One of the significant challenges faced during electrochemical CO₂ reduction (ECO₂RR) is the low selectivity of the products obtained. The best example is polycrystalline Cu, which can electrochemically produce hydrocarbons and alcohols but with poor selectivity.^{1, 2} To date, high selectivity has been achieved only towards CO and formate on Au and Sn surface, respectively but scalability and commercialization are limited owing to their cost and stability. Metal-nitrogen-carbon (MNC) materials are comparable to Au or Ag catalysts, albeit with their lower overpotentials, higher mass activity and high selectivity toward CO.^3,4 During the pyrolysis of N-containing carbon molecules, different chemical functionalities such as pyridinic, pyrrolic and graphitic N atoms can be formed, each behaving as a product-specific active site. ⁵ It is still debatable whether pyrdinic or pyrrolic N is responsible for CO₂RR activity; thus, it remains unclear how to favor CO₂ reduction over hydrogen evolution.

In this work, we aimed to synthesize C₂N-like covalent organic frameworks, with 8-10 Å pore size, tailored to host dual atoms coordinated to nitrogen. Such sites are expected to favor C-C coupling and produce multi-carbon products during ECO₂RR. A step-by-step synthetic approach was employed to optimize the CO₂RR:HER ratio through:

• Tuning carbon:nitrogen ratio using two different synthetic approaches.
• Reducing nitrogen in the carbon matrix through pyrolyzing at different temperatures (700, 800 and 900 ^oC),
• Doping phosphorus and nitrogen into the carbon matrix
• Tuning CO₂RR product distribution through different metals (Fe and Ni) doping.

EXAFS and STEM studies revealed the presence of a mixture of single and dual atomic sites. XPS and ICP results showed that Fe and Ni loadings <2 wt% could be obtained in the C₂N materials. Higher hydrogen evolution, owing to the higher N-content, was observed at a lower pyrolysis temperature which decreased at a pyrolysis temperature of 900 ^oC. Both Fe-NC and Ni-NC selectively produced CO, thus suppressing FE_H2 to <5%. Fe-NC also exhibited lower overpotential for CO production compared to its Ni-NC counterpart. A small percentage of ethanol and 2-propanol (FE<5%) was observed indicating the favoured C-C coupling on the dual-atomic sites. Further optimization of the metal-N coordination environment may allow for an improved selectivity for >C2 products.

References

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