# Electrosynthesis of HKUST-1 with Flow-Reactor Post-Processing
## 3 Results and Discussion
### Electrosynthesis of HKUST-1
The synthesis parameters were 0.078 M H\({}_{3}\)BTC solution, ambient temperature, and 0.250 Hz polarity reversal frequency except for the parameter that was being studied. SEM images (Figure 5) demonstrate that crystal morphology is typical of HKUST-1 when the H\({}_{3}\)BTC concentration was between 0.05 and 0.078 M. When this concentration was 0.1 M bar-shaped structures were detected, and greater concentrations resulted in fibrous products without any typical HKUST-1 crystal morphologies visible. In the highest H\({}_{3}\)BTC concentrations, a white precipitate was formed on the electrode surface. Nitrogen adsorption isotherm results were consistent with the SEM analysis, as increasing the H\({}_{3}\)BTC concentration above 0.01 M resulted in products with a lower BET surface area (Table 3). The chemical structure of the fibrous products was not studied in more detail. [PERSON] et al. [14] reported similar structures in their research and according to [PERSON] et al. [25], a one-dimensional coordination polymer [CuII(HBTC)(H\({}_{2}\)O)\({}_{3}\)] is formed when water was used as the solvent instead of an ethanol-water mixture due to partial deprotonation of H\({}_{3}\)BTC. Our results show that increasing the H\({}_{3}\)BTC concentration past a certain point will result in the formation of these fibrous products. This may be an effect of pH, as an increased H\({}_{3}\)BTC concentration will result in a more acidic solution where H\({}_{3}\)BTC cannot be sufficiently deprotonated for formation of HKUST-1. This idea would be in agreement with previous studies which demonstrate that over-coordinated species can be formed at high H\({}_{3}\)BTC concentrations [26]. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**H\({}_{3}\)BTC Concentration (M)**} & \multicolumn{1}{c}{**Product on the Electrodes (g)**} & \multicolumn{1}{c}{**Product in the Solution (g)**} \\ \hline
0.050 & 0.5136 & 0.6353 \\
0.078 & 0.3455 & 0.7315 \\
0.100 & 0.5418 & 0.1910 \\
0.200 & 0.4765 & 0.2836 \\
0.300 & 0.6403 & 0.3704 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The weight of the products on the electrodes and in the solution after 150 min electrosynthesis time with varying H\({}_{3}\)BTC concentrations using 0.250 Hz polarity reversal and 0.8 A current. Figure 5: SEM images of the products formed at (**a**) 0.78 M H\({}_{3}\)BTC solution at room temperature using 0.250 Hz frequency compared to (**b**) a higher H\({}_{3}\)BTC concentration of 0.300 M, (**c**) a higher temperature of 60 \({}^{\circ}\)C, and (**d**) a lower frequency of 0.05 Hz. The effect of temperature on electrosynthesis was studied at 20 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, 60 \({}^{\circ}\)C, and 80 \({}^{\circ}\)C. An increase in temperature improved the conductivity of the solution and hence reduced the initial cell voltage. However, the cell voltage increased rapidly indicating faster passivation of the electrodes at elevated temperatures compared to the room temperature. Warmer temperatures resulted in lower product yields in the solution. The product formed at 20 \({}^{\circ}\)C consisted of small crystals that had typical HKUST-1 morphology. In contrast, products synthesised at elevated temperatures were less pure, exhibiting small HKUST-1 crystals mixed with larger bar-shaped products. The optimisation of current density, polarity reversal frequency, H\({}_{3}\)BTC concentration, and temperature are important for efficient batch electrosynthesis of HKUST-1. Results show that there are three major reactions taking place; anodic copper ion release, HKUST-1 formation, and copper ion reduction to the metallic state. Low current density, low H\({}_{3}\)BTC concentration, and low temperature improve the yield of HKUST-1 and reduce the number of side-products. The optimum polarity reversal frequency is 0.250 Hz. At high frequencies, the proportion of the capacitive current to faradaic current is higher and therefore less copper is released into the solution.