Potential For Oxygen Reduction Reaction Research Articles

Carbon-supported cobalt (III) complex for direct reduction of oxygen in alkaline medium

Transition metal-nitrogen-carbon catalysts obtained from pyrolysis of simple sources of metal and nitrogen or metal-organic frameworks are envisioned as promising replacement of Pt-based catalysts for oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. However, the lack of clarity on the active site structure is a fundamental problem in developing efficient catalysts for the complete reduction of oxygen with maximum active site density. In this study, we have synthesized a simple metal-organic complex, [Co(bpy)2CO3]NO3 ⋅5H2O, 1 (bpy = 2, 2′-bipyridine) and studied its ORR activity in 0.1 M KOH supporting on Ketjenblack EC-600JD, 1/C. The crystal structure of 1 was unambiguously determined using single-crystal X-ray diffraction. The onset potential of ORR, Tafel slopes, kinetic current density, the fraction of oxygens reduced to peroxyl ion (χHO2−) and the electron transfer number (n) were compared with relevant literature and commercial 20 wt % Pt/C. Primarily, 1/C reduces O2 through the direct 4-electron pathway. ‘n’ and χHO2−on 1/C were found to be 3.87 and 7% at 0.7 ViR-freevs. RHE, respectively. The turn-over frequencies for the 4- and 2-electron process were found to be 0.124 and 0.001 electrons [Co]−1 s−1 at 0.7 ViR-freevs. RHE, respectively. Density-functional theory analysis reveals that the preferential activation of the side-on mode of adsorption on 1 is responsible for high selectivity in the direct 4-electron pathway.

Towards Synergy of rGO and Ni doped CeO2 in their copmposite as Efficient Catalyst for Oxygen Reduction Reaction

AbstractThe persistently increasing energy consumption and the deficiency of conventional fuels have raised serious concern in the recent years. The advanced energy conversion technologies like fuel cells have been benefitted by the development of suitable catalytic materials that can effectively carry out appropriate electrochemical processes like oxygen reduction reaction (ORR). Direct Methanol Fuel cells (DMFCs) act as an energy conversion device and the efficiency can be enhanced with active ORR catalysts. Here, we have reported the ORR activity of Ni doped CeO2/rGO synthesized by microwave mediated hydrothermal method. We have observed that the synergistic interaction between Ni doped CeO2 and rGO is responsible for the enhanced ORR activity. The optimized 5% Ni‐CeO2/rGO was evaluated as the most efficient ORR catalyst among all other composites with onset ORR potential −0.17 V vs. Ag/AgCl and limiting current density 3.67 mA/cm2. The 5% Ni‐CeO2/rGO catalyst showed better stability and methanol tolerant when compared with commercial Pt/C.

An Fe-N/S-C hybrid electrocatalyst derived from bimetal-organic framework for efficiently electrocatalyzing oxygen reduction reaction in acidic media

Heteroatoms doped Fe-N-C electrocatalysts have been widely acknowledged as one of the most promising candidates to replace Pt-based materials for electrocatalyzing oxygen reduction reaction (ORR). However, the complicated synthesis method and controversial catalytic mechanism represent a substantial impediment as of today. Herein, a very facile strategy to prepare Fe-N/S-C hybrid through pyrolyzing Zn and Fe bimetallic MOFs is rationally designed. The electrocatalytic ORR performance shows a volcano-type curve with the increment of added Fe content. The half-wave potential (E1/2) for ORR at optimized Fe-N/S-C-10% (10% = n (Fe)/(n (Fe) + n (Zn)), n (Fe) and n (Zn) represent the moles of Fe2+ and Zn2+ in the precursors, respectively) shifts significantly to the positive direction of 19.6 mV with respect to that of Pt/C in acidic media, as well as a high 4e selectivity and methanol tolerance. After 10,000 potential cycles, E1/2 exhibits a small negative shift of ~27.5 mV at Fe-N/S-C-10% compared favorably with Pt/C (~141.0 mV). This can be attributed to: (i) large specific surface area (849 m2/g) and hierarchically porous structure are favorable for the rapid mass transfer and active sites exposure; (ii) the embedded Fe-containing nanoparticles in porous carbon are difficult to be moved and further agglomerated during the electrochemical accelerated aging test, further improving its stability; (iii) there exist small Fe-containing nanoparticles, uniformly doped N and S, abundant Fe-N as efficiently active sites. This work represents a breakthrough in the development of high-efficient non-precious-metal catalysts (NPMCs) to address the current Pt-based electrocatalysts challenges.

(Invited) Thermoplasmonic Generation of Carbon Nanohydrids for the Oxygen Reduction Reaction

Local heating from plasmonic nanostructures has shown great potential in photothermal therapy, optical trapping, heterogeneous catalysis, and synthesis of nanostructures. Titanium nitride (TiN) is one the most promising plasmonic materials for thermal applications (thermoplasmonics) due to its high thermostability (i.e. refractory), CMOS compatibility, and resistance in chemically harsh environments. On the other hand, hydrogen peroxide (H2O2) has a large annual production approaching 4 ×106 tons worldwide and the selective reduction of O2 via electrochemical oxygen reduction reaction (ORR) using renewable electricity is a promising way for H2O2 production. Carbon-based materials have shown great potential for the two-electron ORR to hydrogen peroxide. In this contribution, I will show a general approach to use the highly intense local temperature generated from plasmonic TiN nanocrystals for generating TiN/carbon nanohybrids that show enhanced efficiency and selectivity toward the ORR. The general applicability of plasmon-driven synthesis of nanocarbons to electrocatalysis will be discussed.

(Invited) Boosting Non-Precious Metal Catalysts Toward the Oxygen Reduction Reaction through Ionic Liquid Modification

Finding cost effective catalysts for the oxygen reduction reaction (ORR) is one of the most overriding challenges in the field of electrochemistry [1]. Carbon-based materials emerge as one of the promising candidates due to their remarkable advantages of low cost, abundant structural variety, tailorable surface chemistry, and good conductivity. Tremendous efforts have been made to improve the performance of carbon-based materials for the ORR, mainly through modifying the inherent structures of carbon by doping heteroatoms (e.g., B, N, P) or combining with metal/metal oxides. We have demonstrated recently that modifying Pt-based catalysts by a minor amount of ionic liquid (IL, e.g., [BMIM][NTf2], [MTBD][NTf2]) could significantly boost their ORR activity [2-5]. This innovative strategy has also been successfully transferred to some non-precious metal Fe-N-C catalysts, on which an ionic liquid ([BMMIM][NTf2]) modification is found to boost both their activity and stability [6]. In the current contribution, we will present our latest progress toward boosting the emerging zeolitic imidazolate framework (ZIF)-derived carbons (ZDC) for the ORR using IL modification strategy. First we verified again the boosting effect of IL toward the ORR on the as-synthesized ZDC materials. The half wave potential of ORR on these ZDCs is positively shifted by up to 18 mV and their intrinsic kinetic current (@ 0.85 V) is increased by a factor of 2 after being modified with [BMMIM][NTf2]. At the same time, it is observed that the boosting effect from IL modification is highly sensitive to the exact loading amount of the IL. Exceeding a specific loading amount, however, leads to a mass transport related activity drop. Moreover, it is also unraveled that the IL takes effect by increasing electrochemically accessible surface area, as reflected by the significantly enhanced double layer capacitance of ZDCs after IL modification. These results again demonstrate the great promise of the IL modification strategy as a generic method to improve ORR catalysts, and at the same time are anticipated to form the basis for an unprecedented perspective in developing high-performing non-precious metal catalysts for low temperature fuel cell applications.

The Activity of Oxygen Reduction Reaction on the Noble Metal Doped Titanium Oxide in Acidic Electrolyte

Titanium oxide with the oxygen vacancies is expected as one of the candidates for the non-platinum catalyst of polymer electrolyte fuel cells. The reasons why the activity of titanium oxide improves by the presence of oxygen vacancies are assumed the formation of the active site and the micro electron-conducting path [1, 2]. From the viewpoint of enhancement of the electron conductivity, it is well known that doping a transition metal to titanium oxide is also effectiveness method. Thus, many papers had reported about the niobium doped titanium oxide to increase the electron conductivity. However, not only the doping transition metals but also the oxygen vacancies are necessary to obtain high activity of the titanium oxide. We put forward a possibility that the crystal distortion acts as the active site [3]. Now, we think that the majority role of transition metal doping is the formation of the micro electro-conducting path to the active site. Hence, we were attending to the electron conductivity of transition metal-doped titanium oxide. From the results of a theoretical calculation, the electron conductivity of niobium doped titanium oxide, which is researched well as the non-platinum catalyst, is relatively low [4]. And, ruthenium and iridium as doping species give high electron conductivity to titanium oxide. Ruthenium and iridium are famous for interacting with titanium oxide in the electrolysis field. In this study, the activity of oxygen reduction reaction (ORR) of ruthenium or iridium doped titanium oxide was evaluated. Ruthenium doped titanium oxide (Ru-TiO2) and iridium doped titanium oxide (Ir-TiO2) were synthesized by the sol-gel method, and both doping titanium oxides were supported on the Ketjen Black 300 J. The loading of both doping titanium oxides was less than 20 wt%. In addition, the molar ratio of titanium and ruthenium or iridium was 95:5. Electrochemical studies were conducted in Ar- or O2-saturated 0.1 M HClO4 at 60ºC with carbon plate counter-electrode and Ag/AgCl reference. The ORR activity was evaluated by the difference current between the current in Ar-saturated condition from the current in O2-saturated condition. The crystal structure of Ru-TiO2 was almost the same as the anatase structure of TiO2. The intensity of diffraction peaks of Ir-TiO2 was weaker than that of Ru-TiO2, although its crystal structure was the anatase. Both samples showed no diffraction peaks attributed to the noble metal oxide and metallic phase. The atomic valence of ruthenium in Ru-TiO2 was mainly tetravalent from the results of XPS analysis. But a full width at half maximum of Ru 2p was wide. Thus, the ruthenium atom may have many oxidation states. Titanium atom at the surface showed tetravalent, and it inside of a particle indicated the reduction state. Now, we thought that the ruthenium atom was atomically mixing with titanium oxide structure because no evidence observed ruthenium oxide and ruthenium metal by our analysis method.ORR activity of titanium oxide improved by the addition of 5 mol% ruthenium. ORR activity of titanium oxide is well known to increase by introducing the oxygen vacancies. ORR activity of Ru-TiO2 was higher than that of titanium oxide with the oxygen vacancies. Additionally, Ru-TiO2 showed also high ORR activity after the reduction process using hydrogen gas as well as titanium oxide. Ir-TiO2 showed the highest ORR current in this study, and its onset potential of ORR was also a high value at about 0.87 V.From the above results, the addition of a small quantity of noble metal was very effective for the enhancement of ORR activity of titanium oxide.[1] A. Ishihara, et al., J. Phys. Chem. C, 117, 18837 (2013).[2] A. Ishihara, et al., J. Phys. Chem. C, 123, 18150 (2019).[3] T. Saida, et al., MRS Advances, 4, 1851 (2019).[4] M.-C. Tsai, et al., Mol. Syst. Des. Eng., 2, 449 (2017).

A pair of metal organic framework (MOF)-derived oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts for zinc-air batteries

Metal organic frameworks (MOFs) are amazing precursors for the development of functional catalysts due to their ability of being modified both structurally and in terms of composition. This work presents a pair of MOF - derived oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts for the advancement of zinc-air batteries. Zeolitic imidazolate frameworks (ZIF) are known as ideal materials to create doped carbons. Combining with a sacrificial tellurium template, we have developed a “pseudo-carbon nanotube” with a ZIF-8 that is doped with a mixture of zinc, cobalt, and iron. The resulting mixed-metal doped porous carbon nanotubes (CoFeZn@pCNT) show excellent catalytic activity for ORR with a half-way ORR potential (E1/2) of 0.87 V vs. RHE (reverse hydrogen electrode). Iridium-based catalysts are known as the best option for OER so far, but it is not viable for practical applications due to cost restrictions. Previous research into alternative catalysts has focused on metal oxides with first-row transition metals. Metal sulfides provide superior conductivity compared to metal oxides and are a new frontier in the search for affordable, active, and stable OER catalysts. By combining cobalt, iron, and nickel into a MOF structure (ZIF-67), followed by a sulfurization process, a MOF-derived mixed-metal sulfide catalyst (FeCoNi–S@ZIF) has been developed in this study. The resulting catalysts show an onset OER potential of 1.65 V vs. RHE at 10 mA cm−2 and exhibit remarkable stability for the operation of zinc-air batteries.

Synthesis and characterization of reduced graphene oxide-iron oxide nanocomposite as a potential fuel cell electrocatalyst

Driven by the high demand for commercialization of fuel cell (FC) technology, a design of potential oxygen reduction reaction electrocatalayst based on reduced graphene oxide (rGO) and iron oxide (Fe3O4) nanocomposite has been described and denoted as rGO/Fe3O4. The nanocomposite was synthesized by means of facile one-pot process. The resultant rGO/Fe3O4 was physically and electrochemically characterized by using Fourier Transform Infrared Spectroscopy (FT-IR), X-Ray Diffractrogram (XRD), Scanning Electron Microscopy (SEM), Cyclic Voltammetry, (CV) and Electrochemical Impedance Spectroscopy (EIS). The FTIR analysis shows the formation of rGO/Fe3O4 from the presence of C=C, C-C and Fe-O bonds in the spectrum of the indicating the synthesis material is successfully obtained. XRD analysis also confirms the presence of rGO and Fe3O4 in the composite by hematite structure indexed peak of diffractogram. Scanning Electron Microscopy (SEM) image depicts the attachments Fe3O4 onto the surfaces of rGO. The composite was then dissolved in the solvent and drop-casted on the glassy carbon electrode (GCE) for electrochemical analysis. Cyclic Voltammetry (CV) shows increment in current responses of nearly two and half folds for rGO/Fe3O4/GCE compared to bare GCE. Electrochemical Impedance Spectroscopy (EIS) shows a stable electron transfers process with lower charge transfer resistance (Rct) of the nanocomposite modified electrode which due to the synergistic effect between rGO and Fe3O4. The results of the analysis show the compound could be a promising candidate as an electrocatalyst for fuel cell.

FeNx/FeSx-Anchored Carbon Sheet–Carbon Nanotube Composite Electrocatalysts for Oxygen Reduction

Even though various Pt-free electrocatalysts for oxygen reduction reaction (ORR) have been introduced, many of them are found to be active only in alkaline conditions. Considering Nafion, phosphoric acid-doped polybenzimidazole (PBI), and so on as the prominent ionomer membranes, used in the commercially available polymer electrolyte membrane fuel cells (PEMFCs), it becomes important that any development on the Pt-free catalysts should ensure the better ORR performance under acidic conditions. The present work effectively tackles this issue, where an ORR-based catalyst could be prepared with simultaneous incorporation of both Fe–N and Fe–S active sites on in situ generated carbon sheets which are spatially separated by the carbon nanotube (CNT) network. This catalyst shows ability to perform under both acidic and basic conditions. This has been achieved by growing a polyethylenedioxythiophene polymer network in the presence of CNT and melamine followed by its pyrolysis under an inert atmosphere. The catalyst formed at 900 °C (PMCNT-900) displays 0.94 V onset potential for ORR under acidic electrolyte conditions, which corresponds to 60 mV overpotential compared to its 40 wt % Pt/C counterpart. Interestingly, in single cell demonstration of Nafion-based PEMFC with PMCNT-900 as the cathode catalyst, the system delivered a maximum power density (PD) of 500 and 275 mW/cm2 at 60 °C under H2–O2 and H2–air feed conditions, respectively. On the other hand, in a single cell test in the anion exchange membrane fuel cell (AEMFC) mode, a maximum power density of 65 mW/cm2 at 50 °C could be achieved with the same cathode catalyst, which is a comparable value obtained while employing Pt/C as the cathode. These results, thus, infer to the efficiency of the catalyst to facilitate ORR under the extreme pH conditions, and particularly its performance under acidic condition reveals its prospect as a potential Pt-free electrocatalyst to serve in the Nafion-based systems.

Novel bi-functional electrocatalysts based on the electrochemical synthesized bimetallicmetal organic frameworks: Towards high energy advanced reversible zinc–air batteries

Advanced zinc‐air batteries (ZABs) are garnered renewed-attention as one of the most sustainable choices to power energy grids and electric vehicles. Yet, slow ORR (oxygen reduction reaction) and OER (oxygen evolution reaction) kinetics demonstrate a limiting factor for energy conversion performance of high-tech ZABs. Hence the development of effective catalysts for ORR and OER at air electrode to remarkably reduce the polarization loss in metallic-air batteries that seriously prevents the rate capability, operating life and energy performance is very important. We illustrate the powerful ZABs with innovative electrochemical-synthesized benzene-1,3,5 tricarboxylate (BTC)metal–organic frameworks (MOFs) bi-functional oxygen electrocatalysts as compared to routine MOFs. Ni/Fe-BTC-MOF shows superior electrocatalytic activity (0.595 V) with half-wave potential (0.964 V) for ORR and over-potential (1.521@10 mA cm−2) for OER, outperforming commercialRuO2and Pt/C. Ni/Fe-BTC-MOF cathode represents a high open circuit potential (1.55 V), discharge peak power density (182 mW cm2) and a large specific capacity (775 mAh g−1 at 10 mA cm−2) compared to the previous reported. Also Ni/Fe-BTC-MOF shows the excellent lifetimes for 594 h (65.80% round trip efficiency @10 mA cm−2) with 5264 cycles. These encouraging results demonstrate the great potential of these novel MOFs for marketable execution of rechargeable Zn‐air batteries.

In-situ formation of N doped hollow graphene Nanospheres/CNTs architecture with encapsulated Fe3C@C nanoparticles as efficient bifunctional oxygen electrocatalysts

The exploration of cost-effective and efficient bifunctional catalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is currently a critical obstacle in the development of regenerative fuel cells and rechargeable metal-air batteries. In this reported study, we detail the synthesis of an N-doped hollow graphene nanospheres/CNTs architecture with encapsulated Fe3C@C nanoparticles (Fe3C@C-NGns-NCNTs) via a facile, in situ fabrication method. The resulting catalyst had a high specific surface area (450 m2 g−1) with a hierarchical meso-macroporous structure. In an alkaline electrolyte, the catalyst manifested remarkable bifunctional catalytic activity with a half-wave potential for ORR of ca. 23 mV that was higher than that produced by a commercial Pt/C and the former had a small overpotential of 412 mV at a current density of 10 mA cm−2 for the OER. The ΔE (oxygen electrode activity parameter) value for the Fe3C@C-NGns-NCNTs was only 0.787 V. We posit that the high electrocatalytic performance of this catalyst was due to the following factors: (1) a synergistic effect, induced by the presence of multiple types of active sites, including Fe3C@C nanoparticle, nitrogen dopant and possible Fe-Nx-C sites; (2) the unique hollow graphene nanospheres/CNTs architecture, which facilitated the adsorption/diffusion of reactants/products and rapid electron transfer; and (3) the high degree of graphitization of the catalyst that improved the electrocatalytic stability.

Ultrafine Cu6Sn5 nanoalloys supported on nitrogen and sulfur-doped carbons as robust electrode materials for oxygen reduction and Li-ion battery

Ultrafine Cu6Sn5 nanoalloys supported on porous N and S-doped carbons (Cu6Sn5@N–S–Cs) were prepared by annealing Cu–Sn-contained covalent organic frameworks (COFs). The Cu6Sn5 alloys with particle size of about 2–10 nm were formed on the surface of N–S–Cs in a high dispersion state. The Cu6Sn5@N–S–Cs were found to efficiently catalyze oxygen reduction reaction (ORR) in both alkaline and acidic media, with half-wave ORR potential (E1/2) of 0.86 V in 0.1 M KOH and 0.67 V in 0.1 M HClO4. The ORR performance of Cu6Sn5@N–S–C could be competitive to the best Fe (or Co)-based non-precious metal electrocatalysts and commercial Pt/C. Cu6Sn5@N–S–C also showed better durability for ORR in both media. Moreover, Cu6Sn5@N–S–C also possessed electrochemical lithium ion storage capacity of 905 mA h g−1 in an initial discharge at the current density of 50 mA g−1, with the outstanding coulomic efficiency. The synergistic effects of ultrafine Cu6Sn5 nanoalloys and porous N–S–Cs contributed to the impressive ORR performance and lithium storage capacities. COFs-derived Cu6Sn5@S–N–C was demonstrated to be a promising bifunctional electrode material for proton exchange membrane fuel cells and lithium ion batteries.

Investigation of Potential of Oxygen Reduction Reaction at Aluminum Doped Carbon Nanocage (Al-C72) as a Catalyst

In this work, potentials of aluminum doped carbon nanocage (Al–C72) as navel catalyst to oxygen reduction reaction in acidic environment are investigated. In present study, the acceptable reaction paths are examined, and optimum reaction mechanism of oxygen reduction reaction on the surface of Al-doped C72 is recognized. Results indicate that oxygen reduction reaction on Al-doped C72 surface can be proceed through Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms. In this work, the calculated initial potential to oxygen reduction reaction on Al-doped C72 surface is 0.30 V, which is smaller than initial potential the on surfaces of platinum-based catalysts (0.45 V). Results indicated that in acidic environment the initial potential for oxygen reduction process can be evaluated as 0.79 V, that correspond to 0.30 V as minimum overpotntial for oxygen reduction process. Results demonstrated that Al-doped C72 is an acceptable catalyst to oxygen reduction process with high performance.

Integrated Ultrafine Co0.85 Se in Carbon Nanofibers: An Efficient and Robust Bifunctional Catalyst for Oxygen Electrocatalysis.

Transition-metal selenides are emerging as alternative bifunctional catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR); however, their activity and stability are still less than desirable. Herein, ultrafine Co0.85 Se nanoparticles encapsulated into carbon nanofibers (CNFs), Co0.85 Se@CNFs, is reported as an integrated bifunctional catalyst for OER and ORR. This catalyst exhibits a low OER potential of 1.58 V vs. reversible hydrogen electrode (RHE) (EJ=10, OER ) to achieve a current density (J) of 10 mA cm-2 and a high ORR potential of 0.84 V vs. RHE (EJ=-1, ORR ) to reach -1 mA cm-2 . Thus, the potential between EJ=10, OER and EJ=-1, ORR is only 0.74 V, indicating considerable bifunctional activity. The excellent bifunctionality can be attributed to high electronic conduction, abundant electrochemically active sites, and the synergistic effect of Co0.85 Se and CNFs. Furthermore, this Co0.85 Se@CNFs catalyst displays good cycling stability for both OER and ORR. This study paves a new way for the rational design of hybrid catalysts composed of transition-metal selenides and carbon materials for efficiently catalyzing OER and ORR.

Mn Nanoparticles Encapsulated within Mesoporous Helical N‐Doped Carbon Nanotubes as Highly Active Air Cathode for Zinc–Air Batteries

AbstractThe practical application of clean energy conversion and storage technologies, such as fuel cells and metal–air batteries, have been significantly impeded by the high cost and scarcity of precious metal catalysts used for the oxygen reduction reaction (ORR). Transitional metal/carbon compounds are a promising alternative to precious metal catalysts for the ORR. Herein, Mn nanoparticles encapsulated within mesoporous helical N‐doped carbon nanotubes (Mn@HNCNTs) are developed as highly active ORR catalysts. In terms of the onset potential and half‐wave potential for ORR, the optimized Mn@HNCNTs exhibit an excellent ORR performance, which is comparable to Pt/C. When the Mn@HNCNTs are assembled as air cathodes, with zinc foil as the anode in primary zinc–air cells, the obtained cell delivers a higher cell voltage and capacity than the zinc–air cell using Pt/C as the air cathode, as well as many reported cells using TM/CNT‐based air cathodes. The electrocatalytic performance in the zinc–air cell demonstrates that Mn@HNCNTs have great potential for use as ORR catalysts in metal–air batteries.

An alternate aqueous phase synthesis of the Pt3Co/C catalyst towards efficient oxygen reduction reaction

AbstractCarbon supported Pt-Co alloys are among the most promising electrocatalysts towards oxygen reduction reaction (ORR) for the application in low temperature fuel cells and beyond, thus their facile and green synthesis is highly demanded. Herein we initially report an alternate aqueous phase one-pot synthesis of such catalysts (containing nominally ca. 20 wt.% Pt) based on dimethylamine borane (DMAB) reduction. The as-obtained electrocatalyst (denoted as Pt3Co/C-DMAB) is compared with the ones obtained by NaBH4 and N2H4·H2O reduction (denoted as Pt3Co/C-NaBH4 and Pt3Co/C-N2H4·H2O, respectively) as well as a commercial Pt/C, in terms of the structure and electrocatalytic property. It turns out that Pt3Co/C-DMAB exhibits the highest ORR performance among all the tested samples in an O2-saturated 0.1 mol/L HClO4, with the mass activity (specific activity) ca. 4 (6) times as large as that for Pt/C. After 10000 cycles of the accelerated degradation test, the half-wave potential for ORR on Pt3Co/C-DMAB decreases only by 4 mV, in contrast to 24 mV for that on Pt/C. Pt3Co/C-NaBH4 or Pt3Co/C-N2H4·H2O shows a specific activity comparable to that for Pt3Co/C-DMAB, but a mass activity similar to that for Pt/C. ICP-AES, TEM, XRD and XPS characterizations indicate that Pt3Co nanoparticles are well-dispersed and alloyed with a mean particle size of ca. 3.4 plusmn; 0.4 nm, contributing to the prominent electrocatalytic performance of Pt3Co/C-DMAB. This simple aqueous synthetic route may provide an alternate opportunity for developing efficient practical electrocatalysts for ORR.

Hierarchical Hollow Co/N-C@NiCo2O4 Microsphere as an Efficient Bi-functional Electrocatalyst for Rechargeable Zn–Air Battery

Zn–air batteries (ZABs) are one of the promising power sources for portable and wearable electronic devices because of their high theoretical specific energy density and low cost. However, their practical performance with high energy density and power density is difficult to maintain under high mass-loading due to the compact restacking that seriously hinders mass/charge diffusion. Herein, we reported an efficient bi-functional electrocatalyst of Ni-Co mixed metal oxides incorporated cobalt/nitrogen doped carbon with hierarchical hollow nanostructure (H-Co/N-C@NiCo2O4). Benefiting from this structural and compositional merits, the as-synthesized H-Co/N-C@NiCo2O4 exhibits excellent electrocatalytic activity and long-term stability for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The potential gap between half-wave potential in ORR and overpotential at a current density of 10 mA cm−2 in OER reaches within 0.8 V, much smaller than that of most reported bi-functional electrocatalysts. Moreover, H-Co/N-C@NiCo2O4 is constructed as an air electrode for rechargeable ZAB, delivering a high power density and long cycling stability. A good linear relationship has been achieved between the power density and various mass-loading of H-Co/N-C@NiCo2O4 on the electrode, indicating well preserved performance even under high dense packing of catalysts, which provides a promising paradigm for the practical applications in ZAB.

TiC supported amorphous MnOx as highly efficient bifunctional electrocatalyst for corrosion resistant oxygen electrode of Zn-air batteries

Zn-air batteries (ZABs) represent promising candidates for the next generation energy conversion and storage systems based on their superior features to those of lithium-ion batteries, including high theoretical energy density, low cost, and high safety. However, their further development and application is severely lagged due to the lack of high efficient and durable bifunctional oxygen electrocatalysts. The widely applied carbon-based catalysts are thermodynamically instable during battery charging. Herein, TiC supported amorphous MnOx (a-MnOx/TiC) is reported for the first time as electrocatalyst for the corrosion resistant oxygen electrodes of ZABs. A-MnOx/TiC delivers a remarkable activity and stability toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with a high half-wave potential (0.8 V) for ORR and a low potential (1.56 V) at 10 mA cm−2 for OER, which far outperforms the state of the art ORR catalyst (Pt/C) and OER catalyst (IrO2), as well as the Pt/C-IrO2 and a-MnOx/C bifunctional catalysts. The excellent bifunctional activity of a-MnOx/TiC can be attributed to the efficient synergistic effect between the active amorphous MnOx catalyst and the highly conductive and stable TiC support. More impressively, a-MnOx/TiC demonstrates an outstanding electrochemical stability in strong alkaline electrolyte under OER condition in contrast to the readily oxidized carbon-based a-MnOx/C catalysts. ZAB with a-MnOx/TiC delivers a greater discharge performance with a peak power density of 217.1 mW cm−2 than that of Pt/C-based ZAB, and a surpassing discharge and charge cycling performance and stability to ZABs with Pt/C-IrO2 and a-MnOx/C. Furthermore, a-MnOx/TiC can be applied for solid-state ZABs which exhibit excellent mechanical flexibility and cycle stability under their flat and bent states. The a-MnOx/TiC bifunctional electrocatalyst with extraordinarily high activity and electrochemical stability provides a promising approach for exploring corrosion resistant electrocatalysts for Zn-air batteries with high efficiency and long-term cycling stability.

Co3O4/C and Au supported Co3O4/C nanocomposites – Peculiarities of fabrication and application towards oxygen reduction reaction

The purpose of this paper was to investigate the influence of fabrication conditions of the carbon based cobalt(II, III) oxide (Co3O4/C) and Au supported Co3O4/C (AuCo3O4/C) catalysts on their electrocatalytic activities towards the oxygen reduction reaction (ORR). The catalysts were prepared using impregnation and microwave irradiation methods. It has been found, that the combination of both of these methods was a more efficient way for fabrication of high catalytically active Co3O4/C and AuCo3O4/C catalysts. The synthesized Co3O4/C and AuCo3O4/C catalysts showed high electrocatalytic activity towards the oxygen reduction reaction in an alkaline medium. The ORR onset potential for the investigated catalysts was comparable to theoretical ORR potential values between 0.7 and 0.9 V vs. reversible hydrogen electrode (RHE).

Intrinsic properties of nitrogen-rich carbon nitride for oxygen reduction reaction

The intrinsic electronic and catalytic properties of two-dimensional (2D) graphitic carbon nitride (g-C3N4) and its heterostructures were investigated as a potential oxygen reduction reaction (ORR) electrocatalyst. g-C3N4 is rich in native pyridinic N and shows favorably interactions toward ORR intermediates, but lacks the ability for efficient charge transfer for practical electrochemical processes. The electronic structures of freestanding g-C3N4 and hetero-bilayers paired with graphene (GN) and hexagonal boron nitride (hBN) monolayers were evaluated using Density Functional Theory (DFT). The energy band gap for the g-C3N4/GN bilayer, calculated from the HSE06 hybrid functional, is found to be significantly narrowed.. Moreover, a four-coordinated Fe atom, with Fe anchored by the pyridinic N (Fe-N-C) exists, resembling the GN-based FeN4 motif. The bindings of ORR intermediates at these single Fe atom sites were also found to be tunable, suggesting that there are opportunities to fine-tune g-C3N4-based materials to achieve superior performance as Pt-free electrocatalysts.