Heterogeneous molecular catalysts based on transition metal complexes supported on conductive substrate have received increasing attention for their potential application in electrocatalysis and electrosynthesis. In this talk I will report some exemplary work on tuning the electrocatalytic activity of such heterogeneous molecular catalysts. The heterogenization of molecular catalyst was first demonstrated by anchoring molecular Co2+ ions onto N/S/O doped graphene. The S dopant of graphene is found to be most effective in improving the intrinsic activity of the Co sites.  In another demonstration based on Ni molecular catalyst, we found for the first time that the presence of Fe³⁺ ions in the solution could bond at the vicinity of the Ni sites, generating heterogeneous Ni-Fe dual sites anchored on doped graphene. These Ni-Fe sites exhibited drastically improved oxygen evolution activity. Using CO₂ reduction on well-defined molecular catalysts as a probe, we also demonstrated the importance of axial ligand, relay molecule and microenvironment in improving interfacial electron transfer and subsequent electrocatalytic activity.  

Electrode interface is the important place of electrochemical reactions inside batteries, properties of which has an influence on performance and states of batteries. To obtain and precisely evaluate state of charge (SoC) and state of health (SoH) of the energy storage batteries, exploring reliable In-situ characterization techniques to monitor the interfacial electrochemical dynamics and properties variation should have significant importance. At present, applied interface characterization techniques are used to analysis the components and morphology, Scanning Electrochemical Microscope (SECM) is the only detection technique that can provide spatially resolved information of the kinetics of interfacial reactions [1,2], and ultrasonics is a nondestructive technique to locate interface based on differentiate acoustic impedance of polyphase system, which has been widely used in medical examination and diagnose. Some attempts of ultrasonics detection for batteries have also made recently [3,4].

    Here in this work, SECM and an ultrasonic longitudinal wave reflection nondestructive testing technology was used for different mainstreamed secondary batteries like lithium-ion batteries, sodium-ion batteries, liquid metal batteries and aqueous zinc metal batteries.

In the current growth of the electric vehicle (EV) market, the roll-to-roll dry coating process, enabling mass production of high-thick electrode (≥10mAh cm^-2), stands out as an innovative and practical approach to fabricate the low-cost and high-energy-density Li-ion batteries (LIBs). However, as the thickness of the electrode fabricated by dry coating process becomes greater (≥10 mAh cm^-2), Li-ion migration resistance (R_ion) and charge-transfer resistance (R_ct) in the electrode dramatically increase due to long diffusion lengths for Li-ion and electron. This is primarily associated with the non-uniform distribution of electrode components and severe fractures of cathode active material during the dry coating process. Therefore, it is crucial to reduce diffusion lengths in the thick electrode to achieve high energy density LIBs. In this study, we identified the factors associated with the increase in ionic resistance through electrochemical analysis. Then, manipulation of material and process parameters was conducted to optimize the microstructure of the dry electrode. Finally, we successfully developed a 10mAh cm^-2 dry electrode with low ionic resistances, exhibiting the enhanced of Li⁺/e^- kinetics and improved high C-rate capability.

Wearable energy storage devices are indispensable corner stones for future wearable electronics. Current energy storage technologies are based on materials and devices that are rigid, bulky, and heavy, making them difficult to wear. On the other hand, fibers are flexible and lightweight materials that can be assembled into different textiles and have been worn by human beings thousands of years. Different from conventional two-dimensional thin films and foils, the three-dimensional fibre and textile structures not only provide superior wearing ability, but also much larger surface areas. This talk will introduce how our research group makes use of the attributes of fibres for high-performance wearable energy storage devices. We develop textile composite electrodes that not only address the flexibility challenge, but also provide additonal benefit to the electrochemical properties of batteries. We will demonstrate the strategies and discuss the perspectives to modify fibers and textiles for making wearable capacitators and batteries with excellent mechanical durability, electrochemical stability, and high energy/power density.

 All-solid-state batteries (ASSBs) with sulfide-based solid electrolytes with high ionic conductivity are regarded as the ultimate next-generation energy storage systems due to their enhanced safety and energy density by enabling the use of metallic anodes. Li metal is considered a promising anode material for ASSBs because of its high theoretical specific capacity (3860 mAh/g) and the lowest electrochemical potential (-3.04 V versus standard hydrogen electrode). However, its practical use has been hindered by several issues related to the interface, such as contact loss during cycling, which accelerates Li dendrite growth, and chemical instability between Li and sulfide-based solid electrolytes. In this talk, the fundamental degradation mechanisms of the ASSBs underlying electrochemical and mechanical aspects are introduced first. Subsequently, we introduce our strategies to stabilize the Li metal and sulfide-based solid electrolytes interface. The designed ASSBs could effectively retard the Li dendrite growth and unwanted side reaction and shows much enhanced electrochemical performance.

Sustainable energy storage is the bottleneck for the integration of high-ratio renewable energy to the grid. The all-liquid-structure and membrane-free liquid metal batteries (LMBs), with the merits of low-cost, long-lifespan and easy-scale-up, are promising for large-scale energy storage applications. Previously reported lithium LMBs exhibit excellent electrochemical performance, however, the practical application of lithium LMBs was hindered by the sealing issue of highly corrosive lithium vapor, along with the very limited reserve of lithium.

Instead of lithium, due to the weaker corrosion and much higher abundance, sodium LMBs show great potential for long-term and large-capacity energy storage applications. However, sodium has a high dissolution in single-cationic molten sodium halide mixtures salt electrolytes, which causes severe self-discharge and low coulombic efficiency of batteries. Here, for the first time, high-performance molten salt electrolyte and alloy electrodes are designed for the realization of practical sodium LMBs. A multi-cationic ternary chloride (LiCl-NaCl-KCl) with a low melting point (~390 ℃) and low sodium halide content (5 mol% NaCl) is rationally designed as the electrolyte, which significantly inhibits the dissolution of sodium in the molten salt electrolyte. Based on the designed cationic electrolyte, cooperating with a dual-active Bi₉Sb alloy positive electrode, a sodium LMB is constructed and the battery stably cycles over 700 cycles with a coulombic efficiency of 97% at 100 mA cm^-2 and 450 ℃, indicating the design of coupled cation electrolyte and alloy electrode is feasible to inhibit the self-discharge of batteries. Meanwhile, the adoption of electrolytes with low active substance content in this work also provides new directions for the design of electrolytes for high-performance electrochemical cells based on molten salts for high-efficiency energy conversion and storage. The calculation based on MW energy storage system indicates that the estimated Levelized Cost of Storage (LCOS) of the sodium LMB is lower than 0.029 $/kWh. To further reduce the cost, LiCl-NaCl-CaCl₂ (28:32:40 mol%) and NaCl-CaCl₂ (50:50 mol%) have been investigated for their potential as electrolytes for sodium LMBs. The significant reduction of Li⁺ content resulted in batteries with lower cost (reduced by 32%), but the interaction of the anode (Na) with the electrolyte led to difficulties in releasing the capacity of the battery and poor stability. To address this issue, the structure and composition of the anode were optimized, which achieved the thermodynamic compatibility between the anode and the electrolyte and the stable operation of the battery. The results demonstrate that sodium LMB is a promising technology for grid-scale energy storage applications.

Electrochemical energy storage technologies have been brought into the spotlight as they provide elegant and efficient approaches to storing, transporting, and delivering energy harvested from sustainable energy resources.^[1-2] The demand for power and energy supply is equally imperative in actual use and is keen to expand in the future. Thus, it is highly desirable to design new electrode materials or rationally re-construct the recognized electrode materials for energy storage devices to mitigate the power-energy tradeoff. In this talk, I will present our recent efforts in exploring 2D layered materials for sustainable energy storage applications.^[3] Specifically, I will present several interlayer engineering strategies for inorganic 2D layered materials to regulate the ion transport behaviors and boost the power-energy performance of the assembled energy storage devices.^[4-7]

References:

[1] Yu et al., Chem. Soc. Rev. 2021, 50, 2388-2443.

[2] Yu et al., Joule 2019, 3, 338-360.

[3] Yu et al., J. Am. Chem. Soc. 2020, 142, 12903-12915.

[4] Yu et al., Angew. Chem. Int. Ed. 2021, 60, 896-903.

[5] Yu et al., Nat. Commun. 2020, 11, 1348.

[6] Yu et al., Adv. Mater. 2022, 34, e2108682.

[7] Yu et al., Adv. Energy Mater. 2023, accepted.

Supercapacitors are considered optimal storage devices for electric vehicles and wireless technology due to their superior power density, cycle stability, and charge-discharge efficiency in comparison to traditional batteries [1]. Their inability to act as primary energy sources, however, is hindered by their low energy density. Research efforts are thus concentrated on nanostructured materials that exhibit enhanced specific capacitance, with a focus on carbon-based materials such as activated carbon, carbon nanotubes (CNTs),  carbon aerogels and graphene [2]. Supercapacitors operate via electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs store energy at the electrode/electrolyte interface through ion adsorption, relying on carbon-based electrodes [3]. Although graphene has superior electrical and thermal conductivity, its theoretical capacitance leads to limitations, despite showing promise. Adjusting the alignment of graphene from horizontal to vertical, as discovered in previous studies, leads to a 38% increase in capacitance, thereby overcoming some of the limitations [4]. Vertical graphene nanowalls (GNWs), manufactured via plasma-enhanced chemical vapor deposition (PECVD), provide superior electrical conductivity and a three-dimensional structure that makes them ideal scaffolds for supercapacitor electrodes [5], [6]. Pseudocapacitors, comprising of metal oxides and polymers, demonstrate a higher specific capacitance in comparison to EDLCs but necessitate a carbon substrate for better charge transfer [7]. The amalgamation of carbon materials and metal oxides manifests a synergistic effect, fulfilling vital requisites for novel energy storage devices. Nevertheless, the elevated cost and toxicity linked with most metal oxides present obstacles. Zinc oxide (ZnO) is a promising electrode material for supercapacitors due to its natural abundance, eco-friendliness, cost-effectiveness, and long cycling life [8]. The specific capacitance of ZnO can be improved by preserving oxygen vacancies, which can be attained by annealing in inert atmospheres [9]. Although integrating ZnO with graphene-based structures has exhibited notable advancements, it remains a complicated and expensive process.

In this study, GNWs represent a promising porous structure, offering a substantial surface area for active sites and facilitating rapid ion diffusion. To enhance their specific capacitance, we present a supercapacitive improvement through a hierarchical arrangement of defect-engineered ZnO nanorods (ZNRs) anchored onto the GNWs. This hierarchical structure is synthesized through a multi-step process involving inductively coupled plasma-chemical vapor deposition, magnetron sputtering, and hydrothermal methods. The presence of oxygen vacancy (OV) defects within the ZNRs, induced by argon annealing, has been characterized utilizing X-ray photoelectron spectroscopy. The emergence of OV defects below the conduction band of the ZNRs results in a narrowing of the bandgap within the hybrid structure, thereby enhancing its conductivity and increasing the reaction sites. In the capacity of supercapacitor electrodes, the ZNRs/GNWs hybrids were evaluated in an aqueous KOH electrolyte solution, operating within a voltage range of 0.5 V and at a current density of 0.1 mA cm^-2. This assessment yielded an area capacitance of 21.45 mF cm^-2, signifying a 1.5-fold increase in capacitance compared to GNWs grown on graphite sheets. The ZNRs/GNWs hybrid demonstrates remarkable electrochemical performance and exhibits substantial potential for energy storage applications. Our work is expected to offer valuable insights for the enhancement of electrochemical properties in various composite and hybrid materials.

Low temperature fuel cells are electrochemical devices that convert chemical energy directly to electricity. They have great potential for both stationary and transportation applications and are expected to help address the energy and environmental problems that have become prevalent in our society. Despite their great promise, commercialization has been hindered by lower than predicted efficiencies and high loading of Pt-based electrocatalysts in the electrodes. For more than five decades, extensive work has been focused on the development of novel electrocatalysts for fuel cell reactions. In this talk, I will present recent progress in developing advanced electrocatalysts and their fuel cell performance in my group, with an emphasis on core-shell and non-precious metal materials. The results demonstrate that core-shell structure not only enhances the mass activity of Pt, but also its stabilit in a real fuel cell environment. Our hydrbid catalysts consiting of Pt-Fe nanoparticles and Fe-N-C support don't decacy even after 100,000 potential cycles in a fuel cell testing.