## 3 Electrodeposition
### Properties Relevant to Applications
#### 3.3.2 Catalytic Activity
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Alloy** & **Structure** & \(\mathbf{\eta_{HER}}\)**, \(\mathbf{mV/i}\), \(\mathbf{mA/cm^{2}}\)** & \(\mathbf{\eta_{OER}}\), \(\mathbf{mV/i}\), \(\mathbf{mA/cm^{2}}\) & **Ref.** \\ \hline FeCoCrNi & amorphous & – & 295/10 [2] & [24] \\ \hline NiFeCoMnSn & amorphous & 4/10 [2] & – & [23] \\ \hline FeMnCuCo & FCC+BCC & – & 226/10 [2] & [60] \\ \hline FeCoNiCuMn & FCC & 200/100 [2] & 251/100 [2] & [51] \\ \hline FeNiCoCrMn & FCC & 168/10 [2] & 231/10 [2] & [42] \\ \hline FeNiCoCrMn & FCC & 191/10 [2] & – & [52] \\ \hline NiFeCoMnV & FCC & 7/10 [2] & – & [25] \\ \hline FeCoNiMnW & FCC & 15/10 [1] & 512/10 [1] & [54] \\ \hline NiFeCuCoW & FCC & – & 247/10 [2] & [21] \\ \hline NiFeCoMoW & amorphous & 171/10 [2] & – & [22] \\ \hline CuMoNiCoFe & no data & 12/10 [2] & 290/10 [2] & [53] \\ \hline NiFeCuCoCe & FCC & – & 219/10 [2] & [61] \\ \hline PtPdNiCoMn & FCC & 23/10 [2] & – & [37] \\ \hline \hline \end{tabular} \({}^{1}\) In 0.5M H\({}_{2}\)SO\({}_{4}\). \({}^{2}\) In 1M KOH.  
\end{table} 
Table 3: Catalytic activity of some high-entropy alloys—overpotentials \(\eta\) for HER and OER at given current density i in acid \({}^{1}\) or alkaline \({}^{2}\) solutions. the presence of high-valence manganese, which promotes O-O bond formation, further enhancing catalytic performance. In turn, [PERSON] et al. [23] investigated the role of tin atoms incorporated into NiFeCoMnSn alloys (DC, sulfate-citrate bath). They achieved excellent HER electrocatalytic performance, which was attributed to several factors: (i) the three-dimensional porous structure of the nickel foam substrate significantly increased the active surface area, (ii) the incorporation of tin atoms (much larger than the other constituent metals) into the high-entropy alloy induced strong lattice distortion due to atomic size mismatch, which shifted the d-band center and tuned the surface electronic structure, and (iii) the combined synergistic effects and lattice distortion facilitated the adsorption and desorption of reaction intermediates, thereby accelerating the reaction kinetics. [PERSON] et al. [24] highlighted the crucial role played by the amorphous structure in FeCoCrNi alloys for efficient water splitting catalysis. They noted that the disordered atomic arrangement of amorphous materials leads to abundant structural defects, which provide numerous active sites and facilitate ion diffusion. During the OER process, surface reconstruction promotes the formation of oxygen defects and mixed metal oxides/hydroxides, whose synergistic interactions significantly enhance catalytic performance. A large active surface area and tunable electronic structure were identified as key factors in improving OER activity. [PERSON] et al. [37] designed PtPdNiCoMn high-entropy alloys (PD, organic bath) for HER catalysis by combining noble and non-noble metals, guided by five key descriptors: hydrogen absorption energy, surface M-H bond length variance, mixing enthalpy, electronegativity difference, and cationic preference. Manganese contributed redox activity and cost reduction, while cobalt and nickel stabilized the FCC phase due to their magnetic properties. Substituting platinum and palladium with non-isoelectronic metals introduced chemical heterogeneity, maintaining catalytic activity while reducing reliance on precious metals. The high configurational entropy improved the distribution and accessibility of active sites, resulting in excellent HER activity, even in challenging media like simulated and real seawater. High-entropy alloys have also demonstrated promising catalytic performance in the electrochemical oxidation of organic compounds such as methanol [62; 63] and glycerol [64]. For instance, [PERSON] et al. [63] synthesized a mesoporous PtPdRhRuCu alloy via soft-template-assisted electrodeposition. They used a polystyrene-poly(oxyethylene) block copolymer in a tetrahydrofuran-water system to form metal-decorated micelles, which migrated to the cathode and were reduced into a film.