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Nanomaterials 2020, 10, 1407 12 of 26 Some effective ways to address the problems that DMFCs face is to operate the fuel cell at high temperatures (above 120 ◦C) and provide the cell a mixture of methanol and water in the vapor phase. Vapor phase DMFCs show significant advantages, i.e., they show higher energy efficiency, enhanced mass transfer and anode electrode kinetics, they do not require methanol dilution (which makes the fuel tank smaller), and they show higher cathode tolerance to methanol crossover, while methanol crossover in the vapor phase decreases with temperature [123]. In this framework, CNHs were used as an electrocatalyst support in a vapor phase high temperature DMFC (HT-DMFC) and its performance was assessed [124]. The CNH-based electrocatalyst showed superior methanol electrochemical oxidation than CB and oxygen reduction reactions for voltages lower than 0.65V vs. RHE. The improved performance was attributed to higher water vapor adsorption and/or electrode morphology. Moreover, CNH-based electrodes presented improved performance and longer stability in a vapor phase HT-DMFC environment. Additionally, CNHs and carbon black supports were compared in membrane electrode assemblies (MEA) for high temperature PEMFCs [125]. Similar peak power densities were obtained in both cases operating at 160 ◦C. Nevertheless, Pt/CNHs hybrids showed a higher ohmic resistance compared to carbon black-based MEA, most likely attributed to the hydrophobic character of CNHs. Furthermore, the Pt/CNH anode exhibited lower charge transfer resistance, while the Pt/CNH cathode electrode presented similar cathode charge-transfer resistance than the corresponding CB electrode [126]. The electrocatalytic properties, characteristics, and performance of CNH-based materials towards MOR are summed up in Table 2. Table 2. Electrocatalytic properties, characteristics, and performance of CNH-based materials towards MOR. Electrocatalyst Pt/CNHs Pt/CNHs PtRu/N-CNHs Pt/CNHs Pt/CNHs Reaction/Conditions MOR/CV in 0.5 M H2SO4/1.0 M CH3OH MOR/CV in 1.0 M KOH/1.0 M CH3OH Formic acid oxidation reaction/CV in 0.5 M H2SO4/0.5 M HCOOH MOR/CV in 0.5 M H2SO4/1 M CH3OH MOR/CV in 0.5 M H2SO4/1 M CH3OH MOR/CV in 0.5 M H SO /1 M CH OH MOR Performance Oxidation peak: 0.7 V vs. SCE Mass activity: 350 mA mg−1 Durability: 90% mass activity loss after 1800 s Oxidation peak: −0.22 V vs. Ag/AgCl Mass activity: 0.49 mA μg−1 Durability: 13% peak current loss after 100 cycles Oxidation peak: 0.35 V vs. Ag/AgCl Mass activity: 81 mA mg−1 Durability: 60% mass activity loss after 3600 s Oxidation peak: 0.7 V vs. RHE Mass activity: 850 mA mg−1 Oxidation peak: 0.7 V vs. Ag/AgCl Oxidation peak: 0.69 V vs. Ag/AgCl Current density: 127 mA cm−2 Mass activity: 184 mA mg −1 Ref. [118] [119] [120] [121] [122] 2 4 3 Pt 2.3. Water Splitting—Hydrogen Evolution and Oxygen Evolution Electrolysis is the process where water electrocatalytically dissociates to hydrogen, via the hydrogen evolution reaction (HER) and oxygen, via the oxygen evolution reaction (OER). Electrolysis is a keystone reaction toward a society that aims on becoming independent from fossil fuels. Hydrogen is considered to be a promising candidate for a secondary source of energy. Currently, 96% of hydrogen is produced by steam reforming from hydrocarbon fuels. An alternative way to address dependence on finite resources is the power-to-gas strategy, where intermittent energy resources are transferred and stored as hydrogen, mostly generated from water splitting. The latter reaction is a straightforward, inexpensive, and highly efficient approach to produce hydrogen and takes place in a unit called anPDF Image | Carbon Nanohorn-Based Electrocatalysts for Energy Conversion
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