The development of high-performance and low-cost oxygen reduction and evolution catalysts that can be easily integrated into existing devices is crucial for the wide deployment of energy storage systems that utilize O...The development of high-performance and low-cost oxygen reduction and evolution catalysts that can be easily integrated into existing devices is crucial for the wide deployment of energy storage systems that utilize O2-H2O chemistries, such as regenerative fuel cells and metal-air batteries. Herein, we report an NHB-activated N-doped hierarchical carbon (NHC) catalyst synthesized via a scalable route, and demonstrate its device integration. The NHC catalyst exhibited good performance for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), as demonstrated by means of electrochemical studies and evaluation when integrated into the oxygen electrode of a regenerative fuel cell. The activities observed for both the ORR and the OER were comparable to those achieved by state-of-the-art Pt and Ir catalysts in alkaline environments. We have further identified the critical role of carbon defects as active sites for electrochemical activity through density functional theory calculations and high-resolution TEM visualization. This work highlights the potential of NHC to replace commercial precious metals in regenerative fuel cells and possibly metal-air batteries for cost-effective storage of intermittent renewable energy.展开更多
Nickel-iron layered double hydroxide (NiFe-LDH) nanosheets have shown optimal oxygen evolution reaction (OER) performance; however, the role of the intercalated ions in the OER activity remains unclear. In this wo...Nickel-iron layered double hydroxide (NiFe-LDH) nanosheets have shown optimal oxygen evolution reaction (OER) performance; however, the role of the intercalated ions in the OER activity remains unclear. In this work, we show that the activity of the NiFe-LDHs can be tailored by the intercalated anions with different redox potentials. The intercalation of anions with low redox potential (high reducing ability), such as hypophosphites, leads to NiFe-LDHs with low OER overpotential of 240 mV and a small Tafel slope of 36.9 mV/dec, whereas NiFe-LDHs intercalated with anions of high redox potential (low reducing ability), such as fluorion, show a high overpotential of 370 mV and a Tafel slope of 80.8 mV/dec. The OER activity shows a surprising linear correlation with the standard redox potential. Density functional theory calculations and X-ray photoelectron spectroscopy analysis indicate that the intercalated anions alter the electronic structure of metal atoms which exposed at the surface. Anions with low standard redox potential and strong reducing ability transfer more electrons to the hydroxide layers. This increases the electron density of the surface metal sites and stabilizes their high-valence states, whose formation is known as the critical step prior to the OER process.展开更多
As one of the most important families of porous materials,metal–organic frameworks(MOFs)have well-defined atomic structures.This provides ideal models for investigating and understanding the relationships between str...As one of the most important families of porous materials,metal–organic frameworks(MOFs)have well-defined atomic structures.This provides ideal models for investigating and understanding the relationships between structures and catalytic activities at the molecular level.However,the active sites on the edges of two-dimensional(2D)MOFs have rarely been studied,as they are less exposed to the surfaces.Here,for the first time,we synthesized and observed that the 2D layers could align perpendicular to the surface of a 2D zeolitic imidazolate framework L(ZIF-L)with a leaf-like morphology.Owing to this unique orientation,the active sites on the edges of the 2D crystal structure could mostly be exposed to the surfaces.Interestingly,when another layer of ZIF-L-Co was grown heteroepitaxially onto ZIF-L-Zn(ZIF-L-Zn@ZIF-L-Co),the two layers shared a common b axis but rotated by 90°in the ac plane.This demonstrated that we could control exposed facets of the 2D MOFs.The ZIF-L-Co with more exposed edge active sites exhibited high electrocatalytic activity for oxygen reduction reaction.This work provides a new concept of designing unique oriented layers in 2D MOFs to expose more edge-active sites for efficient electrocatalysis.展开更多
文摘The development of high-performance and low-cost oxygen reduction and evolution catalysts that can be easily integrated into existing devices is crucial for the wide deployment of energy storage systems that utilize O2-H2O chemistries, such as regenerative fuel cells and metal-air batteries. Herein, we report an NHB-activated N-doped hierarchical carbon (NHC) catalyst synthesized via a scalable route, and demonstrate its device integration. The NHC catalyst exhibited good performance for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), as demonstrated by means of electrochemical studies and evaluation when integrated into the oxygen electrode of a regenerative fuel cell. The activities observed for both the ORR and the OER were comparable to those achieved by state-of-the-art Pt and Ir catalysts in alkaline environments. We have further identified the critical role of carbon defects as active sites for electrochemical activity through density functional theory calculations and high-resolution TEM visualization. This work highlights the potential of NHC to replace commercial precious metals in regenerative fuel cells and possibly metal-air batteries for cost-effective storage of intermittent renewable energy.
基金This work was supported by the National Natural Science Foundation of China (NSFC), the National Key Research and Development Project (Nos. 2016YFF0204402 and 2016YFC0801302), the Program for Changjiang Scholars, and innovative Research Team in the University, and the Fundamental Research Funds for the Central Universities, and the long term subsidy mechanism from the Ministry of Finance and the Ministry of Education of China. S. S. gratefully acknowledges Villum Foundation.
文摘Nickel-iron layered double hydroxide (NiFe-LDH) nanosheets have shown optimal oxygen evolution reaction (OER) performance; however, the role of the intercalated ions in the OER activity remains unclear. In this work, we show that the activity of the NiFe-LDHs can be tailored by the intercalated anions with different redox potentials. The intercalation of anions with low redox potential (high reducing ability), such as hypophosphites, leads to NiFe-LDHs with low OER overpotential of 240 mV and a small Tafel slope of 36.9 mV/dec, whereas NiFe-LDHs intercalated with anions of high redox potential (low reducing ability), such as fluorion, show a high overpotential of 370 mV and a Tafel slope of 80.8 mV/dec. The OER activity shows a surprising linear correlation with the standard redox potential. Density functional theory calculations and X-ray photoelectron spectroscopy analysis indicate that the intercalated anions alter the electronic structure of metal atoms which exposed at the surface. Anions with low standard redox potential and strong reducing ability transfer more electrons to the hydroxide layers. This increases the electron density of the surface metal sites and stabilizes their high-valence states, whose formation is known as the critical step prior to the OER process.
基金the support from the National Natural Science Foundation of China(grant nos.21975148,21875149,21835002,21875140,and 21773146)the Fundamental Research Funds for the Central Universities+3 种基金the Research Funds of Shaanxi Normal University,Shanghai Natural Science Fund(no.17ZR1418600)ChEM,SPST of ShanghaiTech University(no.EM02161943)the Swedish Research Council Formas(no.2020-00831,Z.H.)the University of Calgary’s Canada First Research Excellence Fund Program,the Global Research Initiative in Sustainable Low Carbon Unconventional Resources.
文摘As one of the most important families of porous materials,metal–organic frameworks(MOFs)have well-defined atomic structures.This provides ideal models for investigating and understanding the relationships between structures and catalytic activities at the molecular level.However,the active sites on the edges of two-dimensional(2D)MOFs have rarely been studied,as they are less exposed to the surfaces.Here,for the first time,we synthesized and observed that the 2D layers could align perpendicular to the surface of a 2D zeolitic imidazolate framework L(ZIF-L)with a leaf-like morphology.Owing to this unique orientation,the active sites on the edges of the 2D crystal structure could mostly be exposed to the surfaces.Interestingly,when another layer of ZIF-L-Co was grown heteroepitaxially onto ZIF-L-Zn(ZIF-L-Zn@ZIF-L-Co),the two layers shared a common b axis but rotated by 90°in the ac plane.This demonstrated that we could control exposed facets of the 2D MOFs.The ZIF-L-Co with more exposed edge active sites exhibited high electrocatalytic activity for oxygen reduction reaction.This work provides a new concept of designing unique oriented layers in 2D MOFs to expose more edge-active sites for efficient electrocatalysis.