Cobalt,sulfur, nitrogen co-doped carbon as highly active electrocatalysts towards oxygen reduction reaction

Rational design of efficient, cost-effective electrocatalyst towards oxygen reduction reaction (ORR) is of vital importance to the wide application of polymer electrolyte membrane fuel cells. In this work, a novel and simple Na₂SO₄-assisted pyrolysis strategy with ZIF-12 as the precursor is reported for the synthesis of cobalt, sulfur, nitrogen, co-doped carbon (termed as CoSNC-xNa₂SO₄-T) materials towards ORR. Different from CoNC-800 derived from pure ZIF-12, with the presence of Na₂SO₄, the derived CoSNC-0.5Na₂SO₄-800 material exhibits layered flake morphology with hierarchical meso-microporous structure. Besides, CoSNC-0.5Na₂SO₄-800 material shows higher content of pyridinic-N and graphitic-N, higher relative intensity of Co-N_x, higher content of carbon defect, as well as larger specific surface area in comparison with CoNC-800, which results in higher activity of CoSNC-0.5Na₂SO₄-800. The CoSNC-0.5Na₂SO₄-800 material displays a half-wave potential of 0.88 V, which is superior to that of commercial Pt/C (half-wave potential being 0.86 V). Moreover, CoSNC-0.5Na₂SO₄-800 demonstrates a better durability compared with Pt/C. The pyrolysis temperature and the amount of Na₂SO₄ are found to affect the physiochemical properties and electrochemical performance of the CoSNC-xNa₂SO₄-T materials. This work not only provides a facile and novel synthesis approach for the preparation of highly active Co, S, N co-doped carbon materials for ORR, but also disclosing the key structure properties for enhancing the performance of these catalysts.     

Phosphorus-doped hierarchical porous carbon as efficient metal-free electrocatalysts for oxygen reduction reaction

Recently, fuel cells and metal-air batteries have attracted extensive attentions. Researching and developing non-noble metal catalyst with high electrocatalytic activity and low cost is one of the important challenges for these energy storage and conversion devices. In this study, phosphorus doped hierarchical porous carbon (P-HPC) has been firstly synthesized via a hard template method. The prepared PHPC possesses a unique porous structure which consists of micropores, mesopores and macropores simultaneously. The electrocatalytic activity of the PHPC toward ORR in KOH solution has been studied and compared with the ordinary structured phosphorus doped carbon (PC) and the commercial Pt/C by means of rotating ring-disk electrode (RRDE) technique. The prepared PHPC exhibits an excellent electrocatalytic performance toward ORR in terms of the electrocatalytic activity, the reaction kinetics, the durability and the methanol tolerance. And the high electrocatalytic activity and durability of PHPC could be attributed to the special hierarchical porous structure. This research demonstrates that the rational design of the microstructures for catalyst plays significant roles in improving the catalytic activity for the ORR.     

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.     

Nitrogen and sulfur doped ZnAl layered double hydroxide/reduced graphene oxide as an efficient nanoelectrocatalyst for oxygen reduction reactions

In this work, to synthesize an efficient and low-cost electrocatalysts for Oxygen Reduction Reaction (ORR), the combination of N,S-rGO and ZnAl-LDH with several concentrations is studied for the first time. For this purpose, six electrocatalysts including Graphene Oxide (GO), functionalized reduced graphene oxide with nitrogen and sulfur atoms (N,S–rGO), Zinc–Aluminum layered double hydroxides (ZnAl-LDH), and ZnAl-LDH/N,S–rGO hybrids in three weight ratios of 1:1, 1:3, and 1:5 (the weight ratio of N,S-rGO is 1) are synthesized by the hydrothermal method. The physical properties, morphology, and structure of the synthesized electrocatalysts are determined by using X-Ray Diffraction (XRD) analysis, Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-Ray Analysis (EDX), the Fourier Transform Infrared Spectroscopy (FTIR) and Raman analysis. Electrochemical measurements are implemented by using Cyclic Voltammetry (CV), Linear Scanning Voltammetry (LSV), and chronoamperometric. Also, the electron transfer number is calculated by K-L plot. The obtained results for all samples are compared with the %20 Pt/C commercial catalyst. Based on the results of the physical tests, in addition to the uniform distribution and the correct deposition of the synthesized electrocatalysts, the particle size also reached the nanometer range. According to the electrochemical results, among the synthesized electrocatalysts, the ZnAl-LDH/N,S–rGO with 1:1 wt ratio has the best electrochemical activity. This result indicates a well synergistic and interaction effect between N,S–rGO and ZnAl-LDH for the ORR. The onset potential is obtained to be −0.01 V vs Ag/AgCl. The average of electron transfer number by this electrocatalyst is 3.60, which indicates that it is close to the 4e pathway for the ORR. The electrocatalytic stability was favorable in the alkaline medium. It can be concluded that the Layered Double Hydroxides (LDHs) improve the electrical conductivity, the electrocatalytic activity, the active surface area, and the stability for the oxygen reduction reaction after the combination with carbon bases. To be clear, the combination of N,S-rGO and ZnAl-LDH with several concentrations has been investigated for the first time on the ORR applications. The sensitivity analysis is implemented to determine the optimal concentration. This study proposes a new approach for using N, S-rGO composite to improve the low electron conductivity of LDHs.     

NiCo alloy nanoparticles encapsulated in N-doped 3D porous carbon as efficient electrocatalysts for oxygen reduction reaction

A series of N-doped three dimensional porous carbons loaded with NiCo alloy nanoparticles (NiCo@NpCs) have been successfully fabricated by a template-assisted in-situ pyrolysis strategy and the ORR activities of different-concentration alloy supported N-doped carbons are systematically investigated. The optimized sample exhibits a positive half-wave potential of 0.78 V (vs. RHE), a high diffusion-limited current density (5.120 mA cm⁻²), and a high durability over 92%, which is superior to the Pt/C. The excellent activity and stability are mainly due to synergistic effect between carbon matrix and NiCo alloy. The N-doped porous carbon support with high surface area and good conductivity not only provides more active sites but also promotes electron transport and mass transfer process. Furthermore, the unique core-shelled structure NiCo@NpCs can effectively avoid the dissolution and corrosion of alloy particles and the surface peeling of particles during the catalyze reaction, which is beneficial to improving the activity and stability.     

Trimetallic PtNiCo nanoflowers as efficient electrocatalysts towards oxygen reduction reaction

Nanostructures of PtNiCo alloy have been prepared using a simple solvothermal process followed by annealing at higher temperature and studied for electrochemical oxygen reduction reaction (ORR) kinetics. PtNiCo/C catalyst has demonstrated an interesting trend of enhancement in the ORR activity along with long-term durability. The specific activity of 2.47 mA cm^?2 for PtNiCo-16h/C (PtNiCo/C prepared at reaction time of 16 h) is ～12 times higher than that of Pt/C (0.2 mA cm^?2). Further, X-ray diffraction, transmission electron microscopy and X-ray photo electron spectroscopy studies have been carried out systematically to understand the phase formation, morphology along with surface defects and elemental analysis respectively. The durability of the catalyst was evaluated over 10,000 potential cycles using standard triangular potential scan in the lifetime regime. Interestingly, after 10k durability cycles, PtNiCo-16h/C electrocatalyst showed enhanced ORR activity (32% higher activity; Im@10k cycles = 0.716 A mg_Pt^?1) and stability compared to commercial Pt/C signifying the retention of Ni and Co due to higher lattice contraction in PtNiCo alloy electrocatalyst.     

Direct synthesis of nitrogen and phosphorus co-doped hierarchical porous carbon networks with biological materials as efficient electrocatalysts for oxygen reduction reaction

We described a process of the preparation of N, P co-doped hierarchical porous carbon by one-step pyrolysis of the chitosan/phytic acid (CS/PA) precursor without extra activation processes, and the nitrogen and phosphorus were successfully incorporated into the carbon framework. Experimentally, the best performance was identified with NPC-1000 which possessed the highest BET specific surface area of 1117.2 m² g^?1. This NPC-1000 showed a half-wave potential of 50 mV difference with commercial Pt/C, better tolerance to methanol and a superior stability comparable to commercial Pt/C catalyst. The results suggest that it is a simple, feasible, and economical route to synthesis of hierarchical porous carbon which can be used as metal-free catalysts for oxygen reduction.     

Duckweed derived nitrogen self-doped porous carbon materials as cost-effective electrocatalysts for oxygen reduction reaction in microbial fuel cells

Cost-effective metal-free electrocatalysts for oxygen reduction reaction were incredible significance of improvement about microbial fuel cells. In this research, a novel nitrogen self-doped porous carbon material is effectively inferred with KOH activation from a natural and renewable biomass, duckweed. Self-doped nitrogen in carbon matrix of nitrogen-doped porous carbon at 800 °C provides abundant active sites for oxygen reduction and improves the oxygen reduction kinetics significantly. Moreover, the porous structure of nitrogen-doped porous carbon at 800 °C encourages the transition of electrolyte and oxygen molecules throughout the oxygen reduction reaction. Oxygen on the three-phase boundary is reduced to water according to a four-electron pathway on nitrogen-doped porous carbon electrocatalyst. The single-chamber microbial fuel cell with nitrogen-doped porous carbon as electrocatalyst achieves comparable power density (625.9 mW m⁻²) and better stability compared to the commercial Pt/C electrocatalyst. This simple and low-cost approach provides a straightforward strategy to prepare excellent nitrogen-doped electrocatalyst derived from natural and renewable biomass directly as a promising alternate to precious platinum-based catalysts in microbial fuel cells.     

Nanocrystaline tungsten carbide supported Au–Pd electrocatalyst for oxygen reduction

Au–Pd nanobimetallic particles supported on nanocrystaline tungsten carbide as electrocatalysts for oxygen reduction were prepared by an intermittent microwave heating (IMH) method. XRD measurement revealed that AuPd alloy formed during the IMH process. We showed these novel electrocatalysts could offer the activities that surpass that of the state-of-the-art Pt-based electrocatalysts for oxygen reduction reaction. The AuPd–WC/C electrode showed an over 70 mV shift towards more positive potentials compared to Pt/C electrode for ORR. The advantage seemed to come from the novel support of tungsten carbide which itself has the catalytic activity to enhance the catalytic activity of the metal electrocatalysts.     

Nitrogen and sulfur co-doped graphene supported PdW alloys as highly active electrocatalysts for oxygen reduction reaction

Nitrogen and sulfur co-doped graphene (NSG) is prepared by a facile microwave irradiation method and palladium-tungsten (PdW) alloy nanoparticles are supported on the NSG substrate. Several techniques, including X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry and scanning electrochemical microscopy etc. are used to characterize the physical and electrochemical properties of the as-prepared samples. It is found that the PdW alloy nanoparticles are uniformly dispersed on the surface of NSG and the electrochemical performance of PdW/NSG is much better than those of Pd/NSG and Pd/G. The reason for the improved electrochemistry performance of PdW/NSG is considered to be the strong interactions and synergetic effects between PdW nanoparticles and NSG.     

Structurally ordered PtFe intermetallic embedded in N-doped carbon as a highly active and durable electrocatalyst for oxygen reduction reaction

The structurally ordered PtM with surface coating layers strategy has drawn increasing attention. In this work, we synthesize a structurally ordered PtFe@NC-X-PDA catalyst modified with nitrogen-doped carbon coating layers by confined space annealing strategy. Compared with the current commercial Pt/C catalyst, the structurally ordered PtFe@NC-X-PDA catalyst shows better catalytic activity and stability. Especially, the mass activity and specific activity of the synthesized PtFe@NC-0.06-PDA sample with the optimized poly-dopamine feeding mass content (0.06 g) exhibit 9.95 and 11.53 times higher than that of commercial Pt/C catalyst. In addition, after 20,000 CV cycles, the PtFe@NC-0.06-PDA sample achieves the minimum activity loss (7%). The PtFe alloy catalyst with the different thickness NC shell (PtFe@NC-X-PDA) possesses the enhanced ORR activity and stability owing to the protection of nitrogen carbon shell (NC) and the strong electronic interaction of the ordered PtFe NPs. The improved ORR activity and stability of the structurally ordered PtFe@NC-X-PDA catalyst provide a promising direction for the development of fuel cells.     

Characterization of composite materials of electroconductive polymer and cobalt as electrocatalysts for the oxygen reduction reaction

Platinum-free electrocatalysts based on electroconductive polymer, modified with cobalt, were prepared and characterized for the oxygen reduction reaction (ORR). The carbon-supported materials were: carbon/polyaniline/cobalt, carbon/polypyrrole/cobalt and carbon/poly(3-methylthiophene)/cobalt. Also the corresponding cobalt-free precursors were studied. EDAX studies show that in cobalt-modified catalysts, significant percentages of cobalt, between 5 and 7% in weight, are present. FTIR, TGA, and EDAX studies confirmed that the addition of cobalt modifies the chemical structure of C–Pani, C–Ppy, and C–P3MT materials. Cyclic voltammetry shows reduction peaks corresponding to the ORR for all materials and kinetic parameters were calculated based on lineal voltammetry using RDE at different rotating speeds. It was found that C–P3MT–Co has highest exchange current densities, followed by C–Ppy and C–Ppy–Co. All samples have Tafel slopes between −110 and −120 V/dec, indicating that the first electron transfer is the decisive step in the global ORR. Potentiostatic tests showed an adequate stability of cobalt-modified samples in acid medium at ORR potentials. Based on the potential range at which ORR occurs, the exchange current density and stability tests, it is concluded that the best material for potential application as fuel cell cathode catalyst is C–Ppy–Co.     

Electrospun iron and nitrogen co-containing porous carbon nanofibers as high-efficiency electrocatalysts for oxygen reduction reaction

One-dimensional carbon nanofibers hold great promise to be potential candidates as high-efficiency electrocatalysts for oxygen reduction reaction (ORR). However, the catalysts in powder form always aggregate when preparing catalyst layer, which significantly hinders the extensive application. Herein, we report the facile synthesis of iron and nitrogen co-containing porous carbon nanofibers derived from PAN nanofibers existing as a flexible film via the electrospinning method, which could avoid aggregation when fabricating catalyst layer of fuel cells. The Fe–N/N–C NFs exhibit onset potential of 0.95 V and the half-wave potential of 0.78 V in 0.1 M KOH solution, suggesting superior electro-catalytic activity for ORR. Meantime, for Fe–N/N–C NFs catalyst, it shows a nearly four electron transferring process and the current density only decreases by 14.2% after 40,000 s. This easy and facile method provides a new idea for synthesis of oxygen reduction reaction with high-activity and good-stability.     

Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction

Novel nano-crystalline Fe₃PtN supported on nitrogen-doped carbon materials are synthesised via double pyrolysis, under Ar and NH₃ with two sets of temperatures namely, 800 and 900 °C. An improved catalytic activity has been observed in terms of higher values of onset and half-wave potentials, with larger kinetic currents and low hydrogen peroxide yields. The activity upon comparing with Pt/C with simultaneous experiments shows impressive results. Within the double annealed samples a comparative study has been done on the basis of active sites available for the oxygen reduction reactions. We have revealed the origin of its activity by intensively investigating the composition and the structure of the catalyst and their correlations with the electrochemical performance.     

Iron chelates as low-cost and effective electrocatalyst for oxygen reduction reaction in microbial fuel cells

Iron-chelated electrocatalysts for the oxygen reduction reaction (ORR) in a microbial fuel cell (MFC) were prepared from sodium ferric ethylenediamine-N,N′-bis(2-hydroxyphenylacetic acid) (FeE), sodium ferric diethylene triamine pentaacetic acid (FeD) supported on carbon Vulcan XC-72R carbon black and multi-walled carbon nanotubes (CNTs). Catalyst morphology was investigated by TEM; and the total surfaces areas as well as the pore volumes of catalysts were examined by nitrogen physisorption characterization. The catalytic activity of the iron based catalysts towards ORR was studied by cyclic voltammetry, showing the higher electrochemical activity of FeE in comparison with FeD and the superior performance of catalysts supported on CNT rather than on Vulcan XC-72R carbon black. FeE/CNT was used as cathodic catalyst in a microbial fuel cell (MFC) using domestic wastewater as fuel. The maximum current density and power density recorded are 110 (mA m⁻²) and 127 ± 0.9 (mW m⁻²), respectively. These values are comparable with those obtained using platinum on carbon Vulcan (0.13 mA m⁻² and 226 ± 0.2 mW m⁻²), demonstrating that these catalysts can be used as substitutes for commercial Pt/C.     

Highly microporous nitrogen doped graphene-like carbon material as an efficient fuel cell catalyst

Nitrogen-doped carbon materials are known to be promising candidates as oxygen reduction reaction electrocatalysts used in fuel cells. However, developing metal-free catalysts with high performance and stability still remains a big challenge. Herein we report a new route by using the Maillard reaction, to caramelize ribose in a dispersing salt matrix, followed by carbonization of this caramel to synthesize metal-free catalysts. This catalytic material has the morphology of microporous nitrogen doped graphene-like carbon, and a highest surface area of 1261 m² g^?1 with a large amount of micropores. Such microporous structure offers numerous defects that generate a large number of reactive sites. As a result, when used as the cathode materials in fuel cells, the fuel cell shows a high power density of 547 mW cm^?2 under 1.0 atm back pressure with good stability with only 12.5% loss after 250 h. Such catalyst has good performance in the class of metal-free oxygen reduction reaction catalysts, and is possible for commercial use.     

Nitrogen doped lotus stem carbon as electrocatalyst comparable to Pt/C for oxygen reduction reaction in alkaline media

The nitrogen doped carbon with high content of pyridine N and porous structure indicates high activity for oxygen reduction reaction (ORR). In this paper, nitrogen doped lotus stem carbon (N-LSC) with 6.3 at% of N (containing 52 at% of pyridine N) and porous structure is developed by using lotus stem as carbon source and dopamine hydrochloride as nitrogen source. The ORR activity, stability and methanol tolerance are characterized. The results show that the N-LSC has comparable activity to Pt/C, and much better methanol tolerance and stability than Pt/C. The porous structure and high content of pyridine N are believed to lead to the high ORR performances of the N-LSC.     

Nitrogen doped porous carbon with iron promotion for oxygen reduction reaction in alkaline and acidic media

Nitrogen doped water-hyacinth graphite with little iron (NFe-WHG) is synthesized by using water hyacinth as carbon source, dopamine hydrochloride as N source and Fe(NO₃)₃ as Fe source. The water hyacinth is carbonized to porous carbon; the addition of Fe increases pore diameter, graphitization degree, total N and pyridinic N content. The characterizations indicate that the doping N contributes great on ORR activity, yet the residual Fe species themselves show inconspicuous catalytic effect on ORR. The NFe-WHG with the above features displays superior ORR activity in alkaline media and comparable ORR activity to commercial Pt/C in acidic media. Due to the graphite matrix and that most of the Fe species have been removed, the NFe-WHG shows excellent stability in both alkaline and acidic media with excellent anti-methanol and anti-CO performances.     

Recent progress in heteroatom doped carbon based electrocatalysts for oxygen reduction reaction in anion exchange membrane fuel cells

Oxygen reduction reaction (ORR) is a key step in many electrochemical devices such as fuel cells and metal-air batteries. However, the reaction proceeds at a significant overpotential requiring Pt-based catalysts. The scarcity and economical challenges associated with Pt is one of the major limitations for the commercialization of the devices. In this context, the electrochemical research community is constantly exploring other low-cost and earth-abundant materials as ORR catalysts. Carbon nanomaterials are identified as promising electrocatalysts due to their superb electronic conductivity together with high specific surface area. However, the low reactivity of carbon is the major limiting factor in the fabrication of ORR catalysts. Recent studies have proved that chemical modification of the carbon network (substitution of foreign atoms, Ex: N, S, B, F, P) could alter the reactivity of carbon nanomaterials for ORR. Many doping strategies have been proposed including single atom doping, co-doping and multi-atom doping. The heteroatom doped carbons have delivered promising results towards ORR in alkaline media. This review presents a rational approach of doping methods and the electrochemical properties of heteroatom doped carbons, and we believe that this review could be a guiding material to design advanced non-noble catalysts for ORR in the coming future.     

A noble silver nanoflower on nitrogen doped carbon nanotube for enhanced oxygen reduction reaction

An electrodeposition-approach for the synthesis of silver nanoflowers (AgNFs) on nitrogen doped carbon nanotubes (NCNTs) for the oxygen reduction reaction (ORR) in alkaline media has been developed. The as prepared material (NCNTs-AgNFs) has been characterized by various instrumental methods. The morphological analysis shows the unique rose-like AgNFs are placed onto the NCNTs with better dispersion. The higher population of AgNFs has also been observed onto NCNTs coated glassy carbon (GC) rather than bare GC plate. The X-ray photoelectron spectroscopy shows chemical reduction and N-doping has done successfully with the restoring sp² domain in carbon network. The electrocatalytic activities have been verified using cyclic voltammetry (CV) and hydrodynamic voltammetry techniques in 0.1 M KOH electrolyte. The resulting catalyst system, NCNT-AgNFs, surpasses the performance of Pt/C, in terms of a kinetic current density, better fuel selectivity and durability. It is also noteworthy that the NCNT-AgNFs exhibits a four-electron reduction pathway for ORR with lowering H₂O₂ yield. The admirable performance of NCNT-AgNFs catalyst along with higher durability holds great potential for application in various fuel cells as cathode catalyst.