Diffusion-limited Current Density Research Articles

Preparation of cathode catalysts for efficient direct lignin fuel cells by nitrogen doping reduction of oxidized graphene with phthalocyanine iron and Kraft lignin

Direct lignin fuel cells (DLFC) are one of the important forms of high value-added utilization of lignin. In this study, lignin was studied not only as a fuel but also as a catalyst. Specifically, Kraft lignin was modified with ZnCl2, KOH and THF (Tetrahydrofuran) respectively, and added to the catalyst after activation. The results of scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectrometer (EDS), Brunauer − Emmett − Teller (BET), X-ray photoelectron spectroscopy (XPS), X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FT-IR) and Raman spectra shown that AL/FePc-NrGO (activated lignin/iron phthalocyanine/nitrogen-doped reduction of graphene oxide) three-dimensional composite catalyst has been synthesized. The results showed that KOH-activated Kraft lignin had the best performance as an oxygen reduction reaction (ORR) catalyst, with a half-wave potential (E1/2) of 0.73 V and a limiting diffusion current density of 4.3 mAcm−1. The THF-modified catalyst showed similar stability and methanol resistance to 20 % Pt/C at ORR. The ORR catalyst applied to the DLFC has the best electrical performance with an open circuit voltage (OCV) was 0.53 V and the maximum power density it could reach 95.29 mWm−2 when the catalyst was modified with THF. It is encouraging that the AL/FePc-NrGO catalyst has better-generated electricity performance than 20 % Pt/C. This work has provided a new idea for developing non-noble metal catalysts and studying direct biomass liquid fuel cells.

Synthesis of NbC@C(Nx) nanoparticles using DC arc discharge plasma for highly efficient oxygen reduction reaction

Development of the low-cost, high-efficiency, and durable non-precious metal electrocatalysts for cathodic oxygen reduction reaction (ORR) is of vital importance for fuel cells and metal-air batteries. Herein, the carbon-coated NbC nanoparticles (NbC@C NPs) were prepared using DC arc discharge plasma under a methane atmosphere. Then, the nitrogen-doping was conducted by heat treatment, resulting in nitrogen-doped carbon-coated NbC nanoparticles (NbC@C(Nx) NPs). Benefiting from the merit of unique core-shell structure and best synergy between the nitrogen-doped carbon shell and the NbC core, the obtained highest nitrogen-doped NbC@C(N1.36) NPs showed an onset potential (Eonset) of 0.89 V (vs. RHE) and a half-wave potential (E1/2) of 0.79 V, which are comparable to those of commercial Pt/C (20 wt%) (Eonset = 0.90 V, E1/2 = 0.80 V), and its diffusion-limited current density (JL) of 5.99 mA·cm−2 is even higher than that of commercial Pt/C (5.43 mA·cm−2). Moreover, it exhibited a remarkable long-term durability and impressive methanol resistance than Pt/C catalyst. In addition, the real-time optical emission spectrum (OES) analysis of arc plasma shows that the electron temperature (Te) is 7562.9 K in local thermal equilibrium (LTE) plasma. Therefore, the use of the DC arc discharge plasma method opens a fast and simple way to develop cost-effective and highly efficient non-precious metal catalysts for ORR.

Nanoengineering of Porous 2D Structures with Tunable Fluid Transport Behavior for Exceptional H2O2 Electrosynthesis.

Precision nanoengineering of porous two-dimensional structures has emerged as a promising avenue for finely tuning catalytic reactions. However, understanding the pore-structure-dependent catalytic performance remains challenging, given the lack of comprehensive guidelines, appropriate material models, and precise synthesis strategies. Here, we propose the optimization of two-dimensional carbon materials through the utilization of mesopores with 5-10 nm diameter to facilitate fluid acceleration, guided by finite element simulations. As proof of concept, the optimized mesoporous carbon nanosheet sample exhibited exceptional electrocatalytic performance, demonstrating high selectivity (>95%) and a notable diffusion-limiting disk current density of -3.1 mA cm-2 for H2O2 production. Impressively, the electrolysis process in the flow cell achieved a production rate of 14.39 mol gcatalyst-1 h-1 to yield a medical-grade disinfectant-worthy H2O2 solution. Our pore engineering research focuses on modulating oxygen reduction reaction activity and selectivity by affecting local fluid transport behavior, providing insights into the mesoscale catalytic mechanism.

Synthesis of MnOOH and its application in a supporting hexagonal Pd/C catalyst for the oxygen reduction reaction.

With the expansion of global energy problems and the deepening of research on oxygen reduction reaction (ORR) in alkaline media, the development of low cost and high electrocatalytic performance catalysts has become a research hotspot. In this study, a hexagonal Pd-C-MnOOH composite catalyst was prepared by using the triblock copolymer P123 as the reducing agent and protective agent, sucrose as the carbon source and self-made MnOOH as the carrier under hydrothermal conditions. When the Pd load is 20% and the C/MnOOH ratio is 1 : 1, the 20% Pd-C-MnOOH-1 : 1 catalyst obtained by the one-step method has the highest ORR activity and stability in the alkaline system. At 1600 rpm, the limiting diffusion current density and half-wave potential of the 20% Pd-C-MnOOH-1 : 1 electrocatalyst are -4.78 mA cm-2 and 0.84 V, respectively, which are better than those of the commercial 20%Pd/C catalyst. According to the Koutecky-Levich (K-L) equation and the linear fitting results, the electron transfer number of the 20%Pd-C-MnOOH-1 : 1 electrocatalyst for the oxygen reduction reaction is 3.8, which is similar to that of a 4-electron process. After 1000 cycles, the limiting diffusion current density of the 20%Pd-C-MnOOH-1 : 1 catalyst is -4.61 mA cm-2, which only decreases by 3.7%, indicating that the 20%Pd-C-MnOOH-1 : 1 catalyst has good stability. The reason for the improvement of the ORR performance of the Pd-C-MnOOH composite catalyst is the improvement of the conductivity of the carbon layer formed by original carbonization, the regular hexagonal highly active Pd particles and the synergistic catalytic effect between Pd and MnOOH. The method of introducing triblock copolymers in the synthesis of oxides and metal-oxide composite catalysts is expected to be extended to other electrocatalysis fields.

PREDICTION OF ELECTROCHEMICAL PROCESSES OF LOCAL DISSOLUTION OF STAINLESS STEELS. PART 1. SPECTRAL –FREQUENCY ANALYSIS OF POTENTIAL FLUCTUATIONS

The propagation of localized corrosion of stainless steels was investigated by spectral frequency analysis (HPA) of potential fluctuations in the modes of stationary and cyclic galvanostatic polarization. Corrosion-resistant austenitic and austenitic-ferritic chromium-nickel steels were studied: 12X18N10T, 10X17N13M2T, 08X22N6T, resistant to pitting corrosion, in an aqueous solution of 0.1 M NaCl with a cyanide-containing complexing agent, a corrosion stimulator - 0.01 g/l (K3[Fe(CN)6]). The studies were carried out in the range of low densities (0.5–10 µA /cm2) due to the small values of the limiting diffusion current density in oxygen. To find the time of the origin of macropittings, it is proposed to use the NAV of chronopotentiograms with their subsequent statistical processing (Fourier transform). It has been established that fluctuations in the boundary polarization regime are characteristic of the process of the origin, development and passivation of macropittings. In the mode of stationary galvanostatic polarization, frequency characteristics were studied in the range of 1.0 ± 0.5 µA/cm2. When analyzing spectral density graphs, it was found that the dominant frequency characterizes the time of formation of the macropitting, and the sections of the chronopotentiogram should be divided into sections with a duration of no more than 50-100 s. In non-stationary conditions of galvanostatic polarization in the range of 1.0-10 µA/cm2 in a cyclic mode, it was found that the processes of nucleation and passivation of macropittings can be traced with a changing value of the polarizing current. Based on the frequency range of chronopotentiograms, it was found that the average macropitting lifetime is 250-300 s (frequency range 0.033 - 0.004 Hz). The dominant frequencies are determined to identify the characteristic dependencies of the envelopes along the upper (fhg) and lower (fhg) boundaries of the potential change, based on them, an algorithm is proposed for selecting the values of the parameters of the prediction mode of the formation of macropitting processes.

Modeling of water film generation in up-inclined low liquid loading natural gas pipeline elbows and its implication on internal corrosion occurrence

Condensed water often accompanies natural gas in the transmission process. The water forms thin film when parameters such as temperature and pressure change. This accelerates pipeline corrosion and makes the pipeline wall become thinner. Severe corrosion can lead to pipeline rupture, endangering the pipeline safety and leading to economic losses. The distribution of a water film plays a significant role in the corrosion process, So the work of predicting the distribution of a water film and flow characteristics becomes a key priority when analyzing corrosion in a pipe elbow. In this study, Eulerian, RNG (re-normalization group) k-ε and Eulerian wall film models are employed to establish a pipe elbow model that predicts the locations of water phase accumulation. Combining with a limiting diffusion current density, this model can be used to predict the potential corrosion locations and extent. The proposed model is validated by using velocity and water film distributions and it shows substantial agreement with previous models and results in the literature. From the simulation outcomes, a water film is mainly distributed at bottom while the thickness of the water film increases significantly at the pipe elbow. At the elbow bottom the inlet gas velocity increases, this shifts the high corrosion rate zone from elbow rear to front. Corrosion at the elbow front is more serious regardless of the changes in the inlet liquid velocity, elbow angle or water holdup. Meanwhile, when the gas velocity or elbow angle is large, the water film gradually spreads from the elbow bottom to the side and top parts which are in a high corrosion rate zone and more dangerous. Therefore, weak positions where pipeline elbow corrosion failures may occur can be quickly and conveniently computed and determined by the proposed model.

Sterical Self-Consistency of Carbonaceous Nanopolyhedra Triggered by Introduced CNTs to Optimize ORR Performance

The electrocatalyst of oxygen reduction reactions is one of the basic components of a fuel cell. In addition to costly Pt/C benchmark catalysts, cost-effective carbon-based catalysts have received the most attention. Enormous efforts have been dedicated to trade off the catalyst performance against the economic benefit. Optimizing composition and/or structure is a universal strategy for improving performance, but it is typically limited by tedious synthesis steps. Herein, we have found that directly introducing CNT into MOF-derived carbonaceous nanopolyhedra, i.e., introduced carbon nanotubes (CNTs) penetrated porous nitrogen-doped carbon polyhedra (NCP) dotted with cobalt nanoparticles (denoted as CNTs-Co@NCP), can optimize the catalytic activity, stability, and methanol tolerance. The hierarchical architecture combines the 0D/1D/3D Co/CNT/NCP interfaces and 1D/3D CNT/NCP junctions with the frameworks with a greatly exposed active surface, strengthened mass transport kinetics, stereoscopic electrical conductivity networks and structural robustness. The sterical self-consistency of MOF-self-assembly triggered by introduced CNTs demonstrates tactful ORR electrocatalytic activity regulation. Eventually, the CNTs-Co@NCP showed a half-wave potential (E1/2) of 0.86 V and a diffusion-limited current density (JL) of 5.94 mA/cm2 in alkaline electrolyte. The CNTs-Co@NCP was integrated into the cathode of a direct methanol fuel cell (DMFC) with an anion-exchange membrane, and an open-circuit voltage (OCV) of 0.93 V and a high power density of 46.6 mW cm−2 were achieved. This work successfully developed a catalyst with competitive ORR performance through plain parameter fine-tuning without complex material design.

N-doped carbon nanotube/particle composite as highly efficient electrocatalyst towards oxygen reduction reaction

Optimizing the microstructure is crucial to design the N-doped carbon-based electrocatalysts towards oxygen reduction reaction (ORR). Herein, a 1D/0D composited catalyst (L-Fe-CN-C) is prepared by using graphitic carbon nitride, iron chloride and carbon black as the raw materials. The resultant catalyst is consisted of N-doped carbon nanotubes and nanoparticles. The nanotubes are dramatically shrunk and smaller carbon nanoparticles are simultaneously generated with the pre-addition of carbon black, both contributing to the large specific surface area, high space utilization and to construct a mesopore-dominated microstructure. Moreover, the content of active pyridinic N is increased in the resultant catalyst. Benefiting from the modulation, the catalyst exhibits an excellent ORR activity with 0.850 V of half wave potential, 6.224 mA cm−2 of limited diffusion current density and high selectivity to 4e-path of ORR in 0.1 M KOH. The zinc-air battery with the catalyst on air electrode outputs a high peak power density, high specific capacity and excellent long-term durability, being comparable to those of the benchmark Pt/C based battery. This work provides a rational strategy to controllably synthesize an N-doped carbon ORR catalyst with hierarchical structure and preferable performance.

Multi‐Component PtFeCoNi Core‐Shell Nanoparticles on MWCNTs as Promising Bifunctional Catalyst for Oxygen Reduction and Oxygen Evolution Reactions

AbstractThe development of commercially viable fuel cells and metal‐air batteries requires effective and cheap bifunctional catalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Multi‐component Pt−Fe−Co−Ni nanoparticles on multi‐walled carbon nanotubes (MWCNTs) were synthesized by wet chemistry route via NaBH4 reduction of metal salts, followed by sintering at different temperatures. The catalyst demonstrates an excellent ORR activity and a promising OER activity in 0.1 m KOH, with a bi‐functional over‐potential, ΔE of 0.83 V, which is comparable to the values of Pt/C or RuO2. Furthermore, it shows outstanding long‐term stability in ORR and OER, namely diffusion limited current density at a potential of 0.3 V decreased just by 5.5 % after 10000 cycles in ORR. The results of the PFCN@NT300 indicate a significant effect of the substitution of Pt by the transition metal (TM) and the formation of nanoparticles on the catalytic performance, especially in the OER.

Tailoring CoNi Alloy-Embedded Carbon Nanofibers by Coaxial Electrospinning for an Enhanced Oxygen Reduction Reaction

A flexible CoNi@CNF electrochemical catalyst was developed using coaxial electrostatic spinning technology. The distribution and content of CoNi alloy nanoparticles on the surface of carbon fibers were adjusted by regulating the feed speed ratio of the outer and inner axes of coaxial electrostatic spinning. The results indicate that the content of the CoNi alloy distributed on the carbon fiber surface increased from 26.7 wt.% to 38.4 wt.% with an increase in the feed speed of the inner axis. However, the excessive precipitation of the CoNi alloy on the carbon fiber surface leads to the segregation of the internal CoNi alloy, which is unfavorable for the exposure of active sites during the electrolytic reaction. The best electrocatalytic performance of the composite was achieved when the rate of the outer axis feed speed was constant (3 mm/h) and the rate of the inner axis was 1.5 mm/h. The initial oxygen reduction potential and half-slope potential were 0.99 V and 0.92 V (VS RHE), respectively. The diffusion-limited current density was 6.31 mA/cm−2 and the current strength retention was 95.2% after the 20,000 s timed current test.

Two-Dimensional Polyphosphazene Nanosheet-Derived N,P Doubly Doped Carbon Nanotubes for Electrocatalytic Oxygen Reduction

Two-dimensional (2D) covalent organic polymers (COPs) featuring large surface areas and exposed active sites are desirable in the modification of electrode materials for electrochemical reactions. However, the rigid and planar conformation of 2D COPs limits the wrap of one-dimensional electrodes. Herein, we have proposed a molecular strategy to regulate in situ encapsulation of carbon nanotubes (CNTs) with 2D polyphosphazene. The stoichiometric control over the nucleophilic substitution reaction between hexachlorocyclophosphazene and amines allows for the oriented evolution around the surface of CNTs. The formed COP sheath is an ultrathin and uniform coating without granular deposition. Upon carbonization, the well-defined core/shell CNT composite was converted into the N,P-codoped carbon, exhibiting outstanding electrocatalytic oxygen reduction performance with a diffusion-limiting current density of 5.4 mA cm–2 and an electron transfer number of 3.97. The improved electrocatalytic activity originates from a high content of active N,P-containing catalytic sites uniformly distributed on porous carbon. This study provides a viable strategy to develop functional composite materials by modulating the flexibility of 2D COPs for 1D substrates, contributing to broad applicability.

Two-Dimensional Covalent Framework Derived Nonprecious Transition Metal Single-Atomic-Site Electrocatalyst toward High-Efficiency Oxygen Reduction

Designing an active, stable, and nonprecious metal catalyst substitute for Pt in the oxygen reduction reaction (ORR) is highly demanded for energy-efficient and cost-effective prototype devices. Single-atomic-site catalysts (SASCs) have been widely concerning because of their maximum atomic utilization and precise structural regulation. Despite being challenging, the controllable synthesis of SASCs is crucial for optimizing ORR activity. Here, we demonstrate an ultrathin organometallic framework template-assisted pyrolysis strategy to synthesize SASCs with a unique two-dimensional (2D) architecture. Electrochemical measurements revealed that Fe-SASCs displayed an excellent ORR activity in an alkaline media, having a half-wave potential and a diffusion-limited current density comparable to those of commercial Pt/C. Remarkably, the durability and methanol tolerance of Fe-SASCs were even superior to those of Pt/C. Furthermore, Fe-SASCs displayed a maximum power density of 142 mW cm-2 with a current density of 235 mA cm-2 as a cathode catalyst in a zinc-air battery, showing its great potential for practical applications.

Enhancing oxygen reduction activity by Mn-pyridinic N bonding in SmMn2O5/nitrogen-doped carbon black hybrid catalysts

In order to overcome the poor conductivity of transition metal oxides, design of hybrid catalyst with carbonaceous materials is an effective strategy to improve oxygen reduction reaction (ORR) performance. Here, the hybrid catalysts composed of SmMn2O5 and nitrogen-doped carbon black (SMO/NC-x) were synthesized in situ by hydrothermal method. When SMO content is about 21.6 wt%, SMO/NC-2.5 exhibits excellent ORR performance, the half-wave potential increases from 0.69 V for pure SMO to 0.77 V for SMO/NC-2.5, the diffusion-limited current density of SMO/NC-2.5 is 4.9 mA cm−2, higher than 3.8 mA cm−2 for pure SMO. Combining with X-ray photoelectron spectroscopy and Raman analysis, it is confirmed that the efficient ORR activity of the hybrid catalyst is mainly attributed to the synergic effect generated by Mn and pyridinic N Bonding, which is conducive to the storage and transfer of electrons at the interface. In addition, the detailed mechanism of synergic effect was well discussed.

Transient Polarization and Dendrite Initiation Dynamics in Ceramic Electrolytes

Solid-state electrolytes, by enabling lithium metal anodes, may significantly increase the energy density of current lithium-ion batteries. However, similar to their liquid counterparts, these hard and stiff electrolytes can still be penetrated by soft Li metal, above a critical current density (CCD). The prevailing method to determine the CCD employs step-wise galvanostatic cycling, which suffers from inconsistent active interfacial areas due to void formations after repeated stripping and plating, leaving large variance in the reported data that preclude precision understandings. Here, we combine a one-way polarization technique with electrochemical impedance spectroscopy to uncover, for the first time, the existence of significant polarization dynamics in ceramic electrolytes. In contrast to the diverging transient current due to metal penetration, the current peaks we observed suggest a diffusion-limited mechanism that follows the classic Randles-Sevcik equation for analyzing the diffusion-limited processes in liquid electrolytes. Our results allow a rigorous self-consistent analysis to reveal that the CCD is a diffusion-limited current density, while the system-specific limiting current density for ceramic electrolytes is still lower than CCD, which suggests that the ion transport mechanism preceding the dendrite penetration in ceramic electrolytes is unifiable with that in liquid electrolytes.

Trace Cr(OH)3 modified α-MnO2 electrocatalyst intercalated by Ag+ with superior activity and stability for oxygen reduction reaction

To achieve the practical application of MnO2-based oxygen reduction electrocatalysts, two important issues in terms of slow electrochemical kinetics and low stabilities must be resolved. In this work, α-MnO2 nanorods intercalated by Ag+ and decorated by trace amounts of Cr(OH)3 were designed by a simple two-step solution method to improve the electrocatalytic activity. The resulting catalysts exhibited efficient oxygen reduction reaction (ORR) activity with a high half-wave potential (E1/2 = 0.85 V vs RHE) and diffusion-limiting current density (Jlim = -5.99 mA cm−2) comparable to commercial 20 wt% Pt/C catalysts. More importantly, the catalysts displayed a more durable performance after 10 h in terms of the decrease in electrochemical current (3.5 %), much lower than that of Pt/C catalysts (9.1 %). The excellent electrocatalytic activity was attributed to the synergistic effects between MnO2, Ag+ and Cr(OH)3, reduced the charge transfer resistance of α-MnO2, adjusted the ratio of Mn3+/Mn4+, generated abundant oxygen vacancies, and promoted the adsorption and activation of surface O2. Furthermore, density functional theory calculations were also carried out to study the coupling interactions to reveal the mechanism of the excellent electrocatalytic performance. This study provides a co-modification strategy for the design of MnO2-based efficient and stable oxygen reduction electrocatalysts.

MOF-derived N-doped carbon nanosticks coupled with Fe phthalocyanines for efficient oxygen reduction

The rational design and preparation of atomic-level dispersed non-platinum catalysts to achieve efficient oxygen reduction reactions (ORR) is worth exploring for clean energy conversion and storage technology. Herein a simple self-templating strategy is demonstrated to obtain MIL-68-NH2-x series using pyridine as a surfactant to realize micron-scale to nanosized conversion. Among them, the as-prepared MIL-68-NH2-300 owns a large specific surface area and abundant pore defects after thermal treatment, which can effectively adsorb and anchor Fe phthalocyanines (FePc). In this case, MOF-derived N-doped carbon nanosticks coupled with FePc molecules (FePc@NCNS) exhibit an excellent electrocatalytic ORR activity with a positive half-wave potential (0.904 V), a large diffusion-limited current density (6.17 mA cm−2) and a high current retention after 20 h (95.7%). Furthermore, the high performance of ORR is confirmed by the relevant theoretical calculations on O2 adsorption ability. These above results will pave the way for MOF-derived carbon nanomaterials with specific morphological dimensions for high-performance energy conversion and storage.

Co Nanoparticles Confined in Mesoporous Mo/N Co-Doped Polyhedral Carbon Frameworks towards High-Efficiency Oxygen Reduction.

Exploiting effective non-noble metal electrocatalysts for oxygen reduction reaction (ORR) is crucial for fuel cells and metal-air batteries. Herein, we designed and fabricated Co nanoparticles confined in Mo/N co-doped polyhedral carbon frameworks (Co-NP/MNCF) derived from polyoxometalate-encapsuled metal-organic framework, which showed comparable ORR performance with commercial Pt/C and a larger diffusion-limited current density. Moreover, the Co-NP/MNCF also exhibited excellent ORR stability and methanol tolerance. These appealing performances can be attributed to the porosity regulation and heteroatom doping of metal-organic framework derived polyhedral carbon frameworks, which could be beneficial for the exposure of more active sites, the optimization of electronic structure and the mass transfer of electrolyte/electron/ion.

Co2P Nanoparticles Encapsulated in N-Doped Carbon Nanotubes as a Bifunctional Oxygen Catalyst for a High-Performance Rechargeable Zn–Air Battery

A significant but challenging task is the development of a rechargeable Zn–air battery bifunctional catalyst. Here, a supramolecular self-assembly of poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) was designed to construct Co2P nanoparticles encapsulated in N-doped carbon nanotubes (Co2P@NCNT-x). The results show that P123 is an important factor in the formation of N-doped carbon nanotubes coated with Co2P nanoparticles. The Co2P@NCNT-4 catalyst exhibits the maximum limiting diffusion current density (5.2 mA·cm–2 under 808.9 hPa) and a more positive onset potential (0.90 V vs RHE) and half-wave potential (0.84 V vs RHE) due to the greater specific surface area and higher graphitization degree. Its potential gap is only 0.87 V versus RHE which is between the half-wave potential of the oxygen reduction reaction and the Ej = 10 of the oxygen evolution reaction. The Co2P@NCNT-4-based rechargeable Zn–air battery shows the highest peak power density (217 mW·cm–2), the highest specific capacity (831.25 mA h·gZn–1 at 10 mA·cm–2), and remarkable cycle stability. This preparation method can provide ideas for designing other transition metal phosphide catalysts in the future.

Aligned silver nanoparticles anchored on pyrrolic and pyridinic-nitrogen induced carbon nanotubes for enhanced oxygen reduction reaction

Comprehension of the metal-nanostructure arrays is of great significance for the rational design of the cathode catalyst for the oxygen reduction reaction (ORR). Therefore, we fabricated a cathode catalyst consisting of the small-size, aligned silver nanoparticles (Ag NPs) embedded into the pyrrolic and pyridinic-Nitrogen functionalized carbon nanotubes. The N-defected structure was prepared by acidification of carbon nanotubes (CNTs) to avoid inherited stacking of its layers followed by the introduction of 2,7 diaminophenazine (DPA) using a hydrothermal process, while small-size Ag NPs were aligned through electrodeposition onto the N-defected carbon nanotube (CNT-DPA-Ag). The individual role of pyrrolic and pyridinic -N in the electrocatalytic ORR performance, synthesis of small size, and distribution behavior of Ag NPs were evaluated. To maximize the ORR performance of the catalyst, the advantage was taken from the presence of carbon atoms next to the pyridinic-N which is a Lewis base and acts as an adsorption and protonation site for the oxygen (O2) molecule and eventually ensures a direct 4e− transfer route for the electrochemical conversion of O2 into water (H2O). Transmission electron microscopy (TEM) analysis of CNT-DPA-Ag revealed a small size, aligned Ag NPs (average size 2 nm) resulting in a large surface area and rich electroactive sites for adsorption of O2. The CNT-DPA-Ag showed a remarkable ORR performance in terms of positive onset potential (-0.052 V), high diffusion-limiting current density (-5.54 mAcm−2), and noteworthy methanol tolerance, and long-term durability in comparison to the commercial 20 wt% Pt/C.

Collaborative integration of ultrafine Fe2P nanocrystals into Fe, N, P-codoped carbon nanoshells for highly-efficient oxygen reduction

The rational design and facile synthesis of cost-effective and high-performance carbon-based catalysts for oxygen reduction reaction (ORR) is of great significance but remain challenging. Herein, we report the crafting of a robust Fe,N,P-codoped, ultrafine Fe2P nanocrystals decorated, highly hierarchical porous carbon nanoshells (denoted Fe,N,P-CNSs/Fe2P) via a simple yet robust one-step synergetic phosphorization and pyrolysis approach for highly-efficient ORR. Specifically, well-defined polypyrrole-co-polyaniline hollow nanospheres with a shell thickness of ≈ 43 nm (PPy-co-PANI-HNs) are first elaborately synthesized via a facile soft-templating (Triton X-100 micelles) copolymerization of pyrrole and aniline monomers. Subsequent one-step synergetic phosphorization and pyrolysis of PPy-co-PANI-HNs pre-impregnated with NaH2PO2·H2O and FeCl3 (H2PO2-,Fe3+@PPy-co-PANI-HNs) yields Fe,N,P-CNSs/Fe2P with a well-crafted ultrathin nanoshell (shell thickness of ≈13 nm) architecture containing abundant hierarchical porosity. Remarkably, an alkaline electrolyte capitalizing on the resulting Fe,N,P-CNSs/Fe2P manifests excellent ORR performance with the half-wave and onset potentials (E1/2 of 0.854 V and Eonset of 0.955 V) comparable to Pt/C, diffusion limited current density (JL of −5.98 mA cm−2) higher than Pt/C, and long-time durability and methanol resistance better than Pt/C, demonstrating great potential as air cathode in zinc-air batteries (maximum power density of 170 mW cm−2 and specific capacity of 824.5 mA h g−1). Combined experimental and theoretical studies reveal that the impressive ORR performance of Fe,N,P-CNSs/Fe2P originates from simultaneous compositional (i.e., Fe,N,P-codoping and ultrafine Fe2P nanocrystals decorating) and structural (i.e., hierarchically porous ultrathin nanoshell architecture) tailoring enabled by the judicious one-step synergetic phosphorization and pyrolysis.