Water-soluble binuclear aquo Cu(ii) complex with an amide ligand as an electrocatalyst for the OER, HER and CO2RR

Abstract

A dinuclear copper complex (1) with the formula Cu₂L₂ (where L = [3-((pyridin-2-ylmethyl)carbamoyl)isonicotinic acid]) was synthesized and characterized by single-crystal XRD and mass spectrometry. The complex showed a high order of electrocatalytic hydrogen evolution reaction (HER) and carbon dioxide reduction reaction (CO₂RR) activity in acidic media, along with a moderate oxygen evolution reaction (OER) activity in the pH range of 7 to 13.5. The efficiency of 1 for the HER was calculated as follows: TOF = 1679 s^-1, TON = 586 and F.E. = 83% in 56 equivalents of perchloric acid. For the CO₂RR, a TOF value of 4 h^-1, a TON of 18 and an F.E. of 92% were observed, with a percentage selectivity of ∼99.99% for CO₂ to (COOH)₂. The best water oxidation activity was accomplished at pH 13.5 with the following electroanalytical efficacy parameters: TOF = 9 s^-1 (peak current method), TOF_FOWA = 31 s^-1, TON = 66 and F.E. = 84%. The high stability of the molecular catalyst was analyzed through CV, FESEM, EDX and DLS.

Fig. 1. ORTEP diagram of the ligand L1 and dinuclear Cu-complex: 1.

Fig. 2. (A) CV curve and DPV of the cathodic segment of 1 in water; c = 0.5 mM, scan rate = 100 mV s⁻¹, internal reference: Ag/AgCl (aqueous), and working electrode: glassy carbon. (B) CV curves of 0.5 mM complex 1 in water with the addition of 0–56 equivalents of perchloric acid.

Fig. 3. Comparison of the CV curves of complex 1 in water without acid and with 56 equivalents of acid.

Fig. 4. CV and DPV of the anodic segment of 1 in water; c = 0.25 mM, scan rate = 100 mV s⁻¹, and internal reference: Fc/Fc⁺.

Fig. 5. (A) CV curves of complex 1 in phosphate buffer from pH 7–13.5 at a scan rate of 100 mV s⁻¹ and 0.5 mM concentration using glassy carbon as the working electrode. (B) Pourbaix diagram of complex 1.

Fig. 6. (A) CV of 0.5 mM complex 1 in phosphate buffer at pH 13.5 with increasing scan rates from 2 mV s⁻¹ to 100 mV s⁻¹. (B) The plot of i_cat/i_p as a function of the inverse of the square root of the scan rates used in the experiment of complex 1.

Fig. 7. The cyan line shows the CV curve of complex 1 (0.5 mM) at pH 13.5. The red line indicates the data points used for the FOWA. The i/i_pvs. 1/{1 + e[(E^0,ap − E)(F/RT)]} plot assuming a WNA. The fitting points for the extraction of rate constants at the foot of the wave are taken from the WNA graph. The red line indicates the points taken into consideration for the calculation of TOF.

Fig. 8. CV curves of 0.5 mM complex 1 in water after 60 minutes of purging with CO₂ and consecutive addition of perchloric acid. Scan rate = 100 mV s⁻¹, internal reference: Ag/AgCl (aqueous), working electrode: glassy carbon.

Fig. 9. (A) Optimized structure of complex 1. (B) Theoretically calculated HOMO–LUMO gap of complex 1.