## 4 Noble Metal-Based Electrocatalysts for ORR and HER
### Pd-Based Catalysts in ORR and HER  
Pd-based nanomaterials have emerged as potential contenders to replace Pt-based electrocatalysts. Pd is in the same group as Pt in the periodic table and possesses nearly similar physiochemical properties as that of Pt [52]. In addition, Pd is more economical and abundant than Pt, which might address the cost issues. Although Pd-based nanomaterials are a possible alternative, there is a gap in catalytic performance compared to Pt. Numerous strategies have been adopted to change the electronic structure of Pd for filling this gap. The literature says that catalytic performance of Pd-based catalysts can be significantly improved by several orders via incorporating Pd with late-transition metals in various morphologies and compositions [53], creating atomic vacancies, tuning the size, optimizing support materials, and so on [54; 55]. Among the aforementioned approaches, various  
Figure 7: **(a)** The cyclic voltammetry and **(b)** linear sweep voltammetry curves of pristine (Cu@Pd/Pt) NCs and NCs after annealing (Cu@Pd/Pt-H). **(c)** The cyclic voltammetry and **(d)** linear sweep voltammetry curves of Cu@Pd/Pt-H NCs at stability test cycles. **(e)** Normalized current densities of Cu@Pd/Pt, Cu@Pd/Pt-H, and commercial J.M.-Pt/C catalysts at selected accelerated degradation test (ADT) cycles. (Reproduced with permission from [49], ACS Publications, 2019)studies reported that alloying Pd with foreign transition metals is a widely accepted strategy to tune the electronic properties of Pd for better ORR and HER performance. For example, [PERSON] et al. synthesized Pd nanocrystals adjacent to Ni-oxide in an epitaxial manner with an ordered local structure in both of the Ni and Pd domains [56]. Such a unique nanostructure outperforms a commercial Pt-catalyst with the mass activity of 231.2 mAmg\({}^{-1}\) and specific activity is 0.492 mAcm\({}^{-2}\). A similar kind of structure consisting of epitaxially grown Pd nanoparticles on tetrahedral symmetric Ni oxide (NiO\({}_{\text{T}}\)) was synthesized via a wet chemical reduction method on a carbon nanotube (CNT) support. As-prepared NiPd-CNT NC efficiently catalyzed the HER in an acidic medium with an overpotential of 46 mV and a low Tafel slope of 38.0 mV dec\({}^{-1}\). Of special relevance, the NiPd NC maintained \(\sim\)95% performance when operated for 1000 cycles (Figure 8) [57]. [PERSON] et al. synthesized Pd-Co alloy catalysts on carbon supports by using an impregnation method, where sodium borohydride was used as the reducing agent. The ORR performance of as-prepared Pd-Co alloys was further improved by heat treatment in the temperature range of 300 \({}^{\circ}\)C to 700 \({}^{\circ}\)C. The optimal ORR performance was demonstrated by the Pd-Co alloy catalysts heat-treated at 300 \({}^{\circ}\)C [58]. The same group also demonstrated the effect of reducing agents on the morphology and corresponding ORR activity of Pd-Co alloys. Three different reducing agents including ethylene glycol, formaldehyde, and sodium borohydride were utilized to synthesize Pd-Co alloys on the carbon support. Various microscopic and spectroscopic techniques were employed to unveil the morphological differences in Pd-Co alloy nanoparticles synthesized with different reducing agents. These Pd-Co alloys were employed to boost ORR performance and the preferred order was found to be ethylene glycol \(>\) sodium borohydride \(>\) formaldehyde [59]. The Co-Pd alloy catalysts also  
Figure 8: (**a**) The linear sweep voltammetry curves, (**b**) Tafel plots, and (**c**) corresponding overpotential values of NiPd-CNT NC compared with those of control samples. (**d**) The stability test results of NiPd-CNT NCs. (Reproduced with permission from [57], Royal Society of Chemistry, 2016)  
exhibited tremendous activity towards HER. [PERSON] et al. synthesized Co-Pd alloys via co-reduction of [Co(NH\({}_{3}\))\({}_{6}\)]\({}^{3+}\) and [Pd(NH\({}_{3}\))\({}_{4}\)]\({}^{2+}\) complexes for HER.