Unique activity of a Keggin POM for efficient heterogeneous electrocatalytic OER

Abstract

Polyoxometalates (POMs) have been well studied and explored in electro/photochemical water oxidation catalysis for over a decade. The high solubility of POMs in water has limited its use in homogeneous conditions. Over the last decade, different approaches have been used for the heterogenization of POMs to exploit their catalytic properties. This study focused on a Keggin POM, K₆[CoW₁₂O₄₀], which was entrapped in a sol-gel matrix for heterogeneous electrochemical water oxidation. Its entrapment in the sol-gel matrix enables it to catalyze the oxygen evolution reaction at acidic pH, pH 2.0. Heterogenization of POMs using the sol-gel method aids in POM's recyclability and structural stability under electrochemical conditions. The prepared sol-gel electrode is robust and stable. It achieved electrochemical water oxidation at a current density of 2 mA/cm² at a low overpotential of 300 mV with a high turnover frequency (TOF) of 1.76 [mol O₂ (mol Co)^-1s^-1]. A plausible mechanism of the electrocatalytic process is presented.

Keywords: catalysis; energy materials; materials chemistry.

Graphical abstract

Figure 1

Physical characterization of Co-POM-TMOS (A) Powder-XRD of Co-POM-TMOS compared with simulated PXRD pattern of Co-POM. (B) XPS of Co-POM-TMOS sol-gel electrode and Co-POM.

Figure 2

Electrochemical characterization of Co-POM-TMOS CVs in solutions containing 0.20 M NaClO₄ at pH 2, at a scan rate of 20 mV/s. (A) Co-POM-TMOS sol-gel electrode and TMOS sol-gel electrode as working electrodes. (B) Glassy carbon electrode as a working electrode. The solution contains 1.0 mmol Co-POM.

Figure 3

Tafel plot of oCo-POM-TMOS iR-corrected, Recorded in 0.2 M NaClO₄, pH 2.

Figure 4

Electrochemical surface area (ECSA) for the Co-POM-TMOS and TMOS sol-gel electrodes Linear fits for the anodic current @ 0.41 V (vs. NHE) for CO-POM-TMOS and TMOS sol-gel electrodes.

Figure 5

EIS measurements of Co-POM-TMOS and TMOS Nyquist plot of (A) Co-POM-TMOS and (B) TMOS sol-gel electrodes, recorded in 0.2 M NaClO₄ and pH 2.

Figure 6

Electrochemical stability of Co-POM-TMOS electrode (A) Constant Potential Electrolysis (CPE) recorded at 1.6 V for Co-POM-TMOS sol-gel electrode. (B) Comparison of LSVs of Co-POM-TMOS before and after continuous reusing for electrochemical study for 30 days, scan rate: 50 mV/s, 0.2 M NaClO₄, pH 2.

Figure 7

Structural stability of Co-POM-TMOS electrode Spectral analysis of Co-POM-TMOS before and after the electrochemical study using (A) Raman analysis, (B) FT-IR analysis.

Figure 8

Morphological stability of Co-POM-TMOS electrode SEM images of the Co-POM-TMOS sol-gel electrodes were recorded (A and C) before and (B and D) after the electrochemical analysis.