Novel Materials and Advanced Characterization for Energy Storage and Conversion

Global climate changes, such as frequent extreme weather, continuous temperature increase, and melting glaciers, constantly press us to reduce our dependence on the traditional carbon-based energy resources. Currently, renewables, including solar, wind, and tidal energy, are widely used for power generation. Their costs are falling rapidly. However, renewables are undermined by their intermittency, which makes coupling them with a dispatchable storage capacity necessary to provide an uninterrupted electricity supply. Therefore, stable energy storage technologies are crucial to increase the market penetration of renewables. Given electricity plays a pivotal role in our life; it only accounts for 20% of the total energy consumption. The remaining 80% is from fuel [1]. Hydrogen is a clean fuel with a high calorific value and a high energy density up to 33.3 kW/kg, and it is generated mainly from fossil fuel. Less than 4% of the global hydrogen production comes from electrolysis; most of it comes from chlor-alkali electrolysis, rather than water [2]. So, the catalytic hydrogen production by water electrolysis will be the later focus of green fuel research in years to come. On the other side of the same coin, research of CO₂ capture and its conversion into high value-added chemicals and fuels is gaining strong momentum, as we are aiming for the grand mission of carbon neutrality by 2050.

As they are a star representative of energy storage, lithium-ion batteries are ubiquitous these days. In recognition of their contribution to the development of lithium-ion batteries, the Nobel Prize in chemistry for 2019 was awarded to John B. Goodenough, M. Stanley Whittingham and Akira Yoshino. Currently, almost half of the lithium-ion battery demand is for electric vehicles. It is critical to develop lithium-ion batteries with high specific energy and safety [3]. Commercial lithium-ion batteries, nowadays, consist of a graphite anode, polypropylene fiber membrane, lithium-containing organic electrolyte, and LiFePO₄, LiCoO₂ or ternary oxide cathodes, etc. Although silicon-containing materials are also used as negative electrodes, they are not yet ready for large scale production due to the volume expansion and pulverization of silicon electrodes during charging and discharging. To meet the need of the largest application market of lithium-ion batteries, that is, electric vehicles with a cruising range > 500 KM at a market-vital cost [4], researchers have replaced the negative electrode with metal lithium to form a lithium metal battery and changed the positive electrode materials from traditional ternary oxides to high-nickel ternary oxides and lithium-rich manganese-based oxides [5,6]. However, these high-capacity materials all face a series of safety issues. In this regard, all-solid-state lithium-ion batteries have attracted widespread attention due to their high degree of safety. When they are compared to the current commercial lithium-ion batteries which have only a 250–300 Wh/Kg energy density, all-solid-state lithium-ion batteries have an almost two times higher energy density in current technologies. Moreover, other Li-based batteries such as Li-S or Li-O₂ batteries have a theoretical energy density of 2600 and 3500 Wh/Kg, respectively [7,8,9], which are closer to that of gasoline (~12 kWh/kg) [10]. In addition, to overcome the shortage of lithium resources and their uneven geographical distribution, sodium/potassium-ion batteries are considered to be a good complement to lithium-ion batteries. Because they have similar structures to those of lithium atoms, research models and methods of lithium-ion batteries can be transferred to the study of sodium/potassium-ion batteries. Although both of the batteries are not suitable for the electrical vehicles due to their poor rate performance, they can reduce the waste that is caused by abandoned light and wind and play an important role in the power grid. In addition to these monovalent alkali metal ion batteries, multivalent metal-ion batteries with a higher capacity, such as Zn²⁺, Mg²⁺, Ca²⁺ and Al³⁺, are receiving more and more attention [11,12]. It is worth noting that, due to the complexity of the electrochemical processes that are associated to these ions, most of these batteries are still in the early stage of development. There is a long way to go before their practical application.

As it is an energy carrier, hydrogen can transform chemical energy into electrical energy without causing pollution or carbon emissions. To reduce the carbon footprint during the hydrogen production, water splitting is a promising technique to convert the electrical and/or thermal energy from renewables to pure hydrogen fuel [13]. Currently, electrochemical water splitting is carried out via two routes, i.e., low-temperature and high-temperature water electrolysis. The first route includes alkaline water electrolysis, proton exchange membrane water electrolysis and anion exchange membrane water electrolysis. The second route can be divided into the proton-conducting solid oxide electrolysis cell and the oxygen-conducting solid oxide electrolysis cell. However, both routes have their own drawbacks. The low-temperature water electrolysis has a large overpotential due to the sluggish kinetics and large energy barrier on the electrodes. Moreover, the electrocatalysts are made of a precious metal and/or noble metal oxides. Large-scale applications of these devices are still cost-prohibitive. The high-temperature water electrolysis runs at around 800 °C. Although this technology can avoid the cost of electricity, its harsh operating conditions reduce the durability and stability of its operation [14,15,16]. The development of low-cost, high-efficiency and harshness-resistant materials is of great importance for electrolytic hydrogen.

In order to study the electrochemical workings and the materials evolution mechanisms of both batteries and electrolysis, advanced in situ as well ex situ characterization techniques are essential. It is becoming more and more important to characterize materials by large science facilities, such as X-ray absorption spectroscopy and neutron diffraction, etc. Their stronger X-ray beam and other unique features can capture the minor changes of the crystal and electronic structure which are beyond the detection limit of the conventional lab-level facilities. The genuine structure change can also be monitored in real time by these techniques during their electrochemical operation. Multi-scale in situ/operando characterization technologies are being constructed to provide in-depth understanding of the materials. Nevertheless, there are still many challenges to overcome before the advanced multi-scale characterizations can reach their full potential.

This special issue, Novel Materials and Advanced Characterization for Energy Storage and Conversion, provides a platform to promote problems-solving and cooperation between different disciplines such as materials studies, chemistry, physics and engineering, etc. The topics include but are not limited to energy storage materials and devices; the development and utilization of materials in the fields of batteries, fuel cell, electrocatalysis, photocatalysis, thermocatalysis, etc.; advanced mechanisms studies; the application of advanced characterization techniques. There are still many challenges that should be addressed. However, we will definitely overcome the bottleneck of communication on this issue, thus achieving the next-generation energy storage system.