To drive the further development of electrochemical CO2 reduction technologies, there is an urgent need for highly active catalysts that minimize unwanted side reactions and that also possess a large specific surface area. While nanostructured catalysts typically fulfill the latter requirement, they often use porous carbon supports that improve the nanoparticles’ dispersion but can shift the product selectivity towards undesirable H2 formation.[1] This challenge could be solved by using unsupported aerogels consisting of interconnected nanodomain networks of highly porous materials such as nano-wires or -particles, which provide a large surface area while minimizing unwanted side reactions.[2] So far, precious metals such as gold (Au) and silver (Ag) have shown remarkable activity and selectivity as CO2-reduction catalysts for CO production.[3] However, due to the high cost of such noble metals, improving their mass-specific activity is essential. One strategy to attain this, especially for Au, is to reduce the adsorption energy of the catalyst’s surface towards CO by changing the electronic structure of the d-band through alloying with other metals. In this context, ordered AuCu structures have proven to be promising candidates for improving the CO2-to-CO activity and selectivity.[4] With the motivation to combine both of the above approaches (i.e., Au-alloying with Cu and the absence of a C-support), in this study we present an AuCu aerogel with an average domain size of ≈ 7 nm that exhibits an exceptionally high faradaic efficiency of 87 % for CO at -0.6 V versus the reversible hydrogen electrode (RHE). This corresponds to a ≈ 2-fold higher Au-mass-specific partial current density for CO when compared to an equivalent, monometallic Au aerogel. Notably, this enhanced activity and selectivity are achieved by performing a potential cycling procedure prior to the CO2 reduction potential hold that involves cyclic voltammetry (CV) between 0.1 and 1.7 V vs. RHE at a scan rate of 50 mV/s until a stable voltammogram is achieved. To investigate the changes in the electronic and structural properties which have happened to the catalyst during this potential cycling, we performed an in situ grazing incidence X-ray absorption spectroscopy (XAS) measurements in the course of these CVs. Chiefly, these spectroelectrochemcial tests were carried out in the same cell used for the assessment of the CO2 reduction performance, ensuring for the first time that the mass-transport conditions encountered by the catalyst during these XAS measurements are identical to those in the CO2-reduction activity and selectivity tests. Figure 1 illustrates the changes observed throughout the potential cycling procedure in the Cu K- and Au L3-absorption edges, whereby the acquired spectra were submitted to a multivariate curve resolution (MCR) analysis. For the Cu K spectra, a total of three components were identified to describe the whole dataset, and it becomes evident that as the number of cycles increases, the copper oxide phase (component 1 in Figs. 1b and 1c) diminishes while a metallic phase (component 2, identified as an AuCu alloy through EXAFS fitting) becomes more prominent. In the case of the Au L3 data, only two components were discerned to describe the dataset, and both of them were identified as distinct AuCu alloy phases through EXAFS fitting. With an increasing number of cycles, component 2 (featuring a higher oxide content than component 1) becomes dominant at positive potentials, suggesting an increasing Au content on the aerogel’s surface as the potential cycling procedure progresses. In summary, in this contribution we present an AuCu aerogel catalyst exhibiting a high activity and selectivity for CO production that is achieved by cyclic voltammetry treatment prior to holding CO2 reduction potential. The results derived from the in situ XAS measurements on this material indicate that this process effectively removes the copper oxide domains initially present in the catalyst, forming a novel AuCu alloy phase while simultaneously enriching the aerogel’s surface with Au. References Baturina, O.A., et al., CO2 Electroreduction to Hydrocarbons on Carbon-Supported Cu Nanoparticles. ACS Catalysis, 2014. 4(10): p. 3682-3695. Cai, B. and A. Eychmuller, Promoting Electrocatalysis upon Aerogels. Adv Mater, 2019. 31(31): p. e1804881. Hori, Y.M., A.; Kikuchi, K.; Suzuki, S, Electrochemical Reduction of Carbon Dioxides to Carbon Monoxide at a Gold Electrode in Aqueous Potassium Hydrogen Carbonate. J. Chem. Soc., Chem. Commun., 1987. 10: p. 728-729. Liu, K., et al., Electronic Effects Determine the Selectivity of Planar Au-Cu Bimetallic Thin Films for Electrochemical CO(2) Reduction. ACS Appl Mater Interfaces, 2019. 11(18): p. 16546-16555. Figure 1

The increase in regulations on per- and polyfluoroalkyl substances (PFAS) due to health and environmental concerns,1 has resulted in an increasing need for alternatives to polyfluoroalkylsulfonic acid (PFSA) based proton exchange membranes (PEMs). An alternate class of polymers are functionalized polyarylenes, such as sulfonated polyphenylenesulfone (sPPS)2, sulfonated phenylated polyphenylenes (sPPP)3, and the co-polymer sulfonated polyphenylene quinquephenylene (SPP-QP)4. Despite good progress made in reaching comparable performance to Nafion, radical induced degradation of phenyl(sulfonate) rings during fuel cell operation remains prevalent. In a recently published study,5 we investigated the potential of using a Cu(II)-porphyrin complex as an antioxidant for aromatic sulfonate compounds. First, γ-radiolysis was used on aqueous solutions containing the model aromatic compound potassium 4-(tert-butyl)-2-methoxyphenylsulfonate with varying amounts of antioxidant. The exposure of water-based solutions to high-energy radiation results in the formation of hydroxyl radicals, which in turn react with the model compound. Resulting degradation of the compounds was analyzed using high performance liquid chromatography (HPLC). In the second part of the study, we incorporated the Cu(II)-porphyrin into PEMs to test membrane stability during fuel cell operation using an accelerated degradation protocol. The measurements combined with post-mortem analysis show that membranes containing antioxidants have a five-fold increased stability compared to reference non-stabilized membranes. References: (1) Ackerman Grunfeld, D.; Gilbert, D.; Hou, J.; Jones, A. M.; Lee, M. J.; Kibbey, T. C. G.; O’Carroll, D. M. Underestimated Burden of Per- and Polyfluoroalkyl Substances in Global Surface Waters and Groundwaters. Nat. Geosci. 2024, 17 (4), 340–346. https://doi.org/10.1038/s41561-024-01402-8. (2) Araujo, C. C. de; Kreuer, K. D.; Schuster, M.; Portale, G.; Mendil-Jakani, H.; Gebel, G.; Maier, J. Poly(p-Phenylene Sulfone)s with High Ion Exchange Capacity: Ionomers with Unique Microstructural and Transport Features. Phys. Chem. Chem. Phys. 2009, 11 (17), 3305–3312. https://doi.org/10.1039/B822069G. (3) Adamski, M.; Skalski, T. J. G.; Britton, B.; Peckham, T. J.; Metzler, L.; Holdcroft, S. Highly Stable, Low Gas Crossover, Proton-Conducting Phenylated Polyphenylenes. Angew. Chem. Int. Ed. 2017, 56 (31), 9058–9061. https://doi.org/10.1002/anie.201703916. (4) Miyake, J.; Taki, R.; Mochizuki, T.; Shimizu, R.; Akiyama, R.; Uchida, M.; Miyatake, K. Design of Flexible Polyphenylene Proton-Conducting Membrane for next-Generation Fuel Cells. Sci. Adv. 2017, 3 (10), eaao0476. https://doi.org/10.1126/sciadv.aao0476. (5) de Wild, T.; Wurm, J.; Becker, P.; Günther, D.; Nauser, T.; Schmidt, T. J.; Gubler, L.; Nemeth, T. A Nature-Inspired Antioxidant Strategy Based on Porphyrin for Aromatic Hydrocarbon Containing Fuel Cell Membranes**. ChemSusChem 2023, 16 (21), e202300775. https://doi.org/10.1002/cssc.202300775.

As global warming takes place at an unprecedented pace, it becomes increasingly important to develop negative emission (i.e., CO2-depleting) technologies to achieve the hoped-for net-zero target in 2050. The electrochemical CO2-reduction reaction (CO2RR) to carbon monoxide (CO) or formate is expected to be an economically viable approach to close the carbon cycle while reducing greenhouse gas emissions.[1] In this context, palladium (Pd) has been identified as an interesting CO2RR-catalyst owing to its ability to selectively produce formate vs. CO in the lower vs. higher overpotential regimes [i.e., at -0.1 to -0.4 vs. -0.5 to -0.9 V vs. the reversible hydrogen electrode (RHE), respectively].[2] To this day, the reasons for this potential-induced change in product selectivity remain poorly understood and a subject of open debate in the literature. In this regard, several publications have suggested that PdHx forms at the negative potentials at which the CO2RR takes place, and that this hydride acts as the active phase in the reduction of CO2 to formate.[3, 4] To elucidate this catalytic mechanism, we have investigated two different types of Pd catalysts: one consisting of dispersed Pd-nanoparticles supported on a carbon black, and a second one in the form of unsupported Pd nanoparticles tridimensionally interconnected into a network structure (i.e., a so called aerogel).[5] To track these materials’ potential-dependent PdHx-formation using X-ray absorption spectroscopy (XAS) at the Pd K-edge under CO2RR-conditions and link it to their partial current densities (pCDs) towards formate, we employed a newly designed spectroelectrochemical operando XAS flow cell. The latter enables spectral acquisition in a grazing incidence (GI) configuration allowing the use of thin layer electrodes (i.e., with a thickness < 1 μm) which in turn minimizes the accumulation of evolved gaseous bubbles along the CL-thickness, thus avoiding spectral artifacts related to the present of such bubbles. Moreover, the implementation of an ion-conductive membrane to separate the working- and counter-electrode compartments enables the accurate quantification of gaseous products via mass spectrometry and gas chromatography, as well as of liquid products by collecting aliquots of the electrolyte over time (which, in the case of formate, are subsequently analyzed through ion chromatography). Using this combination of spectroelectrochical and analytic techniques, we found that the two catalysts exhibit significantly distinct behaviors, as illustrated in Figure 1. Specifically, while for the C-supported Pd nanoparticles the stable pCD towards formate observed at -100 mV and -200mV vs. RHE is accompanied by a quick PdHx-formation (stabilizing at hydride stoichiometries of x ~ 0.6 vs. ~ 0.5, respectively), the unsupported Pd aerogel features a negligible formate- production capability at the same potentials, and the corresponding hydride phases only form gradually in the course of the potential holds and reach x-values of ~ 0.35 and x ~ 0.4, respectively. Moreover, the fact that for the C-supported Pd nanoparticles the higher pCD towards formate observed at -200 mV vs RHE corresponds to a hydride phase with a lower H-content compared that at -100 mV vs RHE indicates an indirect correlation between the formate- production rate and the nanoparticles H-content. In conclusion, these results provide valuable new insights into the important role of the time-dependent formation of PdHx on these materials’ CO2-to-formate selectivity. References Durst, J., et al., Electrochemical CO2 Reduction - A Critical View on Fundamentals, Materials and Applications. Chimia (Aarau), 2015. 69(12): p. 769-776. Diercks, J.S., et al., An Online Gas Chromatography Cell Setup for Accurate CO2-Electroreduction Product Quantification. Journal of The Electrochemical Society, 2021. 168(6). Min, X. and M.W. Kanan, Pd-catalyzed electrohydrogenation of carbon dioxide to formate: high mass activity at low overpotential and identification of the deactivation pathway. J Am Chem Soc, 2015. 137(14): p. 4701-8. Rahaman, M., A. Dutta, and P. Broekmann, Size-Dependent Activity of Palladium Nanoparticles: Efficient Conversion of CO2 into Formate at Low Overpotentials. ChemSusChem, 2017. 10(8): p. 1733-1741. Diercks, J.S., et al., Interplay between Surface-Adsorbed CO and Bulk Pd Hydride under CO2-Electroreduction Conditions. ACS Catalysis, 2022: p. 10727-10741. Figure 1

Hydrocarbon-based materials are of interest as next-generation proton exchange membranes (PEMs) for polymer electrolyte fuel cells (PEFCs). The biggest drawback of aromatic hydrocarbon PEMs is the presence of aromatic groups contained within the polymer, which make these materials susceptible to radical induced degradation reactions. In the fuel cell community antioxidant action is usually equated with scavenging of potent radicals, i.e. HO•, by various additives. In this work, however, we report on the repair of damaged membranes. This is tested by performing in situ accelerated stress tests at open circuit voltage with high H2 and O2 partial pressures. Membranes with aromatic sulfonate groups are synthesized as these are common constituents in hydrocarbon-based membranes. Two different approaches to incorporate the repair agent, cerium(III), are explored: 1) ionic bonding of Ce(III) to sulfonate groups and 2) covalent attachment through stable complexes of crown ether and Ce(III). We report that, during fuel cell operation, polymer degradation can be significantly reduced by Ce(III) when immobilized in the membrane. Ionic attachment did not yield in the desired repair effect as a result of cerium loss from the membrane.

We demonstrate performance and durability of a fluorine-free poly(phenylene-sulfone) membrane for proton exchange membrane fuel cells. Sulfonated poly (phenylene-sulfone) (EW 220 g center dot eq(-1); sPPS-220) has been prepared and processed by acid/base blending with 25 wt% poly(benzimidazole) (sPPS-220/PBI-O). A blend membrane with a nominal EW 463 g center dot eq(-1)(ion exchange capacity of 2.16 meq center dot g(-1)) has been fabricated by solution co-casting on a commercial production line. Blend component fractions are chosen to meet Nafion 211's specific conductivity and obtain mechanical robustness. Blend membrane has significantly lower gas permeability than Nafion under both dry (RH = 30%) and wet (RH = 95%) conditions, significantly higher storage modulus and no softening at high temperature. Like pure-sPPS, blend membrane is hydrolytically stable up to T = 180 degrees C in water. Single-cell H-2/air fuel cell tests demonstrate higher open-circuit voltage and similar performance under load conditions compared to Nafion. Still, high mass transport polarization is likely due to water condensation at the cathode side. This increases membrane swelling and thus electroosmotic water drag reducing hydration at the anode side and leading to flooding of the cathode. Under accelerated stress conditions at low relative humidity, sPPS-220/PBI-O is more durable than Nafion. Possible explanations are i) lower gas crossover reduced radical formation rate and ii) high intrinsic stability of sPPS.

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.

In an operating polymer electrolyte fuel cell (PEFC) radical intermediates are formed as a result of the interaction of H 2 and O 2 on the surface of the Pt-based electrocatalyst. Of the various radical intermediates, • OH is of particular concern owing to its high oxidative strength, E °( • OH,H + /H 2 O) = 2.72 V) [1]. In PFSA ionomers, • OH reacts relatively slowly with the polymer and has a lifetime on the order of microseconds. This leaves sufficient time for additives, such as Ce(III), to scavenge the radicals and thus mitigate radical induced damage of the ionomer. In hydrocarbon-based ionomers containing aromatic units, • OH reacts within nanoseconds with polymer constituents [2]. Radical scavenging can therefore not be effective. In the development of alternative antioxidant strategies for hydrocarbon-based ionomers, it is crucial to elucidate polymer degradation mechanisms triggered by radical attack. In addition to studying the initial reactions of primary radicals, i.e. • OH, it is equally important to characterize the nature of the formed polymer intermediates, their lifetime, stability and reactivity. Sufficiently long-lived intermediates can be repaired by suitable additives, thereby restoring the original ionomer [3]. In the fuel cell, the thus protected membranes show a much lower rate of degradation in an accelerated stress test (AST) at open circuit voltage (OCV) [4]. In this contribution, we provide an overview of methods to study reaction of radicals with small-molecule model compounds and ionomer constituents [5, 6]. The interaction of ionizing radiation (MeV electrons or photons) with water leads to the formation of radicals, such as • OH, at known rates. Water radiolysis can therefore be used to study the kinetics of radical attack and follow-up reactions as a function of pH, electron density of the aromatic ring, and presence of additives. Examples relevant to the acidic conditions in the proton exchange membrane (see Figure, Panel a) as well as the alkaline conditions in an anion exchange membrane (AEM) will be given. Moreover, the use of suitable antioxidants to mitigate degradation through the repair of intermediates will be discussed. Fuel cell tests with hydrocarbon-based membranes containing polymer-bound antioxidants will be shown (see Figure, Panel b) and prospects for long-term stability of non-fluorinated ionomers assessed. Figure caption: a) Analysis of the effect of Ce(III) ions on the extent of degradation of 1 mM of 4-cumenesulfonate (4CS) and 4-(tert-butyl)phenylsulfonate (BPS) at pH 0 (1 M H 2 SO 4 ) in 1 mM H 2 O 2 upon exposure to a radiolytic dose provided by a 60 Co-source of 800 Gy (corresponding to a cumulative amount of • OH of 0.22 mM). ‘Excess degradation’ indicates the difference to the extent of degradation in the absence of Ce(III) and H 2 O 2 . Data for BPS from [5]. b) Ohmic resistance of single cells (average of several experiments) with partially fluorinated membrane containing a tethered crown ether with and without Ce(III) metal center during an accelerated stress test. Ion exchange capacity at the end of test was measured to represent remaining membrane state of health. Conditions: open circuit voltage (OCV), H 2 /O 2 , 80 °C, 2.5 bar a , and 100% R.H. [4]. References: H. A. Schwarz, R. W. Dodson, J. Phys. Chem., 88, 3643 (1984). L. Gubler, W.H. Koppenol, in: The Chemistry of Membranes Used in Fuel Cells: Degradation and Stabilization, S. Schlick (Ed.), 107-138, John Wiley & Sons (2018). T. De Wild, T. Nemeth, T.M. Nolte, T.J. Schmidt, T. Nauser, L. Gubler, J. Electrochem. Soc., 168, 054514 (2021). T. de Wild, T. Nemeth, P. Becker, D. Günther, T. Nauser, T.J. Schmidt, L. Gubler, J. Power Sources 560, 232525 (2023). T. Nemeth, T. de Wild, L. Gubler, T. Nauser, Phys. Chem. Chem. Phys. 24, 2, 895 (2022). T. Nemeth, T. de Wild, L. Gubler, T. Nauser, J. Electrochem. Soc. 169, 054529 (2022). Figure 1

An antioxidant strategy for aromatic hydrocarbon based proton exchange membranes for fuel cells and electrolyzers requires an understanding of the mechanisms of radical attack and follow-up reactions. We provide experimental evidence that radical scavenging is ineffective. Instead, focus has to be placed on repairing intermediates. Recent studies of exposure of model compounds to radicals formed in a γ-irradiation cell are reported. Fuel cell tests of membranes containing polymer-bound antioxidants with cerium and copper metal complexes show improved chemical stability in an accelerated stress test. Requirements for a successful implementation of a damage-repair mechanism in hydrocarbon membranes are discussed.

The Front Cover shows that during the operation of a proton exchange membrane fuel cell harmful radical species (HO⋅, H⋅ and HOO⋅) form that attack the membrane. The use of hydrocarbon-based proton-conducting membranes in fuel cells is currently hampered by the insufficient durability of the material in the device. In our work, we implement a nature-inspired antioxidant strategy by covalently modifying the membrane with metalated porphyrins. Porphyrin-based antioxidants are suitable for transferring damage and thus greatly enhance membrane stability against radical-induced degradation with a minimal impact on fuel cell performance. More information can be found in the Research Article by T. de Wild et al.

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.

Hydrocarbon-based materials are of interest as next-generation proton exchange membranes (PEMs) for the polymer electrolyte fuel cell (PEFC). The biggest drawback of aromatic hydrocarbon PEMs is the presence of aromatic groups contained within the polymer, which make these materials susceptible to radical induced degradation reactions.In this work, we test the hypothesis whether transition metal ions, such as Ce(III), can act as a repair agent in hydrocarbon-based membranes by reducing harmful radical intermediates formed during operation. This is tested by performing in situ accelerated stress tests at open circuit voltage with high H 2 and O 2 partial pressures. Membranes with aromatic sulfonate groups are synthesized as these are common constituents in hydrocarbon-based membranes. Two different approaches to incorporate cerium are explored: 1) ionic bonding of Ce(III) to sulfonate groups and 2) covalent attachment through stable complexes of crown ether and Ce(III). We report that, during fuel cell operation, polymer degradation can be significantly reduced by Ce(III) when immobilized in the membrane. Ionic attachment did not yield in the desired repair effect, most likely due to cerium migration effects.

Harmful radical species (HO•, HOO•) are formed during fuel cell operation. Out of these species, the highly electrophilic hydroxyl radical is the most reactive (1.8 – 2.7 V vs. NHE). Consequently, the most electron-rich constituents, aromatic rings, are preferentially attacked and undergo rapid degradation. Hydroxyl radical formation rates depend on operating conditions and cell parameters, but even with low formation rates the damage accumulates over time and can lead to thinning or embrittlement of the membrane, and will cause irreversible damage to the cell. Due to these effects, long-term operation becomes challenging. Therefore, membranes with enhanced radical stability are increasingly desirable. In our previous work, we have shown, using kinetics measurements, that it should be possible for Ce(III) to act as a repair agent for formed damaged intermediates (cf. Figure).[1] In our follow-up work we demonstrate that Ce(III) can indeed reduce degradation rates.[2] These experiments were performed in a fuel cell setup using accelerated stress tests at open circuit voltage (OCV), a cell temperature of 80 °C, 100% RH and high H2 and O2 partial pressures of 2 bar. In-situ measurements and post-test analyses unequivocally show that bound Ce(III) can mitigate degradation of poly(α-methylstyrene sulfonate)-based membranes. We believe that this approach could also be applicable for other aromatic-based polymers if a sufficiently long-lived intermediate is formed upon radical attack. References: [1] T. de Wild et al., J. Electrochem. Soc. 2021, 168, 054514. [2] T. de Wild et al., J. Power Sources, submitted Figure 1

During the operation of aromatic hydrocarbon-based proton exchange membrane fuel cells, formed radical species attack the membrane. The most deleterious radical formed is HO · , both strongly electrophilic and oxidising. Oligomers of α -methylstyrene sulfonates (PAMSS) were used as model compounds. We report on the complex reaction cascade following the oxidative attack on aromatic cores bearing proton conductive sulfonate groups. UV-absorption bands of initial oxidation products indicate the formation of radical adducts and aromatic cation radicals. Subsequently, a transformation associated with an absorbance build-up at 580 nm is observed, presumably also related to aromatic cation radicals. Build-up and decay are significantly accelerated at high ionic strength levels that are also typical in fuel cells. Increased ionic strength causes phase separation: dynamic light scattering experiments indicate particle formation that is dependent both on chain length and on ionic strength. Aromatic cation radicals are known strong oxidants. With a presumed redox potential of E °((PAMSS-580 nm) ·+ /PAMSS) ∼ 2 V this oxidizing species should react also with mediocre reductants. Here, Mn(II) was oxidised to Mn(III) with rate constants of (5–10) × 10 6 M −1 s −1 . Implications for experimental design of kinetics experiments and understanding chemical mechanisms are discussed.

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.

Polymer electrolyte fuel cell (PEFC) membranes are subject to radical-induced degradation. Antioxidant strategies for hydrocarbon-based ionomers containing aromatic units can focus on intermediates that are formed upon attack by hydroxyl radicals (HO·). Among the different intermediates, the cation radical P·+ is the most promising target for repair, for example by cerium(III). For the “repair” reaction of Ce(III) with radicals of a poly(α-methylstyrene sulfonate) oligomer we determined an activation energy of (9 ± 2) kJ mol−1 and a rate constant of 1.6 · 108 M−1 s−1 at 80 °C by pulse-radiolysis. For the reduction of Ce(IV) by hydrogen peroxide the activation energy was determined by stopped-flow as (30 ± 1) kJ mol−1 with a rate constant of 4.8 · 106 M−1 s−1 at 80 °C. These parameters are fed into a kinetics model to estimate the efficacy of the cerium (III)/(IV) redox couple as a catalytic repair agent in hydrocarbon-based fuel cell membranes. While cerium can mitigate polymer degradation, repair efficacy depends on the polymer degradation pathway and the nature and lifetime of the intermediates.

The search for a piezoelectric elastomer that generates an electrical signal when pressed and stretched has increased significantly in the last decade as they hold great promise in harvesting energy from human motion and monitoring human activities. Here, the excellent elasticity of polydimethylsiloxane-based elastomers and the piezoelectric properties of lead zirconate titanate (PZT) were combined and, using a thermally activated poling process, elastic piezoelectric composites were obtained. For this, two polydimethylsiloxane (PDMS) matrices with a molar mass of 139 kDa and 692 kDa and PZT fillers with particle sizes of 2 and 20 mu m were used. For the same poling conditions, an increase in the piezoelectric response with increasing amount of filler, filler size and molar mass of the polymer matrix was observed. Overall, d(33)* and d(31)* values of 2.7-40 pC N-1 and 16-48 pC N-1 were achieved in this work with filler contents ranging from 37-72 vol%. A composite material with a PZT filler content of 38 vol% (20 mu m particle size) in a commercially available PDMS with a M-w = 139 kg mol(-1) exhibited high flexibility, good elasticity with long-term mechanical stretchability and high longitudinal and transverse piezoelectric coefficients of 3.6 pC N-1 and 30 pC N-1, respectively. The higher transverse piezoelectric constant d*(31) can be explained by an additional capacitor effect of the composite film structure. These properties are interesting features for energy conversion from human motion, monitoring human activities, and stretchable electronics. The functionality of the newly developed material is demonstrated in a pressed sensor.

The self-assembly of small colloidal clusters, so-called colloidal molecules, into crystalline materials has proven extremely challenging, the outcome often being glassy, amorphous states where positions and orientations are locked. In this paper, a new type of colloidal molecule is therefore prepared, assembled from poly(N-isopropylacrylamide) (PNIPAM)-based microgels that due to their well documented softness and temperature-response allow for greater defect tolerance compared to hard spheres and for convenient in situ tuning of size, volume fraction and inter-particle interactions with temperature. The microgels (B) are assembled by electrostatic adsorption onto oppositely charged, smaller-sized microgels (A), where the relative size of the two determines the valency (n) of the resulting core-satellite ABn-type colloidal molecules. Following assembly, a microfluidic deterministic lateral displacement (DLD) device is used to effectively isolate AB4-type colloidal molecules of tetrahedral geometry that possess a repulsive-to-attractive transition on crossing the microgels' volume phase transition temperature (VPTT). These soft, temperature-responsive colloidal molecules constitute highly promising building blocks for the preparation of new materials with emergent properties, and their optical wavelength-size makes them especially interesting for optical applications.