet conditions. This approach is validated using ex situ wetting experiments, which aid the study of the transient saturation of a CL in a flow cell configuration in situ. The azimuthally integrated scattering data are fitted using 3D morphology models of the CL under dry conditions. Different wetting scenarios are realized in silico, and the corresponding SAXS data are numerically simulated by a direct 3D Fourier transformation. The simulated SAXS profiles of the different wetting scenarios are used to interpret the measured SAXS data which allows the derivation of the most probable wetting mechanism within a flow cell electrode.

Carbonate Regeneration Using a Membrane Electrochemical Cell for Efficient CO2 Capture.

The use of atmospheric CO2 as a chemical feedstock is a promising way to decarbonize the chemical and transportation sectors, which currently rely heavily on fossil fuels. This transition demands new technologies to reduce the energy required to capture and separate CO2. Here, we develop and demonstrate an alternative method of carbonate solution regeneration using an anion exchange membrane electrochemical cell. This process simultaneously regenerates the CO2 capture solution on the feed side, while enriching a stream of H2 with CO2 on the permeate side of the cell. Preliminary results show a CO2 transport faradaic efficiency of 50% (100% CO3 2- transport) when supplying a pure K2CO3 solution at current densities up to 60 mA·cm-2. A small cathode gap benefited cell operation by preventing membrane transport of OH-, although with an increased ohmic resistance. This represents a step forward in the application of electrochemistry to drive processes that are critical to CO2 valorization.

On the Radical-Induced Degradation of Quaternary Ammonium Cations for Anion-Exchange Membrane Fuel Cells and Electrolyzers.

Four benzylic-type quaternary ammonium (QA) compounds with different electron density at the phenyl group were evaluated for their susceptibility against degradation by radicals. Time-resolved absorption spectroscopy indicated that radicals with oxidizing and reducing character were formed upon oxidation by HO⋅ and O⋅- (conjugate base of HO⋅). It was estimated that, dependent on the QA, 18-41 % of the formed radicals were oxidizing with standard electrode potentials (E0 ) above 0.276 V and 13-23 % exceeded 0.68 V, while 13-48 % were reducing with E0 <-0.448 V. The stability of these model compounds against oxidation and reductive dealkylation was evaluated at both neutral and strongly alkaline conditions, pH 14. Under both conditions, electron-donating groups promoted radical degradation, while electron-withdrawing ones increased stability. Therefore, durability against radical-induced degradation shows an opposite trend to alkaline stability and needs to be considered during the rational design of novel anion-exchange membranes for fuel cells and electrolyzers.

EPR Study on the Oxidative Degradation of Phenyl Sulfonates, Constituents of Aromatic Hydrocarbon-Based Proton-Exchange Fuel Cell Membranes.

Sulfonated aromatic hydrocarbon-based ionomers are potential constituents of next-generation polymer electrolyte fuel cells (PEFCs). Widespread application is currently limited due to their susceptibility to radical-initiated oxidative degradation that, among other intermediates, involves the formation of highly reactive aromatic cation radicals. The intermediates undergo chain cleavage (dealkylation/dearylation) and the loss of protogenic sulfonate groups, all leading to performance loss and eventual membrane failure. Laser flash photolysis experiments indicated that cation radicals can also be formed via direct electron ejection. We aim to establish the major degradation pathway of proton-exchange membranes (PEMs). To this end, we irradiated aqueous solutions of phenyl sulfonate-type model compounds with a Xe arc lamp, thus generating radicals. The radicals were trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and the formed adducts were observed by electron paramagnetic resonance (EPR). The formed DMPO spin adducts were assigned and relative adduct concentrations were quantified by simulation of the experimental EPR spectra. Through the formation of the DMPO/•SO3 - adduct, we established that desulfonation dominates for monoaromatic phenyl sulfonates. We observed that diaryl ether sulfonates readily undergo homolytic C-O scission that produces DMPO/•aryl adducts. Our results support the notion that polyphenylene sulfonates are the most stable against oxidative attack and effectively transfer electrons from DMPO, forming DMPO/•OH. Our findings help to identify durable moieties that can be used as building blocks in the development of next-generation PEMs.

Reaction of SO4˙⁻ with an oligomer of poly(sodium styrene sulfonate). Probing the mechanism of damage to fuel cell membranes.

Clarification of the mechanism of degradation of model compounds for polymers used in polymer electrolyte fuel cells may identify intermediates that propagate damage; such knowledge can be used to improve the lifetime of fuel cell membranes, a central issue to continued progress in fuel cell technology. In proton-exchange membranes based on poly(styrene sulfonic acid), hydroxycyclohexadienyl radicals are formed after reaction with HO˙ and thought to decay to short-lived radical cations at low pH. To clarify subsequent reactions, we generated radical cations by reaction of SO(4)˙(-) with oligomers of poly(styrene sulfonic acid) (MW ≈ 1100 Da). At 295 K, this reaction proceeds with k = (4.5 ± 0.6) × 10(8) M(-1) s(-1), both at pH 2.4 and 3.4, and yields benzyl radicals with an estimated yield of ≤60% relative to [SO(4)˙(-)]. The radical cation is too short-lived to be observed: based on a benzyl radical yield of 60%, a lower limit of k > 6.8 × 10(5) s(-1) for the intramolecular transformation of the aromatic radical cation of the oligomer to a benzyl radical is deduced. Our results show that formation of the benzyl radical, an important precursor in the breakdown of the polymer, is irreversible.

Damage to fuel cell membranes. Reaction of HO* with an oligomer of poly(sodium styrene sulfonate) and subsequent reaction with O(2).

An understanding of the reactivity of oligomeric compounds that model fuel cell membrane materials under oxidative-stress conditions that mimic the fuel cell operating environment can identify material weaknesses and yield valuable insights into how a polymer might be modified to improve oxidative stability. The reaction of HO˙ radicals with a polymer electrolyte fuel cell membrane represents an initiation step for irreversible membrane oxidation. By means of pulse radiolysis, we measured k = (9.5 ± 0.6) × 10(9) M(-1) s(-1) for the reaction of HO˙ with poly(sodium styrene sulfonate), PSSS, with an average molecular weight of 1100 Da (PSSS-1100) in aqueous solution at room temperature. In the initial reaction of HO˙ with the oligomer (90 ± 10)% react by addition to form hydroxycyclohexadienyl radicals, while the remaining abstract a hydrogen to yield benzyl radicals. The hydroxycyclohexadienyl radicals react reversibly with dioxygen to form the corresponding peroxyl radicals; the second-order rate constant for the forward reaction is k(f) = (3.0 ± 0.5) × 10(7) M(-1) s(-1), and for the back reaction, we derive an upper limit for the rate constant k(r) of (4.5 ± 0.9) × 10(3) s(-1). These data place a lower bound on the equilibrium constant K of (7 ± 2) × 10(3) M(-1) at 295 K, which allows us to calculate a lower limit of the Gibbs energy for the reaction, (-21.7 ± 0.8) kJ mol(-1). At pH 1, the hydroxycyclohexadienyl radicals decay with an overall first-order rate constant k of (6 ± 1) × 10(3) s(-1) to yield benzyl radicals. The second-order rate constant for reaction of dioxygen with benzyl radicals of PSSS-1100 is k = (2-5) × 10(8) M(-1) s(-1). We discuss hydrogen abstraction from PSSS-1100 in terms of the bond dissociation energy, and relate these to relevant electrode potentials. We propose a reaction mechanism for the decay of hydroxycyclohexadienyl radicals and subsequent reaction steps.