Durability of perfluorinated ionomer membrane against hydrogen peroxide

The durability of perfluorinated ionomer membrane, Nafion^® 117, with various counter ions against hydrogen peroxide was investigated as a degradation factor of polymer electrolyte fuel cells (PEFC). After protonated Nafion membrane was tested in 30% H₂O₂ solution at 80 °C, small amounts of fluoride ion and sulfate ion, which are derived from the CF bonds and the sulfonic acid groups, respectively, of the membrane, were detected in the solution. This fact indicated a potential vulnerability of the electrolyte membrane to H₂O₂ formed in the cell. The durability of Nafion with alkali and alkaline earth metal ions as counter ions were similar to that of protonated Nafion, and hence these cations do not have any specific effects on membrane degradation. In contrast, the presence of ferrous and cupric ions as counter ions significantly enhanced the decomposition rate of Nafion. This is due to the formation of strongly nucleophilic radicals such as hydroxy and hydroperoxy radicals upon decomposition of H₂O₂ at these catalyst ions. The results of FT-IR and ¹⁹F NMR measurements of deteriorated Fe²⁺-Nafion membrane revealed that both the main and side chains are decomposed at similar rates by radical attack, most probably because the decomposition proceeds through radical de-polymerization (so-called un-zippering mechanism).     

Kinetic study of Nafion degradation by Fenton reaction

To predict the durability of polymer electrolyte membranes in fuel cells, the degradation reactions of Nafion 117 films were studied as oxidation reactions with hydroxyl radicals as oxidation accelerators. The radical species were generated by the Fenton reaction between hydrogen peroxide (H₂O₂) and iron ions (Fe²⁺). The Nafion degradation kinetics were estimated by fluorine ion (F⁻) generation. The H₂O₂ and Nafion degradation reactions fit a pseudo-first-order rate constant. The values of the activation energy and frequency factor are 85 kJ mol⁻¹ and 3.97 × 10⁸ s⁻¹ for H₂O₂ decomposition in the presence of a Nafion film and 97 kJ mol⁻¹ and 9.88 × 10⁸ s⁻¹ for F⁻ generation. The Nafion surface morphology became rough after reaction for 12 h; small cracks, approximately 100 μm in length, were observed at temperatures below 60 °C. These cracks connected to make larger gaps of approximately 1 mm at temperatures above 70 °C. We also found a linear relationship between H₂O₂ consumption and F⁻ generation. The rate constant is temperature dependent and expressed as ln(d[F⁻]/d[decomposed H₂O₂]) = −19.5 × 10³ K⁻¹ + 42.8. F⁻ generated and H₂O₂ consumed along with the Nafion degradation conditions can be predicted using this relation.     

The effect of platinum in a Nafion membrane on the durability of the membrane under fuel cell conditions

The effect of platinum on free radical generation and membrane degradation in proton exchange membrane (PEM) fuel cells is investigated using three typical cell configurations. Examinations of the fluoride emission rates (FERs) under different testing conditions indicate that platinum deposited in the membrane plays an important role as a catalytic center for the formation of H₂O₂ and HO free radicals, leading to PEM degradation. The chemical durability of the membranes is tested in accelerated Fenton tests. It confirms the formation of free radicals in the presence of platinum in the decomposition of H₂O₂ by colorimetric method with dimethyl sulfoxide (DMSO) as the trapping agent. In addition, structural and morphological changes of the membranes are characterized using FT-IR spectroscopy and scanning electron microscopy (SEM).     

Synergistic effects of advanced oxidization reactions in a combination of TiO2 photocatalysis for hydrogen production and wastewater treatment applications

In this paper, the synergistic effects of advanced oxidization reactions in a combination of TiO₂ photocatalysis are comparatively investigated for hydrogen production and wastewater treatment applications. An experimental study is conducted with a photoelectrochemical reactor under a UV-light source. TiO₂ is selected as the photocatalyst due to the high corrosion resistant nature and ability to form hydroxyl radicals with the interaction with photons. The synergetic effects of advanced oxidization processes (AOPs) such as Fenton, Fenton-like, photocatalysis (TiO₂/UV) and UV photolysis (H₂O₂/UV) are investigated individually and in a combination of each other. The Fenton type reagent in the reactor is formed by anodic sacrificial of stainless-steel electrode with the presence of H₂O₂. The influences of various parameters, including pH level, type of the electrode and electrolyte and the UV light, on the performance of the combined system are also investigated experimentally. The highest chemical oxygen demand (COD) removal efficiency is observed as 97.9% for the experimental condition which combines UV/TiO₂, UV/H₂O₂ and photo-electro Fenton type processes. The maximum hydrogen production rate from the photoelectrolysis of wastewater is obtained as 7.0 mg/Wh for the experimental condition which has the highest rate of photo-electro Fenton type processes. The average enhancement with the presence of UV light on hydrogen production rates and COD removal efficiencies are further calculated to be 3% and 20%, respectively.     

Hydrogen-based sono-hybrid catalytic degradation and mitigation of industrially-originated dye-based pollutants

Hazardous pollutants in water bodies have increased global concern due to their considerable toxicity and threat to the environmental matrices. Conventional remediation approaches are futile for eliminating various toxic dyes and other related pollutants. Regulations compliances for wastewater expulsion have forced scientists to either introduce new methods or upgrade present technologies to attain operative deprivation and mineralization of pollutants. Advanced oxidation processes (AOPs) relying on the generation of highly reactive oxidizing radicals, like ^?O, and ^?OH are considered efficient to attain high mineralization of a large number of dye pollutants and many other organic contaminants. Compared to conventional AOPs, including photocatalysis, Fenton, photo-ferrioxalate, ozone/UV, ozonation, H₂O₂/UV, etc., sonolysis is a comparatively newer AOP that implicates the use of ultrasound irradiation for generating oxidizing radicals, leading to the degradation of recalcitrant dyes. Due to no chemical catalyst requirement and being executed at ambient pressure and temperature, ultrasound-assisted AOPs have become robust hybrid AOPs to degrade environmental contaminants. Ultrasound treatments to mitigate pollutants are important because of the cavitation phenomenon. This review focuses on the degradation of dyes through ultrasound-based advanced oxidation processes. Firstly, we have described the ecotoxicity and health hazards of dye pollutants, then different sono-based methods such as sono-hybrid Fenton (US + Fe⁺²/H₂O₂), Fenton like (Fe⁺³/H₂O₂) process, sono-hybrid photo-Fenton process (US + Fe²⁺/H₂O₂/UV system, sono-hybrid hydrogen peroxide (US + H₂O₂), sono-hybrid catalytic (photo/electro) processes have been examined in details for their efficacy for degradation of dyes in wastewater. Future perspectives of ultrasound-assisted AOPs for dyes removal have also been discussed.     

In situ Fenton-enhanced cathodic reaction for sustainable increased electricity generation in microbial fuel cells

This study reports that Fenton's reaction is capable of facilitating cathodic reaction and thus increasing the current output in microbial fuel cells (MFCs). The hydroxyl radicals (OH) produced via Fenton's reaction are demonstrated to be vital to the enhancement of electricity generation in MFCs. In a two-chamber MFC employing expanded polytetrafluoroethylene (e-PTFE) laminated cloth as a separator, the power output is enhanced approximately four times with Fenton's reaction. However, the enhancement lasts only a short time period due to the rapid consumption of Fenton's reagents. To overcome this problem, a Fe@Fe₂O₃/carbon felt (CF) composite cathode is made, which results in a greater and, more importantly, sustainable power output. In the composite cathode, Fe@Fe₂O₃ functions as a controllably releasing Fenton iron reagent and CF functions as an air-fed cathode to electro-generate H₂O₂. This newly developed MFC with a Fenton system can ensure a continuous high power output, and also provides a potential solution to the simultaneous electricity generation and degradation of recalcitrant contaminants.     

Effect of H2O2 on Nafion properties and conductivity at fuel cell conditions

During PEM fuel cell operation, formation of H₂O₂ and material corrosion occurs, generating trace amounts of metal cations (i.e., Fe²⁺, Pt²⁺) and subsequently initiating the deterioration of cell components and, in particular, PFSA membranes (e.g., Nafion). However, most previous studies of this have been performed using conditions not relevant to fuel cell environments, and very few investigations have studied the effect of Nafion decomposition on conductivity, one of the most crucial factors governing PEMFC performance. In this study, a quantitative examination of properties and conductivities of degraded Nafion membranes at conditions relevant to fuel cell environments (30-100%RH and 80 °C) was performed. Nafion membranes were pre-ion-exchanged with small amounts of Fe²⁺ ions prior to H₂O₂ exposure. The degradation degree (defined as loss of ion-exchange capacity, weight, and fluoride content), water uptake, and conductivity of H₂O₂-exposed membranes were found to strongly depend on Fe content and H₂O₂ treatment time. SEM cross-sections showed that the degradation initially took place in the center of the membrane, while FTIR analysis revealed that Nafion degradation preferentially proceeds at the sulfonic end group and at the ether linkage located in the pendant side chain and that the H-bond of water is weakened after prolonged H₂O₂ exposure.     

Cesium substituted 12-tungstophosphoric (CsxH3−xPW12O40) loaded on ceria-degradation mitigation in polymer electrolyte membranes

A novel multifunctional catalyst Cs_xH_3−xPW₁₂O₄₀/CeO₂ was prepared to mitigate the free radical attack to membranes in fuel cell environment. Cs_xH_3−xPW₁₂O₄₀/CeO₂ nanoparticles synthesized by solution-based hydrothermal method and two-step impregnation method were dispersed uniformly into the Nafion^® resin, and then the composite membrane was prepared using solution-cast method. The particles prepared were characterized by X-ray powder diffraction (XRD), TEM and FT-IR to evaluate the crystallite size, distribution of the nanopaticles and the crystal structure. The membrane degradation was investigated via ex situ Fenton test and in situ open circuit voltage (OCV) accelerated test. In the durability tests, the fluoride emission rate (FER) reduced nearly one order of magnitude by adding Cs_xH_3−xPW₁₂O₄₀/CeO₂ nanoparticles into the Nafion membrane, suggesting that Cs_xH_3−xPW₁₂O₄₀/CeO₂ catalyst has a promising application to greatly improve the proton exchange membrane (PEM) durability.     

Preparation and performance evaluation of a Nafion-TiO2 composite membrane for PEMFCs

Nafion/TiO₂ composite membranes were studied for the application in proton exchange membrane fuel cell (PEMFC) to be used with the humidified or dry reactant gases of H₂ and O₂. Composite membranes were prepared by carrying out in-situ sol–gel reaction of Ti (OC₄H₉)₄ in Nafion perfluorosulfonic acid films, such as Nafion112, 1135 and 115. The influence of the concentration of Ti (OC₄H₉)₄ isopropyl alcohol solution on the Ti content in the membranes of different thicknesses was investigated. The X-ray diffraction (XRD) analysis demonstrated that TiO₂ in the composite membranes had a structure of anatase with an average particle size of 4.0 nm. The energy dispersive spectra (EDS) analysis indicated a symmetrical distribution of the TiO₂ particles in the modified membranes. The water retention ability and electrochemical performance of Nafion/TiO₂ composite membranes were evaluated using a single PEMFC operated with humidified or dry gas reactants during a long period.     

Experimental determination of some hydrogen combustion characteristics,its performances as a clean fuel compared to fossil fuel ones

The present study was undertaken to acquire an improved understanding of the mechanisms involved in the energy transfer from oxy-fuel flames to solids. Industrial fuels like hydrogen, propane and acetylene have been investigated. Three different measurement techniques were used to compare potential of these fuels. ESR spectroscopy and IR spectroscopy were used to obtain respectively H radical concentration and OH concentration and temperature. Heat fluxes were measured using a differential calorimeter. We obtained, for the three oxy-fuel flames, curves showing distributions of radicals concentrations, temperature and heat transferred from different parts of the flame. These results for H₂-O₂ flames point out: (1) that experimental values are different from the predicted values for an adiabatic flame, (2) the heat transfer efficiency is for some equivalence ratios as well as or better than for other hydrocarbon - O₂ flames. This study gives also elements to create new flames using hydrogen fuel to replace in part fossil fuels.     

A degradation study of Nafion proton exchange membrane of PEM fuel cells

The durability and degradation behavior of the Nafion NR111 proton exchange membranes (PEMs) is investigated in detail under various mechanical, chemical and polarization conditions. It was found that the fatigue strength or the safety limit of the cyclic stress for Nafion NR111 membrane is ∼1.5 MPa that is 1/10 of the tensile strength of the membrane. The cyclic stress and dimensional change (or strain) induced by the water uptake can be substantial and are the main causes for the mechanical degradation and failure of the membrane. For example, in the case of RH cycling of water soaked state to 25% RH state, the cyclic stress of the Nafion membrane was as high as 2.23 MPa and the dimensional change was ∼11%. Both FTIR and NMR analysis indicate that the decomposition of the Nafion polymer in the H₂O₂ solution in the presence of trace Fe, Cr and Ni ions started from the ends of the main chain, resulting in the loss of the repeat units and the formation of voids and pinholes in the membrane. The high degradation rate of the membrane at the open circuit voltage most likely results from the attack of H₂O₂ formed between O₂ and H₂ crossovered through the membrane.     

Microelectrode-based hydrogen peroxide detection during oxygen reduction at Pt disk electrode

We investigated the diffusion process of H₂O₂ generated during O₂ reduction at a Pt microdisk electrode used as a generator for scanning electrochemical microscopy (SECM). First, the amount of O₂ consumption and generated H₂O₂ at the Pt generator electrode are estimated using a detector electrode installed in the SECM. Based on the results, a large amount of O₂ consumption and generated H₂O₂ are detected at the center of each generator electrode. According to the measurement, the O₂ starvation and H₂O₂ detection currents can be defined. Subsequently, the O₂ starvation and H₂O₂ detection currents are measured by varying the generator size. As a result, these currents decrease with a decrease in the generator electrode size, however, the H₂O₂ diffusion process is changed for the generator diameter of less than 50 μm. Finally, the O₂ starvation and H₂O₂ detection measurements were conducted using a Nafion-coated Pt microdisk electrode. The amount of O₂ consumption is not suppressed, while the amount of generated H₂O₂ decreases with the Nafion layer prepared on the Pt electrode. This result indicates that the thickness of the H₂O₂ diffusion layer in the H₂SO₄ solution is dramatically diminished by coating the Nafion layer on the Pt generator.     

Fenton法预活化污泥制备磁性活性炭的实验研究

利用Fenton法预活化二沉池剩余污泥能够有效改善污泥活性炭的性质,制备性能良好的污泥磁性活性炭.通过考察H_2O_2投加量、H_2O_2/Fe~(2+)投加质量比、活化pH、预活化时间对污泥前驱体和污泥磁性活性炭的影响,探索Fenton法预活化污泥的作用机理.结果表明:Fenton试剂在酸性pH条件下产生羟基自由基,具有强氧化性的·OH破坏污泥胞外聚合物,同时将大分子有机物氧化成中间体和小分子有机物,少量孔隙随着CO_2和H_2O的逸出而形成;Fenton试剂的使用引入了铁,而铁盐是污泥热解的催化剂,能够促进焦油的裂解,加快有机物大分子键的断裂,从而促使更多孔隙的生成.     

A durable and robust Fe–N–C electrocatalyst for oxygen reduction reactions by introducing Ti3C2–TiO2 as radical scavengers

Modern Fe–N–C electrocatalysts are promising as alternatives to expensive Pt-based catalysts for oxygen reduction reactions (ORR). Although the activity of this type of electrocatalyst have been improved over the years, their durability and longevity need critical enhancements for practical applications in fuel cells. Typically, the incomplete oxygen reduction inevitably generates reactive oxygen species, including ·OH and HO₂· radicals, which will fiercely attack the carbon support and directly damage active sites in Fe–N–C electrocatalysts. Herein, a durable and robust Fe–N–C@Ti₃C₂–TiO₂ electrocatalyst for high-efficiency ORR is synthesized, in which Ti₃C₂–TiO₂ could effectively scavenge ·OH radicals or decompose H₂O₂ molecules, and synergistically work with Fe–N–C catalysts to improve the durability. Consequently, the Fe–N–C@Ti₃C₂–TiO₂ electrocatalyst shows prominent ORR performance in both alkaline and acidic electrolytes, low H₂O₂ yield, and long-term stability. This work provides great prospects for the design of highly stable ORR electrocatalysts by introducing radical scavengers as an active defense to proactively eliminate H₂O₂ and its radicals.     

Effects of H2 and H preferential diffusion and unity Lewis number on superadiabatic flame temperatures in rich premixed methane flames

The structures of freely propagating rich CH₄/air and CH₄/O₂ flames were studied numerically using a relatively detailed reaction mechanism. Species diffusion was modeled using five different methods/assumptions to investigate the effects of species diffusion, in particular H₂ and H, on superadiabatic flame temperature. With the preferential diffusion of H₂ and H accounted for, significant amount of H₂ and H produced in the flame front diffuse from the reaction zone to the preheat zone. The preferential diffusion of H₂ from the reaction zone to the preheat zone has negligible effects on the phenomenon of superadiabatic flame temperature in both CH₄/air and CH₄/O₂ flames. It is therefore demonstrated that the superadiabatic flame temperature phenomenon in rich hydrocarbon flames is not due to the preferential diffusion of H₂ from the reaction zone to the preheat zone as recently suggested by Zamashchikov et al. [V.V. Zamashchikov, I.G. Namyatov, V.A. Bunev, V.S. Babkin, Combust. Explosion Shock Waves 40 (2004) 32]. The suppression of the preferential diffusion of H radicals from the reaction zone to the preheat zone drastically reduces the degree of superadiabaticity in rich CH₄/O₂ flames. The preferential diffusion of H radicals plays an important role in the occurrence of superadiabatic flame temperature. The assumption of unity Lewis number for all species leads to the suppression of H radical diffusion from the reaction zone to the preheat zone and significant diffusion of CO₂ from the postflame zone to the reaction zone. Consequently, the degree of superadiabaticity of flame temperature is also significantly reduced. Through reaction flux analyses and numerical experiments, the chemical nature of the superadiabatic flame temperature phenomenon in rich CH₄/air and CH₄/O₂ flames was identified to be the relative scarcity of H radical, which leads to overshoot of H₂O and CH₂CO in CH₄/air flames and overshoot of H₂O in CH₄/O₂ flames.     

Radical-induced degradation mechanism of perfluorinated polymer electrolyte membrane

The degradation mechanism induced by radicals was investigated for Nafion^®-117 by solution analysis. Nafion^® was exposed independently to three kinds of radicals, OH, H and O₂⁻ which were produced separately by γ-irradiation. Based on the eluted elements, the scission site in the membrane was analyzed. The results showed that the scission site was classified into two and these locations were closely relating to oxidative and reductive reactions. The decreasing rate of proton conductivity was more significant under the influence of reductive radicals. The progression of the unzipping reaction of main chain was suggested to be initiated by the production of tertiary carbon radical by reductive radicals such as H and O₂⁻ with the aid of OH. The structural degradation such as collapse of cluster and the cluster decomposition as well as the performance degradation was found to be initiated by such reductive radicals.     

Paramagnetic centers in particulate formed from the oxidative pyrolysis of 1-methylnaphthalene in the presence of Fe(III)2O3 nanoparticles

The identity of radical species associated with particulate formed from the oxidative pyrolysis of 1-methylnaphthalene (1-MN) was investigated using low temperature matrix isolation electron paramagnetic resonance spectroscopy (LTMI-EPR), a specialized technique that provided a method of sampling and analysis of the gas-phase paramagnetic components. A superimposed EPR signal was identified to be a mixture of organic radicals (carbon and oxygen-centered) and soot. The carbon-centered radicals were identified as a mixture of the resonance-stabilized indenyl, cyclopentadienyl, and naphthalene 1-methylene radicals through the theoretical simulation of the radical’s hyperfine structure. Formation of these radical species was promoted by the addition of Fe(III)₂O₃ nanoparticles. Enhanced formation of resonance stabilized radicals from the addition of Fe(III)₂O₃ nanoparticles can account for the observed increased sooting tendency associated with Fe(III)₂O₃ nanoparticle addition.     

A review on ethanol steam reforming for hydrogen production over Ni/Al2O3 and Ni/CeO2 based catalyst powders

Hydrogen is contemplated as an alternative clean fuel for the future. Ethanol steam reforming (ESR) is a carbon-neutral, sustainable, green hydrogen production method. Low cost Ni/Al₂O₃ and Ni/CeO₂ powder catalysts demonstrate high ESR activity. However, acidic nature of Al₂O₃ and instability of CeO₂ lead to deactivation of the catalysts easily. This article examines the research articles published on the modification of Ni by various noble and non-noble metals and on alteration of the supports by different metal oxides in detail and their effect on ESR all through 2000–2021. The ESR reaction mechanisms on Ni/Al₂O₃ and Ni/CeO₂ powder catalysts and basic thermodynamics for different possible reactions and H₂ yield are explored. Manipulation of catalyst morphology (surface area and particle size) via preparation method, selection of active metal promoter and support modifier are found to be significantly important for H₂ production and minimizing carbon deposition on catalysts.     

Thermostable ionomeric filled membrane for H2/O2 fuel cell

Dispersion of submicronic particles of phosphatoantimonic acid fillers (H3) with a 4.3 H⁺/kg (4.3 eq/kg) cationic exchange capacity (cec), in a solution of sulfonated polysulfone (PSS) of 1.07 H⁺/kg gives viscous suspension allowing the `filled' material to be shaped in thin films. From the conductivity measurements, a synergic effect between the acidity of PSS and H3 has been highlighted. Conductivity values close to those of the Nafion 117 have been determined in the same experimental conditions, i.e., 96% relative humidity at 80°C. Furthermore, the inorganic filler improves both the mechanical strength and the gas impermeability of the filled membrane as compared to an unfilled PSS membrane. A PSS–H3 membrane of 1.07 H⁺/kg cec filled with 7.1% in H3 provided 80% of Nafion performances in a H₂/O₂ fuel cell for 500 h at 80°C and 4 bars pressure of H₂ and O₂.     

Enhancing oxygen reduction reaction performance in acidic media via bimetal Fe and Cr synergistic effects

Currently, it is a great challenge to improve the stability of Fe-based oxygen reduction reaction (ORR) catalysts in acidic medium, because the Fenton reaction between Fe-based catalyst and H₂O₂ will reduce the stability of the catalyst. In this study, Fe, Cr and nitrogen-doped carbon (FeCr-N-C) was synthesized via co-impregnation of biomass walnut shells with metal precursor solutions. The FeCr-N-C catalyst has an onset potential of 0.88 V vs. RHE and outperforms the Fe-N-C catalyst in acidic media. Moreover, FeCr-N-C shows negligible activity decay (ΔE_1/2 = 14 mV) in 0.1 M HClO₄ after 20 000 cycles. The experimental results proved that the bimetal synergism can produce low yield of H₂O₂ (<2%), which might attribute to variations of local electronic structure. The reactive oxygen species of the catalyst were analyzed by Ultraviolet-visible (UV-Vis) absorption spectra. It was proved that the presence of bimetal inhibited the Fenton reaction between Fe²⁺/Fe³⁺ and H₂O₂, and further improved the stability of the catalysts. Hence, this study proposes an efficient strategy to facilitate the practical application of Fe-based catalysts in acidic media.