Constructing two-dimensional (2D), free-standing, nonprecious, and robust electrocatalysts for oxygen evolution reactions (OERs) is of primary importance in the commercial water-splitting technology. Herein, we have constructed a 2D heterostructured NiFe 2 O 4 /NiFe layered double hydroxides (LDH) mixed composite on a low-cost stainless-steel mesh substrate using a lowtemperature one-step wet chemical synthesis method and have also investigated the effect of starting material concentration on the formation of the NiFe 2 O 4 /NiFe LDH mixed composite. The as-prepared NiFe 2 O 4 /NiFe LDH-25 electrocatalyst drives a 100 mA/cm 2 OER with the lowest reported overpotential of 190 mV and a Tafel slope 21.5 mV/dec and drives a stable 100 mA/cm 2 OER process in 1 M KOH. These OER activities are superior to that of the state-of-the-art RuO 2 OER electrocatalyst. The excellent OER activity appears to be due to the synergetic effect of NiFe LDHs and NiFe 2 O 4 . In addition, the vertically aligned heterostructure of the NiFe 2 O 4 /NiFe LDH composite thin sheets provides a large number of active edge sites, directly attached to the highly conducting substrate, which contributes to improving the electronic conductivity of the electrocatalyst. This work provides valuable insight into the design and one-step synthesis of NiFe 2 O 4 /NiFe LDH bimetallic mixed oxide and hydroxide composite thin films with enhanced OER activity and stability as well as deciphering the origin of the OER enhancement by metal oxides and metal hydroxides.
This review article discusses the recent advances in rechargeable metal–CO2 batteries (MCBs), which include the Li, Na, K, Mg, and Al-based rechargeable CO2 batteries, mainly with nonaqueous electrolytes. MCBs capture CO2 during discharge by the CO2 reduction reaction and release it during charging by the CO2 evolution reaction. MCBs are recognized as one of the most sophisticated artificial modes for CO2 fixation by electrical energy generation. However, extensive research and substantial developments are required before MCBs appear as reliable, sustainable, and safe energy storage systems. The rechargeable MCBs suffer from the hindrances like huge charging–discharging overpotential and poor cyclability due to the incomplete decomposition and piling of the insulating and chemically stable compounds, mainly carbonates. Efficient cathode catalysts and a suitable architectural design of the cathode catalysts are essential to address this issue. Besides, electrolytes also play a vital role in safety, ionic transportation, stable solid-electrolyte interphase formation, gas dissolution, leakage, corrosion, operational voltage window, etc. The highly electrochemically active metals like Li, Na, and K anodes severely suffer from parasitic reactions and dendrite formation. Recent research works on the aforementioned secondary MCBs have been categorically reviewed here, portraying the latest findings on the key aspects governing secondary MCB performances.
All-solid-state sodium oxygen (ASS Na–O2) batteries have received interest due to their higher theoretical energy density, lower cost, higher safety level, and nonflammability compared with liquid electrolyte and Li–O2 batteries. Here, we report the application of carbon nanotube (CNT) and Ru/CNT cathodes, succinonitrile with a NaClO4 (SN + NaClO4) interlayer, a Na3Zr2Si2PO12 (NZSP) solid electrolyte, and a Na film anode for ASS Na–O2 batteries. Results showed that the SN + NaClO4 interlayer plays a crucial role in the tri-conductive cathode, ionic conductivity, and interfacial charge transfer kinetics between the Ru/CNT cathode and NZSP electrolyte. The ASS Na–O2 batteries with Ru/CNT and SN + NaClO4 tri-conductive cathodes exhibited a long cycling performance of 100 cycles (current density of 100 mA g–1 and limited capacity of 500 mA h g–1), a discharge capacity of 11 034 mA h g–1 (current density of 100 mA g–1), and a small overpotential gap of 1.4 V. These values were better than those for CNT and SN + NaClO4 tri-conductive cathodes (maximum discharge capacity of 2413 mA h g–1, 27 cycles, and potential gap of 1.7 V) with a Na2O2 discharge product. Ex situ analysis showed that the Ru/CNT cathode achieved superior reversibility deposition and decomposition of the Na2O2 discharge product. Therefore, the ASS Na–O2 battery system is safe and stable for energy storage applications.
With magnesium being a cost-effective anode metal compared to the other conventional Li-based anodes in the energy market, it could be a capable source of energy storage. However, Mg−O 2 batteries have struggled its way to overcome the poor cycling stability and sluggish reaction kinetics. Therefore, Ru metallic nanoparticles on carbon nanotubes (CNTs) were introduced as a cathode for Mg−O 2 batteries, which are known for their inherent electronic properties, large surface area, and increased crystallinity to favor remarkable oxygen reduction reactions and oxygen evolution reactions (ORR and OER). Also, we deployed a first-of-its-kind, conducive mixed electrolyte (CME) (2 M Mg(NO 3 ) 2 :1 M Mg(TFSI) 2 /diglyme). Hence, this synergistic incorporation of CME-based Ru/CNT Mg−O 2 batteries could unleash long cycle life with low overpotential, excellent reversibility, and high ionic conductivity and also reduces the intrinsic corrosion behavior of Mg anodes. Correspondingly, this novel amalgamation of CME with Ru/CNT cathode has displayed superior cyclic stability of 65 cycles and a maximum discharge potential of 25 793 mAh g −1 with a small overvoltage plateau of 1.4 V, noticeably subjugating the findings of conventional single electrolyte (CSE) (1 M Mg(TFSI) 2 /diglyme). This CME-based Ru/CNT Mg−O 2 battery design could have a significant outcome as a future battery technology.
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