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Abstract: The process of hydrogen evolution reaction (HER) through water electrolysis is an important technology for establishing the so called "hydrogen economy". Here we will cover different systems for electrocatalytic HER. Transition metal carbides and metal phosphides are alternative to platinum (Pt) and offer excellent electrocatalytic activity for HER. Pyrolysis of hexacarbonyl tungsten, W(CO)6, in 1-octadecene has been used to prepare colloidal tungsten, W, nanoparticles (NPs) [1]. The obtained W NPs has been spin-coated on graphite (C) electrodes. Heat treatment of the W/C electrodes at elevated temperatures (≥ 900°C) allows the preparation of metallic W and tungsten carbide (W2C@WC) thin films. The obtained W2C@WC electrodes were used for hydrogen evolution studies (HER) in 0.5M H2SO4. Cyclic voltammetry tests for 1000 cycles showed that W2C@WC exhibit long term stability without significant drop in current density. The overpotential defined at 10 mA/cm2 is 310 mV vs. RHE giving an excellent catalytic activity for HER. Iron phosphide electrocatalysts were synthesized using a triphenylphosphine (TPP) precursor. Different iron phosphide phases were synthesized at 300°C (Fe2P) and at 350°C ( FeP ) [2]. To enhance the catalytic activities of obtained iron phosphide particles heat-treatments were carried out at elevated temperatures. Annealing at 500°C under reductive atmosphere induced structural changes in the samples: (i) Fe2P provided a pure Fe3P phase (Fe3P−500°C) and (ii) FeP transformed into a mixture of iron phosphide phases (Fe2P/FeP−500°C). The lowest electrode potential of 110 mV vs. a reversible hydrogen electrode (RHE) at 10 mA cm−2 was achieved with Fe2P/FeP−500°C catalyst
Keywords: Fe2P, electrocatalysis, hydrogen, ammonia
Published in RUNG: 13.12.2023; Views: 574; Downloads: 3
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Abstract: A catalytic system based on iron phosphide (Fe2P) has exhibited electrocatalytic activity toward N2-reduction reaction in alkaline medium (0.5 mol dm−3 NaOH). Based on voltammetric stripping-type electroanalytical measurements, Raman spectroscopic and spectrophotometric data, it can be stated that the Fe2P catalyst facilitates conversion of N2 to NH3, and the process is fairly selective with respect to the competing hydrogen evolution. A series of diagnostic electrocatalytic experiments (utilizing platinum nanoparticles and HKUST-1) have been proposed and performed to control purity of nitrogen gas and to probe presence of potential contaminants such as ammonia, nitrogen oxo-species and oxygen. On the whole, the results are consistent with the view that the interfacial reduced-iron (Fe0) centers, while existing within the network of P sites, induce activation and reduction of nitrogen, parallel to the water splitting (reduction) to hydrogen. It is apparent from Tafel plots and impedance measurements that mechanism and dynamics of nitrogen reduction depends on the applied electroreduction potential. The catalytic system exhibits certain tolerance with respect to the competitive hydrogen evolution and gives (during electrolysis at -0.4 V vs. RHE) the Faradaic efficiency, namely, the selectivity (molar) efficiency, toward production of NH3 on the level of 60%. Under such conditions, the NH3-yield rate has been found to be equal to 7.5 µmol cm−2 h−1 (21 µmol m−2 s−1). By referring to classic concepts of electrochemical kinetic analysis, the rate constant in heterogeneous units has been found to be on the moderate level of 1-2*10−4 cm s−1 (at -0.4 V). The above mentioned iron-phosphorous active sites, which are generated on surfaces of Fe2P particles, have also been demonstrated to exhibit strong catalytic properties during reductions of other electrochemically inert reactants, such as oxygen, nitrites and nitrates.
Keywords: nitrogen reduction, alkaline medium, iron phosphide catalyst, ammonia, electrochemical determinations
Published in RUNG: 30.11.2023; Views: 623; Downloads: 4
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Abstract: The electrochemical reduction reaction of the nitrate ion (NO3−), a widespread water pollutant, to valuable ammonia (NH3) is a promising approach for environmental remediation and green energy conservation. The development of high-performance electrocatalysts to selectively reduce NO3− wastes into value-added NH3 will open up a different route of NO3− treatment, and impose both environmental and economic impacts on sustainable NH3 synthesis. Transition metal phosphides represent one of the most promising earth-abundant catalysts with impressive electrocatalytic activities. Herein, we report for the first time the electrocatalytic reduction of NO3− using different phases of iron phosphide. Particularly, FeP and Fe2P phases were successfully demonstrated as efficient catalysts for NH3 generation. Detection of the in-situ formed product was achieved using electrooxidation of NH3 to nitrogen (N2) on a Pt electrode. The Fe2P catalyst exhibits the highest Faradaic efficiency (96 %) for NH3 generation with a yield (0.25 mmol h−1 cm-−2 or 2.10 mg h−1 cm−2) at − 0.55 V vs. reversible hydrogen electrode (RHE). The recycling tests confirmed that Fe2P and FeP catalysts exhibit excellent stability during the NO3− reduction at − 0.37 V vs. RHE. To get relevant information about the reaction mechanisms and the fundamental origins behind the better performance of Fe2P, density functional theory (DFT) calculations were performed. These results indicate that the Fe2P phase exhibits excellent performance to be deployed as an efficient noble metal-free catalyst for NH3 generation.
Keywords: iron phosphide, electrocatalysts, nitrates reduction ammonia, DFT calculations
Published in RUNG: 02.02.2023; Views: 1288; Downloads: 4
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