# Electrosynthesis of HKUST-1 with Flow-Reactor Post-Processing
## 3 Results and Discussion  
### 3.1. Anodic Dissolution of Copper during Electrosynthesis  
The theory of anodic dissolution of copper has been well established in corrosion literature [21; 22]. Copper can dissolve via one-electron and two-electron mechanisms. In aqueous solutions, a cuprous oxide (Cu\({}_{2}\)O) layer is typically formed on the electrode passivating the surface; however, the high current densities typically used in MOFs electrosynthesis processes will make the surface trans-passive. In the trans-passive region, the corrosion current is increased and a more soluble cupric oxide (CuO) is formed. The highly oxidizing environment of the trans-passive region constantly generates oxides and corrosion products that are released into the solution.  
Typically, the interelectrode distance of the anode and cathode has not been reported for the HKUST-1 electrosynthesis studies [12; 23]. Additional electrolytes, such as tetrabutylammonium tetrafluoroborate or tributammonium methyl sulfate, are usually required to enhance the conductivity of the solution, and this electrolyte needs to be removed from the product in the subsequent washing stages. In this research, an interelectrode distance of 0.5 mm was used without any additional electrolytes. A short electrolyte path between the electrodes reduced the cell voltage but the system was more prone to blocking and short-circuiting (Figure 2a). Electrosynthesis with direct current (DC) resulted in HKUST-1 crystal growth on the electrode surfaces. Anode surfaces were covered with blue precipitate, indicating HKUST-1 formation, whereas cathode plates were covered with dark precipitate. Passive layers that formed on the electrode surfaces inhibited the current flow and increased the cell voltage, eventually leading to blocking or short-circuiting of the cell. A similar short-circuit phenomenon was detected in a smaller electrochemical test cell in our earlier research where XRD analysis indicated that small copper particleswere deposited onto the cathode, eventually creating a conductive pathway between the electrodes [15].  
A study on the formation of HKUST-1 on copper surfaces identified four stages in the process: initial nucleation, growth of the island, intergrowth, and detachment [24]. At low current densities, the nucleation does not take place on the electrode surface but rather in the solution because the nucleation threshold on the surface is not reached. In this research, we used two currents of 0.8 A and 1.6 A. Doubling the current density did not increase the copper concentration in the solution (Figure 2b). The weight of the products on the electrodes was 0.3455 g when 0.8 A current was used and increased to 0.6334 g with 1.6 A current. Visual observation and weight analysis of the products on the plates and the solution after the tests indicated that high current density increased only the product mass on the electrode surface, not in the solution. This is consistent with the findings of [PERSON] et al. [24] which demonstrate that higher current densities will favour deposition of HKUST-1 on the electrode surface [24].