Electrochemical reduction of nitrite and CO₂, and oxidation of organic fuels

Akther, Jasmeen (2023) Electrochemical reduction of nitrite and CO₂, and oxidation of organic fuels. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In today's world, the need for sustainable technology is more crucial than ever. With the continuous growth in demand for resources and energy, the environment is under severe strain due to climate change, depletion of natural resources, and environmental degradation. Electrochemical techniques offer a promising solution for sustainable development. One such approach is using carbon dioxide as a renewable, non-fossil-based feedstock to produce fuels and value-added chemicals via electrochemical processes that use renewable energy sources. In addition, coelectrolysis of carbon dioxide with environmental pollutants such as NO₂-, NO₃-, and NO has shown promising results for producing of sustainable fuels, commodity chemicals, and fertilizers while reducing environmental pollutants. Our research focuses on coreduction of CO₂ with nitrite (NO₂-) to produce ammonia and urea simultaneously using renewable power sources. Among the effective catalysts for this process, metallophthalocyanines (M-Pc) have been shown to be successful, especially iron-based phthalocyanine, with a high current efficiency. We investigated the electrochemical coreduction of NO₂- and CO₂ at carbon-supported iron-based phthalocyanine electrocatalysts to produce ammonia and urea under ambient conditions. To understand the electrochemical behavior of the electrodes, we used both cyclic voltammetry and chronoamperometry in 0.1 M NaHCO₃ and 5 mM NaNO₂ solution under N₂ and CO₂ environments. The produced ammonia and urea concentrations were determined using two different spectrophotometric techniques, and secondary analytical techniques, liquid-chromatography-mass spectrometry (LC-MS) and proton nuclear magnetic resonance spectrometric (1H-NMR), were used to confirm the accuracy of the results. Our results indicate that it is possible to produce urea at low overpotentials at various ironbased phthalocyanine electrocatalysts in NaHCO₃ as an electrolyte. However, our experiments revealed that ammonia was the primary electrolysis product when using carbon-supported iron phthalocyanine (FePc/C) as an electrocatalyst. At a potential of -0.347 V vs RHE, 85% of the current was used for NH₃ production, while only 4.1% was utilized for urea production. Nevertheless, we observed a significant amount of urea production at FePc/C, with a maximum yield of 5.8% at the lowest overpotential (-0.047 V vs RHE). We also observed that carbon supported sulfonated iron(III) phthalocyanine (FeTSPc/C) produced the highest faradaic yield (54.8%) of urea at a potential of +0.053 V vs RHE, with 25% coproduction of NH3. In a PEM electrolysis cell, the FePc/C catalyst demonstrated the potential to produce urea and ammonia simultaneously using very low NO₂- concentrations. The faradaic efficiency for urea was increased from 2.8% to 15.9% compared to the normal three-electrode cell. In addition to producing commodity chemicals, research has also focused on developing electrocatalysts for fuel cell applications. PtBi/C and PtPb/C catalysts were prepared by the surface decoration of a commercial Pt/C catalyst, and their catalytic activities for electrochemical oxidation of formic acid, methanol and ethanol were compared. It was found that the currents at 0 V vs SCE for formic acid oxidation at the PtBi/C and PtPb/C catalysts were ~ 6 and ~ 2 times higher, respectively, compared to the unmodified Pt/C catalyst. In addition, the PtBi/C catalyst also showed slightly higher activity for ethanol oxidation at low potentials compared to the unmodified Pt/C.

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