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Nanomaterials 2020, 10, 1407 9 of 26 nanoscale. The CNH-based electrocatalyst showed excellent activity and selectivity for ORR to H2O2 at low overpotentials, outperforming current metal-based electrocatalysts. Efficient control of crucial parameters like high surface area and porosity as well as the optimal distribution of pyridinic-N and pyrrolic-N due to the polydopamine dopant led to the superior activity of the modified CNHs [111]. Table 1 summarizes the electrocatalytic properties, characteristics, and performance of CNH-based materials towards ORR. 2.2. Methanol Oxidation Direct methanol fuel cells (DMFCs) are a relatively new entry into the family of fuel cells technology and are considered a subcategory of PMFCs because they use a polymer membrane as an electrolyte. DMFCs were first invented and developed in the 1990s to tackle the problem of hydrogen storage and to eliminate the need of a reformer that converts fuel to hydrogen. In DMFCs, pure methanol is used as fuel and reacts directly at the anode. However, in addition to platinum, other catalysts like Ru must be added to break the methanol bond in the anodic reaction. Methanol offers several advantages as a fuel, namely, it is cheap and has low energy power density per mass unit (20 MJ/kg), while it can be easily transported, handled, and stored. This means methanol can be supplied to the fuel cell unit from a liquid refillable reservoir, which can be kept topped up, or in cartridges which can be quickly changed out when spent. DMFCs operate in temperatures between 60 and 130 ◦C and are mostly used in applications with modest power requirements, such as mobile electronic devices or chargers and portable power packs. Their usage as power units in materials handling vehicles even sees commercial traction in replacing the battery power in forklift trucks. However, DMFCs are held back due to two major drawbacks: (i) the poor oxidation kinetics of the fuel, and (ii) the diffusion of methanol into the cathode. When the proton exchange membrane becomes permeable to the fuel, methanol molecules can diffuse through the membrane and are directly oxidized by oxygen on the positive electrode. This can cause a mixed potential and reduce the cathode potential and consequently, the overall performance of the cell, which eventually raises the cost of the electrode Additionally, it forces the use of dilute methanol solutions at the anode (typically 0.5–1.0 M), demanding larger quantities of water, making the size and complexity of the system bigger, thus, impractical for portable devices. These problems can be addressed by using membranes with lower permeability towards methanol and by using efficient cathode catalyst materials for oxygen reduction, which simultaneously show high tolerance toward methanol chemisorption. Several strategies have been developed to take advantage of carbon supports as anode catalysts [112]. The unique structure of CNHs is advantageous to support nanoparticles due to the thousand nanospaces between the cones of the aggregates [113]. Actually, the use of CNHs as supports to Pt and PtRu nanoparticles in DMFCs and PEMFCs has shown higher catalytic activities of 60% compared to carbon black [114–116]. Oxidized CNHs were used to achieve better dispensability of Pt nanoparticles and they were used as cathodes in DMFCs [117]. Markedly, CNHs-based electrocatalysts reached a power density of 76 mW/cm2 operating at 40 ◦C. In another work, different assistant agents were studied for the preparation of Pt nanoparticles supported on CNHs [118]. Results showed that ionic liquids improved the dispersibility and size uniformity of Pt nanoparticles than 4,4-bipyridine. The same went for oxidized CNHs compared to intact CNHs, while the electrocatalytic activity of Pt on oxidized CNHs for methanol oxidation was higher compared to that of Pt on CNHs [118]. However, Pt nanoparticles stabilized on oxidized CNHs exhibited poor durability. Apart from this, a novel approach was developed for the preparation of Pt nanoparticles on CNHs supports (Pt/CNHs) [119]. Most synthetic procedures require the introduction of defects and/or oxygenated groups to CNHs for the effective deposition of Pt nanoparticles, which may end up damaging their structure and result in poor properties. Contrarily, in the discussed work, formic acid was employed as a reducing agent for the preparation and immobilization of Pt nanoparticles on pristine CNHs. Moreover, Pt/CNHs exhibited higher catalytic activity and better long-term stability for the optimal amount of Pt on CNHs, for both methanol and formic acid oxidation reactions than commercial Pt/C. In the same concept, an anode catalyst consisting of Pt/Ru nanoclusters supportedPDF Image | Carbon Nanohorn-Based Electrocatalysts for Energy Conversion
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