Fluorinated MAX Phases for Photoelectrochemical Hydrogen Evolution

Photoelectrochemical generation of hydrogen from water is considered to be the most appealing solution for the replacement of fossil fuels as a source of energy. For this reason, the study of novel and affordable materials with high energy conversion efficiencies is currently a crucial objective for the scientific community. Chemical modification of two-dimensional (2D) and layered materials, such as fluorination, can play a decisive role in tuning the properties for energy-related applications, as it was documented in the past by fluorination of graphite and graphene. MAX phases (MAX) are a class of layered ternary compounds that is well known for their interesting physical properties but still underexplored as a photoelectrocatalyst for energy conversion. Herein, a set of MAX, namely, Ta₂AlC, Cr₂AlC, Ti₂AlC, and Ti₃AlC_2, was exposed to fluorine gas in a controlled environment and their photoelectrocatalytic properties were tested for the hydrogen evolution reaction with illumination by a visible light source of 660 nm wavelength. All of the mentioned compounds showed excellent hydrogen evolution performances under illumination, in particular after the fluorination process. Fluorinated Cr₂AlC is the phase that showed the lowest overpotential, and fluorinated Ti₂AlC and Ti₃AlC₂ showed the most prominent photoelectrocatalytic enhancement upon fluorination. The fluorinated MAX phases should find broad applications to photoelectrochemistry, as their fluorinated graphene counterparts did in the past.