Z-scheme-inspired tandem photoelectrochemical (PEC) cells have received attention as a sustainable platform for solar-driven CO2 reduction. Here, continuously 3D-structured, electrically conductive titanium nitride nanoshells (3D TiN) for biocatalytic CO2-to-formate conversion in a bias-free tandem PEC system are reported. The 3D TiN exhibits a periodically porous network with high porosity (92.1%) and conductivity (6.72 × 104 S m−1), which allows for high enzyme loading and direct electron transfer (DET) to the immobilized enzyme. It is found that the W-containing formate dehydrogenase from Clostridium ljungdahlii (ClFDH) on the 3D TiN nanoshell is electrically activated through DET for CO2 reduction. At a low overpotential of 40 mV, the 3D TiN-ClFDH stably converts CO2 to formate at a rate of 0.34 µmol h−1 cm−2 and a faradaic efficiency (FE) of 93.5%. Compared to a flat TiN-ClFDH, the 3D TiN-ClFDH shows a 58 times higher formate production rate (1.74 µmol h−1 cm−2) at 240 mV of overpotential. Lastly, a bias-free biocatalytic tandem PEC cell that converted CO2 to formate at an average rate of 0.78 µmol h−1 and an FE of 77.3% only using solar energy and water is successfully assembled.