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Original Article
ARTICLE IN PRESS
doi:
10.25259/AJC_538_2025

Application of polyaspartic acid-based porous carbon loaded with bimetals Cu and Ni in hydrogen adsorption

, , , , ,
 Department of School of Chemistry and Chemical Engineering, Tarim University, Xinjiang, Aral, China

*Corresponding author: E-mail address: zjb1102@outlook.com (J. Zhao)

Received: , Accepted: ,
© 2026 Arabian Journal of Chemistry - Published by Scientific Scholar
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This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

This study investigated the effect of simultaneous loading of bimetals nickel and copper on the hydrogen adsorption performance of polyaspartic acid (PASP)-based porous carbon materials. Four groups of materials with different metal ratios, namely PASP-based porous carbon loaded with bimetals copper and nickel, were prepared. A two-step calcination process was adopted. First, PASP-based porous carbon was prepared, and then the bimetal solution was impregnated, followed by a second calcination. Through characterizations and analyses using Fourier transform infrared (FTIR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), it was confirmed that copper and nickel were successfully loaded and uniformly dispersed on the PASP-based porous carbon, mainly functioning in the elemental form. The results of the hydrogen adsorption test showed that C-PASP-Ni6/Cu4 had the best adsorption effect, with a maximum hydrogen adsorption capacity of up to 240.87 cm3 g-1. Loading the bimetals copper and nickel can also significantly enhance the hydrogen storage capacity. Moreover, due to the relatively low price of copper, the cost is reduced while improving the performance.

Keywords

Polyaspartic acid
Porous carbon
Bimetal
Hydrogen adsorption

1. Introduction

With the gradual depletion of global energy resources and the increasing deterioration of the ecological environment, green and low-carbon development has become the mainstream direction of global development [1-3]. To achieve the goals of carbon peak and carbon neutrality, the development and utilization of renewable new energy sources are essential [4,5]. Among them, hydrogen energy plays an indispensable role in the field of new energy. It has an abundant source, and its product after use is only water, which greatly reduces the burden on the ecological environment. The utilization of hydrogen energy provides an important direction for achieving green, low-carbon, and sustainable development. To improve the utilization rate of hydrogen energy as a substitute for traditional fossil fuels, the storage of hydrogen has become a key issue. There are numerous hydrogen storage methods, and relevant reports have been published both at home and abroad [6,7]. Among them, physical adsorption hydrogen storage in solid-state hydrogen storage has become a research hotspot. Most of the materials for physical adsorption hydrogen storage are porous materials. Among them, porous carbon materials exhibit outstanding performance in gas storage due to their high specific surface area and the controllability of the carbonization process. To increase the active sites of porous carbon materials, loading metals is an effective method. Sometimes, the efficiency of loading only one metal can no longer meet the application requirements. It is often necessary to load bimetals or even multiple metals to supplement the efficiency [8-10]. The synergistic effect of bimetals can further improve the stability and activity of porous carbon materials. There is a saturation value and an optimal metal concentration when loading metals onto porous carbon. Attention should be paid to the concentration ratio of each metal during preparation. The rich pore structure of porous carbon also provides more sites for the loading of bimetals. During the loading process, the bimetals can be effectively loaded onto their respective fixed sites and are relatively evenly dispersed without easy agglomeration [11,12].

Common porous carbon materials include biomass carbon materials [13], nanocarbon materials [14], porous polymers [15], and metal-organic frameworks [16], etc. However, they have problems such as poor stability and high cost. Therefore, it is crucial to develop new, efficient hydrogen adsorption materials with practical application potential. In this study, polyaspartic acid hydrogel (PASP hydrogel) was mainly used as the precursor of porous carbon. PASP hydrogel is a new type of cross-linked material with a 3D network structure. It is non-toxic, harmless, and easily degradable, making it an environmentally friendly material. Moreover, the PASP hydrogel molecule contains a large number of functional groups, such as amino and carboxyl groups. After carbonization, some polar groups are retained on the surface of the porous carbon, which can generate a strong interaction with hydrogen molecules, enhancing the hydrogen adsorption effect and improving the hydrogen storage performance. These polar groups can also promote the diffusion and adsorption of hydrogen on the surface of the material [17-19]. This paper mainly studied a PASP-based porous carbon material loaded with bimetals copper and nickel. A series of characterization tests and hydrogen adsorption tests were carried out to explore the synergistic effect of different metal ratios, providing new ideas for the development of hydrogen storage materials.

2. Materials and Methods

2.1. Materials

Phosphoric acid (H3PO4, AR) was purchased from Yantai Shuangshuang Chemical Co., Ltd. Ethylene glycol diglycidyl ether (EGDE, AR) was purchased from Changzhou Runxiang Chemical Co., Ltd. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and copper nitrate trihydrate (Cu(NO3)2·3H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. Hydrochloric acid (HCl, AR) and potassium hydroxide (KOH, AR) were purchased from Tianjin Xinbotte Chemical Co., Ltd. Polysuccinimide (PSI, 99.5%) was provided by the Bioprocessing Laboratory of Beijing University of Chemical Technology.

2.2. Experimental work

2.2.1. Preparation of PASP hydrogels

A hydrogel with a cross-linking degree of 40% was prepared. A KOH solution with a concentration of 4.54 mol L-1 was used, and the volume of distilled water was 15 mL. In a balance, 6 g of PSI was weighed and poured into completely dissolved KOH solution. A magnetic stirrer bar was put in, and it was placed on a magnetic stirrer at room temperature for 1-2 h until dissolved. After complete dissolution, the pH of the solution was adjusted to 4.8 with phosphoric acid. Then, 2.109 g of the cross-linking agent ethylene glycol diglycidyl ether was added. It was placed in a constant-temperature magnetic water bath, the temperature was set to 50°C, and the rotation speed to 700-800 rpm. After stirring for about 5-7 h, the PASP hydrogel was obtained.

2.2.2. Preparation of PASP porous carbon

The prepared PASP hydrogel was cut into pieces and placed in a petri dish. They were placed in a freeze dryer for 48 h to dry. The dried hydrogel was crushed and sieved through a 200-mesh sieve to obtain PASP hydrogel powder. A portion of the PASP hydrogel powder was put in a magnetic boat and then into a tube furnace for calcination under the protection of nitrogen. The heating rate of the tube furnace was set to 5°C min-1, the carbonization temperature to 600°C, and the holding time was 1 h. The calcined PASP-based porous carbon was soaked in 0.5 mol L-1 hydrochloric acid overnight, and then washed with a large amount of distilled water until it was neutral. Finally, the obtained wet porous carbon was put into a vacuum drying oven and dried at 60°C for 24 h. Then, it was ground and sieved through a 60-mesh sieve to obtain C-PASP.

2.2.3. Preparation of PASP-based porous carbon loaded with bimetals nickel and copper

Four portions of 0.3 g of PASP-based porous carbon were weighed and placed in a 50 mL beaker. The mixed metal salt solutions were prepared with different concentrations. The specific preparation has been shown in Table 1. The porous carbon was immersed in the mixed metal salt solution, and the PASP-based porous carbon was fully impregnated through ultrasonic treatment and stirring. The excess filtrate was filtered out, and the wet PASP-based porous carbon was allowed to stand overnight. Then, it was put into a vacuum drying oven and dried at 104°C for 4 h. It was then put in a magnetic boat and a tube furnace for calcination under a nitrogen atmosphere. The heating rate was 5°C/min, the holding time was 2.5 h, and the holding temperature was 900°C. Then, it was switched to carbon dioxide and held for 30 min for reduction. After naturally cooling to room temperature, the porous carbon material was ground and sieved. The obtained PASP-based porous carbon materials are named C-PASP-Ni, C-PASP-Ni8/Cu2, C-PASP-Ni6/Cu4, and C-PASP-Ni4/Cu6. The technical route diagram of their preparation has been shown in Figure 1.

Table 1. C–PASP–Ni/Cu with different metal concentration ratios.
Sample Roasting temperature (°C) Cu/Ni (molar ratio) Metal loading amount (wt%)
Ni(NO3)2 Cu(NO3)2
C-PASP-Ni 900 0 10 0
C-PASP-Ni8/Cu2 900 0.25 8 2
C-PASP-Ni6/Cu4 900 0.67 6 4
C-PASP-Ni4/Cu6 900 1.50 4 6
The preparation technical route diagram of C–PASP–Ni/Cu.
Figure 1.
The preparation technical route diagram of C–PASP–Ni/Cu.

2.3. Characterization of C-PASP and C-PASP-Ni/Cu

In this study, a series of characterization tests were conducted on the C-PASP and C-PASP-Ni/Li materials. The samples were scanned within the wavenumber range of 400-4000 cm-1 using a fourier transform infrared spectrometer (FTIR), the Thermo Fisher Scientific Nicolet iS20 model from the United States. After performing gold sputtering treatment on the samples, their microscopic morphologies were observed using a scanning electron microscope (SEM), the ZEISS Sigma 300 model from Germany. The transmission electron microscope (TEM) test of the porous carbon materials was carried out using the FEI Talos F200X instrument from the United States. Samples were prepared with a molybdenum mesh, and the magnification was adjusted for the test. The X-ray diffraction (XRD) characterization test of the porous carbon materials was performed using the Rigaku SmartLab SE model instrument from Japan. With a copper target, the test was conducted at a scanning speed of 1° min-1 within the angular range of 10-90°. Under the conditions of a temperature of 77.3K and a pressure of 1.1 MPa, a nitrogen adsorption and desorption experiment was carried out using a specific surface area analysis tester from 3Flex Company to explore the specific surface area and pore structure characteristics of the porous carbon. Tests such as sample morphology shooting and energy spectrum mapping were performed using a ZEISS GeminiSEM 300 scanning electron microscope. When shooting the morphology, the acceleration voltage was set to 3 kV, and when shooting the energy spectrum mapping, the acceleration voltage was adjusted to 15 kV. The SE2 secondary electron detector was used to further analyze the elemental composition and content at different sites. The X-ray photoelectron spectroscopy (XPS) test of the porous carbon materials was carried out using the Thermo Scientific K-Alpha instrument from the United States. With Al Kα as the radiation source, the power was set at 500W, the target voltage and target current were 15 kV and 10 mA respectively, the air pressure in the vacuum chamber was lower than 2×10⁻⁶ Pa, the transmission energy of the analyzer was 50 eV, the measurement step size was 0.1 eV, the sputtering rate was 0.2 nm s-1, and the sputtering area was 2 mm×2 mm.

2.4. Hydrogen adsorption by C-PASP and C-PASP-Ni

The multi-station full-function adsorption instrument of Micromeritics from the United States was used to test the porous carbon samples. High-purity hydrogen was selected as the adsorption medium, and the degassing treatment was carried out overnight at a temperature of 473 K under vacuum conditions. According to the hydrogen adsorption isotherm curve of the tested samples obtained at a low temperature (77K) and a pressure of 1.1 Mpa for H2, the hydrogen adsorption capacity of the porous carbon samples was obtained.

3. Results and Discussion

3.1. FT-IR testing of C-PASP, C-PASP-Ni and C-PASP-Ni/Cu

Figure 2 shows the infrared spectra of three groups of samples, namely C-PASP, C-PASP-Ni and C-PASP-Ni/Cu. The first curve in the figure corresponds to the C-PASP sample, which has a peak at 3500 cm-1, corresponding to the stretching vibration of N-H, indicating the presence of amide structure in the material. There is a weak peak at 2900 cm-1, which is formed by the stretching vibration of C-H in the porous carbon framework. In addition, there are two characteristic peaks at 1610 cm-1 and 1100 cm-1, corresponding to the stretching vibrations of C=O and C-O in the porous carbon material respectively, indicating the presence of carboxyl groups and ether bonds in the structure [20]. The presence of these functional groups may form hydrogen bonds with hydrogen, thus increasing the adsorption of hydrogen by the material. In the C-PASP-Ni material, the peak at 3500 cm-1 can be observed to become narrower and sharper, indicating that the N in the material also combines with the metal Ni to form a coordination effect. The peak at the position of C=O at 1600 cm-1 also becomes narrower and sharper, indicating that Ni also combines with the O in the material and exists in the form of metal oxide. In the infrared spectrum of the C-PASP-Ni/Cu material, the peak at 3500 cm-1 becomes even sharper, indicating that in addition to the metal nickel, the metal Cu also combines with the N in the structure. The absorption peak at 1610 cm-1 is weaker compared with that of C-PASP-Ni, indicating that part of the metal Ni combines with Cu to form an alloy structure [21,22]. The peak at 1100 cm-1 almost disappears, indicating that the bimetals cause the reduction of the surface of the porous carbon material, which further increases the active sites on the surface of the material. This makes it easier for the metal to form metal hydrides with hydrogen to enhance the adsorption of hydrogen. After adding the metals, the ways of hydrogen adsorption of the PASP porous carbon increase, and the hydrogen storage performance of the material is improved.

Infrared spectra of C - PASP, C - PASP – Ni, and C - PASP - Ni/Cu.
Figure 2.
Infrared spectra of C - PASP, C - PASP – Ni, and C - PASP - Ni/Cu.

3.2. Scanning electron microscopy (SEM) images of C-PASP-Ni and C-PASP-Ni/Cu

The PASP molecular chain contains many amino and carboxyl functional groups. During calcination, many chemical bonds break with a continuous rise in temperature, and decomposition occurs. The amino and carboxyl groups decompose to produce gases, such as carbon dioxide and water vapor, which escape from the inside to the outside of the material. Moreover, PASP molecular chains are arranged relatively regularly. Therefore, as the gas escapes outward, ordered and regular pores will be formed. The PASP hydrogel porous carbon has a cross-linked 3D network structure. In this complex structure, gas accumulates inside the material during the pyrolysis process, causing it to form many pores. At the same time, the 3D structure also increases the stability of the material, preventing excessive decomposition and collapse of the material as the temperature rises. Potassium hydroxide, an activator, is added during preparation, and carbon dioxide activation is also conducted in the later stage [23]. As an activator, potassium hydroxide will react with the PASP porous carbon during the calcination process to generate more active sites, further corroding the material to form microporous and mesoporous structures. After loading the nickel and copper, the pores of the PASP porous carbon material may be blocked or reduced. Then, carbon dioxide activation is helpful in increasing the material’s pore structure. Carbon dioxide activation is a physical activation process during which carbon dioxide is reduced to produce gas, which can create pores on the surface and inside of the material. Through the above ways of forming the pore structure, the PASP porous carbon material will form a more complex pore structure [24,25]. The rich pore structure helps to adsorb more gas and increase the subsequent hydrogen storage.

Figure 3 shows the SEM images of PASP porous carbon loaded with nickel and copper at different metal concentrations. It can be seen from the figure that the surfaces of the four samples are rough with wrinkles of different degrees, and there are also many pores of different sizes on the surface. The pores of all samples are small; there are no large pores with a large area, and the surface has not collapsed. This is because after the metal is loaded, the pore structure of the porous carbon material becomes smaller. Figure 3 (a) is the SEM image of PASP porous carbon loaded only with nickel. Figure 3(b) to (d) show PASP porous carbon loaded with nickel and copper at different metal concentrations. The pore structures of the three samples shown in the figures have little difference in size. Among them, the pores in Figure 3(c) are more uniform, and most of them are micropores and mesopores. Compared with Figures 3(b) to (d), the number of pores formed on its surface is less, and the pores are larger. Through calculation, the porosity of C-PASP-Ni is 0.36, and the porosities of C-PASP-Ni8/Cu2, C-PASP-Ni6/Cu4, and C-PASP-Ni4/Cu6 are 0.42, 0.43, and 0.41, respectively. The results of the porosity are basically consistent with the figures. The porosities of the four samples are appropriate, and most of them are micropores and mesopores, which is beneficial to the adsorption and storage of hydrogen, and the C-PASP-Ni6/Cu4 material has the best effect.

SEM images of (a) C-PASP-Ni, (b) C-PASP-Ni8/Cu2, (c) C-PASP-Ni6/Cu4, and (d) C-PASP-Ni4/Cu6.
Figure 3.
SEM images of (a) C-PASP-Ni, (b) C-PASP-Ni8/Cu2, (c) C-PASP-Ni6/Cu4, and (d) C-PASP-Ni4/Cu6.

3.3. TEM characterization of C-PASP-Ni and C-PASP-Ni/Cu

Figure 4 shows the TEM images of PASP porous carbon loaded with metal nickel and PASP porous carbon loaded with bimetals copper and nickel. Figures 4(a), (b), and (c) are the TEM images and particle size distribution diagrams of the materials loaded with bimetals nickel and copper, and Figures 4(d), (e), and (f) are the TEM images and particle size distribution diagrams of the materials loaded only with metal nickel. It can be seen from the figures that the metal particles are very evenly distributed, and there are no large particles or large-area agglomeration phenomena. Analyzing Figure 4(b), it can be known that the frequency of particles with a particle size between 8-10 nm and 10-12 nm is the lowest, which is 8%, and the distribution frequency between 12-14 nm is 20%. The number of metal particles between 14-16 nm and 16-18 nm is the largest, accounting for 25% of all particles, and the particle size distribution frequency between 18-20 nm is 14%. When the bimetals copper and nickel are loaded, the average particle size of the particles is 15 nm. In Figure 4(c), the lattice fringes of the metal particles formed by Cu and Ni can be observed. Through measurement and calculation, the lattice spacing d is obtained as 0.211 nm. Analyzing Figure 4(e), it can be known that the distribution frequencies of metal particles between 8-10 nm and 18-20 nm are the lowest, which are 7% and 3%, respectively. The distribution frequency is the highest at 10-12 nm, which is 32%, indicating that the average particle size of the metal particles is mostly in this range. The particle distribution frequencies between 12-14 nm, 14-16 nm, and 16-18 nm are 23%, 25%, and 10%, respectively. In Figure 4(f), the lattice fringe spacing of the metal Ni formed is also d = 0.202 nm. The lattice spacing corresponding to the (111) crystal plane of copper is 0.208 nm, and the lattice spacing corresponding to the (111) crystal plane of metal nickel is 0.202 nm; the (111) crystal planes are all in the form of metal nickel’s [26-28]. Thus, it can be concluded that when only metal nickel is loaded, the average particle size d is 11 nm. The difference in the average particle sizes of the two materials is not large, indicating that the metal particles of the PASP porous carbon material loaded with bimetals copper and nickel do not agglomerate and have a good dispersion degree. The average particle size is slightly larger than that of the PASP porous carbon material loaded only with single metal nickel because of the presence of a mixture of metal nickel and copper. This indicates that when only nickel is loaded, metallic nickel is present on the material surface. After loading both copper and nickel bimetals, metallic particles exist on the material surface in the form of either elemental metals or a mixture of the two metallic elements [29]. The good distribution of metal particles is very beneficial to the adjustment of the carbon skeleton and can further improve the hydrogen storage efficiency.

(a) TEM images of C-PASP-Ni / Cu. (b) is the particle size distribution of C-PASP-Ni/Cu. (c) Locally amplified TEM images of C-PASP-Ni/Cu. (d) TEM image of C-PASP-Ni. (e) The particle size distribution of C-PASP-Ni. (f) is a locally enlarged TEM image of C-PASP-Ni.
Figure 4.
(a) TEM images of C-PASP-Ni / Cu. (b) is the particle size distribution of C-PASP-Ni/Cu. (c) Locally amplified TEM images of C-PASP-Ni/Cu. (d) TEM image of C-PASP-Ni. (e) The particle size distribution of C-PASP-Ni. (f) is a locally enlarged TEM image of C-PASP-Ni.

3.4. XRD characterization of C-PASP-Ni and C-PASP-Ni/Cu

Figure 5(a) shows the XRD characterization patterns of C-PASP-Ni, 6 C-PASP-Ni4/Cu6 C-PAPS-Ni6/Cu4 and C-PASP-Ni8/Cu2. As shown in Figure 5(b) C-PASP-Ni also has diffraction peaks at the positions of 2θ=44.6° and 51.9°. It can be seen from the figure that all four groups of materials exhibit a relatively broad diffraction peak in the range of 2θ between 20° and 30°. This is the (002) [30] diffraction peak of carbon in the porous carbon framework, corresponding to graphitic amorphous carbon. The intensity of the diffraction peak of the materials loaded with bimetals copper and nickel is relatively weaker compared to that of the material loaded with only single metal nickel. This indicates that after adding an additional metal, copper, the degree of graphitization decreases and the disorder of carbon increases. At this time, new sites and micropores may be generated in the porous carbon framework, which will further enhance the hydrogen adsorption performance of the porous carbon material. C-PASP-Ni also has diffraction peaks at the positions of 2θ=44.6° and 51.9°. These two peaks correspond to the (111) and (200) [31] crystal planes of metallic nickel, respectively. This indicates that the metal mainly exists in the form of an element on the PASP porous carbon material. The peaks of C-PASP-Ni4/Cu6, C-PASP-Ni6/Cu4, and C-PASP-Ni8/Cu2 at 2θ = 44.6° and 51.9° shift slightly to the left collectively, and the intensity of the diffraction peaks gradually weakens as the concentration of added metallic copper increases. This indicates that after the addition of metallic copper, an alloy is formed with metallic nickel, so the corresponding diffraction peaks also change accordingly [26]. C-PASP-Ni4/Cu6 also has a tiny characteristic peak at the position of 2θ=36.7°, which corresponds to the diffraction peak of CuO [32,33], while the other two groups of PASP porous carbon materials with a lower concentration of loaded metallic copper do not show the diffraction peak of copper oxide. This indicates that when the loading amount of metallic copper is small, the metallic element has good dispersion characteristics on the surface of the porous carbon. The three groups of PASP porous carbon loaded with bimetals copper and nickel also exhibit a characteristic peak at 2θ=74.2°, which corresponds to the diffraction of metallic copper element [34,35]. This indicates that in the bimetallic porous carbon material, metallic copper mainly exists in the form of an element and as an alloy formed with nickel. The existence form of the element makes it easy to form metal hydrides with hydrogen, enabling the porous carbon material to have more sites for adsorbing hydrogen, which is more conducive to the storage of hydrogen by the material [36-38].

(a) XRD characterization of C-PASP-Ni and C-PASP-Ni/Cu. (b) The local enlarged XRD patterns of C-PASP-Ni and C-PASP-Ni/Cu at 2θ of 30-60° were obtained.
Figure 5.
(a) XRD characterization of C-PASP-Ni and C-PASP-Ni/Cu. (b) The local enlarged XRD patterns of C-PASP-Ni and C-PASP-Ni/Cu at 2θ of 30-60° were obtained.

3.5. BET characterization of C-PASP-Ni/Cu

The BET characterization was carried out on three groups of PASP porous carbon materials loaded with bimetals copper and nickel. The pore structure of the porous carbon material plays a very important role in hydrogen adsorption. Table 2 shows the specific surface area and pore size distribution of the three groups of materials. It can be seen from the table that the specific surface areas of C-PASP-Ni4/Cu6, C-PASP-Ni6/Cu4, and C-PASP-Ni8/Cu2 are 435.17 m2 g-1, 712.63 m2 g-1, and 500.68 m2 g-1, respectively. Among them, C-PASP-Ni6/Cu4 has the highest specific surface area, and its total pore volume is 0.45, which is also the largest among the three groups of materials, indicating that it forms the most pore structures. On the other hand, C-PASP-Ni4/Cu6 has the smallest specific surface area, and its total pore volume is 0.17, which is also the smallest, suggesting that the pore structure of the material is not rich enough, imposing certain limitations on gas adsorption. In porous materials, pores with a diameter smaller than 2 nm are micropores, those with a diameter between 2 and 50 nm are mesopores, and those larger than 50 nm are macropores. Microporous and mesoporous structures are most favorable for gas adsorption [39-43]. The average pore diameters of the three groups of materials are also listed in the structure table. The corresponding average pore diameters of C-PASP-Ni4/Cu6, C-PASP-Ni6/Cu4, and C-PASP-Ni8/Cu2 are 3.69, 2.56, and 2.82, respectively. The average pore diameters are not large, indicating that the pore structures of the three groups of materials are mostly micropores and mesopores. Among them, C-PASP-Ni6/Cu4 has the smallest average pore diameter, indicating that it contains the most micropores compared with the other two materials, and is more conducive to hydrogen adsorption. The t-plot analysis also lists the specific surface area of micropores and the micropore volume. C-PASP-Ni6/Cu4 has the largest specific surface area of micropores and micropore volume, which are 661.34 m2 g-1 and 0.32 cm3 g-1, respectively, further indicating that its adsorption performance is higher than that of the other two groups of materials. By comparing with other porous carbons and bimetallic catalysts, it can be observed that the PASP-derived porous carbon loaded with nickel and copper bimetals still maintains certain advantages in terms of specific surface area and pore size distribution, which is more favorable for hydrogen adsorption [44-46].

Table 2. Pore volume and pore size distribution of different materials.
Sample SBET (m2 g-1) VTotal (cm3 g-1) Average pore size (nm) t-plot analysis
Smic(m2 g-1) Sext(m2 g-1) Vmic(cm3 g-1)
C-PASP-Ni4/Cu6 435.17 0.17 3.69 439.98 41.81 0.17
C-PASP-Ni6/Cu4 712.63 0.45 2.56 661.34 51.29 0.32
C-PASP-Ni8/Cu2 500.68 0.35 2.82 453.51 47.16 0.18
ACNC40-900 [44] 579.4 0.34 2.35 —— —— ——
A-Cu-MOF/ SNW-1 [45] 341.12 0.12 —— —— —— ——
NiPd-rGO-200 [46] 137.89 —— 2.5 —— —— ——

Figure 6 shows the nitrogen adsorption and desorption curves and pore size distribution diagrams of three groups of PASP porous carbon materials loaded with bimetals copper and nickel. Figure 6(a) shows that although the three groups of materials have different metal loading ratios, their nitrogen adsorption and desorption curves all have obvious hysteresis loops, indicating the presence of many mesopores in the materials. During nitrogen adsorption, capillary condensation occurs between the gas and the pores, which is a typical Type IV adsorption isotherm. When the relative pressure is less than 0.1, the three groups of samples change sharply with slight changes in the relative pressure, indicating that the pressure has a great influence on their nitrogen adsorption. When the relative pressure is greater than 0.1 and less than 0.9, the adsorption and desorption curves of the PASP porous carbon materials loaded with bimetals copper and nickel are misaligned, and the curves are relatively flat, indicating that the adsorption reaches equilibrium at this time, and the change in pressure has little effect on gas adsorption. When the relative pressure is greater than 0.9, the gas adsorption capacities of the three groups of materials increase sharply, indicating that increasing the pressure is beneficial to the gas adsorption of the materials. The nitrogen adsorption and desorption curve of the C-PASP-Ni6/Cu4 material is significantly higher than those of the other two groups of materials. Its adsorption capacity reaches 231.61 cm3 g-1 when the adsorption reaches equilibrium, and continues to increase with further pressure increase until it reaches saturation, with a maximum value of 294.67 cm3 g-1, which corresponds to the analysis in Table 1. The equilibrium adsorption capacities of C-PASP-Ni4/Cu6 and C-PASP-Ni8/Cu2 are 83.50 cm3/g and 136.13 cm3/g, respectively. Although the equilibrium adsorption capacity of C-PASP-Ni8/Cu2 is greater than that of C-PASP-Ni4/Cu6, the gas adsorption capacity of C-PASP-Ni4/Cu6 increases more rapidly with the increase of pressure. When the adsorption reaches saturation, its maximum adsorption capacity is 258.80 cm3 g-1, which has exceeded the maximum adsorption capacity of C-PASP-Ni8/Cu2 (227.47 cm3 g-1). This indicates that there is a saturation state when the amount of metal Cu replacing metal Ni gradually increases, and when the amount of metallic copper is excessive, the gas adsorption effect of the porous carbon material will decrease instead. Figure 6(b)-(d) are the pore size distribution diagrams of PASP porous carbon after being loaded with bimetals copper and nickel. Figures 6 (b) and (c) are the pore size distribution diagrams of the materials C-PASP-Ni6/Cu4 and C-PASP-Ni8/Cu2. It can be seen from the figures that there are strong absorption peaks when the pore diameter is less than 2 nm, and there are also absorption peaks at a pore diameter of 4 nm. This indicates that the materials mainly function through micropores and mesopores, and the pore diameters of the mesopores are relatively small and close to those of the micropores. Figure 6(d) is the pore size distribution of C-PASP-Ni4/Cu6. There is no obvious peak when the pore diameter is less than 2 nm, indicating that there are fewer micropores in the material. There is an obvious absorption peak at a pore diameter of 20 nm, indicating that the material contains many mesoporous structures. However, the pore diameter of 20 nm is much larger than the 4 nm of C-PASP-Ni6/Cu4 and C-PASP-Ni8/Cu2. Although they all belong to mesopores, their relatively larger pore diameters will reduce their gas adsorption effect to some extent.

(a) Nitrogen adsorption and desorption curves of C-PASP-Ni/Cu, (b)-(d) Pore size distribution diagrams of C-PASP-Ni6/Cu4, C-PASP-Ni4/Cu6, and C-PASP-Ni8/Cu2.
Figure 6.
(a) Nitrogen adsorption and desorption curves of C-PASP-Ni/Cu, (b)-(d) Pore size distribution diagrams of C-PASP-Ni6/Cu4, C-PASP-Ni4/Cu6, and C-PASP-Ni8/Cu2.

3.6. EDS characterization of C-PASP-Ni/Cu

Table 3 and Figure 7 show the EDS results of the C-PASP-Ni6/Cu4 sample. The results indicate that the metals copper and nickel have been successfully loaded onto the porous carbon. Table 3 lists the distribution data of the contents of various elements in C-PASP-Ni6/Cu4. It can be seen from the table that the main elements contained in C-PASP-Ni6/Cu4 are C, N, O, Ni, and Cu, with their contents being 70.31%, 10.31%, 11.56%, 5.94%, and 3.07%, respectively. When designing the material, the percentages of nickel and copper added were 6% and 4%, respectively. There is some loss in the results, but basically, they have been loaded onto the porous carbon. It can also be seen from Figure 7 that there are several peak positions for both metallic nickel and copper. Although the corresponding peak intensities are relatively lower than those of C and O, they still indicate that both metals have been successfully loaded and exist in multiple valence states. When the skeleton of the porous carbon itself is used for hydrogen adsorption, its high specific surface area and special pore structure lay the foundation for its adsorption. After the metals are loaded onto the carbon skeleton, they can serve as active sites themselves and continue to form metal hydrides with hydrogen to further increase hydrogen storage [47-52]. The adsorption of the carbon skeleton is a physical adsorption, while the adsorption of hydrogen by the metals to form chemical bonds belongs to chemical adsorption. This combination of physical and chemical adsorption is very beneficial for hydrogen storage.

Table 3. EDS analysis results of C-PASP-Ni/Cu.
Element Weight % Atomic %
C K 70.31 79.96
N K 10.31 9.47
O K 11.56 9.46
Ni K 5.94 0.88
Cu K 3.07 0.22
EDS spectra of C-PASP-Ni/Cu.
Figure 7.
EDS spectra of C-PASP-Ni/Cu.

3.7. XPS characterization of C-PASP-50% and C-PASP-Ni50%

Figure 8 shows the XPS characterization of C-PASP-Ni/Cu. Figure 8(a) is the overall spectrum of PASP porous carbon loaded with bimetals copper and nickel. It can be seen from the figure that the elements contained in the material are C, N, O, Ni, and Cu, which is consistent with the results of the EDS spectrum of the material, indicating that Ni and Cu have been successfully loaded onto the porous carbon. Among them, the peak of C1s is the highest, followed by the peaks of O1s and N1s. These are the main elements in the hydrogel PASP, and the skeleton of the porous carbon is mainly composed of the C element.

(a) The total XPS spectrum of C-PASP-Ni / Cu, and (b)-(f) is the spectrum of C1 S, O1 S, N1 S, Ni2p and Cu2p.
Figure 8.
(a) The total XPS spectrum of C-PASP-Ni / Cu, and (b)-(f) is the spectrum of C1 S, O1 S, N1 S, Ni2p and Cu2p.

Figure 8(b) to (f) correspond to the XPS spectra of C1s, O1s, N1s, Ni2p, and Cu2p of PASP porous carbon loaded with bimetals copper and nickel, respectively. The C1s spectrum in Figure 8 (b) has four characteristic diffraction peaks at 283.31 eV, 284.80 eV, 286.10 eV, and 287.60 eV, respectively. The diffraction peak corresponding to 283.31 eV represents that C in the porous carbon forms a chemical bond with the metal elements Ni or Cu, and this chemical bond is very beneficial for hydrogen storage. The peak corresponding to 284.80 eV is for the C-C bond or C-H bond, indicating that the porous carbon material contains a large amount of amorphous carbon. The peak corresponding to 286.10 eV is for the C-O bond in the porous carbon, and the peak at 287.60 eV corresponds to C=O, indicating that the material of PASP porous carbon loaded with bimetals copper and nickel still contains functional groups such as hydroxyl groups, carboxyl groups, and carbonyl groups. The presence of these functional groups can improve the polarity and stability of the material, etc., and can increase the hydrogen storage capacity of the material [53,54]. The O1s in Figure 8 (c) correspond to two characteristic peaks at 530.95 eV and 535.23 eV, respectively. 530.95 eV represents the formation of a Cu-O or Ni-O bond, and the O-H bond formed at 535.23 eV corresponds to the C1s spectrum [55]. The chemical bond formed between the metal and oxygen can provide more active sites for the material and increase its hydrogen adsorption capacity. Figure 8(d) corresponds to the N1s spectrum, which also has two characteristic diffraction peaks at 397.03 eV and 398.93 eV. The peak corresponding to 397.03 eV is for the chemical bond formed between the metal and the nitrogen element, which is the interaction between pyridine nitrogen and the metal [56]. The peak corresponding to 398.93 eV is for pyrrole nitrogen, which is the diffraction peak formed by the nitrogen in the amino group of the porous carbon material [57]. Figure 8(e) is the diffraction spectrum corresponding to Ni2p. 853.82 eV and 871.85 eV correspond to the 2p3/2 peak and 2p1/2 peak of nickel, respectively, and the peaks at 859.73 eV and 878.81 eV correspond to the satellite peaks of the 2p3/2 peak and 2p1/2 peak [58,59]. This indicates that the metallic nickel in the porous carbon material mainly exists in the form of metal ions and metallic elements. The metallic element can further combine with hydrogen to form chemical bonds, thereby improving the hydrogen storage performance of the porous carbon [60]. Figure 8(f) is the Cu2p spectrum. It can be seen from the figure that Cu2p has three characteristic peaks at 931.26 eV, 941.66 eV, and 951.27 eV, respectively. The characteristic peaks at 931.26 eV and 951.27 eV correspond to the 2p3/2 peak and the 2p1/2 peak of copper, respectively, corresponding to elemental copper, and its function is the same as that of elemental nickel formed by metallic nickel. The characteristic peak formed at 941.66 eV indicates that part of the metallic copper still exists in the form of Cu2+, indicating that the loaded metallic copper also combines with the nitrogen or oxygen elements in the porous carbon material, which is also consistent with the results of the N1s and O1s spectra analysis [61].

3.8. Application of C-PASP-Ni and C-PASP-Ni/Cu in hydrogen adsorption

Figure 9 shows the hydrogen adsorption and desorption curves of PASP porous carbon loaded with bimetals copper and nickel. The figure shows that the hydrogen adsorption and desorption curves of the four groups of materials basically coincide. As the relative pressure continuously increases, the hydrogen adsorption capacity gradually increases. When the relative pressure is less than 0.1, the hydrogen adsorption capacity is greatly affected by the change in pressure. When the relative pressure is greater than 0.1, the increase in the hydrogen adsorption capacity slows down with the increase in pressure. The equilibrium adsorption capacity of C-PASP-Ni is 118.98 cm3 g-1, and the maximum adsorption capacity is 130.71 cm3 g-1; the equilibrium adsorption capacity of C-PASP-Ni4/Cu6 is 181.47 cm3 g-1, and the maximum adsorption capacity is 213.21 cm3 g-1; the equilibrium adsorption capacity of C-PASP-Ni8/Cu2 is 193.21 cm3 g-1, and the maximum adsorption capacity is 227.69 cm3 g-1; the equilibrium adsorption capacity of C-PASP-Ni6/Cu4 is 202.76 cm3/g, and the maximum adsorption capacity is 240.87 cm3 g-1. Among the four groups of materials, C-PASP-Ni6/Cu4 has the best hydrogen adsorption performance. The PASP porous carbon material loaded with metals copper and nickel has a certain improvement in hydrogen adsorption performance compared with that loaded with only one metal, nickel. This indicates that the bimetals play a synergistic role in the porous carbon material. At the same metal concentration, the performance is better when two metals are loaded. However, as the proportion of metallic copper in the porous carbon increases, the hydrogen adsorption capacity initially increases, but when the concentration of metallic copper is too high, the overall hydrogen adsorption performance of the material decreases. Therefore, there is an optimal ratio of the loading amounts of the two metals in the bimetal loading system. In PASP porous carbon, when the metal concentration is 10% for both, the optimal concentration is 4% for metallic copper and 6% for metallic nickel. In addition, metallic copper is relatively cheaper than metallic nickel in terms of price. Therefore, metallic copper can be used to replace part of the more expensive metallic nickel to achieve the same hydrogen storage performance effect, which broadens the research direction of porous carbon in hydrogen energy storage.

Hydrogen adsorption and desorption curves of C-PASP-Ni, C-PASP-Ni4/Cu6, C-PASP-Ni6/Cu4, and C-PASP-Ni8/Cu2.
Figure 9.
Hydrogen adsorption and desorption curves of C-PASP-Ni, C-PASP-Ni4/Cu6, C-PASP-Ni6/Cu4, and C-PASP-Ni8/Cu2.

To contextualize the performance of C-PASP-Ni6/Cu4 within a broader perspective, we compared it with several types of reported advanced hydrogen adsorption materials. For instance, in the category of biomass-derived carbon materials, Ezaty et al. achieved an adsorption capacity of 188 cm3 g-1 by impregnating nickel into kenaf [62]; Turhan et al. prepared porous carbon via ZnCl2 activation of peanut shells, achieving a maximum adsorption capacity of 238 cm3 g-1 [63]. Among metal-organic frameworks (MOFs) and their derivatives, Li et al. reported that the Cu-based bimetal MOFs Cu0.625Ni0.375(BDC)TED0.5 exhibited the best hydrogen storage capacity of 231 cm3/g at 77 K [64], while Seema et al. obtained a maximum adsorption capacity of 133 cm3 g-1 for Ni-MOF-74 derived carbon hybrid materials [65]. In comparison with the aforementioned studies, the C-PASP-Ni6/Cu4 material in this work achieved a hydrogen adsorption capacity of 240.87 cm3 g-1, demonstrating certain advantages.

4. Conclusions

A PASP porous carbon material loaded with bimetals copper and nickel was prepared. Three types of PASP porous carbon materials with different concentration ratios of metallic nickel and copper were designed, and a comparative experiment was carried out with the PASP porous carbon material loaded only with metallic nickel. It showed good performance in hydrogen storage. The infrared results, scanning electron microscopy images, and BET tests of C-PASP, C-PASP-Ni, and C-PASP-Ni/Cu indicate that the porous carbon material was successfully prepared and a good pore structure and specific surface area were formed. Among them, C-PASP-Ni6/Cu4 has the largest specific surface area, the smallest average pore diameter, and the best nitrogen adsorption and desorption effect. The transmission electron microscopy, XRD characterization, EDS characterization, and XPS characterization results of C-PASP-Ni and C-PASP-Ni/Cu show that the metals copper and nickel have been successfully loaded onto the PASP porous carbon, and the metal particles are relatively evenly distributed without large-area agglomeration. The metals mainly exist in the form of metallic elements, further increasing the activity of the porous carbon material. The hydrogen adsorption results are consistent with the BET results. C-PASP-Ni6/Cu4 has the best hydrogen adsorption performance, with an equilibrium adsorption capacity and a maximum adsorption capacity of 202.76 cm3 g-1 and 240.87 cm3 g-1, respectively. The doping of bimetals can further increase the hydrogen storage capacity to a certain extent, and metallic copper has a lower cost than metallic nickel, making it suitable for large-scale production. In the future, efforts should be continued to explore how to achieve mass production and industrial application, and continue to contribute to the development of new energy and technological progress.

CRediT authorship contribution statement

Zhao Jianbo: Conception and initiation of the project, project management and supervision, and acquisition of funds; Lu Mingqiu: Experimental design, verification of carbon material synthesis, formal data analysis, and drafting of the initial manuscript; Huang Yu: Detection and analysis (characterization by means such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), etc.) and data processing; Jiang Meilin: Research on the loading of metals onto carbon materials, investigation, and data collation; Zhu Huimin: Investigative research and data collation; Jiang Meilin: Verification and writing (analysis of XPS patterns). Liu Ling: Investigative research and literature collection.

Acknowledgments

The author thanks the Tarim University of Technology for its assistance to the project. The work of this paper is supported by the National Natural Science Foundation of China (Grant No. 22268038 and 21865026) and the Corps Science and Technology Plan Project (2022DB025).

Declaration of competing interest

There are no conflicts of interest.

Data availability

The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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