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Environmentally friendly energy sources alternatives to fossil fuels such as solar and wind are strategic for meeting the needs of an increasingly energy demanding society, despite their periodic/intermittent nature. Thus, urge the development of clean and renewable energy sources such as based on solar energy and water in a cyclic way, by photoinduced water-splitting and regeneration in fuel cells. In this context, energy storage devices such as hybrid supercapacitors become fundamental for realization of a sustainable society. In this review, the early discovery and recent advances concerning synthetic strategies, hierarchical structures, and oxygen evolution reaction (OER)/hydrogen evolution reactions (HER) catalytic performances of nickel-vanadium double hydroxides (NiV-LDHs) based nanomaterials are summarized. A discussion about the role of vanadium ions in HER/OER was also included, highlighting the recent progress in theoretical calculations in this field. Finally, some hybrid supercapacitor electrode materials based on NiV-LDHs are described, including the strategies to circumvent the parasitic oxygen evolution reaction during charge-discharge of those energy storage devices. In short, catalysts for HER/OER and hybrid supercapacitor electrode materials based on NiV-LDHs were reviewed considering their key multifunctional role in the way to a more sustainable society.

Electrochemical capacitors show high power density but suffer from low energy density. The use of pseudocapacitive material can increase the cell capacitance and therefore increase the device energy density. In this work, we show that using microtubular conducting polymers, as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT), alongside activated carbon (AC) in asymmetrical devices, and highly concentrated aqueous electrolyte (or water-in-salt electrolyte, WiSE), is possible to maintain the polymers electroactivity at the negative polarization and take advantage of the electrolyte electrochemical stability. The polymers electrochemical behaviours were compared in WiSE and aqueous acidic solution, which hinted that the polymers charge compensation are different in the different electrolytes. PPy-AC and PEDOT-AC cells can reach 2.6 and 2.8 V, respectively, and electrodes mass balance greatly depends on the polymers open circuit potential before and after cycling

The use of water-in-salt electrolytes (WiSEs) in electrochemical capacitors is an interesting alternative to state-of-the-art electrolytes based on organic solvents. However, making use of their wide electrochemical stability in electrochemical capacitors has been challenging, as shown in battery materials. In this work, we used three concentrations of LiTf2N in water (7, 15 and 21 mol kg−1) with activated carbon, and the determination of the maximum operating voltage showed stability up to 2.2, 2.4 and 2.6 V, respectively. This determination suggested positive/negative-electrode mass balances of 2.1, 1.5 and 1.2, but the experiments showed that the cycle life of devices using WiSEs of 15 and 21 mol kg−1 can be improved. Thus, we find that increasing the amount of active material in the positive electrode gave higher capacitance retention over cycling. With the optimization for a longer cycle life, the observed capacitance retentions for the devices with WiSEs of 7, 15 and 21 mol kg−1 were 97%, 70% and near 100%, respectively, after 10000 cycles. The higher amount of active material in the positive electrode penalized the energy-power performance, but the higher electrochemical stability compensated for it, and WiSEs of 7, 15 and 21 mol kg−1 stored 15, 16 and 21 W h kg−1 when they operated at 0.1 A g−1.

We propose novel pseudocapacitors that can store energy related to the partial entropy change associated with proton concentration variations following neutralization reactions. In this situation, it is possible to obtain electrochemical energy after the complete charge/discharge cycle conducted in electrolytic solutions with different proton concentrations. To this end, we prepared modified electrodes from phosphomolybdic acid (PMA), poly(3,4-ethylenedioxythiophene/poly(styrenesulfonate) (PEDOT-PSS), and polyallylamine (PAH) by the layer-by-layer (LbL) method and investigated their electrochemical behavior, aiming to use them in these neutralization pseudocapacitors. We analyzed the potentiodynamic profile of the current density at several scan rates, to evaluate the reversibility of the proton electroinsertion process, which is crucial to maximum energy storage efficiency. On the basis of the proposed reaction mechanism and by using frequency-domain measurements and models, we determined rate constants at different potentials. Our results demonstrated that the conducting polymer affects the self-assembled matrixes, ensuring that energy storage is high (22.5 kJ mol(-1)). The process involved neutralization of a hydrochloric acid solution from pH = 1 to pH = 6, which corresponds to 40% of the neutralization enthalpy.

Conselho Nacional de Desenvolvimento Científico e Tecnológico Motivated by the current demand of storage devices more energy efficient and sustainable, in this work were manufactured electrodes for supercapacitor based on carbon nanotubes (CNT) and nickel oxide (NiO) nanocomposites supported on indium tin oxide (ITO) substrate. Electrochemical capacitors, knowed as supercapacitors, are energy storage devices characterized by their high energy density, high power density, long life cycle and short charge and discharge time. In this context, nanocomposites based on carbon nanotubes and metal oxides can exhibit excellent electrochemical performance in applications for supercapacitor electrodes. The total capacitance of electrode nanocomposite is the addition of both contributions: the high capacitance of the electric double layer, due to high specific surface area of carbon nanotubes and pseudocapacitance performance of nickel oxide, due stored energy electrochemically by faradaic processes. There are several metal oxides that have pseudocapacitive features, but many are too expensive. The nickel oxide has a comparable performance with other pseudocapacitive oxides and conductive polymers, and have a low cost. To produce the NiO/CNT/ITO electrodes in this work, first nanotube films were prepared by electrophoretic deposition on ITO substrate. Then nickel metallic was electrodeposited on the nanotubes, and finally the electrode passed through a thermal treatment to form the oxide. The NiO/CNT/ITO electrode developed in this study showed a high electrochemical performance for the potential ranging from 0.0 V to 0.8 V, with ~ 35 F/g of maximum capacitance. This nanocomposite showed ~ 15 kJ/kg of maximum power densities and ~ 5 kW/kg of maximum power. These densities are consistent with expected values for supercapacitors according to the Ragone Plot. Moreover, this nanostructured electrode showed higher cyclic stability than oxide without carbon nanotubes. These properties make this nanocomposite a promising material for the development of high energy density and high power density supercapacitors devices. Motivado pela demanda atual de dispositivos de armazenamento de energia mais eficientes e sustentáveis, nesse trabalho foram fabricados eletrodos de supercapacitor baseado em nanocompósitos de nanotubos de carbono (CNT) e óxido de níquel (NiO) em substrato de óxido de índio-estanho (ITO). Capacitores eletroquímicos ou supercapacitores são dispositivos de armazenamento de energia caracterizados por sua alta densidade de energia, alta densidade de potência, longo ciclo de vida e curto tempo de carga e descarga. Neste contexto, nanocompósitos baseados em CNTs e NiO podem apresentar excelente desempenho eletroquímico para aplicação em eletrodos de supercapacitores. A capacitância total desse tipo de nanocompósito é a soma de duas contribuições: a elevada capacitância da dupla camada elétrica, devido a alta área superficial específica dos CNTs e a pseudocapacitância do NiO, que, por meio de processos faradaicos, contribue para o armazenamento de energia eletroquimicamente. Neste trabalho, para produzir os eletrodos NiO/CNT/ITO, primeiramente foram depositados filmes de nanotubos sobre substrato de ITO através da técnica de deposição eletroforética. Em seguida, níquel metálico foi eletrodepositado sobre os nanotubos, e finalmente o eletrodo passou por tratamento térmico para a formação do óxido. O eletrodo de NiO/CNT/ITO desenvolvido nesse estudo apresentou alto desempenho eletroquímico, sendo dominantemente capacitivo na faixa de potencial de 0,0 V à 0,8 V, com capacitância máxima de ~35 F/g. Tal nanocompósito apresentou densidades de energia máxima de ~15 kJ/kg e potência máxima de ~ 5 kW/kg. Segundo o Diagrama de Ragone, estas densidades são compatíveis com os valores esperados para supercapacitores. Além disso, o eletrodo com nanocompósito apresentou maior estabilidade cíclica que o eletrodo com somente óxido, sem nanotubo. Devido às propriedades apresentadas, esse nanocompósito é um material promissor para o desenvolvimento de dispositivos supercapacitores com alta densidade de energia e alta densidade de potência.