An enhanced non-noble perovskite-based oxygen electrocatalyst for efficient oxygen reduction and evolution reactions

Here, the hybrid of NiCo2S4 nanoparticles grown on graphene in situ is first described as an effective bifunctional nonprecious electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the alkaline medium. NiCo2S4@N/S-rGO was synthesized by a one-pot solvothermal strategy using Co(OAc)2, Ni(OAc)2, thiourea, and graphene oxide as precursors and ethylene glycol as the dispersing agent; simultaneously, traces of nitrogen and sulfur were double-doped into the reduced graphene oxide (rGO) in the forms of pyrrolic-N, pyridinic-N, and thiophenic-S, which are often desirable for metal-free ORR catalysts. In comparison with commercial Pt/C catalyst, NiCo2S4@N/S-rGO shows less reduction activity, much better durability, and superior methanol tolerance toward ORR in 0.1 M KOH; it reveals higher activity toward OER in both KOH electrolyte and phosphate buffer at pH 7.0. NiCo2S4@graphene demonstrated excellent overall bicatalytic performance, and importantly, it suggests a novel kind of promising nonprecious bifunctional catalyst in the related renewable energy devices.

Orange peel derived activated carbon supported Ni–Co3O4 as an efficient electrocatalyst for oxygen evolution and reduction reaction in alkaline media

Bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are the heart of metal-air batteries, fuel cells, and other energy storage systems. Here, we report a series of a novel class of redox-active viologen-based ionic covalent organic networks (vCONs) which are directly used as metal-free bifunctional electrocatalysts towards ORR and OER applications. These vCONs (named vGC, vGAC, vMEL and vBPDP) were synthesized by the well-known Zincke reaction. The installation of redox-active viologen moieties among the extended covalent organic architectures played a crucial role for exceptional acid/base stability, as well as bifunctional ORR and OER activities, confirmed by the cyclic voltammetry (CV) curves. Among all of them, vBPDP showed high ORR efficiency with a half-wave potential of 0.72 V against a reversible hydrogen electrode (RHE) in 1 M KOH electrolyte. In contrast, vMEL demonstrated high OER activity with an overpotential of 320 mV at a current density of 10 mAcm-2 and a Tafel slope of 109.4 mV dec-1 in 1 M KOH electrolyte solution. This work is exceptional and unique in terms of directly used pristine ionic covalent organic networks that are used as bifunctional (ORR and OER) electrocatalysts without adding any metals or conductive materials.

The effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

Efficient nonprecious‐metal oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts are key for the commercial viability of fuel cells, metal–air batteries, and water‐splitting systems. Thus, high‐performance ORR and OER electrocatalysts in acidic electrolytes are needed to support high‐efficiency proton exchange membrane (PEM)‐based systems. Herein, we report a new approach to design and prepare an ultrathin N‐doped holey carbon layer (HCL) on a graphene sheet that exhibits outstanding bifunctional ORR/OER activities in both alkaline and acidic media. The edge sites of HCL are utilized to achieve selective doping of highly active pyridinic‐N. The sandwiched graphene sheet provides mechanical support, stabilizes HCL structure and promotes charge transfer. The synergetic effect of the catalyst structure overcomes the drawbacks of holey graphene approaches. The resulting ORR and OER performances are equal to or better than the top‐ranked electrocatalysts.

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.

Designing highly efficient electrocatalyst for ORR and OER of transition metals doped g-C9N7: A density functional theory study

Zinc–air batteries (ZABs) have attracted extensive attention due to their remarkable high theoretical energy output. They represent one of the most promising future power sources. However, many barriers restrict their application on a large scale. One of the main challenges is the sluggish rates of the oxygen‐reduction reaction (ORR) and oxygen‐evolution reaction (OER), which govern the discharging and charging processes of the battery, respectively. Here, recent advances related to oxygen electrocatalyst materials for ZABs are discussed. Detailed discussions will focus on unifunctional ORR electrocatalysts and bifunctional ORR and OER electrocatalysts. Pt‐based nanomaterials, as the best ORR electrocatalysts, possess the virtue of high activity, but have the disadvantages of high cost, scarcity, and poor stability. Thus, materials based on transition metals (alloys, metal oxides, metal nitrides, and spinel oxides) and metal‐free materials are widely investigated as nonprecious ORR catalysts owing to their promising catalytic activities. As for bifunctional ORR and OER electrocatalysts, the following two categories are introduced: (i) metal‐based materials, including single metal/metal‐oxides‐based materials and mixed‐metal/metal‐oxides‐based materials; and (ii) metal‐free materials. Finally, perspectives on the continuous research and limitation of the current ZAB technology are provided.

Efficient, robust and cost-effective bifunctional oxygen electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are of vital importance to the widespread utilization of Zn-air batteries. Here we report the fabrication of a bubble-like N,S-codoped porous carbon nanofibers with encapsulated fine Fe/Fe5C2 nanocrystals (∼ 10 nm) (FeNSCs) by a facile one-pot pyrolysis strategy. The novel FeNSC nanostructures with high Fe content (37.3 wt.%), and synergetic N and S doping demonstrate remarkable ORR and OER catalytic activities in alkaline condition. Particularly for ORR, the optimal FeNSC catalyst exhibits superior performance in terms of current density and durability in both alkaline and acidic media. Moreover, as catalysts on the air electrodes of Zn-air batteries, the optimal FeNSCs show a high peak power density of 59.6 mW/cm2 and extraordinary discharge-charge cycling performance for 200 h with negligible voltage gap change of only 8% at current density of 20 mA/cm, surpassing its noble metal counterpart (i.e. Pt). The impressive battery stability can be attributed to favorable electron transfer resulting from appropriate graphitization of the bubble-like carbon nanofibers and thorough protection of Fe/Fe5C2 nanoparticles by carbon wrapping to prevent oxidation, agglomeration and dissolution of Fe nanoparticles during battery cycling. The present FeNSC catalyst, which is highly active, robust yet affordable, shows promising prospects in large-scale applications, such as metal-air batteries and fuel cells.

Electronic structure modulation of molybdenum-iron double-atom catalyst for bifunctional oxygen electrochemistry

CoNi nanoparticles anchored inside carbon nanotube networks by transient heating: Low loading and high activity for oxygen reduction and evolution

3D porous cellular NiCoO2/graphene network as a durable bifunctional electrocatalyst for oxygen evolution and reduction reactions

The effect of the crystallographic form of MnO2 on the kinetics of oxygen reduction and evolution reaction

ConspectusThe oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are key in numerous electrochemical technologies, such as water electrolyzers, CO2 electrolyzers, low-temperature fuel cells, regenerative fuel cells and some metal-air batteries. The OER and ORR tend to be sluggish and catalyzed by scarce and expensive materials, the durability of which is often insufficient. For two decades, computational methods have been regarded as a cost-effective means to explain experimental observations, test hypothesis, and design new materials for these two reactions.Currently, the most widely used computational model is based on the free energies of the intermediates (*O, *OH, *OOH) and the scaling relations among them. Since the publication of two seminal papers in 2011, the scaling relation between the adsorption energies of *OOH and *OH was assigned all the responsibility for the experimental inefficiencies of OER and ORR electrocatalysts. This triggered a research paradigm based on breaking such scaling relation that still lasts until this day (see the diagram next to this text). After noting in 2018 that breaking the scaling relation between *OOH and *OH does not necessarily entail an improvement of the OER overpotential, my group moved away from the mainstream and has since been devising alternative descriptors and methods to enhance OER electrocatalysts and bifunctional OER/ORR electrocatalysts.In this Account, I will describe when and why we introduced the concepts of electrochemical symmetry, delta-epsilon optimization, bifunctional volcano plot, and error awareness, among others, aiming to provide quantitative tools for the computational design and optimization of electrocatalysts.

Mn doping of cobalt oxynitride coupled with N-rGO nanosheets hybrid as a highly efficient electrocatalyst for oxygen reduction and oxygen evolution reaction