Knowledge on the mechanisms of oxygen reduction reaction (ORR) and descriptors linking the catalytic activity to the structural and electronic properties of transition metal oxides enable rational ...design of more efficient catalysts. In this work ORR electrocatalysis was studied on a set of single and complex Mn(III) oxides with a rotating disc electrode method and cyclic voltammetry. We discovered an exponential increase of the specific electrocatalytic activity with the potential of the surface Mn(IV)/Mn(III) red-ox couple, suggesting the latter as a new descriptor for the ORR electrocatalysis. The observed dependence is rationalized using a simple mean-field kinetic model considering availability of the Mn(III) centers and adsorbate-adsorbate interactions. We demonstrate an unprecedented activity of Mn2O3, ca. 40 times exceeding that of MnOOH and correlate the catalytic activity of Mn oxides to their crystal structure.
Carbon materials are widely applied as conductive additives in studies of the oxygen evolution reaction (OER) in alkaline media catalyzed by transition metal oxides. In this work we investigate the ...anodic behavior of three representative carbon materials: furnace black (Vulcan XC-72R), acetylene black, and pyrolytic carbon of the Sibunit family (Sibunit-152), in 1 M NaOH at high potentials (ranging from the OER onset and up to ca. 2 V vs. RHE) in the time span from several minutes to several hours. We apply the rotating ring-disk electrode (RRDE) to separate the OER current from the carbon corrosion current. We then use transmission electron microscopy (TEM) to visualize changes in the carbon morphology resulting from corrosion. Finally, we study the OER performance of composite electrodes comprising carbon materials mixed with a La0.5Sr0.5Mn0.5Co0.5O3−δ perovskite OER catalyst, and discuss possible influence of the oxide on the carbon corrosion.
In this work we studied the effect of carbon in LaMnO3- and Mn3O4- carbon composites synthesized through the In Situ Auto Combustion (ISAC) route for the Oxygen Reduction Reaction (ORR). For this ...study, we prepared LaMnO3 perovskite and Mn3O4 spinel composites using different types of carbon materials: carbon blacks, pyrolytic carbons, and catalytic filamentous carbon. Various analytical methods such as cyclic voltammetry, rotating ring disc electrode, Brunauer-Emmett-Teller (BET) method, X-Ray Powder Diffraction (XRD), thermogravimetric analysis (TGA), X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS) were used to shed light on the influence of carbon in LaMnO3- and Mn3O4 - carbon composites during the ORR. These allowed us to define key characteristics of carbon materials, which determine the ORR activity and selectivity of the composites, among them sub-structural characteristics of carbon materials and pore accessibility. We conclude that (i) the ISAC route allows to significantly increase the number of accessible oxide sites and the ensuing ORR activity, (ii) the oxidation degree of Mn in LaMnO3 depends on the type and wt.% of carbon material and sub-structural order of carbon. (iii) We further claim that sub-structural characteristics of carbon materials, their pore size and size distribution affect the ORR activity of LaMnO3- and Mn3O4 - carbon composites. (iv) We see that an optimum combination of the carbon and the metal oxide allows obtaining the highest ORR activity. (v) Vulcan XC72 carbon seems to be the best suited for the ISAC synthesis of metal oxide-carbon composites due to its high structure consisting of highly branched flexible reticulate agglomerates.
We consider compositional and structural factors which can affect the activity of bixbyite α-Mn2O3 towards the oxygen reduction reaction (ORR) and the stability of this oxide in alkaline solution. We ...compare electrochemistry of undoped, Fe and Al-doped α-Mn2O3 with bixbyite structure and braunite Mn7SiO12 having bixbyite-related crystal structure, using the rotating disk electrode (RDE), the rotating ring-disk electrode (RRDE), and cyclic voltammetry (CV) techniques. All manganese oxides under study are stable in the potential range between the ORR onset and ca. 0.7 V vs. Reversible Hydrogen Electrode (RHE). It is found that any changes introduced in the bixbyite structure and/or composition of α-Mn2O3 lead to an activity drop in both the oxygen reduction and hydrogen peroxide reactions in this potential interval. For the hydrogen peroxide reduction reaction these modifications also result in a change in the nature of the rate-determining step. The obtained results confirm that due to its unique crystalline structure undoped α-Mn2O3 is the most ORR active (among currently available) Mn oxide catalyst and favor the assumption of the key role of the (111) surface of α-Mn2O3 in the very high activity of this material towards the ORR.
Transition metal oxides are attractive noble metal-free catalysts of the oxygen reduction for application at the cathode of alkaline membrane fuel cells or metal-air batteries. However, despite of a ...rapidly increasing number of publications devoted to the oxygen electrocatalysis on transition metal oxides, a clear picture regarding the relations between their structure and composition on the one hand and electrocatalytic activity on the other hand is lacking. This short review discusses challenges facing researchers seeking to understand electrocatalysis of the oxygen reduction reaction on transition metal oxides.
•Factors affecting ORR activities of transition metal oxides are briefly reviewed.•Guidelines are provided on how to avoid pitfalls in establishing structure–activity relations.•Influence of carbon on the electrocatalytic activity is highlighted.•Oxygen reduction pathways and mechanisms on transition metal oxides are analyzed.
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 LaMnO3+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.
•In situ autocombustion route: a highly clean and efficient route to produce perovskite-carbon nanocomposites.•A superior ORR activity of nanocomposite prepared by in situ autocombustion compared to the composite manually mixed.•A smaller size of the nanocomposite prepared by in situ auto-combustion route against the unsupported oxide.•A better distribution of the nanocomposite perovskite in carbon favouring closer interaction between the two components.
•The In Situ Autocombustion allows easy preparation of oxide–carbon composites for OER.•Co3O4 nanoparticles in composites are smaller and less agglomerate than bulk one.•The Co3O4-BDD composite is a ...competitive electrocatalyst for the alkaline OER.•Higher stability of the BDD compared either to Vulcan or Sibunit conventional carbons.•BDD is a promising support for alkaline or anion exchange membrane electrolysis cells.
In this work we studied the anodic stability of nanostructured Boron-Doped Diamond (BDD) in view of its eventual utilisation as support for metal oxide–carbon composites prepared by the In Situ Autocombustion (ISAC) method. The anodic behaviour of the BDD was investigated by the rotating ring-disc electrode (RRDE) method in 1 M NaOH electrolyte to determine its corrosion current and compare the latter with that of the commercial Vulcan XC-72 carbon from Cabot and Sibunit pyrolytic carbon, the latter known for its high stability against electrochemical corrosion. The BDD has proven to be significantly more stable than either the Sibunit or the Vulcan carbon. The Co3O4-BDD and Co3O4-Sibunit composites prepared by the ISAC method were then tested as catalysts of the oxygen evolution reaction (OER). The Co3O4-BDD composites appear to be competitive electrocatalysts for the OER in alkaline medium, showing activity comparable to the literature and much higher support stability towards oxidation. Further research is required to decrease the size of Co3O4 spinel nanoparticles and improve their distribution over the BDD support and hence increase the mass-specific OER activity of the BDD-based ISAC composites.
The glucose oxidation reaction (GOR) was studied on Au and Ni electrodes by cyclic voltammetry (CV), rotating disk electrode (RDE) measurements coupled with Koutecky–Levich analysis, Differential ...Electrochemical Mass Spectrometry (DEMS), in situ Fourier Transform Infrared spectroscopy (FTIRS) and High Performance Liquid Chromatography (HPLC) analysis of the reaction products after electrolysis in potentiostatic mode under continuous flow conditions. This study allowed to identify the reaction products and propose tentative reaction mechanisms. On gold, glucose is adsorbed on metallic sites through its anomeric function (C1) resulting in the formation of gluconate as the main GOR product at potentials close to 0.6 V vs. RHE, with a selectivity towards gluconate close to 100% and a projected faradaic efficiency of ca. 70%. The conversion rate is rather low, close to 20%, due to poisoning of the surface, leading to a strong deactivation. At potentials above 0.7 V vs. RHE, the selectivity towards gluconate decreases and non-selective GOR oxidation proceeds through CC bond cleavage. On nickel, the GOR occurs at high potentials (close to 1.2 V vs. RHE) on Ni(OH)2 and NiOOH sites, and proceeds through C1C2 bond breaking, resulting in arabinose and formate. At higher potentials, the selectivity towards arabinose decreases, formate being the main reaction product.
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•Gold selectively catalyzes the glucose oxidation into gluconate at 0.6 V vs. RHE.•Glucose oxidation on gold leads to molecular hydrogen release as by product.•The GOR occurs on Ni(OH)2/NiOOH but not on metallic Ni sites.•Arabinose and formic acid are the main products from glucose oxidation on Ni.
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•Carbon in nanocomposites reduces the particle size of perovskite and decrease the agglomeration degree in composite, allowing a better dispersion and an increase of the number of ...active sites.•Strong interaction of the perovskite oxide with carbon is evidenced by the carbon-fraction dependent mean oxidation state of Mn.•The ORR activity measured for the ISAC nanocomposites increases with the fraction of carbon in nanocomposites.•The ORR activity is also influenced by the mean oxidation state of Mn of the perovskite in the composites.
In this study, the authors synthesized perovskite-carbon composites by the In Situ AutoCombustion (ISAC) method. By physical-chemical characterization it was observed that nanoparticles of the LaMnO3±δ perovskite phase are well dispersed in the carbon matrix and are primarily located in the mesopores and small macropores of the Vulcan carbon. By electrochemical methods it was concluded that the oxygen reduction reaction (ORR) activity improvement of ISAC composites relative to perovskite/carbon composites prepared by mixing is due to the ISAC synthesis route and to the presence of carbon in the composites, which act by reducing the particle size of the perovskite, and decreasing their agglomeration degree. These result in the higher dispersion of perovskite particles and the ensuing increase of the number of catalytically active sites. Results of temperature programmed reduction reaction, X-ray photoelectron and electron energy loss spectroscopies suggest that perovskite particles in ISAC composites have lower oxygen stoichiometry compared to the unsupported LaMnO3 and are reduced by H2 at lower temperatures.
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