A novel two-dimensional (2D)/2D heterostructure of oxygen-defective OV-Ti3AlC2 MAX with proton-rich functionalized carbon nitride (f-C3N4) was fabricated for photocatalytic CO2 conversion into energy-rich solar fuels, CH4, in the presence of H2O/H-2 under visible light. During CO2 photoreduction with H2O over f-C3N4, CO was produced rapidly; however, incorporating OV-Ti3AlC2, selective CH4 evolution was observed. The CH4 yield rate up to 786 mu mol g cat(-1)h(-1) was achieved via photodriven CO2 hydrogenation over the composite catalyst, which are 13- and 15-fold higher than Ti3AlC2 and graphitic carbon nitride (g-C3N4) catalysts, respectively. This superior performance confirms the higher electrical conductivity and efficient interfacial charge transfer due to dual mediators with oxygen clusters. Thermodynamic analysis reveals that both CO and H-2 intermediates greatly contribute to boosting CH4 generation, while the highest energy is consumed during water splitting. In other words, H-2 is necessary for CO2 hydrogenation, which can be injected directly or can be produced by water splitting. The optimized H-2/CO2 feed ratio of 2.0 further confirmed the adsorption competition of reactants before converting to CH4 Cyclic experiments show the highest catalytic reactivity with durability for continuous CH4 production without obvious deactivation. With the OV-Ti3AlC2 MAX framework structure, significant opportunities exist in terms of improved catalyst reactivity for single-step CO2 methanation to solar fuels.