###### Abstract  
We propose a simple and novel system to form precipitation patterns of Cu-Fe-based Prussian blue analogues (Cu-Fe PBA) in agarose gel through coupled electrochemical reactions, reactant ion diffusion influenced by electric field, and precipitation reactions. The spatiotemporal evolution, spatial distribution, and crystallite morphologies of the precipitates were investigated by visual inspection, Fe _Ka_ intensity distribution measurements, and optical and scanning electron microscope observations. The observed precipitation patterns and their evolution depended on the applied voltage. Multicolored periodic precipitation bands were stochastically formed under cyclic alternating voltage (4 V for 1 h and then 1 V for 4 h per cycle). The distances between adjacent bands were randomly distributed (0.30 \(\pm\) 0.25 mm). The sizes and shapes of the crystallites generated in the gel were position-dependent. Cubic but fairly irregular crystallites (0.1-0.8 um) were formed in the periodic bands, whereas definitely cube-shaped crystallites (1-3 um) appeared close to the anode. These cube-like reddish-brown crystallites were assigned to Cu-FeII PBA. In some periodic bands, plate-like blue crystallites (assigned to Cu(OH)\({}_{2}\)) were also present. Future issues for potential applications of the observed periodic banding for selective preparation of Cu-Fe PBA crystallites were discussed. p 2021 24 May 2021 2  
and alternating electric fields [2; 12]. A schematic illustration of the RD setup used for examining Liesegang banding under an electric field is shown in Figure 1b. In this study, we propose a simple reaction-diffusion-reaction (RDR) setup, where electrochemical reactions, diffusion of reactant ions influenced by electric field, and precipitation reactions are coupled to form periodic precipitation patterns in agarose gel (Figure 1c), which has not yet been investigated. Although the proposed RDR setup appears similar to the RD setup in Figure 1b, the processes occurring within these systems are quite different. Instead of diffusion from the outer electrolyte column, the reactant ions are produced at the electrodes through electrochemical reactions. Specifically, in a tube containing agarose gel, Cu\({}^{2+}\) and [Fe(CN)\({}_{6}\)]\({}^{4-}\) ions are generated at the anode and cathode, respectively, when the applied voltage exceeds the sum of the superpotential of the electrodes (typically around 0.5 V in aqueous solutions at 25 \({}^{\circ}\)C [13]) and the potentials of the electrode reactions (Cu\({}^{2+}\) + 2e\({}^{-}\) = Cu + 0.340 V and [Fe(CN)\({}_{6}\)]\({}^{3-}\) + e\({}^{-}\) = [Fe(CN)\({}_{6}\)]\({}^{4-}\) + 0.361 V [14]), i.e., -1 V. The reactant ions are transported under the influences of the electric field and diffusion (similar to the setup in Figure 1b in this aspect) and react with each other to form precipitates of Cu-Fe-based Prussian blue analogues (Cu-Fe PBA), which are represented by the formula K\({}_{x}\)Cu\({}_{y}\)[Fe(CN)\({}_{6}\)]\({}_{\square-}\):\(\mathrm{m}\)H\({}_{2}\)O. Note that here \(\square\) indicates Fe(CN)\({}_{6}\) vacancies, i.e., defects arising from missing Fe(CN)\({}_{6}\) moieties (\(x\) = 2, \(y\) = 2 = 1, and \(n\) = 0 for Cu-Fe\({}^{\mathrm{II}}\) PBA with no Fe(CN)\({}_{6}\) vacancies, whereas \(x\) = \(y\) = \(z\) = 1 and \(n\) = 0 for Cu-Fe\({}^{\mathrm{III}}\) PBA with no Fe(CN)\({}_{6}\) vacancies). Crucially, the concentrations of reactant ions can be controlled in situ by varying the voltage of the electrodes. The proposed RDR setup could expand the applications of precipitation patterning in material science and engineering. Because the reactant ions are generated from the electrodes, various changes can be made to their concentrations during the experiment to yield different precipitation patterns. For instance, pulsatile concentration changes are easily achievable by tuning the voltage of the power supply. Such tuning is capable of generating diverse precipitation patterns.