Electrochemical Analysis of the Mechanism of Potassium‐Ion Insertion into K‐rich Prussian Blue Materials

Abstract

The electrochemical insertion patterns of potassium‐rich Prussian blue (PB) materials with different compositions and particle sizes were systematically compared, which allows us to deduce correlations between the influence of particle morphology and material structure on the potassium‐ion insertion mechanisms in aqueous solutions. Although structural analysis indicates that no first‐order phase transitions occur for nanosized K‐rich Prussian blue particles upon potassium‐ion (de)insertion, the electrochemical data (galvanostatic charge/discharge, cyclic voltammetry, small‐ and large‐amplitude potential step experiments) suggest other outcomes related to the two‐phase insertion mechanism for all explored PB samples. However, even in a case whereby the phase transformation is not accompanied by abrupt changes in the crystal structure, the two‐phase mechanism dominates all of the essential practical characteristics of performance of this cathode material, such as hysteresis (overpotential) between charge and discharge steps and the measured diffusivities of potassium‐ions during charge and discharge. The formalism presented herein provides a basis for quantitatively assessing ion insertion and deinsertion parameters unique to PB analogues.

Resolving the Seeming Contradiction between the Superior Rate Capability of Prussian Blue Analogues and the Extremely Slow Ionic Diffusion

“…The coulometrically estimated thickness of the film was 0.5 μm. The CV of the PB film is similar to the voltammogram of K-FeHCF particles (Figure 3D), and the differential capacitance versus potential plot (Figure 3E) shows a sharp peak corresponding to a phase transition, 50 which indicates that the film properties are similar to those of the K-rich PB (the determined K:Fe ratio in the assynthesized film is ∼0.22:1). The electrodeposited film is unlikely a compact one, as SEM analysis shows distinct nanoparticles with sizes in the range of 20−50 nm, similar to the diameters of primary particles of the PBAs prepared via a co-precipitation route (Figure 3F and Figure S8).…”

The Misconception of Mg2+ Insertion into Prussian Blue Analogue Structures from Aqueous Solution

“…The asymmetric shape of CVs for K‐NiHCF and K‐FeHCF electrodes reflects the phase transformations to occur upon the insertion of potassium in these structures. For the K‐NiHCF electrodes, the structure changes during K + de/insertion could not be unambiguously detected with XRD (see the Supporting Information, Figure S3), while for K‐FeHCF a cubic‐to‐monoclinic phase transformation takes place [36] . The (de)insertion of K + in the NiHCF, FeHCF and CuHCF structures is a single‐phase process, as follows from the symmetry of anodic and cathodic branches of the CVs including our results and previous reports as well as from earlier reported XRD analysis of the electrodes showing no changes of the initial cubic structure [52–54] .…”

“…The PBA materials K 2 Ni[Fe(CN) 6 ] (K‐NiHCF), Ni 3 [Fe(CN) 6 ] 2 (NiHCF), Cu 3 [Fe(CN) 6 ] 2 (CuHCF), K 2 Fe[Fe(CN) 6 ] (K‐FeHCF) were prepared by precipitation from aqueous solutions of potassium hexacyanoferrate and transition metal salts. Fe 4 [Fe(CN) 6 ] 3 (FeHCF) was prepared by a hydrothermal route (see the Supporting Information, section A) [36] . Films of K‐NiHCF on Au electrodes were obtained by electrodeposition from the solution of 0.5 mM K 3 [Fe(CN) 6 ], 0.5 mM NiSO 4 and 1 M KNO 3 [37] .…”

A new programmable dipping robot

“…Metal hexacyanometallates (MHCMs) represent a class of inorganic coordination network compounds known for a long time. [ 15,16 ] Despite their long history, there is a very vivid current research interest in those materials due to their interesting and not completely understood structural (chemical and electronical) features, [ 17 ] electrochemistry, [ 18,19 ] catalytic properties, [ 20–22 ] which perspective for applications for charge storages, [ 23 ] gas storage, [ 24 ] sensors, [ 22,25 ] environmental cleaning [ 26 ] and others.…”