CoMoO4 Microspheres Synthesized Hydrothermally: Superior Electrode Performance for High‑Rate Supercapacitors

Abstract

Single‑phase CoMoO4 was produced by a facile hydrothermal method followed by calcination at 400 °C. Morphological, structural, and electrochemical characteristics of samples synthesized for 1, 4, 8, 12, and 24 h were systematically investigated. All samples exhibit a microsphere morphology composed of nanoflakes. The specific capacitances at 1 A g^–1 were 151, 182, 243, 384, and 186 F g^–1 for the 1, 4, 8, 12, and 24 h samples, respectively. The 12‑h sample (CMO‑12) shows a remarkable rate capability, retaining 45 % of its initial capacitance when the current density rises from 1 to 8 A g^–1. All samples retain over 95 % of their capacitance after 1 000 charge–discharge cycles at 8 A g^–1, confirming excellent long‑term stability. These results demonstrate that CoMoO4 microspheres are promising electrode materials for high‑performance supercapacitors.

Background

Renewable energy conversion and storage are critical for mitigating the depletion of fossil fuels. Supercapacitors, with their high power density, rapid charging, and long cycle life, have become attractive energy‑storage devices. Their charge‑storage mechanisms fall into two categories: electrochemical double‑layer capacitors (EDLCs) and pseudocapacitors (PCs). While EDLCs rely on ion adsorption at the electrode/electrolyte interface, PCs depend on fast redox reactions on the electrode surface, offering higher energy density.

Metal oxides such as NiO, Co₃O₄, CuO, MnO₂, and SnO₂ have been extensively studied as supercapacitor electrodes. Molybdenum and cobalt oxides, in particular, stand out due to their multiple oxidation states, high theoretical capacitance, and low cost. CoMoO4 has attracted attention for its non‑toxicity, low cost, and superior electrochemical performance. Reported specific capacitances range from 133 F g^–1 (plate‑like CoMoO4) to 2 100 F g^–1 (dandelion‑shaped CoMoO4) under various conditions.

In this study, CoMoO4 nanoflakes were synthesized via a hydrothermal approach with varying reaction times, followed by calcination. Electrochemical tests (CV, GCD, EIS) in 2 M KOH reveal that the 12‑h sample delivers the highest specific capacitance and excellent rate and cycle stability.

Experimental

Synthesis of CoMoO4

CoMoO4 was prepared by dissolving 0.4410 g Co(NO₃)₂·6H₂O and 0.2675 g (NH₄)₆Mo₇O₂₄·4H₂O in 30 mL of deionized water. After 10 min of stirring, 0.3621 g of urea was added and the mixture was stirred for an additional hour to achieve a homogeneous solution. The solution was transferred to a 50 mL Teflon‑lined autoclave and heated at 180 °C for 1 h. For longer reaction times (4, 8, 12, 24 h) the same procedure was followed. The resulting precipitates were washed with water and ethanol, dried at 60 °C for 10 h, and calcined at 400 °C for 2 h to yield the final products, labeled CMO‑1, CMO‑4, CMO‑8, CMO‑12, and CMO‑24.

Material Characterization

Phase purity was confirmed by X‑ray diffraction (Bruker D8 Discover, 40 kV/40 mA). Morphology was examined by field‑emission SEM (Zeiss SUPRA 40) and TEM (JEM‑2100). Nitrogen adsorption–desorption isotherms (Autosorb‑iQ) provided BET surface areas and BJH pore distributions.

Electrode Preparation and Electrochemical Measurements

The working electrodes were fabricated by mixing the active material, acetylene black, and PTFE in a 70:20:10 weight ratio. The paste was coated onto 1 cm² nickel foam, dried at 50 °C, and pressed at 10 MPa. Electrochemical performance was measured using a CS 350 workstation (CorrTest) in 2 M KOH with a three‑electrode setup (CoMoO4 as working, Pt as counter, SCE as reference). CV scans were performed from –0.2 to +0.6 V at 5–100 mV s^–1; GCD tests were carried out at 1–8 A g^–1; EIS was recorded from 0.01 Hz to 100 kHz.

Results and Discussion

Structural and Morphological Characterization

XRD patterns match the standard CoMoO4 phase (JCPDS No. 21‑0868). Peak broadening indicates reduced crystallinity, which is beneficial for pseudocapacitive behavior. SEM images reveal uniform microspheres built from interwoven nanoflakes. The nanoflake thickness increases with reaction time up to 12 h, then decreases at 24 h. EDS mapping of CMO‑12 confirms a 1:1:4 Co:Mo:O ratio, consistent with CoMoO4 stoichiometry.

TEM and HRTEM of CMO‑12 show clear lattice fringes at 0.34 nm and 0.27 nm, corresponding to the (002) and (–131) planes. The SAED pattern confirms single‑crystalline nature.

BET analysis shows surface areas of 18.4, 29.2, 42.8, 74.1, and 26.2 m² g^–1 for CMO‑1 to CMO‑24. The 12‑h sample exhibits the highest surface area (74.1 m² g^–1) and a well‑defined mesoporous structure (pore sizes around 22–75 nm), providing abundant active sites and efficient ion transport.

Electrochemical Characterization

CV curves display distinct redox peaks, confirming pseudocapacitive behavior. Peak positions shift with scan rate, indicating quasi‑reversible kinetics influenced by ion diffusion. The specific capacitance derived from CV increases from 171.3 to 315.7 F g^–1 as the synthesis time extends to 12 h, then drops to 178.7 F g^–1 at 24 h.

GCD measurements at 1 A g^–1 show the longest discharge time for CMO‑12, yielding a capacitance of 384 F g^–1. Even at 8 A g^–1, CMO‑12 retains 172 F g^–1, outperforming all other samples and many literature reports.

Cycling tests at 8 A g^–1 demonstrate excellent stability: 102.9, 87.8, 101.5, 94.2, and 100.5 % retention for CMO‑1 to CMO‑24. CMO‑12 shows the highest capacity retention (94.2 %) and consistent coulombic efficiency above 99 % over 1 000 cycles.

EIS analysis reveals the lowest charge‑transfer resistance (R_ct = 0.23 Ω) and series resistance (R_s = 1.22 Ω) for CMO‑12, confirming superior electronic and ionic conductivity.

Conclusions

Hydrothermal growth followed by calcination yields single‑phase CoMoO4 microspheres composed of nanoflakes. The 12‑h synthesis (CMO‑12) delivers exceptional supercapacitor performance: 384 F g^–1 at 1 A g^–1, 172 F g^–1 at 8 A g^–1, and >94 % capacitance retention after 1 000 cycles. The high BET surface area, mesoporous architecture, and low resistance synergistically enhance ion accessibility and electron transport, making CoMoO4 microspheres a compelling electrode material for next‑generation energy‑storage devices.

Abbreviations

BET

Brunauer–Emmett–Teller

CV

Cyclic voltammetry

EDS

Energy‑dispersive spectroscopy

EIS

Electrochemical impedance spectroscopy

FE‑SEM

Field‑emission scanning electron microscopy

GCD

Galvanostatic charge–discharge

PTFE

Polytetrafluoroethylene

SAED

Selected area electron diffraction

SCE

Saturated calomel electrode

TEM

Transmission electron microscopy

XRD

X‑ray diffraction