B and N codoped nanodiamond – A novel electrocatalyst for the selective electrochemical reduction of CO2 to ethanol

Ethanol is a clean and renewable liquid fuel with high heating value. Use of ethanol as a fuel is a viable strategy in terms of resource utilization and mitigating problems with regard to global warming. Electrochemical reduction of CO₂ can be considered as a possible route to obtain ethanol. Transition metal oxides and chalcogenides, B doped diamond, N doped carbon nanotubes and graphene were hitherto explored as potential electrocatalysts for CO₂ reduction. In spite of their good activity and durability, they reduce CO₂ to CO, HCHO or HCOO^– as the major products with a Faradic efficiency of 74.0 to 87.0%. Nevertheless, development of durable electrocatalysts for selective conversion of CO₂ to CH₃CH₂OH with high Faradic efficiency remains a big challenge.

Researchers from Dalian University of Technology, China and California Institute of Technology, USA have reported that B and N codoped nanodiamond (BND) could function as an efficient and stable electrode for the selective reduction of CO₂ to ethanol (Yanming Liu et al., Angew. Chem. Int. Ed. 10.1002/anie.201706311)

The BND film was deposited on Si by hot filament chemical vapor deposition method using a gas mixture of CH₄/B₂H₆/N₂/H₂ that had 2.5% CH₄. Three different BNDs with same B₂H₆ content (12.5%) but different N₂ levels (2.5%, 5.0% and 10.0% denoted as BND1, BND2 and BND3) were prepared.

All the three BNDs possess a similar crystal structure, morphological features and B content (Fig. 1(a)). All of them are found to be active for electrocatalytic reduction of CO₂ and they preferentially convert CO₂ to CH₃CH₂OH. The BNDs present a more negative H₂ evolution potential, which is favorable for CO₂ reduction with higher Faradic efficiency. The production rate of CH₃CH₂OH is significantly increased with a negative shift in potential from -0.8 V to -1.1 V (Fig. 1(b)) while the extent of formation of CH₃OH and HCOO^– remains low. The synergistic effect of B and N codoping is considered responsible for the better activity and high selectivity for the conversion of CO₂ to CH₃CH₂OH. Among the BNDs, maximum Faradic efficiency for the conversion of CO₂ to CH₃CH₂OH is achieved on BND3 (93.2% at -1.0 V) (Fig. 1(c)), suggesting that the higher the N content, the greater the electrocatalytic effect. The high durability of BND3 for electrocatalytic reduction of CO₂ is evidenced by its ability to show a Faradaic efficiency of ~93.2% during 16 consecutive experiments (Fig. 1(d)). Since a higher N content is likely to promote H₂ evolution, a balance between N content and H₂ evolution must be maintained to achieve better results.

Fig. 1 (a) Surface morphology of BND3; (b, c) production rates of CH₃CH₂OH, CH₃OH and HCOO^– in CO₂ saturated 0.1 M NaHCO₃ and the corresponding Faradaic efficiencies on BDN3; and (d) Faradic efficiency for CO₂ reduction during 16 consecutive runs on BND3 at -1.0 V

Based on the experimental results and density function theory (DFT) calculations, the possible pathway for the multi-electron reduction of CO₂ to CH₃CH₂OH proceeds as follows: CO₂ → *COOH → *CO → *COCO → *COCOH → *COCHOH → *COCH₂OH → *CHOCH₂OH → *CH₂OCH₂OH → CH₃CH₂OH

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Yanming Liu et al., Selective Electrochemical Reduction of Carbon Dioxide to Ethanol on a Boron- and Nitrogen-Co-doped Nanodiamond, Angew. Chem. Int. Ed. 10.1002/anie.201706311