Smith, Daniel E.
(2020)
Electrocatalysis in protic and aprotic ionic liquids for energy applications.
PhD thesis, University of Nottingham.
Abstract
This thesis documents a study of electrocatalytic reactions pertinent to electrochemical energy conversion using protic ionic liquids (PILs) and aprotic ionic liquids (AILs) as electrolytes. The effect of proton-donating and proton-accepting species on electrocatalytic reactions in ionic liquids are investigated, and the implications for the use of PILs as intermediate-temperature fuel-cell electrolytes are considered. A fundamental understanding of proton-shuttling mechanisms in PIL-based fuel cells is presented.
PILs were synthesised and characterised and AILs (used as received) were characterised. A method of synthesising stoichiometric PILs by addition of 0.1 mol dm−3 aqueous acid to 0.1 mol dm−3 aqueous base with slight excess of base, is presented. The presence of excess neutral acid is detected in PILs voltammetrically, and results from a commonly used synthesis method. The acid does not have a significant effect on PIL ionic conductivity, particularly when compared to the effect of water.
The presence of strong acid in PILs causes the oxygen-reduction reaction (ORR) to shift ≈0.9 V positive, and the presence of a strong base causes the hydrogen-oxidation reaction (HOR) to shift ≈0.9 V negative. Voltammetric experiments show that the fuel-cell half-cell reactions (HOR and ORR) occur at a similar potential in pure stoichiometric PILs, meaning that stoichiometric PILs cannot be used as fuel-cell electrolytes. When acid or base are present in PILs, voltammetric experiments predict a fuel-cell open circuit potential (OCP) of around 0.9 V, which is comparable to that in conventional hydrated systems. The presence of acids and bases in PILs changes the mechanism of the ORR and HOR, respectively. Adding acids of increasing strengths (by aqueous pKa) to O2-saturated diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]) shifts the ORR half-wave potential positive by ≈ 50 mV per unit decrease in pKa over the range 10.6 to −3. O2 reduction can therefore be used to assess relative acidity of protic species added to PILs over this range.
In all AILs tested, the ORR shifts to more positive potentials in the presence of added acid. The greatest shift in ORR to a more positive potential is observed in 1-methyl,1-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([pyrr][NTf2]) in the presence of 0.1 mol dm−3 hydrogen bis(trifluoromethanesulfonyl)imide (HNTf2). In the absence of acid, the ORR in [pyrr][NTf2] occurs at a significantly more negative potential than the HOR indicating that without acid, [pyrr][NTf2] is not a viable fuel-cell electrolyte. In acidified [pyrr][NTf2] the difference between the ORR and HOR is around 0.5 V, predicting a lower fuel-cell OCP than in conventional hydrated systems. Voltammetry in O2-saturated N,N,N,N-tetraoctylammonium bis(trifluoromethanesulfonyl)imide ([N8888][NTf2]) is similar to that in O2-saturated N,N,N,N-tetraoctylphosphonium bis(trifluoromethanesulfonyl)imide ([P8888][NTf2]) in the absence and presence of acid.
Formic acid oxidation (FAO) and water oxidation were studied in [dema][TfO]. FAO was investigated as a probe reaction to assess basicity in PILs. However, all but one of the bases chosen were found to passivate the platinum electrode used. In the presence of diethylmethylamine (dema), the FAO voltammetric peak magnitude increases, confirming that formate is the active species in direct FAO. Voltammetric evidence for oxygen evolution from water splitting in [dema][TfO] positive of 2.2 V is also presented, along with evidence of the adsorption (on Pt) of a triflic acid-based species at highly oxidising potentials.
Fuel-cell tests confirm the predictions made in Chapter 4, that pure PIL electrolytes cannot sustain a current density because a viable proton shuttling mechanism is not present. When acid or base is added to [dema][TfO], a viable proton shuttling mechanism allowed fuel-cell performance comparable to that of conventional hydrated systems. When base is present at the anode and acid at the cathode, a fuel-cell OCP of around 1.8 V is measured, confirming the understanding of the reaction thermodynamics and mechanisms of the ORR and HOR in PILs presented throughout this thesis. Heating PILs for extended periods at ‘intermediate-temperatures’ (150 °C), discolours them and generates a protic species, likely triflic acid, which causes the ORR to shift to a more positive potential, which could explain the favourable fuel-cell performance reported in the literature using [dema][TfO] as an electrolyte.
The conclusions of this work, and new understanding of electrocatalytic reactions in ILs, have significant implications for the use of PILs as fuel-cell electrolytes because of the need for added acid or base to provide a viable proton shuttling mechanism. New challenges now arise to identify electrolyte compositions that retain the advantages offered by PILs (non-volatility and non-corrosivity), while supporting fast proton transport between the anode and cathode of fuel cells.
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