Nickel–cobalt oxalate as an efficient non-precious electrocatalyst for an improved alkaline oxygen evolution reaction

The quest for developing next-generation non-precious electrocatalysts has risen in recent times. Herein, we have designed and developed a low cost electrocatalyst by a ligand-assisted synthetic strategy in an aqueous medium. An oxalate ligand-assisted non-oxide electrocatalyst was developed by a simple wet-chemical technique for alkaline water oxidation application. The synthetic parameters for the preparation of nickel–cobalt oxalate (Ni2.5Co5C2O4) were optimized, such as the metal precursor (Ni/Co) ratio, oxalic acid amount, reaction temperature, and time. Microstructural analysis revealed a mesoporous block-like architecture for nickel–cobalt oxalate (Ni2.5Co5C2O4). The required overpotential of Ni2.5Co5C2O4 for the alkaline oxygen evolution reaction (OER) was found to be 330 mV for achieving 10 mA cmgeo−2, which is superior to that of NiC2O4, CoC2O4, NiCo2O4 and the state-of-the-art RuO2. The splendid performance of Ni2.5Co5C2O4 was further verified by its low charge transfer resistance, impressive stability performance, and 87% faradaic efficiency in alkaline medium (pH = 14). The improved electrochemical activity was further attributed to double layer capacitance (Cdl), which indefinitely divulged the inferiority of NiCo2O4 compared to Ni2.5Co5C2O4 for the alkaline oxygen evolution reaction (OER). The obtained proton reaction order (ρRHE) was about 0.80, thus indicating the proton decoupled electron transfer (PDET) mechanism for OER in alkaline medium. Post-catalytic investigation revealed the formation of a flake-like porous nanostructure, indicating distinct transformation in morphology during the alkaline OER process. Further, XPS analysis demonstrated complete oxidation of Ni2+ and Co2+ centres into Ni3+ and Co3+, respectively under high oxidation potential, thereby indicating active site formation throughout the microstructural network. Additionally, from BET-normalised LSV investigation, the intrinsic activity of Ni2.5Co5C2O4 was also found to be higher than that of NiCo2O4. Finally, Ni2.5Co5C2O4 delivered a TOF value of around 3.28 × 10−3 s−1, which is 5.56 fold that of NiCo2O4 for the alkaline OER process. This report highlights the unique benefit of Ni2.5Co5C2O4 over NiCo2O4 for the alkaline OER. The structure–catalytic property relationship was further elucidated using density functional theory (DFT) study. To the best of our knowledge, nickel–cobalt oxalate (Ni2.5Co5C2O4) was introduced for the first time as a non-precious non-oxide electrocatalyst for alkaline OER application.


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1.0 Experimental Section 1.1 Materials: All chemicals were analytical grade and treated without any purification.  3 .6H 2 O] were dissolved in 50 ml of deionised water with constant stirring for 15 min at 80°C. Then, 30 ml of oxalic acid (1.512 g) aqueous solution was added drop wise manner and again stirred for 120 min. The hot-viscous solution was quenched in ice-water and kept in room temperature (30°C) for 30 min. The precipitate was centrifuged and washed with methanol. Finally, it was dried in oven at 60°C for 12 hour. For the controlled experiment, same procedure was repeated with variation in temperature, time, amount of oxalic acid and Co/Ni ratio.

Preparation of Nickel Oxalate (NiC 2 O 4 ):
For the synthesis of nickel oxalate, same procedure was repeated without [Co(NO 3 ) 3 .6H 2 O].

Preparation of Nickel-Cobalt Oxide (NiCo 2 O 4 ):
The dry sample of nickel cobalt oxalate was collected and annealed for 350°C with calcination rate of 2°C/min and dwell time for 2 hour. All the samples are abbreviated in Table S1.

Characterization:
The phase investigation was done by X-ray diffraction (XRD) study using Rigaku-Smartlab diffractometer (Cu Kα source at 35 mA and 70 kV). Thermal features was examined by differential thermal analysis (DTA) and thermogravimetry (TG) experiment (Netzsch STA 449C, Germany). The phase analysis was further supported by FTIR (Perkin-Elmer Spectrum RX1 spectrophotometer) and RAMAN (Horiba Jobin-Yvon LabRAM HR800) study. Phase purity and composition analysis was investigated by XPS measurement using Thermo Fisher Scientific system. For microstructural characterization, FESEM study was performed by SUPRA 55-VP instrument equipped with EDAX (GEMINI column technology). TEM (JEM 2100F field emission transmission electron microscope operating at 200 kV) study was carried out for further details regarding morphology and porosity. Elemental analysis was studied by ICP-AES (ARCOS, Simultaneous ICP Spectrometer manufactured by SPECTRO Analytical Instruments GmbH, Germany). Nitrogen adsorption-desorption experiment was studied for textural engineering properties such as surface area, porosity and pore size distribution. Samples were degassed at 80°C for 4 hours in vacuum and treated for analysis by Multi-point BET (Micromeritics Gemini VII-2390t) at 77K. Electrochemical activity was measured by Biologic SP300 electrochemical workstations.

Electrochemical Techniques:
First of all, 2 mg catalyst was added in 200 µL ethanol and 40 µL Nafion mixture. Then, it was kept for sonication around 15 min. Next, 60 µL homogeneous ink was drop casted each side of carbon paper (geometric surface area was 0.25 cm 2 ) and dried in oven at 50°C.
Loading of catalyst was maintained as 4 mg/cm 2 . These electrodes were treated as working electrode and used in three electrode cell measurement. Platinum (Pt) wire and Ag/AgCl (3.5 M KCl) was employed as counter electrode and reference electrode respectively. Catalytic S4 performance was investigated by sweep voltammetry (LSV) and cyclic voltammetry (CV) technical mode in 1(M) KOH (pH=14). Working electrodes were initially treated for 40 precondition cycle (with a scan rate 50 mV/sec) within required potential scale. Catalytic activity was performed by LSV mode (with scan rate 5 mV/sec) using iR-compensation technique from uncompensated solution resistance (R u   respectively, as recommended by the previous studies. [3][4][5][6] The ion-electron interactions were treated using the projector augmented wave potential (PAW). The DFT-D2 empirical correction method proposed by Grimme was applied for describing the effect of van der Waals interactions. 7 The kinetic energy cut off was set to be 500 eV in the plane-wave expansion for all the calculations. All the structures were fully relaxed (both lattice constant and atomic position) using the conjugated gradient method and the convergence threshold was set to be 10 -4 eV in energy and 0.01 eV/Å in force. The Brillouin zone was sampled using a 5×5×1 Monkhorst-Pack k-point mesh for geometry optimization while a higher 7×7×1 Monkhorst-Pack grid was used to calculate electronic density of states (DOS          1.14 , 28.5 1.14 , 28.5 1.14 , 28.5 1.14 , 28.              Best performance was achieved for the 120 min reaction period