# Porous Electrodeposited Cu as a Potential Electrode for Electrochemical Reduction Reactions of CO2
## 3 Results and Discussions  
### Electrodeposition of Porous Copper  
All 27 sets of the porous Cu samples were successfully deposited with uniform surface appearance. The morphology of the samples and correlations of process parameters and pore characteristics are presented in Figures 2 and 3, respectively. Figure 2a shows the morphology of the samples in group A, prepared by various applied current densities and deposition times. It is evidenced that the distribution of pore sizes as observed from the surface is rather uniform, and that both of the two parameters significantly affect the apparent porosity and apparent pore size observed at the surface. Particularly, the decrease of applied current and the increase of deposition time led to enlargement of apparent pore size. This may be attributed to slow rates of hydrogen evolution in conjunction with bubble coalescence in such conditions [5; 35; 36]. The effects of the two parameters on apparent porosity are however inconclusive, and the values of apparent porosity vary less significantly as compared to pore size. Figure 1: Schematic of a 3-electrode H-cell setup for the CO\({}_{2}\) reduction reaction (CO\({}_{2}\)RR) test. Figure 2: Surface morphology of the Cu electrodeposits in group (**a**) A, (**b**) B, and (**c**) C, prepared by varying applied current densities and deposition times. Figure 2b shows the morphology of the samples in group B. Similar to those in group A, applied current density and deposition time appear to influence apparent pore size more than porosity. Overall, considering the same sets of plating parameters, the group B samples exhibit either similar or higher apparent pore size and apparent porosity as compared to group A samples. At first glance, this appears counter-intuitive, since the ratio of H\({}_{2}\)SO\({}_{4}\) (which contribute H\({}^{+}\)) and Cu\({}_{2}\)SO\({}_{4}\) is higher for group A. This may however imply that the group A samples comprise of a higher number of smaller-sized pores, in a nano regime, which were not clearly captured by image J software. Indeed, upon a closer examination using BET, it is found that the true surface area of the sample from group A is much higher than that from group B (Table 2), indicating a large number of nano-pores distributed along the struts of the porous Cu structure. The magnified microstructure of the group A sample (Figure 4), composing of clusters of sub-micron particulates and nano-pore in between each particulate, underlines this observation. As shown in the micrographs of the samples in group C (Figure 2c), it is evident that their apparent pore size and apparent porosity are somewhat larger than those of the other two groups. This is clearly owed to the effect of HCl addition in the plating bath that promotes hydrogen generation and bubble coalescence. Although the effects of applied current and deposition time on the apparent pore size and apparent porosity are inconclusive, the increase of applied current and deposition time generally lead to reductions of apparent pore size and apparent porosity, especially for the relatively high applied current conditions. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Characteristics** & **A** & **B** & **C** \\ \hline Apparent pore size (\(\upmu\)m) & 58 & 52 & 340 \\ Apparent porosity (\%) & 31 & 37 & 51 \\ BET surface area (m\({}^{2}\)/g) & 19.56 & 4.00 & 3.75 \\ Apparent density (g/cm\({}^{3}\)) & 0.34 & 0.47 & 0.26 \\ True porosity (\%) & 96.26 & 94.77 & 97.07 \\ EDX Cu content (at.\%) & 89.0 & 87.2 & 71.3 \\ EDX O content (at.\%) & 10.5 & 12.8 & 27.8 \\ \hline \hline \end{tabular}
\end{table}