Low-cost Oxygen Reduction Reaction Research Articles

Iron-Cobalt Phosphide Encapsulated in a N-Doped Carbon Framework as a Promising Low-Cost Oxygen Reduction Electrocatalyst for Zinc-Air Batteries.

The oxygen reduction reaction (ORR) plays a vital role in many next-generation electrochemical energy conversion and storage devices, motivating the search for low-cost ORR electrocatalysts possessing high activity and excellent durability. In this work, we demonstrate that iron-cobalt phosphide (FeCoP) nanoparticles encapsulated in a N-doped carbon framework (FeCoP@NC) represent a very promising catalyst for the ORR in alkaline media. The core-shell structured FeCoP@NC catalyst offered outstanding ORR activity with a half-wave potential (E1/2) of 0.86 V vs reversible hydrogen electrode (RHE) and excellent stability in a 0.1 M KOH electrolyte, outperforming commercial Pt/C and many recently reported noble-metal-free ORR electrocatalysts. The superiority of FeCoP@NC as an ORR electrocatalyst relative to Pt/C was further verified in prototype zinc-air batteries (ZABs), with the aqueous and flexible ZABs prepared using FeCoP@NC offering excellent stability, impressive open circuit voltages (1.56 and 1.44 V, respectively), and high maximum power densities (183.5 and 69.7 mW cm-2, respectively). Density functional theory calculations revealed that encapsulating FeCoP nanoparticles in N-doped carbon shells resulted in favorable electron penetration effects, which synergistically regulated the adsorption/desorption of ORR intermediates for optimal ORR performance while also boosting the electronic conductivity. Our findings offer valuable new insights for rational design of transition metal phosphide-based catalysts for the ORR and other electrochemical applications.

Tuning the charge distribution and crystal field of iron single atoms via iron oxide integration for enhanced oxygen reduction reaction in zinc-air batteries

Metal-air batteries face a great challenge in developing efficient and durable low-cost oxygen reduction reaction (ORR) electrocatalysts. Single-atom iron catalysts embedded into nitrogen doped carbon (Fe-N-C) have emerged as attractive materials for potential replacement of Pt in ORR, but their catalytic performance was limited by the symmetrical electronic structure distribution around the single-atom Fe site. Here, we report our findings in significantly enhancing the ORR performance of Fe-N-C by moderate Fe2O3 integration via the strong electronic interaction. Remarkably, the optimized catalyst (M−Fe2O3/FeSA@NC) exhibits excellent activity, durability and good tolerance to methanol, outperforming the benchmark Pt/C catalyst. When M−Fe2O3/FeSA@NC catalyst was used in a practical zinc-air battery assembly, peak power density of 155 mW cm−2 and specific capacity of 762 mA h gZn−1 were achieved and the battery assembly has shown superior cycling stability over a period of 200 h. More importantly, theoretical studies suggest that the introduction of Fe2O3 can evoke the crystal field alteration and electron redistribution on single Fe atoms, which can break the symmetric charge distribution of Fe-N4 and thereby optimize the corresponding adsorption energy of intermediates to promote the O2 reduction. This study provides a new pathway to promote the catalytic performance of single-atom catalysts.

FeNC Catalysts with High Catalytic Activity and Stability for Oxygen Reduction Reaction

Although Pt-based catalysts have been considered as the most effective electrocatalyst for the cathodic oxygen reduction reaction (ORR) of direct methanol fuel cells (DMFCs), they still suffer from the drawbacks of high cost, poor long-term stability and methanol/ CO poisoning effects. Thus, developing low-cost ORR catalysts with high efficiency, durability and antipoisoning ability is of paramount importance. Herein, a series of non-noble metal FeNC materials are prepared through a facile pyrolysis process and used as the electrocatalysts toward ORR in alkaline electrolyte. Results show that the Fe0.50NC-800-1h catalyst pyrolyzed at 800 ºC for 1 h with the mass ratio of Fe(NO3)3•6H2O to melamine being 0.50 exhibits the highest catalytic performance among the as-prepared FeNC catalysts. The half-wave potential of ORR is ca. 0.81 V, which is only 38 mV lower than that on the noble metal Pt/C catalyst. Besides, it also displays higher stability and methanol tolerance than Pt/C. There is almost no change in the current during the chronoamperometric test when methanol is added in the electrolyte whereas significant decrease is found on Pt/C catalyst. This study of FeNC catalysts provides new insights on understanding the ORR mechanism and suggests a promising strategy to develop low-cost and highly efficient non-noble metal electrocatalysts for ORR.

Oxygen reduction reaction catalysts prepared by platinizing thermally activated zeolitic imidazolate frameworks

Highly efficient and low-cost oxygen reduction reaction (ORR) catalyst is essential to improve the cost competitiveness of the proton-exchange membrane fuel cells (PEMFCs). One potential approach of cost reduction is to apply low loading Pt over a catalytically active support made of platinum metal group free (PGM-free) material to compensate the overall ORR activity through synergistic catalysis between Pt and PGM-free support. In this report, we investigated a series of catalysts prepared by adding low-loading Pt over ORR active support prepared through thermally activated Co/Zn methyl-imidazolate framework (Co/Zn-ZIF) at various Co/Zn ratios. Catalytic activity measurement and structural characterization were performed in an attempt to gain better understanding of certain structural factors that could influence the catalytic performance.

Tuning active sites for highly efficient bifunctional oxygen electrocatalysts of rechargeable zinc-air battery

High activity, excellent durability, and low-cost oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) bifunctional catalysts are highly required for rechargeable zinc (Zn)-air batteries. Herein, we designed an electrocatalyst by integrating the ORR active species of ferroferric oxide (Fe3O4) and the OER active species of cobaltous oxide (CoO) into the carbon nanoflower. By well regulating and controlling the synthesis parameters, Fe3O4 and CoO nanoparticles were uniformly inserted into the porous carbon nanoflower. This electrocatalyst can reduce the potential gap between the ORR and OER to 0.79 V. The Zn-air battery assembled with it exhibited an open-circuit voltage of 1.457 V, a stable discharge of 98 h, a high specific capacity of 740 mA h g−1, a large power density of 137 mW cm−2, as well as good charge/discharge cycling performance, exceeding the performance of platinum/carbon (Pt/C). This work provides references for exploring highly efficient non-noble metal oxygen electrocatalysts by tuning ORR/OER active sites.

Highly Active Porous Carbon-Supported CoNi Bimetallic Catalysts for Four-Electron Reduction of Oxygen

Great interest has been taken in oxygen reduction reaction (ORR), as a common cathode reaction in new energy sources such as fuel cells and metal–air batteries, to improve the current energy structure and develop new green energy sources. Noble metal Pt is still considered the best catalyst for this reaction. However, its disadvantages such as high cost, low selectivity, poor stability, and susceptibility to impurity poisoning severely limit its further industrial applications. Therefore, development of efficient and low-cost ORR reduction catalysts is particularly critical. M-Nx-C materials have been extensively studied to achieve a high ORR activity close to that of Pt catalysts. In this study, CoNi bimetallic alloy carbon-based oxygen reduction catalysts named CoNi-950 (Zn) were successfully synthesized, using a metal–organic framework (MOF) as a carbon precursor and a nitrogen source. The experimental results show that CoNi-950 (Zn) exhibits good ORR catalytic performance with an ultimate current density of 6.27 mA cm–2, which even exceeds that of some Pt/C catalysts with a mass loading of 20 wt % (5.92 mA cm–2). In addition, the active site of the catalyst was also identified by acid leaching and SCN– poisoning experiments, showing two sites in the form of CoNi-Nx and the CoNi alloy or its oxides in the material. Further experiments demonstrate that these two active sites have distinct ORR catalytic mechanisms, generating positive synergism in ORR. A mechanism in which O2 was catalytically reduced by CoNi-950 (Zn) in a four-electron transfer, i.e., a continuous two-step dual-electron transfer reduction process in alkaline electrolytes, is proposed.

Confinement of CoOx-CoP nanoparticles inside nitrogen doped CNTs: A low-cost ORR electrocatalyst

Non-noble metal containing catalysts for oxygen reduction reaction (ORR) are being targeted due to their low cost and availability. The rarity, high-cost, and self-poisoning of noble metals are the main drawbacks for the large-scale applications. Herein, a facile and simple synthetic strategy is adopted to prepare non-noble metal-based nitrogen doped carbon nanotubes (N-CNTs). The cobalt oxides (CoOx) nanoparticles are in-situ encapsulated inside these N-CNTs which are then treated with acid (HCl) and phosphidized to obtain CoOx-CoP/N-CNTs. High resolution transmission electron spectroscopy (HR-TEM) results reveal that the ends of N-CNTs are closed with nanoparticles. X-ray photoelectron spectroscopy (XPS) confirms the presence of cobalt oxide and cobalt phosphide nanoparticles. The catalyst CoOx-CoP/N-CNTs2 demonstrated Eonset and E1/2 of 0.96 V and 0.81 V, respectively in alkaline environment which is very much comparable to the state of art catalyst (20 wt% Pt/C). Acid treatment significantly enhances the electrocatalytic activity for ORR in comparison to material synthesized without prior treatment with acid. The catalyst exhibits excellent durability over 10,000 cycles and methanol tolerance to 0.5 M methanol addition. The enhanced ORR activity is attributed to synergistic effect of carbon matrix and encapsulated nanoparticles.

Unravelling the role of interface of CuOx-TiO2 hybrid metal oxide in enhancement of oxygen reduction reaction performance

Energy security and clean energy are of increasing importance given the current scenario. Hence fuel cells and metal-air batteries are gaining significant attention in recent times. However, the major problem associated with these technologies is the sluggish nature of oxygen reduction reaction (ORR) and the expense associated with the catalyst. This study reports nitrogen-doped carbon-supported CuOx/TiO2-based heterostructure (CuOx/TiO2-10 wt%/NC-800) as an efficient, durable, and low-cost ORR catalyst. CuOx/TiO2-10 wt%/NC-800 catalyst is synthesized using a simple hydrothermal method. The ORR experiments are performed in an alkaline medium (0.1 M KOH). The catalyst has excellent ORR activity with onset potential and half-wave potential of 0.90 V and 0.80 V vs. RHE, respectively. It also shows outstanding limiting current density of 5.80 mA cm−2. The catalyst offers excellent stability (with 92% current retention after 24 h) and durability (only 25 mV half-wave potential shift after 2000 CV cycles). The catalyst follows the 4e− transfer process (n = 3.94) with rather nominal amount of hydrogen peroxide formation (3.5%). The enhanced ORR activity is attributed to the strong synergistic effect between CuOx/TiO2 and NC. The unique structure formation also helps to get excellent electronic conductivity, large surface area, appropriate porosity, and desirable interface charge transfer between CuOx and TiO2.

First-principles screening of Pt doped Ti2CNL (N = O, S and Se, L = F, Cl, Br and I) as high-performance catalysts for ORR/OER

Developing high-activity and low-cost oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) bifunctional catalysts is strategic to metal-air batteries. Herein, based on the idea that functionalized atoms can adjust the electronic properties of MXenes-based catalysts, we have screened some catalysts with high catalytic activity for ORR/OER using first-principles calculations. Specifically, we firstly explored the stability of Pt doped Ti2CNL ((N = F, Cl, Br and I, L = O, S and Se)) and Ti2CN(L)2 though calculating the vacancy formation energy and binding energy. Moreover, compared the overpotential of Pt doped Ti2CN(L)2 and Ti2CNL for ORR and OER, the Janus-MXenes configurations can tune the d-band center, work-function, density of state (DOS) and charge distribution of the catalysts, and then improve the catalytic performance of catalysts. Pt-VS-Ti2CSBr, Pt-VI-Ti2COI and Pt-VS-Ti2CSBr as ORR, OER and ORR/OER bifunctional catalysts were revealed due to their ultra-low overpotential of 0.35, 0.34 and 0.79 V, respectively. Additionally, compared to H ions, the catalyst preferentially adsorbs O2, thus ensuring the ORR advantage to proceed rather than hydrogen evolution reaction (HER). Even so, Pt-VO-Ti2COI, Pt-VS-Ti2CSI and Pt-VSe-Ti2CSeI have high activity for HER. Our work not only screened out catalysts with encouraging catalytic performance but also guided the design of high-performance catalysts.

Molecular MnN4-Complex immobilized on carbon black as efficient electrocatalyst for oxygen reduction reaction

Owing to rich-redox chemistry, metal-N4-molecular catalysts have been found as promising electrocatalysts for oxygen reduction reaction (ORR), however, de-metalation of the metal atom from N4-cavity in the electrolyte limits their activity and endurance. Herein, a MnN4-macrocyclic complex was prepared and characterized via various techniques. The results of analytical and theoretical studies suggest that the MnN4-macrocyclic complex has saddle-shaped geometry. Further, this complex was incorporated with black carbon (MnN4@CB) and studied for electrocatalytic ORR in alkaline media. The MnN4@CB composite displayed comparable ORR formal potential (−0.14 V vs Ag/AgCl) to the traditional 20% Pt/C catalyst (−0.13 V vs Ag/AgCl) and remarkable methanol tolerance. Theoretical studies revealed that the highest occupied molecular orbital (HOMO) of MnN4 has almost the same energy as the π∗ orbital of an O2 molecule, resulting in optimal coupling between HOMO and π∗ orbital, leading to efficient ORR. The charge distribution and 2D Fourier optimization of the MnN4-complex suggested that the N-donor atoms increased the electron density on Mn-atom, and transferred into the π∗ orbital of O2, weakening the O–O bond during ORR. In addition to developing low-cost ORR electrocatalysts, this work will contribute to defining their activity descriptors.

Boosting ORR performance by single atomic divacancy Zn–N3C–C8 sites on ultrathin N-doped carbon nanosheets

Single-atom catalysts (SACs) show great promise to improve the performance of catalysis because of their spatially isolated single-atom sites with unique electronic properties. Herein, we construct single Zn atoms anchored on ultrathin two-dimensional (2D) N-doped carbon nanosheets (Zn–SAs/UNCNS) as an efficient electrocatalyst for oxygen reduction reaction (ORR). The microenvironment of Zn–SAs/UNCNS with super ORR intrinsic activity was identified as the divacancy Zn–N 3 C–C 8 by both experiments and theoretical simulations. Density functional theory (DFT) calculations reveal that the divacancy Zn–N 3 C–C 8 sites exhibit near-Fermi electronic states distinct from those of graphene-enclosed Zn–N 4 –C 10 and divacancy trans -Zn–N 2 C 2 –C 8 sites, which greatly facilitate the ORR process. Furthermore, compared with 3D architecture, the single atomic divacancy Zn–N 3 C–C 8 sites anchored on ultrathin 2D carbon nanosheets show more active site exposure and fast electron transport, which collectively boost the ORR performance, showing a high half-wave potential of 0.91 V versus reversible hydrogen electrode [RHE] and a super turnover frequency (4.99 e − site −1 s −1 ). • The coordination environment of the Zn single-atom catalyst is regulated • Zn–SAs/UNCNS with divacancy Zn–N 3 C–C 8 sites demonstrate high ORR performance • Zn–SAs/UNCNS delivers a maximum primary Zn–air battery power density of 282 mW cm −2 Zn–air batteries are promising next-generation energy-storage devices. A major challenge for further boosting the efficiency of Zn–air batteries is to develop highly efficient and low-cost oxygen reduction reaction (ORR) catalysts. Due to the complexity of screening catalysts, it is of great importance to discover the underlying principles to guide the rational design of cost-effective catalysts. In this work, we identify the coordination environment of the Zn single-atom catalyst with the best ORR performance. By changing the pyrolysis temperature, the relationship between the coordination environment of the Zn single-atom catalyst and ORR activity is discovered. Additionally, we demonstrate the support effect of ultrathin 2D carbon that promotes the active sites’ exposure and electron transport compared with the bulk one. The insights of the coordination environment-activity relationship obtained can guide the further development of efficient and earth-abundant single-atom catalysts for ORR. By anchoring atomically dispersed divacancy Zn–N 3 C–C 8 sites on ultrathin two-dimensional N-doped carbon nanosheets (Zn–SAs/UNCNS), the Zn–SAs/UNCNS exhibits an excellent four-electron oxygen reduction reaction performance with a high half-wave potential of 0.91 V versus RHE and a super turnover frequency (4.99 e − site −1 s −1 ) at 0.90 V (versus RHE).

A theoretical roadmap for the best oxygen reduction activity in two-dimensional transition metal tellurides.

Developing highly active and cost-effective electrocatalysts to replace Pt-based catalysts for the sluggish oxygen reduction reaction (ORR) is a major challenge in the commercialization of fuel cells. Although two-dimensional (2D) transition-metal tellurides have recently been proposed as alternative low-cost ORR catalysts, a fundamental study on the origin of the activity is required to further optimize their composition and performance. Herein, we investigated the electronic properties and ORR catalytic performances of a series of exfoliable 2D transition-metal tellurides to uncover the underlying mechanisms by means of density functional theory simulations. Our in-depth analysis shows that the activation of the ORR mainly depends on the partially filled p z state of active Te atoms, which can simultaneously accept and donate electrons behaving similarly to both the occupied and unoccupied d orbitals of Pt atoms. This results in a linear relationship between the p z -band center and the adsorption free energies of O2 and intermediates, indicating that the p z -band center might be used as an effective descriptor to probe the performance of telluride catalysts. On this basis, we predicted several 2D transition-metal tellurides with promising catalytic performance and reduced precious-metal contents, where NbRhTe4 reaches the top of the activity volcano with a limiting potential of 0.96 V. This study provides theoretical guidance to design high-performing 2D telluride ORR catalysts, and its principle might be applicable to other electrochemical reactions in 2D chalcogenides.

Synthesis and Characterization of Nitrogen Doped Reduced Graphene Oxide Based Cobalt-ZIF-8 Catalysts for Oxygen Reduction Reaction

Non-Platinum Group Metal (non-PGM) catalysts have recently attracted huge attention for use in the Polymer Electrolyte Membrane Fuel Cell (PEMFC) vehicles, as the demand for alternative and sustainable transportation is increasing exponentially. Carbon-based catalysts have advantages such as relative ease of manufacturing, low cost of synthesis, high electrochemical durability, and high electrical conductivity which makes them a potential alternative. While there are several ways to improve the catalytic activity of carbon catalysts, one promising technique is doping heteroatoms into the lattices of the sp2 carbon structure. The most commonly used doping is by nitrogen (N-doping) where a nitrogen precursor is used to incorporate N atoms to help improve electrochemical performances. Transition Cobalt-Nitrogen-Carbon (Co-N-C) catalysts show promising catalytic performance and durability due to their fast electron transfer rates leading to increased kinetic reaction rates. In this paper, we report the synthesis and characterization of Nitrogen-doped Graphene/Graphene oxide with Metal-Organic Framework (MOF) material using wet chemical and pyrolysis techniques for the development of an efficient catalytic material to tackle the sluggish cathodic Oxygen Reduction Reaction (ORR) in fuel cells (FCs). Transition metal, Cobalt is doped into Zeolite Imidazole Frameworks (ZIF-8) using Cobalt Nitrate as precursor to enhance the mass and kinetic rates of the ORR reaction at the cathode. The synthesized metal-doped MOF catalysts (Co-ZIFx) are further pyrolyzed by one-step pyrolysis in an argon atmosphere at 600oC for 2 hours. These catalysts are produced with varying amounts of precursor metals (different molar ratios of metal precursor to ZIF-8 such as 0.6, 0.9 and 1.2 respectively; x denotes molar ratio values) to analyze the effects of each element proportion on the electrochemical performance. The pyrolyzed electrocatalysts (named Co-ZIFx-600 respectively) were further synthesized chemically using nitrogen-doped graphene (N-Gr) catalysts through wet chemical synthesis procedure and are named as N-Gr-Co-ZIFx and N-Gr-Co-ZIFx-600 (x changes with the molar ratios of metal precursor). Doping of graphene carbon lattice with nitrogen heteroatoms has proven to influence charge densities and adsorption mechanism which further enhances the catalytic activity. Nitrogen loading effects were also analyzed to study the electronic and electrochemical behavior of individually varied graphene materials. Various characterization techniques, viz., FTIR, Raman spectroscopy, XRD, SEM and EDS were performed to analyze various physical and chemical properties affecting the electrochemical performances. FTIR of N-Gr-Co-ZIF-600 catalysts showed the presence of cobalt-doped ZIF-8 peaks around 420 cm-1 and the presence of nitrogen bonds at a stretching frequency of around 3000 cm-1. XRD results showed the formation of crystalline phases of the Cobalt-doped ZIF-8 with a (100) orientation. Further, it was observed that the crystallinity was affected with an increase in the metal loading. SEM examination of the Cobalt-doped ZIF-8 samples showed an increase in grain size from around 100 nm to 2 μm with an increase in cobalt composition. EDS results of the catalysts indicated uniform distribution of the various elements such as carbon, nitrogen, cobalt, oxygen, and zinc (from ZIF-8) in varying proportions as we alter the metal precursor doping in ZIF-8. Cyclic Voltammetry is performed in 0.1M HClO4 aqueous solution to evaluate the electrochemical performances of the synthesized catalysts. It is found that the addition of the transition metal plays a major role in the enhancement of electrochemical performance compared to that of non-transition metal carbon-based catalysts. the N-Gr-Co-ZIFx-600 catalyst showed relatively better ORR activity in acidic media with onset and half-wave potentials only 24 and 51 mV lower than those of the Pt/C catalyst. Overall, the synthesized N-Gr-Co-ZIFx-600 is a potential candidate as a high activity, stable, and low-cost ORR catalyst for FCs. All the results of the characterizations will be discussed in detail for different metal precursor loadings (0.6, 0.9 and1.2 molar ratios) of different MOF derived carbon-based electrocatalysts.

Facile synthesis of hemin-based Fe-N-C catalyst by MgAl-LDH confinement effect for oxygen reduction reaction

Because oxygen reduction reaction (ORR) is vital in high-capacity energy conversion and storage systems such as metal–air batteries and fuel cells, developing high-performance, low-cost ORR catalysts is crucial. Hemin containing Fe–N4 macrocycle is an inexpensive and readily available natural material widely present in animal blood. Herein, an efficient and environment-friendly method for preparing hemin-based carbon material is proposed based on the characteristic of the flexible interlayer space of layered double hydroxides (LDH). Hemin (Hm) was initially intercalated into the interlayer region of LDH. Subsequently, high-temperature carbonization was used to prepare Hm-LDH-700, a material with a graphene-like 2D lamellar structure with high ORR catalytic activity. Characterization results showed that the material has a two-dimensional porous network structure and a large specific surface area (1065.79 m2 g−1), which provides numerous active sites in the ORR reaction. Electrochemical tests revealed that Hm-LDH-700 exhibits greater ORR catalysis than Pt/C, with a higher half-wave potential (0.86 V vs. reversible hydrogen electrode (RHE)), a higher limiting current density (6.21 mA cm−2), and a lower Tafel slope (74 mV dec−1). This study presents a simple and effective strategy for preparing high-performance ORR hemin-based catalysts, which is crucial in the practical applications of sustainable clean energy.

Defect-rich and metal-free N, S co-doped 3D interconnected mesoporous carbon material as an advanced electrocatalyst towards oxygen reduction reaction

Researching of high performance and low-cost oxygen reduction reaction (ORR) electrocatalysts is one of the core issues for the rapid development of energy conversion and storage devices. Herein, a N and S dual-doped three-dimensional (3D) interconnected mesoporous carbon catalyst (CNS) was constructed via a facile hydrothermal-pyrolysis strategy. Benefiting from the 3D interconnected mesoporous architecture, CNS was endowed with a fast oxygen mass transfer rate together with plentiful of accessible active centers on the surface. Its ORR half-wave potential (E1/2， 0.87 V) surpasses commercial Pt/C catalyst 50 mV in alkaline media. Its E1/2 negatively shifts only 2 mV after 8000 cycles in half-cell tests. More importantly, the zinc-air battery with CNS as cathode catalyst delivers an excellent high peak power density of 144 mW cm−2. DFT calculation results show that when N and S are co-doped, the spin density of adjacent carbon atoms can be changed, thereby increasing the CNS's binding energy to oxygen and conductivity. The excellent comprehensive performance of CNS undoubtedly discloses its potential practical application in the energy conversation and storage area.

Soybean powder enables the synthesis of Fe–N–C catalysts with high ORR activities in microbial fuel cell applications

The development of microbial fuel cells (MFCs) into a new type of carbon-neutral wastewater treatment technology requires efficient and low-cost oxygen reduction reaction catalysts in air cathodes. The use of raw soybean powder was investigated for synthesizing Fe–N–C ORR catalysts in a sacrificial SiO2 support method. ZnCl2 etching in the synthesis was found to facilitate the formation of hierarchical porous structures of Fe–N–C catalysts. Fe–N–C(1-1) catalyst synthesized with an optimal soybean/ZnCl2 mass ratio of 1:1 exhibited the highest ORR activity in air cathodes. The use of the obtained Fe–N–C(1-1) catalyst enables a maximum power production of ~0.480 mW cm−2 in MFCs, higher than commercial Pt/C (0.438 mW cm−2) with the same catalyst loading of 2 mg cm−2. Long-term MFC operations demonstrated that the Fe–N–C synthesized from raw soybean have high stability and toxic tolerance, indicating that abundant low cost soybean biomass is a potential material for ORR catalyst development in MFC applications.

Atomic Zn Sites on N and S Codoped Biomass-Derived Graphene for a High-Efficiency Oxygen Reduction Reaction in both Acidic and Alkaline Electrolytes

Zn–N–C catalysts with nitrogen-coordinated atomic Zn in carbon are recently found to be an alternative to Pt catalysts for oxygen reduction reaction (ORR). Nevertheless, the activity of the so far reported atomic Zn catalysts is far from satisfactory with the origin of their ORR activity still remaining ambiguous. Herein, a catalyst with atomic Zn sites anchored on N and S codoped biomass-derived graphene (named A-Zn@NSG) has been developed. The as-prepared A-Zn@NSG catalyst exhibits superior ORR performance with a half-wave potential of 0.805 V vs reversible hydrogen electrode (RHE) in 0.1 M HClO4 and 0.905 V vs RHE in 0.1 M KOH, surpassing all the reported Zn–N–C-based catalysts and comparable to the state-of-the-art nonprecious-metal-based ORR catalysts. Experimental results together with the computational results reveal the Zn–N4 active site nature of the A-Zn@NSG catalyst for ORR. More importantly, the surrounding S atoms of the Zn–N4 active sites were found to be able to donate electrons to atomic Zn centers, which enhances the binding of adsorbed O species, promotes the cleavage of the O–O bond of the adsorbed OOH species, and in turn results in a heightened ORR activity of A-Zn@NSG. The present work reports the design and development of low-cost ORR catalysts for both basic studies and realistic use.

MnO<sub>2</sub> nanosheets@Ni-N doped graphene aerogel for Zn-air battery

Zinc-air battery has the advantages of low cost and high theoretical specific capacity (1084 Wh kg−1). The sluggish kinetic of oxygen reduction reaction (ORR) of air cathode limits the reaction rate of zinc-air battery and restricts its practical applications. Moreover, scarce precious metal-based catalysts such as Pt/C are exprensive. Therefore, it is necessary to obtain low-cost ORR catalysts. The conductivity of catalysts affects electron transfer rate in ORR process and consequently affects the catalytic activity. MnO2 has been studied as a promising ORR catalyst for its advantages of low cost, abundant resources and environmental friendliness. However, the poor electronic conductivity of MnO2 limits its applications in ORR related energy storage devices including zinc-air battery. The conductivity can be improved by assembling MnO2 with highly conductive metals and carbon materials. Graphene and its derivatives are often used as substrates of electrochemical materials due to its excellent electronic conductivity and good electrochemical stability. By doping N atoms into graphene skeleton, electronic state can be improved. Doped N atoms make the nearby C atoms become active adsorption sites. Besides, the introduction of oxygen vacancies (OVs) can also improve electronic conductivity of the materials. In this work, a coassembly of MNSs@Ni-NGA showing high ORR catalytic activity was fabricated containing Ni nanoparticles of 10−15 nm in diameter, MnO2 nanosheets (MNSs) of 5−10 nm in sheet thickness at the ratio of exposed surficial to total unit cells of 37.4%, the ratio of OVs to total O atoms of 46.9at% and NGA about 300 nm in wall thickness. Ni nanoparticles, MNSs and NGA all have ORR activity, providing plenty of active sites for ORR. Ni nanoparticles, NGA and OVs improve the conductivity of coassembly and facilitate electron transfer. At the same time, NGA prevents Ni nanoparticles from falling off and aggregating. The big specific surface area (208.0 m2 g−1) of MNSs@Ni-NGA benefits sufficient exposure of active sites. Plenty of mesoporous pores in MNSs@Ni-NGA benefit fast mass transfer during electrochemical process. The basic ORR was characterized by rotating disk electrode (RDE) in electrochemical workstation. Performances of Zn-air battery were studied by electrochemical workstation and LAND battery test system. Through synergistic effects of Ni nanoparticles, MNSs, OVs and NGA, MNSs@Ni-NGA has an onset potential of 0.98 V and a half wave potential of 0.84 V in alkaline electrolyte, close to those of commercial Pt/C (0.98 and 0.85 V). To demonstrate practical application, a Zn-air battery with MNSs@Ni-NGA as air cathode material was assembled, showing an open circuit voltage of 1.53 V, a maximum power density of 114.8 mW cm−2, a specific capacity of 801.9 mAh gZn−1 and a specific energy of 962.3 Wh kgZn−1 (88.8% of the theoretical specific energy), higher than those of commercial Pt/C+IrO2. This work provides a strategy for designing and assembling high performance ORR catalysts with non precious metallic nanoparticles and OVs-containing MnO2 nanostructures for energy storage devices.

In-situ synthesis of heteroatom co-doped mesoporous dominated carbons as efficient electrocatalysts for oxygen reduction reaction

Developing high-performance and low-cost oxygen reduction reaction (ORR) catalysts plays a significant role in the practical application of energy-related devices. Herein, heteroatom (N/B, N/P and N/S) co-doped mesoporous dominated carbons are in-situ synthesized through a facile method. As-prepared NB-C-3, NP-C-2 and NS-C-4 all exhibit outstanding ORR electrocatalytic performance via 4e− pathway in neutral medium (50 mM PBS). Moreover, NS-C-4 (onset potential: 0.769 V (vs. RHE), half-wave potential: 0.613 V (vs. RHE)) possesses more excellent ORR activity than that of NB-C-3 and NP-C-2, and even than that of the commercial Pt/C catalysts. Notably, the cost of raw material (soybean) for these carbons is 2.15 × 10−3 $ g−1, which is nearly a hundred thousand times cheaper than that of Pt/C catalysts (20 wt%, 220.04 $ g−1). Therefore, NB-C-3, NP-C-2 and NS-C-4 should be promising ORR catalysts to replace Pt/C catalysts in the application of energy conversion and storage.

Atomically dispersed iron on nitrogen-decorated carbon for high-performance oxygen reduction and zinc-air batteries

Designing low-cost oxygen reduction reaction (ORR) electrocatalysts to replace platinum group metal holds great promise for boosting the wide use of metal-air batteries. Herein, atomically dispersed iron supported on nitrogen-doped carbon (A-Fe-NC) was fabricated via an effective ligand-stabilized pyrolysis strategy. Owing to the high density of the active sites and the two-dimensional (2-D) nanosheet structure, the as-prepared A-Fe-NC electrocatalyst delivers excellent ORR performance with a positive half-wave potential of 0.865 V in alkaline solution, which is superior to that of Pt/C (0.82 V vs RHE). Moreover, the primary Zn-air battery assembled with the A-Fe-NC delivered an ultrahigh specific power of 132.2 mW cm−2 and stable long-term operation. It also displayed robust charging-discharging cycling performance with voltage gap of 0.83 V after 240 h, lower than that of the Pt/C + RuO2 electrode (0.95 V). To reveal the catalytic mechanism of A-Fe-NC, density functional theory calculation was conducted and the results revealed that the interaction between Fe and nitrogen regulated the electronic structure of active sites, thus improving charge transfer and further promoting the electrocatalytic reactivity for ORR. This work provided a facile and effective strategy for constructing Fe/N codoped 2-D carbon nanocomposite as high performance electrocatalysts for energy conversion.