Hollow FeNi@NCG Materials Prepared with a Double-template Strategy as Highly Efficient Catalysts for Rechargeable Zn-air Batteries

Abstract

One-step pyrolysis approach is the most common used method to synthesize the cathode catalysts of Zn-air batteries (ZABs). However, it is still a challenge to control the structural elements. Herein, a double-template strategy has been established by fabricating GO-doped porous g-C3N4 supported FeNi-MOF arrays as templates to synthesize the FeNi@NCG-700 architecture as highly efficient electrocatalysts. The obtained FeNi@NCG-T catalysts exhibit well-defined hollow architecture with directional arrangement tendency, which may be advantageous to construct a tri-phase region so as to accelerate the O2-containing species transmission and increase the accessibility of internal active sites, thus boosting the catalytic performances and kinetics. The optimal Fe1Ni1@NCG-700 catalyst delivers excellent electrochemical activity with overpotential of merely 217 mV (after iR correction) for OER (1M KOH) and a remarkable small potential gap of 0.625 V between ORR and OER. The Fe1Ni1@NCG-700 based aqueous ZAB exhibits high peak power density of 210.79 mW·cm-2 and high specific capacity of 784.84 mAh·g-1 and long term charge/discharge stability. The corresponding button-shaped solid-state ZAB also presents excellent charge/discharge durability (>466 cycles) with outstanding performances. Most importantly, two as-synthesized button solid-state ZABs catalysts can afford a 2 V LED lamp over 36 h, proving the strong pratical application prospect.

Active Sites of Mixed-Metal Core–Shell Oxygen Evolution Reaction Catalysts: FeO4 Sites on Ni Cores or NiN4 Sites in C Shells?

“…We herein avoid the purely metallic Ni 0 /Fe 0 model typically used for encapsulated catalysts ,,,, and the Ni 1– x Fe x OOH model typically used for fully exposed catalysts. − ,− , Instead, we take an active-site model constituted of an Fe atom sitting on a Ni oxide layer (in the form of FeO 4 or more specifically FeO x H y ; Figure ), which was grown on the fcc-NiFe (111) surface under a strong alkaline condition computationally (i.e., by finding the lowest-energy structure as a series of OH is sequentially introduced to the surface) . Indeed, most X-ray diffraction (XRD) experiments on core–shell catalysts have identified an fcc crystal structure for NiFe metal core before OER operation, and most X-ray spectroscopy (XPS) studies have shown dominant peaks for metallic Ni 0 /Fe 0 on the core surface (unless synthesized with KOH activation), which are gradually oxidized to Ni 2+ /Fe 3+ during the electrochemical cycles. − ,− …”

“…Randomly exposed active sites of these Ni 1– x Fe x OOH catalysts, however, are unstable and eventually dissolve when exposed to harsh electrolytes, keeping these catalysts still inappropriate for long-term operations. − Recently, their stabilities and activities have been improved by encapsulating the catalytic nanoparticles with thin graphitic layers, which expose only an optimum amount of active sites to electrolytes (Figure , right). For example, a Ni–Fe bimetallic analogue of Fe­(III) 4 [Fe­(II)­(CN) 6 ] 3 Prussian blue has yielded high-activity-high-stability hybrids between NiFe metal cores and N-doped carbon (NC) shells (NiFe@NC) via a simple pyrolysis, during which the nitrile ligands decompose and form the NC layers encapsulating the metal in core–shell nanostructures. − A number of other NiFe@NC core–shell catalysts − have also shown enhanced OER activities and stabilities.…”

“…However, the exact OER mechanism of these catalysts as well as the exact roles of core–shell encapsulation, Fe doping in the core, and N doping in the NC shell are still unclear. Even the exact location of the OER active site is still under debate between the core ,− ,,,, and the shell. ,,,,, Since the OER activity is completely lost with the NC shells alone (e.g., after washing away core metals by acids), ,,,,,, the role of the NC shell may be simply to enhance the electrical conductivity between the active core nanoparticles , or to protect the active metal cores from excessive oxidation, dissolution, or agglomeration by exposing only an optimum amount of active sites on the core surfaces to harsh electrolytes. ,,, The NC shell may also indirectly alter the electronic structure of the active site on the enclosed core or promote their binding to key intermediates . On the other hand, the NC shells, after electron penetration or metal atom (M) dispersion (as M–N–C bridges) from the metal cores, can also actively participate in the key OER steps, serving as the active site. ,,,− ,, …”

“…However, the exact OER mechanism of these catalysts as well as the exact roles of core–shell encapsulation, Fe doping in the core, and N doping in the NC shell are still unclear. Even the exact location of the OER active site is still under debate 54 between the core 48 , 56 − 59 , 62 , 66 , 68 , 69 and the shell. 51 , 53 , 55 , 63 , 64 , 71 Since the OER activity is completely lost with the NC shells alone (e.g., after washing away core metals by acids), 48 , 55 , 56 , 58 , 62 , 66 , 68 the role of the NC shell may be simply to enhance the electrical conductivity between the active core nanoparticles 61 , 62 or to protect the active metal cores from excessive oxidation, dissolution, or agglomeration by exposing only an optimum amount of active sites on the core surfaces to harsh electrolytes.…”

Insights on advanced g‐C3N4 in energy storage: Applications, challenges, and future

“…The excellent performance was mainly attributed to the following factors: (1) formation of more cobalt active sites and uniform distribution; (2) inserting cobalt ions during the polymerization pyrolysis process, which facilitates the formation of catalytic active sites. Long and colleagues 378 prepared GO‐doped porous g‐C 3 N 4 ‐loaded FeNi‐MOF arrays as templates for the synthesis of FeNi‐MOF@NCG double‐template materials. Then, the hollow FeNi@NCG‐ T ( T = 700°C) architecture, with its directional arrangement, facilitates charge transferred and increases the number of active sites, which could be obtained by calcining the FeNi‐MOF@NCG template at high temperature.…”