A microtubular all CNT gas diffusion electrode

We report a microtubular gas diffusion electrodes made of multi-walled carbon nanotubes (MWCNT). The electrodes were prepared by inside-out cake filtration of an aqueous MWCNT suspension onto a microfiltration hollow fiber (HF) membrane, followed by washing out the surfactant, drying and removal of the all CNT microtube from the HF membrane. Length, outer diameter, and wall thickness of the tubular electrodes are: up to 44 cm, ~ 1.7 mm and 275 μm, respectively. The BET surface area is 200 m²/g with a porosity of 48–67% and an electrical conductivity of ~ 20 S/cm. Application of this microtubular Gas Diffusion Electrodes (GDE) was studied for the oxygen reduction reaction (ORR) in divided and undivided electrochemical cells. Oxygen supply into the lumen of the tubular electrodes resulted in much higher current densities for ORR than in experiments where the electrolyte was saturated by bubbling with pure oxygen. Within the 0.25–1.0 bar pressure (gauge) region, higher ORR rates were achieved at lower pressure. We also show that H₂O₂ production is possible using the new GDE. We propose to use such novel electrodes for the fabrication of tubular electrochemical reactors, e.g. fuel cells, H₂O₂ generators, CO₂ reduction and other processes that involve GDE application.     

Platinum nanoparticles supported on nitrobenzene-functionalised graphene nanosheets as electrocatalysts for oxygen reduction reaction in alkaline media

A systematic study on the electrocatalytic properties of Pt nanoparticles supported on nitrobenzene-modified graphene (Pt-NB/G) as catalyst for oxygen reduction reaction (ORR) in alkaline solution was performed. Graphene nanosheets were spontaneously grafted with nitrophenyl groups using 4-nitrobenzenediazonium salt. The electrocatalytic activity towards the ORR and stability of the prepared catalysts in 0.1 M KOH solution have been studied and compared with that of the commercial Pt/C catalyst. The results obtained show that the NB-modified graphene nanosheets can be good Pt catalyst support with high stability and excellent electrocatalytic properties. The specific activity of Pt-NB/G for O₂ reduction was 0.184 mA cm⁻², which is very close to that obtained for commercial 20 wt% Pt/C catalyst (0.214 mA cm⁻²) at 0.9 V vs. RHE. The Pt-NB/G hybrid material promotes a four-electron reduction of oxygen and can be used as a promising cathode catalyst in alkaline fuel cells.     

Metal (Co,Fe) tribenzotetraazachlorin–fullerene conjugates: Impact of direct π-bonding on the redox behaviour and oxygen reduction reaction

Novel hexabutylsulphonyltribenzotetraazachlorin–fullerene (C₆₀) complexes of iron (FeHBSTBTAC–C₆₀) and cobalt (CoHBSTBTAC–C₆₀) have been synthesized and their electrochemistry and oxygen reduction reaction (ORR) compared with their octabutylsulphonylphthalocyanine analogues (FeOBSPc and CoOBSPc). It is proved that electron-withdrawing substituents (–SO₂Bu and C₆₀) on phthalocyanine macrocycle exhibit distinct impact on the solution electrochemistry of these metallophthalocyanine (MPc) complexes. The more electron-withdrawing C₆₀ substituent suppressed ORR compared to the –SO₂Bu in alkaline medium. FeOBSPc showed the best ORR activity involving a direct 4-electron mechanism, a rate constant of ～1 × 10⁸ cm³ mol^?1 s^?1 and a Tafel slope of ?171 mV dec^?1.     

O2 surface concentration change and its implication on oxygen reduction mechanism and kinetics at platinum in acidic media

O₂ concentration near Pt surface during oxygen reduction reaction (ORR) in 0.1 M HClO₄ has been monitored by rotating ring-disk electrodes system. At 0.8 V < E < 1.0 V (vs. RHE), O₂ concentration near Pt surface increases with potential accompanying with the decrease of ORR current at the disk electrode; O₂ concentration in the negative-going scan is larger than that at the same potential in the positive-going scan, while ORR current shows the opposite trend at ω > 400 rpm. At E > 0.8 V accumulation of O_ad|OH_ad at Pt disk electrode with ORR time is evident, revealing that O_ad|OH_ad formation rate is faster than that for the removal of OH_ad to H₂O under such conditions. At relatively lower rotation speed and faster scan rate, the cathodic current during ORR in the negative-going scan can be larger than that in the positive-going scan with a current peak at ca. 0.8 V, which is attributed to the superimposition of ORR current increase due to change of O₂ concentration near the surface and the additional reduction of O_ad|OH_ad formed from decomposed O₂ at higher potentials.     

Synthesis of efficient Vulcan–LaMnO3 perovskite nanocomposite for the oxygen reduction reaction

In situ autocombustion has been developed as a novel and efficient route for the synthesis of perovskite–carbon nanocomposites for the oxygen reduction reaction (ORR) in alkaline media. We demonstrate the synthesis of crystalline LaMnO_3 + δ perovskite–Vulcan composite with a high accessibility of active sites and high electronic conductivity required for efficient electrocatalysis. The rotating disc electrode measurements evidenced an excellent activity of the composite for the ORR.     

Ternary non-noble metal chalcogenide (W–Co–Se) as electrocatalyst for oxygen reduction reaction

In this study, a catalyst based on a novel ternary non-noble metal chalcogenide, W–Co–Se, was synthesized for the oxygen reduction reaction (ORR) in acidic medium. The non-noble metal chalcogenide catalyst was electrochemically stable in the potential range of 0.05–0.8 V versus NHE in 0.5 M H₂SO₄ aqueous solution. This catalyst demonstrated significant catalytic activity towards the ORR, showing the ORR onset potential at 0.755 V versus NHE in 0.5 M H₂SO₄ at 25 °C. Such high activity might be attributed to the electronic structure of non-noble metals modified by chalcogen.     

FeAgMo2O8 — A novel oxygen evolution catalyst material for alkaline metal–air batteries

This paper reports about FeAgMo₂O₈ — a novel oxygen evolution catalyst material for secondary (rechargeable) metal–air batteries. Bifunctional air electrodes were made using FeAgMo₂O₈ as a charging catalyst for oxygen evolution reaction (OER) and silverized carbon black (Ag/C) was employed as a discharging catalyst for oxygen reduction reaction (ORR). Corresponding air electrodes were investigated using 10 M KOH as an electrolyte. At current densities between 20 and 50 mA per cm² we observed discharging and charging voltages of 1.20 to 1.15 V and 1.96 to 2.05 V, respectively.     

Electrochemical performance of a novel CoTETA/C catalyst for the oxygen reduction reaction

In this communication, we report a novel CoTETA/C catalyst for the oxygen reduction reaction (ORR) which was prepared from a carbon-supported cobalt triethylenetetramine chelate, followed by heat treatment in an inert atmosphere. Electrochemical performances were measured using rotating disk electrode (RDE) technique and a PEM fuel cell test station. For a H₂–O₂ fuel cell system, the maximum output power density reached 162 mW cm^?2 at 25 °C with non-humidified reaction gases. We found a nanometallic face-centered cubic (fcc) α-Co phase embedded in the graphitic carbon after pyrolysis, based on X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) measurements. These results indicated that CoTETA/C is a promising catalyst for the ORR.     

Oxygen reduction behavior of RuO2/Ti,IrO2/Ti and IrM (M: Ru,Mo, W,V) Ox/Ti binary oxide electrodes in a sulfuric acid solution

Some oxide catalysts, such as RuO₂/Ti, IrO₂/Ti and IrM(M: Ru, Mo, W, V)O_x/Ti binary oxide electrodes, were prepared by using a dip-coating method on a Ti substrate. Their catalytic behavior for the oxygen reduction reaction (ORR) was evaluated by cyclic voltammetry in 0.5 M H₂SO₄ at 60 °C. These catalysts were found to exhibit considerably high activity, and the most active one among them was Ir_0.6V_0.4O₂/Ti prepared at 450 °C, showing onset potential for the ORR at about 0.86 V–0.90 (vs RHE).     

Investigation of the electrocatalytic activity of boron-doped diamond electrodes modified with palladium or gold nanoparticles for oxygen reduction reaction in basic medium

The paper reports on the electrocatalytic activity of boron-doped diamond (BDD) electrodes electrochemically modified with palladium (Pd) or gold nanoparticles (Au NPs) towards oxygen reduction reaction (ORR) in alkaline medium. The BDD/Pd NP interface shows a well-defined diffusion-controlled voltammetric oxygen reduction peak at −0.25 V vs. Ag/AgCl. This is more positive than the ORR peak at −0.59 V vs. Ag/AgCl observed on BDD/Au-NP composite electrodes. The ORR proceeds via a four-electron process in both cases.     

A non-precious metal catalyst for oxygen reduction prepared by heat-treating a mechanical mixture of carbon black,melamine and cobalt chloride

A non-precious metal catalyst CoMe]C for the oxygen reduction reaction is prepared by heat-treating a mechanical mixture of carbon black, melamine and cobalt chloride at 600 under nitrogen atmosphere for 2 h. The catalytic activity of CoMe/C is characterized by the electrochemical linear sweep voltammetry technique. The onset reduction potential of the catalyst is 0.55 V （vs. SCE） at a scanning rate of 5 mV/s in 0.5 mol/L H2SO4 solution. The formation of the ORR activity sites of CoMe/C is facilitated by metallic β- cobalt.     

Improved catalytic performance of nitrided Co–Ti and Fe–Ti catalysts for oxygen reduction as non-noble metal cathodes in acidic media

M–Ti/carbon black electrocatalysts (M: Mn, Fe, Co, Ni, Cu, Mo) were prepared by the polymerized complex (PC) method and subsequent nitridation under ammonia flow, and were investigated as cathodes for the oxygen reduction reaction (ORR) in 0.1 M H₂SO₄ aq. Among the metals investigated, Co and Fe gave high onset potentials of 0.86 and 0.87 V vs. RHE, respectively. In comparison with the Co-only catalyst, the Co–Ti catalyst showed higher current, reflecting high density of Co-related active sites. The presence of Ti was also essential for the electrode stability under severe acidic and oxidizing conditions.     

Transition metal-nitrogen-carbon catalysts for oxygen reduction reaction in neutral electrolyte

Platinum group metal-free (PGM-free) catalysts based on M-N-C types of materials with M as Mn, Fe, Co and Ni and aminoantipyrine (AAPyr) as N-C precursors were synthesized using sacrificial support method. Catalysts kinetics of oxygen reduction reaction (ORR) was studied using rotating ring disk electrode (RRDE) in neutral pH. Results showed that performances were distributed among the catalysts as: Fe-AAPyr > Co-AAPyr > Mn-AAPyr > Ni-AAPyr. Fe-AAPyr had the highest onset potential and half-wave potential. All the materials showed similar limiting current. Fe-AAPyr had an electron transfer involving 4e⁻ with peroxide formed lower than 5%. Considering H₂O₂ produced, it seems that Co-AAPyr, Mn-AAPyr and Ni-AAPyr follow a 2 × 2e⁻ mechanism with peroxide formed during the intermediate step. Durability test was done on Fe-AAPyr for 10,000 cycles. Decrease of activity was observed only after 10,000 cycles.     

Synthesis and characterization of carbon-supported transition metal oxide nanoparticles — Cobalt porphyrin as catalysts for electroreduction of oxygen in acids

Unpyrolyzed, non noble metal catalysts for Oxygen Reduction Reaction (ORR), denoted MeOx–CoP/C, were obtained using a two-step procedure. The procedure consisted of a synthesis of carbon-supported transition metal (Me═Co, or Ni, or Fe) nanoparticles, followed by adsorption of cobalt porphyrin (CoP). TEM and XPS analyses confirm the formation of nanoparticles and the presence of transition metal oxides. Rotating disk electrode measurements showed that the as-synthesized materials exhibit catalytic ORR activity in acidic medium toward oxygen reduction, which is higher than that of cobalt porphyrin on carbon. This reveals that the metal oxide nanoparticles enhance the activity of the metalloporphyrin without being electroactive themselves. The catalytic activity follows the sequence: CoOx–CoP/C > NiOx–CoP/C > FeOx–CoP/C, showing the influence of nature of the transition metal on the enhancing effect. The presence of a cobalt center incorporated in the macrocycle was found to be essential to the oxygen reduction reaction, appearing thus to be the catalytic active site of the reaction. Our data suggest the ORR occurs at a single active site.     

Supportless PdFe nanorods as highly active electrocatalyst for proton exchange membrane fuel cell

The PdFe nanorods (PdFe-NRs) with tunable length were synthesized by an organic phase reaction of [Pd(acac)₂] and thermal decomposition of [Fe(CO)₅] in a mixture of oleyamine and octadecene at 160 °C. They show a better proton exchange membrane fuel cell (PEMFC) performance than commercial Pt/C in working voltage region of 0.80–0.65 V, due to their high intrinsic activity to oxygen reduction reaction (ORR), reduced cell inner resistance, and improved mass transport.     

Role of rhodium doping into lanthanum cobalt oxide (LaCoO3) perovskite and the induced bifunctional activity of oxygen evolution and reduction reactions in alkaline medium

Transition metal oxides, especially perovskites, have been considered effective electrocatalysts for the oxygen evolution (OER) and oxygen reduction (ORR) reactions in an alkaline solution. Here, a series of lanthanum cobalt rhodium oxide perovskites with the chemical formula LaCo_1-xRh_xO₃ (LCRO, 0.1 ≤ x ≤ 0.70) were prepared through the approach of solid-phase synthesis and their bifunctional electrocatalytic activity was assessed for both the OER and ORR. The crystallinity, morphology, surface, and electrocatalytic features of the LCRO were significantly correlated with the rhodium content. The LaCo_0.7Rh_0.3O₃ electrocatalysts with x = 0.3 showed enhanced electrocatalytic bifunctional performance with a substantially lower OER/ORR onset potential of 1.38/0.73 V vs HRE, smaller Tafel slope (116/90 mV/dec), and low charge-transfer resistance, which is the most efficient catalyst among the other studied ratios and superior to the pristine lanthanum cobalt oxide benchmark electrocatalysts. The LaCo_0.7Rh_0.3O₃ electrode exhibit good bifunctional electrocatalytic behavior and long-term durability with an OER and ORR onset potential gap (ΔE = E_OER ? E_ORR) of only 0.65 V, which could be credited to the enriched oxygen vacancies, lattice expansion and the improved electrical conductivity upon the doping of larger size of Rh ions. The LaCo_1-xRh_xO₃ catalysts are obtained from abundant materials that have the potential of highly-active bifunctional OER and ORR electrocatalysts.     

Boosting Pt oxygen reduction reaction activity by tuning the tin oxide support

The influence of the dopant element on the physicochemical properties of the SnO₂ support and on the oxygen reduction reaction (ORR) activity of the supported Pt nanoparticles was systematically studied. The dopant element and concentration significantly influence the SnO₂ crystal structure and the electrical conductivity, as well as the ORR activity in 0.1 M HClO₄ of the supported Pt catalysts. ORR activities significantly exceeding that of Pt deposited on carbon were observed for Pt nanoparticles supported on Nb- and W-doped SnO₂. No linear correlation between the support conductivity and the ORR activity was observed, suggesting that other factors, such as (electronic) metal–support interactions, could play a role in the ORR activity of Pt catalysts supported on metal oxides.     

Influence of the proton transport on the ORR kinetics and on the H2O2 escape in three-dimensionally ordered electrodes

In this study, we use rotating ring disc electrode measurements to investigate the influence of the proton transport on the kinetics of the oxygen reduction reaction (ORR) in a 3D nanostructured catalytic layer based on Pt nanoparticles supported on vertically aligned carbon nanofibers. The results confirm that protons are involved in the rate determining step of the ORR in acidic media. For pH ≥ 3, the ORR occurs in two successive reduction waves. The first current plateau is limited by the proton diffusion and is followed by the second reduction wave attributed to the mechanism involving water dissociation. The shape of the H₂O₂ escape current curve is strongly affected by the pH of the solution and shows a pronounced maximum when the pH value is increased. These experimental features are discussed with the help of a kinetic model.     

Dealloyed carbon-supported PtAg nanostructures: Enhanced electrocatalytic activity for oxygen reduction reaction

Dealloyed PtAg/C nanostructures, prepared by selective electrochemical etching of Ag in 0.5 M H₂SO₄ from a series of alloyed Pt_mAg/C samples with atomic Pt/Ag ratio m = 0.1, 0.5, 1.0 and 1.5, were employed as cathode electrocatalysts for oxygen reduction reaction (ORR) in 0.5 M KOH. Compared with their as-prepared counterpart alloy catalysts, the dealloyed catalysts showed higher half-wave potentials (E_1/2) and significantly higher Pt mass-specific activity (MSA) data. The intrinsic activity (IA) of Pt increased more or less after the dealloying treatment but was strongly dependent on the composition (m) of the alloyed sample. The Pt IA numbers were comparable for the dealloyed catalysts derived from Pt_mAg/C of m = 0.5, 1.0 and 1.5, which were nearly twice that for E-TEK Pt/C catalyst and 3 times that for the dealloyed catalyst derived from Pt_0.1Ag/C.     

Crucial effect of rubidium cation on catalytic activity of a polycrystalline gold electrode toward oxygen reduction reaction in alkaline media

Catalytic activity of a polycrystalline gold electrode toward oxygen reduction reaction (ORR) in aqueous alkaline media in the presence of various alkali-metal sulfates (M₂SO₄, M = Li, Na, K, Rb and Cs) was investigated by hydrodynamic voltammetry. The fraction of 4e^? pathway in low overpotentials (? 0.1 to ? 0.3 V) depended on the alkali-metal cations (Rb ? Na, K, Cs, Li). A complete 4e^? reduction of O₂ was only attained in the presence of Rb⁺ cation in the solution, which was comparable or even superior to that reported at the Au(100) single crystal electrode.