Tailored Metal‐Porphyrin Based Molecular Electrocatalysts for Enhanced Artificial Nitrogen Fixation to Green Ammonia

Abstract Electrochemical nitrogen reduction (E‐NRR) is one of the most promising approaches to generate green NH3. However, scarce ammonia yields and Faradaic efficiencies (FE) still limit their use on a large scale. Thus, efforts are focusing on different E‐NRR catalyst structures and formulations. Among present strategies, molecular electrocatalysts such as metal‐porphyrins emerge as an encouraging option due to their planar structures which favor the interaction involving the metal center, responsible for adsorption and activation of nitrogen. Nevertheless, the high hydrophobicity of porphyrins limits the aqueous electrolyte–catalyst interaction lowering yields. This work introduces a new class of metal‐porphyrin based catalysts, bearing hydrophilic tris(ethyleneglycol) monomethyl ether chains (metal = Cu(II) and CoII)). Experimental results show that the presence of hydrophilic chains significantly increases ammonia yields and FE, supporting the relevance of fruitful catalyst‐electrolyte interactions. This study also investigates the use of hydrophobic branched alkyl chains for comparison, resulting in similar performances with respect to the unsubstituted metal‐porphyrin, taken as a reference, further confirming that the appropriate design of electrocatalysts carrying peripheral hydrophilic substituents is able to improve device performances in the generation of green ammonia.


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Figure S4.CV Co-TPP-TEG (left) and Cu-TPP-TEG catalysts in a 0.1 M tetrabutylammonium perchlorate solution in dichloromethane as a supporting electrolyte using a glassy carbon working electrode, a Pt wire as a counter electrode Ag/AgNO3 in 0.1 M tetrabutylammonium perchlorate solution in acetonitrile as pseudo-reference electrode.

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Figure S5.CV Co-TPP-EH (left) and Cu-TPP-EH catalysts in a 0.1 M tetrabutylammonium perchlorate solution in dichloromethane as a supporting electrolyte using a glassy carbon working electrode, a Pt wire as a counter electrode Ag/AgNO3 in 0.1 M tetrabutylammonium perchlorate solution in acetonitrile as pseudo-reference electrode.

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Figure S6.Cross-sectional images of porphyrins film on carbon paper: a i-ii) Cu/Co-TPP b i-ii) Cu/Co-TPP-EH c i-ii) Co/Cu-TPP-TEG.

Figure S11 .
Figure S11.Observed peaks in IC chromatograms after 2 h under Ar atmosphere.

Figure S12 .
Figure S12.CV (multiple cycles) of the investigated porphyrins on CP electrode as a working electrode, a Pt wire as a counter electrode and Ag/AgCl 3.5 M KCl pseudo-reference electrode in 0.1 M HClaq.