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Nanomaterials 2020, 10, 1407 4 of 26 properties and render them suitable candidates in electrocatalysis [41]. Specifically, CNHs have been engaged principally in vital electrocatalytic reactions involved in fuel cell technologies and/or production of renewable fuels, mostly as supports for the realization of hybrid materials, contributing to higher performances and better stability and durability. This review highlights the advances made with CNH-based materials in electrocatalysis towards oxygen reduction, methanol oxidation, and hydrogen evolution as well as oxygen evolution from water splitting. 2. CNHs in Electrocatalysis 2.1. Oxygen Reduction Electrocatalysis provides a sustainable and efficient energy process, with electrolysis and fuel cells being auspicious application-platforms, for energy conversion. Fuel cells are electrochemical-based devices that are considered to be the most promising power sources for stationary and portable electronic devices as well as transportation. Analogous to batteries, fuel cells convert chemical energy of a fuel directly into electric energy. Most proton exchange fuel cells (PEMFCs) are powered by hydrogen, which can be fed to the fuel cell directly or can be generated within the system by reforming hydrogen-rich fuels. Unlike batteries, fuel cells do not need recharging as they require continuous sources of fuel (and oxygen) in order to keep the process going. Conversely, when fuel cells use hydrogen as fuel, only electricity, water, and heat are produced. Their high efficiency, no environmental pollution, and unlimited reactant sources boost them on top of other thermal engines. Notably, PEMFCs have received great attention in recent years for potential use in vehicles, portable electronics, and into combined heat and power systems due to their simplicity, high power density, quick start-up, and low working temperature. Fuel cells are being developed and tested in trucks, buses, boats, motorcycles, and bicycles, among other kinds of vehicles, and are expected to be widely commercially used as a solution in global energy problems [46]. The main reactions involved in PEMFCs are fuel oxidation at the anode and oxygen reduction at the cathode. At the anode, hydrogen is oxidized, and electrons and protons are produced (H2 → 2H+ + 2e−) and transferred to the cathode through an external circuit and the proton exchange membrane, respectively. At the cathode, oxygen reacts with protons and electrons and is reduced to produce water (1/2O2 + 2H+ + 2e− → H2O). Both anode and cathode electrodes consist of platinum (Pt) to promote the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR). The HOR occurs extremely fast and requires a low Pt loading, typically less than of 0.05 mg/cm2 [47]. However, at the cathode, on the other hand, ORR is characterized by sluggish kinetics and much more Pt loading is required, of about ~0.4 mg/cm2, in order to achieve a good fuel cell performance [48]. It is, therefore, quite clear that the cathodic reaction needs to be optimized. Although Pt is the most effective electrocatalyst at the moment, its high cost and limited durability and stability have made the exploration of alternative electrocatalysts essential in order to limit or even fully replace Pt-based materials. The ORR can occur through two possible pathways in aqueous media. One involves direct water production via a four-electron reduction route according to the following equation: O2 +4H+ +4e− →2H2O (1) The other is a two-electron reduction route, which involves the formation of hydrogen peroxide (H2O2) as an intermediate according to the following equations: O2 +2H+ +2e− →H2O2 (2) H2O2 + 2H+ + 2e− → 2H2O (3) The most efficient and preferable reaction for the fuel cell technology is the four-electron pathway, while the formation of hydrogen peroxide is considered to be undesirable, as it leads to low efficiency. On the other hand, hydrogen peroxide is a versatile chemical with high commercial value, withPDF Image | Carbon Nanohorn-Based Electrocatalysts for Energy Conversion
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