High Active Material Loading in Organic Electrodes Enabled by an in‐situ Electropolymerized π‐Conjugated Tetrakis (4‐Aminophenyl) Porphyrin

Abstract

Porphyrin complexes have been widely studied as promising electrode material in diverse energy storage systems and chemistries. However, like other organic electrodes, porphyrins often suffer from low conductivity and, consequently, require a significant amount (typically 40 %) of electrochemically inactive conductive carbon that occupies volume and mass without storing energy. In this study, we investigate [5,10,15,20 tetrakis(4‐aminophenyl)‐porphyrin] (TAPP) and its metal complexes as redox‐active cathode materials to address the aforementioned issues. The lithium‐ion cells prepared with a high content of CuTAPP active material (70 wt %) demonstrate a stable discharge capacity of ∼120 mAh/g when cycling with a constant current density of 1000 mA/g. The material also showed superior rate capability, e. g., ∼60 mAh/g at 8 A/g. The results of DFT calculations and experimental analysis indicate that the degree of planarity of the metalloporphyrins directly correlates to their cycling stability. Moreover, the contribution from the central metal redox during the cycling is found to be the reason for the significantly higher performance of the Cu‐complex compared to the metal‐free complex. The findings of this study show a general approach for facing common conductivity challenges of organic electrodes and open up a pathway for practical application of organics electrode materials in energy storage application.

A π‐Conjugated Porphyrin Complex as Cathode Material Allows Fast and Stable Energy Storage in Calcium Batteries

“…The altered structure of CuTPyP−MOF, in comparison to H 2 TPyP (Figure S24), results in a large contribution from diffusion‐controlled reaction, indicating a more battery‐like material character. This stands in contrast to previous studies, which showed dominant surface reactions upon the introduction of a copper ion [25a,29,41a] . Interestingly, the stronger faradaic character of CuTPyP−MOF does not influence the rate capability.…”

“…A comparative CV measurement (Figure S20) suggests that an additional redox peak, which occurs at 2.7 V in the anodic scan and reverses at 2.5 V in the cathodic scan, could be attributed to the copper atom in the porphyrin core structure. Previous research also reported on partial Cu(II) to Cu(I) redox reaction in Cu−porphyrins [25a,29,41a,42] . However, in this study, we observe the reaction to be more reversible and complete than previous studies have suggested.…”

“…No plateaus were observed in the charge‐discharge cycles of porphyrinic cells (Figures 6d, S23), indicating a fast redox reaction at porphyrin sites and pseudocapacitive behavior, that have been well documented in literature [25a,29,41a] . To further investigate this pseudocapacity and gain a more comprehensive understanding of the underlying processes in porphyrin−calcium cells, a detailed kinetic analysis of these materials was performed (Figure 7, according to Equation S1) in order to disentangle diffusion and surface‐controlled processes.…”

“…A common problem of porphyrins as well as other organic materials that impacts the electrochemical performance is the high solubility of active compounds in the electrolytes [40] . In earlier studies it was already found that introducing metal atoms into the porphyrin structure might be beneficial for closing the solubility gap [25a,41] . Also, in the case of H 2 TPyP, which has an inherently low solubility, the dissolution in DME was further reduced after copper complexation (Figures S4 and S5).…”