RNA as a Precursor to N-Doped Activated Carbon

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Article

RNA as a Precursor to N-doped Activated Carbon Kiankeat Lee, Tamara Church, and Niklas Hedin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00589 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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ACS Applied Energy Materials

RNA as a Precursor to N-Doped Activated Carbon Kian Keat Lee, Tamara L. Church, and Niklas Hedin*

Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden.

*E-mail: [email protected]

Keywords: N-doped activated carbons, CO2 sorption, supercapacitors, capacitance, hydrothermal, RNA

Abstract

Activated carbons (ACs) have applications in gas separation and power storage, and N-doped ACs in particular can be promising supercapacitors. In this context, we studied ACs produced from yeast-derived ribonucleic acid (RNA), which contains aza-aromatic bases and phosphate-linked ribose units, and is surprisingly inexpensive. The RNA was hydrothermally carbonized to produce hydrochars that were subsequently activated with CO2, KOH, or KHCO3 to give ACs. The ACs adsorbed up to ~7 mmol/g at 0 °C and 1 bar, and had capacitances as high as ~300 F/g in a three-electrode cell and a 6-M KOH(aq) electrolyte. The material that displayed the best capacitance was tested in a two-electrode cell, which displayed a specific capacitance of 181 F/g even at a current density of 10 A/g. The ACs with the highest uptake of CO2 and the highest capacitance were those activated with KOH and KHCO3; however, CO2 activation is arguably less expensive and more suitable for industrialization.

Introduction

The high energy densities and relatively long discharge times of supercapacitors make them suitable for applications where neither batteries nor conventional capacitors are ideal.1 Electrical double layer capacitors (EDLCs) are a class of supercapacitors that operate via the electrical double layer partitioning of ions on charged interfaces, and electrode materials used for this purpose require high specific surface area and a relatively high electrical conductivity. For this reason, activated carbons (ACs) are commonly used as electrode materials in EDLCs,2 and N-doped ACs in particular have yielded fast and cyclable supercapacitors.3-6 N-doped AC supercapacitors can be produced either by the post-synthetic 1 ACS Paragon Plus Environment

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functionalization of an ordered carbon or char with a nitrogen-containing precursor, or by the direct pyrolysis of carbon- and nitrogen-containing precursors. The latter method has the advantage of procedural simplicity, and can produce a more uniform N-doping throughout the material.6 The performance of an N-doped AC as a supercapacitor is related to, among other factors, its precursor, textural properties, N content, and the type of N species present.6 In particular, the most abundant pore size of the material should be slightly larger than the effective size of the adsorbing ion to facilitate attractive surface–adsorbate interactions. 7, 8

Though N-containing synthetic polymers and ionic liquids can be used as precursors to N-doped AC supercapacitors,6 there are manifold potential environmental benefits to using N-doped ACs produced from biomass. N-doped AC supercapacitors have recently been derived from the direct pyrolysis of precursors including chitin,9 anchovy powder,10 fallen flowers,11 or a combination of sugarcane bagasse pith and chitosan.12 Another approach to the synthesis of ACs from biomass or biomolecules is to start with a precursor hydrochar derived from the hydrothermal carbonization of biomass or biomolecules. The hydrothermal carbonization process can be used to transform wet biomass, generates only a small amount of CO2 or other gas molecules, and is exothermic, which contributes positively to the overall heat balance of carbonization,13-18 and the product hydrochar can be pyrolyzed in the presence of an activating agent to give AC. Various types of saccharides and polysaccharides have been studied as precursors for hydrothermal chars that have been activated to give ACs and N-doped ACs.19-31 Such ACs have been studied as potential electrode materials in EDLCs,32-42 and high capacitances have been reached. Nitrogen-containing biopolymers such as proteins43, 44 or chitosan45 have been used to produce N-doped ACs with ≥5 wt% N content via hydrothermal carbonization and subsequent pyrolysis, and these results, combined with the known utility of saccharides in hydrothermal carbonization (vide supra) inspired us to examine hydrothermal char from ribonucleic acid (RNA) as a precursor to N-doped ACs. The individual azacyclic nucleobases found in RNA have been used as supplementary N sources for ionic-liquid-derived Ndoped ACs,46 but to the best of our knowledge, whole RNA has not. Not only is RNA a renewable biopolymer, it is also surprisingly inexpensive (search for “RNA powder” on an online marketplace), making it of potential interest for the production of N-doped ACs for CO2 uptake and supercapacitor applications.

Experimental Section 2 ACS Paragon Plus Environment

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Materials

RNA from yeast (Roche) [CAS 63231–63–0], polytetrafluoroethylene (60 wt. % dispersion in H2O) [CAS 9002–84–0], 2-propanol (≥99.8 %) [CAS 67–63–0], hydrochloric acid (ACS reagent 37%) [CAS 7647–01–0] and potassium bicarbonate (ACS reagent, 99.7%) [CAS 298–14–6] were supplied by Sigma-Aldrich. KOH pellets (AnalaR NORMAPUR® Reag. Ph. Eur. analytical reagent) [CAS 1310–58–3] were obtained from VWR. Nickel meshes were purchased from Goodfellow. All chemicals were used as received.

Sample preparation

In one series of experiments, aqueous suspensions of of RNA (0.2–0.6 g/mL) were prepared and placed in Teflon-lined stainless steel autoclaves, which were subsequently heated at 200 °C for 24 h. In another series, aqueous suspensions of 0.5 g/mL RNA were prepared and heated at different temperatures (160–240 °C) for 24 h. After hydrothermal treatment, the autoclaves were allowed to cool in air. The solid HTC-RNA was recovered by filtration, washed with deionized water and dried at 110 °C.

Three activating agents were used to prepare ACs from the HTC-RNAs: CO2, KOH, and KHCO3. For CO2activated ACs, 2.5 g of HTC-RNA were loaded into a vertical reactor and exposed to 200 standard cm3/min CO2 while being heated at 10 °C/min to 800 °C and then maintained at that temperature for 4–16 h. After the reactor had cooled to room temperature, samples were collected and treated with hydrochloric acid to remove ash components and then rinsed with deionized water and dried.

Potassium-hydroxide activated ACs were prepared by stirring the HTC-RNAs, which had been prepared via hydrothermal carbonization at 200 °C for 24 h, in KOH(aq) (mKOH:mHTC-RNA = 4 or 1) overnight at room temperature. The suspensions were subsequently dried in an oven, ground, and loaded into a vertical reactor. The reactor was exposed to a flow of 200 standard cm3/min N2, heated to 700 or 800 °C, and held at that temperature for 2 h before being allowed to cool. The resulting ACs were washed with HCl(aq) until the supernatant had neutral pH, then dried at 110 °C.

Potassium-bicarbonate activated ACs were prepared by mixing the HTC-RNA (prepared via hydrothermal carbonization at 200 °C for 24 h) with KHCO3 powder (mKHCO3:mHTC-RNA = 8 or 2) using a mortar and pestle. 3 ACS Paragon Plus Environment


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The resulting solid mixtures were subjected to the same thermal treatment and washing processes used for the KOH activation.

Materials characterization

The textural properties of the ACs activated with KHCO3 and KOH were determined from the volumetric N2 and CO2 adsorption–desorption isotherms recorded at –196 and 0 °C, respectively, on a Micromeritics ASAP 2020 Physisorption Analyzer. The ACs were degassed under dynamic vacuum at 300 °C for 10 h prior to analysis. The textural properties of the CO2-activated ACs were evaluated from the volumetric N2 and CO2 adsorption–desorption isotherms recorded at –196 and 0 °C, respectively, on a Gemini VII Physisorption Analyzer. Those ACs were degassed in a flow of N2 for 1 h at 90 ºC, then for 6 h at 350 ºC. Specific surface areas were calculated by applying the Brunauer–Emmett–Teller model47 (SBET) to the N2 adsorption isotherms at relative pressures (P/P0) that ensured a positive C constant and that the composite Vads(1–P/P0) increased with P/P0. The total pore volume, Vt, was determined from the last point of the adsorption isotherm, at P/P0 = 0.98. Micropore volumes and surface areas were calculated from N2 adsorption data using the t-plot (Vµp-t and Sµp-t) and Dubinin–Radushkevich (DR) (VDR-N2 and SDR-N2) methods. The characteristic energy E0 calculated by the DR method was used to derive the average micropore width L0-N2 using the equation L0N2 (nm)

= 13.7/(E0 – 9.7 kJ/mol), and the geometrical surface area for slit shaped pores, Sslit, was calculated

from VDR-N2 and L0-N2 using Sslit = 2000VDR-N2/L0-N2. The carbon black statistical thickness method (STSA) was used for the t-plot analysis of micropores and the Barrett–Joyner–Halenda (BJH, based on adsorption data) analysis of mesopores. The mesopore surface areas and volumes Smp-BJH and Vmp-BJH are given for pores with d = 2–100 nm. The ultramicropore volumes and surface areas (VDR-CO2 and SDR-CO2) were calculated with the DR method from CO2 adsorption data. Ultramicroporosity is defined as the porosity in pores with d < 1 nm (predominantly d = 0.4–0.8 nm), accessible by CO2 molecules at 0 °C. Pore size distributions (PSDs) were calculated from N2 adsorption data using the Micromeritics non-local density functional theory (NLDFT) functional for carbon-slit pores, and a surface area SNLDFT was calculated from this data as the surface area of pores with d = 0.5–50 nm. Ultramicropore size distributions were calculated from CO2 sorption data using density functional theory and the Micromeritics functional for CO2 sorption, and an ultramicropore surface area SDFT-CO2 was calculated as the surface area of pores with d = 0.4–1.0 nm. 4 ACS Paragon Plus Environment

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Elemental analysis of the HTC-RNA samples was performed by MEDAC Ltd., UK. CHN (or CHNS for some samples) composition was determined by conventional combustion analysis. Raman spectra were recorded using a Horiba LabRAM HR 800 Raman spectrometer with Nd:YAG laser (532 nm/50 mW). Scanning electron microscope (SEM) images were recorded with a JEOL JSM-7000 F microscope using accelerating voltages of 5 kV. X-ray photoelectron spectra were recoded with a Kratos AXIS Ultra

DLD

spectrometer using a monochromatic Al-based X-ray beam. The relative surface compositions (in atomic %) were quantified by spectra recorded for each element using established procedures.

Electrochemical studies

Electrodes composed of 80 wt.% active material (AC), 10 wt.% polytetrafluoroethylene (PTFE) and 10 wt.% acetylene black were prepared. First, 2-propanol was added dropwise to a mixture of AC, PTFE, and acetylene black, and the mixture was homogenized for 15 min using an agate mortar and pestle. The formed paste was spread on a Ni mesh, dried, and immersed in the 6-M KOH(aq) electrolyte for at least one day to ensure complete impregnation with the electrolyte. On the electrodes, the coverage of ACs was 5–8 mg/cm2.

Electrochemical studies were carried out at room temperature using a 6-M KOH(aq) electrolyte and a BioLogic SP-50 potentiostat. The electrochemical capacitances of selected samples were initially evaluated in a three-electrode setup. An electrochemical half-cell was assembled in glassware with a Pt foil as the counter electrode, Ag/AgCl (3 M KCl) as the reference electrode, and the prepared materials on Ni mesh as working electrodes. The corresponding specific capacitance Cm,3E was calculated from the CV curves based on equation 1:

, =

∆

(1)

where m is the mass of active material (AC), R is the scan rate, ∆V is the voltage window of scanning, and is the integral area under the CV curve.

For the electrochemical studies with a two-electrode setup, a spring-loaded symmetrical two-electrode cell (ECC-Std electrochemical cell from EL-Cell GmbH) was used. A laboratory-quality filter paper (Munktell) was used as a separator between two electrodes having similar loading. Cyclic voltammetry (CV) and 5 ACS Paragon Plus Environment


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chronopotentiometry (CP) were performed in the voltage range of 0–1.0 V. The specific capacitance (Cm,2E) was calculated with two methods:

(i)

CV, according to equation 2:

, = 4

∆

(2)

where , m, R, ∆ are the integrated area under the CV curve, the total mass of active material (AC) in both electrodes, scan rate, and voltage range, respectively.

(ii)

CP (discharge curve), according to equation 3:

, = 4

∆ ∆

(3)

where I, ∆, m, ∆ are the applied current, discharge time, the total mass of active material (AC) in both electrodes, and the voltage range, respectively.

Results and Discussion

Black solids (HTC-RNAs) precipitated upon hydrothermal treatment of yeast-derived RNA, as expected due to the polyribose nature of RNA and from studies of other sugars and biomass.13, 14, 48, 49 Ribose reactivity in water is high; it forms dark reaction products as rapidly as fructose in water at 65 °C with an oxalic acid catalyst.50 The yield of the precipitated HTC-RNAs increased on increasing the concentration of RNA (Figure S1a), and reached nearly 22% (Figure S1a) for the HTC of 0.5 mg/mL RNA at 180 °C. A similar increase in the yield upon increasing the concentration of precursor has been observed in the hydrothermal carbonization reactions of starch and glucose.51 The yield of HTC-RNA did not vary significantly with the carbonization temperature over the range 180–240 °C (Figure S1b). Further studies of the chemical reactions undergone by RNA under HTC conditions could lead to increased yields of HTC-RNA; however, we have not focused on optimizing yield here.

Hydrothermal carbons synthesized from RNA

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The carbon concentration in the HTC-RNA increased with temperature, from 55 wt% after hydrothermal treatment at 160 °C to 67 wt% after treatment at 240 °C (Table S1 and Figure S2), consistent with literature findings for other sugars.52 The O concentration decreased more significantly with temperature than the N concentration, and the H/C ratio increased slightly with the temperature of treatment. The infrared spectrum (Figure S3) of HTC-RNA prepared at 160 °C showed many more peaks in the region 2000–400 cm–1 than the spectra of the samples prepared at higher temperature, and we interpret this to indicate that the molecular nature of the azacyclic bases and sugars were best retained at low temperature.

Prior to use as a gas adsorbent or supercapacitor electrode, a hydrothermal char is activated to give a more porous carbon material with increased surface area.53 This is accomplished by heating the char in the presence of a weakly oxidizing gas such as CO2 or steam, or in the presence of a condensed-phase acid or base. Activation with condensed-phase etchants can generally be performed using shorter reaction times, and gives higher yields; however, it necessitates subsequent washing steps.54 Further, the surface chemistry of the activated carbon varies depending on the activating agent used.55 Here, we tested both types of activation.

CO2-activated carbons

The CO2-activated ACs derived from HTC-RNA (here labelled AC-HX-Y, where X is the HTC temperature in °C and Y the duration of CO2 activation in h) occurred as intergrown/merged spheres with characteristic dimensions of 0.2–5 µm (Figure 1). Hydrothermal carbons from glucose, saccharose, and biomass form spherical colloids during hydrothermal treatment under certain circumstances.28, 52, 55-58 In this study, we also confirmed that CO2 activation did not disturb the morphologies of HTC biomass or HTC sugar particles in general.22, 55 AC-H160 formed smaller particles than AC-H200 and AC-H240, consistent with the formation of small spheres in the hydrothermal carbonization of glucose at low temperature28, 52 and concentration.59 Samples activated in CO2 for extended periods developed significant ash contents (up to almost 30 wt%), thus, a hydrochloric acid treatment was applied to reduce the ash content of all of the as-synthesized ACs produced using this method (see examples Table S2).



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Figure 1. Scanning electron microscopy images of (a) AC-H160, (b) AC-H200, and (c) AC-H240 prepared by CO2 activation for 16 h, as well as AC-H200 prepared by heating in CO2 for (d) 4 h, (e) 8 h, (f) 12h. Scale bar: 1 µm.

The ACs derived from the activation of HTC-RNA in CO2 displayed IUPAC Type Ib60 N2-sorption isotherms (Figure S4), indicating that the materials were microporous. The surface areas SBET and micropore surface areas (as given by SDR-N2 or Sµp-t) (Table S3) were strongly correlated (R = 0.87) to the duration of CO2 activation, and the sample that had undergone hydrothermal carbonization at 240 °C prior to pyrolysis for 16 h took up noticeably less N2 than those that had been hydrothermally carbonized at 160 or 200 °C and pyrolyzed for 16 h. Thus the surface area was highest for AC-H200-16 (SBET = 1000 m2/g, SDR-N2 = 1070 m2/g; Table S3); the SBET of 1000 m2/g measured for this sample is somewhat lower than the highest SBET values obtained for CO2-activated ACs prepared from HTC glucose by Thomberg et al.36 and Romero-Anaya et al.55 The PSD for AC-H200-16 (Figure S5a) showed mainly ultramicropores (7 mmol CO2 at 1 bar and 0 °C (Figure 3b).

The N2-sorption isotherms measured for AC-800-K1 and -K4 (obtained in 23 and 7.7% yield respectively) were also Type Ib (see Figure 4a), but in contrast to those for AC-700-K1 and -K4, revealed pore size distributions (Figure 4b) that contained large micropores, and AC-800-K4 also had some narrow mesopores (mostly with diameters of ~2.3 nm, but some as large as 4 nm).60 This is consistent with literature reports that increased temperature and KOH loading during activation enhance the formation of mesopores.21, 65 AC-800K1 and -K4 took up less CO2 (Figure 4c) and thus had less ultramicropore volume (Figure 4d, SDR-CO2 in Table 1) than the AC-700 samples. The trend of a lower activation temperature promoting the formation of micropores and ultramicropores has been observed by others21, 65, 66 and us.35

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(c)

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Figure 3. Gas sorption data for activated carbons prepared by KOH-activation of hydrothermal char from RNA (HTC-RNA) at 700 ºC. (a) N2 adsorption–desorption isotherms; (b) pore size distributions derived from NLDFT treatment of the N2 adsorption– desorption isotherms; (c) CO2 adsorption–desorption isotherms; (d) pore size distributions derived from DFT treatment of the CO2 adsorption–desorption isotherms. Samples are labelled AC-700-KY, where Y is mKOH:mHTC-RNA used for activation. In isotherms, filled circles are adsorption points, whereas open circles give desorption points.



(a)

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AC-800-K1 AC-800-K4

AC-800-K1

1.2

50

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pore

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[Å]

Figure 4. Gas sorption data for activated carbons prepared by KOH-activation of hydrothermal char from RNA (HTC-RNA) at 800 ºC. (a) N2 adsorption–desorption isotherms; (b) pore size distributions derived from NLDFT treatment of the N2 adsorption– desorption isotherms; (c) CO2 adsorption–desorption isotherms; (d) pore size distributions derived from DFT treatment of the CO2 adsorption–desorption isotherms. Samples are labelled AC-800-KY, where Y is mKOH:mHTC-RNA used for activation. In isotherms, filled circles are adsorption points, whereas open circles give desorption points.

Table 1. Surface areas, pore volumes, and average pore sizes of the activated carbons, calculated from their N2 and CO2 adsorption isotherms.a Sample Surface area Sb Pore volume Vb L0 c (m2/g) (cm3/g) (nm) ACSBET SDR-N2 SNLDFT Sµp-t Sslit Sext Smp-BJH SDR-CO2 SDFT-CO2 Vt Vµp-t VDR-N2 VDR-CO2 Vmp-BJH L0-N2 700-K1 1630 1750 1770 1580 1210 55 16 890 490 0.675 0.623 0.623 0.358 0.019 1.03 700-K4 2010 2170 1450 1950 1480 63 20 1020 540 0.836 0.771 0.771 0.409 0.029 1.05 800-K1 2390 2420 1490 2320 1290 74 24 720 360 1.02 0.952 0.859 0.289 0.025 1.33 800-K4 2110 2030 1140 1870 774 242 69 450 190 1.06 0.845 0.721 0.181 0.081 1.86 800-KC2 2210 2250 1310 2080 1160 130 34 890 330 0.988 0.865 0.800 0.356 0.046 1.38 800-KC8 2400 2450 1360 2240 1340 160 43 1060 410 1.07 0.917 0.870 0.423 0.056 1.30 a Activated carbons were derived from hydrothermally carbonized ribonucleic acid (HTC-RNA) by activation for 2 h under flowing N2. Samples activated with KOH and KHCO3 are labelled AC-X-KY and AC-X-KCY, respectively, where X is the activation temperature and Y is the mass ratio of activating agent to HTC-RNA. b BET = Brunauer–Emmett–Teller, DR = Dubinin–


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Radushkevich, µp-t = micropores, calculated using the t-plot method, slit = geometric surface area of a slit-shaped pore, ext = external, BJH = Barrett–Joyner–Halenda; see experimental section for further details. c L0-N2 = average pore width, calculated from the characteristic adsorption energy E0 determined from DR treatment of N2 adsorption data according to L0-N2 = 13.7/(E0 – 9.7 kJ/mol).

KHCO3-activated carbons

The activation of chars with KHCO3 is much less explored than that with KOH; however, it has been used to derive high-surface area ACs,67 and HTC-glucose has recently been activated with KHCO3.68 KHCO3 is arguably less hazardous than KOH, and we therefore activated HTC-RNA (prepared by hydrothermal carbonization at 200 °C for 24 h) with KHCO3 at 800 °C. Based upon the ratios mKHCO3:mHTC ≤ 8 used by Sevilla and Fuertes,68 we activated HTC-RNA in the presence of 2 or 8 mass equivalents of KHCO3 to give the activated carbons AC-800-KCY (Y = 2 or 8). These materials retained the spherical morphologies of the HTC-RNA precursors (Figure 5), consistent with the findings of Sevilla and Fuertes.68

Figure 5. Scanning electron microscopy images of (a) AC-800-KC2 and (b) AC-800-KC8, which were prepared by activating hydrothermally carbonized ribonucleic acid (HTC-RNA) with 2 and 8 mass equivalents of KHCO3, respectively, at 800 °C. Scale bar: 1 µm.

N2 adsorption–desorption isotherms (Figure 6a) of the AC-800-KCY materials were Type Ib, revealing them to be microporous (Figure 6b). AC-800-KC8 took up 6 mmol/g CO2 at 1 bar and 0 °C (Figure 6c), which is somewhat less than the uptake of AC-700-K4 (vide supra), but still high. The PSDs were similar for these two carbons (Figures 3b,d and 6b, d).



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AC-800-KC2 AC-800-KC8

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Figure 6. Gas sorption data for activated carbons prepared by KHCO3-activation of hydrothermal char from RNA (HTC-RNA) at 800 ºC. (a) N2 adsorption–desorption isotherms; (b) pore size distributions derived from NLDFT treatment of the N2 adsorption– desorption isotherms; (c) CO2 adsorption–desorption isotherms; (d) pore size distribution derived from DFT treatment of the CO2 adsorption–desorption isotherms. Samples are labeled AC-800-KCY, where Y is mKHCO3:mHTC-RNA used for activation.

Electrochemical studies of selected activated carbons

The electrochemical performances of the ACs were studied in the manner applied in one of our recent studies.35 The four ACs with the highest ultramicropore volume, as well as AC-H200-16, which had the highest surface area among the carbons activated in CO2, were chosen for initial investigations using a threeelectrode cell in 6-M KOH(aq). At a low sweep rate (2 mV/s), AC-800-KC8 showed the best specific capacitance of 302 F/g (Table S6, Figure S7a and b), comparable to other high-performing electrodes based on ACs derived from HTC sugars or HTC biomass.37, 68, 69 AC-800-KC8 also showed the best retention of specific capacitance and rate capability (percentage of specific capacitance retained at higher scan rates) at 14 ACS Paragon Plus Environment

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scan rates up to 40 mV/s (Figure S7b). Based solely on the level of ultramicroporosity, AC-700-K4 would be expected to display the highest capacitance, but it displayed a capacitance of only 254 F/g at 2 mV/s, and lost more capacitance upon increasing from 2 to 40 mV/s than AC-800-KC8 did. Tentatively, we ascribe the high value for AC-800-KC8 to a combination of the high ultramicroporosity and a high atomic ratio nC/nO (Table S4), which may be a result of the higher activation temperature used for this sample.

The surface N content of AC-800-KC8 and two additional ACs were measured using X-ray photoelectron spectroscopy (Table S5). This method was chosen over elemental analysis as it had been somewhat complicated for us to fully combust the ACs produced in this and related studies. AC-800-KC8 had a surface N content of 1.1 at%, which was highest among the samples measured here. Higher values (3–6 at%) have been obtained for ACs derived from the hydrothermal carbonization of biomass or biomass-derived compounds followed by pyrolysis in the presence of KOH at 600 °C,32, 44, 70 which is expected given that Ncontaining functional groups are less stable above 500–600 °C. However, in only one of these cases did the resulting AC display higher capacitance than AC-800-KC8.44 This comparison illustrates the trade-off between a high N content and a high degree of ultramicroporosity, which favors both capacitance and CO2 uptake capacity, in ACs activated at high temperature; however, we cannot eliminate the possibility that nucleobase-derived N is less stable than other forms of N under pyrolysis conditions. A waste-seed-derived AC that was synthesized via hydrothermal carbonization and subsequent pyrolysis with KOH and that had both higher N content and higher capacitance than AC-800-KC8 was reported recently, but an added N source (urea) was used in that case.71

The electrode material AC-800-KC8, containing 1.1 at% N and having showed promising results in the initial electrochemical experiments, was further studied in a symmetrical two-electrode setup in order to better emulate an actual supercapacitor.72 The same 6-M KOH(aq) electrolyte was employed. CV curves measured for sweep rates up to 200 mV/s (Figure 7a and b) were used to analyze the sweep rate dependency of the capacitance (Figure 7c). Further, galvanostatic discharge curves were recorded at different specific currents (Figure 7d and e), and the resulting curves were used to determine the specific capacitance as a function of the specific current used. At specific currents of ≥2 A/g, the charge and discharge times were below 60 s, in the recommended measurement range corresponding to typical applications.72 At current 15 ACS Paragon Plus Environment


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densities of 5 and 10 A/g (Figure 7e), an IR drop was visible upon the initiation of discharging. As these measurements were performed on a small supercapacitor cell, the calculated resistive drop of 2.6–3.1 Ω can be considered a reasonable measure of the equivalent series resistance of the cell.72, 73 The AC-800-KC8 retained its capacitance well for currents up to 10 A/g, where it had a capacitance of 181 F/g (79% of the value measured at 0.32 A/g). Thus AC-800-KC8 compares well to reported carbon-based supercapacitors that have been tested under similar conditions, i.e. in a two-electrode cell using a concentrated KOH(aq) electrolyte and with information reported for current densities of at least 5 A/g (Table S7), which have shown specific capacitances between 60 and almost 300 F/g at current densities of 10 A/g. Notably, it outperforms

some

supercapacitors

made

from

N-rich

synthetic

polymers

like

polyvinylpyrrolidone/polyacrylonitrile composites74,75 or some melamine/formaldehyde resins.76, 77 On the other hand, some biopolymer-derived activated carbons, such as those derived from gelatin78, 79 or casein,80 have shown higher energy-storage capacities under these conditions.

Figure 7. Electrochemical data measured for AC-800-KC8 in a symmetrical 2-electrode setup in 6-M KOH(aq). (a, b) CV curves at scan rates of 2–200 mV/s; (c) specific capacitances (Cm,2E) calculated from the CV curves in (a) and (b); (d) galvanostatic discharge curves at 0.3–2.2 A/g; (e) galvanostatic charge–discharge curves at 5 and 10 A/g; (f) specific capacitances (Cm,2E) calculated from the galvanostatic discharge curves in (d) and (e).

Finally, the cyclic stability of AC-800-KC8 was also tested. Thus the electrode was subjected to 4000 scans at a scan rate of 100 mV/s, then an additional 4000 scans at a more rapid scan rate of 150 mV/s, and lastly


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for another 2000 scans at a scan rate of 100 mV/s. No deterioration of the capacitance was observed (Figure 8).

Figure 8. (a) Specific capacitance of AC-800-KC8 over multiple cycles, as well as capacitance normalized to the value at Cycle 5. Scan rates were 100 mV/s in Cycles 1–4000 and Cycle 8001–10000, and 150 mV/s in Cycles 4001–8000). (b) Comparison of the fifth and 10000th CV curves.

Conclusions N-doped ACs were derived from the activation of HTC-RNA and tested for their ability to adsorb CO2 and store electrical energy. One of the N-doped ACs took up more than 7 mmol/g CO2 (at 1 bar and 0 °C), a high value related to its high ultramicropore volume. Another N-doped AC, which also had very high ultramicroporous and microporous volumes, and which contained 1.1 at% N at its surface, displayed a high specific capacitance of ~300 F/g in a three-electrode cell. This material was also tested as part of a twoelectrode cell, and displayed a specific capacitance of 181 F/g at a current density of 10 A/g. Overall, the general relation between ultramicroporosity, low-pressure CO2 uptake, and specific capacitance was supported by this study. However, some minor differential trends were observed related to the uptake of CO2 and the specific capacitance. Overall, RNA derived from yeast was confirmed to be a potential source of functional N-doped activated carbons.

Conflicts of interest

There are no conflicts of interest to declare.



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Acknowledgement

We acknowledge support from the Carl Trygger Foundation for a postdoctoral fellowship for KKL. Anas Afkir and Marc Lombard are acknowledged for their experimental support. Marie Ernstsson and Mikael Sundin are acknowledged for the XPS analyses.

References

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TOC Graphic:


RNA as a Precursor to N-Doped Activated Carbon

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