## 4 Noble Metal-Based Electrocatalysts for ORR and HER
### Pt-Based Catalysts in ORR and HER  
It is a well-known fact that Pt is by far the most widely accepted catalyst for ORR and HER due to the appropriately balanced adsorption energy of intermediate species on the Pt surface. However, the high materials cost and limited reserves in the Earth's crust are major issues severely hampering its industrial scalability. To this end, there is an urgent need to minimize Pt-loading while maintaining comparable performance. Potential engineering efforts have been directed to surpass the aforementioned deep-rooted issues of Pt-based catalysts in ORR and HER. The electronic structure of Pt strongly affects the adsorption energy of intermediate species during HER and ORR on Pt surfaces. Alloying Pt with foreign transition metals effectively manipulates the electronic structure of surface-active sites, resulting in a downshift in the Pt d-band center, which further weakens the interaction with intermediate adsorbates and largely facilitates the HER and ORR kinetics. Previously reported studies demonstrated effective strategies to manipulate the electronics structure of Pt-based alloys via composition and morphology control [29]. Out of various pioneer studies, Pt\({}_{\text{x}}\)Co alloys are the most intriguing catalysts for HER and ORR. [PERSON] et al. prepared Pt\({}_{\text{x}}\)Co alloy nanoparticles in different compositions (PtCo\({}_{3}\), PtCo, and Pt\({}_{3}\)Co) on high surface area carbon (HSAC) supports by using a liquid precursor impregnation-freeze-drying-annealing method and demonstrated the structure-performance correlation. The as-prepared Pt\({}_{\text{x}}\)Co alloys were tested in both alkaline and acidic medium for ORR. Unsurprisingly, the Pt\({}_{\text{x}}\)Co alloys exhibited different behaviors in alkaline and acidic mediums. The electrochemical active surface areas (ECSA) of Pt\({}_{\text{x}}\)Co decreased with increasing content of Co in an alkaline medium, while it increased in an acidic medium. The mass activities of Pt\({}_{\text{x}}\)Co alloys follow different trends in different mediums. In acidic medium, Pt\({}_{\text{x}}\)Co alloys follow  
Figure 3: **(a)** The relationship between exchange current density (\(\beta_{0}\)) and metal-H\({}^{\text{ads}}\) bond strengths. The volcano shape shows that Pt stands nearly at the middle of the \(x\)-axis (i.e., \(\Delta\)G\({}_{\text{H}}\)) indicating the appropriately balanced H\({}^{\text{ads}}\) bond strength. **(b)** A volcano plot (oxygen reduction reaction (ORR) activity vs. energy of oxygen interaction) indicating the ORR activity of different metals on the (111) surface plane. (Reproduced with permission from [23; 24], Royal Society of Chemistry, 2013, American Chemical Society, 2004)  
the trend of PtCo \(<\) Pt\({}_{3}\)Co \(<\) PtCo\({}_{3}\), while in an alkaline medium, the trend is PtCo\({}_{3}\)\(<\) PtCo \(<\) Pt\({}_{3}\)Co, which reveals that ORR performance of Pt\({}_{\text{x}}\)Co alloys strongly depends upon the composition [30]. The same group also undertook a similar kind of study with Pt\({}_{\text{x}}\)Cu alloys. They prepared PtCu\({}_{3}\), PtCu, and Pt\({}_{3}\)Cu alloys and found that the mass activities of Pt\({}_{\text{x}}\)Cu alloys followed opposite trends in alkaline and acidic mediums [31]. Pt\({}_{\text{x}}\)Co nanoparticles are also employed to catalyze HER. [PERSON] et al. synthesized highly efficient and stable PtCo alloy nanoparticles encapsulated in carbon nanofibers (PtCo/CNFs) by integrating the electrospinning and graphitization processes. As-prepared PtCo/CNFs with a Pt-loading of 5 wt.% effectively catalyzed the HER in 0.5 M H\({}_{2}\)SO\({}_{4}\) solution with a low overpotential and a small Tafel slope of 28 mVdec\({}^{-1}\)[32]. Alloying with Pt is not limited to Co only. [PERSON] et al. prepared octahedral Pt-Ni alloy nanoparticles on carbon supports with variable compositions of Pt (Pt\({}_{4}\)Ni, Pt\({}_{3}\)Ni, Pt\({}_{2}\)Ni, Pt\({}_{1.5}\)Ni, and PtNi).