ORIGINAL PAPER

Hierarchically porous carbon from foamed Mg chelatefor supercapacitor and capacitive deionization

Shuhui Liu1& Yingna Chang1

& Biao Han1& Yuge Zhao1

& Guoxin Zhang2& Zheng Chang1

Received: 25 November 2019 /Accepted: 17 April 2020# Springer-Verlag GmbH Germany, part of Springer Nature 2020

AbstractPore hierarchy facilitates the mass transportation/exchange between the interior surface and bulk solution, which is critical for theenhancement of capacitive performance. Herein, by applying in situ foamedMg chelates as precursors, we managed the scalablefabrication of hierarchically porous carbon (HPC) materials and explored their capacitive applications. Particularly, citric acidfirst reacted with magnesium nitrate to form Mg chelate while the generated gaseous HNO3 molecules bubbled the intermediatecarbon framework to produce abundant open pores. The as-made precursors were then submitted to potassium hydroxideactivation for a high carbonization degree and rich meso-/micropores. The optimized sample (HPC-2) exhibited very highspecific capacitance of 213.5 F g−1 in neutral NaCl solution and a high rate capability of ~ 67.5% at 10.0 A g−1. Furthermore,it showed impressive capacitive deionization performance regarding high removal efficiency (67.1%), large capacity of1810.1 mg g−1 (in 2200 mg L−1 NaCl solution), and robust cycling stability.

Keywords FoamedMg chelate . Hierarchically porous carbon . Supercapacitor . Capacitive deionization

Introduction

Nowadays, excessive consumption of fossil fuels has causedseveral time-ticking energy and environmental issues [1].Electrical double-layer capacitance (EDLC)-orientedsupercapacitors and the capacitive deionization (CDI), asclean and efficient technologies for energy storage and cleanwater, have drawn intensive worldwide attention from bothresearch and practical fields. Meanwhile, the capacitive deion-ization (CDI) technologies have been regarded as one of thecurrently available strategies for the desalination of seawater[2–9]. The performance for both EDLC and CDI is ruled by

the electrostatic adsorption of electrolyte ions on carbon-basedelectrode material with large specific surface areas. Hence, itis critical to exploit electrode materials with beneficial porousstructures.

Carbonaceous materials (CMs), taking advantage of abun-dant resources and their high adjustability of structure, mor-phology, and porosity, have been regarded as the most prom-ising candidates for the fabrication of EDLC [10–13]. It hasbeen reported that over 80% of commercially availablesupercapacitors are fabricated based on carbon materials[14]. Considering the efficient management of open poresfor CMs, the most common strategy is to plant either solidor liquid templates before the carbonization of precursors[15–18]. The advantages of these ways are obvious becausethe resulted pores can be programed via pre-designed tem-plates [19]; however, tedious post-treatments are often re-quired to remove the templates. Recently, a gas-bubblingmethod for the efficient synthesis of porous CMs involvingvolatile reagents such as NH4Cl has gained lots of attentiondue to its easy operation and simple post-treatment [20–22].To combine the advances of both solid and gas, templates maypotentially open up the scalable low-cost production of high-performance hierarchically porous carbon (HPC) electrodematerials for practical applications, especially supercapacitorand CDI [23].

Electronic supplementary material The online version of this article(https://doi.org/10.1007/s11581-020-03584-8) contains supplementarymaterial, which is available to authorized users.

* Guoxin Zhangzhanggx@sdust.edu.cn

* Zheng Changchangzheng@mail.buct.edu.cn

1 State Key Laboratory of Chemical Resource Engineering, BeijingUniversity of Chemical Technology, Beijing 100029, China

2 College of Electrical Engineering and Automation, ShandongUniversity of Science and Technology, Qingdao 266590, China

https://doi.org/10.1007/s11581-020-03584-8

/ Published online: 14 May 2020

Ionics (2020) 26:4713–4721

In this study, inexpensive biomass of citric acid (CiA) wasapplied to react with magnesium nitrate (Mg(NO3)2) underadjusted temperature (180 °C) in air ambiance to form Mgchelate. The reaction between CiA and Mg (NO3)2 could besymbolized by Formula (1) [24] displayed below, in whichMg ions played a key role in the formation of carbon frame-works. The bubbling of in situ generated gaseous HNO3 andH2O molecules caused carbon frameworks to form rich openpores. Hierarchically porous carbon (HPC) could be obtainedafter the precursors were further annealed at 600 °C with thepresence of KOH to gain high conductivity and abundantmeso-/micropores. The reaction mechanism was shown inFormula (2) [25] and (3) [26]. The optimized HPC-2 sample,as revealed by electrochemical measurements, showed a highspecific capacitance of 213.5 F g−1 (1.0 A g−1) in 1.0 mol L−1

NaCl and maintained over 67.5% of its capacitance at highcurrent density (10.0 A g−1). Furthermore, the HPC-2 alsoperformed reasonably good CDI performance toward NaClin terms of desalination capacity, removal efficiency, and cy-cling stability.

2C6H8O7⋅H2Oþ 3Mg NO3ð Þ2 � 6H2O →180°C

Mg3 C6H5O7ð Þ2þ6HNO3↑þ 20H2O↑

ð1Þ

Mg3 C6H5O7ð Þ2 →25−600°C;N2

3 MgOð Þ � xCþ yCO↑þ zCO2↑

þ5H2O↑ xþ yþ z ¼ 12ð Þð2Þ

6KOHþ C→Δ2K þ 3H2↑þ 2K2CO3 ð3Þ

Experimental

Material

Citric acid monohydrate (CiA) and magnesium nitrate hexa-hydrate (Mg(NO3)2·6H2O) were used as starting materials.CiA was purchased from Beijing Chemical ReagentsCompany. Mg(NO3)2·6H2O was purchased from TianjinGuangfu Fine Chemical Institute. Potassium hydroxide(KOH) was purchased from Beijing Lark Technology Co.Ltd. All the reagents were of AR grade used without anypurification, and deionized (DI) water was used in allexperiments.

Preparation of hierarchically porous carbon

In a typical procedure, 15.0 mL aqueous solution containing0.01mol L−1Mg(NO3)2 and 0.02mol L−1 CiAwas placed in a200-mL capacity beaker. After magnetic stirring for 10 min at

room temperature, the solution became clear. Then, the beakerwas transferred to an oven and heated at 180 °C for 2 h to getthe precursors. After hand grinding in an agate mortar, themixture of KOH and the precursor (their weight ratios were15 wt%, 25 wt%, and 50 wt%, respectively.) was transferredto a tube furnace and heated at 600 °C for 2 h with a heatingrate of 10 °C min−1 under N2 protection. Afterward, the prod-uct was ground to fine powders and washed with diluted hy-drochloric acid, water, and ethanol several times, followed bydrying at 80 °C overnight. According to the KOH-to-precursor ratios of 15 wt%, 25 wt%, and 50 wt%, the finalproducts were denoted as HPC-1, HPC-2, and HPC-3, respec-tively. For comparison, we dissolved 0.01 mol L−1 magne-sium citrate in 15.0 mL aqueous solution to obtain precursorsfor HPC; the precursive solution was also heated at 180 °C for2 h.

Characterization

The morphologies of the materials were examined using scan-ning electron microscopy (SEM, Zeiss SUPRA 55) and trans-mission electron microscopy (TEM, FEI Technai G2 F20 mi-croscope). Chemical compositions were determined by X-rayphotoelectron spectroscopy (XPS, Thermo ElectronESCALAB 250). The nitrogen adsorption and desorption iso-therms were measured at 77 K with a QuantachromeAdsorption Instrument (Quantachrome Autosorb-1C-VP).Before the measurements, the samples were degassed at200 °C in a vacuum for 3 h. The specific surface area wascalculated from the adsorption branch according to theBrunauer-Emmett-Teller (BET) method. The pore size distri-bution plot was derived from the desorption branch of theisotherm based on the Barrett-Joyner-Halenda (BJH) method.The X-ray diffraction (XRD) data were collected with aShimadzu XRD-6000 diffractometer with Cu Ka radiation(λ = 1.5418 Å). Raman spectra were recorded from 100 to4500 cm−1 on the LabRAM ARAMIS Raman system with a532-nm argon-ion laser for excitation.

Electrode preparation

HPCs were used as electrode materials. In a typical procedure,HPCs, carbon black, and polytetrafluoroethylene (PTFE,Aldrich; 60 wt% dispersion in water) were mixed in an appro-priate volume of ethanol according to the proportions of80 wt%, 15 wt%, and 5 wt% and then sonicated for 1 h. Forthe preparation of supercapacitor electrodes, 20.0 μL suspen-sion containing 0.2 mg HPCs was dropwise onto the glassycarbon electrode (diameter = 0.98 cm). For the preparation ofCDI electrodes, 10.0 mL suspension containing 60 mg HPCswas first drop coated on a graphite sheet with dimensions of6 cm × 7 cm (the sample loading on current collector is about

4714 Ionics (2020) 26:4713–4721

1.43 mg cm−2). The as-fabricated electrodes were dried at80 °C overnight to remove any remaining solvent.

Electrochemical measurements

Cyclic voltammetry (CV), galvanostatic charge/discharge(GCD), and electrochemical impedance spectroscopy (EIS)measurements were performed in 1.0 mol L−1 NaCl aqueoussolutions on a CHI 660D electrochemical station (ChenhuaInstrument Company) using a three-electrode configured set-up that consisted of a working electrode, a Pt counter elec-trode, and a calomel reference electrode. CV and GCD mea-surements were carried out in a potential window of 0–1.0 V,and the frequency range of EIS impedance was 100 kHz–0.01 Hz. The cycling stability was also measured at a currentdensity of 10.0 A g−1 in a potential window of 0–1.0 V. Thespecific capacitances (Cs) were calculated from GCD curvesaccording to the equation:

Cs ¼ I �Δtm� V

ð4Þ

where I is the discharge current density, Δt is the dischargetime, m is the loaded mass of the active materials, and V is thedischarge voltage difference.

Capacitive deionization measurements

In a typical CDI measurement [27], 100.0 mL NaCl aqueoussolution with an initial conductivity of ~ 100.7 μS cm−1

(45.0 mg L−1) in a beaker was continuously fed into the CDIunit using a peristaltic pump with a constant flow rate of25.0 mL min−1 and the effluent was returned to this beaker.Meanwhile, the applied voltage was 1.4 V. The solution con-centration was monitored by a conductivity meter (Type 308F,Leici Company) located at the outlet of the CDI unit. Thedesalination capacities (W) were calculated according to theequation:

W ¼ C0−Cð Þ � Vm

ð5Þ

whereC0 andC are the initial and final concentrations, respec-tively, calculated from the corresponding conductivities, V isthe total solution volume, and m is the mass of the activematerials in both electrodes. The accumulated ion removalrate was given by the accumulated mass of desalination ca-pacities (W) divided by the operation time (t) [28], as shownbelow.

U ¼ Wt

ð6Þ

Results and discussion

Fabrication and characterizations of hierarchicallyporous carbon

As briefed in the introduction section, the formation of thechelate structure was induced by Mg ions [24, 29]. The heattreatment on the fully contacted CiA and Mg(NO3)2 wouldlead to a fast and uniform generation of gaseous byproductsincluding HNO3 and H2O that could bubble the carbon frame-works into porous materials. As visualized in Fig. 1a, b, thevolume expanded after heat treatment was almost 20-fold ofthe initial reactants. Figure. 1c shows the typical morphologyof the as-obtained precursors, in which macropores from gasbubbling could be clearly observed. If directly starting withmagnesium citrate (Mg3(Cit)2), only minor volume expansionoccurs (Fig. S1A/B), and no such macropores were generated(Fig. S1C). It suggested that the roles of CiA and Mg(NO3)2were critical to the porosity of the foamed precursors.Furthermore, residual nitrate salt could be reduced by adjacentcarbon species during high-temperature annealing, leading tonitrogen doping. After KOH activation at high temperature,the overall morphology of HPCs showed little variation fromtheir precursors, as typified by the SEM image of HPC-2(heated at 600 °C with the KOH-to-precursor ratio of25 wt%) in Fig. 1d. The texture of HPC-2 was further revealedby the TEM image in Fig. 1e, revealing plentiful mesoporesprobably originated from in situ formed MgO or K2CO3 [24].Similar morphologies with embedded macropores were alsoobserved for HPC-1/3 (respectively with the KOH/precursorratios of 15/50 wt%), while their mesoporous characters werenot obvious (Fig. S2).

The pore hierarchy of HPCs was examined by BET mea-surements as shown in Fig. 2a. Large amounts of mesoporeswere verified in the type IV isotherm of HPC-2; meanwhile,the long vertical curves at P/P0 approaching 0 clearly indicatedthe presence of abundant micropores which could greatly ex-tend the surface area of porous materials [30, 31]. HPC-1/HPC-3 also showed type IV isotherms but their contents ofboth mesopores and micropores were much less than that ofHPC-2, which was possibly due to the balancing effects ofmelting and activation from KOH [32]. In the pore distribu-tion curves of HPCs (Fig. 2b), the high intensity was spanningin the pore diameter range from 0.5 to 100.0 nm. Thus, it wasproved that the HPCs, especially HPC-2, possessed rich hier-archical porosity (simultaneously consisting of micropores,mesopores, and macropores). Correspondingly, HPC-2achieved the highest specific surface area (SSA) among allHPCs samples, reaching 858.0 m2 g−1. Two other samples,HPC-1 and HPC-3, obtained SSAs of 279.4 and220.8 m2 g−1, respectively. Additionally, electrochemical sur-face area (ECSA) measurements were performed to reveal theaccessible surface area in the electrochemical analysis. As

4715Ionics (2020) 26:4713–4721

shown in Fig. S3, the ESSA of HPC-2 was calculated to be82.9 mF cm−2, larger than that of HPC-1 (59.3 mF cm−2) andHPC-3 (46.0 mF cm−2). Generally speaking, the accessiblesurface area was highly correlated with capacitance and porehierarchy could facilitate the fast transportation of ions[33–35], which enlightened us that HPC-2 would be expectedto have the best capacitive performance.

XPS measurement was employed to investigate the ele-mental compositions of HPCs. In Fig. 2c of the XPS elementalsurvey, two main types of elements were presented in allHPCs: carbon at ~ 285 eV and oxygen at ~ 533 eV.Meanwhile, a very small peak of nitrogen centering at ~400 eV was also detected. The nitrogen doping might be as-cribed to the reduction of residual nitrate in the precursors by

0.0 0.2 0.4 0.6 0.8 1.0

0

100

200

300

400

500

600

VolumeAd

sorpt io

n( cm

3g-

1 )

Relative Pressure (P/P0)

HPC-1-279.4 m2 g-1

HPC-3-220.8 m2 g-1

HPC-2-858.0 m2 g-1

0.0

0.1

0.2

dV/dlog(w

) PoreVo

l .( cm

3g-

1 )

Pore Diameter (nm)

HPC-1

HPC-3

HPC-2

10 100

290 288 286 284 282

C=OC-O / C-N C-C / C=C

HPC-3

HPC-2

HPC-1

Intens

ity

Binding Energy (eV)396 398 400 402 404 406 408

Graphitic NPyrrolic N

HPC-3

HPC-2

HPC-1

Intens

ity

Binding Energy (eV)

Pyridinic N

0

5

10

15

80

85

90

Percen

tage

(%)

C NO

HPC-1 HPC-2 HPC-3

a b c

d e f

700 600 500 400 300 200 100

440 400 360

C1s

N1sO1s

HPC-3HPC-2HPC-1

Intens

ity

Binding Energy (eV)

Intens

ity

Fig. 2 BET measurements on HPCs: a N2 adsorption/desorption iso-therms and b pore size distributions of HPCs. XPS measurements on

HPCs: c elemental survey, d elemental content analysis, e C1s spectra,and f N1s spectra

Fig. 1 a and bGraphical abstract of fabricating precursors for hierarchically porous carbon (HPC). SEM images of c the precursor for HPCs and dHPC-2. e TEM image of HPC-2

4716 Ionics (2020) 26:4713–4721

adjacent carbon during high-temperature annealing [36]. Thedetails of element contents calculated from XPS are shown inFig. 2d. The contents of carbon and oxygen for all HPCs werekept at a similar level, which was possibly due to the sameannealing temperature applied [37], while HPC-2 showed alittle bit higher N content (3.91 at.%) than HPC-1/HPC-3(2.92 and 3.89 at.%, respectively) did. C1s spectra of HPCsare displayed in Fig. 2e along with their deconvolutions, inwhich the main peak centered at ~ 284.5 eV was attributableto the C–C and C=C bonding taking over 49.1% of the wholecarbon content, whereas the second largely contributed spe-cies centered at ~ 286.5 eV was assigned to C–O and/or C–N.Very low intensity of broadband (~ 287.9 eV) suggested theminor presence of C=O, suggesting that most of the oxygen-ated groups of citric acid had been removed during thermaltreatment [38, 39]. N1s spectra (Fig. 2f) could be fitted intothree N species: pyridine N (~ 398.3 eV), pyrrolic N (~400.1 eV), and graphitic N (~ 401.2 eV) [40, 41]. Pyrrolic Ntook overwhelming possession (> 40.8%), which served asion-anchoring sites to enhance capacitance [42]. XRD profilesin Fig. S4A reveal that the HPCs are in an amorphous state.Their defective nature was verified by the intensive D bands ofHPCs in Raman spectra (Fig. S4B); meanwhile, the HPCsshowed considerable intensity of G bands, which receivedreasonably good electrical conductivity from partial graphiti-zation [43]. Furthermore, HPC-2 revealed an intensity ratiobetween the D band at 1358 cm−1 and the G band at1584 cm−1 (ID/IG) of about 0.95, which also reflected its betterconductivity compared to two other samples.

Supercapacitor performance of HPCs

The HPCs were firstly fabricated into electrodes forsupercapacitor measurements in neutral NaCl electrolyte.There were very few studies on supercapacitors with neutralNaCl electrolytes due to low capacitances while capacitivedeionization performance was highly correlated with capaci-tive behavior in NaCl solution. Herein, their capacitive perfor-mance was evaluated in 1.0 mol L−1 NaCl electrolyte insteadof commonly used 6.0 mol L−1 KOH. Cyclic voltammetry(CV) curves of HPCs were recorded with a scan rate of100.0 mV s−1 in Fig. 3a. The largest loop area of CV curvewas observed for HPC-2, which was in good agreement withSSA and ESSA results [44, 45]. Galvanic charge/discharge(GCD) curves of HPCs collected with varied current densitiesfrom 0.5 to 10.0 A g−1 are displayed in Fig. S5. For compar-ison, we extracted GCD curves with current densities of1.0 A g−1 and 10.0 A g−1. As shown in Fig. 3b, c, the HPCsdisplayed typical EDLC behaviors by performing nearly sym-metric charge/discharge profiles. Among all HPCs, HPC-2showed the largest voltammogram area and the longestcharge/discharge time duration. Remarkably, it obtained animpressive specific capacitance of 213.5 F g−1 (at the current

density of 1.0 A g−1) in a neutral electrolyte of 1.0 mol L−1

NaCl. HPC-1 and HPC-3 also submitted the specific capaci-tances of 149.1 F g−1 and 148.6 F g−1, respectively, indicatingthe dominant effect of KOH activation.

The HPC-2 derived from the foamed Mg precursor per-formed much higher specific capacitance than the controlsample frommagnesium citrate after the sameKOH activation(Fig. S6), suggesting the importance of foaming the Mg/Cprecursor. Meanwhile, HPC-2 maintained about 67.5% of itscapacitance at the high current density of 10.0 A g−1, reaching144.3 A g−1 as shown in Fig. 3d, which suggested that highrate capability was ascribed to the beneficial pore hierarchythat facilitates fast ion transportation. Electrochemical imped-ance spectroscopy (EIS) measurement is recognized as one ofthe principal methods to reveal the behaviors of electrodematerials [46]. Nyquist plots of HPCs were also collected in1.0 mol L−1 NaCl solution. As shown in Fig. 3e and its inset,the resembled curve shapes consisting of a very small reactionimpedance arc and a vertical liner tail could be observed forthe HPCs and HPC-2 performed the most vertical tail imply-ing the best ion transportation behavior between the interiorsurface and bulk solution. Additionally, HPC-2 exhibited re-markably high cycling stability, over 94.4% of its capacitancemaintained after cycling for 5000 times at 10.0 A g−1 (Fig. 3f).Therefore, the HPCs can be trusted as one type of excellentelectrode materials for supercapacitor in neutral electrolytes.

CDI performance of HPCs

The essence of CDI is forming electrical double layers for ionabsorbing on the accessible surface of electrode materials un-der an applied voltage [47, 48]. Using electrode materials withlarge SSA and abundant functionalities for ions to land is ofhigh practical importance to enhance CDI performance [22,49–51]. In this study, we are to emphasize the effects of ben-eficial pore structure. Hierarchical pore structure, as reportedby many literature, can facilitate ion transport by providingshorter diffusion pathways and smaller resistances in the elec-trochemical processes [46, 52]. The ion buffering reservoirsformed in macropores or even bigger open pores can greatlyminimize ion diffusion distances into interior pores [46].Therefore, our porous carbon materials with abundant openpores were fabricated into CDI electrodes. The CDI perfor-mances were evaluated in 45.0 mg L−1 NaCl solution at anapplied voltage of 1.4 V; however, the applied operating volt-age of 1.4 V is lower than the voltage for splitting water. Weperformed ultraslow polarization characterization to detectany oxidation/reduction peaks to determine the extent of fa-radic reaction at the applied voltage of 1.4 V. As shown in Fig.S7, one can observe that the faradic reaction is roughly oc-curred at ~ 1.58 V and becomes apparent at ~ 1.7 V, meaningthat the capacitive deionization processes are free of faradicreaction at 1.4 V.

4717Ionics (2020) 26:4713–4721

A bench-scale CDI testing apparatus consisted of apump, a conductivity meter, an electrical power supply,a self-made CDI unit, and a feed water reservoir, as sche-matically shown in Fig. S8 [27]. The concentrationchange of NaCl solution was calculated from conductivitytransient, and no voltage was applied in the first 20 min toexclude the physical adsorption. Little conductivity de-creases in this period suggested very low physical adsorp-tion capacity of the HPCs, which can benefit the enhance-ment of desalination capacity and recycling stability. Fig.S9 showed the conductivity change plotting to the elaps-ing time on the HPCs obtained with different calcinationtemperatures under the same KOH amount for activation(25 wt%), which revealed that the fast desalination behav-ior was obtained by using the sample calcinated at 600 °Cupon applied voltage (1.4 V).

Fig. 4a shows the conductivity change plotting to theelapsing time on the HPCs at 600 °C with the differentKOH amount for activation. Specifically, HPC-2 per-formed the best desalination behavior for ion removal byshowing sharp conductivity decrease from initial100.7 μS cm−1 to 33.1 μS cm−1, and the correspondingremoval efficiency was as high as ~ 67.1%. The desalina-tion capacity (also known as electrosorption capacity) at230 min stop could be calculated from Fig. 4a, in whichHPC-2 had the highest capacity of 53.1 mg g−1 (in45.0 mg L−1 NaCl), which exceeded most of the currentlyreported desalination capacities of CDI electrodes as listed

and compared in Table 1. Meanwhile, the complete de-sorption processes could be simply achieved by reversingthe applied voltage. The CDI measurements of HPC-2 inthree other NaCl concentrat ions: 220, 450, and2200 mg L−1 were also conducted and their capacitiesare calculated to be 158.9, 206.6, and 1810.1 mg g−1,respectively, as shown in Fig. 4b, which confirmed thegood applicability in high salt concentrations. Obviously,the ion removal rate of HPC-2 was also faster than thoseof HPC-1 and HPC-3, and the maximum accumulated ionremoval rates of HPC-1/HPC-2/HPC-3, calculated fromthe peaks in F ig . 4c , were 0 .276 , 0 .346 , and0.260 mg g−1 min−1, respectively. Finally, cycling stabil-ity of HPC-2 was measured and the conductivity tran-sients of four consecutive adsorption-desorption cyclesare presented in Fig. 4d. Over 93.1% of initial conductiv-ity could be obtained after each desorption process, indi-cating a good cycling stability of HPC-2. Meanwhile,considering the CDI capacity regeneration without anyenergy and secondary pollution, we believed the as-made HPC-2 would be expected for practical CDIapplication.

Conclusions

In summary, the sedimentation reaction between citric acidand Mg(NO3)2 was utilized to synthesize foamed Mg chelate

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-0.004

-0.002

0.000

0.002Current

(A)

Potential (V)

HPC-1HPC-2HPC-3

0 2 4 6 8 100

50

100

150

200

250

HPC-1HPC-2HPC-3

Cap

acita

nce(F

g-1 )

Current density (A g-1)

~60.4%

0 2000 4000 6000 8000 100000

20

40

60

80

100

Reten

tion(%

)

Cycle (n)

10.0 A g-1

92.7%

0 100 200 300 4000.0

0.2

0.4

0.6

0.8

1.0

Potential(V)

Time (s)

j = 1.0 A g-1

HPC-1HPC-2HPC-3

a b

d e f

c

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

j = 10.0 A g-1

Potential (V)

Time (s)

HPC-1HPC-2HPC-3

0 1 2 3 4 5 6 7 80

1

2

3

4

5

6

7

8

0.0 0.2 0.4 0.60.0

0.2

0.4

0.6

-Z''(oh

m)

Z' (ohm)

HPC-1HPC-2HPC-3

-Z''(oh

m)

Z' (ohm)

Fig. 3 Electrochemical measurements of HPCs in 1.0 mol L−1 NaCl: aCVmeasurements at a scanning rate of 100.0 mV s−1, b GCD curves at acurrent density of 1.0 A g−1, c GCD curves at a current density of

10.0 A g−1, d rate capability tests, e EIS curves, and f cycling stabilityof HPC-2 at a current density of 10.0 A g−1

4718 Ionics (2020) 26:4713–4721

precursors for the fabrication of hierarchically porous carbonmaterials. The precursors were then submitted to annealingwith the presence of activation agent KOH to gain both highconductivity and rich meso-/micropores besides the previous-ly formed macropores. We found that the optimized sampleHPC-2 showed impressive performance in capacitive

applications including supercapacitor and CDI, which thanksto its large specific surface area, hierarchical porosity, rich Ndoping, and high graphitization degree. HPC-2 performed rea-sonably high specific capacitance and rate capability underlarge working current streams in neutral NaCl electrolyte.Additionally, it also showed very good CDI performance

Table 1 Summary of a few high-performance CDI electrodematerials

Sample Applied voltage(V)

Initial salt concentration(mg L−1)

Desalination capacity(mg g−1)

Ref.

G/N-CFs 1.2 585 25 [53]

NCPCs-900 1.6 100 11.98 [54]

ACS25 1.2 667.9 13.3 [55]

C@NTO 1.4 500 35.12 [56]

CNT@SBE-b-CDP 1.2 500 6.37 [57]

CSG 1.5 290 9.60 [58]

Na@C 1.2 100 8.75 [59]

LSCK14 1.0 584.4 23.5 [60]

rGO@NTO 1.4 250 48.8 [61]

PC-800 1.4 500 25.16 [62]

HPC-2 1.4 45 53.1 Thiswork1.4 220 158.9

Fig. 4 Capacitive deionizationcharacterizations on HPCs: aplots of conductivity transientversus time, b electrosorptioncapacity of HPC-2 in differentNaCl concentrations, c desalina-tion rate plotting to capacity, andd cycling stability of HPC-2. Allmeasurements in panels a, c, andd were performed in the electro-lyte of 45.0 mg L−1 NaCl at ap-plied voltage of 1.4 V

4719Ionics (2020) 26:4713–4721

regarding high deionization capacity and robust cycling sta-bility. This strategy would fit the scalable production of HPCsunder low costs for practical capacitive applications.

Funding information This work was financially supported by theNational Natural Science Foundation of China (NSFC, 21701101), theProgram for Changjiang Scholars and Innovative Research Team in theUniversity (IRT1205), the Fundamental Research Funds for the CentralUniversities, the Long-Term Subsidy Mechanism from the Ministry ofFinance, the Ministry of Education of PRC, and the Shandong ScientificResearch Awards Foundation for Outstanding Young Scientists (grantnumber ZR2018JL010).

References

1. Harfoot MBJ, Tittensor DP, Knight S, Arnell AP, Blyth S, BrooksS, Butchart SHM, Hutton J, Jones MI, Kapos V, Scharlemann JPW,Burgess ND (2018) Present and future biodiversity risks from fossilfuel exploitation. Conserv Lett 11:e12448

2. Cao J, Wang Y, Chen C, Yu F, Ma J (2018) A comparison ofgraphene hydrogels modified with single-walled/multi-walled car-bon nanotubes as electrode materials for capacitive deionization. JColloid Interface Sci 518:69–75

3. Chen Y-W, Chen J-F, Lin C-H, Hou C-H (2019) Integrating asupercapacitor with capacitive deionization for direct energy recov-ery from the desalination of brackish water. Appl Energy 252:113417

4. Ma J, Wang L, Yu F (2018) Water-enhanced performance in capac-itive deionization for desalination based on graphene gel as elec-trode material. Electrochim Acta 263:40–46

5. Yasin AS, Mohamed IMA, Amen MT, Barakat NAM, Park CH,Kim CS (2019) Incorporating zirconia nanoparticles into activatedcarbon as electrode material for capacitive deionization. J AlloysCompd 772:1079–1087

6. Khan ZU, Yan T, Han J, Shi L, Zhang D (2019) Capacitive deion-ization of saline water using graphene nanosphere decorated N-doped layered mesoporous carbon frameworks. Environ Sci Nano6:3442–3453

7. Yan T, Shen J, Shi L, Zhang J, Zhang D (2019) Capacitive deion-ization of saline water by using MoS2 - graphene hybrid electrodeswith high volumetric adsorption capacity. Environ Sci Technol 53:12668–12676

8. Zhang J, Fang J, Han J, Yan T, Shi L, Zhang D (2018) N, P, S co-doped hollow carbon polyhedra derived from MOF-based core-shell nanocomposites for capacitive deionization. J Mater ChemA 6:15245–15252

9. Yan T, Liu J, Lei H, Shi L, An Z, Park HS, Zhang D (2018)Capacitive deionization of saline water using sandwich-like nitro-gen-doped graphene composites via a self-assembling strategy.Environ Sci Nano 5:2722–2730

10. Chen L, Chen Z, Kuang Y, Xu C, Yang L, Zhou M, He B, Jing M,Li Z, Li F, Chen Z, Hou Z (2018) Edge-rich quasi-mesoporousnitrogen-doped carbon framework derived from palm tree bark hairfor electrochemical applications. ACS Appl Mater Interfaces 10:27047–27055

11. He H, Ma L, Fu S, Gan M, Hu L, Zhang H, Xie F, Jiang M (2019)Fabrication of 3D ordered honeycomb-like nitrogen-doped carbon/PANI composite for high-performance supercapacitors. Appl SurfSci 484:1288–1296

12. Li Y, Qi J, Li J, Shen J, Liu Y, Sun X, Shen J, Han W, Wang L(2017) Nitrogen-doped hollow mesoporous carbon spheres for ef-ficient water desalination by capacitive deionization. ACS SustainChem Eng 5:6635–6644

13. Tiruneh SN, Kang BK, Choi HW, Kwon SB, Kim MS, Yoon DH(2018) Millerite core-nitrogen-doped carbon hollow shell structurefor electrochemical energy storage. Small 14:e1802933

14. Zhai Y, Dou Y, Zhao D, Fulvio PF, Mayes RT, Dai S (2011) Carbonmaterials for chemical capacitive energy storage. Adv Mater 23:4828–4850

15. Liu X, Ma C, Li J, Zielinska B, Kalenczuk RJ, Chen X, Chu PK,Tang T, Mijowska E (2019) Biomass-derived robust three-dimensional porous carbon for high volumetric performancesupercapacitors. J Power Sources 412:1–9

16. Hu X, Wang Y, Ding B, Wu X (2019) A novel way to synthesizenitrogen doped porous carbon materials with high rate performanceand energy density for supercapacitors. J Alloys Compd 785:110–116

17. Yoo Y, Park GD, Kang YC (2019) Carbon microspheres withmicro- and mesopores synthesized via spray pyrolysis for high-energy-density, electrical-double-layer capacitors. Chem Eng J365:193–200

18. Zhou J, Wang M, Li X (2018) Facile preparation of nitrogen-dopedhigh-surface-area porous carbon derived from sucrose for high per-formance supercapacitors. Appl Surf Sci 462:444–452

19. Gao X, Chen Z, Yao Y, ZhouM, Liu Y,Wang J,WuWD,ChenXD,Wu Z, Zhao D (2016) Direct heating amino acids with silica: auniversal solvent-free assembly approach to highly nitrogen-doped mesoporous carbon materials. Adv Funct Mater 26:6649–6661

20. Han F, Qian O, Chen B, Tang H, Wang M (2018) Sugar blowing-assisted reduction and interconnection of graphene oxide into three-dimensional porous graphene. J Alloys Compd 730:386–391

21. Jiang X-F, Wang X-B, Dai P, Li X, Weng Q, Wang X, Tang D-M,Tang J, Bando Y, Golberg D (2015) High-throughput fabrication ofstrutted graphene by ammonium-assisted chemical blowing forhigh-performance supercapacitors. Nano Energy 16:81–90

22. Liu X, Liu H, Mi M, Kong W, Ge Y, Hu J (2019) Nitrogen-dopedhierarchical porous carbon aerogel for high-performance capacitivedeionization. Sep Purif Technol 224:44–50

23. Tsai YC, Doong RA (2016) Hierarchically ordered mesoporouscarbons and silver nanoparticles as asymmetric electrodes for high-ly efficient capacitive deionization. Desalination 398:171–179

24. Chen L, Sun X, Liu Y, Li Y (2004) Preparation and characterizationof porous MgO and NiO/MgO nanocomposites. Appl Catal A 265:123–128

25. Gajbhiye NS, Bhattacharya U, Darshane VS (1995) Thermal de-composition of zinc-iron citrate precursor. Thermochim Acta 264:219–230

26. Zhu Y, Murali S, Stoller MD, Ganesh KJ, Cai W, Ferreira PJ, PirkleA,Wallace RM, Cychosz KA, ThommesM, Su D, Stach EA, RuoffRS (2011) Carbon-based supercapacitors produced by activation ofgraphene. Science 332:1537–1541

27. Chao L, Liu Z, Zhang G, Song X, Lei X, Noyong M, Simon U,Chang Z, Sun X (2015) Enhancement of capacitive deionizationcapacity of hierarchical porous carbon. J Mater Chem A 3:12730–12737

28. Lee J, Kim S, Kim C, Yoon J (2014) Hybrid capacitive deionizationto enhance the desalination performance of capacitive techniques.Energy Environ Sci 7:3683–3689

29. Radanović DD, Rychlewska U, Djuran MI, Warżajtis B, DraškovićNS, Gurešić DM (2004) Alkaline earth metal complexes of theedta-type with a six-membered diamine chelate ring: crystal struc-tures of [Mg(H 2 O) 6 ][Mg(1,3-pdta)] · 2H 2 O and [Ca(H 2 O) 3Ca(1,3-pdta) (H 2 O)] · 2H 2 O: comparative stereochemistry ofedta-type complexes. Polyhedron 23:2183–2192

30. Lei H, Yan T, Wang H, Shi L, Zhang J, Zhang D (2015) Graphene-like carbon nanosheets prepared by a Fe-catalyzed glucose-blowingmethod for capacitive deionization. J Mater Chem A 3:5934–5941

4720 Ionics (2020) 26:4713–4721

31. Jin Z, Wang Y, Chen S, Li G, Wang L, Zhu H, Zhang X, Liu Y(2016) Influence of a solution-deposited rutile layer on the mor-phology of TiO2 nanorod arrays and the performance of nanorod-based dye-sensitized solar cells. RSC Adv 6:10450–10455

32. Luo H, Liu Z, Chao L, Wu X, Lei X, Chang Z, Sun X (2015)Synthesis of hierarchical porous N-doped sandwich-type carboncomposites as high-performance supercapacitor electrodes. JMater Chem A 3:3667–3675

33. Ling Z,Wang Z, ZhangM, Yu C,Wang G, Dong Y, Liu S,Wang Y,Qiu J (2016) Sustainable synthesis and assembly of biomass-derived B/N co-doped carbon nanosheets with ultrahigh aspect ratiofor high-performance supercapacitors. Adv Funct Mater 26:111–119

34. Shi W, Li H, Cao X, Leong ZY, Zhang J, Chen T, Zhang H, YangHY (2016) Ultrahigh performance of novel capacitive deionizationelectrodes based on a three-dimensional graphene architecture withnanopores. Sci Rep 6:18966

35. Wen X, Zhang D, Yan T, Zhang J, Shi L (2013) Three-dimensionalgraphene-based hierarchically porous carbon composites preparedby a dual-template strategy for capacitive deionization. J MaterChem A 1:12334

36. Zhang H, Li Y, Zhang G, Xu T, Wan P, Sun X (2015) A metallicCoS2 nanopyramid array grown on 3D carbon fiber paper as anexcellent electrocatalyst for hydrogen evolution. JMater ChemA 3:6306–6310

37. Zhang G, Wang L, Hao Y, Jin X, Xu Y, Kuang Y, Dai L, Sun X(2016) Unconventional carbon: alkaline dehalogenation of poly-mers yields N-doped carbon electrode for high-performance capac-itive energy storage. Adv Funct Mater 26:3340–3348

38. Chen H, Chang X, Chen D, Liu J, Liu P, Xue Y, Lin H, Han S(2016) Graphene-karst cave flower-like Ni–Mn layered double ox-ides nanoarrays with energy storage electrode. Electrochim Acta220:36–46

39. Niu R, Li H, Ma Y, He L, Li J (2015) An insight into the improvedcapacitive deionization performance of activated carbon treated bysulfuric acid. Electrochim Acta 176:755–762

40. Sun F, Liu J, Chen H, Zhang Z, Qiao W, Long D, Ling L (2013)Nitrogen-rich mesoporous carbons: highly efficient, regenerablemetal-free catalysts for low-temperature oxidation of H2S. ACSCatal 3:862–870

41. Zhang J, Qu L, Shi G, Liu J, Chen J, Dai L (2016) N,P-Codopedcarbon networks as efficient metal-free bifunctional catalysts foroxygen reduction and hydrogen evolution reactions. AngewChem Int Ed Engl 55:2230–2234

42. Wen Z,Wang X,Mao S, Bo Z, KimH, Cui S, LuG, Feng X, Chen J(2012) Crumpled nitrogen-doped graphene nanosheets with ultra-high pore volume for high-performance supercapacitor. Adv Mater24:5610–5616

43. Li Y, Li Z, Shen PK (2013) Simultaneous formation of ultrahighsurface area and three-dimensional hierarchical porous graphene-like networks for fast and highly stable supercapacitors. Adv Mater25:2474–2480

44. Fan X, Yu C, Yang J, Ling Z, Hu C, Zhang M, Qiu J (2015) Alayered-nanospace-confinement strategy for the synthesis of two-dimensional porous carbon nanosheets for high-rate performancesupercapacitors. Adv Energy Mater 5:1401761

45. Zhang G, Wang L, Hao Y, Jin X, Xu Y, Kuang Y, Dai L, Sun X(2015) Unconventional carbon: alkaline dehalogenation of poly-mers yields N-doped carbon electrode for high-performance capac-itive energy storage. Adv Funct Mater 26:3340-3348.

46. Zhang G, Luo H, Li H, Wang L, Han B, Zhang H, Li Y, Chang Z,Kuang Y, Sun X (2016) ZnO-promoted dechlorination for

hierarchically nanoporous carbon as superior oxygen reductionelectrocatalyst. Nano Energy 26:241–247

47. Zhao R, Soestbergen MV, Rijnaarts HHM, Wal AVD, Bazant MZ,Biesheuvel PM (2012) Time-dependent ion selectivity in capacitivecharging of porous electrodes. J Colloid Interface Sci 384:38–44

48. Suss ME, Porada S, Sun X, Biesheuvel PM, Yoon J, Presser V(2015) Water desalination via capacitive deionization: what is itand what can we expect from it? Energy Environ Sci 8:2296–2319

49. Mi M, Liu X, Kong W, Ge Y, Dang W, Hu J (2019) Hierarchicalcomposite of N-doped carbon sphere and holey graphene hydrogelfor high-performance capacitive deionization. Desalination 464:18–24

50. Min X, Hu X, Li X, Wang H, Yang W (2019) Synergistic effect ofnitrogen, sulfur-codoping on porous carbon nanosheets as highlyefficient electrodes for capacitive deionization. J Colloid InterfaceSci 550:147–158

51. Wang Z, Yan T, Chen G, Shi L, Zhang D (2017) High salt removalcapacity of metal–organic gel derived porous carbon for capacitivedeionization. ACS Sustain Chem Eng 5:11637–11644

52. Yan J,Wang Q,Wei T, Fan Z (2014) Recent advances in design andfabrication of electrochemical supercapacitors with high energydensities. Adv Energy Mater 4:1300816

53. Belaustegui Y, Zorita S, Fernández-Carretero F, García-Luis A,Pantò F, Stelitano S, Frontera P, Antonucci P, Santangelo S (2018)Electro-spun graphene-enriched carbon fibres with high nitrogen-contents for electrochemical water desalination. Desalination 428:40–49

54. Li Y, Liu Y, Shen J, Qi J, Li J, Sun X, Shen J, Han W, Wang L(2018) Design of nitrogen-doped cluster-like porous carbons withhierarchical hollow nanoarchitecture and their enhanced perfor-mance in capacitive deionization. Desalination 430:45–55

55. Chen Y, Liu C, Hsu CC, Hu C (2019) An integrated strategy forimproving the desalination performances of activated carbon-basedcapacitive deionization systems. Electrochim Acta 302:277–285

56 . Yue Z , Gao T, L i H (2019 ) Robus t syn t he s i s o fcarbon@Na4Ti9O20 core-shell nanotubes for hybrid capacitivedeionization with enhanced performance. Desalination 449:69–77

57. Ma D, Wang Y, Cai Y, Xu S, Wang J (2019) Multifunctional groupsulfobutyl etherβ-cyclodextrin polymer treated CNTas the cathodefor enhanced performance in asymmetric capacitive deionization.Electrochim Acta 313:321–330

58. Chang L, Hu Y (2019) 3D channel-structured graphene as efficientelectrodes for capacitive deionization. J Colloid Interface Sci 538:420–425

59. Chang L, Hu Y (2018) Highly conductive porous Na-embeddedcarbon nanowalls for high-performance capacitive deionization. JPhys Chem Solids 116:347–352

60. Feng C, Chen Y, Yu C, Hou C (2018) Highly porous activatedcarbon with multi-channeled structure derived from loofa spongeas a capacitive electrode material for the deionization of brackishwater. Chemosphere 208:285–293

61. Zhou F, Gao T, Luo M, Li H (2018) Heterostructuredgraphene@Na4Ti9O20 nanotubes for asymmetrical capacitive de-ionization with ultrahigh desalination capacity. Chem Eng J 343:8–15

62. Wang Z, Yan T, Chen G, Shi L, Zhang D (2017) High salt removalcapacity of metal–organic gel derived porous carbon for capacitivedeionization. ACS Sustain Chem Eng 12:11637–11644

Publisher’s note Springer Nature remains neutral with regard to jurisdic-tional claims in published maps and institutional affiliations.

4721Ionics (2020) 26:4713–4721