Generalised predictability in the synthesis of biocarbons as clean energy materials: Targeted high performance CO 2 and CH 4 storage

This work shows how knowledge of any biomass and choice of carbonisation process can offer a generalised route to predictability in the preparation of activated biocarbons. We demonstrate that based on O/C ratio of carbonaceous matter, it is possible to predictably generate biocarbons with suitable porosity, surface area density, volumetric surface area and packing density targeted towards record levels of CO 2 and CH 4 storage capacity. Highly porous carbons with controlled levels of microporosity of up to 97% of the surface area and 92% of the pore volume are generated. The level of synthetic control is such that it enables, on the one hand, exceptional CO 2 storage at 25 o C and low pressure (1.5 and 5.4 mmol g -1 at 0.15 and 1 bar, respectively) or moderate pressure (23.7 mmol g -1 at 20 bar), indicating superior uptake under both post-combustion and pre-combustion CO 2 capture conditions. The carbons may also be directed towards storing record levels of methane; at 25 o C and 100 bar, volumetric methane uptake of between 309 and 334 cm 3 STP cm -3 was obtained, which values are considerably higher than all current benchmark materials and, moreover, surpass the United States Department of Energy (US DOE) target of 263 cm 3 (STP) cm -3 . Crucially, the carbons also have very attractive working capacity (deliverable methane for 100 – 5 bar) of 262 cm 3 (STP) cm -3 , 234 cm 3 (STP) cm -3 (80 to 5 bar), and 210 cm 3 (STP) cm -3 (65 to 5 bar).


Introduction
Growing concerns regarding climate change and related environmental issues have encouraged considerable efforts to control the emission of CO 2 , a major greenhouse gas. To reduce the amount of emitted CO 2 , there are considerable efforts aimed at carbon capture and storage (CCS) as an intermediate solution. However, the ever increasing global consumption of fossil fuels and rising concerns over the sustainability of oil reserves have stimulated research in alternative energy sources.
In this regard, natural gas, with its better environmental sustainability properties compared to oil-based fuels, has been touted as a cleaner alternative energy source.
However, methane's volumetric energy density at standard temperature and pressure conditions, being only 0.12% of that of gasoline has limited its practical applications. [1][2][3][4][5][6] Strategies for increasing the energy density of methane have included liquefaction or compression. However, both are generally viewed as not being viable under ambient temperature and pressure conditions; compressed natural gas needs high-pressure (typically 200-300 bar) conditions that require expensive holding vessels, while liquefied natural gas depends on costly cryogenic cooling techniques. Adsorbed natural gas is, on the other hand, regarded as a promising way forward as it presents advantages with respect to safety, high gravimetric and volumetric energy density and energy efficiency. In this context, it is necessary to find suitable adsorbent materials that are viable for storage of methane and other energy-related gases. [1][2][3][4][5][6][7] Porous carbons, amongst other materials, have been suggested as promising candidates for gas storage applications related to sustainable energy provision where they are explored in relation to other adsorbents, including zeolites and metal-organic frameworks (MOFs). [1][2][3][4][5][6] Porous carbons, especially activated carbons, can have a competitive edge due to their large-scale availability, low cost, controllable porosity, high thermal and chemical stability, easy preparation, and variable packing density. [7][8][9][10] Activated carbons, in particular, can be readily generated from an extensive range of carbon-containing materials. [11][12][13] Considering the need for sustainability in large-scale gas storage applications of porous carbons, it is worthwhile to prepare them from renewable materials. 14 To this end, biomassderived porous carbonaceous materials have gained attention due to their ready availability, low cost, renewability, and simple preparation methods. [14][15][16][17][18][19] The amount of gas adsorbed and stored on a solid is influenced by the surface area and porosity of the absorbent. 8 In this regard, exploring new trends in the synthesis of tailorable porous materials with large surface area and optimised porosity is one of the long-pursued objectives towards high-performance activated carbons for gas storage applications. The porosity of an activated carbon can be tailored by varying the carbonisation and/or activating processes. 7,10,20 The carbonisation process can dramatically alter the characteristics of both the activatable carbonaceous matter and the final carbon products. Hydrothermal carbonisation (HTC) has long been established as a starting point in transforming biomass into carbon-rich carbonaceous matter that is suitable for activation. The HTC process has the attraction of being relatively simple, only requiring the heating of biomass in water at a typical temperature of 250 °C under autogenous pressure.
HTC provides superheated water conditions under which biomass is converted into so-called hydrochar that is amenable to activation. 18,21,22 Air-carbonisation (AC), on the other hand, involves the transformation of biomass to carbonaceous matter at relatively low temperature of ca. 400 °C in the presence of air. 7,18,23 Carbonised matter from either process can then be activated, which in this report is via a chemical activation step using potassium hydroxide (KOH) as an activating agent.
KOH is a preferred activating agent and is widely used to produce carbons with a range of porosity characteristics that can be tailored for enhanced gas adsorption performance. 9,11,18,24,25 We have recently shown that the carbonisation phase can affect the elemental composition of biomass-derived carbonaceous matter. 7,13,18,23,26 As a consequence, the atomic oxygen/carbon (O/C) ratio is heavily influenced by the nature of the biomass source and the carbonisation process. 7,13,18,23,26 Furthermore, it has also recently been shown that the nature of a carbonaceous precursor has a significant impact on activation behaviour (i.e., susceptibility or resistance to activation) and, consequently, plays a key role in determining the nature of porosity (e.g., micropore/mesopore mix) in the resulting carbons. 7,26 These recent advances are important because the ability to intentionally select or generate targeted biomass-derived carbonaceous precursors can provide activated carbons with predictable and tailored properties for specific applications.
More generally, extensive research findings have demonstrated that biomass-derived activated carbons can show real-world application potential for gas storage. 7,10,13,20,25,27 To this end, biomass-derived activated carbons have been explored for methane storage. 7,28 A practical target for methane storage has recently been set by the US Department of Energy (DOE) at 350 cm 3 (STP) cm -3 of volumetric storage capacity and 0.5 g (CH 4 ) g -1 of gravimetric storage capacity at room temperature and pressure of 35 to 100 bar. It is worth noting that the 350 cm 3 (STP) cm -3 target was set at that level based on the crystallographic density of MOFs. 3,4 MOFs have a crystallographic density at least 25% higher than their actual packing density. Hence, this target allows for a 25% reduction in volumetric capacity (to ca. 263 cm 3 (STP) cm -3 ) due to the need to pack MOFs into a storage tank. It is important to note that, in the case of activated carbons, no reduction is anticipated as the volumetric uptake can be obtained using experimentally determined packing density. This means that the target for methane storage in carbons can be taken to be 263 cm 3 (STP) cm -3 . An adsorbent's density is key in determining volumetric storage capacity because the adsorbent must be confined in a specific volume (e.g. in a tank), and therefore the higher the adsorbent density, the higher the amount of material that can be restricted in a tank and thus the higher the storage capacity. 3,4 To achieve a high packing density, an adsorbent's porosity should arise predominantly from micropores, which may be accompanied by the presence of some small mesopores.
This work demonstrates clear predictability in the synthesis of biomassderived activated carbons that are intentionally targeted to have properties suitable for CO 2 and CH 4 storage. Clove (Syzygium aromaticum) was selected as starting material because it has a relatively low elemental oxygen content. The carbonisation process (AC or HTC) was used along with variation in the activation temperature and the amount of activating agent, to control the textural properties of the resulting activated carbons. The motivation of the study is that cloves, based on their elemental composition and in particular oxygen content and O/C atomic ratio, can be used to predictably generate activated carbons with the appropriate porosity and high packing density that are suited for achieving exceptional levels of CO 2 and CH 4 storage capacity. Although cloves have been used to demonstrate the predictability, the implications are more general and point to the use of either (i) biomass starting material with a low O/C ratio (such as cloves), which yield activateable carbonaceous matter with low O/C ratio or (ii) any biomass that can be transformed into activateable carbonaceous matter with low O/C ratio. In this regard, the cost of producing activated carbon in a predictable manner (from cloves or any other suitable biomass) should be no more expensive compared to that of already used biomass sources for any commercially available carbons.

Synthesis of biomass-derived activated carbons
Air carbonisation: 2 g of cloves were placed in an alumina boat and heated in a horizontal tube furnace to 400 °C under a nitrogen atmosphere at a heating ramp rate of 10 °C min -1 . Once at 400 °C, the cloves were briefly (5-10 min.) exposed to a flow of air, after which the furnace was left to cool under a nitrogen flow (Supporting Scheme S1). The resulting carbonaceous matter was designated as air carbonised clove, ACC.
Hydrothermal carbonisation (HTC): 4.6 g of cloves were dispersed in 20 ml of deionised water and placed in a stainless-steel autoclave, heated up to 250 °C, maintained at the target temperature for 2 h, and then cooled to room temperature (Supporting Scheme S2). The resulting solid product, denoted as hydrochar, was obtained via filtration, washed abundantly with deionised water, and dried at 100 °C for 24 h. The resulting hydrochar was designated as HCC -hydrochar from cloves.

Chemical activation:
The required amount of KOH was thoroughly mixed with the carbon precursor (ACC or HCC) in an agate mortar at a KOH/carbon precursor ratio of 2 or 4. The resulting mixture was loaded onto an alumina boat, placed inside a tubular furnace, and heated at a ramp rate of 3 °C min -1 to 600, 700 or 800 °C under a flow of nitrogen. The furnace was held at the final temperature for 1 h, and then allowed to cool under an atmosphere of nitrogen gas. The resulting activated carbons were washed with 20% HCl at room temperature and then filtered, following which they were washed severally with deionised water until neutral pH was achieved for the filtrate. The carbons were then dried in an oven at 100 °C. The activated carbons was designated as ACCxT for air-carbonised carbon-derived samples and HCCxT for hydrochar-derived samples, where x is the KOH/carbon precursor ratio, and T is the activation temperature.

Material characterisation
Elemental, CHN, analysis was performed on an Exeter Analytical CE-440 Elemental Analyser. A PANalytical X'Pert PRO diffractometer was used to perform powder XRD analysis using a Cu-K light source (40 kV, 40 mA) with a step size of 0.02 o and 50 s time step. Nitrogen sorption analysis (at -196 °C ) with a Micromeritics 3FLEX sorptometer was used for porosity assessment and determination of textural properties. Prior to analysis, the carbon samples were degassed under vacuum at 200 °C for 16 h. The surface area was calculated using the Brunauer-Emmett-Teller (BET) method applied to adsorption data in the relative pressure (P/P o ) range of 0.02 -0.22, and pore volume was estimated from the total nitrogen uptake at close to saturation pressure (P/P o ≈ 0.99). The relative pressure range for the determination of surface area was monitored in all cases such that there was a positive y-axis intercept from multipoint BET fitting (i.e., C > 0) and also that V ads (1 − p/p o ) would rise with P/P o . 29 The micropore surface area and micropore volume were determined via t-plot analysis. The pore size distribution (PSD) was determined using Non-local density functional theory (NL-DFT) applied to nitrogen adsorption data. The determination used SAIEUS software wherein the applied 2D-NLDFT heterogeneous surface kernel allowed adequate consideration of the

Yield and elemental composition of activated carbons
The yields of air-carbonised clove (ACC), clove-derived hydrochar (HCC) and activated carbons are summarised in Table 1 and resistant to activation with KOH due to having a lower O/C ratio as confirmed in Table 1 and 2. 7,18 Similar trends in yield between AC and HTC routes have previously been observed for activated carbons derived from other biomass sources such as date seed 7 or sawdust. 18 In general, the carbon yield decreases at greater levels of activation (i.e., higher amounts of KOH and/or activation temperature).  The primary aim of the carbonisation process is to enrich the carbon content of the resulting carbonaceous matter. The elemental composition of the raw clove, the carbonized matter (ACC and HCC), and activated carbons is given in Table 1 and air-carbonised sawdust 18 and so-called CNL1 carbon. 23 In all cases, activation of both ACC and HCC increases the C content, with the rise being generally more significant at higher levels of activation. The low O/C ratio of the ACC and HCC offers an opportunity to predictably target the porosity and packing density of the resulting carbons as described in the following sections. The expectation is that activated carbons derived from ACC and HCC, by virtue of the low O/C ratio, will be dominated by micropores and therefore exhibit both a high surface area density and enhanced packing density.

Structure and morphology of activated carbons
X-ray diffraction (XRD) was performed to ascertain the nature of the carbons and their purity with respect to the absence of any crystalline inorganic phases. This is important if any inferences are to be made on the link between O/C ratio of precursors with porosity (especially the surface area density) and packing density.
Any inferences require that both the precursors (ACC and HCC) and activated carbons be fully carbonaceous with no inorganic matter. The XRD patterns of the raw clove, air-carbonised clove (ACC), clove-derived hydrochar (HCC) and activated carbons are shown in Supporting Figures S1, S2 and S3. The XRD pattern of the HCC and ACC show a broad peak at 2 = 22°, which may arise from minor graphitic/turbostratic carbon domains. The XRD patterns for all the carbons are featureless except for low intensity and broad peaks at 2 = 22° and 44°, which are typically attributed, respectively, to the (002) and (100) diffractions related to graphitic/turbostratic carbon (Supporting Figure S1). The low intensity and broad nature of the peaks suggests the lack of planarity of graphitic domains. 18 According to the XRD patterns of the activated carbons (Supporting Figure S2 and S3), the amount of KOH has no significant impact on the graphitic/turbostratic nature of the carbons. At any given activation temperature, the XRD patterns indicate a comparable level of graphitic ordering or graphene stacking. 32 Crucially, all the XRD patterns show no sharp peaks, which confirms the absence of any inorganic matter. Thus, according to the XRD patterns, ACC, HCC and the activated carbons are fully carbonaceous.
Cloves have a bulky morphology with a compact surface lacking any conspicuous porous architecture (Supporting Figure S4). After air carbonisation, cavities or cracks appear on the external surface of the ACC sample (Supporting Figure S4). However, when clove is converted to hydrochar, some of the clove's original morphology appears to be preserved (Supporting Figure S4). Conversely, the morphology of activated carbons shows irregularly shaped particles with relatively smooth surfaces and randomly distributed craters and pores (Supporting Figure S5 and S6). 14 Such cavities are consistent with generation of porosity via gasification processes. 33 It is interesting to note that this morphology is similar to that of most previously reported activated carbons. This is consistent with the fact that it is now well-recognised that activated carbons produced by KOH activation have similar morphology and that the type of precursor material used has little effect on particle shape.

Porosity and textural properties
The nitrogen sorption isotherms and the pore size distribution (PSD) curves of aircarbonised activated carbons (ACCxT) are displayed in Figure 1 and Figure 2. All the carbons exhibit type I isotherms, which indicates their microporous nature.
Although the quantity of nitrogen adsorbed increases with the severity of activation (i.e. higher activation temperature), all ACC2T carbons ( Figure 1A) show no variation in the shape of the isotherm. All ACC2T carbons have a type I isotherm with a sharp adsorption knee wherein virtually all nitrogen sorption occurs at very low relative pressure (P/Po < 0.01). A sharp knee indicates the presence of a significant proportion of microporosity and the absence of pores larger than the micropore range (up to 20 Å). As shown in Figure 1B, the porosity of the ACC2T carbons is dominated by 5-20 Å pore channels, with all pores being less than 20 Å in diameter. Despite the non-changing shape of the isotherms, the level of porosity, as measured by the amount of nitrogen adsorbed, increases modestly for samples generated at higher activation temperature. The isotherms of ACC4T carbons ( Figure 2A) are consistent with a predominantly microporous nature but with a broader knee. Knee broadening, which is greater at higher activation temperature (i.e., more severe activation), indicates presence of larger pores. This is confirmed by the PSD curves in Figure 2B. Unlike ACC2T carbons ( Figure 1B), the ACC4T set ( Figure 2B) possess a greater proportion of wider micropores and some small mesopores.

Energy & Environmental Science Accepted Manuscript
Open The nitrogen sorption isotherms and PSD curves of hydrochar-derived activated carbons (HCCxT) are shown in Figure 3 and Figure 4. The isotherms of HCC2T carbons ( Figure 3A) are type I, with a pronounced sharp adsorption knee at low relative pressure, which is characteristic of essentially microporous materials.
The sample activated at 800 °C (HCC2800) exhibits a gentle adsorption knee, implying the presence of supermicropores (pore channels with diameters ranging from 7-20 Å) in addition to micropores. This is confirmed by the PSD curves in The textural properties of both sets of activated carbons are given in Table   3. In the context of all known activated carbons, the surface area and pore volume are moderate to high, depending on the severity of activation. The surface area of ACC2T carbons gradually increases from 1500 m 2 g -1 for ACC2600 to 2150 m 2 g -1 for ACC2800, and from 2229 m 2 g -1 for ACC4600 to 3175 m 2 g -1 for ACC4800.
This modest increase in surface area for activation at higher temperature is consistent with the resistant to activation nature of ACC. 7 A similar trend is observed for pore volume, which is in the range of 0.63 to 0.94 cm 3 g -1 for ACC2T carbons and up to 1.65 cm 3 g -1 for sample ACC4800. It is worth noting that the aircarbonised samples possess a very high proportion of surface area and pore volume arising from micropores, which for ACC2T samples is typically ca. 96% of the surface area and ca. 87% of pore volume, while for ACC4T samples it is 81 -89% of surface area and 71 -79% of pore volume. It is remarkable that the most severely activated sample (ACC4800), still has a proportion of microporosity at 81% (surface area) and 71% (pore volume). For samples prepared via hydrothermal carbonisation (HCCxT), the surface area of HCC2T carbons ranges from 1396 to 2414 m 2 g -1 , and the pore volume is in the range of 0.57-1.13 cm 3 g -1 , with a very high proportion of micropore surface area of 97%, while micropore pore volume is between 80% and 92%. After the severest activation, sample HCC4800 has the highest surface area and pore volume of 3116 m 2 g -1 and 1.75 cm 3 g -1 , respectively, with relatively high microporosity; 70% of surface area and 56% of pore volume. The surface area density (SAD) of the present carbons, which is the ratio of total surface area to total pore volume, is given in Table 3. The SAD of activated carbons is related to the susceptibility or resistance to activation of the carbonaceous precursor from which they are derived. Under any given activation conditions, a high SAD can be well matched with low O/C ratio for the precursor meaning resistance to activation and consequently a tendency to generate micropores rather than mesopores. 7 The O/C ratio can, therefore, be used as a predictor for SAD (i.e., the balance of microporosity and mesoporosity). Moreover, both O/C and SAD may be used to predict the packing density of activated carbons. 7,34,35 Given the low O/C ratio of both ACC and HCC, the expectation was that the resulting activated carbons would have high SAD, which is indeed confirmed in Table 3. The SAD is in the  diminution of porosity or textural properties. As shown in Table 4, in comparison to the data in Table 3 above, there are only minor changes in the textural properties of the carbons after compaction; both surface area and pore volume are largely retained along with the proportion of microporosity, which is enhanced in some cases. As shown in Table 4

CO 2 storage
The CO 2 capture capacity was measured at 25 °C and a pressure range of 0 to 20 bar. The CO 2 uptake isotherms for ACCxT and HCCxT carbons are shown in Figure   6 and Figure 7, respectively, and Table 5 summarises the CO 2 uptake at various pressures (0.15 bar, 1 bar and 20 bar). Generally, the CO 2 uptake isotherms of the ACC2T and HCC2T carbons prepared at KOH/precursor ratio of 2 approach saturation at 20 bar, whereas those prepared at a ratio of 4 (ACC4T and HCC4T) are far from saturation, which indicates that they can reach greater storage capacity at higher pressures. As discussed above, the porosity of the ACC2T and HCC2T carbons is dominated by micropores, while ACC4T and HCC4T samples have larger micropores and some small mesopores of size up to ca. 30 Å. Comparing the porosity data and CO 2 uptake reveals that the CO 2 uptake at low pressures of 0.15 bar and 1 bar is determined by the pore size rather than the total surface area, wherein carbons having narrow micropores show the higher uptake. Narrow micropores have been proven to be more effective at creating stronger interactions between CO 2 molecules and adsorbents than is possible for larger micropores and mesopores. 23 The CO 2 uptake of ACC2T samples at 1 bar ranges from 4.5 mmol g -1 for ACC2600 to a high of 4.9 mmol g -1 for ACC2700. The uptake of ACC2800 is the lowest at 4.2 mmol g -1 , which is consistent with the widening of the pore size for this sample ( Figure 3B). The HCC2T set of samples show a similar trend; the CO 2 uptake at 1 bar being 4.3 mmol g -1 (HCC2600), 5.4 mmol g -1 (HCC2700) and ACC2T and HCC2T set of carbons, it is samples ACC2800 and HCC2800 that have the highest storage capacity (Figure 6 and 7, and Table 5).
Pressure (bar)     10,13,18,26,27,32,34,37,38,[40][41][42] hence, showing the potential of these carbons as post-combustion CO 2 storage materials. The uptake of HCC2700 is exceptional and one of the highest ever reported for carbons at ambient temperature and pressure, and is due to the sample having both the highest level of microporosity (97% of surface area and 92% of pore volume), and relatively high surface area for such a highly microporous material. Such a porosity combination, which is highly suited for low pressure CO 2 uptake, is unique to the extent that porous carbons rarely show uptake higher than ca. 4.8 mmol g -1 at 1 bar and 25 o C (Supporting Table S1). [37][38][40][41][42][43][44] Uptake as high as 5.4 mmol g -1 has seldom been observed (Table S1) and matches the record values reported to date, namely, 5.8 mmol g -1 for compactivated carbons derived from sawdust, 45 5.67 mmol g -1 for fern-derived carbons, 46  uptake at a pressure of 20 bar or above, but have much lower uptake at a low pressure (≤1 bar). Furthermore, materials characterised by low to moderate surface area and having excellent low-pressure CO 2 uptake generally show low uptake at high pressure. This trend has been ascribed to the fact that the main determinant of CO 2 uptake at low pressure is pore size (and, consequently, the interaction between the gas molecules and pore walls), while the uptake capacity at high pressure is significantly dependent on surface area or space filling. Previous trends are, therefore, somewhat bucked for the present carbons that exhibit superior CO 2 uptake under conditions relevant to both pre-combustion and post-combustion CO 2 capture.
Such unique CO 2 uptake is possible for the present carbons because they simultaneously achieve high surface area (and pore volume) and a high level of microporosity. The former ensures good CO 2 uptake at 20 bar while the latter is responsible for attractive low pressure (< 1 bar) uptake.
Although the present carbons show promise for both pre and post-combustion CO 2 uptake, their microporous nature, especially for ACC2T and HCC2T samples, is best suited for the latter (i.e., post-combustion CO 2 capture). We therefore further explored the low pressure CO 2 uptake of the ACC2T and HCC2T series of samples under conditions that mimic post-combustion CO 2 capture from flue gas streams. Table 5 shows the gravimetric uptake of the present carbons along with a comparison with benchmark carbons (Table S1). To better understand the performance of the present carbons we also determined their volumetric CO 2 uptake. Volumetric uptake is important given that for application in CO 2 capture, the carbons would be packed into a column with limited space (i.e., volume) and therefore the amount of CO 2 stored as a function of the volume occupied by the adsorbing carbon should be optimised. The volumetric uptake takes into account the packing density of the carbons and their gravimetric uptake (Supporting Table S2).
Similar to previous reports on biomass-derived carbons, the present clovederived carbons exhibit good regeneration and recyclability. Regarding recyclability, of particular interest is the amount of CO 2 that can be sequestered and delivered, i.e., the working capacity, over several cycles of use and reuse. The adsorption and regeneration cycles can be effected via pressure swing operations in the form of a pressure swing adsorption (PSA) process or vacuum swing adsorption (VSA) process. [51][52][53] To work out the working capacity for the present carbons, we considered the following swing adsorption processes; PSA with adsorption at 6 bar and desorption at 1 bar, and VSA with adsorption at 1.5 bar and desorption at 0.05 bar. 53 Cognisant of the nature of flue gas streams from fossil fuel power stations, we determined the working capacity for two scenarios, namely, from a pure CO 2 stream, and from a flue gas stream in which CO 2 constitutes 20% of the gas flow so as to mimic real post-combustion flue gas stream conditions. The gravimetric working capacity is presented in Table 6 along with data for current benchmark activated carbons, 45 high performing MOFs (Mg-MOF-74 and HKUST-1), 54 and zeolite NaX. 55 For a pure CO 2 stream, the PSA working capacity of the present carbons is between 4.3 and 8.1 mmol g -1 , and thus is higher than that of Mg-MOF-74 (3.5 mmol g -1 ), benchmark carbons (3.4 -4.0 mmol g -1 ) and zeolite NaX (1.6 mmol g -1 ), and at the high end also surpasses that of HKUST-1 (7.8 mmol g -1 ). For flue gas conditions, the PSA uptake of the present carbons is between 3.1 and 4.2 mmol g -1 , which matches the performance of HKUST-1 (4.5 mmol g -1 ). The VSA uptake of the present carbons is also very attractive with sample HCC2700 reaching 6.1 mmol g -1 and 2.3 mmol g -1 , under pure CO 2 and 20% CO 2 conditions, respectively, which when taken together compares favourably with all the other benchmark materials (Table 6). Table 6. Gravimetric working capacity for pressure swing adsorption (PSA) and vacuum swing adsorption (VSA) of CO 2 on clove-derived activated carbons compared to benchmark porous materials at ca. 25 o C for a pure CO 2 gas stream and a 20% partial CO 2 pressure flue gas stream. a 1 bar to 6 bar for PSA; 0.05 bar to 1.5 bar for VSA. b 0.2 bar to 1.2 bar for PSA; 0.01 bar to 0.3 bar for VSA.
Furthermore, the volumetric working capacity of the present carbons for both PSA and VSA processes is generally higher than that of the benchmark materials (Supporting Table S3). For pure CO 2 , the PSA volumetric working capacity of the clove-derived carbons is exceptionally high ranging from 185 g l -1 (94 cm 3 cm -3 ) to a high of 257 g l -1 (131 cm 3 cm -3 ) compared to between 142 and 153 l g l -1 (72 -78 Pure CO 2 uptake a (mmol g -1 ) Flue gas CO 2 uptake b (mmol g -1 ) Reference Sample

Energy & Environmental Science Accepted Manuscript
Open Given that flue gas streams contain majority N 2 , it is important to understand the extent to which the present carbons are selective in adsorbing CO 2 over N 2 . We therefore determined the selectivity for a representative sample (HCC2700) by comparing the relative uptake at 25 o C and 1 bar of CO 2 and N 2 . The comparison (Supporting Figure S8) shows that at 1 bar the N 2 uptake is 0.25 mmol g -1 compared to CO 2 uptake of 5.4 mmol g -1 . This gives an equilibrium CO 2 /N 2 adsorption ratio of 22, which is higher than typical ratios of 5 -11 for carbon materials. 11,34,42 The selectivity for CO 2 can also be estimated by considering a simulated postcombustion flue gas stream containing ca. 15% CO 2 with the remainder as N 2 by comparing the relative uptake of CO 2 at 0.15 bar and N 2 at 0.85 bar. This comparison can give a realistic estimation of selectivity for CO 2 from a scenario that closely mimics real application conditions. Determination of selectivity relies on the ideal adsorbed solution theory (IAST), which is the established model for estimating the relative uptake (or selectivity) by an adsorbent for any two gases in a binary gas mixture. 56 The selectivity (S) for CO 2 can be derived using the IAST precursor can embed predictability in the activation process thus making the synthesis of activated carbons more rational rather than being a random process that is based on trial and error or hit and miss.

Methane storage
An efficient adsorbent for methane storage should have high surface area and pore volume arising from pore channels of size in the range of 8 to 15 Å, significant microporosity that is ideally above 85% of the total surface area and/or pore volume, with the rest being small mesopores. [1][2][3][4][5][6][7] The present carbons should be ideal candidates to attain high methane storage capacity at moderate to high pressures, particularly given their combination of micro and mesoporosity (Table 4) and high surface area density and volumetric surface area. The methane uptake capacity of the carbons was determined at 25 °C and pressures of between 0 and 100 bar. The methane uptake measurements facilitated direct determination of the excess uptake. The total methane storage capacity was then worked out from the excess data by taking into account the methane density at any given temperature and pressure, and the total pore volume of the activated carbon according to the following equation; where θ T is the total methane uptake, θ Exc is the measured excess methane uptake, d CH4 is the methane gas density (g cm -3 ) at the prevailing conditions (temperature and pressure) as obtained from the National Institute of Standards and Technology website (http://www.nist.gov/), and V T is the total pore volume (cm 3 g -1 ) of the activated carbon. Figure 8 shows the excess and total methane uptake isotherms of the CHCCxT carbons, and Table 7 summarises the methane storage capacity at 35,65 and 100 bar. At low pressure, the methane uptake increases sharply with pressure, while a gradual increase occurs in the medium-to-high pressure ranges, and the isotherms are fully reversible. The excess uptake isotherms indicate that the carbons approach saturation at ca. 60 bar. The excess uptake follows the trend in surface area, i.e., CHCC2800 < CHCC4700 < CHCC4800. At 35 bar, the excess uptake is in the range of 10.8 to 12.2 mmol g -1 , which increases to between 12.7 and 14.7 mmol g -1 at 65 bar, and rises further to 13.1 -15.5 mmol g -1 at 100 bar. The excess methane uptake compares favourably with data from previous reports. [1][2][3][4][5][6][7]39,50,[57][58][59][60][61][62][63] The excess uptake is within a relatively narrow range, which is consistent with the spread of the porosity of the compacted carbons. The total uptake shows a wider range due to the impact of pore volume in its computation and is between 12.8 and    Figure 9 shows the volumetric methane storage isotherms, and    Figure 10 that the uptake of the present carbons surpasses that of current benchmark carbons and MOFs. This is despite the use of crystallographic density rather than true packing density in calculating values for powder forms of MOFs. It is now accepted that application of crystallographic density overestimates volumetric uptake for MOFs and envisages an impractical scenario where MOFs are packed as single crystals into storage tanks. In practice the actual packing density of MOFs tends to be much lower than crystallographic density with the consequence that the volumetric uptake values for MOFs in Figure 10 (and Supporting Table S4) are overestimated by between 25 and 50%. Thus a more realistic comparison is presented is where reductions of 25% are applied to the values of powder MOFs (Supporting Figure S9). Comparison with recently reported monolithic forms of MOFs, namely mono HKUST-1 and mono UiO-66_D, 1,79 removes the ambiguity arising from the use of crystallographic density. It is clear from Figure 10 (and Supporting Figure S9, and Table S4) that the present carbons outperform the monolithic mono HKUST-1 and related mono UiO-66_D, both of which are claimed to be the current MOF record holders for methane storage at 25 o C and pressure of up to 100 bar. 1,79 Furthermore, the present carbons also have much higher gravimetric uptake, which is almost twice as high compared to mono HKUST-1 and mono UiO-66_D as shown in Table S4.  To fully evaluate the performance of the carbons for methane storage applications, it is crucial to consider the amount (gravimetric and volumetric) of CH 4 that can be delivered, which is commonly referred to as the 'working capacity' or 'deliverable capacity'. In this study, the working capacity is taken as the difference between the adsorbing pressure (35 bar or above) and 5 bar as the desorbing pressure. The volumetric working capacity of the present carbons is given in Table 8, and Table S5 compares their performance to that of a suite of materials.
Whilst the present carbons outperform the current benchmark MOF and carbon materials (Table S5), the most relevant comparison is with mono HKUST-1, which is considered to be the current record holder for volumetric methane storage in porous materials and is claimed to be 50% better than any other MOF. 1 The highest deliverable CH 4 at 100 bar uptake pressure is 262 cm 3 (STP) cm -3 ) is for sample CHCC4700 compared to 198 cm 3 (STP) cm -3 ) and 253 cm 3 (STP) cm -3 ) for mono HKUST-1 and mono UiO-66_D, respectively.

Conclusions
Highly microporous activated carbons were generated from readily-available biomass precursors, cloves (Syzygium aromaticum) via either hydrothermal carbonisation or flash air-carbonisation followed by chemical activation with KOH.
Both carbonisation routes yield carbonaceous matter with low O/C ratio and consequently on activation offer advantages with respect to carbon yield and suitable porosity for exceptional performance in CO 2 and CH 4 storage. The resulting activated carbons have high surface area of up to 3175 m 2 g -1 and pore volume of up to 1.85 cm 3 g -1 , and depending on activation conditions, present extremely high levels of microporosity of up to 97% of surface area and 92% of the pore volume.
The activated carbons can simultaneously display high CO 2 uptake of 5.4 mmol g -1

Supporting Information
Two schemes depicting the carbonisation steps, nine additional figures; XRD patterns, TEM images, nitrogen sorption isotherms and pore size distribution curves, comparative gravimetric and volumetric methane uptake isotherms, and five tables; comparative CO 2 and methane uptake.

Energy & Environmental Science Accepted Manuscript
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