Modification of Hematite Photoanode with Cobalt Based Oxygen Evolution Catalyst via Bifunctional Linker Approach for Efficient Water Splitting

The modification of hematite photoanode with cheap, scalable, and efficient oxygen evolution catalyst is an essential step for its practical application for solar fuel production. In this paper, a simple and water-based method has been developed for the modification of hematite surface with cobalt based oxygen evolution catalyst using 3-aminopropionic acid (APA) as a bifunctional linker. APA exists in an aqueous solution at pH 6.1 in zwitterionic form and thus it has a positive charge on the protonated amino group and a negative charge on the carboxylate group. The carboxylate groups of APA molecules can thus interact with the protonated surface hydroxyl groups of hematite whereas the amino groups tether the cobalt ions. The hematite photoanodes were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy and inductively coupled plasma optical emission spectroscopy. The photoelectrochemical measurements indicated that Co/APA-hematite photoanode exhibit 3.8-, 3.2-, and 2-fold higher photocurrent at 1.23 V vs RHE than bare, Co-, and CoPi-hematite photoanodes, respectively. Moreover, the onset potential of photoelectrochemical water oxidation on Co/APA-hematite photoanode is cathodically shifted by 290 mV in comparison to that obtained on bare hematite photoanode. This finding has been explained by measuring the transient photocurrent and intensity modulated photocurrent spectroscopy responses. On the basis of these measurements, it was found that the rate constant of electron transfer at Co/APA-hematite photoanode/liquid interface is higher than that measured for bare and CoPi-hematite photoelectrodes. This explains the higher photoelectrochemical activity of Co/APA-hematite photoanode and reflects on the potential application of this simple approach for the modification of different metal oxide photoelectrodes with cobalt-based oxygen evolution catalyst.