Surface morphology and electrochemical impedance correlation in Co-Pi modified TNA photoanodes

Hydrogen production via photocatalytic water splitting is one of the most auspicious strategies so as to overcome the energy crisis. Despite the existence of various pathways for the conversion of water and sunlight into hydrogen, photoelectrochemical (PEC) water splitting with semiconductor photoelectrodes has attracted much interest [1,2] .One main issue to increase solar hydrogen production in semiconductor PEC systems is reducing required over potential in water oxidation half reaction [3]. Here, an optimized photoanode was designed and electrochemically fabricated in which TiO2 nanotube arrays (TNA) were fabricated by the anodizing approach and then modified with cobalt phosphate fine particles as electrocatalyst using a potentiostat electro-deposition method. The amount of loaded electrocatalyst was changed by variation of deposition time duration from 10 min to 16 hours and its influence on the photoanode surface morphology and related charge transport resistance was studied using field emission scanning electron microscopy (FESEM) and electrochemical impedance spectroscopy (EIS), respectively. As illustrated in Figure 1a, charge transfer resistance of the bare TNA was about 5.5 kΩ which decreased to 4.5 , 4.2 and 4.0 kΩ for the samples modified for 10, 20 and 60 min, respectively and then increased to 5.0 kΩ for the 16 hours Co-Pi electrodeposited TNA. According to FESEM results, the addition of co-catalyst led to filling interspace of nanotubes as well as thickening their walls (Figure 1b, 1c and 1d). More amount of loading resulted in tube mouth closing and accumulation of Co-Pi particles on the whole electrode surface (Figure 1e). The latter case caused the increase in the charge transfer resistance of the sample modified for 16 hours.