Recent advances in metal pyrophosphates for electrochemical supercapacitors: A review

Over the last few decades, a wide upraise in clean and sustainable energy sources is witnessed to meet the surging energy demand. The electrochemical energy storage (EES) devices have momentously enhanced, as an exceeding variety of advanced electrode active materials and new device, architectures have been developed. Supercapacitors (SCs) have engrossed worldwide attention among all the energy storage devices for their utilization in electric power transport, portable electronics and biomedical devices. The SCs show great potential in reducing the size and volume of devices, amending the charge storage capacity and minimizing the material and fabrication cost while bringing the advantage of additional functions to the system. The recent electrode materials including carbon-based materials, metal oxide/hydroxide-based materials, metal pyrophosphates, metal chalcogenides, conducting polymer-based materials, and their composites have been discovered to obtain the significant capacitance value and hence, energy density. Herein, the latest research progress on several metal pyrophosphates (MPPs) as electrode materials for SCs are reviewed with an emphasis on the material aspect. MPPs have shown great potential in the field of energy storage devices due to their abundant crystal chemistry, redox-active nature, good reversibility, inexpensive, naturally abundant, extensive working potential easy to synthesize and stable layered structure. This review consists of an understanding of the pyrophosphates family in terms of their versatile applications in SCs. The article concentrates on modern research findings in the synthetic routes of nanostructured MPPs in connection with the structures, morphologies and their electrochemical energy storage performances. In summary, the challenges and future perspectives for the MPPs are discussed.