Carbon-based Electrocatalysts For Oxygen Reduction Reaction Research Articles

Enhancing oxygen reduction on a bimetallic Ag-Fe-N-C electrocatalyst for a zinc-air battery

Bi-metallic organic frameworks (MOFs) have been rarely reported to be functioned as non-platinum (Pt) electrocatalysts for oxygen reduction reaction (ORR). In this work, here we synthesize a carbon-based bimetallic Ag-Fe-N-C electrocatalyst for the ORR by jointly introducing Ag and Fe ions into the internal structure of MOFs derived from zinc acetate and 2-methylimidazole (2-MIM). The Ag-Fe-N-C catalyst possesses a porous carbon network structure with an specific surface area of 961.3 m2g−1, which is woven by carbon nanotubes/layers-encapsulated Fe3C or Ag nanoparticles. This catalyst has demonstrated a distinguished ORR catalytic activity with a half-wave potential of 0.883 V (vs. RHE) and an outstanding stability in an alkaline electrolyte. In addition, it gained a higher peak power density of 211 mWcm−2 and specific energy density of 875 Whkg−1Zn, a narrower charge-discharge potential gap of 77 mV and a better catalytic durability compared to the corresponding Pt/C-based zinc-air batteries. Our results may provide a facile strategy for activity enhancement of carbon-based ORR electrocatalysts via the controllable doping of bi-metallic ions.

Exploring the underlying oxygen reduction reaction electrocatalytic activities of pyridinic-N and pyrrolic-N doped graphene quantum dots

In this study, density functional theory was used to investigate the differences of oxygen reduction reaction catalytic activities between pyridinic-N and pyrrolic-N doped graphene quantum dots (GQDs) and the influence of four ions/groups: OH–, H3O+, OH and H3O on their electronic properties. Free energy change calculations show that pyridinic-N doped GQDs (pN-GQDs) have better catalytic activities than pyrrolic-N doped GQDs (prN-GQDs). Moreover, the catalytic activity of both pN-GQDs and prN-GQDs can be enhanced via the functionalization of OH and H3O groups, which greatly alters electronic structures of the GQDs and thereby the adsorption strength of reactants can be effectively controlled. The findings can shed light on the effects of different N doping configurations and adsorbed species on electrocatalytic activities and provide theoretical guidelines for design and development of carbon-based oxygen reduction reaction electrocatalysts for fuel cells and metal-air batteries.

Unveiling the inter-molecular charge transfer mechanism of N-doped graphene/carbon nanotubes heterostructure toward oxygen reduction process for Zn-air battery

Heteroatom-doped carbon-based materials have been deemed as highly efficient electrocatalysts for cathodic oxygen reduction reaction (ORR) in the sustainable energy conversion technologies. However, the limited active sites caused by the aggregation of carbon layers ineluctably hinder the mass transfer, thereby generating insufficient ORR performance. Herein, we offer a rational and facile method to manufacture a hybrid heterostructure of N-doped graphene and carbon nanotubes at the pyrolysis temperature of 900 °C (N-G/CNTs-900), which exhibits a better ORR activity than most of reported carbon-based ORR electrocatalysts, and comparable to that of Pt/C. Thus-fabricated N-G/CNTs-900 features hierarchically porous architecture, high specific surface area, abundant defects, and high active N content, and what’s more, it also affords active sites for oxygen activation and subsequent 4e− reduction processes due to the hetero-interlayer charge transfer and the electron accumulations of the carbon atoms on the surface layer by theoretical calculations. Compared with commercial Pt/C, N-G/CNTs-900-based Zn-air battery presents a higher peak power density (133.60 mW/cm2) and a larger specific capacity (707 mAh/gZn). This work provides a novel idea to design the high-efficiency and long-term durability ORR catalyst, which is benefit for boosting the commercial applications of metal-free carbon-based materials in the renewable energy conversion devices.

Nitrogen-doped carbon nanotubes filled with Fe3C nanowires for efficient electrocatalytic oxygen reduction

It is of great significance to develop the highly active, durable and inexpensive oxygen reduction reaction (ORR) electrocatalysts for fuel cells and metal-air batteries. Nitrogen-doped carbon nanotube (NCNTs) was considered to be one of the most significant non-noble metal ORR electrocatalysts, but its catalytic activity has some shortcomings due to its unfavorable ORR intermediates adsorption characteristics. Herein, we report a new kind of nitrogen-doped carbon nanotubes, which have filled with Fe3C nanowires (NCNTs@Fe3C NWs), working as a stable and effective ORR catalyst. The half-wave potential of the obtained catalyst in a 0.1 M KOH solution is 0.88 V, which is significantly higher than the pristine NCNTs (0.79 V) and the commercial Pt/C (0.85 V). Compared with NCNTs, the improvement in ORR activity of NCNTs@Fe3C NWs can be attributed to the strain effect on the outermost carbon layer due to the high filling of Fe3C NWs. Most importantly, NCNTs@Fe3C NWs shows higher durability and better methanol tolerance than the commercial Pt/C catalyst. Density functional theory (DFT) calculation shows that the tensile strain can optimize the binding energy of ORR intermediates to a certain extent. This work provides new research ideas for the rational design of high-efficiency carbon-based ORR electrocatalysts through the direction of strain engineering.

N-Doped Colloid Imprinted Carbons As Promising ORR Catalysts for Alkaline Applications

The oxygen reduction reaction (ORR) has long been of interest in relation to its many energy applications and interesting multi-pathway mechanisms. The ORR is a key reaction in a range of electrochemical energy conversion and storage devices, such as hydrogen fuel cells and metal-air batteries, respectively. These devices are expected to play an ever-increasing role in the global transition to net zero emissions. Metal-air batteries, such as Zn/air batteries, can operate at room temperature, use recyclable materials, are environmentally friendly, and are preferred in relation to consumer safety than batteries relying on organic solvents and reactive electrode species.1 High rates of the ORR are crucial for the development of high performance Zn/air batteries and catalysts play a significant role, with lowering of the catalyst cost also of increasing importance.The costs of conventional Pt-based ORR catalysts are high. Therefore, metal-free carbon-based ORR electrocatalysts are viewed as increasingly promising alternatives, especially as they are lower in cost due to the availability of the precursor materials. Carbon is also a good electrical conductor and support material, is chemically stable, and can have large surface areas. However, the ORR kinetics are sluggish on carbon and chemical and physical modifications are required to enhance its activity. In typical Zn/air batteries, the ORR occurs at the three-phase boundary (TPB) formed between the solid electrode, liquid electrolyte, and gaseous oxygen. The porosity of the catalyst layer, the wettability of the catalyst/electrolyte interface, and the gas permeability and hydrophobicity of the gas diffusion layer (GDL) are thus also important, significantly influencing the cathode performance and durability. The catalyst layer (CL) must therefore be constructed with both a high-performance catalyst and an optimized TPB length to provide high performance without compromising durability.In the current work, we have doped nitrogen into the lattice of a family of nanoporous colloid imprinted carbon (CIC) powders to increase its ORR activity. The CICs are unique for their versatility in terms of pore size control and ease of surface functionalization.2 Pore sizes in the range of 12 to 100 nm were examined and their effect on the ORR activity and mass transport limitations were investigated. To carry out N-doping, the CICs were exposed to ammonia at 800 ˚C for 7 hr. Catalyst inks were then prepared by mixing the CICs with a binder in an isopropyl alcohol/water solution. Aliquots of the ink were drop-casted on the disc of an RRDE system, or were spray coated or drop-casted on a GDL to determine the ORR performance in an in-house Zn/air battery testing cell, with the N-doped CIC catalyst layer sandwiched between an electrolyte chamber and a graphite current collector. In this setup, O2 gas was flowed through the pores in the GDL to the catalyst/electrolyte interface, a Zn wire installed in the electrolyte chamber was used as the reference electrode, and a Ni sponge was used as the counter electrode.The RRDE experiments showed that, after N doping of the CIC powders, the production of peroxide decreased significantly and the ORR onset potential increased to a very respectable value of ca. 0.9 V vs RHE, indicating the successful activation of the ORR. Electron transfer numbers were found to be greater than 3.5, indicating that either a direct or pseudo- 4 electron transfer ORR pathway is dominant. In agreement with the literature, the ORR currents in the kinetic regions increased linearly with mass loading, expected to be proportional to the total active N-doped CIC surface area.3 The N-doped CIC samples retained excellent performance up to a loading of 0.350 μg/cm2 without losing mechanical stability.Similar N-doped CICs and binders of different hydrophobicity were tested in the Zn/air battery testing system. Electrodes made with a hydrophobic NCS microporous layer (MPL) showed much better ORR performance and durability than hydrophilic NCS MPLs. Although electrodes made using hydrophobic polytetrafluoroethylene (PTFE) as the binder in the catalyst layer showed a similar initial performance to those made using hydrophilic Nafion binders, the PTFE based electrodes exhibited better durability. Furthermore, the temperature and pressure used during electrode fabrication were also found to have a significant impact on the binder distribution and ORR performance. Once optimized, a very good correlation was obtained between the N-doped CIC catalyst performance in the RRDE setup and in the Zn/air battery testing system. References J. Pan et al., Adv. Sci., 5, 1700691 (2018).X. Li et al., ACS Appl. Mater. Interfaces, 10, 2130–2142 (2018).N. Gavrilov et al., J. Power Sources, 220, 306–316 (2012).

Graphitic carbon nitride on reduced graphene oxide prepared via semi-closed pyrolysis as electrocatalyst for oxygen reduction reaction

Graphitic carbon nitride (g-C3N4)/graphene hybrids were shown to be promising low-cost and durable substitutes for Pt-based oxygen reduction reaction (ORR) electrocatalysts in metal-air batteries and fuel cells. However, most of the synthesis procedures found in the literature are tedious, resource-intensive, and make use of toxic precursors. In this paper, we present a facile method for preparing g-C3N4 on reduced graphene oxide (g-C3N4@rGO) via semi-closed pyrolysis of graphene oxide and urea under an ambient air atmosphere. The resulting electrocatalyst was characterized using microscopic, structural, and spectroscopic techniques. Electrochemical characterization showed that the onset and half-wave potentials (vs. Ag/AgCl), as well as ORR electron transfer number, were found to be – 0.248 V, – 0.281 V, and 3.16, respectively. Although g-C3N4@rGO has relatively lower electrocatalytic activity compared to Pt/C, the former was shown to exhibit better electrochemical stability. Nonetheless, the low-cost and more sustainable method of manufacturing g-C3N4@rGO in this study advances the economic viability of carbon-based ORR electrocatalysts.

Nitrogen-doped carbon nanowalls/diamond films as efficient electrocatalysts toward oxygen reduction reaction

Substitution of commercial Pt/C electrocatalysts with efficient carbon-based ones for oxygen reduction reaction (ORR) still remains a huge challenge. For practical ORR applications it is significant to design robust 3D network nanostructures in that they do not require polymer binders. For conventional powder catalysts, they must be combined with substrate, leading to their shedding and degradation. In this work, vertically-aligned N-doped carbon nanowalls/diamond (N-CNWs/D) films are synthesized by means of a microwave plasma chemical vapor deposition technique, where nitrogen doping is conducted during the growth process and a subsequent facile annealing treatment under Ar atmosphere. The obtained Ar treated N-CNWs/D film exhibits an ORR onset potential of 835 mV (versus reversible hydrogen electrode) in 0.1 mol l−1 KOH solution in a four-electron reaction pathway. It also displays excellent tolerance toward methanol crossover and long-term stability (e.g. a current density loss of only 10% even after 16 h measurement). The boosting ORR performance can be attributed to the activated pyridinic N dopant at abundant edge sites and enlarged electrochemical surface areas of N-CNWs/D films. This work not only develops a controllable strategy to fabricate binder-free carbon-based ORR electrocatalysts, but also paves a way to in-depth understand actual active sites in terms of ORR pathway mechanisms.

Transforming C60 Molecules into Polyhedral Carbon Micro-Nano Shells for Electrochemically Producing H2O2 in Neutral Electrolytes.

The electrochemical production of hydrogen peroxide (H2O2) via the two-electron oxygen reduction reaction (ORR) can realize the customer-oriented onsite synthesis of H2O2 in a green and sustainable method. The ongoing challenge that needs to be solved is the fabrication of robust electrocatalysts of excellent performance. In this work, C60 was selected as a precursor due to its uniform structure and abundant pentagon rings. Thanks to the strong interaction between C60 and thiophene, after heteromolecule assembly in the liquid reaction and subsequent reconstruction of the carbon topological structure in solid calcination, C60 was successfully transformed into polyhedral carbon micro-nano shells (PCMNS) with an effective pore structure for the first time, which exhibited excellent capacity for production of H2O2 via two-electron ORR, especially in neutral media. In addition to the high onset potential (0.49 V vs reversible hydrogen electrode (RHE)) and low Tafel slope (72 mV dec-1), its selectivity reached >90% within the potential range of 0.30-0.45 V and maintained >80% after constant potential electrolysis for 10 h. The yield rate of H2O2 was 1102.5 mmol gcat-1 h-1, determined by an H-type electrolytic cell, which was one of the highest values of metal-free carbon-based ORR electrocatalysts ever reported. Such excellent two-electron ORR performance of PCMNS was attributed to its abundant accessible active sites and hierarchical pore structures.

Facile regulation of porous N-doped carbon-based catalysts from covalent organic frameworks nanospheres for highly-efficient oxygen reduction reaction

Exploiting inexpensive, highly active and durable electrocatalysts for highly-efficient oxygen reduction reaction (ORR) remains an urgent need. Herein, uniformed covalent organic frameworks (COFs) nanospheres were prepared as the precursors of N-doped porous carbon-based electrocatalyst for ORR reaction. Investigations into the COF-derived N-doped carbon nanospheres (N–CNS) demonstrated that the porous and crystalline frameworks with precise N positions in COF precursors facilitated the regulation of surface area, conductivity, active sites (defects) and their accessibility (pores) in N–CNS via optimizing the pyrolysis temperature. As a result, an excellent ORR catalyst with the activity mostly approximating to that of the commercial 20 wt% Pt/C catalyst among the reported metal-free N-doped carbon-based catalysts to our best knowledge, was obtained, which also exhibited good durability and excellent methanol tolerance. Moreover, Fe3+ was facilely coordinated into COF nanospheres, which therefore derived an ultra-small Fe nanoparticles-decorated N–CNS (Fe–N–CNS) by the simple one-step pyrolysis. This Fe–N–CNS can further enhance its ORR activities (onset potentials, half-wave potentials and limit current density) to be comparable with those of 20 wt% Pt/C catalyst via the synergistic Fe–N active sites. Also, it exhibited good durability and methanol tolerance. All these paved a way to derive COF into the carbon-based electrocatalysts for highly-efficient ORR.

γ-Fe2O3 clusters embedded in 1D porous N-doped carbon matrix as pH-universal electrocatalyst for enhanced oxygen reduction reaction

Carbon-based materials have shown promising results in replacing commercial Pt/C as highly efficient oxygen reduction reaction (ORR) catalysts in renewable energy devices. Designing unique structures of carbon matrix and introducing more active sites are essential for their application as electrode materials. Herein, a feasible approach is reported to synthesize dispersive γ-Fe2O3 clusters embedded in 1D porous carbon nanofibers (CNFs) (γ-Fe2O3@CNFs-12) through electrospinning, impregnation and pyrolysis processes. The resulted γ-Fe2O3@CNFs-12 exhibits excellent ORR performance in alkaline electrolyte with a surprising half-wave potential (E1/2) of 0.905 V and merely 5 mV drop of the potential after 5000 cycles, outperforming that of commercial Pt/C. Meanwhile, it also shows considerable ORR performance in acidic and neutral media. The remarkable ORR catalytic activity and durability are primarily attributable to the well-designed 1D hierarchical porous structure and accessible γ-Fe2O3 clusters that embedded in carbon nanofibers. γ-Fe2O3@CNFs-12 is also capable to act as an air cathode for Zn-air batteries with an open circuit voltage of 1.428 V. This work provides a new perspective to the novel design and synthesis of carbon-based ORR electrocatalysts.

Cube-shaped metal-nitrogen–carbon derived from metal-ammonia complex-impregnated metal-organic framework for highly efficient oxygen reduction reaction

The non-platinum metal-nitrogen-carbon (M-NC) system is a class of highly active oxygen reduction reaction (ORR) electrocatalysts that are well known and widely used in fuel cell applications. Herein, one simple synthesizing method of efficient M-NC electrocatalysts is proposed by the direct pyrolysis of a pretreated core-shell structure of ZnO@ZIF-8 containing metal-ammonia complex (MAC, [M(NH3)x]n+) from the polydisperse self-sacrificing MOF-5 cubes. The as-pyrolyzed M-NC (M = Co, Ag, Cu, and Ni) electrocatalysts showcase high electrocatalytic ORR activity in an alkaline medium. Among them, Co-NC catalyst exhibits an excellent ORR performance where its E1/2 = 0.80 V, JL = 5.88 mA cm−2, as well as Tafel slope of 67.0 mV dec−1 are close to commercial Pt/C and Ag-NC, better than the other two catalysts of Cu-NC and Ni-NC. Impressively, zinc-O2 batteries assembled with M-NC materials exhibit the better discharge performance, and have great potential in the practical energy conversion and storage. The experimental demonstration of metal-based nanoparticles and active M-N sites in the carbonaceous matrix can be efficiently used to promote the ORR activity, which is derived from a synergistic contribution of its particular hollow structure, large specific surface area, rich M-N active sites, and high degree of graphitization. This attractive preparation approach provides us a powerful contribution to the construction of high-performance carbon-based ORR electrocatalysts.

New insight to the role of edges and heteroatoms in nanocarbons for oxygen reduction reaction

Oxygen reduction reaction (ORR) is the cornerstone for clean and sustainable energy conversion/storage technologies, depending on effective and robust catalysts. In the rational design of efficient and renewable carbon-based ORR electrocatalysts, the effect of intrinsic carbon edge sites on ORR remains elusive. In this study, nano-cutting is applied to multi-walled carbon nanotubes (MWCNTs) to synthesize a series of edge-engineered nanoribbon/nanotube hybrids with variable edge contents to probe the role of carbon edges. The previously overlooked but certainly emerged difference in conductivity in the edge creation process among the dopant-free carbocatalysts is taken into account towards the ORR activity, aiming to distinguish the function of edges. The carbocatalyst with a higher edge content is proven to be more reactive for ORR, with the premise of catalyst conductivity higher than ~70 S m−1. Further, with heteroatoms (N and S) introduced into the carbocatalysts, a positive correlation between the accommodated heteroatom content and the edging degree is revealed, indicating that edge sites afford anchoring sites for heteroatoms into the carbon framework, which further accelerates the ORR kinetics. This study provides new insights to the intrinsic role of the edge sites in nanocarbons as well as the synergy with heteroatom doping for promoted activity of carbocatalysts in ORR.

Nitrogen-doped graphene layers for electrochemical oxygen reduction reaction boosted by lattice strain

Nitrogen-doped graphene is a promising non-precious metal electrocatalyst for the oxygen reduction reaction (ORR). We report that distorted nitrogen-doped graphene layers encapsulating Fe3Co nanoparticles supported on carbon (Fe3Co@NG-C) display excellent activity toward ORR in alkaline media. Compared with a commercial Pt/C electrocatalyst at a loading of 80 μgPt/cm2 on a rotating disk electrode, the Fe3Co@NG-C exhibits an onset potential positively shifted by 50 mV at 0.1 mA/cm2 and a nearly identical half-wave potential. The Fe3Co@NG-C has minimal activity degradation during accelerated durability testing and superior tolerance to methanol than commercial Pt/C. Density functional theory calculations combined with poisoning experiments reveal that the high activity of the Fe3Co@NG-C mainly arises from strain in nitrogen-doped graphene induced by encapsulation of Fe3Co nanoparticles with exposed (1 1 0) planes, which promotes stabilization of the key OOH* intermediate involved in ORR. Our study shows the rational design of improved carbon-based electrocatalysts for ORR can be achieved by using strain engineering.

N,O-codoped 3D graphene fibers with densely arranged sharp edges as highly efficient electrocatalyst for oxygen reduction reaction

To replace the noble-metal Pt catalysts for oxygen reduction reaction (ORR), developing efficient and earth-abundant electrocatalysts is of great importance. Both the morphology and composition engineering of graphene could effectively modify the electronic structure to optimize its electrocatalytic performance for ORR. Here, we report an effective method to dope carbon materials with N, by which the N doping concentration and form could be well controlled. We first grow 3D graphene fibers (3DGFs) by thermal chemical vapor deposition, which are then treated with acid or heated in air and heated in NH3 in succession, obtaining N,O-codoped 3DGFs. The codoped 3DGFs exhibit outstanding electrocatalytic performance toward ORR with onset potential of 1.01 V, half-wave potential of 0.883 V, long-term operation stability with 90% current retention after 50 h, and a good methanol tolerance in alkaline solutions, which are superior to 20 wt% Pt/C catalyst and other reported advanced metal-free catalysts. The excellent catalytic performance of the 3DGFs probably arises from the synergic effect of the morphology and composition engineering, e.g., the edges and doping, especially the pyridine N. The present work is expected to open up new approach to design outstanding metal-free carbon-based electrocatalysts for ORR.

Platinum-Free Electrocatalyst for Oxygen Reduction Reaction Derived from the Direct Pyrolysis of Watermelon Peels

Carbon-based nanomaterials derived from the pyrolysis of heteroatom-containing carbonaceous precursor was shown to exhibit significant electrocatalytic activity towards oxygen reduction reaction (ORR). Biomass from agricultural waste could be seen as possible low-cost precursor for the preparation of carbon-based ORR electrocatalysts. In this study, watermelon peels (WP) were directly pyrolyzed under N2 atmosphere to produce novel carbon-based ORR electrocatalysts. WP were oven-dried and pulverized using a mechanical grinder to produce WP powder. A weighed amount of the resulting powder was then transferred into a ceramic boat and inside a quartz tube furnace. The WP powder was pyrolyzed from room temperature to pyrolysis temperature T at a rate of 5 °C/min under N2 atmosphere. The temperature was held at T for an hour, followed by natural cooling back to room temperature. The pyrolysis temperature was optimized, with T = 800 °C, 900 °C, and 1000 °C. The resulting pyrolysis products were characterized using cyclic voltammetry (CV), rotating disc electrode voltammetry (RDE), and scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDX). CV revealed that WP pyrolyzed at 1000 °C gave the most positive onset potential (Eonset = 0.40 V) and highest current density (j = 10.79 mA/cm2), compared to those pyrolyzed at 800 °C (Eonset = 0.30 V; j = 2.047 mA/cm2) and 900 °C (Eonset = 0.30 V; j = 3.494 mA/cm2). Levich analysis from RDE data showed that the electrocatalysts prepared at 1000 °C proceeded closest to the ideal four-electron route, compared to those prepared at 800 °C and 900 °C. SEM-EDX analysis showed the changes WP underwent in terms of surface morphology and elemental composition, which are then correlated with the apparent electrocatalytic activities of pyrolyzed WP. These findings present the feasibility of using WP as low-cost precursor for ORR electrocatalysts, which could be utilized in fuel cells.

Iron and nitrogen codoped carbon catalyst with excellent stability and methanol tolerance for oxygen reduction reaction

A series of non-precious metal FexNC electrocatalysts for oxygen reduction reaction (ORR) were successfully synthesized using Fe(NO3)3, glucose, and melamine as the Fe, C, and N sources, respectively. The effects of the pyrolysis temperature and Fe/N contents on the catalytic performances are comprehensively investigated. Electrochemical results reveal that among the FexNC catalysts, Fe1.5NC-900-2 pyrolyzed at 900°C with the mass ratio of FeC to melamine being 1:10 proves the highest catalytic performance. The half-wave potential (E1/2) of ORR was 821 mV (vs reversible hydrogen electrode (RHE)) and only 36 mV lower than that on commercial Pt/C catalyst (857 mV). More importantly, Fe1.5NC-900-2 catalyst shows excellent stability and methanol tolerance. After 1000 sequential cycles, the E1/2 on Pt/C catalyst shifts negatively by approximately 60 mV, while for Fe1.5NC-900-2 catalyst, this shift is only 28 mV although the number of sequential cycles is increased to 8000. In the presence of methanol, the current decay in the chronoamperometric response at 1000 seconds is only 8% and also much lower than that on Pt/C catalyst (46%). The high catalytic performances arise from the abundant Fe3N active sites embedded in the carbon matrix of the FexNC catalysts. These findings can be used to discuss the catalytic mechanism of ORR on the FexNC catalysts and design the nonprecious metal carbon-based electrocatalysts for ORR.

Improved oxygen reduction activity and stability on N, S-enriched hierarchical carbon architectures with decorating core-shell iron group metal sulphides nanoparticles for Al-air batteries

To rational design non-noble catalysts with oxygen reduction reaction (ORR) activity and durability superior to that of Pt/C, we report the fabrication of iron group metal sulphides immobilized inside hierarchically porous N,S-enriched carbon architecture using a facile double-phase encapsulation approach followed by a pyrolysis process. The as-prepared hybrid architectures exhibit significantly enhanced electrocatalytic activity towards the ORR, including a positive onset potential, low Tafel slope, and high electron transfer number. In particular, the ORR activity of FeS@G/NSC is even superior to that of the Pt/C benchmark, which possesses an outstanding onset potential (0.99 V vs RHE) and a high discharge voltage plateau (∼1.47 V at discharge current density of 50 mA cm−2 in the Al-air battery test) with excellent stability. These hybrid carbon architectures provide a new direction for carbon-based ORR electrocatalysts performance optimization for next-generation air batteries.

Low-Cost Sulfonated Phthalocyanines-Derived Hierarchical Porous Co-Cu-N-S-Doped Carbons for Efficient Oxygen Electroreduction

Low-cost commercial sulfonated phthalocyanines were converted into a series of porous multi-doped carbons with Co/Cu and N/S heteroatoms (Co-Cu-N-S-Cs). The water-soluble sulfonated copper (II) phthalocyanines and sulfonated cobalt (II) phthalocyanines could be coated on commercial silica nano-spheres with the interlinking of the acidized chitosan polymers, and the direct pyrolysis of such composites produced hierarchical porous carbons with abundant multi-doped sites. The Co-Cu-N-S-Cs exhibited the efficient electro-catalytic performance for oxygen reduction reaction (ORR) in both alkaline and acidic electrolytes. In particular, the ORR half-wave potential over Co-Cu-N-S-C-900 reached 0.85 V in 0.1 M KOH, 10 mV higher than Pt/C (20 wt%) electro-catalysts. Co-Cu-N-S-C-900 also demonstrated better stability and methanol-tolerance properties, in contrast to commercial Pt/C catalysts (JM, 20wt%). The mixed using of sulfonated Cu and Co phthalocyanine precursors help to balance the graphitization-related electron transportation and doping defects-related active site density in Co-Cu-N-S-C-900. These results indicated that low-cost and high-efficency multi-doped carbon-based electrocatalysts for ORR could be facile prepared using commercial sulfonated phthalocyanines.

Recent Progress on MOF-Derived Heteroatom-Doped Carbon-Based Electrocatalysts for Oxygen Reduction Reaction.

The oxygen reduction reaction (ORR) is the core reaction of numerous sustainable energy‐conversion technologies such as fuel cells and metal–air batteries. It is crucial to develop a cost‐effective, highly active, and durable electrocatalysts for ORR to overcome the sluggish kinetics of four electrons pathway. In recent years, the carbon‐based electrocatalysts derived from metal–organic frameworks (MOFs) have attracted tremendous attention and have been shown to exhibit superior catalytic activity and excellent intrinsic properties such as large surface area, large pore volume, uniform pore distribution, and tunable chemical structure. Here in this review, the development of MOF‐derived heteroatom‐doped carbon‐based electrocatalysts, including non‐metal (such as N, S, B, and P) and metal (such as Fe and Co) doped carbon materials, is summarized. It furthermore, it is demonstrated that the enhancement of ORR performance is associated with favorably well‐designed porous structure, large surface area, and high‐tensity active sites. Finally, the future perspectives of carbon‐based electrocatalysts for ORR are provided with an emphasis on the development of a clear mechanism of MOF‐derived non‐metal‐doped electrocatalysts and certain metal‐doped electrocatalysts.

Milk powder-derived bifunctional oxygen electrocatalysts for rechargeable Zn-air battery

Sustainable oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalysts are desirable for many energy storage and conversion systems. Australian dairy industry generates a large amount of expired milk powders every year, which are often dumped as food wastes. Here, we report a highly efficient bifunctional O2 electrocatalyst by incorporating mixed metal oxide (NiFeOx) nanoparticles into N (8.5at%) and P (1.9at%) dually doped carbon materials derived from milk powders. This catalyst demonstrates an onset potential of 0.90V vs. reversible hydrogen electrode (RHE), a kinetic limiting current density of 12.9mAcm−2 at 0.4V vs. RHE, and an electron transfer number of 3.86 for ORR, it also shows an onset potential of 1.48V vs. RHE, an overpotential of 0.32V at 10mAcm−2, a Tafel slope of 59.03mV/dec in 1.0M KOH for OER. Its catalytic performance is comparable to that of the best metal-free carbon-based ORR electrocatalysts and OER electrocatalysts based on hybrids of metal oxides and carbon materials. Rechargeable Zn-air batteries were also fabricated using air electrodes made of this catalyst, demonstrating an open-circuit potential of 1.39V, a specific capacity of 688mAhg−1 (corresponding to an energy density of 853WhkgZn−1), and excellent rechargeability over 150 cycles with a small performance loss. The milk powder-derived carbon materials can be used to produce eco-friendly and high value-added electrocatalysts for energy conversion and storage applications while reducing food wastage.