3.1 INTRODUCTION Widespread deployment of renewable energy has been the indispensable strategy for addressing the issues resulting from the constant use of fossil fuel, such as global climate change, energy security, and sustainability. Among the available renewable energy sources, solar and wind are probably the most abundant and readily accessible, which have been considered as essential components of the future global energy portfolio [1]. However, the nature of solar and wind energy is random and intermittent. Efficient and economic energy storage and conversion technologies are required to harvest and utilize inexpensive renewable energy [2]. Among others, electrochemical energy technologies such as fuel cells and metal-air batteries can provide a temporary medium to store and release electricity when and where it is needed. Importantly, this electrochemical processes could be reversed via the oxidation or reduction of active species so as to convert chemical energy into electrical energy. Polymer electrolyte fuel cells (PEFCs) represent one of the most promising energy conversion technologies for a wide variety of applications (e.g., transportation, portable, and stationary applications), including several advantages over gasoline combustion, such as better overall fuel efficiency and reduction in CO_2 and other emissions. Meanwhile, metal-air batteries can provide significantly enhanced energy densities over traditional lithium-ion batteries. Unlike the traditional intercalation electrodes used in Li-ion batteries, the porous oxygen cathode in the metal-air cell is capable of taking reactant O_2 from the atmosphere, instead of storing bulky reactants in the electrode. As a result, the battery has significantly improved specific energy density. For example, the theoretical energy density of Li-O_2 batteries reach 5200 Wh kg~(-1), the highest value today among studied electrochemical energy devices [3]. Thus, it has high promise to meet and exceed the battery targets set for automotive applications (1700 Wh kg~(-1), derived from the practical energy density of gasoline) [4,5].
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