SLIDE 1 The Nature of Proton Shuttling in Protic Ionic Liquid Fuel Cells
Daniel Edward Smith
Monday 17th February 2020 H2FC Supergen Researcher Conference, University of Nottingham
SLIDE 2 Polymer electrolyte membrane fuel cells (PEMFCs) at intermediate-temperature (100-300 °C)
- Improves rates of electrocatalytic reactions: the hydrogen-oxidation
reaction (HOR) and especially the oxygen-reduction reaction (ORR)
- H2/O2 gases into the fuel cell do not need to be humidified – water is
not required for cell conductivity
- Cell design is greatly simplified and less expensive
- Removal liquid water reaction products to prevent cell flooding no
longer required
- Heat management – exothermic fuel cell reaction is less of a
concern
- Possibility of moving away from noble and rare metal (Pt) ORR
catalysts
SLIDE 3 What makes a good intermediate-temperature PEM electrolyte?
- Must be thermally stable (doesn’t decompose at operating temperatures)
- Must be (highly) ionically conductive
- Must be non-volatile
- Not overly viscous – aids conductivity and diffusion of dissolved gases
- Non-corrosive
SLIDE 4 What makes a good intermediate-temperature PEM electrolyte?
- Must be thermally stable (doesn’t decompose at operating temperatures)
- Must be (highly) ionically conductive
- Must be non-volatile
- Not overly viscous – aids conductivity and diffusion of dissolved gases
- Non-corrosive
[dema][TfO] [dema][NTf2] Protic Ionic Liquids (PILs)
SLIDE 5 What makes a good intermediate-temperature PEM electrolyte?
- Must be thermally stable (doesn’t decompose at operating temperatures)
- Must be (highly) ionically conductive
- Must be non-volatile
- Not overly viscous – aids conductivity and diffusion of dissolved gases
- Non-corrosive
[dema][TfO] [dema][NTf2] Protic Ionic Liquids (PILs)
SLIDE 6 What makes a good intermediate-temperature PEM electrolyte?
- Must be thermally stable (doesn’t decompose at operating temperatures)
- Must be (highly) ionically conductive
- Must be non-volatile
- Not overly viscous – aids conductivity and diffusion of dissolved gases
- Non-corrosive
[dema][TfO] [dema][NTf2] Protic Ionic Liquids (PILs)
SLIDE 7 Synthesis of [dema][TfO] – neat acid to neat base (method 1)
neat acid to neat base (0.05 molar excess of base)
diethylmethylamine diethylmethylammonium trifluoromethanesulfonate [dema][TfO] trifluoromethanesulfonic acid
+
SLIDE 8 Neat acid to neat base – excess triflic acid is present
2TfOH + 2e– → 2TfO– + H2 CVs of Ar-Saturated [dema][TfO] recorded using a 12.5-μm radius Pt ultramicroelectrode. Potential reported versus Pd-H. (A) Neat acid to neat base (method 1)
Goodwin, S.E.; Smith, D.E.; Gibson, J.S.; Jones, R.G.; Walsh, D.A., Electroanalysis of Neutral Precursors in Protic Ionic Liquids and Synthesis of High-Ionicity Ionic Liquids, Langmuir 2017, 33, 8436−8446.
SLIDE 9 Synthesis of [dema][TfO] – 0.1 M acid to 0.1 M base (method 2)
0.1 M aqueous acid to 0.1 M aqueous base (0.05 molar excess of base).
diethylmethylamine diethylmethylammonium trifluoromethanesulfonate [dema][TfO] trifluoromethanesulfonic acid
+
SLIDE 10 No excess triflic acid when using synthesis method 2
(A) Neat acid to neat base (method 1), (B) Aqueous acid to aqueous base (method 2)
CVs of Ar-saturated [dema][TfO] recorded using a 12.5-μm radius Pt ultramicroelectrode. Potentials reported vs. Pd-H.
No TfOH reduction observed
Goodwin, S.E.; Smith, D.E.; Gibson, J.S.; Jones, R.G.; Walsh, D.A., Electroanalysis of Neutral Precursors in Protic Ionic Liquids and Synthesis of High-Ionicity Ionic Liquids, Langmuir 2017, 33, 8436−8446.
SLIDE 11
The effect on ORR potential of acidifying [dema][TfO]
SLIDE 12
The effect on ORR potential of acidifying [dema][TfO]
≈ 50 mV per unit pKa
SLIDE 13
Acidified [dema][TfO] – [TfO]−/TfOH proton shuttle
HOR: 2H2 + 4[TfO]– + → 4TfOH + 4e– ORR: O2 + 4TfOH + 4e– → 2H2O + 4[TfO]– Overall: 2H2 + O2 → H2O
SLIDE 14
Basified [dema][TfO] – dema/[dema]+ proton shuttle
HOR: 2H2 + 4dema → 4[dema]+ + 4e– ORR: O2 + 4[dema]+ + 4e– → 2H2O + 4dema Overall: 2H2 + O2 → H2O
SLIDE 15
Pure [dema][TfO] – electrolytic PIL destruction
HOR: 2H2 + TfO− → 4TfOH + 4e– ORR: O2 + 4[dema]+ + 4e– → 2H2O + 4dema Overall: 4[dema][TfO] + 2H2 + O2 → H2O + 4dema + 4TfOH
SLIDE 16
Pure [dema][TfO] – electrolytic PIL destruction
HOR: 2H2 + TfO− → 4TfOH + 4e– ORR: O2 + 4[dema]+ + 4e– → 2H2O + 4dema Overall: 4[dema][TfO] + 2H2 + O2 → H2O + 4dema + 4TfOH
SLIDE 17
H2/O2 fuel cell testing with [dema][TfO] electrolyte
SLIDE 18
H2/O2 fuel cell testing with [dema][TfO] electrolyte
SLIDE 19
What happens when heating [dema][TfO]?
B C
SLIDE 20 ORR in heated and unheated [dema][TfO]
Figure - Cyclic voltammograms of [dema][TfO] before and after heating to 150 °C for 64 hours recorded at a Pt electrode at 0.1 V s−1. (A) O2-saturated [dema][TfO] at a stationary Pt electrode. (B) O2-saturated [dema][TfO] at a Pt electrode rotating at 1,600 rpm. Blank CVs were recorded in air-free [dema][TfO] containing 10 ppm H2O in a nitrogen- atmosphere glovebox at a Pt electrode.
SLIDE 21
Pure PILs – PIL breaking regime
Overall: 2H2 + O2 + 4[dema][TfO] → H2O + 4dema + 4TfOH
SLIDE 22
Pure PILs – PIL breaking regime
Overall: 2H2 + O2 + 4[dema][TfO] → H2O + 4dema + 4TfOH
SLIDE 23
PILs + acid and base – PIL making regime
Overall: 2H2 + O2 + 4dema + 4TfOH → H2O + 4[dema][TfO]
SLIDE 24
PILs + acid and base – PIL making regime
Overall: 2H2 + O2 + 4dema + 4TfOH → H2O + 4[dema][TfO]
SLIDE 25
Fuel cell tests + acid and base: ‘super’ fuel cell
+ dema HOR + TfOH ORR 1.8 V OCP is possible from a single H2/O2 cell
SLIDE 26 Conclusions PIL-based PEM fuel cells without an added acid or base will not have a viable proton shuttling mechanism and cannot function effectively. PILs with large ∆pKa, without protic additives, will have a negligible HOR and ORR potential difference Cell potentials of ≈ 1 V are possible but only if strong acids (pKa ≤ −3)
- r bases are used to dope the PIL electrolyte in order to shift the ORR
- nset to more positive potentials. Parent acids and bases are best.
Heating PILs causes physical and chemical changes resulting in protic contaminants that affect the ORR The need for additional acids or bases to facilitate proton shuttling has significant consequences for PILs being used as fuel cell electrolytes and, in many cases, directly counters the advantages of PILs
SLIDE 27 Acknowledgements
My thanks to the following:
- Dr Darren Walsh
- Walsh Electrochemistry Group
- EPSRC
- Staff and Students of the Carbon Neutral Laboratory for
Sustainable Chemistry
- University of Nottingham School of Chemistry
- Centre for Doctoral Training in Fuel Cells & Their Fuels
SLIDE 28
Published in
SLIDE 29
Thank you for your attention
SLIDE 30
BBC East Midlands Today broadcast from the CNL
SLIDE 31
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SLIDE 32 Presentation overview
- 1. Why use protic ionic liquids (PILs) in
polymer electrolyte membrane (PEM) fuel cells?
- 2. Synthesis and voltammetric characterisation
- f PILs – why is there an excess of acid?
- 3. The effect of acids on the oxygen-reduction
reaction (ORR)
- 4. Protic ionic liquids in PEM fuel cells –
What’s really going on?
- 5. Conclusions – consequences for PIL-based
PEM fuel cells
SLIDE 33
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