The Nature of Proton Shuttling in Protic Ionic Liquid Fuel Cells - PowerPoint PPT Presentation

The Nature of Proton Shuttling in Protic Ionic Liquid Fuel Cells Daniel Edward Smith H2FC Supergen Researcher Conference, Monday 17th February 2020 University of Nottingham

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)  H 2 /O 2 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

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

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 Protic Ionic Liquids (PILs) [dema][TfO] [dema][NTf 2 ]

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 Protic Ionic Liquids (PILs) [dema][TfO] [dema][NTf 2 ]

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 Protic Ionic Liquids (PILs) [dema][TfO] [dema][NTf 2 ]

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 trifluoromethanesulfonic acid [dema][TfO]

Neat acid to neat base – excess triflic acid is present 2TfOH + 2e – → 2TfO – + H 2 CVs of Ar-Saturated [dema][TfO] recorded using a 12.5- μ m 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.

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 trifluoromethanesulfonic acid [dema][TfO]

No excess triflic acid when using synthesis method 2 No TfOH reduction observed (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- μ m radius Pt ultramicroelectrode. Potentials reported vs. Pd-H. 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.

The effect on ORR potential of acidifying [dema][TfO]

The effect on ORR potential of acidifying [dema][TfO] ≈ 50 mV per unit p K a

Acidified [dema][TfO] – [TfO] − /TfOH proton shuttle Overall: 2H 2 + O 2 → H 2 O HOR: 2H 2 + 4[TfO] – + → 4TfOH + 4e – ORR: O 2 + 4TfOH + 4e – → 2H 2 O + 4[TfO] –

Basified [dema][TfO] – dema/[dema] + proton shuttle Overall: 2H 2 + O 2 → H 2 O HOR: 2H 2 + 4dema → 4[dema] + + 4e – ORR: O 2 + 4[dema] + + 4e – → 2H 2 O + 4dema

Pure [dema][TfO] – electrolytic PIL destruction HOR: 2H 2 + TfO − → 4TfOH + 4e – ORR: O 2 + 4[dema] + + 4e – → 2H 2 O + 4dema Overall: 4[dema][TfO] + 2H 2 + O 2 → H 2 O + 4dema + 4TfOH

Pure [dema][TfO] – electrolytic PIL destruction HOR: 2H 2 + TfO − → 4TfOH + 4e – ORR: O 2 + 4[dema] + + 4e – → 2H 2 O + 4dema Overall: 4[dema][TfO] + 2H 2 + O 2 → H 2 O + 4dema + 4TfOH

H 2 /O 2 fuel cell testing with [dema][TfO] electrolyte

H 2 /O 2 fuel cell testing with [dema][TfO] electrolyte

What happens when heating [dema][TfO]? B C

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) O 2 -saturated [dema][TfO] at a stationary Pt electrode. (B) O 2 -saturated [dema][TfO] at a Pt electrode rotating at 1,600 rpm. Blank CVs were recorded in air-free [dema][TfO] containing 10 ppm H 2 O in a nitrogen- atmosphere glovebox at a Pt electrode.

Pure PILs – PIL breaking regime Overall: 2H 2 + O 2 + 4[dema][TfO] → H 2 O + 4dema + 4TfOH

Pure PILs – PIL breaking regime Overall: 2H 2 + O 2 + 4[dema][TfO] → H 2 O + 4dema + 4TfOH

PILs + acid and base – PIL making regime Overall: 2H 2 + O 2 + 4dema + 4TfOH → H 2 O + 4[dema][TfO]

PILs + acid and base – PIL making regime Overall: 2H 2 + O 2 + 4dema + 4TfOH → H 2 O + 4[dema][TfO]

Fuel cell tests + acid and base: ‘super’ fuel cell + dema + TfOH HOR ORR 1.8 V OCP is possible from a single H 2 /O 2 cell

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 ∆ p K a , without protic additives, will have a negligible HOR and ORR potential difference  Cell potentials of ≈ 1 V are possible but only if strong acids ( pK a ≤ −3) or bases are used to dope the PIL electrolyte in order to shift the ORR onset 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

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

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Presentation overview 1. Why use protic ionic liquids (PILs) in polymer electrolyte membrane (PEM) fuel cells? 2. Synthesis and voltammetric characterisation of 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

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