Energetic Cost for Being “Redox-Site-Rich” in Pseudocapacitive Energy Storage with Nickel – Aluminum Layered Double Hydroxide Materials
As an alternative to rechargeable batteries, pseudocapacitors feature high power density, fast charge – discharge kinetics and long cycling stability for energy storage. Compositional modification by redox site enrichment is an effective strategy to increase the energy density and enhance the Faradaic reactions of pseudocapacitive materials. Transition metal layered double hydroxides (TM-LDHs) feature a layered structure and modifiable transition metal content for pseudocapacitive energy storage. In an earlier report, we demonstrated that for NiAl-LDH materials increase of Ni/Al ratio leads to expanded van der Waals (vdW) gap enabling fast charge– discharge kinetics, degraded crystallinity, and retention of 2D layered structure featuring high cycling stability. Here, coupling near-room temperature acid solution calorimetry, in situ X-ray diffraction (XRD), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), we find that as the Ni/Al ratio increases, both as-made (hydrated) and dehydrated NiAl- LDH samples tend to be less stable evidenced by experimentally measured enthalpies of formation. Moreover, the higher specific capacity of NiAl-LDH-3 (2128 F/g at 1 A/g) is enabled by effective hydration which energetically stabilizes the near-surface Ni redox sites, solvates intercalated carbonate ions, and fills the expanded vdW gap (interlayer space). In other words, for NiAl-LDH, water – LDH interactions pay for the “energetic cost” of being “redox site rich”. Thus, from a thermodynamic perspective this work points out a strategy to introduce and energetically stabilize the redox sites in TM-LDHs through interfacial engineering for pseudocapacitive energy storage performance enhancement, in which other species, including solid state guests may be introduced into the interlayer gallery.