# Porous Electrodeposited Cu as a Potential Electrode for Electrochemical Reduction Reactions of CO2
## 1 Introduction  
High attention has been paid to the development of porous metals in the past decade, because the materials exhibit many unique characteristics including high surface area, low density, and permeability that allow fluid flow. Recently, porous copper (Cu) and Cu alloys are among the porous metals that receive increasing focus, as Cu itself provides many important features required in the engineering and functional applications [1, 2, 3, 4, 5]. These include, for example, high thermal conductivity for heat exchangers and catalytic activity for environmental and energy applications [6, 7, 8]. Introduction of porosity to Cu has been shown to enhance various properties, such as wettability, fluid permeability and catalytic efficiency [9, 10, 11, 12]. Correspondingly, Cu with porous structures, coupled with its metallic copper quality, can find wider uses for thermal transfer systems, water purification, fuel cell components and electro-catalytic applications [13, 14, 15, 16, 17]. One of the possible techniques to generate porosity and thus fabricate porous Cu is the promotion of hydrogen bubbles in the Cu electrodeposition process in the acid-based electrolyte. This so-called \"dynamic hydrogen bubble template (DHBT) method\" was introduced by [PERSON] et al. in the year 2003 and was demonstrated to provide honeycomb-like porous structure with hierarchical porosity, comprising pores in micro- (30-90 \(\upmu\)m) and nano-scales [4; 18]. Specifically, porosity can be generated in a relatively simple way by increasing the rates of hydrogen formation at the cathode surface upon which Cu ions are electrochemically reduced and copper is electrodeposited. This could be done by increasing applied current for electrodeposition or increasing the concentration ratio of sulfuric acid to copper sulphate in the plating bath [4; 18; 19; 20; 21]. The accumulated hydrogen bubbles are then incorporated into the deposit, resulting in pore formation. With high volume of interconnected porosity and large surface area, DHBT porous Cu can find possible uses in the application of electrochemical CO\({}_{2}\) reduction reactions (CO\({}_{2}\)RR), as an electro-catalyst that supports and accelerates the conversion of CO\({}_{2}\) gas to some useful chemical products [2; 10; 22; 23]. This particular application has gained large amounts of attention in recent years owing to the increasing concern on greenhouse gas emissions and associated global warming. Compared to the existing thermochemical, photochemical, and hydrogenation methods, the electrochemical route for CO\({}_{2}\) conversion is advantageous due to its low energy consumption, dispatchability and room temperature operation [24]. In this regard, the 3D open-pore catalytic electrode of high surface could allow CO\({}_{2}\) to concentrate at the electrode and provide more active sites for the electrochemical reaction [1; 2]. Compared to other common electrodes for CO\({}_{2}\)RR (e.g., Au, Ag and Zn that provide CO primarily; and Sn, In, Pb that dominantly yield formate) [25; 26], Cu is known to be a unique catalyst that promote generations of a variety of valuable chemical products, including CO, formate, hydrocarbons (i.e., methane, ethylene, and ethane), and oxygenates (i.e., acetate, acetone, ethanol, formate, and methanol) [6; 25; 26; 27; 28; 29]. A state-of-the-art study by [PERSON] et al. using a flat cell reactor and Cu foil electrodes demonstrated the generation of as many as 16 CO\({}_{2}\)RR products [26], owing to Cu's intermediate CO binding energy. Oxidation states of Cu electrodes' surfaces (i.e., Cu\({}^{+}\), Cu\({}^{2+}\)) have shown to play a role in CO\({}_{2}\)RR. Depending on the oxidation states as controllable by various methods [30] such as thermal annealing, Cu electrodes are promoted to generate different chemical products and current efficiencies [31; 32; 33; 34].