Effects of Metal Dopings on CuCr2O4 Pigment for Use in

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Effects of Metal Dopings on CuCrO Pigment for Use in Concentrated Solar Power Solar Selective Coatings Yongjoon Youn, John Miller, Kathy Nwe, Kyung-Jun Hwang, Chulmin Choi, Youngjin Kim, and Sungho Jin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01976 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Energy Materials

Effects of Metal Dopings on CuCr2O4 Pigment for Use in Concentrated Solar Power Solar Selective Coatings Yongjoon Youn†,1, John Miller†,1, Kathy Nwe†,1, Kyung-Jun Hwang†, Chulmin Choi†, Youngjin Kim†,*, and Sungho Jin†,‡,* †NanoSD,

Inc, 11575 Sorrento Valley Rd, Suite 211, San Diego, CA 92121, United States of California San Diego, 9500 Gilman Dr., La Jolla, CA 92093, United States

‡University

ABSTRACT The process of fabricating the solar absorbing spinel-structured CuCr2O4 black oxide pigment doped with different metals (Mn, Ni, Co, Al, Zn, and Sn) was discovered to improve the solar selective property. Manganese stands out as an ideal dopant to Copper Chromite (CuCr2O4) for highly solar absorptive pigments compared to the other metal dopings. XRD analysis confirmed that various molar ratio of CuCr(2-x)MnxO4 spinel black oxides were successfully fabricated. Mndoped black oxide has the highest absorptivity (the lowest band gap value of 1.35 eV) among other metal doping black oxides produced by the hydrothermal synthesis. Manganese is the only dopant that suppresses reflectance peaks exhibited by the copper chromium oxide at 1 µm and 1.5 µm in the light spectrum, raising the solar absorptivity of the pigment. Different manganese doping compositions are introduced to CuCr(2-x)MnxO4 where x = 0.1, 0.25, 0.5, 1, 1.5, 1.75, and 1.9. The high selective solar absorptivity appears after 100% (x=1) manganese doping with solar absorbance 0.9874 and a Figure of Merit (FOM) value of 0.9284. Keywords: Solar selective coating; Solar-absorbing pigment; Concentrated Solar Power; Black paint; Spinel black oxide.

1These

authors contributed equally to this work.

ACS Paragon Plus Environment

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1. INTRODUCTION A Concentrated Solar Power (CSP) system utilizes an array of mirrors which redirect solar rays to a receiver so that the thermal energy from heat transfer material is converted to electrical energy that can be used in the residential area (1). One of the benefits of CSP over Photovoltaic is that the heated transfer material such as, molten salt which is generated during the day can be stored and used at night when there is no solar radiation (2). However, a significant concern for CSP is a high cost of energy generation. In 2015, the cost of production of CSP was $0.12/kW, with a DOE stated goal to decrease to $0.06/kW by 2020 (3). In order to decrease the generation cost, the energy efficiency must be improved. In the industry of CSP plants, many technology gaps could be filled in to increase the capability of the CSP systems. Those gaps are categorized into four main components: the collector, the receiver, the power block, and the thermal storage unit (4, 5). The collector part where the sunlight is being reflected to another point includes the structural design and dynamics of the mirror. This mirror form can be anything from trough, heliostat, linear Fresnel and to dish systems (4, 6). The receiver, which collects all the solar heat energy, covers the design of the receiver, optical properties of selective coating and stability of heat transfer material (6). The power block includes turbomachinery, power conversion, and cycle techniques (5). Finally, the thermal storage system, which stores excess energy generated during the day for use at night, has topics of corrosion, heat transfer materials and temperature stability (5). Among these categories, our work only focuses on the exterior coating of the receiver which can bring higher efficiency in energy collection for the central tower CSP system. Regarding a solar selective coating for the receiver, high thermal stability of materials has to be considered because it is necessary for the next generation of CSP plants to operate at over 750 °C to increase the efficiency of the plant (7). Other essential properties of the selective coating are a high absorptivity to maximize the energy transferred to the heat exchange fluid and a low emissivity to minimize the heat loss through convection and radiation (8). The solar absorber coating can be critical for plant operation because it can reduce the levelized cost of energy (LCOE) by around 12% compared to the uncoated receiver (9). There are six types of solar absorber coating: intrinsic absorber, semiconductor-metal tandems, multilayer absorbers, metal-dielectric composite, surface texturing and blackbody-like absorber (10). Some application methods for these solar absorber coatings included spin coating, thermal spray, electrodeposited black metals and sputtered ceramic/metals with metals embedded in a dielectric matrix which all have either high costs of deposition on a sixty feet central receiver or impossible to do on a large scale (10-12). Solution based one-layer coating with spectrally selective pigments is the most straightforward and most cost-effective method for depositing on a central receiver (12). For the solar selective pigments, spinel structured ceramic/metal nanoparticles are chosen as excellent material. These spinels have the formula of AB2O4, where A and B are typically transition metals with partially filled d orbitals such as manganese, cobalt, iron, and chromium. Materials with this crystal structure are well known for its temperature durability and oxidation stability (13-15). The property of spinel structures can be improved by doping with other transition metals (16, 17). There have been many developments for selective solar absorber starting from the 1950s of CuO to W-SiO2 to this date (18). Some famous examples for selective solar absorbers are CuCr2O4 and CuFeMnO4, the latter being considered a ternary spinel (17, 18). These spinel


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ceramic oxides can be synthesized by the facile hydrothermal method followed by thermal decomposition (19). To date, Pyromark 2500 paint is the most common coating for CSP receivers. According to the safety data sheet, the pigment of Pyromark 2500 paint is manganese ferrite (20). One of the problems with this paint, however, is its high emissivity (8, 21). It will absorb a high percentage of light, but due to the high emissivity, the energy it absorbs will be lost to the environment quickly. Another problem with Pyromark 2500 paint is temperature stability. According to Sandia National Labs (2013), Pyromark 2500 paint shows thermal degradation after 300 hours of high-temperature exposure (21). For higher efficiency and lower cost of CSP plants, a solar absorber coating must be developed with lower emissivity and higher temperature stability. In this work, we focus on improving the optical properties of the spinel CuCr2O4. Compared with the Pyromark 2500’s pigment, manganese ferrite, the copper chromite pigment is well known for its durability and temperature stability (22). One of the problems with copper chromite is a high reflectance peak at the 1 µm and 1.5 µm peak in the spectrum, which lowers the absorptivity of the pigment since the solar spectrum is mainly in the lower wavelength region (23). Considering the solar spectrum and blackbody emission spectra at 800 °C, the cut-off wavelength for the pigment should be around 2 µm (7, 18). Below 2 µm, the reflectance has to be as low as possible, and after 2 µm it has to be as high as possible. The absorbance in UV to NIR range is critical in determining the efficiency compared to emittance. Therefore, it is highly desirable to suppress 1 µm and 1.5 µm reflective peaks. One way to accomplish this feat is through elemental doping and optimizing the ratio of transition metals in the ceramic structure (16, 17, 24). Adding new metal will form double spinel structure which can suppress those reflective peaks and decrease solar reflectivity in the UV-VIS-NIR range. We also examine the effects of metal doping on CuCr2O4 and its ability to absorb solar radiation for CSP.

2. EXPERIMENTAL 2.1. Synthesis of Black Oxide Nanoparticles Spinel black oxide nanoparticles were synthesized by co-precipitation and the hydrothermal synthetic process. Equimolar ratios of CuCl2-2H2O, CrCl3-6H2O, and additional metal chloride precursors were dissolved in distilled water and mixed mechanically. Metal chlorides used experimentally include AlCl3-6H2O, MnCl2-4H2O, NiCl2-6H2O, CoCl2-6H2O, ZnCl2, and SnCl2 (Sigma Aldrich Co., Ltd., USA). Dissolved sodium hydroxide pellets in an equimolar amount to all the metal precursors was slowly titrated to stabilize the solution pH and precipitate out a metal hydroxide. After titration and mechanical mixing, the solution was transferred to Teflon tubes inside of autoclaves and into the oven at 200 °C for 20 hours. After 20 hours, the gel was removed from the autoclave, rinsed, and centrifuged a total of 5 times to remove any contaminants. The gel was then moved to a furnace to bake at 110 °C for 1 hour and sinter at 550 °C for 5 hours. Spinel ceramic is then transferred to a mortar and pestle where it is ground down into nanoparticles (19). For the optically high performing element Mn, the atomic ratio of Cu, Cr, and Mn was varied by the following equation, CuCr(2-x)MnxO4, where x=0.1, 0.25, 0.5, 1, 1.5, 1.75, and 1.9 (25). The method of synthesis for these ratios is the same as the method of synthesis for the equimolar ratios stated above. 2.2. Characterization of Black Oxide Nanoparticles


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All the different spinel black oxide nanoparticle composition structure was identified using FEI Apreo (Thermo Fisher Scientific, United States) and Quanta FEG 250 scanning electron microscopy (SEM; Thermo Fisher Scientific, United States). To do the further examination on each of the different spinel structure, X-ray diffraction analysis was operated by D8 ADVANCE (Bruker Co., Ltd., USA) with the radiation wavelength of 1.54058 Å. The scanning parameter for 2θ angle was from 10° to 80° and the rate was 0.05º/step. The optical absorbance and reflectance data were carried out by UV-Vis-NIR spectroscopy and FT-IR spectroscopy (UV-3600 and IRTracer-100, Shimadzu Co., Ltd., Japan) with the integrating sphere accessories attached (ISR603, Shimadzu Co., Ltd., Japan and PN 048-10XX, PIKE Technologies, Inc., USA). The UV-3600 could get wavelength from 220 nm to 2600 nm. Rest of the spectrum through 25 µm was obtained from IRTracer-100. When powders were being measured, the same amounts were pressed down into the sample holder so that the surface was uniformly flat. 2.3. Performance Evaluation of Figure of Merit for Black Nanoparticles In order to evaluate the performance of a spinel black pigment, a Figure of Merit (FOM) value must be calculated (19). The Figure of Merit weighs the solar absorptivity and thermal emissivity of a sample against the intensity of the entire solar spectra. According to Kirchoff’s Law of Radiation, for a black body absorber in equilibrium, the solar absorptivity equals thermal emissivity (26). The sum of all the transmittance, absorbance, and reflectance of material has to equal 1, which is 100% of light. For opaque objects, there is no transmittance, so the absorptivity, which equals the emissivity, should be 1 minus the reflectivity. With this information, an equation for the FOM can be calculated by utilizing reflective profiles in the UV, visible, near-IR, and IR light ranges, ∞

𝐹𝑂𝑀 =

∞

∫0 (1 ― 𝑅(𝜆))𝐼(𝜆)𝑑𝜆 ― 𝐶 ―1∫0 (1 ― 𝑅(𝜆))𝐵(𝜆,𝑇)𝑑𝜆 ∞

∫0 𝐼(𝜆)𝑑𝑦

(Eq. 1)

where λ is the wavelength of light (nm), R(λ) is the reflectance at wavelength λ, I(λ) is the solar radiation at wavelength λ (ASTM G173), B(λ,T) is the blackbody emissivity at temperature T and wavelength λ, and C is the solar concentration ratio (27). The next generation CSP plants will operate at 800 °C at 1,300 suns, so all calculations were done at T=1,073.15 K and C=1,300. Reflectance profiles were measured between wavelengths of 220 nm and 25,000 nm.

3. DISCUSSION AND RESULTS 3.1. Structure and Composition of Ternary Doped Spinel Crystals As stated earlier, the purpose is to dope CuCr2O4 with different atoms including not only transition metals such as cobalt, manganese, nickel, and zinc but also some other metal elements such as aluminum and tin to enhance the solar absorptivity in the UV-VIS-NIR region. XRD analysis of the samples with different element doping can be seen in Figure 1. To date, the XRD information of all other elements excluding CuCr2O4 and CuCrMnO4 has not been reported with high-quality marks. However, doped CuCr2O4 all share the main peak at a 2θ angle of ~35°, which is the main peak for spinel structure (28). Utilizing the Scherrer equation,


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𝐾𝜆

𝜏 = 𝛽 cos 𝜃

(Eq. 2)

where τ is the average crystal size, K is a shape factor which is 0.9 in this case, λ is the X-ray wavelength which is 0.154058 nm, and β is the Full Width at Half Maximum (FWHM), the crystal size of all the doped copper chromium oxides was calculated and shown in Table 1 (29). The crystal sizes for samples doped with Mn, Ni, Co, Al, Zn, and Sn were calculated to be 38, 52, 34, 32, 38, and 86 nm, respectively. These crystal sizes had no trend with the optical result in Table 2 and Figure 2. From the XRD patterns, the Ni and Sn-doped samples have the largest number of XRD peaks. This emergence in peaks is probably due to a lack of solubility of those materials in the chromite spinel structure, which means the Ni and Sn could not replace the Cr in the crystal lattice (30). Out of all the doped samples, Mn has the fewest intensity peaks, meaning it has the highest degree of Cr substitution compared to all the other dopants. Even though the Mn precursor used was divalent, Mn changes its oxidation state to trivalent at temperatures above 500 °C (31). The trivalent oxidation state best suits the spinel crystal structure. All the SEM images for different doping of materials can be seen in Figure 2. A spinel structure should have single crystal octahedral shape, but all the samples have various shapes (32). The Aldoped sample (CuCrAlO4) in Figure 2(a) has irregular rods and polyhedral shapes. The Sn-doped sample and other doped pigments except for CuCrMnO4 seem to form a flake structure with crystals embedded in those flakes. The original CuCr2O4 and CuCrMnO4 samples in Figure 2(g) and 2(f) respectively show the most contents of the octahedral shape which seem to be more stable than the other compositions. For the optical profiles of the hydrothermally produced nanoparticles in Figure 3 and Table 2, compared to CuCr2O4, Sn-doping accentuates those same peaks as well as generating new, large reflective peaks in the UV-Vis region of light. These peaks give a much lower absorbance value around 0.91 and therefore the lowest FOM of all the dopants around 0.86, nearly 4 percent lower than that of the original CuCr2O4. Ni-doping flattens the reflective profile at 1 µm and 1.5 µm respectively, but also lifts the overall reflectivity. This profile leads to a decrease in performance of around 2.5% from the Ni-doped to the CuCr2O4. Addition of Al into the spinel Cu-Cr structure also highlights the reflectivity peaks at 1 and 1.5 µm, as well as raising the reflectivity profile leaving a solar absorptivity of approximately 0.94 and FOM around 0.88. The Zn-doped sample had lower reflectivity in the UV-Vis range but experiences an increase in reflectivity at 750 nm. This increase brings down the absorptivity and therefore the FOM of the sample significantly to values of 0.9444 and 0.8906 respectively. It is still about 1.5 percent less than the CuCr2O4 standard. Similar to Zn doping, Co doping also has a much lower reflective profile than the CuCr2O4 sample in the UV-VIS range but experiences larger peaks at the 1 and 1.5 µm than the original CuCr2O4. Because of the higher UV-VIS light absorbance though, Co-doped sample has a slightly higher absorbance and lower FOM value of 0.9551 and 0.8990. The only sample able to eliminate the 1 µm and 1.5 µm peaks were the CuCr samples doped with Mn. Not only does Mn suppress these peaks, but the addition of Mn significantly lowers the reflectivity profile of the sample through the UV, VIS, and NIR regions of the light spectrum. The Cu-Cr-Mn sample has solar absorbance over 0.98 and a FOM of 0.9284, which is over 3 percent higher than that of CuCr2O4. The enhanced optical properties of the Mn-doped pigment identify the CuCrMnO4 nanoparticles to be an ideal solar absorbing pigment for high-temperature CSP applications.


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Using the optical profiles of these materials, the band gap energy (Eg) was calculated to check the effect of band gap energy versus reflectance. Among many other formulas (33), the Eg was calculated using Kubelka-Munk method. The equation is as follows:

[

(1 ― 𝑅)2 2𝑅

∗ ℎ𝑣]𝑛

(Eq. 3)

where R is the reflectance, h is Planck’s constant, v is the light frequency and n varies for sample transition (1/2, 3/2, 2 and 3) during the experiment (33). When this equation is plotted against hv in the form of eV, the band gap energy can be calculated using the Tauc plot method (34). From the allowed transitions which are ½ and 2, n value was chosen to be 2 because it had the best linear region in Tauc plot. In Figure 4, the graph shows each of the different materials. The slopes for each graph were extrapolated and x-axis intercept was then calculated. The slope of the graph was chosen as the first curve when multiple curves were present on the Tauc plot. This x-axis intercept by the slope line represents the band gap energy. Inside the parentheses of the graph, Eg can be found. All of the band gap values were over 3.0 eV except CuCrCoO4 and CuCrMnO4. The Co-doped samples optical bandgap can be attributed to its higher absorptivity in the UV-Vis range, even with higher NIR reflectance. The Mn-doped CuCr2O4 experiences much higher optical absorbance in the UVVis-NIR region of light which can be described by its much lower band gap value of 1.35 eV because the lower band gap materials have higher absorption of energy above the band gap (35, 36). 3.2. Structure and Composition of Cu-Cr-Mn-O4 Crystals Upon finding of Mn as an ideal dopant for solar absorber coatings, the Cu-Cr-Mn crystal structure was altered for optimum performance. Figures 5 and 6 show the XRD and SEM results for different compositions of the CuCr(2-x)MnxO4 Spinel where x is 0.1, 0.25, 0.5, 1, 1.5, 1.75, and 1.9. SEM could not make out large differences in the crystal structures of the CuCr(2-x)MnxO4. However, after x=1, the higher ratio of Mn sample had various irregular shapes, which can be seen in Figure 6(d), unlike the lower ratio samples. XRD analysis shows similar peaks for all samples of Mn-doped CuCr2O4. The main peak at 2θ= 35° increases in intensity as the percentage of Mn increases, until reaching around x=1.5 (CuCr0.5Mn1.5O4). For higher percentages of doping, an additional peak near 2θ= 33° begins to emerge and increase in its intensity as the molar ratio of Mn approaches to x=1.9 (CuCr0.1Mn1.9O4). Addition of Mn also causes all peaks at a 2θ value greater than 50° to intensify. Note that the main peak shits slightly towards the higher angle as the Mn doping concentration increases. The peak shift can be explained by the lattice distortion of the structure and ionic radius (36). In Figure 7, which shows the change in lattice parameter as a function of Mn doping concentration, the main plane miller indices shift from (2 1 0) to (3 1 1) as the percentage of Mn increases, indicating a change in crystal structure from body-centered to face-centered which matches with the face-centered structure in ICDD PDF #24-0355. However the x=0.1 Mn-doped sample (CuCr1.9Mn0.1O4) had a much larger lattice parameter and a different main plane miller indices (3 2 1) according to calculation. This does not follow the trend of Mn doping on the CuCr2O4 structure from our experiment, so it was excluded from the graph in Figure 7. This may be because the Mn2+ precursor did not oxidize in the sintering step to Mn3+. At temperatures greater than 500℃, Mn changes its oxidation state from II to III (31), and the Mn3+ ion substitutes for the


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chromium in the spinel crystal. The Mn3+ has an ionic radius of 0.070 nm and Cr3+ has an ionic radius of 0.064 nm (37). Mn2+ has an ionic radius of 0.091 nm (37), much larger than Cr3+ or Mn3+, and the valency is not favorable for substituting in Cr. So, CuCr1.9Mn0.1O4 was determined to be an outlier data in the Figure 7. This lattice parameter data also parallels with the crystal size calculation by the Scherrer equation (29) in Table 3. Mn doping when x=0.1 has larger crystal size as 42.80 nm than most of the other Mn doping ratio. Since there is increment in lattice parameter, more Mn doping into the lattice structure made the size of the crystal unit size continually expand roughly from 20 nm to 50 nm, as well as the increment of Bragg’s Angle (38, 39). This proves that the Mn has doped successfully to the original CuCr2O4. Samples with Mn doping higher than x=1 formed much larger crystals than those with less doping. The x=0.5 Mn-doped sample may have an even lower crystal size because of a higher degree of unit cell packing. Figure 8 and Table 4 shows the reflective profiles of different Mn dopings. From the original CuCr2O4 sample, reduction of the 1 µm and 1.5 µm peaks can be seen with as little as 10 percent of Mn doping, CuCr1.9Mn0.1O4. After doping up to x=0.5 of Mn, CuCr1.5Mn0.5O4, both peaks are completely suppressed. The optimal optical profile occurs with x=1 Mn doping, CuCr1.0Mn1.0O4. At this point, the pigment has the solar absorbance of 0.9874 and a FOM value of 0.9284 which is in Table 4. This corresponds to an approximate increase of 2.5% in solar absorbance and about 2.3% in FOM from the original spinel copper chromite. Increasing the x value in CuCr(2-x)MnxO4 over 1 does not appear to lower the reflectance in the UV to NIR region any further. In fact, more Mn doping increases the reflectivity profile by decreasing the absorbance and emissivity values which is made evident by a higher profile in the UV-VIS region of the spectrum, where a majority of the energy from the solar intensity is generated from. The increase in reflectivity spike for x=1.5, 1.75, and 1.9 leads to an absorptivity decrease between 0.6 and 1.4 percent. In any case, the addition of Mn from x=0.1 to 1.9 (CuCr1.9Mn0.1O4 ~ CuCr0.1Mn1.9O4) has the potential to increase the thermal efficiency of CuCr2O4 pigment anywhere from 0.5-1.5%. Remarkably high considering, we are approaching maximum values for solar absorptivity. The high optical performance of CuCrMnO4 pigment coupled with the high-temperature stability of the spinel crystal structure has a high potential for a solar absorbing coating in next generation CSP Plants.

4. CONCLUSION The spinel copper chromite was doped with different elements (Zn, Al, Sn, Ni, Co, and Mn) to improve the solar absorbance of the material by reducing the reflectivity profile and eliminating peaks at a wavelength of 1 and 1.5 µm. Since Mn was the only element to suppress the 1 and 1.5 µm peaks, the ratio of Mn to Cr was changed to determine the optimal recipe for solar absorption and reflective peak suppression. Experimentation determined that an equimolar ratio of Cu, Cr, and Mn yielded the best optical properties with a solar absorbance value of 0.9874 and a Figure of Merit value of 0.9284. The pigment based on spinel CuCrMnO4 approaches the high solar absorbance and FOM for a sample and would be a highly favorable material for next-generation solar absorber coatings. █ ACKNOWLEDGEMENT


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This research is supported by the Department of Energy through the DOE Sunshot-Apollo Project (DE-EE0007113). █ AUTHOR INFORMATION Corresponding authors *E-mail: [email protected] (Y. Kim); [email protected] (S. Jin)

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Effects of Metal Dopings on CuCr2O4 Pigment for Use in

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