Z-scheme in natural photosynthesis are promising for solar-driven CO 2 conversion. [2] By combining multiple photoelectrodes or photovoltaics (PV), the Z-scheme PEC cells can provide sufficient photopotential to simultaneously drive water oxidation and CO 2 reduction under minimal or no external bias. [3] Nevertheless, lowering the kinetic barrier of thermodynamically inert CO 2 remains a hurdle for efficient CO 2 reduction. The development of CO 2reducing biocatalyst-conjugated cathodes can improve chemoselectivity and increase yield under mild conditions. [4] Compared to synthetic catalysts that often require extreme conditions such as high pressure, pH, or temperature, enzymes show high catalytic activities and specificities under mild conditions, making them a valuable catalyst for sustainable and green applications. In particular, formate dehydrogenase (FDH) is an attractive redox enzyme that reduces CO 2 to formate, an alternative water-soluble feedstock that can be easily converted to other common fuels. [5] Previous studies have focused on mediated electron transfer (MET)-type reactions, [6] in which redox mediators such as nicotinamide adenine dinucleotide cofactor (NADH) and Rh-based complexes shuttle electrons between an electrode and FDH. However, the MET-based biocatalysis requires costly electron mediators and multiple electron transfer steps that cause side reactions and significant losses in efficiency. [7] Here, we report the development of 3D titanium nitride nanoshell (3D TiN) electrodes for biocatalytic PEC cells that convert CO 2 to formate through direct electron transfer (DET), as depicted in Scheme 1a. A highly ordered, porous TiN structure is employed as an electrically conductive scaffold for efficient DET to a W-containing FDH from Clostridium ljungdahlii (ClFDH) (inset, Scheme 1a). TiN was chosen as a scaffold for DET-based bioelectrode because it is highly conductive, electrochemically stable and exhibit high chemical and thermal resistance, as well as exceptional hardness. [8] The 3D TiN electrode simultaneously provides (i) a large electroactive surface area generated from an ultrathin (≈30 nm), 3D nanoshell structure with high porosity (92.1%) for high enzyme loading per geometric area, (ii) a continuous electron transfer network with high electrical Z-scheme-inspired tandem photoelectrochemical (PEC) cells have received attention as a sustainable platform for solar-driven CO 2 reduction. Here, continuously 3D-structured, electrically conductive titanium nitride nanoshells (3D TiN) for biocatalytic CO 2 -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 × 10 4 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 CO 2 reduction. At a low overpotential...

Multienzymatic cascade reactions have garnered the attention of many researchers as an approach for converting CO2 into methanol. The cascade reaction used in this study includes the following enzymes: a formate dehydrogenase (ClFDH), a formaldehyde dehydrogenase (BmFaldDH), and an alcohol dehydrogenase (YADH) from Clostridium ljungdahlii, Burkholderia multivorans, and Saccharomyces cerevisiae, respectively. Because this cascade reaction requires NADH as a cofactor, phosphite dehydrogenase (PTDH) was employed to regenerate the cofactor. The multienzymatic cascade reaction, along with PTDH, yielded 3.28 mM methanol. The key to the success of this cascade reaction was a novel formaldehyde dehydrogenase, BmFaldDH, the enzyme catalyzing the reduction of formate to formaldehyde. The methanol yield was further improved by incorporation of 1-ethyl-3-methylimidazolium acetate (EMIM-Ac), resulting in 7.86 mM of methanol. A 500-fold increase in total turnover number was observed for the ClFDH-BmFaldDH-YADH cascade system compared to the Candida boidinii FDH-Pseudomonas putida FaldDH-YADH system. We provided detailed insights into the enzymatic reduction of CO2 by determining the thermodynamic parameters (K d and ΔG ) using isothermal titration calorimetry. Furthermore, we demonstrated a novel time-dependent formaldehyde production from CO2. Our results will aid in the understanding and development of a robust multienzyme catalyzed cascade reaction for the reduction of CO2 to value-added chemicals.

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