Oxygen Reduction Reaction In Alkaline Solution Research Articles

Crab Shell‐Templated Fe and N Co–Doped Mesoporous Carbon Nanofibers as a Highly Efficient Oxygen Reduction Reaction Electrocatalyst

AbstractCrab shell, a cheap and abundant biomass waste, is used as a template for preparing iron and nitrogen co‐doped mesoporous carbon nanofibers (Fe/N/meso‐CNFs). Impregnation of phenolic resin, triblock copolymer, cyanamide, and ferric chloride inside the nanochannels of crab shell allows simultaneous morphology control of Fe/N/meso‐CNFs and Fe/N co‐doping after the pyrolysis and removal of crab shell. The Fe/N/meso‐CNFs exhibit large surface area, porous structure and accessible active sites. Such structural uniqueness enables Fe/N/meso‐CNFs ideal for use as an electrocatalyst. The large surface area and porous structure play an important role in exposing electrocatalytically active sites. As a noble‐metal free electrocatalyst for oxygen reduction reaction (ORR), Fe/N/meso‐CNFs show onset potential and half wave potential close to Pt/C with the limiting current density even exceeding that of Pt/C in alkaline solution. Additionally, the Fe/N/meso‐CNFs also present high tolerance towards methanol poisoning and long‐term durability, indicating Fe/N/meso‐CNFs can be regarded as a potential substitution for Pt towards ORR in alkaline solution.

Nitrogen-doped graphene-supported molybdenum dioxide electrocatalysts for oxygen reduction reaction

A simple one-step pyrolysis strategy has been developed to prepare molybdenum dioxide supported on nitrogen-doped graphene (MoO2/NG) as electrocatalyst for oxygen reduction reactions (ORR). Homogeneous anchoring of MoO2 nanoparticles on nitrogen-doped graphene was achieved for the composite synthesized at 500 °C (MoO2/NG-500). For the first time, the MoO2/NG composite manifests considerable electrocatalytic activity for ORR in alkaline solution, with the onset potential of 0.91 V and the peak potential of 0.73 V. Furthermore, the MoO2/NG catalyst outperforms commercial Pt/C catalyst in terms of the electrochemical durability for ORR. The facile fabrication, reasonable ORR activity and excellent stability endow our MoO2/NG catalyst with great promise in application of cost-effective fuel cells.

Enhancement of nitrogen self-doped nanocarbons electrocatalyst via tune-up solution plasma synthesis.

The development of a metal-free carbon based electrocatalyst for the oxygen reduction reaction (ORR) is an essential issue for energy conversion systems. Herein, we suggest a tune-up solution plasma (SP) synthesis based on a simple one-step and cost-effective method to fabricate nitrogen self-doped graphitic carbon nanosheets (NGS) as an electrocatalyst. This novel strategy using a low-pass filter circuit provides plasma stability and energy control during discharge in pyridine, determining the graphitic structure of nanocarbons doped with nitrogen. Notably, NGS have a relatively high surface area (621 m2 g−1), and high contents of nitrogen bonded as pyridinic-N and pyrrolic-N of 55.5 and 21.3%, respectively. As an efficient metal-free electrocatalyst, NGS exhibit a high onset potential (−0.18 V vs. Ag/AgCl) and a 3.8 transferred electron pathway for ORR in alkaline solution, as well as better long-term durability (4% current decrease after 10 000 s of operation) than commercial Pt/C (22% current drop). From this point of view, the nitrogen self-doped graphitic carbon nanosheet material synthesized using the tune-up SP system is a promising catalyst for the ORR, as an alternative to a Pt catalyst for application in energy conversion devices.

Graphene Layers‐Wrapped Fe/Fe5C2 Nanoparticles Supported on N‐doped Graphene Nanosheets for Highly Efficient Oxygen Reduction

AbstractSynthesis of highly efficient nonprecious metal electrocatalysts for the oxygen reduction reaction (ORR) superior to platinum (Pt) is still a big challenge. Herein, a new highly active ORR electrocatalyst is reported based on graphene layers‐wrapped Fe/Fe5C2 nanoparticles supported on N‐doped graphene nanosheets (GL‐Fe/Fe5C2/NG) through simply annealing a mixture of bulk graphitic carbon nitride (g‐C3N4) and ferrocene. An interesting exfoliation–denitrogen mechanism underlying the conversion of bulk g‐C3N4 into N‐doped graphene nanosheets is revealed. Owing to the high graphitic degree, optimum N‐doping level and sufficient active sites from the graphene layers‐wrapped Fe/Fe5C2 nanoparticles, the as‐prepared GL‐Fe/Fe5C2/NG electrocatalyst obtained at 800 °C exhibits outstanding ORR activity with a 20 mV more positive half‐wave potential than the commercial Pt/C catalyst in 0.1 m KOH solution and a comparable onset potential of 0.98 V. This makes GL‐Fe/Fe5C2/NG an outstanding electrocatalyst for ORR in alkaline solution.

Fe7C3–Fe3N/FeNxCy Decorated Carbon Material as Highly Efficient Catalyst for Oxygen Reduction Reaction in Al-Air Batteries

Traditionally, oxygen reduction reactions (ORR) are catalyzed by Pt/C, which is expensive but essentially efficient catalyst. So doping of highly active moieties onto carbon supports could be investigated as a practical and inexpensive strategy for catalysis of ORR. The present study reported noncrystalline FeNxCy species, crystalline Fe7C3 and Fe3N nanoparticles co-decorated ketjenblack (KB) carbon catalyst (denoted as Fe–N–KB–HCl) for ORR prepared using ferrous sulfate, melamine and KB as raw materials. Carbon nanotubes (CNT) were observed in the catalyst, which caused to a high specific surface area (1125 m2g–1) of the catalyst. The Fe–N–KB–HCl catalyst fabricated in this study exhibited a catalytic performance similar to that of commercial 20% Pt/C for ORR in alkaline solution. Moreover, it exhibited a higher voltage (1.51 V) at a discharge density of 50 mA cm–2 in an in-house made Al-air battery than that of the commercial 20% Pt/C catalyst (1.45 V). So the Fe–N–KB–HCl catalyst prepared in this study could be used for developing inexpensive but highly efficient catalysts for catalysis of ORR and have potential to be used for commercial purposes.

Fabricating Pt-decorated three dimensional N-doped carbon porous microspherical cavity catalyst for advanced oxygen reduction reaction

Abundant and low-cost materials with high electrochemical catalytic activity for oxygen reduction reaction (ORR) are in urgent demand for energy storage and conversion devices. In this work, an advanced ORR electrocatalyst of Pt-decorated three dimensional (3D) N-doped carbon porous microspherical cavity (PtN/C PMC) was fabricated by using polystyrene microspheres (PS) as template, and dopamine and K2PtCl6 as precursors. Microscopic and spectroscopic characterizations revealed large surface area available and affluent pore structures were observed in the resulted PtN/C PMCs, and numerous Pt nanoparticles (NPs) with active (111) facet were decorated on the surface of N/C PMCs. The proposed PtN/C CMCs presented good onset potential, remarkable mass activity (MA) and specific activity (SA), superb methanol tolerance and excellent durability toward ORR in alkaline solutions. The numerous decorated Pt with high active (111) facet coupled with high content of active pyridinic and graphitic nitrogen synergistically contributed to the enhanced ORR activity, while the unique 3D porous structures facilitated O2 and electrolyte transport during the ORR process. This work may open new avenue for designing advanced and cost-effective Pt-based ORR catalyst.

Nitrogen-Doped Porous Graphdiyne: A Highly Efficient Metal-Free Electrocatalyst for Oxygen Reduction Reaction.

Metal-free catalysts for oxygen reduction reaction (ORR) are the desired materials for low-cost proton exchange membrane fuel cells. Graphdiyne (GDY), a novel type of two-dimensional carbon allotrope, is featured by its sp- and sp2-hybridized carbon atoms, different from the other existing carbon materials. Thus, nitrogen (N) can be doped in new styles by substituting sp-hybridized carbon atoms, effective for ORR, which has been displayed in this study using both experimental and theoretical technologies. The N-doped GDY was synthesized with pyridine and NH3 as N sources successively, expressing an electrocatalytic activity at a potential above 0.8 V similar to that of commercial Pt/C for ORR in alkaline solution and higher stability and better methanol tolerance than those of Pt/C.

Electrochemical synthesis of Au-Ni(OH)2-nanocomposite on glassy carbon electrode as highly active bifunctional electrocatalyst for oxygen evolution and oxygen reduction reactions

In this work, we demonstrated an electrochemical method to prepare Au-Ni(OH)2-nanocomposite on a glassy carbon substrate (Au-Ni(OH)2-NC/GCE). This modified nanocomposite showed high electrocatalytic activity towards oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) compared to similarly prepared Au free Ni(OH)2-NC/GCE. The above nanocomposite was prepared through two step processes; first gold‑nickel hexacyanoferrate nanocomposite film on GCE (Au-NiHCF-NC/GCE) was synthesized by single step electrochemical cycling approach. Later, the Au-NiHCF-NC/GCE was treated electrochemically in alkaline solution to produce Au-Ni(OH)2-NC/GCE. The as-formed Au-Ni(OH)2-NC/GCE was used as a bifunctional electrode to study the OER and ORR in alkaline solution. The resulting nanocomposite exhibits high electrocatalytic activity and fast electron transfer kinetics due to the synergic effect of the formation of more active β-Ni(OH)2 phase and Au nanoparticles (AuNPs) during the electrochemical cycling process.

A facile approach to tailoring electrocatalytic activities of imine-rich nitrogen-doped graphene for oxygen reduction reaction

We report efficient modulation of the electrocatalytic activity of imine-rich nitrogen-doped graphene nanosheets (IRnGs) in the oxygen reduction reaction (ORR) by chemical functionalization. IRnGs are prepared by a simple acid-catalyzed dehydration reaction between graphene oxide and aniline derivatives. Various electron-donating and electron-withdrawing substituents are introduced in the para-position of aniline to afford diverse IRnG electrocatalysts. Cyclic voltammetry and rotating ring disk electrode measurements show that the electrocatalytic activity of IRnGs for ORR is highly sensitive to the electronic characteristics of functionalities present in their chemical structures. Thus, the above ORR activities are significantly improved by increasing the electron-withdrawing capability of substituents, since this promotes the beneficial polarization of electrocatalytically active imine bond (CN) in IRnGs. In addition, metal-free IRnGs electrocatalysts offer additional advantages of high selectivity, good long-term stability and excellent tolerance to methanol crossover for ORR in alkaline solution.

Co3O4 nanoparticles assembled on polypyrrole/graphene oxide for electrochemical reduction of oxygen in alkaline media

The development of highly efficient catalysts for cathodes remains an important objective of fuel cell research. Here, we report Co3O4 nanoparticles assembled on a polypyrrole/graphene oxide electrocatalyst (Co3O4/Ppy/GO) as an efficient catalyst for the oxygen reduction reaction (ORR) in alkaline media. The catalyst was prepared via the hydrothermal reaction of Co2+ ions with Ppy-modified GO. The GO, Ppy/GO, and Co3O4/Ppy/GO were characterized using scanning electron microscopy, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The incorporation of Ppy into GO nanosheets resulted in the formation of a nitrogen-modified GO porous structure, which acted as an efficient electron-transport network for the ORR. With further anchoring of Co3O4 on Ppy/GO, the as-prepared Co3O4/Ppy/GO exhibited excellent ORR activity and followed a four-electron route mechanism for the ORR in alkaline solution. An onset potential of −0.10 V vs. a saturated calomel electrode and a diffusion limiting current density of 2.30 mA/cm2 were achieved for the Co3O4/Ppy/GO catalyst heated at 800 °C; these values are comparable to those for noble-metal-based Pt/C catalysts. Our work demonstrates that Co3O4/Ppy/GO is highly active for the ORR. Notably, the Ppy coupling effects between Co3O4 and GO provide a new route for the preparation of efficient non-precious electrocatalysts with hierarchical porous structures for fuel cell applications.

Catalysis of oxygen reduction on electrochemically activated polycrystalline gold by Pd nanoislands in alkaline solution

Catalytic properties of electrochemically activated polycrystalline gold, Au(poly), modified by spontaneously deposited palladium nanoislands with the coverage below a monolayer were examined for the oxygen reduction reaction (ORR) in alkaline solution using the rotating disc electrode (RDE) technique. Atomic force microscopy (AFM) images have shown that the coverage of randomly distributed nanosized Pd islands ranged from 18% to 77%. Cyclic voltammetry characterization involved hydrogen adsorption/desorption on the deposited Pd islands, as well as their oxidation/reduction at more positive potentials. RDE measurements have shown that, although ORR on Au(poly) proceeds partly through 4e-reduction pathway, in the presence of Pd islands it turns completely to 4e-pathway, exceeding the activity of bare Pd(poly). Among different Pd/Au(poly) nanostructures, the ones with Pd coverage of 18% and 30% have shown the highest activity for ORR in alkaline solution.

The development of cobalt phosphate for bifunctional oxygen electrocatalysis in alkaline solution

Most of the bifunctional, non-noble metal oxygen electrocatalysts for rechargeable metal-air batteries are based on transition metal oxides. Recent studies on metal phosphates have revealed their potential as another class of oxygen electrocatalysts. Among the metal phosphates Co3(PO4)2 has demonstrated fairly good activity for both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). However, the performance of native Co3(PO4)2 as a bifunctional oxygen catalyst is inferior to that of the noble metal catalysts. In this study we found out that the inadequacy of Co3(PO4)2 could be ameliorated by integrating it with reduced graphene oxide (RGO) nanosheets. The oxygen bifunctionality as measured by the potential difference (ΔE) between OER and ORR in alkaline solution can be lowered to 0.86V for an optimized catalyst, which even surpasses the performance of a 20wt% Ir/C benchmark catalyst (0.93V); making it one of the best non-noble metal oxygen electrocatalysts. The good bifunctional performance may be attributed to the strong coupling between the Co3(PO4)2 submicroparticles and the RGO nanosheets which synergizes the oxygen activity of Co3(PO4)2 with the (electron) transport properties of RGO. The experimental results also suggest that Co3(PO4)2 is more active than the popular CoOx catalysts since the performance degradation in OER and ORR was found to correlate positively with the in-situ transformation of surface Co3(PO4)2 to CoOx.

Ag/Fe3O4-N-Doped Ketjenblack Carbon Composite as Highly Efficient Oxygen Reduction Catalyst in Al-Air Batteries

Highly efficient and low-cost catalysts are greatly desirable for oxygen reduction reaction (ORR) to replace the costly Pt/C. Here, we report a novel Ag/Fe3O4-N-doped ketjenblack carbon (KB) composite synthesized by a facile and scalable strategy using silver nitrate, iron nitrate, glucose, urea and KB as raw materials. This method involves an initial one-pot hydrothermal treatment and the subsequent pyrolysis process as well as hydrochloric acid treatment. The as-prepared composite with a low Ag mass ratio of 4.22% shows excellent catalytic activity toward ORR in alkaline solution (half wave potential of 0.82 V, limiting current density of 6.0 mA cm−2). The half-wave potential is only 8.2 mV negative shift after 2000 cycles, even better than the 20% Pt/C, demonstrating remarkable stability. When used as cathode catalyst in home-made Al-air battery, this composite shows a higher discharge voltage of 1.49 V at a current density of 50 mA cm−2 than that of Pt/C (1.45 V). This superior performance should be attributed to the synergistic effects of Ag nanoparticles, carbon surrounded Fe3O4, Fe-N active sites and N-doped KB.

A new method for developing defect-rich graphene nanoribbons/onion-like carbon@Co nanoparticles hybrid materials as an excellent catalyst for oxygen reactions.

Herein, a novel material with an excellent electrocatalytic activity for the oxygen reduction reaction (ORR), composed of onion-like carbon@Co nanoparticles (NPs) incorporated on defect-rich N-doped graphene nanoribbons (C@Co-NGR), is prepared via a combination of nitric acid etching and subsequent high-temperature treatment of the as-synthesized C@Co-N doped carbon nanotubes. The as-prepared C@Co-NGR exhibits excellent electrocatalytic activity for the ORR in 0.10 M KOH with a half-wave potential of 0.830 V (vs. a reversible hydrogen electrode, RHE), which is 21 mV more positive than that of a commercial 20 wt% Pt/C catalyst. Moreover, it shows a higher stability and better methanol tolerance than the commercial Pt/C catalyst in alkaline solution, with almost no shift in the ORR polarization curve after 3000 cycles and no change of the ORR peak in the cyclic voltammogram in the presence of 1.0 M methanol. Most importantly, the C@Co-NGR also exhibits a comparable activity for the oxygen evolution reaction (OER) in 0.10 M KOH solution, in which the overpotential needed to achieve a current density of 10 mA cm-2 is 410 mV, comparable to that of the state-of-the-art commercial RuO2 catalyst (370 mV, vs. RHE). This makes the C@Co-NGR one of the best non-precious-metal catalyst for OER and ORR in alkaline solution ever reported.

Template-directed synthesis of nitrogen- and sulfur-codoped carbon nanowire aerogels with enhanced electrocatalytic performance for oxygen reduction

Heteroatom doping, precise composition control, and rational morphology design are efficient strategies for producing novel nanocatalysts for the oxygen reduction reaction (ORR) in fuel cells. Herein, a cost-effective approach to synthesize nitrogen- and sulfur-codoped carbon nanowire aerogels using a hard templating method is proposed. The aerogels prepared using a combination of hydrothermal treatment and carbonization exhibit good catalytic activity for the ORR in alkaline solution. At the optimal annealing temperature and mass ratio between the nitrogen and sulfur precursors, the resultant aerogels show comparable electrocatalytic activity to that of a commercial Pt/C catalyst for the ORR. Importantly, the optimized catalyst shows much better long-term stability and satisfactory tolerance for the methanol crossover effect. These codoped aerogels are expected to have potential applications in fuel cells.

Metallocorroles as Non‐Precious Metal Electrocatalysts for Highly Efficient Oxygen Reduction in Alkaline Media

AbstractA series of non‐precious metal complexes, composed of five first‐row transition‐metal complexes with β‐pyrrole‐brominated 5,10,15‐tris(pentafluorophenyl)corroles [M(tpfcBr8), M=Mn, Fe, Co, Ni, and Cu], was investigated as catalysts for oxygen reduction in an alkaline solution (0.1 m KOH). The corroles were adsorbed on a high surface area carbon powder (BP2000) prior to electrochemical measurements to create a unique composite material. The comparison between the different metal complexes revealed a high oxygen reduction reaction (ORR) catalytic performance in the case of the Fe‐ and Co‐corroles. These complexes reduce oxygen at very low overpotentials (with E1/2=0.79 V and 0.77 V vs. RHE, respectively), which is better than other well‐defined molecular catalysts and comparable to that of Pt on carbon (XC‐72). The mechanism by which the most active complexes catalyze the ORR in alkaline solutions was also studied, disclosing that the dominant reaction path is a four‐electron reduction of molecular oxygen to hydroxide.

Nitrogen-doped cobalt nanoparticles/nitrogen-doped plate-like ordered mesoporous carbons composites as noble-metal free electrocatalysts for oxygen reduction reaction

In this work, nitrogen-doped cobalt nanoparticles/nitrogen-doped plate-like ordered mesoporous carbons (N/Co/OMCs) were used as noble-metal free electrocatalysts with high catalytic efficiency. Compared with OMCs with long channel length, due to more entrances for catalytic target accessibility and a short pathway for rapid diffusion, the utilization efficiency of cobalt nanoparticles inside the plate-like OMCs with short pore length is well improved, which can take full advantage of porous structure in electrocatalysis and increase the utilization of catalysts. The active sites in N/Co/OMCs for oxygen reduction reaction (ORR) are highly exposed to oxygen molecule, which results in a high activity for ORR. By combination of the catalytic properties of nitrogen dopant, incorporation of Co nanoparticles, and structural properties of OMCs, the N/Co/plate-like OMCs are highly active noble-metal free catalysts for ORR in alkaline solution.

Comparison of nitrogen-doped graphene and carbon nanotubes as supporting material for iron and cobalt nanoparticle electrocatalysts toward oxygen reduction reaction in alkaline media for fuel cell applications

In this work the electrocatalytic performance of nitrogen-doped graphene (NG)-based non-precious metal (NPM) catalysts for oxygen reduction reaction (ORR) have been compared with their counterparts while supported on multiwalled carbon nanotubes (MWCNTs). Fe and Co nanoparticles were precipitated on NG (M/NG, M = Fe, Co) and for comparison on MWCNTs (M/MWCNT, M = Fe, Co) using a modified polyol method. The electrocatalytic properties of all catalysts towards oxygen reduction reaction in 0.1 M KOH solution were investigated. In comparison with M/MWCNTs, M/NG catalysts exhibited higher ORR activity, which indicates a better electrocatalytic performance of nitrogen-doped graphene compared to those which were supported by MWCNTs. Chronoamperometric results also demonstrated that the Co/NG catalyst was more stable for ORR in alkaline solution rather than Co/MWCNT and commercial Pt/C.

Nitrogen-doped amorphous carbon with effective electrocatalytic activity toward oxygen reduction reaction

Highly efficient metal-free catalysts to improve the performance of oxygen reduction reactions (ORR) have attracted significant attention. Here, nitrogen-doped amorphous carbon (NDAC) were synthesized with a one-step method that pyrolyzed dopamine in a N2 atmosphere at 700°C, 800°C, and 900°C (NDAC-700, NDAC-800, and NDAC-900, respectively). The NDAC-700 demonstrates good catalytic activity with terrific positive onset potential (0.96V VS RHE), which is close to Pt-C (1.0V) (20%) toward ORR. Moreover, versus commercial Pt/C (20%) catalyst, it has better long-term operational stability and tolerance to methanol for ORR in alkaline solution. This exciting result increases the possibility of large-scale application of metal-free nanomaterials in fuel cells.

Mechanism of Oxygen Reduction Reaction on Pt(111) in Alkaline Solution: Importance of Chemisorbed Water on Surface

We report a detailed mechanistic study of the oxygen reduction reaction (ORR) on Pt(111) in alkaline solution, combining density functional theory and kinetic Monte Carlo simulations. A complex reaction network including four possible pathways via either 2e– or 4e– transfer is established and is able to reproduce the experimental measured polarization curve at both low- and high-potential regions. Our results show that it is essential to account for solvation by water and the dynamic coverage of *OH to describe the reaction kinetics well. In addition, a chemisorbed water (*H2O)-mediated mechanism including 4e– transfers is identified, where the reduction steps via *H2O on the surface are potential-independent and only the final removal of *OH from the surface in the form of OH–(aq) contributes to the current. For the ORR in alkaline solutions, such a mechanism is more competitive than the associative and dissociative mechanisms typically used to describe the ORR in acid solution. Finally, *OH and **O2 int...