Harnessing Layer-Controlled Two-dimensional Semiconductors for Photoelectrochemical Energy Storage via Quantum Capacitance and Band Nesting

Two-dimensional (2D) transition metal dichalcogenides like molybdenum diselenide (MoSe$_2$) have shown great potential in optoelectronics and energy storage due to their layer-dependent bandgap. However, producing high-quality 2D MoSe$_2$ layers in a scalable and controlled manner remains challenging. Traditional methods, such as hydrothermal and liquid-phase exfoliation, lack precision and understanding at the nanoscale, limiting further applications. Atmospheric pressure chemical vapor deposition (APCVD) offers a scalable solution for growing high-quality, large-area, layer-controlled 2D MoSe$_2$. Despite this, the photoelectrochemical performance of APCVD-grown 2D MoSe$_2$, particularly in energy storage, has not been extensively explored. This study addresses this by examining MoSe$_2$'s layer-dependent quantum capacitance and photo-induced charge storage properties. Using a three-electrode setup in 0.5M H$_2$SO$_4$, we observed a layer-dependent increase in areal capacitance under both dark and illuminated conditions. A six-layer MoSe$_2$ film exhibited the highest capacitance, reaching $96 \mu\mathrm{F/cm^2}$ in the dark and $115 \mu\mathrm{F/cm^2}$ under illumination at a current density of $5 \mu\mathrm{A/cm^2}$. Density Functional Theory (DFT) and Many-Body Perturbation Theory calculations reveal that Van Hove singularities and band nesting significantly enhance optical absorption and quantum capacitance. These results highlight APCVD-grown 2D MoSe$_2$'s potential as light-responsive, high-performance energy storage electrodes, paving the way for innovative energy storage systems.