Characterization of melanin
FTIR test is used to identify the chemical structure of melanin, the FITR spectra result is shown in Fig. 1a. The strong resonance absorption peak at 3404 cm− 1 corresponds to the N-H stretching vibration of indole and O-H group, indicating that it has a typical melanin indole structure. A peak at 2922 cm− 1 corresponds to the stretching vibration of aliphatic C-H groups, shows that cuttlefish melanin has aromatic hydrocarbon structure. This weak peak at 2922 cm− 1 is one of the characteristics of cuttlefish melanin FTIR spectra. The characteristic strong band at 1616 cm− 1 is attributed to the C = O stretching vibration and the C = C vibration of the aromatic ring or the symmetric stretching of the COO- group. This band together with the strong absorption peak at 3400 cm− 1 indicates the existence of COOH structure. Therefore, it can be concluded that squid black pigments contain specific binding sites (O-H, N-H and COOH) of melanin. The UV-visible absorption spectrum (Fig. 1b) indicates that the sample melanin image has no obvious chromogenic peak and has monotonic broad-band absorbance in the ultraviolet to visible wavelength range.
Chemical structure of CMAs and CMPCMs
XPS was used to analysis the element composition of CPCM, CMPCM-2 and CMPCM-4. Compared with the pure CPCM sample, the spectrum of CMPCM clearly shows the presence of nitrogen (Table 1). XPS spectrums obtained from pure CPCM samples (Fig. 2a) showed signals near 532eV and 286.0eV, indicating the presence of oxygen and carbon, respectively. There is also a weak peak at 400eV, which is presumed to be nitrogen-containing impurities. The XPS spectrums of CMPCM-2 (Fig. 2b) and CMPCM-4 (Fig. 2c) also showed strong peaks around 532eV and 286.0eV. A new peak appeared on the right side of the impurity peak of 400eV, and the new peak was more obvious with the increase of melanin content. This indicates that CMPCM is composed of carbon, nitrogen and oxygen, and nitrogen is introduced by the doping of melanin. The analysis of XPS data showed that the addition of melanin increased the nitrogen content of the sample, which was caused by the nitrogen-containing functional groups of melanin.
As shown in Fig. 2d, the high-resolution XPS C 1s spectrum of CPCM shows that the carbon peak consists of C-C (284.8eV), C-O (286.0eV) and C = O (288.0eV). The high-resolution XPS C 1s spectra of CMPCM-2 (Fig. 2e) and CMPCM-4 (Fig. 2f) showed that the signal of carbon peak was contributed by C-C (284.8eV/284.7eV), C-O (286.4eV/286.0eV), C = O (287.8eV/288.5eV) and C-N/N-H (286.2eV). The high resolution XPS N 1 spectra showed that both CMPCM-2 (Fig. 2g) and CMPCM-4 (Fig. 2h) contained N-H (399.6eV) and –NH+- (400.3eV) functional groups. The high resolution XPS O 1s spectrum of CMPCM-4 (Fig. 2i) corresponds to C-O (532.2 eV) and C = O (532.8eV), respectively. The results of XPS further demonstrated that the in-situ polymerization of melanin and CPCM was successful.
Table 1
XPS analysis of the CPCM and CMPCMs
Element | CPCM | CMPCM-2 | CMPMCM-4 |
C (%) | 40.8 | 53.21 | 53.04 |
O (%) | 57.54 | 43.69 | 40.39 |
N (%) | 1.65 | 3.11 | 6.57 |
Morphology of CMAs and CMPCMs
The microstructures of CA, CMAs, CPCM and CMPCMs were studied by SEM, as presented in Fig. 3. CA (Fig. 3a) is porous under microscope, most of the pore sizes are in the range of a few microns to a few hundred microns. The structure of pore wall is nano sheet, and the nano sheet has a certain sense of hierarchy. CMAs (Fig. 3b is CMA-2 and Fig. 3c is CMA-4) are also porous under the microscope, and the pore size is mostly in the range of a few microns to a few hundred microns. The main framework is still CA structure. The only difference is that there are irregular protrusions on the pore wall structure, which are formed by the addition of melanin.
The composite materials CPCM (Fig. 3d) and CMPCMs (Fig. 3e is CMPCM-2 and Fig. 3f is CMPCM-4) are blocky structures under the microscope. PEG fills the whole pore structure and forms blocky structure. It still can see the CA framework in some angle sections (Fig. 3d and Fig. 3f), which is enough to support the judgment. The formation of blocky structure indicates that PEG and CA and CMAs have good compatibility. The porous structure and good compatibility can make the low-density aerogel encapsulate a large number of PEG and maintain a higher energy storage density.
Shape stability of CMPCMs
Shape ability is a very important property of composite phase change materials, which often plays a decisive role in the practical application of such materials. When the temperature is higher than the phase transition temperature, most organic PCMs will show fluidity and leak out from the composites, which is the leakage problem of organic phase change materials. To solve this problem, inorganic or organic carrier materials are often used to encapsulate organic PCMs, so it is necessary to measure the encapsulation ability of the materials. The encapsulation ability can be judged by detecting the leakage of materials at different time in the phase transformation stage. For CPCM and CMPCMs, the leakage material in the phase transformation stage is PEG. In this experiment, the phase transition temperature is 80 ℃, and the conditions of 0 min, 10 min, 20 min and 30 min are recorded. The comparison of test results is shown in Fig. 4.
At 25 ℃, PEG, CPCM and CMPCM-4 were smooth white opaque solid, rough white opaque solid and rough black opaque solid, respectively. After heating at 80 ℃ for 10 minutes, PEG began to melt, and the filter paper was wet. CPCM and CMPCM-4 did not change significantly, and the filter paper remained dry. After heating at 80 ℃ for 20 minutes, PEG melts obviously and cannot keep its original shape, the filter paper is wetter. CPCM and CMPCM-4 began to leakage some liquid, and the filter paper was slightly wet. After heating at 80 ℃ for 30 minutes, a large amount of PEG melts and becomes semi solid and semi liquid, and liquid begins to accumulate on the filter paper. CPCM and CMPCM-4 slightly leakage, the morphology remained unchanged, the filter paper is slightly wet. In general, the difference between CPCM and CMPCMs is not obvious, but compared with pure PEG, it has stronger thermal stability and better encapsulation ability.
Thermal properties of CMPCMs
Thermal properties of materials are the different thermal physical properties of materials in the process of using at different temperatures and conditions. Heat capacity is one of the most important thermal properties of phase change energy storage materials. Differential scanning calorimetry (DSC) can measure the specific heat capacity and related thermal properties of samples by measuring and analyzing the data of heat flow corresponding to time or temperature input to the samples. The heat energy storage ability has a very strong connection between phase change properties. Therefore, in order to quantitatively evaluate the effect of the added melanin particles on the PCM phase change enthalpy, DSC analysis was used. Processed the result with software, the melting enthalpy (ΔHm), crystallization enthalpy (ΔHc), melting and crystallization point (Tm and Tc) were collected in Table 2, and the DSC curves of pure PEG, CPCM and CMPCMs were presented in Fig. 5. As expected, a separate and obvious peak could be observed in both heating and cooling processes for all samples. Moreover, the DSC curves exhibited the endothermic peaks of CPCM and CMPCMs in the range of 66.3–69.8 ℃ and the exothermic peaks in the range of 34.7–38.1 ℃, which were almost the same as pure PEG. The reason is that the melanin particles are only dispersed in the aerogel, and there is no chemical reaction between the PEG and the melanin particles, so the melanin particles will not affect the heat absorption capacity of the PEG. But with the increase of melanin content, the peak of crystallization slightly moves to low temperature, and the peak of melting slightly moves to high temperature. This arises because other substance restricted arrangement of PEG molecules and effect their phase transition process. This effect will hinder the crystallization and melting process of the PEG.
Table 2 Thermal storage property of PEG, CPCM and CMPCMs |
Sample | Tm (℃) | ΔHm (J/g) | Tc (℃) | ΔHc (J/g) |
PEG | 65.9 | 197.6 | 33.9 | 178.4 |
CPCM | 69.8 | 176.3 | 34.7 | 165.9 |
CMPCM-1 | 69.1 | 175.9 | 37.2 | 164.6 |
CMPCM-2 | 66.3 | 173.6 | 36.0 | 163.5 |
CMPCM-3 | 68.0 | 167.1 | 37.6 | 160.1 |
CMPCM-4 | 67.9 | 168.3 | 38.1 | 161.8 |
As shown in Table 2, the ΔHm of pure PEG, CPCM and CMPCMs were 197.6 J/g, 176.3 J/g, 175.9 J/g, 173.6 J/g, 167.1 J/g, 168.3 J/g, respectively and ΔHc were 178.4 J/g, 165.9 J/g, 164.6 J/g, 163.5 J/g, 160.1 J/g, 161.8, respectively. It can be found that melting enthalpy of CPCM is about 11% lower than that of PEG and crystallization enthalpy of CPCM is about 7% lower than of PEG, because CNF and other substances will replace a certain amount of PEG with the same quality, and the overall enthalpy is reduced. With the increase of melanin content, the enthalpy of CMPCMs decreases gradually, which indicates that melanin can also replace PEG, which has a negative impact on the overall energy storage properties of the sample. This also shows that the porous structure of CNF aerogel has good bearing capacity for PEG.
Thermal reliability can reflect the performance changes of materials in long-term use. For phase change energy storage materials, their energy storage performance will generally change with the increase of use times, and the change of performance determines how to apply the materials. The thermal energy storage properties of the sample after 100 times of heating and cooling from 0 ℃ to 80 ℃ are shown in the Table 3. The melting point and crystallization point of CPCM and CPMCMs samples had negligible changes after 100 repeated thermal cycles. Compared with samples before experiment, the melting point and crystallization point of CPCM and CPMCMs samples decreased by about 2% respectively. The results of accelerated thermal cycling measurement show that CPCM and CPMCMs have excellent thermal reliability.
Table 3
Thermal storage property of CPCM and CMPCMs after thermal cycling
Sample | Tm (℃) | ΔHm (J/g) | Tc (℃) | ΔHc (J/g) |
CPCM | 69.8 | 171.9 | 34.6 | 162.1 |
CMPCM-1 | 69.2 | 172.8 | 37.2 | 162.2 |
CMPCM-2 | 66.4 | 169.6 | 36.1 | 160.4 |
CMPCM-3 | 68.0 | 165.1 | 37.5 | 157.8 |
CMPCM-4 | 67.9 | 165.7 | 38.0 | 158.9 |
Thermal stability of CMPCMs
Thermal stability refers to ability of a material to remain stable or resist thermal shock at a certain temperature. Thermogravimetry (TG) is one kind of thermal analysis method to measure the relationship between the mass of sample and temperature under temperature-control program. By thermogravimetry, the stability of the samples under different temperature conditions can be summarized, the temperature range suitable for the normal operation of different samples can be found, and the change of internal structure of different samples can be inferred.
Figure 6 displays the TG graphs of CPCM and CMPCMs. The TG curves of CPCM and CMPCMs are similar, almost all the curves have the characteristics of one-step thermal decomposition, and their detailed Tmax (maximum weight loss rate temperature), T− 50 wt% (50 wt% weight loss temperature) and char residue at 650°C are summarised in Table 4. The decomposition temperature is between 350 ℃ and 420 ℃. It shows that melanin has no chemical reaction with PEG, and PEG will volatilize in this temperature range. The maximum mass loss rate is about 392 ℃. With the increase of melanin, the curve will gradually slope downward at 150 ℃, which means melanin will volatilize from 150℃ to 350℃. The residual weight of each sample is different, which shows melanin can affect the structure of the material, but the distribution of the influence is not uniform. With different structure, the sample will have different residual weight.
Table 4
Thermal stability properties of the CPCM and CMPCMs
Sample | T− 50 wt% (℃) | Tmax (℃) | char yield at 650 ℃ (wt%) |
CPCM | 395.8 | 422.3 | 2.71 |
CMPCM-1 | 393.2 | 418.6 | 13.86 |
CMPCM-2 | 390.2 | 423.8 | 0.57 |
CMPCM-3 | 391.3 | 417.8 | 7.54 |
CMPCM-4 | 391.4 | 418.7 | 9.69 |
Solar-thermal conversion and storage of CMPCMs
Photothermal conversion efficiency reflects the material's ability to use sunlight, which is an important property of photothermal materials. The higher the photothermal conversion efficiency is, the better the thermal storage performance of the composite is, and the practical application is more extensive. In order to test the photothermal conversion and thermal storage capacity of CMPCMs under practical conditions, we carried out simulated sunlight experiments on CPCM and CMPCMs. A cylindrical sample with a diameter of 3cm and a height of 1cm is placed in an insulating foam box and exposed directly to the 250 mW cm− 2 irradiance simulator. Thermocouples are used to measure the temperature of CMPCM over time. A data recorder is connected to collect the temperature of CMPCMs. As shown in scheme 2.
Figure 7 shows the temperature-time curve of CMPCMs under illumination. The temperature of the sample rises gradually from the time of irradiating the sample. When the temperature of each sample rises to the range of 50–55 ℃, the heating curve begins to slow down, and the heating rate drops sharply. This change is due to the endothermic process of PEG phase transition in CMPCMs. After the phase transition, the temperature of the sample continues to rise, and when the measured temperature reaches 80 ℃, the illumination was stopped immediately. After illumination stopped, the actual sample temperature continued to rise to 83 ℃, and then began to cool down. During the cooling process, the solidification temperature of PEG is about 50 ℃. From the image, we can see that with the increase of the mass ratio of CNF and melanin, the phase transition time becomes shorter.
After calculation, the photothermal conversion and storage efficiency of CPCM, CMPCM-1, CMPCM-2, CMPCM-3, and CMPCM-4 are 47.2%, 61.4%, 70.8%, 78.6% and 85.9% respectively. The results show that CPCM after adding melanin can obtain the photothermal properties of melanin, and the photothermal conversion efficiency increases with the increase of the proportion of melanin. Compared with CPCM, the photothermal conversion rates of MPCM-1, MPCM-2, MCM-3, and MPCM-4 are increased by 30.1%, 50.0%, 66.5% and 82.0%, respectively, which can show that melanin can greatly improve the photothermal properties of CPCM and optimize it to a phase change material with high photothermal properties and high energy storage density.