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Bifunctional materials able to catalyze both the oxygen reduction (ORR) and the oxygen evolution (OER) reactions in alkaline media are still a challenge for the progress of energy conversion and storage devices such as metal-air batteries or unitized regenerative fuel cells. In this work, carbon nanofibers synthesized by electrospinning are modified with a combination of cobalt oxide and metallic cobalt (CoO-Co/CNF) and studied as a bifunctional air electrode for metal-air batteries. The performance of CoO-Co/CNF for both reactions is compared with state-of-the-art catalysts such as Pt/C and IrO 2 . The combination of cobalt oxide and metallic cobalt, finely distributed on the surface of graphitic carbon nanofibers, leads to a bifunctional catalyst with a half-wave potential for the ORR slightly better than Pt/C and a reversibility (ΔE OER-ORR ) of 809 mV. The stability of CoO-Co/CNF is assessed by means of different stress tests: polarizations at high electrochemical potentials (2 V vs. RHE), rapid charge-discharge cycles at ±80 mA cm −2 and long durability tests by charging for 12 h at 60 mA cm −2 and discharging for 8 h at −80 mA cm −2 . CoO-Co/CNF shows a remarkable stability, maintaining, at least, an 82% of its performance for the ORR after the stress tests, even when cycled for more than 100 h.

A membrane-electrode assembly based on a 90 μm short-side-chain Aquivion® proton exchange membrane and containing low catalyst loadings, 0.4 mg IrRuOx cm−2 and 0.1 mg Pt cm−2 at anode and cathode, respectively, is investigated for combined thermal and load cycling at high current density (3 A cm−2) in water electrolysis cell. Durability tests under steady-state and load-thermal cycles are compared to evaluate the efficiency losses under specific operating conditions. Ac-impedance spectra and post-operation analyses are carried out to investigate the degradation mechanism. Catalyst degradation occurs more rapidly under cycled operation whereas mass transfer issues are relevant especially under steady-state mode. Membrane thinning appears to be affected by the uptime hours at high current density. The overall cell voltage increase is slightly larger for the cycled operation compared to the steady-state mode. However, this is essentially related to a compensation effect associated to a larger decrease of series resistance during the steady-state durability test. The dynamic electrolysis mode at high current density does not exacerbate significantly the degradation issues of low catalyst loading MEAs compared to a steady-state operation. This confirms the proper dynamic characteristics of the polymer electrolyte membrane electrolyser.

A simple and broadly applicable preparation procedure to obtain silica-supported transition metal (namely Cr, Co and Mn)-based radical scavengers, containing sulfonic acid functionalities, is reported. These systems are widely characterised in terms of structure, bulk and surface composition and morphology by X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), X-Ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM). The scavenger material is loaded in ePTFE reinforced membranes prepared from Aquivion ® perfluorosulfonic acid (PFSA) dispersions. All these composite membranes show longer lifetime in Accelerated Stress Tests (AST) and reduced fluoride release in Fenton's tests than the scavenger-free membranes without any loss in electrochemical performance. The Cr-scavenger-based polymer electrolyte shows a three-time larger stability than the pristine membrane.

A facile synthesis, based on a wet impregnation technique and a thermal treatment, of a novel silica-supported cerium-oxide-based radical scavenger bearing sulfonic acid functionalities is presented. This material is loaded as a filler in ePTFE reinforced membranes (called R79-02S) prepared starting from Aquivion ® Perfluoro-Sulfonic Acid (PFSA) dispersions. The aim is to mitigate the peroxy radicals attack to the polymeric membrane under fuel cell operating conditions. These membranes show much longer (7 times more) life-time in Accelerated Stress Tests (AST) and reduced fluoride release (about one half) in Fenton's tests than the radical scavenger-free membrane without any loss in electrochemical performance. Scavenger-free Aquivion ® PFSA-based membrane durability is about 200 h in AST whereas the same membrane containing the newly developed radical scavenger exceeds 1400 h. These results confirm the stability of the modified membranes and the excellent activity of the composite scavenger in mitigating the polymer electrolyte degradation.

9 páginas.- 1 tabla.- 18 figuras., Several graphite-based electrodes are investigated for vanadium flow battery applications. These materials are characterized both as-received and after chemical or electrochemical treatments in order to vary the content of oxygen functional groups on the electrode surface. The surface properties of the samples are investigated by X-ray photoelectron spectroscopy. Electrochemical performance is evaluated by cyclic voltammetry and electrochemical impedance spectroscopy measurements in a three electrode half-cell. The chemical treatment with HNO3 causes a cleaning of the electrode surface from adsorbed oxygen species or labile bonded functional groups in highly graphitic samples. Whereas, carbonaceous materials characterized by smaller graphitic domains or a higher degree of amorphous carbon show an increase of oxygen functional groups upon chemical and electrochemical pre-treatments. In both cases, an increase of oxygen species content on the surface above 5% causes a decrease of electrochemical performance for the redox battery determined by an increase of ohmic and charge transfer resistance, Authors from CNR-ITAE acknowledge the financial support from “Ministero dello Sviluppo Economico – Accordo di Programma MSE-CNR per la Ricerca del Sistema elettrico Nazionale”.

This paper reports on the development of polymer electrolyte membranes (PEMs) based on sulfonated polysulfone for application in a DMFC mini-stack operating at room temperature in passive mode. The sulfonated polysulfone (SPSf) with two degrees of sulfonation (57 and 66%) was synthesized by a well-known sulfonation process. SPSf membranes with different thicknesses were prepared and investigated. These membranes were characterized in terms of methanol/water uptake, proton conductivity, and fuel cell performance in a DMFC single cell and mini-stack operating at room temperature. The study addressed (a) control of the synthesis of sulfonated polysulfone, (b) optimization of the assembling procedure, (c) a short lifetime investigation and (d) a comparison of DMFC performance in active-mode operation vs. passive-mode operation. The best passive DMFC performance was 220 mW (average cell power density of about 19 mW cm −2 ), obtained with a thin SPSf membrane (70 μm) at room temperature, whereas the performance of the same membrane-based DMFC in active mode was 38 mW cm −2 . The conductivity of this membrane, SPSf (IEC = 1.34 mequiv. g −1 ) was 2.8 × 10 −2 S cm −1 . A preliminary short-term test (200 min) showed good stability during chrono-amperometry measurements.

The effect of barium doping on a fuel cell anode consisting of a Ni 0.53 Cu 0.47 alloy and gadolinia doped ceria cermet (50:50 wt.) was investigated in an intermediate temperature solid oxide fuel cell (IT-SOFC) fed with dry methane. IT-SOFCs containing the same cathode-electrolyte configuration but equipped with different anodes were compared in terms of performance, steady state operation, and formation of carbon deposits on the anode surface. After proper cell conditioning, similar maximum power densities of 284 mW cm −2 and 310 mW cm −2 were obtained in dry methane fuel at 750 °C for the anodic compositions with and without barium, respectively. The anode containing barium did not show significant carbon deposits after a 200 h time-study in dry methane whereas the anode without barium showed the presence of carbon deposits as evidenced by X-ray diffraction (XRD). Both anode formulations showed carbon deposits when exposed to dry CH 4 in ex-situ experiments. The electrochemical profile recorded during the time-study was different in the two systems. In the absence of barium, a slow and progressive increase in performance as a function of time was achieved. On the contrary, the Ba-doped anode achieved optimum performance in a short time. It is determined that the presence of barium reduces carbon deposition under fuel cell conditions and influences the electrochemical behaviour.

An investigation of properties and operating parameters of a passive DMFC monopolar mini-stack, such as catalyst loading and methanol concentration, was carried out. From this analysis, it was derived that a proper Pt loading is necessary to achieve the best compromise between electrode thickness and number of catalytic sites for the anode and cathode reactions to occur at suitable rates. Methanol concentrations ranging from 1 M up to 10 M and an air-breathing operation mode were investigated. A maximum power of 225 mW was obtained at ambient conditions for a three-cell stack, with an active single cell area of 4 cm 2 , corresponding to a power density of about 20 mW cm −2 .

This paper reports the development and characterization of sulfonated polysulfone (SPSf) polymer electrolytes for direct methanol fuel cells. The synthesis of sulfonated polysulfone was performed by a post sulfonation method using trimethyl silyl chlorosulfonate as a mild sulfonating agent. Bare polysulfone membranes were prepared with two different sulfonation levels (60%, SPSf-60 and 70%, SPSf-70), whereas, a composite membrane of SPSf-60 was prepared with 5 wt% silica filler. These membranes were investigated in direct methanol fuel cells (DMFCs) operating at low (30–40 °C) and high temperatures (100–120 °C). DMFC power densities were about 140 mW cm−2 at 100 °C with the bare SPSf-60 membrane and 180 mW cm−2 at 120 °C with the SPSf-60-SiO2 composite membrane. The best performance achieved at ambient temperature using a membrane with high degree of sulfonation (70%, SPSf-70) was 20 mW cm−2 at atmospheric pressure. This makes the polysulfone-based DMFC suitable for application in portable devices.

An anodic cermet of NiCu alloy and gadolinia doped ceria has been investigated for CH 4 electro-oxidation in IT-SOFCs. Polarization curves have been recorded in the temperature range from 650 to 800 °C. A maximum power density of 320 mW cm −2 at 800 °C has been obtained in the presence of dry methane in an electrolyte-supported cell. The electrochemical behaviour during 1300 h operation in dry methane and in the presence of redox-cycles has been investigated at 750 °C; variation of the electrochemical properties during these experiments have been interpreted in terms of anode morphology modifications. The methane cracking process at the anode catalyst has been investigated by analysing the oxidative stripping of deposited carbon species.

High surface area carbon supported bimetallic Pt–Fe catalysts have been investigated for the oxygen electro-reduction process in low temperature direct methanol fuel cells (30–60 °C). The electrocatalysts have been prepared by using a combination of colloidal and incipient wetness methods allowing of synthesis carbon supported bimetallic nanoparticles with a particle size of about 2–3 nm. These materials were studied in terms of structure, morphology and composition using XRD, XRF and TEM techniques. Oxygen reduction properties of these bimetallic catalysts were investigated in single cell DMFC polarizations. The electrochemical data were compared with those obtained in the presence of a Pt/C catalyst prepared by the same procedure and characterised by the same concentration of active phase on the carbon support and a similar particle size. An improvement of the DMFC single cell performance was observed in the activation controlled region in the presence of the bimetallic catalysts. Moreover, the influence of the ionomer loading on the performance of the DMFC was investigated. The highest power density was recorded with electrodes containing 25% Nafion loading (87 mW cm−2).

A La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3 –Ce 0.8 Gd 0.2 O 1.9 (LSFCO–CGO) composite anode material was investigated for the direct electrochemical oxidation of methane in intermediate temperature solid oxide fuel cells (IT-SOFCs). A maximum power density of 0.17 W cm −2 at 800 °C was obtained with a methane-fed ceria electrolyte-supported SOFC. A progressive increase of performance was recorded during 140 h operation with dry methane. The anode did not show any structure degradation after the electrochemical testing. Furthermore, no formation of carbon deposits was detected by electron microscopy and elemental analysis. Alternatively, this perovskite material showed significant chemical and structural modifications after high temperature treatment in a dry methane stream in a packed-bed reactor. It is derived that the continuous supply of mobile oxygen anions from the electrolyte to the LSFCO anode, promoted by the mixed conductivity of CGO electrolyte at 800 °C, stabilises the perovskite structure near the surface under SOFC operation and open circuit conditions.

The surface properties of inorganic fillers employed in composite membranes have been investigated by X-ray photoelectron spectroscopy (XPS), BET surface area and acid–base characterisations. The adsorption density of hydroxyl–hydrogen ions and the pH of slurry varied with the amount and strength of acid–base surface functionalities. A shift towards higher binding energies was observed for the O 1s photoelectron peak as a function of the increase of the filler acidic behaviour. The conductivity of the membranes and the maximum power density at 145 °C for direct methanol fuel cells (DMFCs) increased with the filler surface acidity.

Low cost ethylene–tetrafluoroethylene (ETFE)-based grafted membranes have been prepared by a process based on electron beam irradiation, subsequent grafting, cross-linking and sulfonation procedure. Two different grafted membranes varying by their grafting and cross-linking levels have been investigated for applications in direct methanol fuel cells (DMFCs) operating between 90 and 130 °C. DMFC assemblies based on these membranes showed cell resistance and performance values comparable to Nafion 117. Stable electrochemical performance was recorded during 1 month of cycled operation. Tailoring of grafting and cross-linking properties allows a significant reduction of methanol cross-over while maintaining suitable conductivity and performance levels.

Serpentine (SFF) and interdigitated (IFF) flow fields were investigated with regard to their use in a direct methanol fuel cell (DMFC). The DMFC equipped with SFFs showed lower methanol cross-over, higher fuel utilisation and slightly larger voltage efficiency at low current densities. IFFs enhanced mass transport and membrane humidification allowing to achieve high power densities of 450 and 290 mW cm−2 in the presence of oxygen and air feed, respectively, at 130°C. A fuel efficiency of 90% was obtained with the IFFs in the presence of 1 M methanol feed at 130°C and a current load of 500 mA cm−2.

A study is made of the electrooxidation of methanol in sulfuric acid on carbon-supported electrodes containing platinum-tin bimetal catalysts that are prepared by an in situ potentiometric-characterization route. The catalysts are investigated by employing chemical analyses, X-ray diffraction (XRD), X-ray absorption-near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) data in conjunction with electrochemical measurements. From the electrochemical data, it is inferred that while an electrode with (3:1) Pt-Sn/C catalyst involves a two-electron rate-limiting step akin to platinum-on-carbon electrodes, it is shifted to a one-electron mechanism on electrodes with (3:2)Pt-Sn/C, (3:3)Pt-Sn/C, and (3:4)Pt-Sn/C catalysts. The study suggests that the tin content in the platinum-tin bimetal catalyst produces: (i) a charge transfer from tin to platinum; (ii) an increase in the coverage of adsorbed methanolic residues with increase in the tin content, as indicated by the shift in rest potential of the electrodes towards the reversible value for oxidation of methanol (0.043 V versus SHE), and (iii) a decrease in the overall content of higher valent platinum sites in the catalyst. © 1994.

Minimization of high-cost metal electrocatalyst is necessary to achieve cost-effective green hydrogen productionby proton exchange membrane (PEM) water electrolysis (WE). The large increase of the cost of Ir recently observedrequires individuating alternative catalyst solutions for the oxygen evolution reaction (OER) in PEMWE.The OER is the rate determining step of the electrolysis process requiring high-cost noble metals with significantlyhigh loading. A non-platinum group metal (non-PGM) anode catalyst based on silver and titanium suboxidewas prepared and used for the oxygen evolution reaction in a PEMWE cell. By using a solid-state synthesisprocedure, a silver nitrate and titanium suboxides (TinO2n-1) with Magneli phase powders were mixed and subjectedthereafter to a thermal treatment at 300 °C in a 50% H2/N2 gas stream to promote the inclusion of silverwithin the Ti-suboxides structure. A membrane – electrode assembly (MEA), consisting of the anode Ag/TinO2n−1and conventional Pt/C cathode catalysts deposited on a 212 NAFION® membrane (thickness 50 μm), was investigated to assess the performance and durability of the PGM free oxygen evolution catalyst in an acidic environment.A promising performance, of 0.6 A cm−2 at 2 V/cell at 80 °C, and an excellent stability (degradation rate