Mechanisms of Degradation of Na₂Ni[Fe(CN)₆] Functional Electrodes in Aqueous Media: A Combined Theoretical and Experimental Study

Abstract

Prussian blue analogues (PBAs) are versatile functional materials with numerous applications ranging from electrocatalysis and batteries to sensors and electrochromic devices. Their electrochemical performance involving long-term cycling stability strongly depends on the electrolyte composition. In this work, we use density functional theory calculations and experiments to elucidate the mechanisms of degradation of model Na2Ni­[Fe­(CN)6] functional electrodes in aqueous electrolytes. Next to the solution pH and cation concentration, we identify anion adsorption as a major driving force for electrode dissolution. Notably, the nature of adsorbed anions can control the mass and charge transfer mechanisms during metal cation intercalation as well as the electrode degradation rate. We find that weakly adsorbing anions, such as NO3 –, impede the degradation, while strongly adsorbing anions, such as SO4 2–, accelerate it. The results of this study provide practical guidelines for electrolyte optimization and can likely be extrapolated to the whole family of PBAs operating in aqueous media.

Identifying the charge storage mechanism in polyimide anodes for Na-ion aqueous batteries by impedance spectroscopy

“…To account for the two stages of the mechanism, an adsorption capacitance C a,2 and resistance R a,2 are added to the Randles circuit. 34 Hereby, the model reflects the fact that the reduction of the first carbonyl group determines the second step of the redox process, which unquestionably takes place after the transfer of the first electron. Thus, the two stages are conjugated, and C a,2 , and R a,2 are connected in the circuit accordingly.…”

“…Simultaneously to the primary two-stage conjugated mechanism, a one-electron single-stage mechanism was found to take place. This process can be modeled by a charge transfer resistance R ct,1 and an adsorption capacitance C a,1 34 that are connected in parallel to the elements representing the two-stage mechanism. By including these parameters, the spectra can be fitted with extremely high accuracy for all potentials, as shown in Fig.…”

“…8 M NaClO 4 was used as the electrolyte, as highly concentrated electrolytes are known to enhance electrode stability by preventing active material dissolution. 33,34 Before being run into the cell the electrolyte was purged with argon for approximately 10 minutes. The reference electrode utilized was an Ag/AgCl electrode (SSC, 3 M KCl, SI Analytics, ''B 3420+''), connected to the electrolyte via a Luggin capillary.…”

Open-Circuit Potential Drift in Intercalation Electrodes: Role of Charge Redistribution in a Prussian Blue Analog

“…4 are discussed in section S2 of the supplementary text. [36][37][38] Collectively, the potential drift profiles from Figs. 3b and 4 indicated that there were no significant contributions from electrode dissolution, faradaic processes, or other experimental anomalies.…”

Fast-Charging Capability of Thin-Film Prussian Blue Analogue Electrodes for Aqueous Sodium-Ion Batteries

“…As exemplarily shown in Figure S1, stable performance of the thin-film electrodes is obtained at low and high C-rates along with a coulombic efficiency close to 100%. A more detailed analysis and discussion of the stability of PBAs in aqueous electrolytes is available in our previous work. , Since the composite electrode had a much higher absolute capacity compared to the thin-film electrodes, a symmetric WE-CE configuration was set up using a largely oversized CE, which was previously brought to 50% state-of-charge. The measurement protocols involved CV scans at different rates and galvanostatic cycling with potential limitations at varying C-rates.…”