There is an urgent global need for electrochemical energy storage that includes materials that can provide simultaneous high power and high energy density. One strategy to achieve this goal is with pseudocapacitive materials that take advantage of reversible surface or near-surface Faradaic reactions to store charge. This allows them to surpass the capacity limitations of electrical double-layer capacitors and the mass transfer limitations of batteries. The past decade has seen tremendous growth in the understanding of pseudocapacitance as well as materials that exhibit this phenomenon. The purpose of this Review is to examine the fundamental development of the concept of pseudocapacitance and how it came to prominence in electrochemical energy storage as well as to describe new classes of materials whose electrochemical energy storage behavior can be described as pseudocapacitive.

The presence of structural water in tungsten oxides leads to a transition in the energy storage mechanism from battery-type intercalation (limited by solid state diffusion) to pseudocapacitance (limited by surface kinetics). Here, we demonstrate that these electrochemical mechanisms are linked to the mechanical response of the materials during intercalation of protons and present a pathway to utilize the mechanical coupling for local studies of electrochemistry. Operando atomic force microscopy dilatometry is used to measure the deformation of redox-active energy storage materials and to link the local nanoscale deformation to the electrochemical redox process. This technique reveals that the local mechanical deformation of the hydrated tungsten oxide is smaller and more gradual than the anhydrous oxide and occurs without hysteresis during the intercalation and deintercalation processes. The ability of layered materials with confined structural water to minimize mechanical deformation likely contributes to their fast energy storage kinetics.

Interlayer structural protons in H2Ti3O7 are identified as the key structural feature to enable electrochemical proton intercalation beyond the near-surface because they effectively reduce interconnections of the titanate layers.

The reversible intercalation of multivalent cations, especially Mg, into a solid-state electrode is an attractive mechanism for next-generation energy storage devices. These reactions typically exhibit poor kinetics due to a high activation energy for interfacial charge-transfer and slow solid-state diffusion. Interlayer water in VO and MnO has been shown to improve Mg intercalation kinetics in nonaqueous electrolytes. Here, the effect of structural water on Mg intercalation in nonaqueous electrolytes is examined in crystalline WO and the related hydrated and layered WO·nHO (n = 1, 2). Using thin film electrodes, cyclic voltammetry, Raman spectroscopy, X-ray diffraction, and electron microscopy, the energy storage in these materials is determined to involve reversible Mg intercalation. It is found that the anhydrous WO can intercalate up to ∼0.3 Mg (75 mAh g) and can maintain the monoclinic structure for at least 50 cycles at a cyclic voltammetry sweep rate of 0.1 mV s. The kinetics of Mg storage in WO are limited by solid-state diffusion, which is similar to its behavior in a Li electrolyte. On the other hand, the maximum capacity for Mg storage in WO·nHO is approximately half that of WO (35 mAh g). However, the kinetics of both Mg and Li storage in WO·nHO are primarily limited by the interface and are thus pseudocapacitive. The stability of the structural water in WO·nHO varies: the interlayer water of WO·2HO is removed upon exposure to a nonaqueous electrolyte, while the water directly coordinated to W is stable during electrochemical cycling. These results demonstrate that tungsten oxides are potential candidates for Mg cathodes, that in these materials structural water can lead to improved Mg kinetics at the expense of capacity, and that the type of structural water affects stability.

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