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. 2022 Oct 26;13(1):6349.
doi: 10.1038/s41467-022-33895-5.

Revealing the role of ionic liquids in promoting fuel cell catalysts reactivity and durability

Arezoo Avid  1   2 , Jesus López Ochoa  1   2 , Ying Huang  2   3 , Yuanchao Liu  1   2 , Plamen Atanassov  1   2 , Iryna V Zenyuk  4   5   6
Affiliations

Revealing the role of ionic liquids in promoting fuel cell catalysts reactivity and durability

Arezoo Avid et al. Nat Commun. .

Abstract

Ionic liquids (ILs) have shown to be promising additives to the catalyst layer to enhance oxygen reduction reaction in polymer electrolyte fuel cells. However, fundamental understanding of their role in complex catalyst layers in practically relevant membrane electrode assembly environment is needed for rational design of highly durable and active platinum-based catalysts. Here we explore three imidazolium-derived ionic liquids, selected for their high proton conductivity and oxygen solubility, and incorporate them into high surface area carbon black support. Further, we establish a correlation between the physical properties and electrochemical performance of the ionic liquid-modified catalysts by providing direct evidence of ionic liquids role in altering hydrophilic/hydrophobic interactions within the catalyst layer interface. The resulting catalyst with optimized interface design achieved a high mass activity of 347 A g-1Pt at 0.9 V under H2/O2, power density of 0.909 W cm-2 under H2/air and 1.5 bar, and had only 0.11 V potential decrease at 0.8 A cm-2 after 30 k accelerated stress test cycles. This performance stems from substantial enhancement in Pt utilization, which is buried inside the mesopores and is now accessible due to ILs addition.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Schematic drawing of catalyst layer interface and TEM images of pristine and modified catalyst powders.
a HSA Pt/C catalysts with micropores and smaller mesopores filled with either water or imidazolium-derived ILs, TEM images of b pristine 40 wt% HSA Ketjenblack Pt/C, c Pt/C modified with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (Pt/C-([C2mim]+[NTf2]−)), d Pt/C modified with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (Pt/C-([C4mim]+[NTf2]−)), and e Pt/C modified with 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide (Pt/C-([C4dmim]+ [NTf2]−)). Arrows show the ionic liquids lumps.
Fig. 2
Fig. 2. Ex situ physiochemical characterization of IL-modified samples vs. baseline.
a, b Nitrogen physisorption analysis at 77 K for pore occupancy by three ILs: a Sorption isotherm with log-scale relative pressure to highlight the microporous region, b volumetric pore size distribution, c mass evolution of catalyst layers containing various ILs with IL/C ratio of 1.28 measured using capillary penetration method. d Zeta potential measurements for pristine and IL-modified Pt/C with and without Nafion (error bars represent standard deviation of the mean).
Fig. 3
Fig. 3. Electrochemical characterization of baseline Pt/C vs. IL-modified Pt/C samples in rotating disk electrode setup and IL/C ratio optimization in MEAs containing various loading of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim]+[NTf2]−) evaluated at 80 °C and 100% RH.
a Cyclic voltammograms of baseline Pt/C and IL-modified Pt/C in N2-saturated 0.1 M HClO4, b polarization activity in O2-saturated 0.1 M HClO4 at a rotation speed of 1600 rpm. c Nyquist plots of impedance spectra under N2-saturated 0.1 M HClO4 for Pt/C-Nafion and Pt/C-IL-Nafion catalyst layers. d H2/air polarization curves of MEAs with IL/C ratio of 0.64, 1.28. 2.56 vs. baseline Pt/C in fuel cell, e power density obtained at 150 kPaabs total pressure, f H2/O2 Tafel plots measured at 150 kPaabs total pressure in 5 cm2 differential cells, cathode: HSA Pt/C or HSA Pt/C-IL with loading 0.13 ± 0.02 mg cm−2, anode: LSA Pt/C with loading 0.1 mg cm−2.
Fig. 4
Fig. 4. Electrochemical characterization of IL-containing MEAs obtained after three voltage recovery cycles vs. baseline Pt/C after two voltage recovery cycles evaluated at 80 °C and 100% RH.
a H2/air polarization curves, b power density at 150 kPaabs total pressure, c cyclic voltammograms recorded in H2/N2 and 100 kPaabs total pressure, d H2/O2 Tafel plots at 150 kPaabs total pressure, e CO-displacement at 0.35 V at 100 kPaabs and 40 °C, f correlation between local oxygen mass transport resistance and proton conductivity in 5 cm2 differential cells, cathode: HSA Pt/C or HSA Pt/C-IL with loading 0.13 ± 0.02 mg cm−2, anode: LSA Pt/C with loading 0.1 mg cm−2.
Fig. 5
Fig. 5. Electrochemical characterization of IL-containing MEAs obtained after three voltage recovery cycles vs. baseline Pt/C obtained after two voltage recovery cycles evaluated at 80 °C and 100% RH.
a Mass and b specific activities at 0.9 V, 150 kPaabs and 100% RH, and c local oxygen transport resistance measured at 75% RH, and d ionic conductivity at 100% RH in 5 cm2 differential cells, cathode: HSA Pt/C or HSA Pt/C-IL with loading 0.13 ± 0.02 mg cm−2, anode: LSA Pt/C with loading 0.1 mg cm−2.
Fig. 6
Fig. 6. Comparison of electrochemical properties at the beginning of life (BOL) and after 30,000 cycles (EOL) for Pt/C baseline vs. Pt/C-([C2mim]+[NTf2]−), Pt/C-([C4mim]+[NTf2]−), and Pt/C-([C4dmim]+[NTf2]−) MEAs.
a Comparison of H2/air polarization curves, b cell potential at 0.8 A cm−2, c mass activities obtained at 0.9 V vs. RHE, and d ECSA for all MEAs at BOL vs. EOL (after 30k cycles). BOL electrochemical properties were obtained after two and three voltage recovery cycles for baseline and IL-containing MEAs, respectively. EOL properties were obtained after one voltage recovery cycle for all MEAs.
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