Elucidating the water-molecule transport behavior and its effect in photocatalytic hydrogen-producing hydrogels from all-atom molecular dynamics simulations

Abstract

Efficient photocatalytic hydrogen production in hydrogel composites depends largely on the transport of water molecules therein. Using all-atom molecular dynamics (MD) simulations, this study elucidated the effects of water distribution and diffusion in polyacrylamide (PAM) hydrogels incorporating ZnIn₂S₄ (ZIS) nanosheets. The adsorbed water near photocatalysts was focused on and divided into bound water (BW), intermediate water (IW), and free water (FW). Results showed that at low hydration, BW dominates, whereas FW prevails at higher hydration conditions, creating continuous channels that enhance diffusion. The overall diffusion coefficient was found to be the fraction-weighted sum of BW, IW, and FW values, indicating that greater hydration markedly enhances proton and hydrogen mobility. Furthermore, ZIS incorporation elongated polymer chains and enhanced overall water retention, thereby enriching FW around In-centered active sites and underscoring the importance of local water trapping in charge transport. Mechanical analysis further showed the ZIS/PAM composite is over ten times stronger than pristine hydrogel. Particularly, ZIS/PAM hydrogels at 25–50 % water content exhibited rapid proton and hydrogen diffusion alongside favorable mechanical properties, making them suitable for photocatalysis. These atomistic insights provided valuable insights of hydrogel-particle systems, highlighting their potential for efficient hydrogen production and other energy conversion applications.