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

Designing Natural Cell-Inspired Heme-Spurred Membrane Electrode Assembly for Fuel Cells

Developing highly efficient and durable membrane electrode assemblies (MEAs) is imperative for the widespread implementation of proton exchange membrane fuel cells (PEMFCs). However, the poor mass transfer efficiency and sluggish oxygen reduction reaction (ORR) kinetics have significantly suppressed the power density and longevity of platinum (Pt)-based MEA in PEMFCs, particularly when using an ultralow Pt loading. Inspired by the functional principles of hemoglobin in red blood cells, we present a Heme-cofactor strategy to create a "respiratory proton-transfer chain" for PEMFCs. This strategy can efficiently spur the catalytic activity of Pt while enhancing the mass transfer efficiency of MEAs, in which the multifunctional Heme featuring carboxyl and Fe2+ groups can accelerate the proton and oxygen transport as well as boost the ORR kinetics. As a result, by integrating Heme with typical Pt catalysts (i.e., commercial Pt/C, commercial Pt3Co/C, and homemade PtCo), the peak power density (PPD) and mass activity (MA) of Heme-spurred Pt-type MEAs can be dramatically enhanced by 50 to 109%, respectively. Particularly, with a low Pt loading of 0.1 mgPt cm-2, the Heme-spurred PtCo-based MEA achieves record PPDs of 3.8 W cm-2 (H2-O2) and 1.9 W cm-2 (H2-air), significantly surpassing the previous PPD records set by state-of-the-art MEAs. Meanwhile, our developed Heme-spurred MEA can even be run stably at 1.5 A cm-2 for over 50 days (1250 h) with 93% MA retention. These results underscore the viability of this universal and efficient Heme-cofactor strategy for practical fuel cell applications.

Electrocatalytic methane conversion via in-situ generated superoxide radicals in an aprotic ionic liquid

The electrochemical activation and partial oxidation of methane are highly attractive to enable the direct conversion in a sustainable and decentralized way. Herein, we report an electrochemical system in a non-diaphragm electrochemical bath to convert CH4 to CH3OH and CH3CH2OH at room temperature, in which V3O7·H2O as the anodic catalyst to activate CH4 and an aprotic ionic liquid [BMIM]BF4 as supporting electrolyte to control superoxide radicals (O2-) as the main active oxygen species generated on cathode. As a result, methanol and ethanol were identified as the liquid products, and the superior methanol Faraday efficiency (FE) of 32.2 % and selectivity of 76.8 % can be reached. Molecular dynamics (MD) simulation indicates that interaction between CH4 molecules and [BMIM]BF4, which enhances the mass transfer in electrochemical reaction. Density function theory (DFT) calculation results suggest that the V sites in V3O7·H2O enhanced the chemisorption and dissociation of CH4 molecules on anode surface, then superoxide radicals (O2-) are supposed to be involved in the formation of methanol and ethanol.

Polymer-Scaffold Induced Extensive Hydrogen-Bond Network: Enabling High Transport of Proton and Oxygen in Cathode Catalyst/Ionomer Interfaces

Uneven ionomer coverage in the cathode catalyst layer of proton exchange membrane fuel cells (PEMFCs) impedes proton conduction and oxygen diffusion, particularly at low platinum loadings. Here, a functionalized polymer-scaffold is designed and constructed by using hydroxy-pyridine polybenzimidazole (PyOHPBI) with abundant hydrogen-bond sites, thereby proposing a hydrogen-bond synergistic strategy to address the challenges of optimizing ionomer distribution and enhancing the transport of protons and gas through the catalyst layer. By integrating molecular dynamics simulations, in situ and ex-situ characterization methods, the design achieves 144.4% of the peak power density compared to commercial Pt/C catalysts, alongside an exceptionally low local oxygen transport resistance of only 7.81 s·m-1 in membrane electrode assemblies (MEAs). This study highlights how surface chemical modifications of carbon supports leverage hydrogen bonds to optimize ionomer coverage, significantly enhancing PEMFC performance and offering insights for developing more efficient and sustainable fuel cell technologies.

A hydrophobic ionic liquid enhances electrocatalytic hydrogen evolution reaction on high specific-surface ruthenium-doped cobalt catalysts

The integration of Ru-Co/C with an ionic liquid (IL) boosts the hydrogen evolution reaction by altering hydrogen-bond interactions. The catalyst exhibits an overpotential as low as 26 mV to achieve 20 mA cm-2 in 1 M H2SO4. Molecular dynamics simulations were used to study the hydrogen-bond structures tuned by the IL on the catalyst.

The Incorporation of Nanoconfined Poly(ionic liquid)s with Two-Dimensional Covalent Organic Frameworks to Enhance Proton Conduction

Covalent organic frameworks (COFs) hold promising potential as high-temperature proton conductors due to their highly ordered nanostructures and high specific surface areas. However, due to their limited functional groups and poor membrane-engineering properties, finding practical applications for COF-based proton-conducting materials still remains challenging. Herein, we proposed a universal strategy to fabricate proton-conducting composite membranes by the incorporation of sulfonic acid-bearing COFs and zwitterionic poly(ionic liquid)s (PILs) via in situ polymerization. Zwitterionic PILs with methanesulfonate counter ions can work as the intrinsic proton sources, and the sulfonic acid groups on the COF nanochannels can act as the extrinsic proton suppliers. Benefiting from the spatial nanoconfinement of long-range ordered nanochannels and the enhanced electrostatic interactions with PILs, the COFs with high densities of sulfonic acid groups can endow the as-prepared composite membrane (PIL@TpBD(SO3H)2) with a comparable anhydrous proton conductivity of 3.20 × 10-3 S cm-1 at 90 °C, which is much higher than that of conventional Nafion (~10-5 S cm-1 at 90 °C under anhydrous condition). 1H NMR DOSY spectra reveal that both the diffusion and dissociation of protons can be drastically facilitated upon nanoconfinement, demonstrating the promising efficiency of nanochannels in proton conduction. Moreover, the obtained composite membranes possess outstanding mechanical and thermal stability, which is crucial for their practical application. This study demonstrates proton conduction elevation in nanoconfined PILs and provides a promising insight into the engineering of stable COF-based proton-conducting materials.

A Protic Ionic Liquid Promoted Gel Polymer Electrolyte for Solid-State Electrochemical Energy Storage

This study presents the synthesis of a transparent, flexible gel polymer electrolyte (GPE) based on the protic ionic liquid BMImHSO4 and on polyvinyl alcohol (PVA) through solution casting and electrochemical evaluation in a 2.5 V symmetrical C/C electrical double-layer solid-state capacitor (EDLC). The freestanding GPE film exhibits high thermal stability (>300 °C), wide electrochemical windows (>2.7 V), and good ionic conductivity (2.43 × 10-2 S cm-1 at 20 °C). EDLC, using this novel GPE film, shows high specific capacitance (81 F g-1) as well as good retention above 90% of the initial capacitance after 4500 cycles. The engineered protic ionic liquid GPE is, hopefully, applicable to high-performance solid-state electrochemical energy storage.

Engineering both intrinsic characteristic and local microenvironment of platinum sites toward highly efficient oxygen reduction reaction

The optimization of the adsorption of oxygen-containing intermediates on platinum (Pt) sites of Pt-based electrocatalysts is crucial for the oxygen reduction reaction process. Currently, a large amount of researches mainly focus on modifying the bulk structure of the electrocatalysts, however, the vital role of solvent effect on the phase interfaces is often overlooked. Here, we successfully developed an electrocatalyst in which the ordered PtCo alloy anchors on the cobalt (Co) single-atoms/clusters decorated support (Co1,nNC) and its surface is further optimized using hydrophobic ionic liquid (IL). Experimental studies and theoretical calculations indicate that compressive stress on Pt lattice contributed by intrinsic structure and the local hydrophobicity caused by IL on the surface can suppress the stabilization of *OH on Pt. This synergistic effect affords outstanding catalytic performance, exhibiting a half-wave potential (E1/2) of 0.916 V vs. RHE and a mass activity (MA) of 1350.3 mA mgPt-1 in 0.1 mol/L perchloric acid (0.1 M HClO4) electrolyte, much better than the commercial Pt/C (0.849 V vs. RHE and 145.5 mA mgPt-1 for E1/2 and MA, respectively). Moreover, the E1/2 of IL-PtCo/Co1,nNC only lost 5 mV after 10,000 cyclic voltammetry (CV) cycles due to a strong and synergistic contact of the intermetallic PtCo alloy with the Co1,nNC support and IL. This research provides an effective method for designing efficient electrocatalysts by combining intrinsic structure and surface modification.

Designing a Novel C3-Fe-N Interface Local Coordination Microenvironment for Efficient Electrocatalytic Water Splitting

Developing single-atomic electrocatalysts (SACs) with high activity and stability for electrocatalytic water-splitting has been challenging. Moreover, the practical utilization of SACs is still far from meeting the the theoretical prediction. Herein a facile and easy scale-up fabrication method is proposed for designing a novel carbon-iron-nitrogen (C-Fe-N) electrocatalyst with a single atom electron bridge (C-Fe-N SAEBs), which exhibits lower overpotential and impedance than previously reported electrocatalysts. 0.8-C-Fe-N SAEBs exhibits significant activity and excellent stability in the bi-functional decomposition of water. The excellent performance of the C-Fe-N SAEBs electrocatalyst can be attributed to the strong coupling effect at the interface owing to the formation of a single atom C3-Fe-N local coordination microenvironment at the interface, which enhance the exposure of active sites and charge transfer, and reduced the adsorption energy barrier of intermediates. Theoretical calculation and synchrotron radiation analysis are performed to understand the mechanistic insights behind the experimental results. The results reveal that the active C3-Fe-N local coordination microenvironment at the interface not only improves water-splitting behavior but also provides a deeper understanding of local-interface geometry/electronic structure for improving the electrocatalytic activity. Thus, the proposed electrocatalyst, as well as the mechanistic insights into its properties, presents a significant stride toward practical application.

Fluorine-Doped Carbon Support Enables Superfast Oxygen Reduction Kinetics by Breaking the Scaling Relationship

It is well-established that Pt-based catalysts suffer from the unfavorable linear scaling relationship (LSR) between *OOH and *OH (ΔG(*OOH)=ΔG(*OH)+3.2±0.2 eV) for the oxygen reduction reaction (ORR), resulting in a great challenge to significantly reduced ORR overpotentials. Herein, we propose a universal and feasible strategy of fluorine-doped carbon supports, which optimize interfacial microenvironment of Pt-based catalysts and thus significantly enhance their reactive kinetics. The introduction of C-F bonds not only weakens the *OH binding energy, but also stabilizes the *OOH intermediate, resulting in a break of LSR. Furthermore, fluorine-doped carbon constructs a local super-hydrophobic interface that facilitates the diffusion of H2O and the mass transfer of O2. Electrochemical tests show that the F-doped carbon-supported Pt catalysts exhibit over 2-fold higher mass activities than those without F modification. More importantly, those catalysts also demonstrate excellent stability in both rotating disk electrode (RDE) and membrane electrode assembly (MEA) tests. This study not only validates the feasibility of tuning the electrocatalytic microenvironment to improve mass transport and to break the scaling relationship, but also provides a universal catalyst design paradigm for other gas-involving electrocatalytic reactions.

Unravel-engineer-design: a three-pronged approach to advance ionomer performance at interfaces in proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs), which use hydrogen as fuel, present an eco-friendly alternative to internal combustion engines (ICEs) for powering low-to-heavy-duty vehicles and various devices. Despite their promise, PEMFCs must meet strict cost, performance, and durability standards to reach their full potential. A key challenge lies in optimizing the electrode, where a thin ionomer layer is responsible for proton conduction and binding catalyst particles to the electrode. Enhancing ion transport within these sub-μm thick films is critical to improving the oxygen reduction reaction (ORR) at the cathodes of PEMFCs. For the past 15 years, our research has targeted this limitation through a comprehensive "Unravel - Engineer - Design" approach. We first unraveled the behavior of ionomers, gaining deeper insights into both the average and distributed proton conduction properties within sub-μm thick films and at interfaces that mimic catalyst binder layers. Next, we engineered ionomer-substrate interfaces to gain control over interfacial makeup and boost proton conductivity, essential for PEMFC efficiency. Finally, we designed novel nature-derived or nature-inspired, fluorine-free ionomers to tackle the ion transport limitations seen in state-of-the-art ionomers under thin-film confinement. Some of these ionomers even pave the way to address cost and sustainability challenges in PEMFC materials. This feature article highlights our contributions and their importance in advancing PEMFCs and other sustainable energy conversion and storage technologies.

Overcoming the Limitation of Ionomers on Mass Transport and Pt Activity to Achieve High-Performing Membrane Electrode Assembly

The membrane electrode assembly (MEA) is one of the critical components in proton exchange membrane fuel cells (PEMFCs). However, the conventional MEA cathode with a covered-type catalyst/ionomer interfacial structure severely limits oxygen transport efficiency and Pt activity, hardly achieving the theoretical performance upper bound of PEMFCs. Here, we design a noncovered catalyst/ionomer interfacial structure with low proton transport resistance and high oxygen transport efficiency in the cathode catalyst layer (CL). This noncovered interfacial structure employs the ionomer cross-linked carbon particles as long-range and fast proton transport channels and prevents the ionomer from directly covering the Pt/C catalyst surface in the CL, freeing the oxygen diffusion process from passing through the dense ionomer covering layer to the Pt surface. Moreover, the structure improves oxygen transport within the pores of the CL and achieves more than 20% lower pressure-independent oxygen transport resistance compared to the covered-type structure. Fuel-cell diagnostics demonstrate that the noncovered catalyst/ionomer interfacial structure provides exceptional fuel-cell performance across the kinetic and mass transport-limited regions, with 77% and 67% higher peak power density than the covered-type interfacial structure under 0 kPagauge of oxygen and air conditions, respectively. This alternative interfacial structure provides a new direction for optimizing the electrode structure and improving mass-transport paths of MEA.

Dynamics and thermochemistry of the negatively charged clusters in a 2-hydroxyethylhydrazinium nitrate ionic liquid system

The formation and fragmentation of negatively charged 2-hydroxyethylhydrazinium nitrate ([HOCH2CH2NH2NH2]+NO3-, HEHN) ionic liquid clusters were examined using a guided-ion beam tandem mass spectrometer furnished with collision-induced dissociation of selected ions with Xe atoms. Measurements included the compositions of cluster ions formed in the ionization source, and the dissociation products, cross sections, and 0 K threshold energies for individually selected cluster ions. To identify the structures of the main cluster ion series [(HEHN)n(HNO3)0-1NO3]- formed, molecular dynamics simulations were employed to create initial geometry guesses, followed by optimization at the ωB97XD/6-31+G(d,p) level of theory, from which global minimum structures were identified for reaction thermodynamics analyses. A comparison was made between the cluster formation and fragmentation in the negatively charged 2-hydroxyethylhydrazinium nitrate with those in the positive mode (reported by W. Zhou et al., Phys. Chem. Chem. Phys., 2023, 25, 17370). In both modes, the cluster ions were predominantly composed of m/z below 350; loss of a neutral 2-hydroxyethylhydrazinium nitrate ion pair represents the most important cluster fragmentation pathway, followed by intra-ion pair proton transfer-mediated 2-hydroxyethylhydrazine and HNO3 elimination; and all clusters started to dissociate at threshold energies less than 1.5 eV. The overwhelming similarities in the formation and fragmentation chemistry of positively vs. negatively charged 2-hydroxyethylhydrazinium nitrate clusters may be attributed to their inherent ionic nature and high electric conductivities.

Piperazinium Poly(Ionic Liquid)s as Solid Electrolytes for Lithium Batteries

Poly(ionic liquid)s combine the unique properties of ionic liquids (ILs) within ionic polymers holding significant promise for energy storage applications. It is reported here the synthesis and characterization of a new family of poly(ionic liquid)s synthesized from cationic piperazinium ionic liquid monomers. The cationic poly(acrylamide piperazinium) in combination with sulfonamide anions like bis(trifluoromethanesulfonyl) imide (TFSI) and bis(fluorosulfonyl) imide (FSI) are characterized as solid polymer electrolytes. The polymer electrolytes in combination with pyrrolidonium ILs and LiFSI show high ionic conductivity, 5×10-3 S cm-1 at 100 °C. Piperazinium polymer electrolytes show excellent compatibility with lithium metal reversible plating and stripping at high current density and low temperature 40 °C.

Structurally Tunable Graphitized Mesoporous Carbon for Enhancing the Accessibility and Durability of Cathode Pt-Based Catalysts for Proton Exchange Membrane Fuel Cells

Low Pt utilization and intense carbon corrosion of cathode catalysts is a crucial issue for high-efficiency proton exchange membrane fuel cells due to the highly demanded long-term durability and less acquisition/application cost. Herein, structurally tunable graphitized mesoporous carbon (GMC) is obtained by direct high-temperature pyrolysis and in situ-controlled mesopore formation; the structure-optimized GMC1300-1800 exhibits a mesopore size of 7.54 nm and enhanced corrosion resistance. Functionalized GMC1300-1800 is loaded with small-sized Pt nanoparticles (NPs) (1.5 nm) uniformly by impregnation method to obtain Pt/GMC1300-1800 and form an "internal platinum structure" to avoid sulfonic acid groups poisoning as well as ensure O2/proton accessibility. Hence, the electrochemically active surface area (ECSA) of Pt/GMC1300-1800 reaches 106.1 m2 g-1 Pt, while mass activity and specific activity at 0.9 V are 2.1 and 1.4 times those of commercial Pt/C, respectively. Notably, the ECSA decay is less than 17% for both 30 000 cycles' accelerated durability tests (ADTs) of Pt attenuation and carbon attenuation. Accordingly, the optimized mesoporous structure of GMC1300-1800 significantly decreases the coverage of sulfonic acid groups on Pt NPs, leading to the highest peak power density in the single-cell test. Density functional theory calculations demonstrate the synergistic effect between graphitization and mesoporosity on enhancing the accessibility and durability of the catalysts.

Potential of ionic liquids as emerging green solvent for the pretreatment of lignocellulosic biomass

Lignocellulosic biomass is available in abundance as a renewable resource, but the major portion of it is often discarded as waste without utilizing its immense potential as an alternative renewable energy resource. To overcome recalcitrance of lignocellulosic biomass, various pretreatment methods are applied to it, so that the complex and rigid polymeric structure can be broken down into fractions susceptible for enzymatic hydrolysis. Effective and efficient biomass processing is the goal of pretreatment methods, but none of the explored pretreatment methods are versatile enough to fulfil the requirement of biomass processing with greater flexibility in terms of operational cost and desired output efficiency. Deployment of green solvents such as ionic liquids for the pretreatment of lignocellulosic biomass has been a topic of discussion amongst the scientific community in recent times. The presented work provides a detailed overview on the deployment of ionic liquid for the pretreatment of lignocellulosic biomass coupled with a brief discussion on other pretreatments methods. The recyclability and reusability along with other unique properties makes an ionic liquid pretreatment different from the other traditional pretreatment methods. Also, this study explores diverse critical parameters that governs the dissolution process of biomass. Hazardous properties of ionic liquids have also been explored. Future perspective and recommendations have been given for an efficient, effective, and eco-friendly deployment of ionic liquid in biomass pretreatment process.

Enhancing Polymer Electrolyte Membrane Fuel Cells with Ionic Liquids: A Review

Polymer electrolyte membrane fuel cells (PEMFCs) represent a promising clean energy solution. However, their widespread adoption faces hurdles related to component optimization. This review explores the pivotal role of ionic liquids (ILs) in enhancing PEMFC performance, focusing on their role in polymer electrolyte membranes, catalyst modification, and other components. By addressing key obstacles, including proton conductivity, catalyst stability, and fuel crossover, ILs provide a pathway towards the widespread commercialization of PEMFCs. In the realm of PEMFC membranes, ILs have shown great potential in improving proton conductivity, mechanical strength, and thermal stability. Additionally, the utilization of ILs as catalyst modifiers has shown promise in enhancing the electrocatalytic activity of electrodes by serving as an effective stabilizer to promote the dispersion of metal nanoparticles, and reduce their agglomeration, thereby augmenting catalytic performance. Furthermore, ILs can be tailored to optimize the catalyst-support interaction, ultimately enhancing the overall fuel cell efficiency. Their unique properties, such as high oxygen solubility and low volatility, offer advantages in terms of reducing mass transport and water management issues. This review not only underscores the promising advancements achieved thus far but also outlines the challenges that must be addressed to unlock the full potential of ILs in PEMFC technology, offering a valuable resource for researchers and engineers working toward the realization of efficient and durable PEMFCs.

Materials Strategies Tackling Interfacial Issues in Catalyst Layers of Proton Exchange Membrane Fuel Cells

The most critical challenge for the large-scale commercialization of proton exchange membrane fuel cells (PEMFCs), one of the primary hydrogen energy technologies, is to achieve decent output performance with low usage of platinum (Pt). Currently, the performance of PEMFCs is largely limited by two issues at the catalyst/ionomer interface, specifically, the poisoning of active sites of Pt by sulfonate groups and the extremely sluggish local oxygen transport toward Pt. In the past few years, emerging strategies are derived to tackle these interface problems through materials optimization and innovation. This perspective summarizes the latest advances in this regard, and in the meantime unveils the molecule-level mechanisms behind the materials modulation of interfacial structures. This paper starts with a brief introduction of processes and structures of catalyst/ionomer interfaces, which is followed by a detailed review of progresses in key materials toward interface optimization, including catalysts, ionomers, and additives, with particular emphasis on the role of materials structure in regulating the intermolecular interactions. Finally, the challenges for the application of the established materials and research directions to broaden the material library are highlighted.

Adsorption and thermal evolution of [C1C1Im][Tf2N] on Pt(111)

In the context of ionic liquid (IL)-assisted catalysis, we have investigated the adsorption and thermal evolution of the IL 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C1C1Im][Tf2N]) on Pt(111) between 100 and 800 K by angle-resolved X-ray photoelectron spectroscopy and scanning tunneling microscopy. Defined amounts of IL in the coverage range of a complete first wetting layer were deposited at low temperature (100-200 K), and subsequently heated to 300 K, or directly at 300 K. At 100 K, the IL adsorbs as an intact disordered layer. Upon heating to 200 K, the IL stays intact, but forms an ordered and well-oriented structure. Upon heating to 250 K, the surface order increases, but at the same time STM and XPS indicate the onset of decomposition. Upon heating to 300 K, decomposition progresses, such that 50-60% of the IL is decomposed. The anion-related reaction products desorb instantaneously, and the cation-related products remain on the surface. Thereby, the surface is partly passivated, enabling the remaining IL to still be adsorbed intact at 300 K. For IL deposition directly at 300 K, a fraction of the IL instantaneously decomposes, with the anion-related products desorbing, opening free space for further deposition of IL. Hence, cation-related species accumulate at the expense of anions, until one fully closed wetting layer is formed. As a consequence, a higher dose is required to reach this coverage at 300 K, compared to 100-200 K.

Advanced Architectures of Air Electrodes in Zinc-Air Batteries and Hydrogen Fuel Cells

The air electrode is an essential component of air-demanding energy storage/conversion devices, such as zinc-air batteries (ZABs) and hydrogen fuel cells (HFCs), which determines the output power and stability of the devices. Despite atom-level modulation in catalyst design being recently achieved, the air electrodes have received much less attention, causing a stagnation in the development of air-demanding equipment. Herein, the evolution of air electrodes for ZABs and HFCs from the early stages to current requirements is reviewed. In addition, the operation mechanism and the corresponding electrocatalytic mechanisms of ZABs are summarized. In particular, by clarifying the air electrode interfaces of ZABs at different scales, several approaches to improve the air electrode in rechargeable ZABs are reviewed, including innovative electrode structures and bifunctional oxygen catalysts. Afterward, the operating mechanisms of proton-exchange-membrane fuel cells (PEMFCs) and anion-exchange-membrane fuel cells (AEMFCs) are explained. Subsequently, the strategies employed to enhance the efficiency of the membrane electrode assembly (MEA) in PEMFCs and AEMFCs, respectively, are highlighted and discussed in detail. Last, the prospects for air electrodes in ZABs and HFCs are considered by discussing the main challenges. The aim of this review is to facilitate the industrialization of ZABs and HFCs.

The cohesive properties and pyrolysis mechanism of an aprotic ionic liquid tetrabutylammonium bis(trifluoromethanesulfonyl)imide

As the cohesive properties (such as the enthalpy of sublimation) of solid organic salts (or ionic liquids, ILs) are unmeasurable, a method of their indirect determination is proposed in this paper. For this purpose, the thermogravimetric analysis (TGA) and differential scanning calorimetric analysis (DSC) were carried out over a wide range of temperatures. In this study, the mathematical relationship of the thermodynamic properties between the liquid and solid phases of ILs is established using the Born-Fajans-Haber cycle, in which the sum of the vaporization enthalpy of ILs, melting enthalpy and the enthalpy of solid-solid phase transition is regarded as the sublimation enthalpy of solid organic salts. With this method, the cohesive properties of tetrabutylammonium bis(trifluoromethanesulfonyl)imide ([N4444][NTf2]), which is an aprotic IL, were successfully obtained. Additionally, the difference between the lattice energy and the cohesive energy was employed to quantitatively calculate the charge separation distance of single ion pair (r12) in the gas phase of ionic liquids for the first time, which can serve as a standard methodology to measure the closeness in distance between the anion and the cation in a gas phase ion pair. The pyrolysis mechanism of [N4444][NTf2] was also explored.

Elucidation of the mechanism of melamine adsorption on Pt(111) surface via density functional theory calculations

The oxygen reduction reaction (ORR) activity of Pt catalysts in polymer electrolyte fuel cells (PEFCs) should be enhanced to reduce Pt usage. The adsorption of heteroaromatic ring compounds such as melamine on the Pt surface can enhance its catalytic activity. However, melamine adsorption on Pt and the consequent ORR enhancement mechanism remain unclear. In this study, we performed density functional theory calculations to determine the adsorption structures of melamine/Pt(111). Melamine was coordinated to Pt via two N lone pairs on NH2 and N- in the triazine ring, resulting in a chemisorption structure with slight electron transfer. Four types of adsorption structures were identified: three-point adsorption (two amino groups and a triazine ring: Type A), two-point adsorption (one amino group and a triazine ring: Type B), two-point adsorption (two amino groups: Type C), and one-point adsorption (one amino group: Type D). The most stable structure was Type B. However, multiple intermediate structures were formed owing to the conformational changes from the most stable to other stable adsorption structures. The resonance structures of the adsorbed melamine stabilise the adsorption, as increased resonance allows for more electron delocalisation. In addition, the lone-pair orbital of the amino group in the adsorbed melamine acquires the characteristics of an sp3 hybrid orbital, which prevents horizontal adsorption on the Pt surface. We believe that understanding these adsorption mechanisms will help in the molecular design of organic molecule-decorated Pt catalysts and will lead to the reduction of Pt usage in PEFCs.

Electrocatalyst Fabrication Using Metal Nanoparticles Prepared in Ionic Liquids

Metal nanoparticle-based electrocatalysts are widely used in electronic devices, which serve for electrochemical reactions like oxygen reduction reaction, alcohol oxidation and CO2 reduction reaction. These catalyst-dependent reactions are the key of the emerging clean energy systems. Catalyst design and synthesis therefore have received keen attention in past decades. We are motivated to study synthesis approaches of metal nanoparticle-based electrocatalysts using ionic liquids (ILs), which are promising solvents for the nanoparticle preparation because of their unique physicochemical properties. In this personal account, we review our previous and present works on nanoparticle preparation in IL and utilization of the obtained nanoparticles as electrocatalysts.

Introduction of Surface Modifiers on the Pt-Based Electrocatalysts to Promote the Oxygen Reduction Reaction Process

The design of Pt-based electrocatalysts with high efficiency towards acid oxygen reduction reactions is the priority to promote the development and application of proton exchange membrane fuel cells. Considering that the Pt atoms on the surfaces of the electrocatalysts face the problems of interference of non-active species (such as OH_ad, OOH_ad, CO, etc.), high resistance of mass transfer at the liquid-solid interfaces, and easy corrosion when working in harsh acid. Researchers have modified the surfaces' local environment of the electrocatalysts by introducing surface modifiers such as silicon or carbon layers, amine molecules, and ionic liquids on the surfaces of electrocatalysts, which show significant performance improvement. In this review, we summarized the research progress of surface modified Pt-based electrocatalysts, focusing on the surface modification strategies and their mechanisms. In addition, the development prospects of surface modification strategies of Pt-based electrocatalysts and the limitations of current research are pointed out.

Surface Hydrophobicity Engineering of Pt-Based Noble Metal Aerogels by Ionic Liquids toward Enhanced Electrocatalytic Oxygen Reduction

Modulating the surface properties of electrocatalysts with ligands could effectively regulate their catalytic properties, while limited in-depth understanding of the surface ligands restricted their rational combination. Herein, ionic liquids (ILs) with different lengths of hydrophobic side chains were employed to regulate the surface hydrophobicity of noble metal aerogels, for comprehending the relationship between surface hydrophobicity and oxygen reduction reaction (ORR) activity and enhancing electrocatalytic ORR. The volcano-like trends between the hydrophobicity and the ORR activity for various Pt-based aerogels indicated that a suitable hydrophobic surface constructed by ILs was most favorable for contacting with oxygen molecules and the desorption of oxygen intermediates. Typically, the PtPd aerogel modified by ILs (PtPd aer-[MTBD][PFSI]) exhibited an inspiring ORR activity, with a 70 mV increase in half-wave potential and a 7.1-fold mass activity compared to the commercial Pt/C. Therefore, the regularity between the surface hydrophobicity and ORR activity of noble metal aerogels was uncovered and will facilitate the modulation of electrocatalysts for practical applications.

Increasing Accessible Active Site Density of Non-Precious Metal Oxygen Reduction Reaction Catalysts through Ionic Liquid Modification

Non-precious metal catalysts show great promise to replace the state-of-the-art Pt-based catalysts for catalyzing the oxygen reduction reaction (ORR), while their catalytic activity still needs to be greatly improved before their broad-based application. Here, we report a facile approach to improving the performance of zeolitic imidazolate framework-derived carbon (ZDC) toward the ORR by incorporating a small amount of ionic liquid (IL). The IL would preferentially fill the micropores of ZDC and greatly enhance the utilization of the active sites within the micropores, which are initially not accessible due to insufficient surface wetting. It is also disclosed that the ORR activity in terms of kinetic current at 0.85 V depends on the loading amount of the IL, and the maximum activity is obtained at a mass ratio of IL to ZDC at 1.2. The optimum stems from the counterbalance between the enhanced utilization of the active sites within the micropores and the retarded diffusion of the reactants within the IL phase due to its high viscosity.