Spin-to-charge conversion of chloroplast-functionalized ZnCo2O4 electrode in photoelectrochemical water splitting

In the pursuit of developing suitable photocatalyst for application in photoelectrochemical (PEC) water splitting and hydrogen generation, the current strategies involve surface modification, heterogenous doping or heterojunction formation by integrating multiple nanomaterials. Spin polarization, on the other hand, has emerged as an attractive technique to bypass these complex processes. Spin polarization assists in enhancing charge transport and reduced recombination rate in PEC systems by leveraging the spin degree of freedom of electrons. The unique electronic configuration of Co³⁺ and Co²⁺ in ZnCo₂O₄ (ZCO) provides a foundation for spin polarization due to the splitting of Co 3d orbitals into e_g and t_2g energy levels, thereby exhibiting distinct spin states. Upon external perturbations such as laser irradiation and an applied bias voltage, these spin polarized electrons can facilitate efficient charge separation. This study, thus, aims at providing the first of its kind analysis of p-type ZCO semiconductors as a photoelectrocatalyst. The spin-to-charge effects on photocurrent density were analyzed using 405 and 808 nm lasers. We observe that under an applied bias potential and simulated solar light exposure, the photocurrent density increases proportional to the porosity and rising calcination temperature of ZCO. This study further compares the influence of conducting substrates, such as carbon paper, with indium tin oxide-based electrodes on the PEC activity of ZCO. Conducting substrates, such as carbon paper, substantially enhance photocurrent density, stability, and hydrogen generation performance. Notably, the calculated hydrogen generation rate of 11.58 mmol s⁻¹ m⁻² was maximized in the spin-down states of induced polarization. The poor visible light absorption of ZCO was improved by chloroplast coating on the photoelectrode, consequently elevating the hydrogen generation rate to 46.31 mmol s⁻¹ m².