Toward the goal of clean and sustainable energy source, the development of a trifunctional electrocatalyst is a boon for energy storage and conversion devices such as regenerative fuel cells and metal-air batteries. MOF-derived semiconducting-metallic core−shell electrocatalyst Co 3 O 4 @Co/NCNT (NCNT = nitrogen-doped carbon nanotube), which was shown to catalyze oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), is also found to be an active electrocatalyst for hydrogen evolution reaction (HER) with a low overpotential of 171 mV. Here, the HER activity of Co 3 O 4 @Co/NCNT is presented and is shown as highly efficient and robust trifunctional electrocatalyst. The detailed theoretical calculation has found N-center of Co−N 4 moiety to be the H + binding active site and thus proves Co 3 O 4 @Co/NCNT to be active for HER. Further, the ORR and OER bifunctionality of Co 3 O 4 @Co/NCNT helped in fabricating secondary Zn-air battery with high power density of 135 mW/cm 2 . Also, an all-solid-state flexible and wearable battery with Co 3 O 4 @ Co/NCNT as cathode and electrodeposited Zn on carbon fiber cloth as anode was shown to withstand its performance even under stressed conditions. Finally, the material being trifunctional in nature was used both as an anode and cathode material for the electrolysis of water, which was powered by the Zn-air batteries with Co 3 O 4 @Co/NCNT as the cathode material. It is believed that the development of a trifunctional catalyst would help in wide commercialization of regenerative fuel cells.
The efficient electrochemical conversion and storage devices can be boosted by the development of cost-effective and durable electrocatalysts. However, simultaneous in-depth understanding of the reaction mechanism is also required. Herein, we report the preparation, characterization, and electrochemical activities of bimetallic Ni x Co1–x NPs and core–shell Ni x Co1–x @Ni x Co1–x O NPs stabilized on N-doped carbon nanotubes (NCNTs). The electrocatalyst is derived from a bimetallic MOF {[Ni0.5Co0.5(bpe)2(N(CN)2)](N(CN)2)·(5H2O)} n (1) via pyrolysis followed by calcination. Pyrolysis of the bimetallic MOF gives rise to bimetallic nanoparticles stabilized on NCNTs, which, when subsequently calcined, leads to the formation of a core–shell structure with a semiconducting oxide shell (Ni x Co1–x O) encapsulating the Ni x Co1–x bimetallic NP core. Detailed evaluation of the electrocatalytic performance of Ni x Co1–x @Ni x Co1–x O/NCNT proves its worth as a bifunctional catalyst with 380 mV overpotential for oxygen evolution reaction at 10 mA cm–2 current density and 0.87 V (vs RHE) onset for oxygen reduction reaction in the alkaline medium. Additionally, the prepared electrocatalyst efficiently catalyzes the hydrogen evolution reaction with a nominal overpotential of 74 mV (vs RHE) for reaching 10 mA cm–2 current density in acidic medium. The practical applicability of this catalyst is further upheld in the fabrication of a zinc–air battery having high specific capacity with high round-trip efficiency and adequate cycle life. DFT calculations establish that the structure of Ni x Co1–x @Ni x Co1–x O/NCNT is crucial for its electrochemical activity since it has the threefold advantages of cooperative charge transfer from Co to Ni, synergistic relationship between the conductive alloy core and semiconducting oxide shell, and a highly conductive N-doped CNT matrix.
Among the earth-abundant catalysts for the airelectrode of metal-air batteries, success with one-component catalyst systems driving the bifunctional oxygen evolution and reduction reactions (OER and ORR) remains elusive. Herein, a cobalt−copper couple addresses this gap-area where the best performing catalyst consists of an electrically conducting backbone of 57 wt % Cu 0.9 Co 0.1 alloy and 10 wt % Co with higher valence surface states of CoO, Cu 2 O, CuO, and Co 3 O 4 with 19.7, 10, 2.9, and 0.3 wt %, respectively. Computational studies show the Cocenter as the active site for OER and ORR, while charge transfer from the Co to Cu center accumulates the electronic charge at the junction between the surface CoO and the alloy core facilitating the adsorption of oxygenated species. The 24 h air-oxidized electrodeposited-catalyst shows OER overpotentials of 225, 290, and 312 mV at 10, 50, and 100 mA cm −2 , respectively, with at least 150 h operational stability at 1.55 V versus reversible hydrogen electrode (RHE). By electrodepositing the catalyst on high-priced Ni-foam, record-low overpotentials of 190, 281, and 298 mV are observed at 10, 50, and 100 mA cm −2 . Since the ORR overpotential is only 550 mV at −1 mA cm −2 on CFP, the bifunctionality index (BI) subsides to 0.775 V. The rechargeable zinc-air battery (ZAB) delivers a stellar 948 mA•h•g Zn −1 specific capacity, 68.8 mW cm −2 power density, and uninterrupted one-week operational stability. Even in the presence of Cl − ions in alkaline artificial seawater, the OER overpotential is retained at 240 mV at 10 mA cm −2 and the ZAB specific capacity is slightly reduced to 759.5 mA•h•g Zn −1. A synergistic bimetallic composition, measured higher valence surface states with singly occupied e g orbitals, and branched morphology, facilitates the optimum adsorption and desorption of the intermediates.
Economic and sustainable (ecological) energy storage forms a major pillar of the global energy sector. Bifunctional electrocatalysts, based on oxygen electrolysis, play a key role in the development of rechargeable metal–air batteries. Pursuing precious metal-free economic catalysts, here, we report K2CoP2O7 pyrophosphate as a robust cathode for secondary zinc–air batteries with efficient oxygen evolution and oxygen reduction (OER||ORR) activity. Prepared by autocombustion, nanoscale K2CoP2O7 exhibited excellent oxygen reduction and evolution reactions among all phosphate-based electrocatalysts. In particular, the OER activity surpassed that of commercial RuO2 with low overpotential (0.27 V). First-principles calculations revealed that the bifunctional activity is rooted in the Co active site with the CoO5 local coordination in the most stable (110) surface. This nanostructured (tetragonal) pyrophosphate can be harnessed as an economic bifunctional catalyst for zinc–air batteries.
Metal-air batteries have emerged as key players in energy storage sector with high energy density. They operate on two underlying processes: namely oxygen reduction reaction (ORR) and oxygen evolution reaction...
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