Gel Electrocatalysts: An Emerging Material Platform for Electrochemical Energy Conversion

Abstract

molecules, and thus converting electrical energy into chemical energy. [5] Rechargeable metal-air batteries, operating based on oxygen reduction reaction (ORR) and OER, corresponding to the discharging and charging processes, have also stimulated great research attention for their high theoretical energy density and renewable fuel resources as they are empowered by oxygen from the air. [6] However, the kinetics of those half-cell processes is relatively slow, severely limiting the performance of corresponding energy devices. [1] The sluggish kinetics mainly originates from the proton-coupled multi-electron transfer during those processes, which needs high activation energy to overcome the reaction barrier. Generally, an optimized electrocatalyst should display high activity, high surface area, good electrical conductivity, and long-term durability. [7] For electrocatalysis, η 10 is a common index of catalytic performance and is defined as the overpotential required to produce a catalytic current density of 10 mA cm −2 (j 10), and Tafel slope is used to evaluate the relationship between j and η. In general, overpotential η can be logarithmically correlated with j and is fit to the Tafel equation: η = a + b log j; where a is the intercept of the Tafel plot and b is the Tafel slope. From the Tafel equation, two important parameters can be obtained, Tafel slope (b) and exchange current density (j 0). A Tafel slope demonstrates how fast the j can increase with a smaller voltage change, and a desirable electrocatalyst usually features lower overpotential and smaller slope. In addition, the value of the Tafel slope b provides insightful information toward the mechanism of catalysis, especially for elucidating the rate-determining step (RDS). [7] The activity of electrodes strongly correlates with the physicochemical properties of the surface of electrocatalysts and the interaction between (catalyst-coated) electrodes and electrolytes. Currently, the bestknown catalysts for the above electrochemical reactions are precious-metal-based materials, such as Pt and Ir, which can deliver satisfactory reaction rates as they require relatively low overpotential. However, the scarcity and relatively low stability greatly hinder their large-scale commercialization. Thus, the development of equally efficient and economical alternative electrocatalysts has become the focus and frontier of clean energy research. To potentially replace precious-metal-based catalysts, considerable research progress has been achieved on non-metal Seeking sustainable and cost-effective energy sources is one of the significant challenges for the sustainable development of modern society. To date, considerable expectations have been held for technologies, such as fuel cells and electrolyzers, where the performance strongly depends on electrochemical conversion processes that can generate and store chemical energy through the breaking or formation of chemical bonds. However, those advanced technologies are severely limited by the efficiency, selectivity, and...

Multifunctional hydrogels for sustainable energy and environment

“…Similarly, via in situ polymerization of crosslinked hydrogels, the contacts between LiFePO 4 cathode materials and PPy-based conductive hydrogels are enhanced, leading to significantly improved rate and cyclic performance of the composite electrode. 38 Hydrogels can be converted into various functional materials, such as carbons, 18,39 metals, 40 alloys 41 and metal oxides, 19 with the inherited hierarchical structures, large surface areas and bicontinuous mass/charge transport channels. Through the hydrogel-template approach, 3D interconnected nanoporous oxide frameworks can be constructed serving as an efficient and stable Li-ion electrolyte.…”

“…45 With various physicochemical properties and structural advantages, hydrogel materials have attracted intensive research interests in electrocatalysis applications, including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction, nitrogen reduction reaction (NRR), and so on. 39 Owing to the high compositional tunability and uniform distribution of metal species, hydrogels and their derivatives can be used as active materials for electrocatalysis. Recently, noble-metal-free electrocatalysts have received much research attention as alternatives to the state-of-the-art noble metal electrocatalysts.…”

Ni(OH)2/NiSe Nanoparticles Supported on Carbon Microspheres for Long‐Life and High‐Performance Asymmetric Supercapacitors

“…Driven by energy shortages and environmental issues, exploring supercapacitors (SCs) as alternative energy storage and conversion systems has increased significantly. [1,2] Investigation on SCs has been sustained owing to their high power density, relatively low cost, environment-friendliness and long cycling stability, when compared with other energy storage devices. But the low energy density of SCs restricts their development in emerging applications, such as electric vehicles and new electronic systems.…”

Nanoarchitectonics of Lotus Seed Derived Nanoporous Carbon Materials for Supercapacitor Applications

“…Most lignocellulose-based materials can be used as precursors for the production of nanoporous activated carbon materials [1]. Activated carbon materials are some of the most versatile and commonly used adsorbents due to their exceptionally high surface areas and micropore volumes [2][3][4][5][6][7], extensive adsorption capabilities, fast adsorption kinetics, and relative ease of regeneration [8]. Currently, chemically activated carbon materials are widely used as high-performance electrochemical double-layer capacitors (EDLCs) or supercapacitor electrodes [9].…”