Bifunctional Oxygen Reduction Reaction Research Articles

Boosting Bifunctional Catalysis by Integrating Active Faceted Intermetallic Nanocrystals and Strained Pt-Ir Functional Shells.

PtIr-based nanostructures are fascinating materials for application in bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysis. However, the fabrication of PtIr nanocatalysts with clear geometric features and structural configurations, which are crucial for enhancing the bifunctionality, remains challenging. Herein, PtCo@PtIr nanoparticles are precisely designed and fabricated with a quasi-octahedral PtCo nanocrystal as a highly atomically ordered core and an ultrathin PtIr atomic layer as a compressively strained shell. Owing to their geometric and core-shell features, the PtCo@PtIr nanoparticles deliver approximately six and eight times higher mass and specific activities, respectively, as an ORR catalyst than a commercial Pt/C catalyst. The half-wave potential of PtCo@PtIr exhibits a negligible decrease by 9mV after 10 000 cycles, indicating extraordinary ORR durability because of the ordered arrangement of Pt and Co atoms. When evaluated using the ORR-OER dual reaction upon the introduction of Ir, PtCo@PtIr exhibits a small ORR-OER overpotential gap of 679mV, demonstrating its great potential as a bifunctional electrocatalyst for fabricating fuel cells. The findings pave the way for designing precise intermetallic core-shell nanocrystals as highly functional catalysts.

A Redox-Mediated Zinc-Air Fuel Cell

As one of the most promising power sources in converting and storing chemical energy, aqueous zinc-air batteries (ZABs) stand out because of their high theoretical energy density, low cost, high safety of aqueous electrolyte, and environmental friendliness. Three main types of ZABs including primary ZABs, mechanically rechargeable ZABs, and electrically rechargeable ZABs have been developed up to now.[1] Primary ZABs for single use are usually discarded after the Zn anode is consumed or passivated although other parts of battery remain intact, leading to enormous waste. To address it, the depleted anode and failure electrolyte can be physically replaced to recover the capacity in mechanically rechargeable ZABs. But the cumbersome disassembly/reassembly process and the performance deterioration after every recharge process limit its practical application. In addition, it is not feasible to replace zinc in some integrated systems, such as the large-scale ZAB stack. Electrically rechargeable ZABs with bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) catalysts could be returned to the original state by electrochemical charging process, but also face some formidable challenges, such as dendritic zinc deposition, electrode deformation, and hydrogen evolution, limiting the long-term cycling performance. Besides, sluggish kinetics of ORR/OER and Zn passivation result in low energy efficiency, limited depth of discharge and power density. Especially in the resource-constrained and off-grid region, there isn’t external electricity source to charge the electrically rechargeable ZABs which limits their applications. Moreover, the reactions inherently take place on the surface of Zn anode and air cathode, in which the storage and conversion capability of the cell would be essentially constrained by the surface area of electrode and the volume of electrode compartment.The concept of redox targeting (RT) of battery materials offers a feasible solution to address these issues by employing redox mediators (RMs). To tackle the above challenges, here we demonstrate a redox-mediated Zn-air fuel cell (RM-ZAFC) based on the RT reactions of cobalt triisopropanolamine complex (CoTiPA) with O2 and 7,8-dihydroxy-2-phenazinesulfonic acid (DHPS) with Zn metal in the catholyte and anolyte, respectively. Both the 4-electron (4 e-) ORR and Zn oxidation reactions are liberated from the electrode surface and occur inside separate reactor tanks resorting to the RT processes. Upon operation, Co(III)TiPA in the catholyte is electrochemically reduced to Co(II)TiPA on the cathode and chemically oxidized back through ORR in the gas diffusion tank (GDT) into which O2 is fed. In conventional ZABs, ORR takes place on the triple-phase reaction interface of the heterogeneous electrocatalyst, and the catalytic performance is closely related to the density of exposed active site, electrical conductivity and reaction energy barrier. As compared, the redox-mediated ORR in GDT occurs homogeneously in the catholyte in which the dissolved RM could provide a large quantity of rection sites with tunable reaction energy barrier. For the negative side, DHPS-2H is firstly oxidized to DHPS on the anode and circulated into anodic tank, where it is regenerated by oxidizing Zn to ZnO. The discharge product ZnO is formed and deposited on the Zn granules loaded in the tank. After fully discharged, the cell can be feasibly refueled by replacing the Zn granules in the tank instead of disassembling and reassembling the whole cell. In this RM-ZAFC, the energy storage unit is decoupled from the power generation unit, thus offering greater operation flexibility and system scalability with modular design than the conventional ZABs. Compared with the commercial hydrogen tank used in hydrogen fuel cells, Zn granules loaded in the anodic tank present much higher volumetric capacity, greater safety and ease for storage and transportation. Therefore, as an energy conversion and storage device, RM-ZAFC would have great promise for automotive, backup, and stationary power applications.[2] In addition, we also propose an efficient and zinc regeneration method by RT reaction between RM and the product ZnO in RM-ZAFC to avoid the waste of ZnO and reduce the cost.

Three-Dimensional Fe-N-C-Supported Amorphous NiFe alloy Nanoparticles as an Efficient Bifunctional Electrocatalyst for Zn-Air Batteries

The development of highly active and durable bifunctional oxygen reduction and evolution reaction (OER/ORR) electrocatalysts is vitally important for rechargeable metal-air batteries. However, little research has been done on the development of amorphous materials as bifunctional catalysts. Here, amorphous NiFe alloy nanoparticles (a-NiFe) coupled to transition metal nitrogen-doped carbon (Fe-N-C) substrate (a-NiFe@Fe-N-C) have been produced and ducted by a simple synthesis method. It is confirmed that the a-NiFe@Fe-N-C composite has significant amorphous characteristics according to XRD, HR-TEM, and SAED characterization methods, and the amorphous structure has abundant defects caused by the disordered arrangement of atoms, which is conducive to accommodate a large mass of active sites and promote the performance of catalysts. Additionally, the large specific surface area, the interconnected multistage pore structure, and the superior electrical conductivity of the Fe-N-C substrate, as well as the synergistic effect between bimetals Ni and Fe, make a-NiFe@Fe-N-C composite electrocatalyst present superior bifunctional catalytic activity and durability. Remarkably, compared with benchmark Pt/C + IrO2, the rechargeable liquid Zinc-air battery using a-NiFe@Fe-N-C as cathode exhibits a superior power density (170 mW·cm−2), specific capacity (687.65 mAh·gZn-1) and cycling durability (100 h).

Copper-cobalt bimetallic conductive metal–organic frameworks as bifunctional oxygen electrocatalyst in alkaline and neutral media

The preparation of low cost bifunctional electrocatalysts that efficiently catalyze oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is a crucial step towards realizing high-performance Zn–air batteries. Herein a bimetallic CuCo-HITP (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) conductive metal-organic frameworks (MOFs) was synthesized as a bifunctional electrocatalyst for OER and ORR through an effective and facile ammonia-assisted method. Structural characterization showed that the metal ions coordinated with HITP ligands through strong coordination bonds of M − N4 to construct a two-dimensional (2D) porous structure. The electrochemical tests demonstrated that CuCo-HITP had superior bifunctional OER and ORR electrocatalytic performance in both alkaline and neutral media. It should be attributed to the bimetallic synergistically effect, which lead to more active sites, faster charge transfer rate and improvement of the catalytic reaction kinetics.

Metal-mediated Schiff base polymer enables metal/nitrogen codoped carbon nanosheets as efficient bifunctional electrocatalyst for durable rechargeable Zn-air batteries

We report a simple yet robust route to bifunctional electrocatalysts comprising FeNi and Co4N hybrid nanoparticles co-embedded within N-doped carbon nanosheets (denoted FeNi/Co4N-NCS) for rechargeable Zn-air batteries (ZABs) with high power density and capacity as well as outstanding cycling stability, markedly outperforming the Pt/C&IrO2 electrocatalyst counterpart. The as-crafted FeNi/Co4N-NCS electrocatalysts display outstanding bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity with a low overall oxygen electrode activity (ΔE = 0.664 V). Density functional theory (DFT) calculations reveal that the synergy of FeNi and Co4N reduces the rate-determining step (RDS) energy barrier, thereby greatly increasing electrocatalytic activity. The aqueous ZAB assembled with FeNi/Co4N-NCS delivers a high power density of 160 mW cm−2, large specific capacity of 812 mAh g−1Zn, and long cycling life of 1450 h at 5 mA cm−2. This study highlights an effective strategy to yield durable bifunctional electrocatalysts for metal-air batteries.

Recent Advances in Nanocarbon-Based Nonprecious Metal Catalysts for Oxygen/Hydrogen Reduction/Evolution Reactions and Zn-Air Battery

Abstract In this review article, we focus on the design and creation of earth-abundant non-precious metal-coordinated eco-friendly catalysts with high oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performances. The development of a cost-effective, efficient and durable bifunctional ORR and OER electrochemical catalyst is a key for future batteries. So far, precious metals, such as Pt and Ir, have been used as efficient catalysts for the ORR and OER, respectively. However, they are expensive and their deposits on earth are limited, which are the major drawbacks of these metals. One more drawback for the practical application is the sluggish reactions of ORR and OER. Since electroconductivity of promising metal-free catalyst candidates are poor, good electronic supporting materials are needed. Owing to the high conductivity, stability and easy handling, carbon nanotubes are a most promising material. After Introduction, we summarize recent advances in i) carbon nanotube/nanocarbon-based ORR and OER catalysts, ii) metal oxide and others/nanocarbon catalysts for ORR, OER, HER and Zn-air electrodes, as well as iii) computational study for such catalyst reactions. Such studies are highly beneficial for future energy materials design and synthesis in batteries and molecular catalysts with high performances.

Atomically Dispersed Fe–N4 Sites and NiFe-LDH Sub-Nanoclusters as an Excellent Air Cathode for Rechargeable Zinc–Air Batteries

The sluggish four-electron processes of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) limit the development of rechargeable Zn-air batteries (RZABs). Highly efficient ORR/OER bifunctional electrocatalysts are therefore highly desired for the commercialization of RZABs in large scale. Herein, the Fe-N4-C (ORR active sites) and NiFe-LDH clusters (OER active sites) are successfully integrated within a NiFe-LDH/Fe,N-CB electrocatalyst. The NiFe-LDH/Fe,N-CB electrocatalyst is first prepared by the introduction of Fe-N4 into carbon black (CB), followed by the growth of NiFe-LDH clusters. The cluster nature of NiFe-LDH effectively avoids the blocking of Fe-N4-C ORR active centers and affords excellent OER activity. The NiFe-LDH/Fe,N-CB electrocatalyst thus exhibits an excellent bifunctional ORR and OER performance, with a potential gap of only 0.71 V. The NiFe-LDH/Fe,N-CB-based RZAB exhibits an open-circuit voltage of 1.565 V and a specific capacity of 731 mAh gZn-1, which is much better than the RZAB composed of Pt/C and IrO2. Particularly, the NiFe-LDH/Fe,N-CB-based RZAB displays excellent long-term charging/discharging cyclic stability and rechargeability. Even at a large charging/discharging current density (20 mA cm-2), the charging/discharging voltage gap is only ∼1.33 V and exhibits an increase smaller than 5% after 140 cycles. This work provides a new low-cost bifunctional ORR/OER electrocatalyst with high activity and superior long-term stability and will be helpful to the commercialization of RZAB in large scale.

Cobalt Nanoparticles Encapsulated with N-Doped Bamboo-Like Carbon Nanofibers as Bifunctional Catalysts for Oxygen Reduction/Evolution Reactions in a Wide pH Range

Nonprecious bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) catalysts in wide pH electrolytes are crucial for the versatile use of electrochemical energy storage and conversion devices. Non-noble metal/nitrogen-doped carbon (M–N/C) has been reported as a promising candidate for efficient and cost-effective bifunctional catalysts. Nevertheless, the stability and catalytic activity of M–N/C bifunctional catalysts in a wide pH range are still limited, especially in neutral and acidic electrolytes. Here, we synthesized directly grown N-doped bamboo-like carbon nanofiber-encapsulated cobalt nanoparticles on carbon flakes (Co–N/C fiber) via pyrolysis of 2D cobalt–zeolitic imidazolate framework and tubular g-C3N4. The in situ growth of N-doped carbon nanofiber incorporated with Co nanoparticles exposes large active sites and enhances charge transfer. Moreover, abundant mesopores with high loading of pyridinic-N, graphitic-N, Co@N/C, and Co–Nx facilitate high catalytic activity in a wide pH condition. Co–N/C fiber shows superior ORR performance in alkaline, neutral, and acidic electrolytes with high onset potentials of 1.003, 0.820, and 0.800 V vs RHE, respectively. A comparable OER performance of Co–N/C fiber to the Ir/C benchmark in alkaline and neutral conditions can be obtained with 397 and 570 mV overpotentials at 10 mA cm–2. Utilizing Co–N/C fiber in alkaline and neutral Zn–air batteries exhibits power densities of 155 and 67 mW cm–2, with excellent stability for over 180 h. This study offers a strategy for developing a bifunctional M–N/C catalyst that is applicable across a wide pH range via hybrid nanostructuring.

Molten salt synthesis of ZIF derived CoFe alloy embedded in 2D ultrathin N-doped porous carbon nanosheets for efficient alkaline media oxygen electrocatalysis

Developing advanced electrocatalysts for cooperative oxygen electrocatalysis is significant to metal-air batteries. Herein, we report a novel and simple molten salt approach for constructing CoFe alloy embedded in 2D ultrathin N-doped porous carbon nanosheets (CoFe/NPC). Furthermore, the pyrolysis temperature and Co/Fe molar ratio were optimized for high activity. As expected, the optimal Co0.5Fe0.5/NPC-900 manifests outstanding catalytic activity and durability for bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). It only needs a positive half-wave potential of 0.89 V to drive ORR and a low overpotential of 318 mV to afford 10 mA·cm−2 for OER, outperforming that of commercial Pt/C and RuO2. More impressively, excellent durability with continuous operation is also achieved. The numerous exposed catalytic sites, the accelerated mass and charge transfer, and the interplay between CoFe alloy and the 2D ultrathin N-doped carbon species account for the enhanced catalytic performance of CoFe/NPC. Similarly, the architecture of porous substrate anchoring CoFe alloy enables long-term durability due to strong metal-support interaction.

Aerophilic Triphase Interface Tuned by Carbon Dots Driving Durable and Flexible Rechargeable Zn-Air Batteries

HighlightsOxygen-respirable sponge was constructed by carbon dots-assisted synthesis strategy.The hydrophilicity and aerophilicity of Co@C–O–Cs active sites boost oxygen diffusion and the bifunctional oxygen reduction reaction and oxygen evolution reaction activities.Co@C–O–Cs-based Zn-air battery (ZAB) displays excellent specific capacity and stability, and the all-solid-state Co@C–O–Cs-based ZAB exhibits excellent flexibility.

Rational design of 2D MBene-based bifunctional OER/ORR dual-metal atom catalysts: a DFT study.

Designing highly active, low-cost, and bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts is urgent for the development of metal-air batteries. Herein, by density functional theory (DFT) calculations, we systematically reported a series of dual-metal atom adsorbed novel two-dimensional (2D) MBenes as efficient bifunctional catalysts for the OER/ORR (namely 2TM/TM1TM2-Mo2B2O2, TM = Mn, Fe, Co, Ni). Our theoretical results show that 2Ni-Mo2B2O2, FeCo-Mo2B2O2 and CoNi-Mo2B2O2 exhibit outstanding OER/ORR catalytic activity with overpotentials of 0.49/0.27 V, 0.38/0.50 V and 0.25/0.51 V, respectively, exceeding those of IrO2(110) for the OER and Pt(111) for the ORR. Additionally, these highly active bifunctional catalysts can effectively suppress the hydrogen evolution reaction (HER), ensuring the absolute preference for the OER/ORR. More importantly, the Bader charge (QTM) of adsorbed dual-metal atoms is used as a descriptor of OER/ORR catalytic activity, which is linearly related to ηORR and volcanically related to -ηOER. Our work not only provides new theoretical guidance for developing noble metal-free bifunctional electrocatalysts but also enriches the application of MBenes in electrocatalysis.

Innovating Rechargeable Zn-Air Batteries for Low Charging Voltage and High Energy Efficiency

The commercialization of rechargeable zinc-air batteries (ZABs) is significantly impeded by their high charging voltage and low energy efficiency. In this work, we apply soluble KI to ZABs as a reaction modifier to change the oxidation pathway in the charging process from oxygen evolution reaction to I– oxidation reaction (IOR), which enjoys lower oxidation potential and faster kinetics. In addition, we surprisingly find that commercial cost-effective XC72R carbon delivers satisfactory bifunctional IOR and oxygen reduction reaction activity. Density functional theory calculation is provided to demonstrate the IOR mechanism by XC72R carbon. Finally, the KI-modified ZABs with XC72R exhibit a low charging voltage of 1.79 V, improved energy efficiency of 65.3% (2.0 V and 60% for conventional ZABs with noble-metal catalysts), and long cycle life of over 120 h at 5 mA cm–2. Further characterizations have illuminated the reason behind the extended battery cycle life as well. This work can significantly facilitate the future use of rechargeable ZABs in energy storage.

NCNT grafted perovskite oxide as an active bifunctional electrocatalyst for rechargeable zinc-air battery

Precious-metal-free electrocatalysts with high activity to bifunctionally activate both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are key enablers for the scalable adoption of zinc-air batteries (ZABs). In this work, a unique hybrid catalyst comprising nitrogen-doped carbon nanotube (NCNT) in-situ grown directly on La0·8Sr0·2Ti0·65Fe0·35O3-δ perovskite oxide (denoted as LSTFO/NCNT) has been developed as a bifunctional ORR and OER electrocatalyst for ZAB applications. The compositionally optimized LSTFO/NCNT hybrid delivers remarkable bifunctional oxygen electrocatalysis performances with a characteristic potential gap (ΔE) of 0.76 V. On a device level, the primary and rechargeable homemade ZABs demonstration using such an LSTFO/NCNT hybrid as an air electrode catalyst exhibit reliable electrochemical performances of high peak power density and cycling stability with a final round-trip efficiency of 62.8% at 10 mA cm−2 during 123 cycles. The active oxygen electrocatalytic activity is believed to be related to the considerable level of pyridinic-N and graphitic-N on NCNT, abundant oxygen vacancies, improved electrical conductivity and the strong electronic interaction between LSTFO and NCNT. The DFT calculation results demonstrated that the electron transfer from NCNT to LSTFO not only reduces the oxygen adsorption and activation energy, but also activates the surface lattice oxygen and favors the formation of O–O bond through reinforcing the Fe/Ti–oxygen covalency, leading to exceptional bi-functionality. This work demonstrates a robust method to obtain exceptional bi-functional oxygen electrocatalysis based on transitional metal oxides for high-performance metal-air batteries.

Atomic Modulation and Structure Design of Fe−N4 Modified Hollow Carbon Fibers with Encapsulated Ni Nanoparticles for Rechargeable Zn–Air Batteries

AbstractExcellent bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) activity and rapid mass transport capability are two important parameters of electrocatalysts for high‐performance rechargeable Zn–air batteries (ZABs). Herein, an efficient atomic modulation and structure design to promote bifunctional activity and mass transport kinetics of an ORR/OER electrocatalyst are reported. Specifically, atomic Fe−N4 moieties are immobilized on premade hollow carbon fibers with encapsulated Ni nanoparticles (Fe‐N@Ni‐HCFs). Synchrotron X‐ray absorption spectroscopy and spherical aberration‐corrected electron microscope analyses confirm the atomic distribution of the active sites and unique lung bubble‐like hollow architecture of the catalyst, while theoretical investigations reveal that the encapsulated Ni nanoparticles can induce electron distribution of the atomic Fe−N4 moieties to reduce reaction energy barriers. As a result, the prepared catalyst possesses enhanced bifunctional ORR/OER activity and well‐constructed gas–solid–liquid interfaces for improved mass transfer. These synergetic advantages endow the binder‐free Fe‐N@Ni‐HCFs electrode with the remarkable power density and cycling stability for ZABs, outperforming the commercial Pt/C+Ir/C benchmark. This exceptional performance suggests that the proposed strategy can be extended to the design and fabrication of electrocatalysts for energy conversion and storage.

CoS2 nanoparticles-embedded N-doped carbon hybrid derived from ZIF-L as bifunctional oxygen electrocatalysts for rechargeable Zn-air batteries

The unique CoS 2 @N-doped carbon hybrids with core-shell structure could effectively improve oxygen bifunctional catalysis for Zn−air batteries. • The ZIF-L (Zn,Co) template-sacrificing and two-steps pyrolysis synthesis method. • Unique CoS 2 @ N -doped carbon hybrids with core–shell structure and small-size pores. • The bifunctional oxygen catalyst with long-term stability for rechargeable Zn-air battery. Zeolitic imidazolate frameworks (ZIFs) with controllable tunability of compositions and morphologies are identified as efficacious self-sacrificial precursors to prepare functionally-oriented nanostructured materials. Pressingly, we urgently need to use highly efficient bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalysts for the enhancement of rechargeable metal air batteries research. Herein, the unique CoS 2 @ N -doped carbon (CoS 2 @NC-X) hybrids are successfully achieved via direct calcination and sulfurization of 2D leaf-like bimetallic imidazole framework (ZnCo-ZIF-L). Owing to the synergic effect between intrinsic activity of CoS 2 nanoparticles, the unique core–shell structure and small-size pores, the optimal CoS 2 @NC-60 is demonstrated to exhibit excellent ORR- and OER- catalytic activity. In particular, the ORR of CoS 2 @NC-60 not only possesses a half-wave potential of 0.84 V (compared to RHE), but also has a limiting current density of 5.18 mA cm −2 . Furthermore, CoS 2 @NC-60 possesses an overpotential (compared to RHE) of 350 mV required to drive the current density of 10 mA cm −2 . As a validation, the CoS 2 @NC-60 catalyst is intended for use in rechargeable Zn-air batteries, where it has the outstanding long-term stability (550 h) and an exceptional power density (170.6 mW cm −2 ). This facile synthetic strategy renders a feasible way to further probe non-noble metal electrochemical catalysts.

Schiff-base polymer derived FeCo-N-doped porous carbon flowers as bifunctional oxygen electrocatalyst for long-life rechargeable zinc-air batteries

Rational design and exploration of low-cost and robust bifunctional oxygen electrocatalysts are vitally important for developing high-performance zinc-air batteries (ZABs). Herein, we reported a facile yet cost-efficient approach to construct a bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) electrocatalyst composed of N-doped porous carbon nanosheet flowers decorated with FeCo nanoparticles (FeCo/N-CF). Rational design of this catalyst is achieved by designing Schiff-base polymer with unique molecular structure via hydrogen bonding of cyanuramide and terephthalaldehyde polycondensate in the presence of metal cations. It exhibits excellent activity and stability for electrocatalysis of ORR/OER, enabling ZAB with a high peak power density of 172 mW cm−2 and a large specific capacity of 811 mA h g−1Zn at large current. The rechargeable ZAB demonstrates excellent durability for 1000 h with slight voltage decay, far outperforming a couple of precious Pt/Ir-based catalysts. Density functional theory (DFT) calculations reveal that high activity of bimetallic FeCo stems from enhanced O2 and OH− adsorption and accelerated O2 dissociation by OO bond activation.

BIAN-based durable polymer metal complex as a cathode material for Li–O2 battery applications

Among several strategies employed to reduce overpotential and achieve reliable reversibility with Li–O2 batteries, the use of atomically dispersed bifunctional carbon catalysts is very attractive. However, most of the methods used to prepare these bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts require high temperatures and exhibit low yields, and it is therefore difficult to predetermine the active sites qualitatively and quantitatively. Here, we propose the use of atomically dispersed metal centers coordinated to diimine moieties of conjugated polymers as bifunctional catalysts without further modification (pyrolysis or composite formation) for Li–O2 battery applications. Poly(bisiminoacenaphthenequinone) (BIAN) iron complex (BP-Fe) catalysts showed high OER activities, which enabled 100% coulombic efficiency for 160 galvanostatic charge discharge cycles with a capacity limit of 500 mAh/g at a current density of 250 mA/g. The overpotential corresponding to charging was as low as ~1.0 V and exhibited almost no change in discharge overpotential across 160 cycles. Additionally, it showed a commendable rate capability with only a 170 mV increase in charge overpotential when the charge‒discharge rate was increased from 100 to 500 mA/g.

Theoretical inspection of TM-P4C single-atom electrocatalysts: High performance for oxygen reduction and evolution reactions

Developing the cost-effective or even bifunctional electrocatalysts for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) with industrially relevant activity is highly desired for metal-air batteries at the current stage. Herein, in this work, the catalytic performances of the single transition metal (TM) atom embeds graphene sheet with the tetra-coordinates Phosphorus (TMP4) for ORR and OER are investigated based on the density functional theory method. The results demonstrate that the most promising ORR and OER catalytic activity can be achieved on the CoP4 with the smallest potential gap ΔE and the lowest overpotential of 0.37 and 0.32 eV among all TMP4 systems, respectively, and the catalytic activity is even better than that of the traditional Pt and IrO2 catalysts. Furthermore, the AIMD and phonon dispersion calculations are conducted to confirm the thermodynamics and dynamics stability of CoP4. This work screens out promising candidates for novel graphene-based bifunctional ORR and OER catalysts and offers detailed microscopic insights into the mechanism of OER/ORR, provides a theoretical guidance for the development of further single-atom catalysts.

Multi-Walled Carbon Nanotube Supported Manganese Selenide As Highly Active Bifunctional OER and ORR Electrocatalyst

Transition metal selenides have attracted intensive interest as cost-effective electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) because of the continuous thrust in sustainable energy conversion. In this article a Mn-based bifunctional electrocatalyst, MnSe has been identified which shows efficient OER and ORR activity in alkaline medium. The catalytic activity could be further enhanced by using multiwalled carbon nanotubes (MWCNT) asadditives which increases the charge transfer and electronic conductivity of the catalyst composite. This MnSe@MWCNT catalyst composite exhibits a very low overpotential of 210 mV at 50 mA cm-2 when deposited on Ni foam, which outperforms state-of-the-art RuO2 as well as other oxide and Mn-based electrocatalysts. Furthermore, the composite’s facile OER kinetics was evidenced by its small Tafel slope of 54.76 mV dec–1 and low charge transfer resistance, indicating quick transport of the reactant species. The MnSe@MWCNT also exhibited efficient electrocatalytic activity for ORR with a Eonset of 0.94 V, which is among the best reported till date for chalcogenide based ORR electrocatalysts. More importantly, this MnSe-based ORR electrocatalyst exhibits high degree of methanol tolerance, showing no degradation of catalyst performance in presence of copious quantities of methanol, thereby out-performing state-of-the-art Pt electrocatalyst. The catalyst compositie also exhibited exceptional functional and compositional stability for OER and ORR after prolonged period of continuous operation in alkaline medium. The surface Raman analysis after OER revealed the retention of manganese selenide surface with evidence of Mn-oxo coordination confirming the formation of mixed anionic (oxy)selenide as the active surface for OER. Such efficient bifunctional OER and ORR activity makes this MnSe based catalyst attractive for overall electrolysis in regenerative as well as direct methanol fuel cells.

Co/Co-N/Co-O Rooted on rGO Hybrid BCN Nanotube Arrays as Efficient Oxygen Electrocatalyst for Zn-Air Batteries.

Developing high-performance non-noble metal bifunctional oxygen reduction and evolution reaction electrocatalysts is a critical factor for the commercialization of rechargeable Zn-air batteries. Herein, Co/Co-N/Co-O rooted on reduced graphene oxide (rGO) hybrid boron and nitrogen codoped carbon (BCN) nanotube arrays (BCN/rGO-Co) is prepared by facile low-temperature precross-linking and high-temperature pyrolysis treatment. Benefit from the synergistic effect of its B/N codoping, Co/Co-N/Co-O bifunctional active sites, 3D hybrid porous structure of BCN nanotubes, and highly conductive rGO sheets. The obtained BCN/rGO-Co exhibits superior bifunctional oxygen catalytic activity with a positive ORR half-wave potential (0.85 V) and a low OER potential (1.61 V) at 10 mA cm-2. Additionally, the BCN/rGO-Co-based liquid Zn-air batteries displays a large peak power density of 157 mW cm-2, and a long charge/discharge cycle stability of 200 h, outdoing the commercial Pt/C+Ru/C catalyst.