No abstract

One challenge with concentrated solar power (CSP) systems is the potential corrosion of the alloys in the receivers and heat exchangers at high-temperature (700-1000 • C), which leads to a reduction of heat transfer efficiency and influences the systems durability. In this work, a corrosion model has been developed to predict the rates and mechanisms for corrosion of a nickel-based alloy that is in contact with a molten salt heat transfer system. In addition to accounting for heat and mass transfer effects on the corrosion, the model takes into account the electrochemical kinetics. Coupled with computational fluid dynamics (CFD), the local electrochemical environment and corrosion rates in a high-temperature molten salt system can be predicted. The kinetic, heat and mass transfer parameters used in the model are based on experimental studies conducted in a thermosiphon. The immersion cell was designed to expose coupons to the molten salt at isothermal or non-isothermal conditions between 700-1000 • C. The model can predict the effect of thermal gradients between the top and the bottom of the reactor which induce natural convection of the molten salt. The model has been validated against experimental results at different isothermal and non-isothermal conditions and good agreement has been achieved between the model predictions of the corrosion rates and corrosion potentials with the experimental observations.

Cathodic protection is one way to mitigate corrosion of metal surfaces of concentrated solar power (CSP) systems, by shifting the potential of the alloy below its open circuit potential (OCP). The behavior of molten salt CSP systems under cathodic protection can be obtained by developing a three-dimensional (3-D) computational corrosion model. A corrosion model was designed for and benchmarked against a thermosiphon reactor. For the cathodic protection case, magnesium (Mg) was added to the salt as a sacrificial anodic species, which reduces the corrosion rate by cathodic polarization of a corroding metal surface. The model then calculated the new corrosion rate at the surface of the coupons. Results were in good agreement with experimental values for the cases with and without the cathodic protection and at isothermal and non-isothermal conditions. The results showed that by adding even small amounts of Mg into the molten salt (KCl-MgCl 2 ) can rapidly reduce the corrosion rate at the surface of the coupons for both isothermal and non-isothermal conditions. The predicted results also showed that the corrosion rate of Haynes-230 in KCl-MgCl 2 containing 1.15 mol% Mg was 35 times lower than baseline tests with no cathodic protection and met the DOE SunShot targets. Concentrating solar power (CSP) plants rely on thermal energy from the sun to generate electricity. Because they do not rely on any fossil fuel backup, CSP plants can be considered a clean energy source of the future and have the potential to replace greenhouse gas emitting fossil fuel power plants. 1 To make this system feasible, heat transfer fluids (HTFs) are required that can operate at high temperatures (above 800• C) for long periods of time without significant degradation reaction of either the HTF or the materials of construction for the system. Among different heat transfer fluids, molten halide salts are promising due to their high thermal conductivity, large specific heat, low melting point, low viscosity, and low vapor pressure.2,3 However, material corrosion has been recognized as an issue for metals in contact with molten salts, particularly at high temperatures (700-1000• C). 4 Although Superalloys have been developed for high-temperature applications, they are not able to meet both the high-temperature strength and the high temperature corrosion resistance simultaneously in some of the most promising molten salts. To increase the lifetime operation of the CSP system, it is important to prevent the corrosion of the superalloys. One of the most commonly used methods of retarding corrosion and extending the life of structures is cathodic protection, and this method was investigated herein. Cathodic protection is a well-known method for preventing alloy corrosion in aqueous solutions by shifting the cathodic alloy potential to the previous alloy corrosion potential, either by using impressed current or by using a sacrificial anode.5 However, cathodic protection has little documented experience in high-temperature molten salt systems outside ...

No abstract

The performance of Pt–Ru catalysts for methanol electrooxidation has been greatly enhanced by replacing the standard carbon XC72 support with oxygen-functionalized multiwalled carbon nanotubes (MWCNTs). Highly dispersed, intimately contacted Pt–Ru nanoparticles were synthesized on MWNT supports by a combination of strong electrostatic adsorption (SEA) and electroless deposition (ED) methods. The catalysts have been characterized by X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), chemisorption, and X-ray photoelectron spectroscopy (XPS) and evaluated by cyclic voltammetry (CV) for the methanol electrooxidation reaction. The results showed that oxygen-functionalized MWCNTs not only influenced the chemical nature and morphology of the surfaces relative to XC72 but also enhanced the electrocatalytic properties of the resulting Pt and Pt–Ru electrocatalysts. STEM images revealed homogeneous dispersion of uniformly sized nanoparticles (NPs) for the two types of functionalized MWCNTs with relatively high particle density and no notable aggregation. The results also showed that the activities of Pt–Ru on functionalized MWCNT catalysts prepared by SEA and ED for methanol oxidation were much higher than those for commercial catalysts. The activities of −OH- and −COOH-terminated Pt–Ru/MWCNT-OH and Pt–Ru/MWCNT-COOH catalysts for methanol oxidation were up to 7 times higher than that for commercial Pt/XC72 and up to 4 times higher than that for a commercial Pt–Ru/XC72 catalyst with a 1:1 = Pt/Ru bulk atomic ratio.