A Soluble Cu(II) Complex of Sodium 1,4-Dihydroxy-9,10-Anthraquinone-2-Sulphonate Electrocatalyzes Water Oxidation with a Large Turn over Frequency
Authors: Roy Sanjay , Guin Partha Sarathi 
Published: February 2020

The electro- and photo- catalytic generations of H2 and O2 from water have been explored as inexpensive ways of producing carbon-neutral fuel. Though various metal complexes have shown significant catalytic activity, however, efforts are still on to find out cheap, efficient and robust water soluble catalysts in producing H2 and O2 from water. In the present study we reported a soluble and inexpensive copper(II) complex of sodium 1,4-dihydroxy-9,10-anthraquinone-2-sulphonate (CuA2) as an electro-catalyst for water oxidation.

Cyclic voltammetry measurements were carried out by using Digi-Ivy Potentiostat (Model DY2312, made in USA) using three- electrode system at 298.15 K. A glassy carbon electrode was used as the working electrode; a platinum wire was used as counter electrode while Ag/AgCl, saturated KCl was used as reference electrode.

The cyclic voltammogram of CuA2 in aqueous phosphate buffer or NaF solutions showed two purely diffusion controlled reduction peaks at +0.635 and -0.265 V which are due to one –electron reduction of Cu(II) and one-electron reduction of one of the two free quinone sites of CuA2, respectively. The cyclic voltammetry measurement of CuA2 in such solutions at pH range 11.4 - 12.5 showed a large, irreversible and pH-dependent oxidation wave which appeared in the range between +1.34 and +1.74 V. A comparison of the present result with earlier clearly showed that the large irreversible anodic wave in the present study was exclusively due to the electrocatalytic evolution of oxygen. The generation of such oxygen was confirmed by suitable analytical cyclic voltammetry method. It is interesting to note that after the electrocatalytic evolution of oxygen, the reduction peak corresponding to Cu(II)/Cu(I) couple disappeared completely though the corresponding oxidation peak of this couple was found. The disappearance of this reduction peak is indicative of the fact that during the electrocatalytic oxidation of OH– to molecular oxygen, Cu(II) of CuA2is reduced to Cu(I) which is in turn oxidized electrochemically in the returned path of the cyclic voltammetry and hence catalyst regains its activity as shown below.

4OH– → O2 + 2H2O + 4e–

CuIIA2 + e– = CuIA2

CuIA2 – e– = CuIIA2(Electrochemical oxidation)

A cyclic voltammogram of aqueous phosphate buffer or NaF solutions as background showed that oxygen formation at the applied potential in the absence of catalyst was negligibly small which suggests that the chance for the oxidation of the working electrode is negligible.

In order to get kinetic information, cyclic voltammograms of CuA2at alkaline pH were recorded at various scan rates. Since the reduction of Cu(II)/Cu(I) in the present study was found to be diffusion controlled and reversible in nature, therefore, the diffusive current may be represented as follows.

id= 0.4633ndFAc√(ndFvD/RT) (1)

where, nd = number of electrons transferred, F = Faraday’s constant, A = area of the electrode, c = bulk concentration of CuA2, D = diffusion coefficient, R = molar gas constant, and T = absolute temperature.

The diffusive peak current in the absence of catalytic process was estimated using the diffusion-controlled Cu(II)/Cu(I) peak. The Cu(II)/Cu(I) peak is reversible in both lower and higher scan rates though reversibility decreases with the increase in scan rate as described above. The ratio of the catalytic current (ic) to the diffusive peak current (id), ic/id, matches the trend from a pure diffusion region to a pure kinetic region with the decrease in scan rate. In the kinetic region, the catalytic peak current in cyclic voltammetry is given as:

ic = ncFACcat√(Dkcat) (2)

where, k
cat = pseudo first-order rate constant, Ccat = bulk concentration of catalyst, nc
= number of electrons transferred = 4, F = Faraday’s constant, A = area of the electrode, D = diffusion coefficient of the catalyst, nc
= number of electrons transferred in the catalytic wave = 4 and v= scan rate.

It is frequently suitable to divide equation (2) by the peak current (id) in the absence of the catalyst (equation (1)), which generates equation (3).

ic/id = 0.359(nc/nd
3/2)√(kcat/v) (3)

(here, nd = 1). By using equation (3) and plot of ic
/id
versus v
−1/2, the pseudo-first-order rate constant for the catalytic oxidation of water or hydroxide at alkaline pH, k
cat, usually known as turnover frequency of the catalyst, was determined as 2927 s-1 at pH 11.90. The overpotential was observed in the present study as ~972 mV vs. NHE.

Comparing the turn over frequency of CuA2 with earlier for soluble metal complexes, it may be said that CuA2 acts as plausibly robust catalyst for water oxidation. However, our efforts are still on to decrease the pH and high overpotential.