Ni_xCo_1–x@Ni_xCo_1–xO/NCNT as Trifunctional ORR, OER, and HER Electrocatalysts and its Application in a Zn–Air Battery

Abstract

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.

Intrinsic Specific Activity Enhancement for Bifunctional Electrocatalytic Activity toward Oxygen and Hydrogen Evolution Reactions via Structural Modification of Nickel Organophosphonates

“…On the other hand, the superior bifunctional electrocatalytic OER and HER performances of the present metal organophosphonate can be attributed to the intrinsic effect of metal ions and the synergistic effect of M···O···P and O···H bond formation (where M is the metal center). ,,,,,,,,,− In addition, distorted octahedral geometry is also responsible for further enhancement of the present bifunctional electrocatalytic activities. Vacant orbitals of phosphorus atoms and aromatic pyrazine ring can accept electrons during electrolysis, therefore enhancing more number of electron flow during the electrochemical reaction, and the electron-donating tendency of nitrogen atoms can directly influence the antibonding orbital filing which makes the metal centers easily accessible for OH – attack, resulting in outstanding catalytic performances of compound II rather than I . ,,,,,, Although the present bifunctional electrocatalyst followed a similar mechanistic pathway as investigated earlier for various metal phosphonate-based electrocatalyst materials, to assess the deeper knowledge of compositions and valence states of the present catalysts in an electrocatalytic environment, X-ray photoelectron spectroscopic (XPS) studies were performed before and after electrocatalytic studies for compound II, as depicted in Figures and S20–S22, Supporting Information.…”

“…The charge-transfer resistances ( R ct ) were determined to be 289 and 403 ohms, respectively, by the help of equivalent circuit fitting. R ct of compound II is lower than that of compound I , which clearly indicates its faster electrocatalytic activity due to the maximal contact area between the electrolyte and the active electrode surface. ,,,, Similarly, a low solution resistance reveals a higher conductivity because of evenly dispersed particles in the solution.…”

“…The OER kinetics of catalyst II is shown to be considerably faster (126 mV/dec) than that of I (134 mV/dec) but quite slower than that of the IrO 2 catalyst (75 mV/dec). Moreover, the charge transfer across the electrodes and the electrolyte interface can also be a deciding factor in the OER activity. ,,,,, Hence, to gain a deeper understanding of reaction kinetics, electrochemical impedance spectroscopy (EIS) was performed at an overpotential of 350 mV in 1.0 M KOH ionic solution for both catalysts (Figure c).…”

“…The stability of a catalyst is also a crucial factor during the fabrication of a suitable electrocatalyst for the anodic oxidation and cathodic reduction of water. ,,,,,,, Hence, a chronopotentiometric stability study was carried out which shows the robust nature of compound II while the current density varies from 20 to 100 mA/cm 2 after stepwise implication at different time intervals (Figure d). Also, a chronoamperometry analysis was carried out to inspect the robust nature of catalyst II at an overpotential of 305 mV for 25 h in a 1.0 M KOH electrolytic solution (Figure e).…”

Pore-Confined π-Chromophoric Tetracene as a Visible Light Harvester toward MOF-Based Photocatalytic CO2 Reduction in Water

“…Figure S7 shows the Zr 3d analysis exhibiting two peaks with binding energies of 182.46 and 184.83 eV, corresponding to Zr 3d 5/2 and 3d 3/2 , respectively . High-resolution C 1s spectra can be deconvoluted into three distinct peaks, indicating the C–C, C–N, and CO linkages at 284.9, 286.1, and 288.9 eV, respectively (Figure c). − Re 4f XPS revealed two characteristic peaks at 41.85 and 44.27 eV, corresponding to 4f 7/2 and 4f 5/2 in the Re(I) oxidation state, respectively (Figure d). , Moreover, the XPS value well matched with the previous reports for the [Re(bpy)(CO) 3 ]Cl complex (bpy = bipyridine). , In addition, from N 1s analysis, the peak corresponding to the coordinated N is also observed at 400.35 eV (Figure e). , From inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis, the amount of coordinated Re(I) was found to be 9.1%. Moreover, the calculated Zr/Re ratio was 6:1.9, which in turn implies that approximately all of the exchanged MBA has been coordinated with the Re(I) center to form [Re(MBA)(CO) 3 Cl] complex inside the MOF.…”