Renewable energy is now facing a challenge that the demand and supply of energy is not stable, which should be balanced by energy storage. The energy can be stored chemically, physically, or in a hybrid. However, most energy storage suffers from one or more of the following: pollution caused by the leakage of the electrolyte in batteries and supercapacitors; limitation to the location of pumped storage; high cost of flywheel battery, etc. Energy storage by a cement-based structure is attractive and promising due to advantages such as low cost, no pollution to the environment, high durability, large energy storage volume, and multi-functional properties.The energy can be stored in a cement-based battery or a cement-based supercapacitor. The mechanism behind them is different. The cement-based battery pertains to the traveling of charges (produced by redox reaction) among the anode, electrolyte, and cathode. The cement-based supercapacitor relies on the interaction of charges and electrodes, providing double-layer capacitance and/or pseudo capacitance. The design of both cement-based energy storage devices involves the design of the structure and composition of electrodes, particularly the active components, the type and concentration of ions in electrolytes, separators, and current collectors. To evaluate the performance of cement-based batteries and supercapacitors, energy-related parameters, such as electrical resistivity, relative permittivity, specific capacitance, power density, energy density, and capacitance retention, are used. In the design of a cement-based battery or supercapacitor, the electronic resistivity of electrodes should be low enough to increase the current density and energy density; the ionic conductivity of electrolytes should be high to facilitate ion movement between two electrodes; the separator should be porous and insulating to allow ions to go through and hinder electrons; the electrical resistivity of the contact materials between electrodes and current collectors should be low to decrease the contact resistance; the porosity of conductive components in cement-based electrodes should be high to increase the specific capacitance, etc.Factors affecting the energy storage performance of cement-based batteries and supercapacitors involve the type of cement-based materials (paste, mortar, and concrete), water-to-cement ratio, electrode materials, and composition of cement-based electrolytes. The performance decays in this order: Cement paste, mortar, and concrete because the electrical resistivity increases in this order. The presence of aggregates weakens the conductive network in cement-based electrodes. The water-to-cement ratio does not affect the open-circuit voltage much, but it affects the current density. Carbon materials (graphene, biochar, etc.) are widely used in the preparation of electrode materials due to their low density, low electrical resistivity, and relatively high porosity. However, the dispersion of conductive materials in cement-based electrodes should be well-designed because poor dispersion weakens both mechanical and conduction behavior. Conductive polymers and ionic liquids are used to increase the ionic conductivity of pore solution in cement-based electrolytes.The cement-based battery and supercapacitor can be in the form of part of structures, such as beams, columns, walls, railings, and bricks. The overall principle is to improve the energy performance of these structures without sacrificing their structural properties. To satisfy this requirement, the structural batteries and supercapacitors should entirely be made of cement-based materials, i.e., cement-based electrodes, cement-based electrolytes, and cement-based separators.
Summary and prospects
Though the cement-based batteries and supercapacitors provide a new way to store the electricity, the related research and techniques are still at the infancy, and more issues need to be addressed in the future: 1) Effects of environment factors on the energy storage performance; 2) Balance between the cost and performance in terms of active components in cement-based electrodes and electrolytes; 3) Prediction of the lifespan of cement-based batteries and supercapacitors; 4) Recycle of some active components in cement-based electrodes, etc. The enhancements observed in the energy storage performance through the utilization of cement-based materials underscore the significance of incorporating conductive additives as a pivotal strategy for increasing energy density, power density, and current density. Nevertheless, several policy implications must be addressed to ensure the successful integration of active components in the development of intelligent, dependable, and sustainable cement-based batteries and supercapacitors. These include fostering effective collaborative partnerships among universities, research institutes, private enterprises, and state-owned entities; implementing monetary incentives such as government subsidies, tax exemptions, and specialized loans to incentivize the recycling of conductive waste in cement-based batteries and supercapacitors; facilitating the exchange of knowledge between scientists and industry stakeholders; integrating sustainable electro-tribological technologies into eco-labeling initiatives and the development of green conductive additives; and providing essential financial backing, technical support, and consultancy services to promote the practical application of cement-based batteries and supercapacitors across the energy sector.