With the surge in demand for energy storage devices, better and safer alternatives are required. Zinc ion hybrid supercapacitor (ZHSC) has a great potential as an alternative to lithium-ion batteries as it combines the high energy capacity of zinc-ion batteries and longevity and high power density of supercapacitors to produce a device that can potentially outperform and outlast conventional batteries. ZHSCs are currently unable to achieve their th. With the surge in demand for energy storage devices, better and safer alternatives are required. Zinc ion hybrid supercapacitor (ZHSC) has a great potential as an alternative to lithium-ion batteries as it combines the high energy capacity of zinc-ion batteries and longevity and high power density of supercapacitors to produce a device that can potentially outperform and outlast conventional batteries. ZHSCs are currently unable to achieve their theoretical specific capacity due to several issues that are stated in the review. This review aims to provide fundamentals of the energy storage mechanism of hybrid supercapacitors and ZHSCs as well as summarize recent developments on ZHSCs. Various types of carbon-based materials along with pseudocapacitive materials that have been utilised as electrode materials for ZHSCs are comprehensively discussed. The zinc anode as well as ways to improve its performance are briefly discussed in the review. Electrolyte for ZHSCs with a focus on hydrogel polymer electrolyte and how it affects ZHSC performance is elaborated. Lastly, a summary of current issues faced by ZHSCs, and future perspectives are discussed.••••Summary of electrode materials for zinc-ion hybrid supercapacitors••Methods of optimisation for zinc anodes and development of suitable electrolytes••Challenges faced by zinc-ion hybrid supercapacitors and their future perspectivesZinc ion hybrid supercapacitorCarbon-based electrodesElectrolytesHydrogel polymer electrolytesAffordable and clean energy is one of the Sustainable Development Goals (SDGs) set by the United Nations Environment Programme. 84.3 % of total energy used globally in 2020 comes from fossil fuels which produce greenhouse gasses that leads to climate change. Moreover, depletion of fossil fuels will result in higher prices as supply will decrease over time. Renewable energy sources such as hydropower, solar, wind, tidal, geothermal, and biomass have been explored widely for clean and sustainable energy. These energy sources generally suffer from one major problem, that is intermittency. Solar energy is highly dependent on the weather and impractical in areas that receive low sunlight throughout the year; wind and tidal power both rely on the wind blowing, which is inconsistent in both wind speed and duration, resulting in inconsistent amount of energy being harvested. One of the ways to overcome this issue is to have a large energy storage device connected to the renewable energy source so that when excess energy is generated, it is stored, and when no energy is generated, the stored energy is released, relieving the intermittency issue. For this, the energy storage device needs to be able to store enormous amounts of energy and store this energy at a high rate without any issues. Lithium-ion battery is the most widely used energy storage device due to its high energy density, high cell voltage, and low self-discharge. However, current lithium-ion batteries are unable to store energy at a high rat. To understand about the hybrid supercapacitor, we must first understand about the basic concepts of supercapacitors. Supercapacitor, also known as ultracapacitors or electrochemical capacitors, is an energy storage-based electrochemical device. There are two energy storage mechanisms for supercapacitors. The first being electric double layer capacitance (EDLC) where the energy storage and release mechanism are based on charge separation at the electrochemical interface formed between the electrodes and electrolyte. EDLCs use electrostatic interaction to accumulate energy in Helmholtz double layers on the interface between the electrodes and electrolyte. The capacitance of an EDLC is based on the potential dependence of the surface energy stored electrostatically at the interface of capacitor electrodes as shown in Fig. 2(a). EDLC-based supercapacitors have very high power density (1–100 kW kg−1) and cycle life (>10,000 charge-discharge cycles) but have relatively low energy density (4–10 Wh kg−1) when compared to commercial batteries (~100 Wh kg−1). The second is pseudocapacitance where inorganic materials exhibit surface or near-surface fast and reversible redox reactions with the charge build-up being proportional to the potential difference. During charge and discharge, redox reactions occur due to the bonds of the compounds and thus result in the energy transfer between electrolyte and electrode as shown in Fig. 2(b). There exists another type of pseudocapacitance named intercalation.
