Preparation of electrochemically reduced graphene oxide-based silver-cobalt alloy nanocatalysts for efficient oxygen reduction reaction

The synthesis and performance of electrochemically reduced graphene oxide-based silver-cobalt (AgCo/ERGO) alloy electrocatalysts for the oxygen reduction reaction (ORR) are discussed in this study. The surface morphology, alloying nature, and chemical changes of the bimetallic precursors within the AgCo/ERGO catalyst has examined in detail. The presence of poly(ethylene glycol) (PEG) and a cobalt precursor during the electroreduction step is a necessary condition for synthesizing a highly active and stable alloy electrocatalyst for the ORR. Morphological analysis demonstrated that the AgCo nanoparticles (NPs) are homogeneously dispersed on the ERGO support with the assistance of PEG, thus resulting in higher electrochemical surface area and mass activity. X-ray analysis also confirmed the successful formation of the AgCo alloy NPs and the electrochemical reduction of graphene. The direct four-electron transfer pathway for ORR with minimal H₂O₂ yield has committed at the AgCo/ERGO catalyst over other catalysts. The as-prepared AgCo/ERGO catalyst has shown better electrocatalytic activity, stability, and tolerance to crossover effects compared to the state-of-the-art Pt/C catalyst for ORR.     

High-performance N,P-CNL nanocomposites as catalyst for oxygen reduction reaction in fuel cell

Transition metal and heteroatom codoped carbon materials have become the most promising materials to replace commercial platinum carbon (Pt / C) catalysts due to their low cost, high stability, and methanol resistance. In this work, iron-nitrogen and phosphorus codoped carbon nanorod-layer composites (N, P-CNL) derived from phosphorus-doped polyaniline (P-PANI) by phytic acid (PA) and iron salt were successfully obtained after high-temperature pyrolysis. As a result, the N, P-CNL materials exhibited good electrocatalytic performance due to abundant active sites. The N, P-CNL with 50% mass filling ratio of iron salt (named as N, P-CNL-1:1) displayed an enhanced limiting current density of −5.97 mA cm⁻² at 1600 rpm and outstanding onset potential (−0.004 V) and oxygen reduction peak potential (−0.144 V). In general, this work can give insights into understanding the mechanism of codoped catalysts and synthesis the catalyst with excellent long-term stability and resistance to methanol crossover and poisoning better than commercial Pt/C.     

Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction

Developing low-cost, efficient and stable catalyst for the oxygen reduction reaction is meaningful and necessary for the industrialization of fuel cells. In this work, we report the controllable synthesis of Co₃O₄ nanospheres uniformly anchored on N-doped reduced graphene oxide (N-rGO) sheets with significant catalytic activity for the oxygen reduction reaction via a simple two-step approach. Results show that there is an interaction between Co₃O₄ nanospheres and N-rGO after riveting on N-rGO, which is beneficial to the electron transfer between them. The Co₃O₄ NS/N-rGO hybrid has excellent catalytic activity, comparable to commercial Pt/C catalyst. The transferred electron number of the oxygen reduction reaction on Co₃O₄ NS/N-rGO is around 3.95. The hybrid has more excellent durability and methanol resistance than commercial Pt/C. The Co₃O₄ NS/N-rGO catalyst in our work provides a cost-effective the oxygen reduction reaction catalyst alternative to the precious metal Pt in fuel cell.     

Reduced graphene oxide supported Fe2B as robust catalysts for oxygen reduction reaction

Transition metal borides have great potential to be low-cost, high-performance catalysts for novel energies despite the synthesis is rather difficult. In this paper, the reduced graphene oxide (rGO) supported iron boride (Fe₂B/rGO) based catalysts are synthesized by a facile reduction method. The successful synthesis of Fe₂B is confirmed by X-ray diffraction, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray photo-electron spectroscopy (XPS) and other tests. HRTEM tests showed that the constructed Fe₂B was embedded in the rGO, where B played the role of coordination atoms which could regulate the electronic structure of the catalysts and improve the catalytic performance towards oxygen reduction reaction (ORR). The electrochemistry tests showed that the peak current intensity of the Fe₂B/rGO catalyzed ORR could be reached up to 7.6 mA/cm², which surpassed that of the Pt/C (20 wt%) catalyst. The current intensity can be kept at 82.47% after continuous running 20,000 s, which is higher than the Pt/C catalyst (79.4%). The onset potential reaches up to 0.95 V, which is only 0.06 V lower than that of Pt/C (20 wt%) catalyst. Both RDE and RRDE tests confirmed that the Fe₂B/rGO catalyzed ORR major happed through 4-electron pathway. The redistributed electron between iron and boron atoms promoted the happening of ORR on Fe₂B/rGO catalysts. The results of this work provide a novel way to develop high performance transition metal boride based catalysts for ORR.     

Soybean milk derived carbon intercalated with reduced graphene oxide as high efficient electrocatalysts for oxygen reduction reaction

Exploring high-performance and low-cost metal-free oxygen reduction reaction (ORR) catalysts from biomass-derived materials is vital to the development of novel energy conversion devices such as fuel cells, etc. Herein, nitrogen-enriched soybean milk derived carbon (BDC/rGO-HT-NH₃) intercalated with reduced graphene oxide (rGO) electrocatalyst is prepared via one-pot hydrothermal synthesis method followed with nitridation by NH₃. The resultant catalyst with high surface area, good conductivity and high content of N (9.4 at.%) shows high electrocatalytic activity towards ORR in alkaline medium, which mainly happens through the direct 4-electron pathway. The onset potential of BDC/rGO-HT-NH₃ catalyzed ORR is 0.96 V vs RHE, which is only 0.11 V lower than that of the commercial Pt/C (20 wt%) catalyst. In addition, the BDC/rGO-HT-NH₃ catalyst shows superior long-term running durability. The desirable catalytic performances enable the facile synthesis approach of BDC/rGO-HT-NH₃ to be a promising methodology for transforming other biomass materials to N-enriched carbon based materials as low-cost and environmental friendly catalysts for ORR.     

The role of Pt loading on reduced graphene oxide support in the polyol synthesis of catalysts for oxygen reduction reaction

Common carbon-blacks have shown insufficient stability as cathodic catalyst supports for proton exchange membrane fuel cells (PEMFCs). In this regard, alternative supports have been proposed and, specifically graphene or reduced graphene oxide (rGO), have attracted special attention. Herein, a set of electrocatalysts using reduced graphene oxide (rGO) as support is synthetized by a modified polyol method. The influence of Pt loading on the support is studied and compared with conventional supports, considering Pt particle morphologies and oxygen reduction reaction (ORR) performance in rotating disk electrode (RDE). Despite Pt average particle size typically increases with the Pt loading, 30 wt% of Pt on rGO is the optimal Pt loading, yielding the highest ORR activity among the rGO-supported electrocatalysts. These results show that both Pt loading and type of support greatly impact on the morphology and electrochemical performance of Pt nanoparticles.     

Facile synthesis of Pd supported on Shewanella as an efficient catalyst for oxygen reduction reaction

While noble metals loaded on carbon-based supports are commonly used as oxygen reduction catalysts for fuel cell cathodes, the preparation process is complicated and expensive cost. In this paper, Pd²⁺ was first adsorbed on Shewanella by its adsorption characteristics, then the Pd supported on Shewanella catalyst was obtained after carbonization at 600 °C and hydrogen reduction at 200 °C. The Shewanella cells retain the rod shape of bacteria following pyrolysis under high temperatures, while N and Pd heteroatoms are uniform distribution on the carbon matrix. As a result, Pd supported on Shewanella catalyst exhibits excellent electrocatalytic activity for ORR via a dominant four-electron oxygen reduction pathway in alkaline medium. More importantly, the mass activity of the prepared catalyst was 5.8 times higher than that of commercial Pd/C, and its stability was also better than the Pd/C, which could be promising alternatives to costly Pt-based electrocatalysts for ORR.     

Transition metal carbides and nitrides as oxygen reduction reaction catalyst or catalyst support in proton exchange membrane fuel cells (PEMFCs)

Fuel cells are potentially efficient, silent, and environmentally friendly tools for electrical power generation. One of the obstacles facing the development and the commercialization of fuel cells is the dependence on the precious metal catalyst, i.e., Platinum (Pt) and Pt - alloy, especially at the cathode where high catalyst loading used to compensate the sluggish oxygen reduction reaction (ORR). Pt is not only an expensive and rare element but also has insufficient durability. The development of an efficient non-precious catalyst, i.e., electrochemically active, chemically and mechanically stable, and electrically conductive, is one of the basic requirements for the commercialization of fuel cells. The bonding to carbon and nitrogen to form metal carbides and nitrides modify the nature of the d-band of the parent metal, thus improve its catalytic properties relative to the parent metals to be similar to those of group VIII noble metals. In this article, we summarize the progress in the development of the transition metal carbides (TMCs) and transition metals nitrides (TMNs) relative to their application as catalysts for the ORR in fuel cells. The preparation of TMCs and TMNs via different routes which significantly affects its activity is discussed. The ORR catalytic activity of the TMCs and TMNs as a non-precious catalyst or catalyst support in fuel cells is discussed and compared to that of the Pt-based catalyst in this review article. Moreover, the recent progress in the preparation of the nano-sized (which is a critical factor for increasing the activity at low temperature) TMCs and TMNs are discussed.     

Ni nanoparticles intercalated LTA-type nanozeolite (KZ) supported on reduced graphene oxide for 4-nitrophenol reduction

We present a novel nanocomposite catalyst, Ni nanoparticles (NPs) intercalated LTA-type nanozeolite (KZ) on reduced graphene oxide (RGO), abbreviated as KZ-Ni/RGO for the reduction of environmental pollutant 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The structure, composition and morphology of the catalyst were characterized by using the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) techniques. The presence of Ni inside the nanocomposite was confirmed by the elemental mapping analysis. 4-NP reduction reaction shows the high catalytic activity (30 min) to KZ-Ni/RGO nanocomposite compared to the KZ-Ni (75 min) with excellent stability up to 5 cycles.     

Enhanced electrocatalytic activity of oxygen reduction by cobalt-porphyrin functionalized with graphene oxide in an alkaline solution

Cobalt[5,15-(p-aminophenyl)-10,20-(pentafluorophenyl)porphyrin] (CoAPFP) functionalized with graphene oxide (GO) by N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC hydrochloride) was used as an amide-coupling reagent (GO-CoAPFP). Electrochemical investigation revealed that ERGO-CoAPFP exhibited much better catalysis and better-improved oxygen reduction reaction (ORR) than GO-CoAPFP. ERGO-CoAPFP is a simple and facile method of improving electrocatalytic activity. The ERGO-CoAPFP catalysts were tested in ORR using electrochemical techniques such as cyclic voltammetry (CV) and rotating-ring-disk-electrode (RRDE) hydrodynamic voltammetry to quantitatively obtain the ORR kinetic constants and the reaction mechanisms on a glassy carbon electrode (GCE) in a 0.1 M KOH solution. The electrocatalytic oxygen reduction reaction of RRDE modified with ERGO-CoAPFP established a pathway of four-electron transfer reactions.     

Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells

Nitrogen doped mesoporous carbons are employed as supports for efficient electrocatalysts for oxygen reduction reaction. Heteroatom doped carbons favour the adsorption and reduction of molecular oxygen on Pt sites. In the present work, nitrogen doped mesoporous carbons (NMCs) with variable nitrogen content were synthesized via colloidal silica assisted sol-gel process with Ludox-AS40 (40 wt% SiO₂) as hard template using melamine and phenol as nitrogen and carbon precursors, respectively. The NMC were used as supports to prepare Pt/NMC electrocatalysts. The physicochemical properties of these materials were studied by SEM, TEM, XRD, BET, TGA, Raman, XPS and FTIR. The surface areas of 11 wt% (NMC-1) and 6 wt% (NMC-2) nitrogen doped mesoporous carbons are 609 m² g^?1 and 736 m² g^?1, respectively. The estimated electrochemical surface areas for Pt/NMC-1 and Pt/NMC-2 are 73 m² g^?1 and 59 m² g^?1, respectively. It is found that Pt/NMC-1 has higher ORR activity with higher limiting current and 44 mV positive onset potential shift compared to Pt/NMC-2. Further, the fuel cell assembled with Pt/NMC-1 as cathode catalyst delivered 1.8 times higher power density than Pt/NMC-2. It is proposed that higher nitrogen content and large pyridinic nitrogen sites present in NMC-1 support are responsible for higher ORR activity of Pt/NMC-1 and high power density of the fuel cell using Pt/NMC-1 cathode electrocatalyst. The carbon support material with high pyridinic content promotes the Pt dispersion with particle size less than 2 nm.     

Porous CoS2 nanostructures based on ZIF-9 supported on reduced graphene oxide: Favourable electrocatalysis for hydrogen evolution reaction

The research and developments of porous, highly active non-noble metal cathode materials are the current hot spots. In our work, ZIF-9 (Zeolitic imidazolate framework-9) as a cobalt source provide porous structure, we have sulfurized the ZIF-9 into CoS₂ by a simple hydrothermal method. Ultimately, the porous CoS₂/RGO cathode material was obtained. Through a series of characterization analyses (powder X-ray diffraction, X-ray photoelectron spectroscopy), it is confirmed that the CoS₂/RGO composite was successfully formed. Furthermore, electrochemical tests demonstrated that the pursued catalyst exhibited remarkable hydrogen evolution reaction (HER) activities with a favorable overpotential (only 180 mV for 10 mA cm^?2 vs reversible hydrogen electrode), a low Tafel slopes (75 mV decade^?1) and high stability in acidic condition (more than 18 h).     

Influence of doping level on the electrocatalytic properties for oxygen reduction reaction of N-doped reduced graphene oxide

Reduced Graphene Oxide (rGO) doped with different nitrogen (N) concentrations (5, 10, and 15 parts in weight) were successfully synthesized via hydrothermal conditions, from graphene oxide (GO) and 3-amino-1,2,4-triazole (amitrole) to obtain N₅-rGO, N₁₀-rGO, and N₁₅-rGO. These N-doped materials were characterized and evaluated for the first time as catalysts for the oxygen reduction reaction (ORR). The physicochemical characteristics confirmed the simultaneous N-doping and reduction processes with a turbostratic re-stacking of graphene layers. Nitrogen was successfully introduced as a mix of pyridine-N, amine like-N, pyrrolic-N, and quaternary bonding species into the carbon lattice. The N content were 8.1, 9.9, and 10.5 at.% for N₅-rGO, N₁₀-rGO, and N₁₅-rGO. High catalytic activity was demonstrated in alkaline media with an onset potential of ~0.88 V and high current density (3.9 mA cm^?2) for N₁₅-rGO, leading the ORR via the 4-electron transfer pathway. The results demonstrated that the amine like-N species enhance the ORR catalytic activity in addition to pyridinic and quaternary.     

Enhancement of the electrocatalytic activity for the oxygen reduction reaction of boron-doped reduced graphene oxide via ultrasonic treatment

Commercial polymer electrolyte membrane fuel cells have relied on scares Platinum to catalyse the kinetically sluggish oxygen reduction reaction occurring at their anodes. Over the last decade organic materials, frequently based on graphitic structures have been demonstrated as promising alternative electrocatalysts to the noble metals. Researchers typically utilize ultrasonic treatment as part of the synthesis procedure to achieve homogeneous dispersion of graphitic carbon prior to. Herein we investigate the implications of the structural and compositional changes induced by the ultrasonication treatment on boron-doped reduced graphene oxide for oxygen reduction reaction. It is shown that ultrasonication pre-treatment prior to the boron doping and reduction of graphene oxide via hydrothermal process step leads to the increase of both substitutional B and electrocatalytic surface area, with associated reduction of average pore size diameter, leading to a significant improvement in the oxygen reduction reaction performance, with respect to the non-ultrasonicated material. It is proposed that the higher degree of substitutional doping of boron is a result of formation of the additional epoxy functionalities on graphitic planes, which act as a doping site for boric acid.     

Carbon nanotubes supported Fe-based compounds as the electrode catalyst for oxygen reduction reaction

An amorphous Fe-based catalyst supported on polypyrrole-modified carbon nanotubes is synthesized by a chemical method. The microstructure, surface composition and morphology are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The synthesized amorphous Fe-based catalyst is composed of amorphous FeOOH and microcrystalline Fe₂O₃. Compared with a crystalline FeOOH catalyst, the amorphous Fe-based catalyst demonstrates higher electrocatalytic activity toward the oxygen reduction reaction (ORR), due to its amorphous structure and large specific surface area. It is considered that amorphization of transition metal compounds could be one of the methods used to improve their catalytic activity toward the ORR.     

Synthesis of silver@platinum-cobalt nanoflower on reduced graphene oxide as an efficient catalyst for oxygen reduction reaction

A novel oxygen reduction reaction (ORR) electrocatalyst silver@platinum-cobalt-rGO nanoflower (Ag@PtCo-rGO NFs) is prepared by hydrothermal reduction method. The formation of silver chloride likes elm leaf microstructure in the reaction process, performing as a template. The Ag@PtCo-rGO NFs exhibit excellent electrocatalytic activity, whose specific activity is 9 times higher than that of commercial Pt/C. More importantly, Ag@PtCo-rGO NFs demonstrate excellent durability than JM Pt/C for ORR in acid media, as evidenced by accelerated durability test after 40,000 cycles. These excellent performances can be attributed to the special structure, atomic steps and synergistic effect between Pt, Ag and Co. The density functional theory calculation shows that Ag@PtCo has the best activity, which is related to the transition state for O–O bond. This work proposes a promising pathway for the synthesis of surfactant-free nanoflowers structure catalysts with high catalytic performance and low cost.     

Comparative methods for reduction and sulfonation of graphene oxide for fuel cell electrode applications

Graphene oxide (GO) was synthetized, reduced and further sulfonated for the preparation of electrodes. GO was obtained using modified Hummer's method from graphite flakes. The partial reduction of graphene oxide (rGO) was performed by chemical and thermal process and subsequently functionalized using sulfuric acid or aryl diazonium salt of sulfanilic acid as sulfonating agents to afford rGO-SO₃H. The influence of the reduction processes on the sulfonation reactions of rGO was evaluated through XRD, TGA, FT-IR and Raman techniques. Electrochemical properties of both rGO and sulfonated rGO materials as modified glassy carbon electrode were evaluated using a K₃FeCN₆ solution as a reference redox system. XRD confirmed the partial reduction of GO by two methods and FT-IR demonstrated that SO₃H groups were successfully grafted on GO. VC results confirmed that both reduction and sulfonated methods leads to a better material with superior electrochemical properties compared to GO and rGO. Thermally reduced GO and functionalized with sulfuric acid [rGO_T-SO₃H(1)] showed the best electron transfer activity compared to those chemically reduced or sulfonated with sulfanilic acid. The electrochemical properties observed for rGO_T-SO₃H(1) suggest than can be a suitable support material of nanoparticles for the preparation of electrodes for fuel cells applications.     

Ultrafine iron oxide nanoparticles supported on N-doped carbon black as an oxygen reduction reaction catalyst

An oxygen reduction reaction (ORR) catalyst comprising ultrafine iron oxide nanoparticles supported on N-doped Vulcan carbon (FeO_1.4/N-C) was prepared via a two-step method. X-ray photoelectron spectroscopy revealed the iron oxide nanoparticles comprised Fe₂O₃ and FeO phases with a combined average oxidation state of 2.8. The FeO_1.4/N-C catalyst produced an ORR onset potential of −0.056 V and a half-wave potential of −0.190 V in alkaline media, which was comparable to that of commercial Pt/C catalyst. Moreover, FeO_1.4/N-C had higher methanol tolerance than Pt/C catalyst and thus affords a promising non-precious metal ORR catalyst for fuel cells.