Investigation on Hydrogen Storage Capacity of Spherical Activated Carbons from Ion Exchange Resins

doi:10.21203/rs.3.rs-2632053/v1

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Investigation on Hydrogen Storage Capacity of Spherical Activated Carbons from Ion Exchange Resins

https://doi.org/10.21203/rs.3.rs-2632053/v1

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In this study, the role of spherical activated carbon from ion exchange resin as an adsorption material for hydrogen storage was researched. Spherical activated carbon samples were prepared from two types of physical activation methods using steam and CO₂. The porosity induced by each method and the resulting hydrogen adsorption performance were compared and evaluated. When the samples had similar BET surface areas, steam activation induced microporous spherical activated carbon to increase hydrogen storage performance at low pressure (~1 bar) under low temperature conditions (under 77 K). CO₂ activation was able to well formation of the 1-2 nm sized pore ratio in the micropores, thus enhancing hydrogen storage performance at high pressure (~200 bar) under ambient temperature (under 298 K). In conclusion, this indicated that there exists a range of spherical activated carbon pore size favorable for hydrogen adsorption, dependent on the pressure range applied.

Spherical activated carbon

ion exchange resin

hydrogen storage

BET surface area

micropore ratio

1-2 nm sized micropore ratio

Hydrogen has become an ideal alternative energy source due to concerns of atmospheric pollution and energy depletion from fossil fuels' continued use [1–11]. Hydrogen is eco-friendly and a sustainable resource as it does not emit pollutants during combustion [7–10, 12–14]. Furthermore, hydrogen is abundantly present and can be more easily obtained than fossil fuels substances such as oil and coal, which need to be excavated from select areas. Therefore, hydrogen fuel has become a major interest for many nations as a next generation energy [14]. There are, however, challenges to overcome, whether they be economical or safety related, for hydrogen fuel to become a norm with the current technology level and infrastructure available [5–6].

Because hydrogen has a low energy density per unit volume, storing hydrogen requires compression technology [9, 15]. Previously studied hydrogen storage technologies had limitations in implementation in its most practical sense: as a high capacity energy source for transportation [2, 5, 12, 14, 16–18]. Traditional hydrogen storage methods such as via cryogenic liquid, hydrogen gas compression, or metal hydrides are not considered attractive [2–3, 8, 10, 12, 14–15, 18–19]. Storing hydrogen through a Cryogenic tank demands a significant amount of energy input due to the need for insulation which requires a temperature of 20 K. Not to mention there is also risk of hydrogen loss from evaporation [6, 14–15, 19–20]. Systems to compress hydrogen require enormous amounts of high pressure in order to realize the demand for high energy storage capacity per volume. Further, safety problems and high costs are incurred due to the large and heavy storage containers that are required to withstand the high pressures [14–15]. Metal hydrides on the other hand has the advantage of being relatively safe compared to the other methods aforementioned, but still require high-temperature conditions to release the stored hydrogen which can incur high costs and safety concerns [14–15].

The U.S. Department of Energy (DOE) presented more than 6.5 wt% weight hydrogen storage density as a target for on-board systems [21]. However, previous hydrogen storage methods have difficulties in securing safety and economic efficiency to access DOE targets [2, 13, 19, 21]. Therefore, hydrogen storage research under conditions of safety and economics is required. Among the methods of storage through the on-board system, solid-state hydrogen storage refers to hydrogen storage that uses material as a medium [21]. The technology of storing hydrogen in porous materials through physical adsorption is the representative example [8–9, 11, 18]. According to the Van der Waals principle, hydrogen storage by physical adsorption has the strength of reversibility and fast kinetic, allowing hydrogen molecules to be adsorbed and easily separated [5, 8, 10]. In addition, hydrogen storage technology using porous materials can reduce cost and weight when compared to other technologies. Porous polymers, metal-organic frameworks(MOFs), zeolites and carbon adsorbents such as carbon nanotube(CNT) and activated carbon have been studied as representative porous materials for hydrogen storage [1–10, 12–17, 19–20, 22–32]. Compared with other materials, carbon adsorbents have advantages in its metrics of high surface area, weight, reversibility, chemical stability, and thermal stability [9, 15, 17, 18, 25]. Activated carbon, which is produced by the carbonization and activation of organic materials, is a frontrunner [9].

Activated carbon is a preferred adsorbent compared to other carbon adsorbents because it has developed a porous structure due to micropore and is easily available at a low cost [12, 15]. Activated carbon exists in various forms, such as powder, granule, and spherical [33–34]. Most previous hydrogen storage studies used powdered activated carbon. To use powdered activated carbon in the commercialization process, however, molding process and particle size control processes are essential. A significant decrease in hydrogen adsorption capacity of these powdered activated carbons is inevitable due to these processes. Therefore, the use of spherical activated carbon is advantageous for commercialization since these additional control processes are unnecessary. Moreover, a spherical activated carbon has the advantages of being superior in various properties which include its abrasion resistance, mechanical strength, smooth surface, liquidity, packing density, micropore volume, and pore distribution [33–34]. These advantages are areas of interest because they can be easily applied to the adsorption process more so than other activated carbon forms [33].

Research is being done around hydrogen storage under low pressure, and low temperature (~ 80 K) environments as both are favorable conditions for activated carbon adsorption [7–8, 18]. However, there are practical limitations because higher pressure and temperature conditions are of the norm in an industrial setting [5, 14, 27]. Research has demonstrated that hydrogen storage performance of activated carbon at ordinary temperatures and pressures has influence on the surface area and the pore size distribution [5, 9, 28–29]. Therefore, one must be able to control the activated carbon's porosity to reap the benefits of hydrogen adsorption under ambient temperature and higher pressure conditions.

Various materials such as coconut shells, rice husks, biomass, and petroleum pitch have been candidates for the manufacturing of activated carbon [7, 9, 11–12, 15, 32–33, 35–36]. However, they are all prepared by powdered activated carbon or are modified in spherical forms through later molding processes. Unlike conventional precursors, ion exchange resins can be manufactured from spherical activated carbon without any additional process. Spherical activated carbon prepared by ion exchange resin has excellent strength and packing density and has the advantage of maintaining its spherical form during its carbonization and activation process [27, 37]. This method also affects porosity by modifying the resin's cross-linking to adjust the pores' structure and volume through activation time [27, 37–38]. This activation process can be done via chemical or physical activation methods. Chemical activation is performed via carbon erosion and oxidation reactions by chemicals such as H₃PO₄, Na₂CO₃, NaOH, and KOH [6–7, 11]. Physical activation is performed by burning and vaporizing carbon through gases such as steams and CO₂ [9, 36]. Both methods can induce higher surface area and microporosity of activated carbon.

Research is being done to induce differences in porosity by doping metal in activated carbon to improve hydrogen adsorption through the spillover effect [2–4, 12–14, 17, 20, 24–26]. However, the doped metal reduces the surface area for physical adsorption in super-activated carbon, diminishing the overall hydrogen adsorption performance as pressure gets higher [24]. One research used hydrogen adsorption performance by inducing porosity improvements via differences in activation conditions [5–9, 18, 31]. As part of this research, You et al. tested hydrogen adsorption performance by producing activated carbon with ion exchange resin as precursor [27]. However, the induction of porosity difference in porosity was limited to the steam activation time of resins with different cross-linking%. In this study, physical activation was done with steam and CO₂ to induce enhanced porosity (higher surface area and pore distribution). Activation time was increased to induce different porosity of its samples. Textual properties and shapes of prepared samples were analyzed using Scanning Electron Microscopy (SEM) and measuring N₂ absorption/desorption at 77 K. The hydrogen adsorption experiments were carried out under 77 K with low pressure and 298 K with high pressure respectively to investigate performance. The study evaluated the favorable condition of hydrogen adsorption and activating condition by measuring the sample's characteristic and performance of hydrogen storage.

2.1. Preparation of the activated carbon adsorbent

The production methods of activated carbon were carried out in two ways: steam activation and CO₂ activation. In this experiment, spherical resin-based activated carbon was prepared by using ion exchange resin, which was named RAC. The resin used gel-type strong acid cation exchange resin (Dow chemical, AMBERLITE™ IR 120H) composed of polystyrene, with an 8% divinylbenzene (DVB) content as a cross-linking agent. Learnings from previous studies indicated that the strong acid cation exchange resin, which has a sulfonic acid as a functional group, can easily form spherical activated carbon because the sulfur can form a sulfonyl bridge [39–40].

Both steam activated carbons and CO₂ activated carbons were prepared by a carbonization and an activation process using rotary tube furnace after drying resin in the oven at 383 K for 24 hours. The same carbonization process was carried out for both samples by placing the dried sample to the rotating tube and raising the temperature up to 1173 K with 6 K/min under an N₂ atmosphere. The steam activated samples were run through the activation process by maintaining the temperature under a N₂ atmosphere containing 65% steam of the carbonized sample when the temperature reached 1173 K. The steam injection time was adjusted differently to 0.5, 1, 2, 2.25, 2.5, 3, 4, and 5 hours to induce porous differences in the samples. They were named RAC-H2O-0.5 and 1, 2, 2.25, 2.5, 3, 4, and 5 respectively. The CO₂ activated carbon samples were produced by performing the activation process: maintaining 1173 K while continuously injecting CO₂ 1 L/min through a quantitative pump. Similar to steam activation, activation time was adjusted to induce distinctions in porosity in samples. During the activation process, the temperature holding time was adjusted differently to 7, 9, 12, 15, 18, 19 and 23 hours in the CO₂ atmosphere. They were named RAC-CO2-7, 9, 12, 15, 18, 19 and 23 respectively.

2.2. Characterization of activated carbon

The porosity of activated carbon, such as surface area, pore volume, and pore size distribution, were analyzed through N₂ adsorption/desorption isotherms measured at 77K using a volumetric adsorption apparatus. The surface area was determined by the Brunauer-Emmet-Teller (BET) formula, and the total pore volume value was measured at a relative pressure of P/P₀ = 0.99. Pore size distribution, micropore volume, and mesopore volume were analyzed through Non-Local Density Functional Theory (NLDFT) method. All samples went through pretreating at 623 K for more than four hours. The analysis of the morphological characteristics of the samples was carried out using Scanning Electron Microscopy (SEM, TM3000 of Hitachi).

2.3. Hydrogen uptake measurements

Measurements of hydrogen storage performance were carried out with hydrogen adsorption up to 1 bar at 77 K and up to 200 bar at 298 K. All adsorption experiments were conducted through volumetric methods by using an adsorption apparatus. Hydrogen uptake at 77 K up to 1 bar was carried out by using a "Micrometrics, 3Flex 3500" instrument. Samples were pre-treated in a vacuum at 623 K for more than 4 hours before adsorption measurement was carried out. Hydrogen uptake at 298 K up to 200 bar was performed with a "Quantachrome, iSorb HP1" high pressure sorption apparatus with a thermostatic circulator attached. Prior to the measurement, samples were pre-treated in a vacuum at 573 K for 4 hours. The hydrogen used for all measurements was ultra-high purity hydrogen (99.999%) in order to reduce the effect of impurity as much as possible.

3.1. Characterization of activated carbons

Table 1 summarizes the textural properties of the spherical activated carbon samples prepared in this study. Both series of RAC-H2Os and RAC-CO2s were produced to be as similar as possible in terms of BET surface area. The CO₂ activation time was maintained longer than the steam-activation. It was judged that the activation time must be maintained for a long time in order to obtain similar BET surface area of the steam activated samples at a temperature of 1173 K due to weak oxidation of activated carbon by CO₂ and release of volatile substances [41]. Burn-off percentages of RAC-H2Os were increased from 68–99% by elevating activation time from 0.5 to 5 hours. As a result, the BET surface area enhanced from 1016 m²/g to 2825 m²/g, and the total pore volume also increased gradually from 0.39 cm³/g to 1.84 cm³/g. RAC-CO2s showed burn-off percentage increased continuously from 70–97% as activation time increased from 7 hours to 23 hours. The BET surface area increased from 1058 m²/g of RAC-CO2-7 to 2222 m²/g of RAC-CO2-18. However, RAC-CO2-19 with more activation time showed a stagnant value of 2225 m²/g and RAC-CO2-23 showed a significant reduction of 1830 m²/g. The total pore volume increased continuously from 0.44 cm³/g of RAC-CO2-7 to 1.60 cm³/g of RAC-CO2-19. However, RAC-CO2-23 showed a lower value of 1.48 cm³/g as its BET surface area decreased. Eventually development of porosity such as enhanced BET surface area and total pore volume of the samples emerged as the steam activation process was continued for a longer period of time. However, the CO₂ activation showed a continuous improvement in porosity of the samples due to an increase in the activation process time and then decreased again. Advanced porosity of activated carbon can have a significant impact on improving hydrogen storage performance [31–32].

Table 1

Textural properties of activated carbon samples
Sample name	Activation time (h)	Burn-Off (%)	S_BET (m²/g)	V_total (cm³/g)	V_micro (cm³/g)	V_meso (cm³/g)	V_micro/ V_total (%)	V_meso/ V_total (%)
RAC	0	57	290	0.13	0.13	0	100	0
RAC-H2O-0.5	0.5	68	1016	0.39	0.39	0	100	0
RAC-H2O-1	1	85	1309	0.57	0.55	0.02	96	4
RAC-H2O-2	2	90	1779	0.85	0.79	0.06	93	7
RAC-H2O-2.25	2.25	93	2089	0.97	0.82	0.16	84	16
RAC-H2O-2.5	2.5	94	2276	1.11	0.85	0.26	77	23
RAC-H2O-3	3	97	2471	1.30	0.86	0.44	66	34
RAC-H2O-4	4	98	2686	1.53	0.78	0.75	51	49
RAC-H2O-5	5	99	2825	1.84	0.75	1.09	41	59
RAC-CO2-7	7	70	1058	0.44	0.42	0.02	95	5
RAC-CO2-9	9	76	1307	0.61	0.53	0.08	87	13
RAC-CO2-12	12	85	1749	0.97	0.69	0.28	71	29
RAC-CO2-15	15	90	2079	1.26	0.74	0.52	59	41
RAC-CO2-18	18	94	2222	1.50	0.69	0.81	46	54
RAC-CO2-19	19	96	2225	1.60	0.67	0.93	42	58
RAC-CO2-23	23	97	1830	1.48	0.56	0.92	38	62
S_BET: BET surface area
V_total: Total pore volume
V_micro: Micropore volume
V_meso: Mesopore volume
V_micro/V_total: Micropore ratio
V_meso/V_total: Mesopore ratio

The relationship between the continuously increased BET surface area and the micropore ratio formed accordingly are shown in Fig. 1. When making a comparison between RAC-H2Os and RAC-CO2s with similar BET surface areas, the micropore ratio was higher for steam activated samples. This indicated that, the steam activation was more effective in forming activated carbon with high micropore ratio than the CO₂ activation when a similar BET surface area is obtained. However, the total pore volume showed a higher value for RAC-CO2s.

Figure 2 compared the difference in total pore volume due to the increase in BET surface area. RAC-CO2s had higher total pore volume than RAC-H2Os with similar BET surface area. This indicated that CO₂ activation was superior in increasing the mesopore ratio. Compared with similar BET surface areas, the packing density of RAC-CO2s was lower than that of RAC-H2Os. This was due to the higher total volume of RAC-CO2s.

The type of pores in activated carbon was determined by the analysis of N₂ adsorption/desorption isotherms curves. N₂ adsorption/desorption isotherms at 77 K for RAC-H2Os and RAC-CO2s are shown in Fig. 3. Adsorption of the low relative pressure area up to 0.1 is caused by existence of micropore [8, 15, 44–45]. However, the area from relative pressure of 0.1 to saturated pressure indicates the effect of the mesopore, where the capillary condensation occurs. Considering the micropore ratios of both RAC-H2Os and RAC-CO2s, the isotherms of samples with more than 80% showed Type I form (IUPAC classification) observed in microporous solid. Since then, activation time had been further increased, and isotherms of samples with less than 80% micropore ratio observed hysteresis loop, a characteristic of the sample with mesopore, occurred in the desorption curve at the point of a relative pressure of 0.4 or higher. Thus, Type IV (IUPAC classification) form that the curves of adsorption and desorption do not match each other was shown. Eventually, as activation time increases, isotherms of RAC-H2Os and RAC-CO2s changed from Type I to Type IV, and the hysteresis phenomenon became apparent. This phenomenon is known to occur as the size of the micropore expands over a longer activation time, or it would develop into a mesopore by collapse [27, 42–43]. Table 1 shows that the micropore ratio of RAC-H2Os decreased from 100–41%, and that of RAC-CO2s decreased from 95–38%, while the mesopore ratio increased from 0–59% and 5–62% for them, respectively. In conclusion, as the longer the activation process, the porosity phase changed from microporosity to mesoporosity.

Figure 4 is the Scanning Electron Microscopy (SEM) images showing the change in morphological characteristics due to the activation time of the samples. The RAC-H2Os images shown in (a)-(c) revealed differences in particle size and surface conservation as the activation time increased from 0.5 to 5 hours. The RAC-H2O-0.5 with the shortest activation time showed a smooth surface (Fig. 4-(a)). Afterwards, in RAC-H2O-2 and 5 with more activation time, the occurrence of rough surfaces and pores on the surface were observed. (Fig. 4-(b)-(c)). In particular, the pores and roughness of surface were more prominent on the sample with the longest activation time (i.e., RAC-H2O-5). Here, the pore on the sample surface is difficult to contribute to the formation of micropore or mesopore, which indicates the loss of large carbon fragments [27]. Likewise, as the activation time of RAC-CO2s was increased from 7 to 23 hours, the samples’ particle sizes became much smaller, and more pores were created on the surface (Fig. 4-(d)-(f)). As the pore size increased noticeably due to the loss of the carbon fragment from RAC-CO2-19, however, RAC-CO2-23, the sample with the longest activation time, showed a complete collapse (Fig. 4-(g)). It was confirmed that the maintenance of spherical activated carbon forms also affected the textural properties shown in Table 1. Both RAC-H2O and RAC-CO2 series showed continuous porosity improvement in samples that maintained the spherical form despite the prolonged activation process. However, RAC-CO2-23, which did not maintained the spherical shape, showed a decreased porosity. The effect of porosity due to maintaining the form may contribute to hydrogen adsorption performance.

International Union of Pure and Applied Chemistry (IUPAC) classified adsorbent pores as micropore (less than 2 nm), mesopore (between 2 nm and 50 nm) and macropore (greater than 50 nm) according to their size [46]. Pores of activated carbon affect porosity according to its size, and it determines adsorption performance of hydrogen. Therefore, it is necessary to produce a proper pore size in order to utilize the material most effectively for hydrogen storage, and it needs to be figure out. The pore size distribution (PSD) of each sample was investigated to determine the change in the pore size due to the adjustment of the activation time of the prepared activated carbon. The PSDs of RAC-H2Os and RAC-CO2s were calculated by Non-Local Density Functional Theory (NLDFT) and they are shown on Fig. 5. Both series of samples showed a noticeable expansion in the range of mesopore with increased activation time. However, many studies have reported that micropore is more advantageous to have superior hydrogen storage performance because pores larger than 2 nm have insufficient energy to store hydrogen [5–6, 8]. The distribution of micropore of RAC-H2Os and RAC-CO2s showed a noticeable difference in the 1–2 nm sized pore distribution of samples with BET surface area of less than 1700 m²/g.

This difference can also be seen in Table 2, which showed the pore volume of 0–1 nm and 1–2 nm sized micropores, named as V_{0 − 1nm} and V_{1 − 2nm}, respectively. The V_{1 − 2nm} of samples with a BET surface area of less than 1700 m²/g was significantly smaller than the V_{0 − 1nm}. Afterwards, samples with BET surface area of more than 1700 m²/g showed the aspect of V_{1 − 2nm} being larger than V_{0 − 1nm}. In particular, the 1–2 nm sized micropore ratio (V_{1 − 2nm}/V_micro) of RAC-H2Os and RAC-CO2s increased consecutively from 8–66% and from 21–69% respectively, in proportion to the BET surface area improved with longer activation times. Among the RAC-CO2s, RAC-CO2-18, 19 showed almost constant in BET surface area, but V_{1 − 2nm}/V_micro was 66% and 69%, respectively, with RAC-CO2-19 showing higher values. This was much better than V_{1 − 2nm}/V_micro (66%) of RAC-H2O-5 with higher BET surface area. Afterwards, RAC-CO2-23 which had decreased porosity showed a lower value of 64% again. From such data, it can be determined that the CO₂ activation had an advantage in enhancing 1–2 nm sized micropore ratio. You et al. reported that this 1–2 nm sized pore, among micropores, can contribute to hydrogen adsorption at high pressure [27].

Table 2

Pore volume of micropores in prepared activated carbons
Sample name	V_{0 − 1nm} (cm³/g)	V_{1 − 2nm} (cm³/g)	V_{0 − 1nm} / V_micro (%)	V_{1 − 2nm} / V_micro (%)
RAC-H2O-0.5	0.36	0.03	92	8
RAC-H2O-1	0.37	0.17	68	32
RAC-H2O-2	0.36	0.43	46	54
RAC-H2O-2.25	0.36	0.46	44	56
RAC-H2O-2.5	0.34	0.51	40	60
RAC-H2O-3	0.31	0.55	36	64
RAC-H2O-4	0.28	0.50	35	65
RAC-H2O-5	0.26	0.49	34	66
RAC-CO2-7	0.33	0.09	79	21
RAC-CO2-9	0.34	0.19	64	36
RAC-CO2-12	0.29	0.40	42	58
RAC-CO2-15	0.26	0.48	35	65
RAC-CO2-18	0.23	0.45	34	66
RAC-CO2-19	0.21	0.46	31	69
RAC-CO2-23	0.20	0.36	36	64
V_{0 − 1nm}: 0–1 nm sized micropore volume
V_{1 − 2nm}: 1–2 nm sized micropore volume
V_{0 − 1nm}/V_micro: 0–1 nm sized micropore ratio
V_{1 − 2nm}/V_micro: 1–2 nm sized micropore ratio

3.2. Hydrogen uptakes behavior due to porosity of activated carbon

The hydrogen adsorption test of prepared activated carbon was conducted to investigate the association of hydrogen storage performance with the modified pore properties of samples following activation conditions. The test was measured in volumetric method at 1 bar and 200 bar respectively. The hydrogen uptake capacity (wt%) up to each maximum pressure is summarized in Table 3.

Table 3

Hydrogen uptake capacity (wt%) of prepared activated carbons measured up to 1bar and 200bar, respectively
Sample name	H₂ uptake (wt%) on 1 bar at 77 K	H₂ uptake (wt%) on 200 bar at 298 K
RAC-H2O-0.5	1.55	0.52
RAC-H2O-1	1.69	0.55
RAC-H2O-2	1.83	0.63
RAC-H2O-2.25	1.92	0.68
RAC-H2O-2.5	1.99	0.69
RAC-H2O-3	2.02	0.76
RAC-H2O-4	2.12	0.79
RAC-H2O-5	2.18	0.81
RAC-CO2-7	1.53	0.53
RAC-CO2-9	1.63	0.59
RAC-CO2-12	1.75	0.65
RAC-CO2-15	1.83	0.72
RAC-CO2-18	1.87	0.75
RAC-CO2-19	-	0.82
RAC-CO2-23	1.77	0.71

First, a low pressure (1 bar) hydrogen adsorption was performed at a temperature of 77 K. As hydrogen has extremely low energy density per unit volume, there is a limitation in hydrogen adsorption in ambient temperature where increasing energy density is difficult at low pressure. Therefore, the adsorption proceeded at 77 K, a temperature favorable to physical adsorption. Measured isotherms were shown in Fig. 6. As Table 3 indicates, RAC-H2Os showed enhanced hydrogen uptake capacities as porosity increased. RAC-CO2s also showed a continuous increase in hydrogen uptake capacities, but RAC-CO2-23, which resulted in a decrease in porosity due to the collapse of the spherical form, showed a decreased value. These results were consistent with the results that hydrogen uptake capacity, already known from previous studies, is proportional to porosity such as BET surface area and total pore volume of adsorbent [47–50]. RAC-H2O-5 which has well developed porosity with the highest BET surface area (2825 m²/g) of all samples showed the highest hydrogen uptake capacity at 2.18 wt%. A previous study have reported high hydrogen uptake capacity of up to 2.4 wt% [7]. Considering that the sample is powdered activated carbon, however, it has a drawback of hydrogen uptake performance reduction due to additional processes such as molding and particle size control for commercialization. In contrast, hydrogen uptake capacity obtained from this study has the advantage of being used in commercial processes without any additional loss due to the property of spherical activated carbon. Between RAC-CO2s and RAC-H2Os, which had similar BET surface area, RAC-H2Os had a higher hydrogen uptake difference from a minimum of 0.02 wt% to a maximum of 0.12 wt% (Table 3). As previously identified in Fig. 1, when the same BET surface area was obtained, it was shown to be due to the results that the steam activation could induce a higher micropore ratio than the CO₂ activation. In addition, the result that the 0–1 nm sized micropore ratio (V_0-1nm/V_micro) of RAC-H2Os was higher than that of the RAC-CO2s with similar BET surface area (Table 2) corresponded with research results that hydrogen adsorption depends on micropore smaller than 1 nm under the same condition [7–8].

Adsorption tests were conducted at 200 bar to measure hydrogen uptake performance at high pressure under ambient temperature, which is a condition applicable to the industry rather than hydrogen storage at extremely low pressure under cryogenic conditions. The experiment was conducted at ambient temperature. The measured isotherms are shown in Fig. 7. Among the RAC-CO2s and RAC-H2Os, which had similar BET surface area, RAC-CO2s had relatively higher hydrogen uptake capacity difference from a minimum of 0.01 wt% to a maximum of 0.06 wt% (Table 3). The RAC-CO2-19 had the highest final hydrogen uptake capacity of 0.82 wt%. As expected, the adsorption capacity of RAC-CO2-23 with porosity degradation decreased. Although the same sample was used, it was much lower than the highest capacity (2.18 wt%) at low pressure because it was measured under room temperature which is unfavorable conditions for physical adsorption. The hydrogen uptake capacity at 200 bar of powdered activated carbon measured in the previous study was 1.2 wt%, better than the results of this study [6]. However, its uptake performance will deteriorate after powder activated carbon experience the additional process. Thus, the spherical activated carbon used in this study can be seen as a candidate to be applied more favorably from the practical perspective. As with the measurement at low pressure (Fig. 6), the hydrogen uptake capacity at high pressure was also proportional to the porosity of activated carbon. Therefore, it could be confirmed that the relationship between hydrogen adsorption performance and porosity is maintained even if the conditions of temperature and pressure vary. However, when similar BET surface areas were obtained, it was different to the view that hydrogen adsorption would be higher in RAC-H2Os, a more microporous structure, similar to the results from low-pressure measurements. If so, it may be possible to determine that the more mesoporous structure of the sample has affected hydrogen adsorption. However, since pores over 2 nm have insufficient energy to store hydrogen [5–6, 8], mesopore is difficult to affect hydrogen adsorption. In order to figure out reason RAC-CO2s had a superior hydrogen uptake performance at high pressure under ambient temperature, the micropore size distribution was observed in the previous experiment (Fig. 5, Table 2). The CO₂ activation acted in favor of increasing 1–2 nm sized micropore ratio over the steam activation, and it decisively served to hydrogen adsorption at high pressure by enhancing the accessibility of hydrogen to the surface of activated carbon.

This study compared the modified porosity and the resulting hydrogen adsorption performance of spherical activated carbons prepared by means of physical activation methods using steam and CO₂. Steam activation obtained a continuous improvement in BET surface area in the range of 1016–2825 m²/g in proportion to the activation time. On the other hand, CO₂ activation showed a continuous improvement of 1058–2222 m²/g, but it experienced stagnation of 2,225 m²/g as the activation time further increased. In the end, the longest activation time sample showed a decreased value of 1830 m²/g, revealing a reduced porosity. When the samples had similar BET surface area, the steam activation increased the micropore ratio, and CO₂ activation induced improvement of the mesopore ratio, resulting in higher total pore volume.

The SEM results showed that the changes in surface shape and reduction in particle size as the activation time of each activated carbon increased. However, RAC-CO2-23 with reduced porosity among CO₂ activated carbon showed a collapse of the spherical form. This failure to maintain the spherical form could be seen to have induced a decrease in porosity.

The pore size distribution, calculated by the NLDFT method, showed that the range of mesopore extended as the activation time increased, over the total pore size range. Both steam activation and CO₂ activation continuously increased the 1–2 nm sized micropore ratio as activation time and BET surface area increased. In particular, the ratio of RAC-CO2-19 with a BET surface area value of 2225 m²/g was 69%, higher than the ratio (66%) of RAC-H2O-5 with a BET surface area of 2825 m²/g. Eventually, it could be seen that CO₂ activation was superior in increasing the 1–2 nm sized pore ratio.

Hydrogen adsorption tests of activated carbon were measured at a low pressure of 1 bar and a high pressure of 200 bar. At 1 bar, steam activated carbons which had more microporous property showed superior hydrogen uptake capacity. The RAC-H2O-5 showed hydrogen uptake capacity of 2.18 wt%, the highest value among all the samples. At 200 bar, CO₂ activated carbon revealed superior adsorption performance as a result of increased hydrogen molecule accessibility at high pressure with an improved 1–2 nm sized micropore ratio due to CO₂ activation. RAC-CO2-19 showed the highest uptake capacity (0.82 wt%). Each of the porosity induced by steam activation and CO₂ activation showed a difference in hydrogen adsorption performance under several pressure conditions. Moreover, given that the porosity and hydrogen adsorption of RAC-CO2-23 where the spherical form collapsed were reduced, it was determined that maintaining the form of spherical activated carbon could also affect hydrogen storage performance.

Conclusively, this study was able to show that spherical activated carbon can be designed to be a possible storage material required for various hydrogen storage conditions, such as low pressure under low temperature and high pressure under ambient temperature. In addition, it can be insinuated that the spherical activated carbon would be a more economical and environmentally friendly method to increase hydrogen storage performance without loss, unlike historically used powdered activated carbons. Additionally, the hydrogen storage performance of spherical activated carbon samples prepared in this study is limited to achieve target capacity (6.5 wt%) of DOE (United States Department of Energy) [21]. However, producing spherical activated carbon using chemical activation at the expense of economic and environmental challenges is expected to increase hydrogen adsorption performance by inducing more porous.

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Editorial decision: Minor Revisions Needed
30 Apr, 2023
Reviewers agreed at journal
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First submitted to journal
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Investigation on Hydrogen Storage Capacity of Spherical Activated Carbons from Ion Exchange Resins

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Chapter 1. Introduction

Chapter 2. Experimental