Polyaniline-Modified Pt Catalyst for Improved Electrochemical Reduction of CO2

Alternative energy sources are being extensively explored such as wind and solar energy.  Due to their intermittency, it will be difficult for grids to handle a large percentage  of their energy coming from these sources.  This energy could be stored in chemical bonds by converting CO₂ and water to fuels and oxygen, creating a carbon neutral cycle.  Heterogeneous catalysts studied for the CO₂ reduction reaction in aqueous environments consist mainly of metals, which produce varied product distributions but all require large overpotentials.¹ Some attempts to improve catalysts include alloys and chemical modification of metal surfaces.^2-4 This study uses surface chemical modification in an effort to tune the product distribution or lower the overpotential required for electrochemical CO₂ reduction on metal catalysts. The metal catalyst explored was platinum, which is actually a poor aqueous CO₂ reduction catalyst since it primarily just reduces the protons to H₂. Polyaniline (PANi) was chosen as the chemical modifier to interact with the CO₂ at the catalyst surface. CO₂ electrolysis experiments were run for 1 hour potentiostatically using a custom electrolysis cell with a large working electrode area to electrolyte volume ratio for increased sensitivity of liquid products. Gas Chromatography (GC) and Nuclear Magnetic Resonance (NMR) were used to detect gas and liquid products, respectively. The PANi was electrodeposited on the Pt foil substrates using cyclic voltametery.  These PANi-Pt catalysts as well as the bare Pt foils were tested at a variety of potentials that spanned current densities from  0.5 to 15 mA/cm². The Pt foils performed similar to literature reports, with H₂ as the dominant product with formate (<0.4%) as a minor product.¹ However, CO, methane, and methanol were also detected in small amounts (<0.2%), which were seen in one report.⁵ The PANi-Pt catalysts show a small but noticeable increase in efficiency for formate (up to 1.5%) and CO (up to 0.8%)  and a decrease in methanol production compared to the pure Pt metal catalyst.  The possibility that the efficiency increased due to degradation of the PANi film was ruled out using a C¹³-labeled CO₂ experiment and H¹ NMR analysis of the formate. The current hypothesis is that the amine groups in the leucoemeraldine form of the PANi interact with the CO₂ intermediates at the surface of the Pt catalyst to lower the barrier for the rate-limiting step for the 2e^- products, CO and HCOO^-. Ongoing work is aimed at exploring this hypothesis to develop the mechanistic understanding needed to develop more efficient catalysts. 

ACKNOWLEDGMENT

The authors would like to thank the Global Climate and Energy Project, Chevron and the Stanford Graduate Fellowship (SGF), and the National Science Foundation (NSF) for funding.

REFERENCES

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