When you look across the urban landscape you realize that one material stands out above all others, be it commercial and residential facilities, bridges, tunnels, or roads, our modern cities are built with cement. The total volume of cement production worldwide amounted to an estimated 4.1 billion tons in 2020, almost three times more than the 1.39 billion tons produced 25 years before. The constant demand for this essential building material is driven by continued population growth and urbanization trends that have made cement the second most consumed commodity in the world after water.
So, what if we could power our buildings and cities with this abundant material? That is the goal of researchers at the Division of Building Technology, within the Department of Architecture and Civil Engineering, at Chalmers University of Technology, in Gothenburg, Sweden. In a 2021 study led by Emma Qingnan Zhang and Luping Tang, a rechargeable cement-based battery was developed, with an average energy density of 7 Wh/m2 (or 0.8 Wh/L) during six charge/discharge cycles. While not anywhere near the energy capacity of lithium-ion and other leading forms of energy storage, the ubiquity and volume of cement in the built environment means that it could support distributed energy provision across our urban landscapes.
“Advanced building materials of the future are being envisioned to provide multifunctional smart features such as self-powering and self-sensing for structural health monitoring applications. Moreover, future building materials could also take on additional functions such as renewable energy sources, which will collect and store renewable energy such as solar and wind energy,” reads the paper. “The concept of using structures and buildings as energy source and storage could be revolutionary because it offers an alternative solution to solve the energy crisis by providing a large amount of energy storage. Due to the large volumes of structures, the capacity of energy storage can be high, even if the energy per unit volume is not high.”
The proposed cement battery would be built from a traditional mixture with the addition of carbon fibers to increase conductivity and flexibility. Also embedded within the mixture would be a metal-coated carbon-fiber mesh, made from iron for the anode and nickel for the cathode. Coupled with renewable energy generation, such as solar panels, energy-storing cement could also become the main energy source for monitoring systems in highways, bridges, and tunnels, for detecting cracks and other critical information. These cement batteries could also be used to power streetlights, traffic sensors, and even entire buildings.
"We have a vision that in the future this technology could allow for whole sections of multi-story buildings made of functional concrete. Considering that any concrete surface could have a layer of this electrode embedded, we are talking about enormous volumes of functional concrete", says Dr. Zhang. "Since concrete infrastructure is usually built to last fifty or even a hundred years, the batteries would need to be refined to match this or to be easier to exchange and recycle when their service life is over. For now, this offers a major challenge from a technical point of view."
The research by Dr. Zhang and Dr. Tang in Gothenburg is by no means the first to explore the energy storage potential of cement. In 2018, a team at the University of Lancaster, UK, proposed a “smart cement” made from fly-ash and chemical solutions. The novel potassium-geopolymetric (KGP) composite achieves conductivity with potassium ions hopping through the crystalline structure without any need for complex or expensive additives. In fact, KGP is cheaper to produce than Ordinary Portland Cement, which is currently the most widely used construction material in the world and has the potential to store and discharge as much as 200 and 500 watts per square meter.
“We have shown for the first time that KGP cement mixtures can be used to store and deliver electrical energy without the need for expensive or hazardous additives,” said the lead researcher Professor Mohamed Saafi. “These cost-effective mixtures could be used as integral parts of buildings and other infrastructure as a cheap way to store and deliver renewable energy, powering street lighting, traffic lights, and electric vehicle charging points.”
KGP is not just cement with energy storage abilities, however. Another key benefit is that the mixture is structurally “self-sensing”. Changes in mechanical stress, caused by things such as cracks, alters the mechanism of ion hopping through the structure and therefore the material’s conductivity. These changes mean the structural health of buildings can be monitored automatically, by measuring conductivity, without the need for the current and inferior manual inspections and external sensors. KPG panels could even be retrofitted to existing buildings for structural, cosmetic, energy storage, and fault monitoring purposes.
Buildings demand both huge amounts of cement and huge amounts of energy storage. Currently, stationary battery systems installed in or around buildings are allowing those facilities to accumulate rooftop solar power to release it during the night. Buildings built with large amounts of either of these conductive cement mixtures could theoretically accumulate excess stored energy to be sold to neighbors or back to the grid, essentially transforming buildings into profitable virtual power plants but without the space or financial investment usually associated with energy storage systems. In the future, we may only consider a building to be truly smart if it is made with smart cement.