Recent Advancements and Future Prospects of Noble Metal-Based Heterogeneous Nanocatalysts for Oxygen Reduction and Hydrogen Evolution Reactions  
[PERSON]  
1 Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan; [EMAIL_ADDRESS] (D.B. ); [EMAIL_ADDRESS] (S.-S.Y. ); [EMAIL_ADDRESS] (C.Y. ); [EMAIL_ADDRESS] (W.-H.H.) [PERSON]  
2 Arity Centre of Nanotechnology, Amity University Haryana, Panchgaon, Guryatnam-122413, India; [EMAIL_ADDRESS]  
[PERSON]  
1 Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan; [EMAIL_ADDRESS] (D.B. ); [EMAIL_ADDRESS] (S.-S.Y. ); [EMAIL_ADDRESS] (C.Y. ); [EMAIL_ADDRESS] (W.-H.H.) [PERSON]  
1 Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan; [EMAIL_ADDRESS] (D.B. ); [EMAIL_ADDRESS] (S.-S.Y. ); [EMAIL_ADDRESS] (C.Y. ); [EMAIL_ADDRESS] (W.-H.H.) [PERSON]  
1 Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan; [EMAIL_ADDRESS] (D.B. ); [EMAIL_ADDRESS] (C.Y. ); [EMAIL_ADDRESS] (W.-H.H.) [PERSON]  
3 Institute of Materials Science and Engineering, National Central University, Taoyuan City 32001, Taiwan; [EMAIL_ADDRESS] (D.B. ); [EMAIL_ADDRESS] (D.B. ); [EMAIL_ADDRESS] (D.B. ); [EMAIL_ADDRESS] (D.B. ); kent108011506@gapp. by virtue of it having the highest gravimetric energy density (142 MJ kg\({}^{-1}\)) of all the carbonaceous species and zero emissions [4]. On Earth, hydrogen is not found in free form and is mainly available in compounds (e.g., hydrocarbons and water), hence, the effective production of hydrogen is of paramount importance for the implementation of the hydrogen economy. At present, nearly 95% of the total hydrogen is produced by coal gasification and steam reforming, while only 4% is obtained from water splitting [5]. Currently, hydrogen production through non-renewable resources (i.e., fossil fuels) severely threatens the environment via massive CO\({}_{2}\) emission [6]. In addition, steam-methane reforming is an intensive energy consumption process, in which hydrocarbon and water react together at high-temperature to generate CO\({}_{2}\) and hydrogen via the following reactions:  
\[\text{CH}_{4}+\text{H}_{2}\text{O}\rightarrow\text{CO}+3\text{H}_{2} \tag{1}\]  
\[\text{CO}+\text{H}_{2}\text{O}\rightarrow\text{CO}_{2}+\text{H}_{2} \tag{2}\]  
In these reactions, every mole of hydrogen produces one mole of CO\({}_{2}\). Neither of these methods for hydrogen production support the reduction of environmental pollution and global warming and, therefore, they are not environmentally benign processes for sustainable hydrogen production. Although electrochemical water splitting is an effective approach to generate cheap and sustainable hydrogen at a large scale, the relatively suppressed efficiency and the lack of efficient, economical, and earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) are major hurdles in this step [7; 8]. Electrochemical water splitting was first observed in 1789 yet contributes only 4% of the H\({}_{2}\) production worldwide, even after 200 years of development, due to sluggish kinetics of HER at the cathode in water splitting [9]. Nevertheless, similar to ORR, the HER also has to surmount a certain energy barrier (known as overpotential) to occur. Generally, for any electrochemical reaction, the difference between the applied and thermodynamic potentials is termed as overpotential. In this event, for lowering the overpotential and promoting the reaction rate, both ORR and HER usually require the assistance of electrocatalysts. To date, noble metals, particularly Pt-based electrocatalysts, are highly utilized in the practice of boosting the ORR and HER kinetics [10]. However, Pt is restricted by its high cost and scarcity for industry applications [11]. Therefore, the development of highly efficient electrocatalysts with reduced Pt-loading and high utilization is an urgent need for the establishment of a carbon-neutral economy.