Study On Fuel Cells’ Performance And Proton Transport Mechanism Of Deep Eutectic Solvents With Methyli-midazole And Quaternary Ammonium As Electrolytes

Ionic liquids are widely used as electrolytes in fuel cells due to their low vapor pressure,wide electrochemical potential window,and high ionic conductivity.However,Midazolium salts,which are the most commonly employed ionic liquids,exhibit suboptimal cell performance and have drawbacks such as high cost and a complicated purification process.In contrast,deep eutectic solvents（DESs）not only possess the properties of ionic liquids but also offer advantages such as low cost,low toxicity,and biodegradability.Therefore,it is necessary to develop novel DESs as electrolytes to improve cell performance.The aim of our work is to develop novel DESs,explore their potential utilization as fuel cell electrolytes,and investigate the proton transport mechanism through quantum chemical calculations.DESs have been successfully prepared by using 1-sulfobutyl-3-methylimidazole hydrogensulfate（B4-1-0）,N,N,N-trimethylbutylsulfonate hydrogensulfate（N4-1-0）,and N,N,N-trimethylpropylsulfonate hydrogensulfate（N3-1-0）as hydrogen bond acceptors along with urea as a hydrogen bond donor in molar ratios of 2:1,1:1,and 1:2.¹H NMR and ¹³C NMR techniques are employed to confirm the chemical structures of B4-1-0,N4-1-0,and N3-1-0.The synthesized DESs were fully characterized by using thermogravimetric analysis and electrochemical tests.The results show that the synthesized DESs exhibit a wide electrochemical potential window and excellent thermal stability,rendering them suitable electrolytes for fuel cells.In the B4-urea system,the peak power densities of B4-1-0,B4-2-1,B4-1-1,and B4-1-2exhibited a gradual increase with increasing temperature.However,as the urea molar ratio increased,the peak power densities of the four samples initially showed an upward trend but subsequently declined.The DES with a molar ratio of B4-1-0 to urea at 1:1（B4-1-1）exhibits a significantly higher peak power density of 35.7 mW·cm^-2 compared to the peak power density(26.6 mW·cm^-2)observed in B4-1-0 without the addition of urea at 70℃,while the B4-1-2displays a much lower peak power density of only 13 mW·cm^-2.The significant difference in peak power density between B4-1-1 and B4-1-2 is attribute to the establishment of a well-structured hydrogen bonding network in B4-1-1,thereby facilitating efficient proton transfer.However,site resistance in the B4-1-2 system becomes an important factor hindering proton transport.The cell performance of B4-urea system was unsatisfactory due to the poisoning effect of the imidazolium cation on the Pt/C catalyst.Therefore,quaternary ammonium salts N4 series containing-SO₃H has been selected as a hydrogen bonding acceptor to improve the cell performance.N4-1-0 without the addition of urea exhibits a higher peak power density of 38.2m W·cm^-2 compared to B4-1-1(35.7 mW·cm^-2).The DES with a molar ratio of N4-1-0 to urea at 2:1（N4-2-1）exhibits the highest peak power density of 80.2 mW·cm^-2 at 50℃.The addition of an appropriate amount of urea facilitated the formation of a robust hydrogen bonding network within DESs and promoted proton transport.The physical properties of ionic liquids can be tailored by adjusting the length of alkyl chains.N3-1-0 can be obtained by reducing one methylene group in the cation of N4-1-0 in order to improve cell performance.N3-1-0 without the addition of urea exhibits a higher peak power density of 48.5 mW·cm^-2 compared to N4-1-0(38.2 mW·cm^-2),demonstrating that short-chain alkyl can improve fuel cell performance.N3-1-1 exhibits the highest peak power density of 92.5 mW·cm^-2 at 50℃,while N3-1-2 displays the lowest peak power density of only 8.7 mW·cm^-2at 30℃,which is higher than that of N4-1-2(1.9 mW·cm^-2).Similarly,the addition of an appropriate amount of urea facilitated the formation of a robust hydrogen bonding network and promoted proton transform.The steric hindrance of N3-1-2 is still an important factor hindering proton transport.Quantum chemical calculation of proton transport in fuel cells shows that proton transport primarily occurs between cations and anions in B4-urea and N4-urea systems.In the N3-urea system,proton transport not only occurs between cations and anions,but also between cations and their corresponding neutral molecules.N3-1-0 has only one less methylene than N4-1-0,but it has a great impact on the cell performance.