Electron Transfer Number For Oxygen Reduction Reaction Research Articles

Spinel Iron Oxide Nanoparticles Decorated on Pyridinic‐N and Carbon Surface: A Highly Efficient Inexpensive Electrocatalyst for Oxygen Reduction and Oxygen Evolution Eeactions

AbstractThe oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) have been established widely owing to their significance in water electrolyzers, fuel cells and metal‐air batteries. The ORR electron transfer number have been calculated from K−L plot as 3.1 (Fe3O4/ N2−xC‐500), 3.8 (Fe3O4/N2−xC‐600), 3.6 (Fe3O4/N2−XC‐700) and 3.2 (Fe3O4/N2−XC‐800). At 3 mA cm−2, the half‐wave potential has been determined as 0.66 V (Fe3O4/N2−XC‐500), 0.76 V (Fe3O4/N2−XC‐600), 0.75 V (Fe3O4/N2−XC‐700) and 0.68 V (Fe3O4/N2−XC‐800). At 10 mA cm−2, the OER potential for Fe3O4/N2−XC‐500, Fe3O4/N2−XC‐600, Fe3O4/N2−XC‐700 and Fe3O4/N2−XC‐800 electrocatalysts were determined as 1.61, 1.58, 1.62 and 1.66 V, respectively. The ORR: OER electrode potentials were found to be 0.95, 0.82, 0.87 and 0.93 V for Fe3O4/N2−XC‐500, Fe3O4/N2−XC‐600, Fe3O4/N2−XC‐700 and Fe3O4/N2−XC‐800 electrocatalysts, respectively by using solar to hydrogen fuel device conversion scale. The ORR/OER Tafel slopes were calculated as 78/89 mV/dec (Fe3O4/N2−XC‐500), 73/78 mV/dec (Fe3O4/N2−XC‐600), 76/83 mV/dec (Fe3O4/N2−XC‐700) and 79/98 mV/dec (Fe3O4/N2−XC‐800). Among the new born oxygen vacancy (Vo) spinel oxide electrocatalysts, Fe3O4/N2−XC‐600 exhibit an exceptional oxygen electrode activity with 0.82 V in 0.1 M KOH due to robust double exchange interconnection (DEI) between stable magnetite spinel Fe3O4 and N2−XC.

Graphitic carbon nitride on reduced graphene oxide prepared via semi-closed pyrolysis as electrocatalyst for oxygen reduction reaction

Graphitic carbon nitride (g-C3N4)/graphene hybrids were shown to be promising low-cost and durable substitutes for Pt-based oxygen reduction reaction (ORR) electrocatalysts in metal-air batteries and fuel cells. However, most of the synthesis procedures found in the literature are tedious, resource-intensive, and make use of toxic precursors. In this paper, we present a facile method for preparing g-C3N4 on reduced graphene oxide (g-C3N4@rGO) via semi-closed pyrolysis of graphene oxide and urea under an ambient air atmosphere. The resulting electrocatalyst was characterized using microscopic, structural, and spectroscopic techniques. Electrochemical characterization showed that the onset and half-wave potentials (vs. Ag/AgCl), as well as ORR electron transfer number, were found to be – 0.248 V, – 0.281 V, and 3.16, respectively. Although g-C3N4@rGO has relatively lower electrocatalytic activity compared to Pt/C, the former was shown to exhibit better electrochemical stability. Nonetheless, the low-cost and more sustainable method of manufacturing g-C3N4@rGO in this study advances the economic viability of carbon-based ORR electrocatalysts.

Fe/N codoped porous graphitic carbon derived from macadamia shells as an efficient cathode oxygen reduction catalyst in microbial fuel cells.

In this study, Fe/N codoped porous graphitic carbon derived from macadamia shells was prepared at different temperatures as cathodic catalysts for microbial fuel cells (MFCs), with K2FeO4 as a bifunctional catalyst for porosity and graphitization. The catalyst prepared at 750 °C (referred to as MSAC-750) showed a large specific surface area (1670.3 m2 g-1), graphite structure, and high pyridine-N and Fe-N X contents. Through the electrochemical workstation test, MSAC-750 shows excellent oxygen reduction reaction (ORR) activity, with an onset potential of 0.172 V and a half-wave potential of -0.028 V (vs. Ag/AgCl) in a neutral medium, and the ORR electron transfer number is 3.89. When applied to the MFCs as cathodic catalysts, a higher maximum power density and voltage of 378.68 mW m-2 and 0.425 V were achieved with the MSAC-750 catalyst and is superior to that of the Pt/C catalyst (300.85 mW m-2 and 0.402 V). In this case, a promising method is hereby established for the preparation of an excellent electrochemical catalyst for microbial fuel cells using inexpensive and easily available macadamia shells.

3D N, S-co-doped carbon nanotubes/graphene/MnS ternary hybrid derived from Hummers' method for highly efficient oxygen reduction reaction

Designing and preparing oxygen reduction reaction (ORR) catalysts with low cost, high activity and strong stability play a crucial function in the application of fuel cells and metal-air batteries. In this work, we use pristine graphite and carbon nanotubes (CNT) mixture as initial carbon source, and for the first time successfully prepare N, S-co-doped carbon nanotubes/graphene/MnS ternary hybrid (NSCNT/NSG/MnS) by modified Hummers' method followed by a pyrolysis process. The morphology, structure, composition and ORR performance of the obtained sample are measured by scanning electron microscope, X-ray diffraction and lots of other techniques. The results show that the successful synthesis of NSCNT/NSG/MnS is achieved by the combination of effective exploitation of residual Mn and S species and strong reducibility of carbon under high temperature. Besides, we find that the resultant NSCNT/NSG/MnS can not only effectively stabilize and disperse MnS nanoparticles, but also possess large specific surface area, high N content and unique 3D architecture. Compared to other counterparts, the obtained 3D NSCNT/NSG/MnS exhibits superior ORR performance (the onset potential moves positively to 1.00 V; limiting-current density achieves 4.93 mA/cm2; ORR electron transfer number is close to four), which is proven to approach that of commercial Pt/C catalyst.

One-Step Construction of Ni/Co-Doped C–N Nanotube Composites as Excellent Cathode Catalysts for Neutral Zinc–Air Battery

Development of a neutral Zn–air battery is of much significance due to the high stability of zinc in a neutral electrolyte. Here, Ni/Co-doped C–N nanotube composites (C–N, Ni/C–N, Co/C–N, and Ni–Co/C–N) as efficient oxygen reduction reaction (ORR) electrocatalysts in a neutral medium have been prepared by direct pyrolysis of Ni/Co salt, dicyandiamide (DCD) and glucose. Among the synthesized catalysts, Ni–Co/C–N presents a high ORR current density of 8.5[Formula: see text]mA[Formula: see text][Formula: see text][Formula: see text]cm[Formula: see text] in a 0.5[Formula: see text]mol[Formula: see text][Formula: see text][Formula: see text]L[Formula: see text] KNO3 solution. The ORR electron transfer number of the catalyst Ni–Co/C–N is 3.8, indicating that O2 is almost completely reduced to H2O. A neutral zinc–air battery utilizing a 0.5[Formula: see text]mol[Formula: see text][Formula: see text][Formula: see text]L[Formula: see text] KNO3 solution has been assembled by using the prepared composite catalyst coated on carbon paper as an air cathode, and Zn plate as an anode. The battery with the cathode catalyst Ni–Co/C–N delivers the open-circuit voltage of 1.13[Formula: see text]V and the maximum power density of 65[Formula: see text]mW[Formula: see text][Formula: see text][Formula: see text]cm[Formula: see text]. The constant discharge current density of 50[Formula: see text]mA[Formula: see text][Formula: see text][Formula: see text]cm[Formula: see text], 100[Formula: see text]mA[Formula: see text][Formula: see text][Formula: see text]cm[Formula: see text] and 150[Formula: see text]mA[Formula: see text][Formula: see text][Formula: see text]cm[Formula: see text] can last 202[Formula: see text]h, 93[Formula: see text]h and 11[Formula: see text]h, respectively. A stable voltage plateau appears at various discharge current densities. The neutral zinc–air battery can be repeatedly discharged after replacing the zinc anode. Results indicate that the synthesized Ni–Co/C–N catalyst is an excellent cathode material applied to a neutral zinc–air battery, showing broad application prospects as a mobile power source.

Preparation and catalytic studies of pyrrole-doped carbon black oxide cathode materials for oxygen reduction reactions

Oxygen reduction reaction (ORR) is an important cathode reaction for alkaline fuel and air-metal batteries. Because of the high cost and low stability of traditional Pt-based cathode materials for ORR, it is important to find an alternative cathode material of high performance and stability and low cost. Nitrogen-doped carbonaceous materials are currently of keen interest among those alternative oxygen cathode materials. The arrangement of carbon atoms in carbon black (CB) is similar to that of graphite, and it is wellknown that CB has a unique advantage over other carbon materials owing to its relatively low price and wide availability. Based on cheap carbon black, pyrrole-doped carbon black oxide (rCBO-Pyrrole) cathode materials were prepared using a facile synthesis method for this article, and their catalytic performances toward ORR were studied. The characterization of the catalysts was explored using a scanning electron microscope (SEM), a transmission electron microscope (TEM), ultraviolet-visible spectroscopy (UV-Vis), and Brunauer-Emmett-Teller (BET) specific surface area and X-ray photoelectron spectroscopy (XPS). The results of these analyses indicate that nitrogen is successfully doped in the rCBO-Pyrrole composite. BET results show that both rCBO and rCBO-Pyrrole have large specific surface areas, which increase significantly after pyrrole doping of carbon black. Further, the results of catalytic performances show that the rCBO-Pyrrole composite induces excellent catalytic activity toward ORR and exhibits the best performance after heat treatment at 700 ℃. In the electrolyte of KOH (1 mol·L-1), the onset potential of rCBO-Pyrrole is 0. 9 V vs RHE, and the limit diffusion current density of this catalyst is 2. 6 m A·cm-2. Moreover, the electron transfer number of ORR on rCBO-Pyrrole is higher than 3. 5, which indicates a preference for the four-electron reduction pathway. These characteristics and results demonstrate that this kind of material has broad potential applicability.

NiCo-doped C-N nano-composites for cathodic catalysts of Zn-air batteries in neutral media

One of the major challenges towards the development of the conventional alkaline Zn-air battery is its spontaneous discharge caused by the unstability of the anode Zn in concentrated alkaline electrolyte. Utilization of a neutral electrolyte as the alternative to alkaline electrolyte is likely to be a promising strategy to overcome this issue because of much high stability of anode Zn in neutral solution. To realize the normal operation of the neutral Zn-air battery however, a crucial issue is how to ensure that the cathodic catalyst can efficiently electro-catalyze oxygen reduction reaction (ORR) in neutral solution. Herein, we report a facile preparation of Ni/Co-doped C-N nano-composite catalysts (NiCo/C-N) from direct pyrolysis of Ni/Co salt, polyaniline (PANI), carbon nanotube (CNT) and dicyandiamide. Among the as-synthesized catalysts, Ni1Co3/CN-3 presents high and stable ORR current density of 5.27 mA cm−2 in 0.5 mol L−1 KNO3 solution at 2000 rpm. Electron transfer number of ORR on Ni1Co3/CN-3 catalyst is 3.78, showing a nearly completed reduction of O2 to H2O. A neutral Zn-air battery with the prepared NiCo/C-N composite coated on carbon paper as the air electrode catalyst and metal Zn as the anode was constructed in 0.5 mol L−1 KNO3 solution. For the catalyst Ni1Co3/CN-3, the open circuit voltage of the neutral Zn-air battery is 1.16 V, and the maximum power density is 38.5 mW cm−2. The galvanostatic discharge time is 345.7, 160.6, 18.1 and 1.3 h as the discharge current density keeps 10, 50, 100 and 150 mA cm−2 respectively. A stable voltage plateau arises at various discharge current densities. The neutral Zn-air battery can repeatedly discharge after the zinc anode has been replaced, indicating that the synthesized ORR catalyst is an excellent cathode material for the neutral Zn-air battery. This kind of neutral Zn-air battery has a wide application prospect as mobile power supply.

Characterization of Fe/N-doped graphene as air-cathode catalyst in microbial fuel cells

This work proposed a simple and efficient approach for synthesis of durable and efficient non-precious metal oxygen reduction reaction (ORR) electro-catalysts in MFCs. The rod-like carbon nanotubes (CNTs) were formed on the Fe–N/SLG sheets after a carbonization process. The maximum power density of 1210 ± 23 mW•m−2 obtained with Fe–N/SLG catalyst in an MFC was 10.7% higher than that of Pt/C catalyst (1080 ± 20 mW•m−2) under the same condition. The results of RDE test show that the ORR electron transfer number of Fe–N/SLG was 3.91 ± 0.02, which suggested that ORR catalysis proceeds through a four-electron pathway. The whole time of the synthesis of electro-catalysts is about 10 h, making the research take a solid step in the MFC expansion due to its low-cost, high efficiency and favorable electrochemical performance. Besides, we compared the electrochemical properties of catalysts using SLG, high conductivity graphene (HCG, a kind of multilayer graphene) and high activity graphene (HAG, a kind of GO) under the same conditions, providing a solution for optimal selection of cathode catalyst in MFCs. The morphology, crystalline structure, elemental composition and ORR activity of these three kinds of Fe–N/C catalysts were characterized. Their ORR activities were compared with commercial Pt/C catalyst. It demonstrates that this kind of Fe–N/SLG can be a type of promising highly efficient catalyst and could enhance ORR performance of MFCs.

Electrochemical study of methanol tolerant oxygen reduction reaction PdxCoy/C-catalysts from organometallic molecular precursors

In this work, we study the mechanism and kinetics of the oxygen reduction reaction (ORR) over Pd/C, Pd2Co/C and PdCo2/C electrocatalysts synthesized by thermal reduction of organometallic precursors, such as [Pd3(μ-3-Ph-pz)6], [Et4N]2[Pd2Co(μ-4-I-3,5-Me2-pz)4Cl4], and [Et4N]2[PdCo2(μ-4-I-3,5-Me2-pz)4Cl4]. The methanol tolerance and formation of peroxide under oxygen reduction reaction conditions were determined using a rotating disk electrode (RDE) and a rotating ring disk electrode (RRDE). The Pd2Co/C catalysts prepared from [Et4N]2[PdCo2(4-I-3,5-Me2-pz)4Cl4] showed the best ORR activity and higher methanol tolerance, compared with commercial platinum catalysts. The ORR electron transfer number for Pd2Co/C was 3.7, similar to the Pd/C commercial catalysts. Compared to the commercial catalysts, Pd2Co/C and PdCo2/C showed the highest hydrogen peroxide formation (%H2O2), 16 and 24%, respectively. Electrogeneration of hydrogen peroxide has potential application in advanced oxidation processes.

A Composite of Pyrrole-Doped Carbon Black Modified with Co3O4for Efficient Electrochemical Oxygen Reduction Reaction

Low-cost catalysts with high performance toward the oxygen reduction reaction (ORR) in energy conversion systems are intensively desired. Here, we report the preparation of a composite catalyst that is composed of pyrrole-doped carbon black (py-CB) and Co₃O₄ nanoparticles (NPs). The synergetic effect of Co₃O₄ and pyrrole on CB particles endows the obtained py-CB/Co₃O₄ hybrid with pronounced catalytic activity toward ORR. The onset potential of the hybrid is 0.88 V vs. RHE and the half-wave potential is only 60 mV negative from that of commercial Pt/C catalyst. The diffusion-limit current density of this catalyst reaches 5.47 mA·cm¯², which is very close to that of Pt/C. The electron transfer number of ORR on the py-CB/Co₃O₄ is 3.9, indicating a desirable four-electron reduction pathway. Moreover, the hybrid catalyst exhibits a much higher durability toward ORR than the commercial Pt/C catalyst. The py-CB/Co₃O₄ composite catalyst is highly competitive compared to other carbon-based catalysts due to its high activity, excellent stability, and especially the use of inexpensive CB and the ease of preparation. This work may open a new avenue for the design and preparation of ORR catalysts based on cheap and commercially available CB.

Oxygen Reduction Reaction of Tetraaza[14]Annulene Iron Complexes

Introduction Alkaline fuel cells (AFC) have attracted much attention because non-Pt-based catalysts can be potentially applied. Metallocomplex-based catalysts are the one of the most prospective candidates of non-Pt-based oxygen reduction reaction (ORR) catalysts. These catalysts have several advantages over Pt catalysts. First, since active sites consist of isolated metal atoms in these catalysts, metal resource can be used economically. Second, the catalytic activity can be improved by modifying structure of the ligand. Among these metallocomplexes, iron complexes-based catalysts are intensively studied because of low cost and relatively high activity. We have been particularly interested in tetraazaannulene (TAA) iron complexes. The TAA complex is one of the N4-type macrocyclic complexes, as shown in Fig. 1. The main advantage of TAA ligands is that the molecular size is smaller than those of other conventional N4macrocyclic ligands such as porphyrins and phthalocyanines. The density of small molecules on an electrode can be increased. Furthermore, the substituents would have strong electronic effects on the active sites due to the small molecular size. Especially, the introduction of a substituent into α- or β- carbon located at diketiminate motif (Fig. 1) is believed to give large electronic effects on the active center. This indicates that the ORR activity of TAA complexes might be enhanced by the introduction of suitable substituents. Experimental studies on ORR activities of iron complexes of TAA have not been conducted to date, while computational studies reported that iron complexes of TAA and its derivatives have good ORR activity1. In this work, we investigate the redox property and ORR of the iron complexes ligated by TAA (Fig. 1 (a)) and TAA-NO2 (an electron withdrawing derivative, Fig. 1 (b)) in alkaline conditions, and discuss the effect of the ligand structure on the electrochemical behavior. Experiments The ligands of TAA2 and TAA-NO2 3 were synthesized according to the reported procedures. The iron complexes abbreviated as Fe-TAA4 and Fe-TAA-NO2 were synthesized by refluxing the ligand with FeCl2･4H2O in an organic solvent in an inert atmosphere. The iron complex-modified carbon materials were prepared as follows. Ketjenblack (KB) was added to the solution of the iron complexes. The mixture was evaporated to dryness. The hydrodynamic voltammetry using a rotating ring(Pt)-disk(GC) electrode was performed in 0.1 M NaOH under 25 °C. A Pt coil and a reversible hydrogen electrode (RHE) were used as a counter electrode and a reference electrode, respectively. The iron complex-modified KB (Fe-TAA/KB or Fe-TAA-NO2/KB) was immobilized on a glassy carbon (GC) electrode. Results and discussion The ORR activities of the Fe-TAA/KB and Fe-TAA-NO2 were evaluated in the alkaline aqueous solution. Strong activities were observed; the onset potential, at which the value of the catalytic current is 0.2 mA cm- 2, reached 0.94 V (Fe-TAA) and 0.97 V (Fe-TAA-NO2) vs. RHE. The onset potential was increased by the introduction of the -NO2 substituents. In the both of the cases, few H2O2was detected at ring electrode. This indicates that the numbers of electron transfer number of ORR by these catalysts are close to 4. The correlation between the onset potentials and redox potentials will also be discussed. Acknowledgement The authors are grateful to Tokuyama Corporation for providing the anion exchange resin (AS-4). References 1) A.L. Pereira Silva, L.F. de Almeida, A.L. Brandes Marques, H.R. Costa, A.A. Tanaka, A.B. Ferreira da Silva, and J.d.J. Gomes Varela Junior, Journal of Molecular Modeling, 20, 2131-2140 (2014). 2) Y.G. Yatluk, and A.L. Suvorov, Chemistry of Heterocyclic Compounds, 23, 316-320 (1987). 3) N. Nishiwaki, T. Ogihara, T. Takami, M. Tamura, and M. Ariga, J. Org. Chem., 69, 8382-8386 (2004) 4) Sustmann, R., Korth, H. G., Kobus, D., Baute, J., Seiffert, K. H., Verheggen, E., Bill, E., Kirsch, M., de Groot, H. Inorg. Chem., 46, 11416-11430 (2007) Figure 1

Synergistic effect between strongly coupled CoAl layered double hydroxides and graphene for the electrocatalytic reduction of oxygen

Precious metal-free electrocatalysts with high efficiency and durability for the oxygen reduction reaction (ORR) are strongly desired in the field of energy technology. Herein, the CoAl layered double hydroxides (CoAl-LDHs)/reduced graphene oxide (rGO) composites were successfully prepared by growing CoAl-LDHs on the surface of GO in situ via coprecipitation and subsequently hydrothermal treatment. The structure, composition, morphology and ORR catalytic activity of the CoAl-LDHs/rGO composites were investigated as a function of mass ratios of CoAl-LDHs and GO. The results show that there is an optimum mass ratio of CoAl-LDHs and GO (wCoAl-LDHs:wGO=1:5) for the ORR catalytic activity, where the electron transfer number for ORR at the CoAl-LDHs/rGO composites reaches to 3.5, closing to the full four-electron process. The synergistic effect between CoAl-LDHs and rGO is discussed in detail and the discussion is instructive for the construction of the better transition metal oxides/carbon composite-based ORR catalysts.

Reduced graphene oxide supported TiO2 as high performance catalysts for oxygen reduction reaction

To overcome the shortcomings of high cost and rare sources of precious metal based catalysts, the catalytic characteristics of reduced graphene oxide (rGO) supported (N, F)-co-doped TiO2 hybrid (TiO2/rGO) for oxygen reduction are investigated. Results show that the onset potential of oxygen reduction reaction (ORR) under the catalysis of TiO2/rGO is about −0.20 V (vs. Hg/Hg2Cl2). The corresponding electron transfer number of ORR is 3.98, which means that the ORR is major happened through a 4-electron process. TiO2/rGO catalyst has high catalytic for ORR and better stability than the commercial Pt/C catalytic performance in alkaline electrolyte. TiO2/rGO catalyst shows great promise as a high activity non-precious metal catalyst for ORR.

High performance and bifunctional cobalt-embedded nitrogen doped carbon/nanodiamond electrocatalysts for oxygen reduction and oxygen evolution reactions in alkaline media

A bifunctional noble metal-free catalyst with a cobalt-embedded nitrogen doped graphitized carbon shell covering a nanodiamond (ND) core (Co–N–C/ND) is synthesized. The resulting Co–N–C/ND exhibits excellent catalytic activities for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline media. The average electron transfer number of ORR on the Co–N–C/ND is 3.82 between −0.4 and −0.7 V (vs. Hg/HgO), indicating a near four-electron transfer mechanism for ORR. Moreover, the catalytic activity of the Co–N–C/ND for ORR is comparable to the 20 wt% Pt reference catalyst supported on carbon black. The OER onset potential on the Co–N–C/ND is 0.43 V (vs. Hg/HgO) and the current density at 0.7 V is 3.19 mA cm−2, demonstrating excellent catalytic activity for OER. In comparison to the Co–N–C derived from carbon black, the Co–N–C/ND exhibits better durability. The superior electrocatalytic performance of the Co–N–C/ND could be attributed to the synergistic effect of the Co–N moieties in the carbon shell and the high stability could be ascribed to the ND core.

Fabrication of the pyrolyzing carbon-supported cobalt–dicyandiamide electrocatalysts and study on the active sites and mechanism for oxygen reduction in alkaline electrolyte

In this work, carbon-supported cobalt–dicyandiamide (Co–DCD) was synthesized by a simple chemical method followed heat-treated at 600,700, 800, 900, and 1000 °C to acquire electrocatalysts with excellent activity for the oxygen reduction reaction (ORR). The resulting catalysts Co–N/C–T pyrolyzed at 600–1000 °C all showed substantial activities to ORR, and the catalyst heat-treated at 800 °C exhibited the best ORR activity. The catalytic performance of Co y –N x /C 0.25–T catalysts synthesized with different amounts of DCD as nitrogen source and Co(OAc)2⋅4H2O as metal precursor were measured by cyclic voltammetry in alkaline electrolyte. The onset potential for oxygen reduction on the optimum catalyst was approximately 0.938 V (vs. RHE) in 0.1 M NaOH solution, higher by 83 mV than that on the commercial catalyst of 20 % JM Pt/C. Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were used to characterize the catalyst in terms of the structure and composition in order to sketch the contours of the catalytic active sites of the catalysts. The characterization studies indicate that pyridinic N–C was the most important of the catalytic active sites and responsible for the ORR catalytic activity of Co–N/C–T in alkaline electrolyte. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) was also used to obtain the overall ORR electron transfer number and electron transfer coefficiency. The overall electron transfer number for ORR catalyzed by the optimum catalyst Co0.2–N2.35/C0.25–800 was determined to be 3.47 by CV and 3.89–3.96 by LSV,respectively, suggesting that the ORR was a mixture of two- and four-electron transfer pathways, but dominated by a four-electron transfer process. Based on these measurements and other references, an ORR mechanism was proposed to facilitate further investigation. The results also show that this novel catalyst with ORR excellent activity would have outstanding methanol tolerance and potential application as a kind of nonprecious metal cathodic ORR catalyst for direct methanol fuel cell.

Preparation and Electrocatalytic Properties of Perovskite Type Oxides CaVO<sub>3</sub> for Oxygen Reduction Reaction

The precursor powder of CaVO_3 was prepared by sol-gel method using vanadium pentoxide,calcium nitrate,citric acid,and oxalic acid as raw materials.The target product CaVO_3 was obtained by calcining the precursor powder in argon atmosphere at 1000 °C.In order to determine the appropriate calcination temperature,thermal stability of the precursor powder was tested.The target product CaVO_3 was characterized by Fourier transform infrared(FTIR) spectrometry,thermogravimetry(TG),X-ray diffraction(XRD),and conductivity.In addition,the properties for oxygen reduction reaction(ORR) were also investigated,and the results showed that the electron transfer number of ORR on the modified electrode was1.5-1.7,which indicating that ORR was a two-electron process.

Homogeneous deposition-assisted synthesis of iron–nitrogen composites on graphene as highly efficient non-precious metal electrocatalysts for microbial fuel cell power generation

This work proposed a novel strategy for synthesizing highly efficient non-precious metal oxygen reduction reaction (ORR) electrocatalysts. Fe complexes were homogeneously deposited (HD) on graphene oxide through in situ hydrolysis of urea, followed by two-step pyrolysis under Ar and NH3 atmospheres, resulting in formation of Fe- and N-functionalized graphene (HD-FeN/G). The morphology, crystalline structure and elemental composition of HD-FeN/G were characterized. ORR activity was evaluated by using a rotary disk electrode (RDE) electrochemical system. HD improved the loading and distribution of the Fe–Nx composites on graphene. The ORR activity of the as-prepared HD-FeN/G in neutral medium was comparable to that of the state-of-the-art commercial Pt/C and significantly superior to a FeN/G counterpart produced via traditional approach. The ORR electron transfer number of HD-FeN/G was as high as 3.83±0.08, which suggested that ORR catalysis proceeds through a four-electron pathway. HD-FeN/G was used as a cathodic electrocatalyst in microbial fuel cells (MFCs), and the resultant HD-FeN/G-MFC showed comparable voltage output and maximum power density to those of Pt/C-MFC. The HD-FeN/G-MFC achieved a maximum power density of 885mWm−2, which was much higher than that of FeN/G-MFC (708mWm−2). These findings demonstrate that HD-FeN/G produced through the novel synthesis strategy proposed in this work would be a good candidate as cathodic electrocatalyst in MFCs.

Facile preparation of modified carbon black-LaMnO3 hybrids and the effect of covalent coupling on the catalytic activity for oxygen reduction reaction

Covalent coupling between LaMnO3 nanoparticles and carbon black to produce a composite catalyst for oxygen reduction reaction (ORR) was achieved by physical mixing of modified carbon and perovskite-type LaMnO3 nanoparticles, followed by sintering at different temperatures. Perovskite- type LaMnO3 nanoparticles were first synthesized via chemical precipitation, and the carbon support (Vulcan XC-72) was modified using graphitization, followed by HNO3 and ammonia treatments. The morphology and electronic states of the carbon black-LaMnO3 hybrid catalyst were characterized by scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The loaded LaMnO3 particles featured rod-like, three bars-like, and bamboo rod-like structures and were homogeneously dispersed in the carbon matrix that featured a hollow spherical structure. At a sintering temperature of about 300 °C, C–O–M (M = La, Mn) bonds formed at the interface between the carbon and LaMnO3 nanoparticles. Electrochemical measurements in 1 mol/L NaOH showed that the carbon-LaMnO3 hybrid prepared at a LaMnO3/GCB mass ratio of 2:3 displayed the highest electrocatalytic activity towards ORR among all the synthesized hybrid catalysts. The electrocatalytic activity was comparable with that obtained by commercial Pt/C catalyst (E-TEK). The average electron transfer number of ORR was ∼3.81, and the corresponding yield of the hydrogen peroxide intermediate was ∼9.5%. The remarkably improved electrocatalytic activity towards ORR was likely because of the formation of covalent bonds (C–O–M) between the LaMnO3 nanoparticles and carbon that can effectively enhance the ORR kinetics. This information is important to understand the physical origin of the electrocatalytic activity of carbon-supported rare earth oxides as catalysts for ORR.

Preparation of Mn-N-C catalyst and its electrocatalytic activity for the oxygen reduction reaction in alkaline medium

Chitosan salicylaldehyde Schiff-base manganese complex (Mn-CS-sal) was synthesized by a simple chemical method and then supported on graphite carbon; the graphite-supported Mn-CS-sal (Mn-CS-sal/C) was heat-treated at different temperatures (t) to obtain the Mn-N-C catalysts (Mn-N-C-t, t = 200, 400, 600, 800, 1000°C). The electrocatalytic activity of Mn-N-C catalysts in the oxygen reduction reaction (ORR) was investigated and their structure and composition were characterized by Fourier Transform Infrared (FT-IR) spectrum, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results indicated that all the Mn-N-C catalysts are highly active in ORR, whereas the Mn-N-C-600 catalyst heat-treated at 600°C exhibits the highest ORR activity. The excellent ORR activity of the Mn-N-C catalysts in alkaline media should be attributed to their Mn-N-C configuration. Two important kinetic parameters, i.e. the overall ORR electron transfer number (n) and electron transfer coefficiency (αnα), were determined by the cyclic voltammetry method. The Mn-N-C-600 catalyst shows an overall electron transfer number of 3.63 for ORR, suggesting that the catalytic ORR is via a mixture of 2- and 4-electron transfer pathways, but dominated by the 4-electron transfer process. Based on these observations, a possible mechanism for ORR over the surface of Mn-N-C modified electrode is proposed.

Kinetics of oxygen reduction reaction on three different Pt surfaces of Pt/C catalyst analyzed by rotating ring-disk electrode in acidic solution

The oxygen reduction reaction (ORR) on a Pt/C-based thin catalyst layer is studied using the rotating ring-disk electrode (RRDE) technique in an O2-saturated acid solution. A modified model for obtaining the ORR constants in different electrode potential ranges is validated using the experimental disk and ring currents. The results calculated from the model and the Koutecky–Levich plots reveal that the ORR constants on three different Pt surfaces (Pt–H, Pt, and PtOX/Pt) are significantly different, among which, pure Pt surface is found to have the best activity towards ORR. This model can also give a detailed formularization of the apparent ORR electron transfer number (n) and the percentage of H2O2 (%H2O2) produced during the ORR process.