MO-Co@N-Doped Carbon (M = Zn or Co): Vital Roles of Inactive Zn and Highly Efficient Activity toward Oxygen Reduction/Evolution Reactions for Rechargeable Zn–Air Battery
TL;DR: In this paper, the MO-Co@N-doped carbon (NC, M = Zn or Co) is developed as a highly active ORR/OER bifunctional catalyst via pyrolysis of a bimetal metal-organic framework containing Zn and Co, i.e., precursor (CoZn).
Abstract: A highly efficient bifunctional oxygen catalyst is required for practical applications of fuel cells and metal–air batteries, as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are their core electrode reactions. Here, the MO-Co@N-doped carbon (NC, M = Zn or Co) is developed as a highly active ORR/OER bifunctional catalyst via pyrolysis of a bimetal metal–organic framework containing Zn and Co, i.e., precursor (CoZn). The vital roles of inactive Zn in developing highly active bifunctional oxygen catalysts are unraveled. When the precursors include Zn, the surface contents of pyridinic N for ORR and the surface contents of Co–Nx and Co3+/Co2+ ratios for OER are enhanced, while the high specific surface areas, high porosity, and high electrochemical active surface areas are also achieved. Furthermore, the synergistic effects between Zn-based and Co-based species can promote the well growth of multiwalled carbon nanotubes (MWCNTs) at high pyrolysis temperatures (≥700 °C), which is favorable for charge transfer. The optimized CoZn-NC-700 shows the highly bifunctional ORR/OER activity and the excellent durability during the ORR/OER processes, even better than 20 wt% Pt/C (for ORR) and IrO2 (for OER). CoZn-NC-700 also exhibits the prominent Zn–air battery performance and even outperforms the mixture of 20 wt% Pt/C and IrO2.
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TL;DR: A wide range of applications based on these materials for ORR, OER, HER and multifunctional electrocatalysis are discussed, with an emphasis on the required features of MOF-derived carbon-based materials for the Electrocatalysis of corresponding reactions.
Abstract: Oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are three key reactions for the development of green and sustainable energy systems. Efficient electrocatalysts for these reactions are highly desired to lower their overpotentials and promote practical applications of related energy devices. Metal–organic frameworks (MOFs) have recently emerged as precursors to fabricate carbon-based electrocatalysts with high electrical conductivity and uniformly distributed active sites. In this review, the current progress of MOF-derived carbon-based materials for ORR/OER/HER electrocatalysis is presented. Materials design strategies of MOF-derived carbon-based materials are firstly summarized to show the rich possibilities of the morphology and composition of MOF-derived carbon-based materials. A wide range of applications based on these materials for ORR, OER, HER and multifunctional electrocatalysis are discussed, with an emphasis on the required features of MOF-derived carbon-based materials for the electrocatalysis of corresponding reactions. Finally, perspectives on the development of MOF-derived carbon-based materials for ORR, OER and HER electrocatalysis are provided.
970 citations
TL;DR: A perspective for design, preparation, and assembly of air electrodes is proposed for the future innovations of Zn–air batteries with high performance.
Abstract: Zn-air batteries are becoming the promising power sources for portable and wearable electronic devices and hybrid/electric vehicles because of their high specific energy density and the low cost for next-generation green and sustainable energy technologies. An air electrode integrated with an oxygen electrocatalyst is the most important component and inevitably determines the performance and cost of a Zn-air battery. This article presents exciting advances and challenges related to air electrodes and their relatives. After a brief introduction of the Zn-air battery, the architectures and oxygen electrocatalysts of air electrodes and relevant electrolytes are highlighted in primary and rechargeable types with different configurations, respectively. Moreover, the individual components and major issues of flexible Zn-air batteries are also highlighted, along with the strategies to enhance the battery performance. Finally, a perspective for design, preparation, and assembly of air electrodes is proposed for the future innovations of Zn-air batteries with high performance.
602 citations
TL;DR: In this article, the fundamental mechanisms of the bifunctional electrocatalysis of ORR/OER and OER/HER are introduced and analyzed, and recent progress in the design, synthesis and emerging applications of nonnoble cobalt (Co)-based materials (e.g., cobalt oxide, cobalt phosphides and cobalt chalcogenides), including layered double hydroxides, Co N-C, Co-N-C and Co-based single atoms and their composites, are summarized and discussed.
Abstract: Electrocatalytic hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are three typical reactions in energy conversion devices, such as water electrolyzers, metal–air batteries and fuel cells However, the sluggish kinetics of the HER/OER/ORR and their dependency on noble-metal-based catalysts (eg Pt, Ir and Ru) hinder the large-scale commercial applications of these devices Hence, the development of low-cost, efficient, stable and scalable electrocatalysts, especially bifunctional HER/OER or OER/ORR electrocatalysts that could simultaneously catalyze HER/OER or OER/ORR, is highly desired but full of great challenges To date, non-noble cobalt (Co)-based materials, including cobalt oxide, cobalt phosphides, cobalt chalcogenides (sulfide and selenides), Co-included layered double hydroxides, Co–N–C, Co-based single atoms and their composites, have been widely investigated as bifunctional electrocatalysts owing to their innate electrochemical capabilities and structural adjustability by mixing, doping or combining with other materials, such as carbon materials In this review, the fundamental mechanisms of the bifunctional electrocatalysis of ORR/OER and OER/HER are introduced and analyzed, and recent progress in the design, synthesis and emerging applications of Co-based materials as bifunctional electrocatalysts for ORR/OER or OER/HER in alkaline electrolytes are summarized and discussed Finally, the strategies and perspectives for designing novel highly efficient bifunctional cobalt-based electrocatalysts are analyzed This analysis is expected to highlight the challenges faced by bifunctional noble-metal-free electrocatalysts for the study and development of next-generation electrocatalysts for the HER/ORR/OER
239 citations
TL;DR: In this paper, the authors synthesize unique hybrid cobalt/cobalt nitride (Co/CoNx) nanoparticles well-integrated with nitrogen-doped carbon (NC) nanoarrays through a facile fabrication using a two-dimensional metal-organic framework (MOF) precursor.
209 citations
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TL;DR: It is reported that vertically aligned nitrogen-containing carbon nanotubes (VA-NCNTs) can act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen reduction in alkaline fuel cells.
Abstract: The large-scale practical application of fuel cells will be difficult to realize if the expensive platinum-based electrocatalysts for oxygen reduction reactions (ORRs) cannot be replaced by other efficient, low-cost, and stable electrodes. Here, we report that vertically aligned nitrogen-containing carbon nanotubes (VA-NCNTs) can act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen reduction in alkaline fuel cells. In air-saturated 0.1 molar potassium hydroxide, we observed a steady-state output potential of –80 millivolts and a current density of 4.1 milliamps per square centimeter at –0.22 volts, compared with –85 millivolts and 1.1 milliamps per square centimeter at –0.20 volts for a platinum-carbon electrode. The incorporation of electron-accepting nitrogen atoms in the conjugated nanotube carbon plane appears to impart a relatively high positive charge density on adjacent carbon atoms. This effect, coupled with aligning the NCNTs, provides a four-electron pathway for the ORR on VA-NCNTs with a superb performance.
6,370 citations
TL;DR: In this paper, the authors report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts for water oxidation.
Abstract: Objective evaluation of the activity of electrocatalysts for water oxidation is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for determining electrochemically active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a current density of 10 mA cm–2 per geometric area, approximately the current density expected for a 10% efficient solar-to-fuels conversion device. Utilizing ...
4,808 citations
TL;DR: The emphasis of this review is on the origin of the electrocatalytic activity of nanostructured catalysts toward a series of key clean energy conversion reactions by correlating the apparent electrode performance with their intrinsic electrochemical properties.
Abstract: A fundamental change has been achieved in understanding surface electrochemistry due to the profound knowledge of the nature of electrocatalytic processes accumulated over the past several decades and to the recent technological advances in spectroscopy and high resolution imaging. Nowadays one can preferably design electrocatalysts based on the deep theoretical knowledge of electronic structures, via computer-guided engineering of the surface and (electro)chemical properties of materials, followed by the synthesis of practical materials with high performance for specific reactions. This review provides insights into both theoretical and experimental electrochemistry toward a better understanding of a series of key clean energy conversion reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The emphasis of this review is on the origin of the electrocatalytic activity of nanostructured catalysts toward the aforementioned reactions by correlating the apparent electrode performance with their intrinsic electrochemical properties. Also, a rational design of electrocatalysts is proposed starting from the most fundamental aspects of the electronic structure engineering to a more practical level of nanotechnological fabrication.
3,918 citations
TL;DR: The high activity of BSCF was predicted from a design principle established by systematic examination of more than 10 transition metal oxides, which showed that the intrinsic OER activity exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an eg symmetry of surface transition metal cations in an oxide.
Abstract: The efficiency of many energy storage technologies, such as rechargeable metal-air batteries and hydrogen production from water splitting, is limited by the slow kinetics of the oxygen evolution reaction (OER). We found that Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3–δ (BSCF) catalyzes the OER with intrinsic activity that is at least an order of magnitude higher than that of the state-of-the-art iridium oxide catalyst in alkaline media. The high activity of BSCF was predicted from a design principle established by systematic examination of more than 10 transition metal oxides, which showed that the intrinsic OER activity exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an e g symmetry of surface transition metal cations in an oxide. The peak OER activity was predicted to be at an e g occupancy close to unity, with high covalency of transition metal–oxygen bonds.
3,876 citations
TL;DR: In this paper, the oxygen reduction reaction (ORR) active site was characterized by using newly designed graphite (highly oriented pyrolitic graphite) model catalysts with well-defined π conjugation and well-controlled doping of N species.
Abstract: Nitrogen (N)-doped carbon materials exhibit high electrocatalytic activity for the oxygen reduction reaction (ORR), which is essential for several renewable energy systems. However, the ORR active site (or sites) is unclear, which retards further developments of high-performance catalysts. Here, we characterized the ORR active site by using newly designed graphite (highly oriented pyrolitic graphite) model catalysts with well-defined π conjugation and well-controlled doping of N species. The ORR active site is created by pyridinic N. Carbon dioxide adsorption experiments indicated that pyridinic N also creates Lewis basic sites. The specific activities per pyridinic N in the HOPG model catalysts are comparable with those of N-doped graphene powder catalysts. Thus, the ORR active sites in N-doped carbon materials are carbon atoms with Lewis basicity next to pyridinic N.
3,201 citations