# Electrosynthesis of HKUST-1 with Flow-Reactor Post-Processing
## 3 Results and Discussion
### Electrosynthesis of HKUST-1  
To prevent passivation of the electrodes and enable continuous electrosynthesis of HKUST-1 the polarity of the electrodes was reversed in controlled frequency. Polarity reversal reduced the build-up of solids on the electrode plates, allowing for longer electrolysis times, but it also affected the composition of the product. Dark black suspensions were formed when the frequency was set to 0.250 Hz whereas DC experiments generated blue suspensions with a small amount of dark precipitate that settled easily (Figure 3). According to our previous work [15], the dark precipitate was a copper powder that forms when copper ions are reduced on the electrode surface. The copper concentration of the suspension was significantly higher when polarity reversal was used and the solution had a darker colour in general. According to visual inspection and gravimetric analysis, the products mostly formed on the electrode surfaces during the DC process whereas polarity reversal seems appears to suspend these otherwise deposited particles into solution. Figure 2: (**a**) The cell voltage of the electrosynthesis system with and without 0.250 Hz polarity reversal frequency with 0.8 A current, and (**b**) the product in the solution when synthesis was done using 0.8 A and 1.6 A currents with 0.250 Hz polarity reversal frequency. The syntheses were done at ambient temperature in 0.078 M BTC solution. The effect of polarity reversal frequency on HKUST-1 yield was studied in the range of 0.05-0.5 Hz (Figure 4). Cell voltage rapidly increased when 0.050 Hz and 0.125 Hz frequencies were used due to the passivation of the electrodes. The lowest cell voltage was measured for the electrosynthesis experiment with 0.250 Hz frequency. The product yield at low frequencies of 0.050 Hz and 0.125 Hz was lower than that of 0.250 Hz and 0.5 Hz (Table 1) which is consistent with the cell voltage results and indicates product build-up on the electrode surface. According to the results, low-frequency polarity reversal is not enough to prevent the passive layer build-up on the electrode surface. The highest mass yield in the solution was measured for the 0.250 Hz polarity reversal frequency. At higher frequencies, the ratio of capacitive current to faradaic current increases, and less copper is released from the electrodes as also shown in our earlier research paper [15]. \begin{table}
\begin{tabular}{c c c} \hline
**Frequency (Hz)** & **Product on the Electrodes (g)** & **Product in the Solution (g)** \\ \hline
0.050 & 0.7360 & 0.1417 \\
0.125 & 0.5563 & 0.0780 \\
0.250 & 0.3455 & 0.7315 \\
0.500 & 0.4808 & 0.3657 \\ \hline \end{tabular}
\end{table} 
Table 1: The weight of the products on the electrodes and in the solution after 150 min electrosynthesis time with varying polarity reversal frequencies in 0.078 M H\({}_{3}\)BTC solution with 0.8 A current. Figure 3: Picture of the test cell and electrodes after synthesis. (**a,b**) were used for DC electrosynthesis, whereas (**c,d**) used 0.250 Hz polarity reversal. Strips on electrode surfaces without product deposition are from the use of spacers to control the inter-electrode distances. The effect of H\({}_{3}\)BTC concentration on HKUST-1 product quality was studied in the range of 0.01-0.30 M. Cell voltage increased rapidly during the electrosynthesis process when the H\({}_{3}\)BTC concentration was 0.1 M or higher (Figure 4) compared to lower concentrations indicating quick passive film formation on the electrode surfaces. The cell voltage reached the recording limit (13 V) of the system in 0.1-0.3 M H\({}_{3}\)BTC solutions. According to the gravimetric analysis of the products (Table 2) the lower H\({}_{3}\)BTC concentrations favoured HKUST-1 formation in the solution whereas products grow mostly on the electrode surface in higher concentration solutions. Figure 4: The cell voltage and product mass of the electrosynthesis at various (**a,b**) polarity reversal frequencies, (**c,d**) H\({}_{3}\)BTC concentrations, and (**e,f**) synthesis temperatures.