Recent advances in rational design of efficient electrocatalyst for full water splitting across all pH conditions
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Introduction Hydrogen is an ideal energy carrier due to its outstanding energy density, abundance, and zero harmful byproducts.1−5 Hydrogen, as a potential energy carrier, serves as a valuable resource that can help to recover air quality and reinforce energy supplies, by extending its application from ammonia production and oil refining to green fuels. Electrochemical water splitting (EWS) is an especially good process for hydrogen production because of the faster production rate than any other known method, it has product purity (≈99.999%) and it does not require high processing temperature and pressure.6,7 EWS consists of electrochemical reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), that originate at the cathode and anode, respectively.8 Typically, the HER activity comprises a two-electron mechanism to form molecular hydrogen, while the OER activity involves a four-proton coupled electron-transfer mechanism to form the O = O bond and cleave the O–H bond.9 Hence, water-splitting systems demand energy to initiate superior OER activity that accompanies the HER activity. According to theoretical predictions, water-splitting systems function at a thermodynamic cell voltage of 1.23 V to electrolyze water into hydrogen and oxygen under room temperature (25°C) and atmospheric pressure (760 mm Hg).10 When investigating the reactions in practical environments, additional resistances
such as ohmic, concentration, and activation in the electrochemical cell act as a barrier to restrict the activity of critical HER and OER reactions, requiring the system to use more energy than computed theoretical estimates.11 Generally, ruthenium oxide (RuO2) and iridium oxide (IrO2) have been engaged as an active electrocatalyst in the anode to promote an upsurge in the sluggish OER activity, while a platinum (Pt) based cathode is a widely used electrocatalyst for HER since it has greater exchange current density and a low Tafel rate.12,13 The expensive and precious nature of these electrocatalysts restricts their widespread usage.14 The monofunctional behavior (i.e., performs either HER or OER) of these costly catalysts is also a drawback. For environmentally friendly mass-hydrogen production, earth-abundant, economical, efficient, and bifunctional transition-metal-based electrocatalysts (i.e., performs HER and OER) are alternatives to noble metals. Transition metals are the best choice due to their unique d-electron configurations, low cost, abundance in the earth, and synergistic effects of multimetal atoms to enhance catalysis. Recently, numerous studies have highlighted transition metal-based electrocatalysts for OER or HER, and as bifunctional catalysts. Despite their electrochemical activity being comparable to or better than noble metals, their limited usage in alkaline or acidic electrolytes thus far has become a negative factor in their commercialization.15−17 The
Gnanaprakasam Janani, Department of Materials Science and Engineering, Chonnam National University, South Korea; janani.gkovai@gma
