# Progress in the Development of Electrodeposited Catalysts for Direct Liquid Fuel Cell Applications
## 4 Shape Control--A Way Forward
#### Deep Eutectic Solvents--A Versatile Approach  
Eutectic solvents are low toxic ecofriendly mixtures with exceptional characteristics such as wider electrochemical potential window, tunable physicochemical properties and biodegradable in nature. He et al. [108] attempted to electrodeposit Pd clusters utilizing ILs with 1-butyl-3-methyl imidazolium [BMIm]\({}^{+}\) as the cation and hexafluorophosphate [PF\({}_{6}\)]\({}^{-}\), tetrafluoroborate [BF\({}_{4}\)]\({}^{-}\) as anions, and tested them for methanol oxidation. Three-dimensional flower-shaped nanoparticles (nanoflowers) have recently emerged as convincing nanomaterial for electrocatalysis due to the presence of certain high-index metal crystal facets that are intrinsically more active to specific fuel cell reactions. The primary emphasis is focused on the synthesis of nanoflowers in non-aqueous systems such as ionic liquids (ILs), deep eutectic solvents [109]. Deep eutectic solvent (DES) is a simple eutectic based system that exhibits properties such as high conductivity, viscosity, surface tension, etc., similar to that of ionic liquids. Moreover, the advantages of DES in terms of non-toxicity, biodegradation, easy preparation at low cost, high purity make them fascinating in the template free synthesis of nanoflowers. Furthermore, DES involve a large amount of electroactive adsorption species that selectively adsorb on specific crystal faces via electrochemical adsorption, functioning as a structure mediating agent in the synthesis of shape-controlled nanocrystals. [PERSON] et al. [110] reported on the shape controlled electrochemical synthesis of Pt nanoflowers in DES using cyclic voltammetric deposition. The parameters such as precursor concentration, cycle number, scan rate, temperature were varied to tune the so formed Pt structures and regulate their growth. For comparison, Pt was also deposited using potentiostatic reduction of Pt precursor in DES. Cyclic voltammogram of Pt deposition on GC electrode in DES solution showed two peaks suggesting that Pt underwent a two-step reduction process. XPS results confirmed that the electrochemical reduction of Pt took place by two steps, i.e., from step (i) tetravalent state (Pt (IV)) to divalent (Pt (II)) and step (ii) divalent (Pt (II)) to metallic state (Pt (0)). SEM images reported by the authors showed the formation of Pt flowers with sharp petals, which varied according to precursor concentration, cycle number, scan rate, and room temperature. Electrochemical studies showed that the Pt nanoflowers with sharp petals synthesized by cyclic voltammetric deposition, using a high precursor concentration at a scan rate of 50 mV s\({}^{-1}\) for 80 cycles exhibited higher electro-catalytic activity and stability than the nano structures synthesized by potentiostatic reduction and commercial catalyst  
Figure 8: Universal strategy showing the palladium structures synthesised by template free cyclic voltammetric electrodeposition approach on different carbon supports and their formic acid oxidation, oxygen reduction reaction performance comparison. Reproduced with permission from [103]. towards ethanol oxidation. The formation of Pt nanoflowers was prominent when deposited at 80 \({}^{\circ}\)C. The decreased mass transportation of reactive species due to the high viscosity in DES hindered the formation of Pt flowers at lower precursor (H\({}_{2}\)PtCl\({}_{6}\)) concentration. Increasing the number of cycles showed a transition from irregular quasi spherical particles to perfect flowers leading to the formation of aggregated flowers. Besides the cycle number, the scan rate also influenced the formation of flowers. A low scan rate resulted in the formation of rough nanosheets on the top of Pt nanostructures crystal surfaces, whereas increasing the scan rate to 50 mV s\({}^{-1}\) resulted in the formation of Pt nanoflowers with sharp petals. Variation in the viscosities of DES with an increase in temperature influenced the morphology of Pt along with the other parameters.