N-doped Carbon Nanotubes Research Articles

Effect of nitrogen species on electrochemical properties of N-doped carbon nanotubes derived from co-pyrolysis of low-density polyethylene and melamine

Heteroatoms in carbon nanotubes can alter their electrochemical properties. Herein, N-doped carbon nanotubes (NCNTs) with gradient doping of graphitic-N, pyridinic-N and pyrrolic-N were prepared by the pyrolysis of different ratios of low-density polyethylene (LDPE) and melamine. The lower fraction of melamine in the feedstock favored the generation of pyridinic-N and graphitic-N with lower formation energies, while the higher fraction favored the generation of pyrrolic-N with higher formation energies. Furthermore, the quantum capacitance theory demonstrated that pyridinic-N had the best energy storage effect at negative potential, and pyrrolic-N had the best energy storage effect at positive potential. The specific capacitance of the 0.6-NCNTs increased by 55 % compared to the undoped sample (1 A g−1, 1 M H2SO4), which was due to the presence of a large amount of pyrrolic-N on its surface. However, as the nitrogen content of the 0.6-NCNTs increased further, the samples suffered from a decrease in specific surface area, resulting in a decrease in specific capacitance thereafter. This paper reveals the effect of different N-doped types on the electrochemical properties of NCNTs, and provides theoretical guidance for adjusting the electrochemical properties of materials in energy storage applications.

Elucidation of the electrocatalytic activity origin of Fe3C species and application in the NOx full conversion to valuable ammonia

Conversion of nitrogen oxides (NOx) into ammonia mediated by electrochemical nitrate/nitrite reduction is an ideal route for the treatment and utilization of NOx pollutants. Herein, pure-phase Fe3C nanoparticle supported on N-doped carbon nanotubes catalysts exhibit an NH3 yield rate of 1034.9 μmol h−1 cm−2 with a Faradaic efficiency of 97.3% for NO3− reduction reaction. The excellent intrinsic activity of Fe3C species is attributed to a superior ability to adsorb and activate NO3− and a proper ability to activate protons. The electrochemical in situ FTIR spectra, in situ Raman spectra and density functional theory calculations reveal the reaction process follows a novel multiple reaction pathway. Eventually, a demonstration device is proposed for a full conversion process from NOx to ammonia synthesis including NOx collection, electrochemical nitrate/nitrite reduction and ammonia collection, which made it possible to integrate the green ammonia synthesis system with distributed nitrogen oxide discharge sites.

Separator modified by a caterpillar-like composite with interconnected N-doped carbon nanotubes decorated Co-MnO heterointerface enabling robust polysulfide adsorption and catalytic conversion for Li-S batteries

To date, realizing satisfactory-performance Li-S batteries (LSBs) by rationally engineering the multifunctional material with good electric conductivity, large specific surface area, and efficient adsorption-diffusion-conversion of lithium polysulfide (LiPSs) remains a challenge. Herein, a self-template and melamine-assisted catalytic growth strategy is developed for the controllable fabrication of a caterpillar-like composite constructed by interwoven N-doped carbon nanotubes (NCNTs) in-situ growth on the Co-MnO backbone. In this architecture, interconnected NCNTs extending outward from the core nanowire can offer sufficient conducting pathways to improve electron transfer and can significantly improve the total surface area of the composite, thus enhancing the contact between the catalyst and LiPSs, facilitating the uniform deposition of Li2S, and suppressing the shuttle effect by physicochemical adsorption of LiPSs. In addition, the experiment and theoretical calculation demonstrated that the Co-MnO heterostructure could significantly promote the “adsorption-diffusion-conversion” process of LiPSs over that of each single-component the Cobalt and MnO, which efficiently enhances the sulfur utilization and reaction kinetics. As a result, the LSB with a Co-MnO/NCNTs modified separator achieves a high capacity (902 mAh g−1 at 1C), enhanced cycling performance (an average capacity fading rate of 0.044% per cycle during overall 800 cycles), and outstanding rate performance (449 mAh g−1 at 5C).

Controlled growth of highly active NiMoN(100)-decorated porous N-doped carbon nanotubes on carbon cloth as efficient electrodes for alkaline media and seawater electrolysis

The design of a cost-efficient, high-performance electrocatalyst for the hydrogen evolution reaction (HER) is paramount for meeting future energy demands. Herein, we report an effective strategy for growing highly active NiMoxN with preferentially exposed (100) planes decorated with porous N-doped carbon nanotubes (PNCTs) grown on carbon cloth (CC) using 3-amino-1,2,4-triazole (AT) as a nitridation agent. The effects of different nitridation agents (AT and dicyandiamide (DA)) and Mo-to-Ni molar ratios were systematically investigated to optimize the exposed active sites, geometric properties, electronic structures, and chemical composition of NiMoxN. Density functional theory (DFT) calculations revealed that the free energy of H* adsorption (ΔGH*) of the NiMoxN(100) plane was 0.31 eV, which renders it the most active plane among the (101) and (001) planes of NiMoxN. The as-prepared bamboo-like NiMoxN/PNCT@CC exhibited excellent HER performance in the electrolysis of alkaline media and seawater in terms of extremely small η10 values of 52 mV (1 M KOH) and 146 mV (seawater), which are similar to those of Pt/C (η10 = 38 and 129 mV, respectively). The outstanding HER performance originates from the synergistic effects between the highly active NiMoxN(100) plane and PNCTs, which led to a very small ΔGH* of 0.05 eV. This study provides a method for preparing high-performance NiMoxN catalysts.

Novel self-sustained flow-through electrochemical advanced oxidation processes using nano-Fe0 confined B, N co-doped carbon nanotubes/graphite felt cathode: Higher mineralization efficiency under wider pH

To improve the mineralization efficiency of pollutants under wide pH ranges by self-sustained electrochemical advanced oxidation processes (EAOPs) without any reagent addition, a flow-through system based on the bifunctional cathode of B, N-doped carbon nanotubes embedded with nano-Fe0 particles that loaded on graphite felt (Fe@BN-C/GF) was developed. Dominant by 1O2 oxidation, the sulfamethazine (SMT) degradation rate and TOC removal in this recirculated flow-through system were 2.6 ∼ 3.3-folds and 1.4 ∼ 2.4-folds that of the batch system under the pH ranges of 3 ∼ 11, respectively. The SMT degradation performance was found not affected in the presence of HCO3–, NO3–, Cl-, K+, or humic acid (HA). The Fe@BN-C/GF cathode had more potential to treat enlarged wastewater by 5-folds during 10 h operation, and the SMT and TOC removal was 99% and 70.31%, respectively. The total phosphorus (TP), ammonia nitrogen (NH3-N) and TOC in reverse osmosis concentrate and secondary wastewater effluent could be simultaneously removed to <0.01 mg L-1 and 8 mg L-1 by this flow-through system with a low electric energy consumption (EEC) of 0.022 and 0.18 kWh gTOC-1, respectively. This EAOPs based on the flow-through Fe@BN-C/GF cathode was self-sustained, and had very promising application prospect of high mineralization of pollutants even for low-conductivity wastewater.

1,2,4-triazole-assisted metal-organic framework-derived nitrogen-doped carbon nanotubes with encapsulated Co4N particles as bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries

The design of high-performance oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) dual-functional catalysts is not only important for the further applications of zinc-air batteries (ZABs) but also a major challenge in the field of energy conversion. The cheap 1,2,4-triazole (1,2,4-TZ) can be decomposed easily by heat, making it a high research value in carbon catalysts derived from metal-organic frameworks (MOFs). Here, Co4N particles encapsulated at the top of N-doped carbon nanotubes (Co4N@NCNTs) were conveniently prepared by 1,2,4-TZ-assisted pyrolysis of Co-MOF-74 for the first time. Owing to the excellent activity of Co4N particles and the highly graphitized N-doped carbon nanotubes (NCNTs), Co4N@NCNTs obtained at 900 °C (Co4N@NCNT-900) exhibited astonishing catalytic performance in both ORR and OER, and high reversible oxygen bifunctional activity (ΔE = 0.685 V). Moreover, Co4N@NCNT-900 displayed a larger discharge power density (122 mW cm−2), a better specific capacity (811.8 mAh g−1), and more excellent durability during the ZAB test, implying that Co4N@NCNT-900 can act as a bifunctional high active catalyst in ZABs.

N-doped carbon nanotubes enhanced charge transport between Ni nanoparticles and g-C3N4 nanosheets for photocatalytic H2 generation and 4-nitrophenol removal

Carbon nanotubes with well conductivity can improve photogenerated carrier separation and transport for enhanced photocatalytic performance. In this paper, Ni nanoparticles embedded N-doped carbon nanotubes (CNTs) were decorated on superior thin g-C3N4 nanosheets as a co-catalyst via a mechano-chemical pre-reaction and thermal polymerization at high temperature. The admirable conductivity and unique one-dimensional structure of Ni-decorated CNTs (NiCNTs) generated more active sites and supplied efficient charge transfer. Abundant unpaired electron and π-conjugated structure of N-doped CNTs promoted the delocalization, retarded recombination, and stabilized charge separation. Ni nanoparticles acted as active sites to trap photogenerated electrons for photoreduction reaction. g-C3N4/NiCNTs composites with an optimized ratio revealed a hydrogen (H2) generation rate as high as 1050.4 μmol g−1 h−1, exhibiting an increase of ∼137 times compared with pure g-C3N4 without using Pt as co-catalyst. The H2 generation rate was even higher than that of g-C3N4/Pt, suggesting that NiCNTs can substitute noble metals completely. In addition, the g-C3N4/NiCNTs sample removed 4-nitrophenol (4-NP) within 10 min under visible light irradiation. The rate constant of g-C3N4/NiCNTs sample was ∼49 times of that of pristine g-C3N4. This work proposed a novel bifunctional catalyst for pollutant treatment and clean energy production.

Tellurium filled carbon nanotubes cathodes for Li-Te batteries with high capacity and long-term cyclability

Li-Te batteries, compared with traditional lithium-ion batteries and Li-other group VIA elements (including O, S, Se) batteries, have shown overwhelming features of high specific volumetric capacity and superior electrical conductivity. These features make the Li-Te batteries possess great importance in modern portable electronics and electric vehicles with limited battery package size. However, the Achilles' Heel in Li-Te batteries is the huge volume change and inevitable dissolution of polytellurides during cycling. Herein, we synthesize a cathode material based on polycrystalline Te encapsulated in N-doped multiwall carbon nanotubes (Te-filled CNTs) by physical vapor transport (PVT) method. With unique spatial restriction of the CNT hosts, the electrochemically active Te content can be well preserved, and the prepared Te-filled CNTs cathodes deliver a high specific capacity of 590 mAh g−1 based on Te content. More importantly, the reaction kinetics and electrochemical reversibility of the cathodes have been significantly improved by using nanoscale cavities (4–6 nm) which stabilize the polytellurides (Li2Ten). Meanwhile, a series of in situ characterizations are carried out to straightforwardly reveal a two-step discharge/charge mechanism for Te-filled CNTs. In addition, theoretical calculations prove that the encapsulated Te ensure a lower energy barrier for lithiation compared with bare CNT. This work sheds light on the nanoconfinement effect of CNT for improving the utilization and cycling stability of Te based cathodes in Li-Te batteries.

A new synthesis of O/N-doped porous carbon material for supercapacitors

In this work, a novel carbonization method using H2SO4 as dehydrating agent to prepare graphitized carbon material with O doping is firstly reported. Starch is used as the carbon source. After dehydration in H2SO4 at 100 °C for 6 h, an O-enriched carbon material (OSC) is prepared. It exhibits a specific capacitance of 354 F g−1 at the current density of 1 A g−1 in 1 M H2SO4 solution. After 20,000 cycles, the capacitance retention remains at 110.2 %. It is proved that appropriate amounts of doping O can provide a certain pseudocapacitance, and also improve the wettability of the material, which therefore improve the electrochemical performance of the carbon material. Further, a vapor deposition method using dicyandiamide is applied to grow N-doped carbon nanotubes on the smooth surface of the OSC, which form a three-dimensional conductive network structure on the surface and get a suitable porous structure with the specific surface area of 355 m2/g (NOSC). The specific capacitance of the NOSC is 759 F g−1 at the current density of 1 A g−1 in 1 M H2SO4 solution. After 10,000 cycles, the capacitance retention still remains at 95.8 %. The assembled all-solid symmetric supercapacitor based on the NOSC can be operated in a 0–1.8 V voltage window. The energy density is 41.12 Wh kg−1 with a power density of 449.95 W kg−1.

Schiff-base polymer derived ultralong FeCo/N-doped carbon nanotubes as bifunctional oxygen electrocatalyst for liquid and flexible all-solid-state rechargeable zinc–air batteries

Controllable designing of robust and cost-effective bifunctional electrocatalysts toward oxygen reduction/evolution reaction (ORR/OER) is essential to zinc-air batteries (ZABs) but still a challenge. Herein, a promising strategy is proposed to construct a bifunctional oxygen electrocatalyst composed of FeCo alloy nanoparticles embedded in ultralong N-doped carbon nanotubes (FeCo-LCNT). The in-situ catalytic generated FeCo-LCNT is yielded via the condensation polymerization of 2,5-dihydroxy-p-benzoquinone and tripolycyanamide, followed by the coordination with Fe3+/Co2+ ions and subsequent pyrolysis. The as-obtained FeCo-LCNT manifests improved bifunctional activity and durability in comparison with commercial Pt/C and IrO2. The experimental results and DFT calculations demonstrate that the interaction between alloy nanoparticles and CNT and the presence of N-doped carbon species are of crucial importance in enhancing the catalytic activities of the FeCo-LCNT. Impressively, the FeCo-LCNT can serve as air-electrode catalyst, endowing the assembled rechargeable liquid and flexible all-solid-state ZABs with good high-rate performance, long cyclic life (for the former) and enhanced stability at various bending angles (for the latter).

Catalytic CO2 Capture via Ultrasonically Activating Dually Functionalized Carbon Nanotubes.

High energy consumption and high cost have been the obstacles for large-scale deployment of all state-of-the-art CO2 capture technologies. Finding a transformational way to improve mass transfer and reaction kinetics of the CO2 capture process is timely for reducing carbon footprints. In this work, commercial single-walled carbon nanotubes (CNTs) were activated with nitric acid and urea under ultrasonication and hydrothermal methods, respectively, to prepare N-doped CNTs with the functional group of -COOH, which possesses both basic and acid functionalities. The chemically modified CNTs with a concentration of 300 ppm universally catalyze both CO2 sorption and desorption of the CO2 capture process. The increases in the desorption rate achieved with the chemically modified CNTs can reach as high as 503% compared to that of the sorbent without the catalyst. A chemical mechanism underlying the catalytic CO2 capture is proposed based on the experimental results and further confirmed by density functional theory computations.

A novel electrochemical sensor based on mesoporous carbon hollow spheres/ZIF-67-derived Co-embedded N-doped carbon nanotubes composite for simultaneous determination of dihydroxybenzene isomers in environmental water samples

Dihydroxybenzene isomers were environmental pollutants which were potential threat to human health. Accurately and rapidly detection of them is very valuable. In this study, a dihydroxybenzene isomer sensor based on mesoporous carbon hollow spheres/zeolitic imidazolate frameworks (ZIF)-derived Co-embedded N-doped CNTs (MCHSs/Co@N-CNTs) was proposed. The MCHSs/Co@N-CNTs, with excellent conductivity and electrocatalytic, presented excellent electrochemical simultaneous detection for phenolic compounds. Under optimal conditions, the MCHSs/Co@N-CNTs/GCE showed wide linear range (CC: 2.5 μM-100 μM, HQ: 1.0 μM −150 μM, RS: 20 μM −1000 μM) and the detection limit (LOD) of CC, HQ and RS were 0.46, 0.27 and 4.21 μM (S/N = 3), respectively. Additionally, the reaction sites and mechanism of phenolic compounds were revealed by analyzing the electrochemical behavior with the help of density functional calculation. Moreover, the sensor showed satisfactory results in the determination of phenolic compounds in environmental water samples.

Rational Design and Engineering of 1D Heterostructured Porous Sn/CoSnx@C Nanotubes for Superior Lithium Storage

Tin is a good anode material for lithium storage because of its high theoretical capacity and good conductivity, but large volume changes during the charging/discharging process lead to poor rate performance and cycling stability. In this work, by assembling ZIF-67 nanosheets on SnO2@PDA nanotubes and following a calcination process, heterostructured Sn/CoSnx (x = 1, 2) alloy anchored N-doped porous carbon nanotubes with high lithium storage performance were rationally designed and successfully prepared (Sn/CoSnx@C). The Co incorporated in CoSnx intermetallic can buffer the internal stress, and combined with the porous structure, the large volume expansion can be effectively alleviated. Besides, coating the ultrafine Sn/CoSnx crystals within one-dimensional N-doped carbon can inhibit particle agglomeration, thus enhancing cyclic stability. Moreover, benefiting from the porous tubular structure that can shorten the mass/charge transport distance, the generated abundant heterointerfaces can promote reaction kinetics, achieving improved rate capacity. Therefore, the tubular structured Sn/CoSnx@C anode shows a high reversible specific capacity of 1713.2 mAh g–1 at a current density of 100 mA g–1, a high rate performance of 1394.1 mAh g–1 at 1.0 A g–1 and 1051.3 mAh g–1 at 5.0 A g–1, and an excellent cycling stability of 444.3 mAh g–1 at 5 A g–1 over 5000 cycles. These results demonstrate an effective strategy for developing high-performance metal alloy-based electrodes in the energy storage system.

Electrochemical sensors based on platinum-coated MOF-derived nickel-/N-doped carbon nanotubes (Pt/Ni/NCNTs) for sensitive nitrite detection.

As excess nitrite has a serious threat to the human health and environment, constructing novel electrochemical sensors for sensitive nitrite detection is of great importance. In this report, platinum nanoparticles were deposited on nickel-/N-doped carbon nanotubes, which were obtained through a self-catalytically grown process with Ni-MOF as precursors. The as-prepared Pt/Ni/NCNTs were applied as amperometric sensors and presented superior sensing properties for nitrite detection. Benefiting from the synergy of Pt and Ni/NCNTs, Pt/Ni/NCNTs displayed much wider detection ranges (0.5-40mM and 40-110mM) for nitrite sensing. The sensitivity is 276.92μAmM-1cm-2 and 224.39μAmM-1cm-2, respectively. The detection limit is 0.17μM. The Pt/Ni/NCNTs sensors also showed good feasibility for nitrite sensing in real samples (milk and peach juice) analysis. The active Pt/Ni/NCNTs composites and facile fabrication technique may provide useful strategies to develop other sensitive nitrite sensors.

N-doped carbon hollow spheres supported N-doped carbon nanotubes for efficient electromagnetic wave absorption

Hierarchical and hollow heteronanostructures had attached wide attention in electromagnetic wave (EMW) absorption fields due to their adjustable impedance matching and loss property. However, fabrication of well-defined heteronanomaterials with hierarchical and hollow features is challenging. Here, we develop a simple method to prepare N-doped carbon hollow spheres supported by N-doped carbon nanotubes (Ni@NCNT/NCHSs). Due to the hierarchical and hollow features, the Ni@NCNT/NCHSs exhibit high-efficiency EMW absorption performance. With a thickness only 1.6 mm, the optimized heteronanostructure has an RLmin of −64.75 dB and an EAB of 4.17 GHz. The experimental results demonstrate that the hierarchical and hollow heteronanostructures have significantly enhanced dielectric loss and conductive loss of the Ni@NCNT/NCHSs as well as the impedance matching characteristics, which are responsible for their enhanced EMW absorption performances. This work presents an effective method to synthesize hierarchical and hollow heteronanostructures for high-performance EMW absorption.

Growth in situ of NiSe2/Ni hybrids in N-doped carbon nanotubes for enhanced overall water splitting

Interface engineering plays an important role to improve the performance of heterostructures. Growth in situ makes the heterostructures with ideal interface natures. Here, the growth in situ of Ni components occurred on N-doping carbon nanotube (Ni-N-CNT) by one-step thermal polymerization for excellent overall water splitting. NiSe2/Ni-N-CNT with well-developed interface was created by selenium treatment of Ni-N-CNT, which improved the carrier transfer efficiency and increased electrochemical active sites. The formation of NiSe2/Ni hybrids improved hydrogen evolution reaction (HER). Ni and Se sites were proved in different electrolytes. The overpotential of NiSe2/Ni-N-CNT at -10 mA·cm−2 was 196 and 220 mV, and Tafel slope is 52 and 70 mV/dec in acidic and alkaline solutions, respectively. NiSe2/Ni-N-CNT revealed well kinetic effect compared with other control samples both in HER and oxygen evolution reaction (OER). The voltage of the electrolytic cell with NiSe2/Ni-N-CNT||NiSe2/Ni-N-CNT electrode was 1.757 and 1.954 V at 10 and 40 mA·cm−2, respectively. NiSe2/Ni-N-CNT as an electrolytic cell electrode was stable for efficient overall water splitting after 9.5 h. The formation of NiSe2/Ni-N-CNT heterostructures supplied a utilizable approach for improving HER and OER to get highly efficient catalysts.

3D Hierarchical-Architectured Nanoarray Electrode for Boosted and Sustained Urea Electro-Oxidation.

Exploring active and durable Ni-based materials with optimized electronic and architectural engineering to promote the urea oxidation reaction (UOR) is pivotal for the urea-related technologies. Herein a 3D self-supported hierarchical-architectured nanoarray electrode (CC/MnNi@NC) is proposed in which 1D N-doped carbon nanotubes (N-CNTs) with 0D MnNi nanoparticles (NPs) encapsulation are intertwined into 2D nanosheet aligned on the carbon cloth for prominently boosted and sustained UOR electrocatalysis. From combined experimental and theoretical investigations, Mn-alloying can regulate Ni electronic state with downshift of the d-band center, facilitating active Ni3+ species generation and prompting the rate-determining step (*COO intermediate desorption). Meanwhile, the micro/nano-hierarchical nanoarray configuration with N-CNTs encapsulating MnNi NPs can not only endow strong operational durability against metal corrosion/agglomeration and enrich the density of active sites, but also accelerate electron transfer, and more intriguingly, promote mass transfer as a result of desirable superhydrophilic and quasi-superaerophobic characteristics. Therefore, with such elegant integration of 0D, 1D and 2D motifs into 3D micro/nano-hierarchical architecture, the resulting CC/MnNi@NC can deliver admirable UOR performance, favorably comparable to the best-performing UOR electrocatalysts reported thus far. This work opens a fresh prospect in developing advanced electrocatalysts via electronic manipulation coupled with architectural engineering for various energy conversion technologies.

CoNi nanocrystal anchoring on MOF derived carbon skeleton autocatalytic growth N-doped carbon nanotubes for efficient bifunctional electrocatalyst towards methanol oxidation/oxygen reduction

In this paper, a novel bifunctional catalyst consisting of nitrogen doped carbon nanotubes (NCNTs) in situ grafted on CoNi-MOF derivative (named CoNi@NC/NCNTs) is controllable synthesized by simple two-steps. Characterization analysis reveal that the interconnected three-dimensional CoNi@NC-NCNTs heterostructure not only atomically dispersed CoNi nanocrystalline, but also autocatalytic grow NCNTs on its surface. The size of CoNi alloy is less than 10 nm, and the crystallization degree of coated graphene carbon layer is extraordinary high. Especially, NCNTs wrapped on CoNi@NC in dihydrodiamine vapour atmosphere form a spatial conductive network, which make the optimized CoNi@NC/NCNTs-400 deliver faster catalytic kinetics toward methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) compared with the CoNi@NC and Co@NC. The peak current density is as high as 58.37 mA cm−2 at the scanning rate is 30 mV s−1 in 0.5 M KOH + 1 M CH3OH solution. Furthermore, the CoNi@NC/NCNTs-400 hybrids display excellent ORR catalytic efficiency in terms of half-wave potential (E1/2 = 0.810 V vs RHE) and high stability. The satisfactory electrocatalytic activity is ascribed to the synergetic effect between CoNi nanoparticles embedded in nitrogen-doped carbon framework and NCNTs, which includes the high conductivity, highly dispersed active sites, optimized electronic configuration and reaction pathways.

Self-catalyzed Co, N-doped carbon nanotubes-grafted hollow carbon polyhedrons as efficient trifunctional electrocatalysts for zinc-air batteries and self-powered overall water splitting

It is still essential and challenging to explore inexpensive and versatile electrocatalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), for the development of rechargeable zinc-air batteries (ZABs) and overall water splitting. Herein, a rambutan-like trifunctional electrocatalyst is fabricated by re-growth of secondary zeolitic imidazole frameworks (ZIFs) on ZIF-8-derived ZnO and the following carbonization treatment. Co nanoparticles (NPs) are encapsulated into N-doped carbon nanotubes (NCNT) grafted N-enriched hollow carbon (NHC) polyhedrons to form the Co-NCNT@NHC catalyst. The strong synergy between the N-doped carbon matrix and Co NPs endows Co-NCNT@NHC with trifunctional catalytic activity. The Co-NCNT@NHC displays a half-wave potential of 0.88 V versus RHE for ORR in alkaline electrolyte, an overpotential of 300 mV at 20 mA cm−2 for OER, and an overpotential of 180 mV at 10 mA cm−2 for HER. Impressively, a water electrolyzer is successfully powered by two rechargeable ZABs in series, with Co-NCNT@NHC as the 'all-in-one' electrocatalyst. These findings are inspiring for the rational fabrication of high-performance and multifunctional electrocatalysts intended for the practical application of integrated energy-related systems.

Local hydroxyl enhancement design of NiFe sulfide electrocatalyst toward efficient oxygen evolution reaction

Herein, a local OH- reactant concentration enhancement design is proposed towards efficient oxygen evolution reaction (OER) electrocatalyst. In this strategy, N-doped carbon nanotubes (NCNT), serving as conductive OH- concentration enhancement matrix, are introduced near the adjacent Fe-NiS2 catalyst to accelerate OER reaction by promoting the local mass transfer process. This OH- enhancement effect is originated from the electropositive OH- absorption sites of NCNT and local-electronic field induced by conductive three-dimensional NCNT network that synergistically works with adjacent Fe-NiS2 nanocatalyst. Accordingly, the Fe-NiS2/NCNT catalyst only requires low overpotential of 247 mV at 100 mA cm−2, outperforms most of recently reported OER catalysts. The anion-exchange membrane water electrolyzer reaches 500 mA cm−2 at 1.70 Vcell. Analysis also reveals that, this enhanced local concentration design promotes the conversion of Fe-NiS2 into S, Fe co-doped NiOOH, which is the real OER active component. This work provides an alternative strategy to develop high-efficiency OER electrocatalysts.