Composition-tunable PtCu porous nanowires as highly active and durable catalyst for oxygen reduction reaction

To accelerate the commercialization of fuel cells, many efforts have been made to develope highly active and durable Pt-based catalyst for oxygen reduction reaction (ORR). Herein, PtCu porous nanowires (PNWs) with controllable composition are synthesized through an ultrasound-assisted galvanic replacement reaction. The porous structure, surface strain, and electronic property of PtCu PNWs are optimized by tuning composition, which can improve activity for ORR. Electrochemical tests reveal that the mass activity of Pt_0.5Cu_0.5 PNWs (Pt/Cu atomic ratio of 1:1) reaches 0.80 A mg_Pt^?1, which is about 5 times higher than that of the commercial Pt/C catalyst. Notably, the improved activity of the porous nanowire catalyst is also confirmed in the single-cell test. In addition, the large contact area with the carrier and internal interconnection structure of Pt_0.5Cu_0.5 PNWs enables them to exhibit much better durability than the commercial Pt/C catalyst and Pt_0.5Cu_0.5 nanotubes in accelerated durability test.     

High performance N-doped moCNTs catalyst with graphene-like nanosheets structure for oxygen reduction

N-doped mildly oxidized CNTs (moCNTs) catalysts for oxygen reduction reaction in proton exchange membrane fuel cells were prepared by a two-step method consisting of polymerization and pyrolyzation. The morphology of the electrocatalyst samples were characterized by scanning electron microscopy (SEM), high resolution transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). It was discovered that the N-doped moCNTs catalyst showed a graphene-like nanosheets nanostructure; electrocatalytic properties of the catalysts were investigated by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). The experimental results exhibited remarkable catalytic activity in an acidic medium, with the onset potential, half-wave potential, and limiting current density reaching 0.87 V, 0.75 V (vs. reversible hydrogen electrode), and 3.5 mA/cm², respectively. It can be concluded that the high catalytic performance is due to the high content of graphitic nitrogen and the graphene-like nanosheets structure.     

Astragali Radix-derived nitrogen-doped porous carbon: An efficient electrocatalyst for the oxygen reduction reaction

The development of biomass-derived nitrogen-doped porous carbons (NPCs) for the oxygen reduction reaction (ORR) is important for sustainable energy systems. Herein, NPCs derived from Astragali Radix (AR) via a cost-effective strategy are reported for the first time. The as-prepared AR-950-5 catalyst shows a stacked layer-like structure and porosity. Notably, the optimized AR-950-5 delivers catalytic activity comparable to that of commercial Pt/C (C-Pt/C), with high onset potential, positive half-wave potential and large limiting current density. It also displays superior long-term stability and methanol tolerance for ORR. This work will pave the way for a new approach in the development of highly active and low-cost NPCs for fuel cells.     

Transitional metal alloyed nanoparticles entrapped into the highly porous N-doped 3D honeycombed carbon: A high-efficiency bifunctional oxygen electrocatalyst for boosting rechargeable Zn-air batteries

Rational development of low-cost, durable and high-performance bifunctional oxygen catalysts is highly crucial for metal-air batteries. Herein, transition metal alloyed FeCo nanoparticles (NPs) embedded into N-doped honeycombed carbon (FeCo@N-HC) was efficiently prepared by a one-step carbonization method in the existence of NH₄Cl and citric acid. Benefiting from the honeycomb-like architectures and the synergistic effects of the FeCo alloy with the doped-carbon matrix, the as-synthesized FeCo@N-HC exhibited outstanding oxygen reduction reaction (ORR) with the more positive onset potential (E_onset = 0.98 V vs. RHE) and half-wave potential (E_1/2 = 0.85 V vs. RHE), coupled with outstanding oxygen evolution reaction (OER) with the lower overpotential (318 mV) at 10 mA cm^?2. Besides, the home-made Zn-air battery has the larger power density of 144 mW cm^?2 than Pt/C + RuO₂ (80 mW cm^?2). This research offers some valuable guidelines for constructing robust oxygen catalysts in clean energy storage and conversion technologies.     

Surface structure and electronic properties of Pt-Fe/C nanocatalysts and their relation with catalytic activity for oxygen reduction

In this work, physical and catalytic properties of Pt-Fe/C nanocatalysts of nominal compositions Pt:Fe 70:30 and 50:50, prepared by a polyol process using a long-chain diol as reducer (hexadecanediol) and oleic acid and oleylamine as stabilizers, are reported. As-prepared materials have very small particle size (2.2 nm), narrow particle size distribution, and homogeneous dispersion on the carbon support. The average compositions determined by energy dispersive X-ray analysis are Pt₇₅Fe₂₅/C and Pt₆₀Fe₄₀/C. Data for samples submitted to heat treatment in hydrogen atmosphere to induce Pt surface segregation are also presented. X-ray diffraction and transmission electronic microscopy are used to examine all as-prepared and heat-treated catalysts. Electronic properties are analyzed based on in situ dispersive X-ray absorption spectroscopy data. Measurements of electrocatalytic activity for oxygen reduction show that all Pt-Fe/C have electrocatalytic activities superior to that of Pt/C. Nanocatalysts with a Pt-rich surface have an enhanced performance for the reduction of oxygen but measurements carried out in methanol containing solutions show that Pt-enriched surfaces have an inferior methanol tolerance.     

Graphitic carbon wrapped Co on Zn,Co, N-codoped 3D tremella-like carbon with abundant nanocages for high-performance oxygen reduction

To develop green and renewable energy, subtle design and synthesis of non-precious metal catalysts with low-price, high-efficiency, and robust stability are of paramount significance for oxygen reduction reaction (ORR). In this study, we synthesized graphitic carbon wrapped Co on Zn, Co, N-codoped 3D tremella-like carbon with abundant nanocages (G-Co/Zn, Co, N-TCs) by one-step coordination pyrolysis in the presence of azacytosine. The pyrolysis temperature and Zn/Co feeding ratio were carefully investigated. Integrating the unique structure and synergistic effect of the bimetals, the resulting G-Co/Zn, Co, N-TCs exhibited remarkable ORR activity (E_onset = 0.99 V; E_1/2 = 0.88 V vs. RHE), and excellent stability (only 13 mV negative shift of E_1/2 after scanning 2000 cycles), outperforming the control groups and commercial Pt/C under alkaline condition. This research presents some precious viewpoints for fabricating advanced N- and transition metal-dual doped carbon electrocatalysts in energy conversion and storage correlated devices.     

Carbon fiber supported Co/Co3O4-embedded N-doped carbon microparticles for efficient overall seawater splitting and oxygen reduction

Due to low cost and abundance, direct seawater splitting to produce hydrogen is an encouraging strategy to alleviate the consumption of fossil energy, while also avoiding freshwater stress. However, when natural seawater is utilized as the electrolyte, the catalysts on the cathode and anode of water splitting should not only have high activity to promote energy efficiency, but also have good stability and durability to resist the corrosion of chloride ions. Herein, a hierarchical carbon-based catalyst for hydrogen and oxygen evolution, ZIF-67/CF-1, was prepared by annealing a composite of ZIF-67 and carbon fiber (CF). It exhibits good electrocatalytic activity and stability for overall splitting in natural seawater and neutral PBS solution. Impressively, when an electrolyzer consisting of ZIF-67/CF-1||ZIF-67/CF-1 is applied to overall seawater splitting, the current density of 10 mA/cm² is achieved with a drive voltage of 2.46 V, which is only 0.28 V higher than precious metal-based electrolyzer (Pt/C||IrO₂). Meanwhile, ZIF-67/CF-1 shows outstanding catalytic ability for oxygen reduction (E_1/2 = 0.84 V, Tafel slope = 66.9 mV/dec), also demonstrating its application potential in rechargeable batteries and fuel cells.     

Nanostructured silver fibers: Facile synthesis based on natural cellulose and application to graphite composite electrode for oxygen reduction

The development of cheaper electrocatalysts for fuel cells is an important research area. This work proposes a new, simpler and low-cost approach to develop nanostructured silver electrocatalysts by using natural cellulose as a template. Silver was deposited by reduction of Ag complexes on the surface of cellulose fibers, followed by heat removal of the template to create self-standing nanostructured silver fibers (NSSFs). X-Ray diffraction (XRD) showed fcc silver phase and X-Ray photoelectron spectroscopy (XPS) demonstrated that the surface was partially oxidized. The morphology of the fibers consisted of 50 nm nanoparticles as the building blocks, and they possessed a specific surface area of about 25 m²/g, which is sufficiently high for electrocatalytic applications. The NSSFs were incorporated in a graphite composite electrode. The resulting modified electrode displayed a good electrocatalytic activity for the reduction of dissolved oxygen in basic media. In an O₂-saturated 0.1 M KOH solution, the overpotential to initiate the oxygen reduction reaction reduced and the limiting current increased by increasing the relative amount of silver fibers from 0 to 5 wt%.     

Highly stable and effective Pt/carbon nitride (CNx) modified SiO2 electrocatalyst for oxygen reduction reaction

We report a durable and active electrocatalyst, Pt/carbon nitride (CN_x) modified silicon dioxide (SiO₂) composite (donated as CN_x/SiO₂), for oxygen reduction reaction (ORR). CN_x/SiO₂ composite is synthesized by calcination of polypyrrole coated SiO₂ (Ppy/SiO₂) at 800 °C. The structure and composition are assessed using Fourier transform infra-red spectroscopy, X-ray diffraction, transmission electron microscope and energy dispersive spectroscopy. Voltammetry is used to study the activities of Pt immobilized on Vulcan XC-72R and CN_x/SiO₂, respectively. The electrochemical data indicate that Pt supported on CN_x/SiO₂ possesses higher electrocatalytic activity and durability for ORR compared with those of Vulcan XC-72R. All these demonstrate that CN_x/SiO₂ is a promising ORR electrocatalyst support for low temperature fuel cells.     

New nanostructured carbons based on porous cellulose: Elaboration,pyrolysis and use as platinum nanoparticles substrate for oxygen reduction electrocatalysis

New nanostructured carbons have been prepared from pyrolysis of recently developed highly porous cellulose, aerocellulose (AC). Aerocellulose is an ultra-light and highly porous pure cellulose material prepared from cellulose gels followed by drying in carbon dioxide supercritical conditions. The carbonized aerocellulose (CAC) materials were obtained after pyrolysis of the aerocellulose under nitrogen flow at 830 °C, and subsequently doped by platinum nanoparticles. The platinum insertion process consisted of (i) thermal activation at various temperatures in CO₂ atmosphere, (ii) impregnation by PtCl₆²⁻ and (iii) platinum salt chemical reduction. The aerocellulose materials and their carbonized counterparts were investigated by scanning and transmission electron microscopy (SEM and TEM), mercury porosimetry and thermogravimetric analysis. The morphology of the platinum particles deposited on the carbonized aerocellulose materials (Pt/CAC) was investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD): the Pt particles are of 4–5 nm size, mainly agglomerated, as a result of the complex surface chemistry of the CAC. Their electrocatalytic activity was investigated by quasi-steady-state voltammetry in the rotating disk electrode (RDE) setup, regarding the oxygen reduction reaction (ORR). The Pt/CAC materials exhibit ORR specific activities comparable with those of commercial Pt/Vulcan XC72R. Their mass activity is lower, as a result of the ca. 10 times smaller specific area of platinum as compared with the commercial electrocatalyst. We nevertheless believe that provided an appropriate pyrolysis temperature is chosen, such green carbonized aerocellulose could be a promising electrocatalyst support for PEM application.     

Heteroatom (B,N and P) doped porous graphene foams for efficient oxygen reduction reaction electrocatalysis

Heteroatoms (B, N and P) doped porous graphene foams are developed via a hard-templating method. We use boric acid as the precursor of boron source, triphenylphosphine as precursor of phosphorus source, cyanamide as the precursor of nitrogen source and ferrous chloride as the precursor of transition metal to synthesize a series of transition metal iron-modified multi-element doped porous graphene foams catalysts. Our results showed that heteroatoms (B, N and P) doped porous graphene foams exhibited excellent ORR performance. The most efficient one, i.e., PGF-Fe-NBP, received the onset potential of 0.95 V and a half-wave potential of 0.84 V in alkaline medium. Even in acidic medium, PGF-Fe-NBP received the onset potential of 0.85 V and a half-wave potential of 0.68 V. In addition, it also obtained superb electro activities of low H₂O₂% and high electron transfer number in both alkaline and acidic medium. Moreover, we found that iron modification can promote doping amount of heteroatoms and increase the degree of graphitization to form a relatively larger specific surface area for more active sites, thus improving the ORR performance of heteroatoms (B, N and P) doped porous graphene foams. Meanwhile, we systematically compare multi-element doping with that of single-element doping and dual-element doping.     

Controllable synthesis of three-dimensional nitrogen-doped graphene as a high performance electrocatalyst for oxygen reduction reaction

Three-dimensional nitrogen-doped graphene (3D-NG@SiO₂) is prepared by pyrolyzing poly (o-phenylenediamine) (POPD) with high nitrogen content. POPD is prepared via an in situ chemical oxidation polymerization of o-phenylenediamine (OPD) in acetic acid with silica colloid as templates. The optimum parameter is OPD:SiO₂ = 1:2, pyrolysis @ 900 °C. SEM and TEM images show the wrinkled and porous graphene structures. Raman spectra indicate that 3D-NG@SiO₂ consists of 4–6 layers graphene. N₂ adsorption–desorption isotherms reveal that the pore size distributions mainly centralize at 5–10 nm. XRD illustrates the amorphous structure. XPS analysis shows that the nitrogen content is 3.6% and nitrogen mainly exists in the form of pyridinic nitrogen and pyrrolic nitrogen. The oxygen reduction reaction (ORR) performance investigated using a rotating disk electrode shows that the initial potential of 3D-NG@SiO₂ is 0.08 V (vs. Hg/HgO). The electron transfer number is 3.92 @ ?0.3 V (vs. Hg/HgO), indicating that 3D-NG@SiO₂ mainly occurs via a four-electron process. The oxygen reduction current density decreases by 21% after 60 h in the chronoamperometry test. The CVs manifests a current density loss of 0.16 mA cm^?2 after scanning for 5000 cycles. The high activity and durability indicate the promising potential of 3D-NG@SiO₂ as ORR catalysts.     

High porosity nitrogen and phosphorous Co-doped carbon nanosheets as an efficient catalyst for oxygen reduction

High porosity nitrogen and phosphorous co-doped carbon nanosheets with high surface area (1057.9 m² g^?1) are fabricated by using polyacrylonitrile and red phosphorous as the precursors. The catalyst exhibits outstanding catalytic performance towards oxygen reduction reaction (ORR), with its half-wave potential 35 mV more superior to that of Pt/C. In addition to high ORR performance, our catalyst also exhibits remarkable methanol tolerance, outstanding stability, as well as high catalytic efficiency (almost 100% selectivity towards four-electron path). Based on the characterization results, we find that moderate P introduction can significantly modify catalysts N compositions, as well as porous structures, which, we suggest, should be the proper origins to our catalyst's outstanding performance.     

N-doped graphene as a bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions in an alkaline electrolyte

In this work, a nitrogen-doped graphene (NG) catalyst was prepared using a hydrothermal method with ammonia as the nitrogen precursor, which was followed by a freeze-dry process. The catalyst was characterized using X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscope, and X-ray photoelectron spectroscopy. The bifunctional catalytic activities for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) were investigated using cyclic voltammetry in an alkaline electrolyte. The results indicate that nitrogen is successfully doped in the NG catalyst, and the catalyst has a loose structure that was produced during the freeze-dry process. The catalyst exhibits an excellent ORR activity with an onset potential of −0.08 V and a high OER activity with an obvious OER current at 0.7 V. The rotating-disk-electrode test results indicate that the ORR process catalyzed by the NG catalyst involves a mix of the two-electron and four-electron transfer pathways. This work preliminarily explores the bifunctional catalytic properties for the ORR and the OER of nitrogen-doped graphene materials in alkaline electrolyte.     

ZIF-derived graphene coated/Co9S8 nanoparticles embedded in nitrogen doped porous carbon polyhedrons as advanced catalysts for oxygen reduction reaction

Development of efficient electrocatalysts for the oxygen reduction reaction (ORR) is vitally important for the commercialization of metal–air batteries. In this work, we demonstrate a novel graphene coated/Co₉S₈ nanoparticles-embedded nitrogen doped porous carbon dodecahedron hybrid (Co₉S₈/NPCP@rGO) prepared by the pyrolysis and sulphuration of precursors composing of graphene oxide and zeolitic imidazolate-frameworks (ZIF). The Co₉S₈/NPCP@rGO hybrid is used as a highly efficient nonprecious metal electrocatalyst for oxygen reduction and exhibits more positive onset potential and half-wave potential, higher limiting current density, lower Tafel slope, and better durability and methanol tolerance in alkaline media in comparison to the commercial 20 wt.% Pt/C catalyst. The greatly improved electrocatalytic performance of Co₉S₈/NPCP@rGO can be attributed to the unique structure with Co₉S₈ nanoparticles dispersed uniformly inside nitrogen doped porous carbon matrix, and the synergistic effect between Co₉S₈/NPCP polyhedral hybrid and rGO.     

Facile synthesis of 3D hierarchical mesoporous Fe-C-N catalysts as efficient electrocatalysts for oxygen reduction reaction

A salt crystal-templating synthesis route is proposed to synthesize a Fe-N-C catalyst with well-controlled mesoporous structure. In the presence of glucose, NaCl-template can efficiently tune the porous structure of catalyst and help to improve the oxygen reduction reaction (ORR) activity. The optimized catalyst possesses a hierarchical mesopore size distribution, a high Brunauer-Emmett-Teller surface area (up to 911.56 m² g^?1) and homogeneous distribution of abundant active sites. As a result, the obtained catalyst shows a desirable ORR activity in alkaline medium (half-wave potential of 0.84 V and kinetic mass activity at 0.8 V of 24.95 A g^?1), high selectivity (electron transfer number >3.92), excellent long term durability (only 16 mV negative shift of half-wave potential after 5000 potential cycles in O₂-saturated 0.1 M KOH) and good tolerance to methanol. The enhanced electrochemical performance enables the proposed catalyst to be the promising electrocatalyst candidate to commercial Pt/C towards ORR.     

Rationale approach of nitrogen doping at defect sites of multiwalled carbon nanotubes: A strategy for oxygen reduction electrocatalysis

A development of electrocatalyst for oxygen reduction reaction (ORR) is one of the crucial reactions for low temperature fuel cell applications. The state-of-the-art catalyst (platinum based) have various limitations such as low abundance, extortionate price and sensitive towards impurities. Therefore, design of high performance non-platinum electrocatalyst is the most challenging issue for low temperature fuel cell. In this work, we discuss the nitrogen doping at defect sites of multiwalled carbon nanotubes (MWCNTs) using melamine foam as a template for efficient CNT assembly with subsequent role of different nitrogen containing agents such as melamine and hexamine. Templated assembly of functionalized CNT through melamine foam provides easy approach towards nitrogen doping that followed effective exposure of active sites (like pyridinic-N and oxidic-N) for electroreduction of oxygen. As-prepared N-CNT catalysts (prepared using both the precursors) show better ORR activity than Pt/C in alkaline medium. A sharp reduction peak in their cyclic voltammogram under O₂-saturated 0.1 M KOH solution proves their activity towards ORR electrocatalysis. More interestingly, the onset potentials of ~0.92 V and ~1.1 V versus RHE for N-CNT obtained by hexamine and melamine respectively indicate superior onsets than that of Pt/C (~1.04 V vs RHE). Furthermore, the best N-CNT catalyst (obtained by melamine) reveals better stability up to 15,000 cycles than Pt/C with zero response towards methanol, exhibiting an excellent methanol tolerance.     

Tuning the oxygen reduction reactivity of interconnected porous carbon by incorporation of phosphorus and activity enhancement through blending with 2D metal dichalcogenides materials

Design and construction of strong oxygen reduction reaction (ORR) electrocatalysts with high activity and durability are the main concerns in proton exchange membrane fuel cells (PEMFCs). In this study, a unique interconnected porous carbon (ICPC) and phosphorus doped ICPC (P-ICPC) were synthesized and utilized as a support matrix for ORR in alkaline medium. The activity of P-ICPC further enhanced by compositing with 2D metal dichalcogenide MoS₂ materials through facile hydrothermal method. The structural characterization indicated that the addition of phosphorus created more defective site in the carbon structure. The MoS₂/P-ICPC catalyst exhibited enhanced ORR activity, and its performance is close to commercial Pt/C catalyst with regards to current density and onset potential. The synthesized MoS₂/P-ICPC catalyst shows better stability regarding activity even after the 2000 cycles of acceleration test. The electron transfer number (n) obtained for MoS₂/P-ICPC is ～3.8, indicating that the oxygen reduction reaction proceeds via 4e^? pathway with the similar kinetics of commercial Pt/C. The current results revealed that the synthesized MoS₂/P-ICPC material might be a better catalyst for oxygen reduction reaction.     

Pt nanodendrites anchored on bamboo-shaped carbon nanofiber arrays as highly efficient electrocatalyst for oxygen reduction reaction

In order to improve the Pt utilization and enhance their catalytic performance in fuel cells, a novel composite electrode composed of single-crystalline Pt nanodendrites and support constructed by bamboo-shaped carbon nanofiber arrays (CNFAs) on carbon paper, is reported. This electrode is designed by growing vertically CNFAs on carbon paper via plasma enhanced chemical vapor deposition, followed by the direct synthesis of Pt nanodendrites using a simple surfactant-free aqueous solution method. Electron microscopy studies reveal that the Pt nanodendrites are uniformly high dispersed and anchored on the surface of CNFAs. Electrochemical measurements demonstrate that the resultant electrode exhibits higher electrocatalytic activity and stability for oxygen reduction reaction than commercial Pt/C catalyst, suggesting its potential application in fuel cells.     

Electrochemical activity and durability of platinum nanoparticles supported on ordered mesoporous carbons for oxygen reduction reaction

A facile procedure for synthesizing platinum nanoparticles (NPs) studded in ordered mesoporous carbons (Pt–OMCs) based on the organic–organic self-assembly (one-pot) approach is reported. These Pt–OMCs, which can be easily fabricated with controllable Pt loading, were found to possess high surface areas, highly accessible and stable active sites and superior electrocatalytic properties pertinent as cathode catalysts for hydrogen–oxygen fuel cells. The enhanced catalytic activity and durability observed for the Pt–OMC electrocatalysts are attributed to the strengthened interactions between the Pt catalyst and the mesoporous carbon that effectively precludes migration and/or agglomeration of Pt NPs on the carbon support.