Synergistic Effects of Nitrogen Doping on MXene for Enhancementof Hydrogen Evolution ReactionThi Anh Le,†,‡ Quoc Viet Bui,‡ Ngoc Quang Tran,†,§ Yunhee Cho,†,‡ Yeseul Hong,‡

Yoshiyuki Kawazoe,∥ and Hyoyoung Lee*,†,‡,§

†Centre for Integrated Nanostructure Physics (CINAP), Institute of Basic Science (IBS), Suwon 440-746, Republic of Korea‡Department of Chemistry and §Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 440-746, Republic ofKorea∥New Industry Creation Hatchery Center, Tohoku University, Sendai 980-8579, Japan

*S Supporting Information

ABSTRACT: Earth-abundant, nonprecious, and efficientelectrocatalysts for effective hydrogen evolution reaction(HER) are crucial for future large-scale green energyproduction. Low-cost two-dimensional MXenes have beenwidely studied in energy-storage devices owing to their uniquechemical and physical properties and have recently attractedscientists in the electrocatalysis field. Nevertheless, theirelectrocatalytic activity still remains unsatisfactory. Herein, wepresent a facile and general strategy using ammonia heattreatment to enhance the hydrogen evolution catalysis ofTi3C2Tx MXenes by modification with a nitrogen heteroatom.Importantly, our approach is focused on revealing: (1) thecontribution of all possible incorporated N species includingTi−N, N−H, and N in O−Ti−N, rather than considering only that of Ti−Nx motifs as previously reported for N-doped MXeneelectrocatalysts, and their role in inducing a change in the electronic configuration of the as-prepared catalysts, which then leadsto increased electrical conductivity and improved intrinsic catalytic reactivity; and (2) the importance of controlling the properamount of N obtained at a suitable calcined temperature to assist the shift of the Gibbs free energy for hydrogen adsorption(ΔHad*) approaching 0 eV (ideal value), as proved by the density functional theory. Moreover, experimental findings indicatethat nitrogen-doped Ti3C2Tx annealed at 600 °C shows superior improved HER electrocatalytic performance compared topristine Ti3C2Tx, with an onset potential of −30 mV and an overpotential as low as 198 at 10 mA cm−2, as well as a muchsmaller Tafel slope of 92 mV dec−1.

KEYWORDS: 2D materials, MXene-based electrocatalysts, nitrogen-doped, hydrogen evolution reaction, DFT calculations

■ INTRODUCTION

Hydrogen, a clean and high-energy-density carrier, isconsidered a renewable energy source. The production ofhydrogen via the electrochemical water-splitting routenormally requires high overpotential, making water electrolysisa low-efficiency process.1−5,41 A completed H2 electrolyzercontains the anodic oxygen evolution reaction (OER) and thecathodic hydrogen evolution reaction (HER).6,7 So far,precious Pt metal and its compounds are among the bestcatalysts for HER because of their ideal hydrogen adsorptionGibbs free energy, which favors water catalysis during theproton-reduction reaction.8,9 Unfortunately, the high cost andscarcity of Pt have severely hindered its industrial utilization.To this end, intensive efforts have been focused on discoveringnovel HER electrocatalysts. For example, transition-metaldichacogenides (TMDs), transition-metal phosphides, tran-sition-metal carbides (TMCs), and nitrides have been reportedas potential alternatives to Pt metal and its alloys.10−12

MXenes, a family of two-dimensional (2D) TMCs, havebeen widely applied in electrochemical applications, especiallyin energy storage (i.e., lithium and sodium ion batteries andsupercapacitors) and recently in energy conversion (i.e.,electrocatalysts and photocatalysts) because of their highsurface area, excellent electronic conductivity, hydrophilicity,and high chemical and mechanical stability.13−18 Since the firstreport of MXene as an electrocatalyst for HER, several studieshave explored if these classes of abundant and cost-effective 2Dmaterials can be efficient hydrogen evolution electrocata-lysts.19,20 Theoretically, it has been demonstrated that basalplanes of MXenes are catalytically active for HER, implyingthat there is a high population of active centers participating inthe catalytic reaction. This is in contrast to the widely studied

Received: August 1, 2019Revised: September 18, 2019Published: September 18, 2019

Research Article

pubs.acs.org/journal/ascecgCite This: ACS Sustainable Chem. Eng. 2019, 7, 16879−16888

© 2019 American Chemical Society 16879 DOI: 10.1021/acssuschemeng.9b04470ACS Sustainable Chem. Eng. 2019, 7, 16879−16888

Dow

nloa

ded

via

SUN

GK

YU

NK

WA

N U

NIV

on

June

1, 2

020

at 0

7:52

:52

(UT

C).

See

http

s://p

ubs.

acs.

org/

shar

ingg

uide

lines

for

opt

ions

on

how

to le

gitim

atel

y sh

are

publ

ishe

d ar

ticle

s.

2D TMDs (MoS2), which have limited active sites at only edgesites.21,22 Unfortunately, the intrinsic catalytic activity ofMXenes is inferior to that of Pt and its compositedcompounds. Therefore, significant wise methodologies arerequired to further enhance the catalytic performance ofMXene-based electrocatalysts for possible practical usage.It is widely established that the number of reactive centers,

intrinsic activity of each active site (i.e., reactivity), andelectrical conductivity are crucial factors that directly affectHER performance.23,24 These factors were therefore alsoconsidered in boosting the reactivity of MXene catalysts. Ofthese, several theoretical and experimental attempts have beenmade by controlling one or all of the key parameters describedabove. For example, Yuan et al. increased the number of activesites by exfoliating ultrathin nanosheets or KOH-activatingnanofibers.25 Despite the positive effect of architecture changeon the catalytic activity by increasing the surface area, thisapproach cannot be used to amend the intrinsic activity. Incontrast, incorporation of promoters can potentially improvecatalyst performance by altering the electronic structure, thusmodulating H-intermediate binding energy to the basal planesof the MXene. To date, several theoretical studies havedemonstrated that the introduction of modulators can not onlyoptimize the Gibbs free energy of hydrogen adsorption(ΔHad*) but also lower the H2 evolution energy barrier,thereby improving HER performance.26,27

Among anion dopants, nitrogen can not only assist inelectronic transfer by inducing graphitization but also boost theelectrocatalytic activity of carbon-related or transition earth-metal-based materials.28−32 The Ti−Nx species were demon-strated to be the electroactive sites for the OER with excellentelectrocatalytic ability.33−36 Recently, Yoon et al. have reportedthat Ti−Nx motifs, obtained by treating Ti2CTx with NaNH2through chemical nitridation, can act as active sites to enhanceelectrocatalytic activity toward HER.37 Although the as-obtained catalyst exhibited much better catalytic activity thanthe pristine Ti2CTx, the direct verification of these results hasnot been revealed yet. In addition, that work mainly considersthe Ti−Nx component, while the possible contribution ofother N-related species has been ignored and remainedambiguous, requiring further studies. In this regard, a direct,simple, and scalable synthetic method that enables thedetermination of the relationship between possible nitrogenspecies coordinated as well as the impact of dopingconcentration in the MXene lattice and HER electrocatalysisis worth exploring.Motivated by the aforementioned considerations, we

prepared nitrogen-doped Ti3C2Tx electrocatalysts by facileheat treatment under an ammonia atmosphere. Throughadjusting the calcination temperatures, we systematicallyinvestigated the structure−activity relation of as-preparedsamples. The results show that nitrogen introduction indeedmodified the electronic configuration by decreasing theelectron densities of oxygen sites through inducing a morefavorable interaction between Ti and N than the Ti−O species.Specifically, samples treated at 600 °C can afford the highestHER performance compared to the one annealed at higher orlower temperatures. We ascribed this enhanced performance tothe synergistic advantageous contributions originating from allnitrogen-coordinated species, leading to ideal ΔHad* (ap-proaching 0 eV) and an improvement in its electronconductance throughout the electrode.

■ EXPERIMENTAL SECTIONChemical Reagents and Precursors. Pristine Ti3AlC2 (99.8%,

385 mesh) was commercially prepared by Forsman Scientific BeijingCo., Ltd., China. Hydrofluoric acid (HF, 48%-51%), ethanol (EtOH),and 3r deionized (DI) water (Rs > 18 MΩ cm) (Direct-Q UV,Millipore) were used.

Synthesis of Ti3AlC2 and Ti3C2Tx. To synthesize Ti3C2Tx, weprepared by reported work described elsewhere.

Synthesis of N−Ti3C2Tx. Nitrogen-doped Ti3C2Tx MXene wasprepared in an ammonia atmosphere using a quartz pipe furnace atvarious temperatures (200, 400, 600, and 800 °C) for 3 h at aconstant ammonia flow of 200 mL min−1. In particular, the furnacewas initially flushed with argon gas for 10 min before and after eachheat-treatment process to prevent any oxidation by air. The rampingtemperature was fixed at 5 °C min−1.

Chemical and Physical Characterization. A scanning electronmicroscope (JSM-6701F/INCA Energy, JEOL) and a transmissionelectron microscope (JEOL JEM 3010) were employed to study themicrostructures of the samples. X-ray diffraction (XRD) analyses wereconducted on a Rigaku Ultima IV X-ray diffractometer with Cu Krradiation. To check the X-ray photoelectron spectroscopy (XPS), aThermo VG Microtech ESCA 2000 with a monochromatic Al KR X-ray source at 100 W was employed. A Micro-Raman machine(Renishaw, RM1000-In Via) with an excited energy of 2.41 eV (514nm) was used to measure Raman spectroscopy.

Electrochemical Characterization. Electrochemical HER cata-lytic activities of the nanosheets were investigated by depositing theas-prepared samples on rotating disk electrode (RDE) (d = 3 mm) asthe working electrode. Particularly, 10 mg of N−Ti3C2Tx and 400 μLof a mixture of DI water and ethanol (volume ratio of 3:1) weresonicated for 1 h before adding 7 μL of Nafion solution, followed bysonication for 1 h. After mixing, 8 μL of suspension was dropped onRDE (mass loading ∼ 0.276 mg cm−2) and dried on a clean bench inair overnight before HER measurements.

VMP3 workstation (Biologic Science Instruments, France) wasused to assess electrochemical performance. HER analysis wasperformed in a three-electrode setup. First, N2-saturated 0.5 MH2SO4 was prepared for 30 min, and 3 M Ag/AgCl as the referenceelectrode and graphite electrode as the counter electrode were used.Electrochemical tests were investigated by LSV at a sweeping rate of 5mV s−1, while the rotating speed of the working electrode was 1600rpm. The electrochemical surface area (ECSA) values were measuredby cyclic voltammetry (CV) in the potential range of 0.10−0.30 Vversus reverse hydrogen electrode (RHE) at various scan rates (20−180 mV s−1). Electrochemical impedance spectroscopy (EIS) wasconducted with an overpotential of −200 mV vs RHE.

Density Functional Theory (DFT) Calculation Details. DFTstudies were conducted by the Vienna Ab initio Simulation Package.The Perdrew−Burke−Ernzerhof generalized gradient approximationwas used to describe electron-exchange correlations. Interactionsbetween ions and electrons were described through the projectoraugmented wave method. Van der Waals configurations weredescribed by Grimme-D3 level. Plane-wave kinetic energy was 400eV, and the k-point mesh was set as (6 × 6 × 1). The convergence inthe energy criterion was set to smaller than 1.0D-6 eV/cell, and theforce convergence criterion was set to 0.01 eV Å−1.

Based on our experimental results, we chose a (2 × 2 × 1) supercellwith a vacuum region of 15 Å along the z-direction to considerhydrogen adsorption to the T3C2Tx surface.

Interpretation of Calculation Results. To describe the stabilityof hydrogen adsorption to MXene, we calculated the adsorptionenergy according to the following formula

Δ = + * − [ − + *] −E E n E n E( H ) ( 1)H12

(H )H 2 (1)

where E(nH + *) and E[(n − 1)H + *] represent the total energy of nand (n − 1) hydrogen atoms adsorbed in the MXene system,respectively, and E(H2) is the sum energy of H2 in the gas phase.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b04470ACS Sustainable Chem. Eng. 2019, 7, 16879−16888

16880

The favorability of adsorbed H bonding on the MXene surface wasassessed by investigating the differential ΔGH of adsorbed hydrogenusing the following formula:

Δ = Δ + Δ − ΔG E E T SH H ZPE H (2)

In this formula, ΔEZPE is the difference at the 0 point vibration energyof the reaction and product sides and ΔSH is the entropy differencebetween the reaction and the product under standard conditions andis approximately half of the entropy of H2 gas of 0.2 eV at T = 300 K.

■ RESULTS AND DISCUSSIONNitrogen-doped titanium carbide MXenes (N−Ti3C2Tx) werefabricated by facile annealing of as-obtained Ti3C2Tx in anammonia atmosphere at different temperatures (200, 400, 600,and 800 °C); the samples were denoted as N−Ti3C2Tx@200,

N−Ti3C2Tx@400, N−Ti3C2Tx@600, and N−Ti3C2Tx@800,respectively. Scheme 1 illustrates the full synthesis processfrom the Ti3AlC2 MAX parent phase by HF etching. Theobtained product was washed several times with DI water andethanol to rinse out all unreacted materials and residual acid.The collected Ti3C2Tx MXene powder was kept in a vacuumoven before calcination in ammonia to yield N−Ti3C2Tx

samples. Details of these processes are provided inExperimental Section. Scanning electron microscopy (SEM)images of Ti3AlC2, Ti3C2Tx MXene, and as-prepared N-dopedMXenes calcined at 200, 400, 600, and 800 °C are displayed inFigure 1a−f. After etching the aluminum layer in MAX,multilayer-Ti3C2Tx characteristics were clearly observed.Interestingly, the morphology of the N-doped samples was

Scheme 1. Illustration of Synthesis of N-Doped MXene from the Ti3AlC2 MAX Phase

Figure 1. Morphology of as-prepared materials: SEM images of (a) Ti3AlC2 parent phase, (b) pristine Ti3C2Tx, (c) N−Ti3C2Tx@200, (d) N−Ti3C2Tx@400, (e) N−Ti3C2Tx @600, and (f) N−Ti3C2Tx@800. (g) SEM−EDX elemental mapping of N−Ti3C2Tx600 and (h) SEM image ofdelaminated N−Ti3C2Tx@600.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b04470ACS Sustainable Chem. Eng. 2019, 7, 16879−16888

16881

not different from that of pristine MXene. Energy-dispersive X-ray (EDX) mappings of N−Ti3C2Tx showed that all mainelements of MXenes were presented and uniformly distributed,namely, Ti, C, O, and F, while N appeared after nitrogenincorporation (Figures 1g and S1, Supporting Information).This indicated successful introduction of nitrogen into the 2Dtitanium carbide.37,40 Transmission electron microscopy(TEM) was employed to further characterize the layer natureof MXene after nitrogen incorporation, and results suggestedthat the original layered structure was well preserved (FigureS2). N−Ti3C2Tx@600 was subsequently exfoliated by ultra-sonication and centrifugation to obtain delaminated N−Ti3C2Tx@600 (d-N−Ti3C2Tx@600) (Figure 1h), which weused to evaluate the effect of increased surface area onelectrocatalytic performance.Crystal and phase structures of nitrogen-doped titanium

carbide MXenes were assessed by X-ray diffraction (XRD). Six

main common peaks (i.e., (002), (006), (008), (0010),(0012), and (110)) representative of bare Ti3C2Tx wereobserved in Figure 2a, consistent with previous studies thathave reported high-quality pristine MXene after acid treat-ment.18,19 After calcination in ammonia, the (002) peakbroadened and shifted slightly for N−Ti3C2Tx@600, while italmost disappeared for N−Ti3C2Tx@800. The other five peakswere maintained, although their intensities decreased gradually,implying a decrease in the crystallinity of N-doped samples andcoverage of the MXene surface with amorphous carbon.17

Notably, as the temperature increased to 600 and 800 °C, anew peak appeared around 41.9°, which we assigned to theformation of a new Ti−N phase due to chemical bonding of Tiwith nitrogen under NH3 atmosphere at high temperature.38

The chemical structure and emergence of disordered carbonon N−Ti3C2Tx were revealed by Raman spectroscopy (Figure2b). In addition to the two peaks at 1357 cm−1 (D band) and

Figure 2. (a) X-ray diffraction (XRD) data and (b) Raman spectra of as-synthesized nitrogen-doped Ti3C2Tx MXene samples annealed at varioustemperatures from 200 to 800 °C.

Figure 3. (a) X-ray photoelectron spectroscopy images of Ti3C2Tx and N-doped Ti3C2Tx MXene annealed at various temperatures; (b) core-levelXPS of the N 1s of N-doped MXene samples annealed at various temperatures; and (c) core-level XPS of the F 1s of Ti3C2Tx and N-dopedTi3C2Tx annealed at various temperatures.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b04470ACS Sustainable Chem. Eng. 2019, 7, 16879−16888

16882

1596 cm−1 (G band), signals representing vibrations ofnonstoichiometric titanium carbide and the anatase phase oftitanium oxide phase were observed in all N-doped MXenesamples as well as pristine MXene (i.e., peaks at 147, 271, 413,and 600 cm−1).13,14,18 These peaks were not present in bareTi3C2Tx. The ratio of D band to G band intensities (ID/IG) iswidely used to judge the degree of graphitization and defectgeneration.39 As the temperature increased from 200 to 800°C, there was a gradual increase (from 0.94 to 0.98) in ID/IG,indicating larger graphitic component/defects and substitutionof nitrogen for carbon atoms on Ti3C2Tx, which could improvethe electrical conductivity of the materials.31−33 The increasein structural distortion and defects in N−Ti3C2Tx@800 can beascribed to the heavy N-doping at a high pyrolysis temper-ature, consistent with the XRD results.X-ray photoelectron spectroscopy (XPS) analyses of N−

Ti3C2Tx catalysts were conducted to elucidate their valencestates and chemical compositions. XPS images of the variousN−Ti3C2Tx samples indicated the presence of Ti, C, O, F, andN signals, except that the N signal was not present in pristineTi3C2Tx (Figure 3a). The atomic concentrations of the N 1sconfiguration of all N-doped samples are presented in TableS1. On adjusting the temperature for samples annealed at 200,400, 600, and 800 °C, the atomic concentration of nitrogen inTi3C2Tx increased from 1.59, 3.27, 7.86, and 10.61 atom %,respectively (Table S1), suggesting that it is easier toincorporate N atoms by substitution at carbon sites or atterminal functional group sites at high temperatures ratherthan at low temperatures. High-resolution N 1s peaks of N−Ti3C2Tx@200, N−Ti3C2Tx@400, N−Ti3C2Tx@600, and N−Ti3C2Tx@800 are displayed in Figure 3b. Specifically, the N 1sof N−Ti3C2Tx@200 and N−Ti3C2Tx@400 was deconvolutedinto two peaks at 398.5 and 400.7 eV, which we attributed to

N−H and anionic N in O−Ti−N, respectively (Figure3b).37,38,40 The more pronounced O−Ti−N peak of thesample annealed at 400 °C is consistent with the higheramount of N incorporated into the MXene structure and thefavorable bonding configuration. Interestingly, when theannealing temperature was increased to more than 600 °C,an extra nitrogen species appeared at about 396.4 eV, whichwas assigned to Ti−Nx binding. The favorable interactionbetween Ti and N (Ti−Nx) reduced the electron density ofnitrogen because the binding energy of O−Ti−N (400.7 eV) ishigher than that of Ti−N (396.4 eV).40 As the temperatureincreased to 800 °C, the O−Ti−N intensity decreaseddramatically due to the large amount of nitrogen incorporated.In particular, the atomic percentage of the O−Ti−N peak ofN−Ti3C2Tx@800 is only 0.44 atom %, while that of N−Ti3C2Tx@600 is 0.61 atom % (Table S1). In addition, Ti−Nwas not found in N−Ti3C2Tx annealed at 200 and 400 °C,indicating greater N dopant incorporation into planar Ti3C2Txnanosheets at higher ammonia treatment temperatures and theTi−N bond can only form at high temperatures from 600 °C.Comparison of the fitted high-resolution XPS O 1s spectra ofTi3C2Tx, N−Ti3C2Tx@600, and N−Ti3C2Tx@800 showedthat the −OH intensity decreased at the expense of O−Ti−N,resulting in N−O bond formation at 531.2 eV after high-temperature calcination, in agreement with the N 1s spectra(Figure S3).38,40 In other words, −OH groups were partlyreplaced by N-related groups because, at a high annealingtemperature (800 °C), oxygen becomes unstable and is mostlyremoved. Hence, mixed phases of TiNx@Ti3C2Tx wereobtained, implying the partial nitration of Ti3C2Tx. Moreover,spectra of F 1s of a series of samples, including pristineTi3C2Tx and as-doped Ti3C2Tx, are shown in Figure 3c. Areduction in the peak intensity was clearly observed as the

Figure 4. Electrochemical performance: (a) HER polarization curves, (b) corresponding Tafel plots, and (c) EIS Nyquist plots of Ti3C2Tx and aseries of different N-doped Ti3C2Tx catalysts (inset: enlarged Nyquist plots in the high-frequency region). (d) Durability test of N−Ti3C2Tx@600by a chronoamperometry (CA) test at a constant overpotential of −0.19 V (inset: photo of a rotating disk electrode (RDE) with the produced H2bubbles on its surface).

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b04470ACS Sustainable Chem. Eng. 2019, 7, 16879−16888

16883

calcination temperature increased, indicating that annealingunder an NH3 atmosphere can reduce the amount of F.Notably, theoretical studies have suggested that fluorine can bedeleterious for the HER;19,20,23 therefore, this result furtherhighlights the synergistic effects of our universal method toimprove the HER catalytic activity. The XPS core levels of Ti2p and C 1s reveal that the ammonia treatment indeed resultedin changes in the oxidation state of Ti, as shown in Figure S4.Specifically, the Ti 2p spectra of N-doped Ti3C2Tx can bedeconvoluted into six main peaks located at 454.5, 455.0,455.7, 456.8, 458.9, and 460.1 eV, corresponding to Ti−C,Ti2+, Ti−N, Ti3+, Ti4+, and Ti−F components (Figure S4a−d).38 As the temperature increased from 200 to 800 °C, theTi4+ component increased at the expense of Ti−C and Ti2+

due to the oxidation of Ti. Furthermore, the C 1s spectra of aseries of N-doped samples also showed the same tendency,where the Ti−C peak component decreased as the calcinedtemperature increased from 200 to 800 °C (Figure S4e−h),which is in strong agreement with the above conclusion.The electrochemical analyses were conducted by a three-

electrode system with the rotating disk electrode (RDE) as theworking electrode to evaluate the HER catalytic properties ofthe electrocatalysts in 0.5 M H2SO4. Polarization curves ofpotential versus reverse hydrogen electrode (vs RHE) withoutIR compensation are displayed in Figure 4a. The commercial20 wt % Pt/C electrocatalyst exhibited the best HER activity,with nearly zero onset potential and low overpotential (@10mA cm−2), whereas pristine Ti3C2Tx displayed far inferiorcatalytic activity, similar to what has been reportedpreviously.19,23,24 N−Ti3C2Tx@600 exhibited significantlysuperior activity to the other catalysts. Specifically, N−Ti3C2Tx@600 afforded an overpotential of 198 mV (@10mA cm−2), while the overpotentials of N−Ti3C2Tx@200, N−Ti3C2Tx@400, and N−Ti3C2Tx@800 were 560, 410, and 390mV, respectively. We ascribed the improvement of HERreactivity after N doping to the synergistic effects of electronicmodulation of active sites coming from all Ti−Nx, N−H, andO−Ti−N species and the decrease of the detrimental Ftermination group as proven by XPS data above. It should benoted that the presence of F functional groups on the basalplans of Ti3C2Tx MXene was demonstrated to have a negativeeffect on HER activity.22 Therefore, our approach is a potentialmethod to enhance HER reactivity by tailoring theterminations on the MXene surface. Surprisingly, the over-potential at 10 mA cm−2 of N−Ti3C2Tx@600 was 198 mV,which was less than that of N−Ti3C2Tx@800 (Figure 4a). Ifthe catalytic activity of N-doped electrocatalysts solely dependson the Ti−Nx component as demonstrated by previous work,37

then the N−Ti3C2Tx@800 with 70.3% should exhibit higherperformance than N−Ti3C2Tx@600 with 33.5% (Table S1).This led us to investigate the possible contribution of other N-related compounds (i.e., N−H and O−Ti−N) that wasneglected in the literature.37 In conjunction with the XPSresults, these HER results suggest that O−Ti−N and N−Hcomponents indeed contributed to HER activity. In particular,the percentages of N−H (49.3%) and O−Ti−N (17.0%)species in N−Ti3C2Tx@600 are all higher than those in N−Ti3C2Tx@800 (23.4 and 0.68% for N−H and O−Ti−N,respectively), which confirmed that these compounds alsocontributed to the improvement of catalytic activity. Thisresult implies that the proper calcination temperature (i.e., 600°C) is important in achieving the beneficial nitrogen-related

species and electronic modulation, thereby boosting the HERactivity of the catalyst.The Tafel slope can provide information about the

mechanism of HER electrocatalysis. The corresponding Tafelslopes of the as-prepared catalysts were calculated to be 268,107, 101, 99, 92, and 35 mV dec−1 for pristine Ti3C2Tx, N−Ti3C2Tx@200, N−Ti3C2Tx@400, N−Ti3C2Tx@600, N−Ti3C2Tx@800, and Pt/C, respectively (Figure 4b). It is wellestablished that the smaller the Tafel slope, the better the HERkinetics. Among the as-N-doped MXene samples, the observedtendency in Tafel slopes to decrease with increasing annealingtemperature was the same as that observed for the over-potential; N−Ti3C2Tx@600 had the lowest Tafel slope,corresponding to the highest catalytic activity. Its Tafel slopecan be attributed to the change of the rate-determining stepduring the HER process. Although the HER mechanism is stillunder debate, a much smaller value of the Tafel slope ofTi3C2Tx@600 suggests that the Ti3C2Tx@600 may follow theVolmer−Heyrovsky mechanism.Moreover, to emphasize the effectiveness of nitrogen doping

on the electrocatalysis of Ti3C2Tx MXene, we conductedcontrolled experiments with the same series of temperatures(200, 400, 600, and 800), but replacing the ammonia gas byinert argon gas. The corresponding HER performance isdisplayed in Figure S5. It can be seen that the overpotential at10 mA cm−2, Ti3C2Tx@200(Ar), Ti3C2Tx@400(Ar),Ti3C2Tx@600(Ar), and Ti3C2Tx@800(Ar) are 595, 540, 531,and 522 mV, respectively. Nevertheless, this improvement isneglectable compared to that of N−Ti3C2Tx. Particularly, N−Ti3C2Tx@200, N−Ti3C2Tx@400, N−Ti3C2Tx@600, and N−Ti3C2Tx@800 can afford a current density of 10 mA cm−2 atoverpotentials of 560, 410, 198, and 390 mV, respectively(Figure S5a). It is obvious that after the calcination, the HERactivity of catalysts obtained by annealing MXene under Arexhibits some improvement, which may have originated fromthe reduction of the F termination group during heat treatmentunder Ar atmosphere.24 Due to the detrimental effect of Ffunctional groups on the HER performance, its reduction canbe attributed to the HER enhancement. This result isconsistent with the reported work.20 And to investigate thekinetics, the corresponding Tafel slopes were derived from theextrapolation of the log of the current density plot versus thelinear region of overpotential (Figure S5b), which obviouslyshowed higher values than those in the case of nitrogen-dopedelectrocatalysts. Overall, the above results reveal that eventhough the calcination under Ar can contribute to the HERactivity of Ti3C2Tx MXene, its effect is much smaller comparedto that of N-doped MXene electrocatalysts. In other words,these results evidently confirm the effectiveness of nitrogendoping on electrocatalysis.To further support that N doping can enhance HER kinetics

of an electrocatalytic reaction, electrochemical impedancespectroscopy (EIS) measurements were performed. It can beseen from the EIS plots in Figure 4c that the charge-transferresistance (Rct) of an electrode, represented by the semicircleparts, decreases dramatically after nitrogen incorporation intothe MXene lattice, indicating the higher rate of charge/iontransports in the internal electrode compared to that of theuntreated one. Among the doped samples (N−Ti3C2Tx@200,N−Ti3C2Tx@400, N−Ti3C2Tx@600, N−Ti3C2Tx@800), N−Ti3C2Tx@600 showed the smallest semicircle, which indicatedthat N−Ti3C2Tx@600 possessed the most favorable charge-transfer resistance (Rct), further suggesting superior charge

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b04470ACS Sustainable Chem. Eng. 2019, 7, 16879−16888

16884

transport kinetics relative to the other electrocatalyst samples(Figure 4c). Additionally, zoom-in of impedance spectra (insetin Figure 4c) shows the real axis intercept in the high-frequency region, corresponding to the interfacial contactresistance between the active catalysts and the substrate forpristine MXene and N-doped samples.24,38 It is clear that the x-intercept of the Nyquist plot increased in the order of N−Ti3C2Tx@600, N−Ti3C2Tx@800, N−Ti3C2Tx@400, N−Ti3C2Tx@200, and Ti3C2Tx. In short, the boosted electron-transfer behavior evidenced by EIS data can be attributed tothe different electronegativity of nitrogen on the surface withlocal elements of MXene. Consequently, this led to theelectronic modulation in the MXene structure, which isprimarily responsible for this reduction of charge-transferresistance.To investigate if there is an increase in the number of active

sites as a result of ammonia heat treatment, we measured theelectrochemical surface area (ECSA) of the catalyst, which issupposed to be linearly proportional to the double-layercapacitance Cdl of the electrocatalyst. Cyclic voltammetry (CV)tests at various scan rates (20−180 mV s−1) were performed in0.5 M H2SO4. As shown in Figure S6, N−Ti3C2Tx@800exhibited the largest Cdl of 3.81 mF cm−2; however, this valuewas relatively close to that of the other N-doped Ti3C2Txsamples (3.79, 3.78, and 3.75 mF cm−2), indicating that thenumber of active sites was not the main determinant of theenhanced HER activity. In other words, heat treatment in anammonia environment did not significantly affect themorphology or the surface area of the catalysts.Long-term stability of an electrocatalyst under acidic

conditions is another critical parameter to consider forcommercial application. Therefore, a duration test of N-doped Ti3C2Tx@600 was conducted in 0.5 M H2SO4 at thestatic overpotential of −0.19 V for 30 h of continuousoperation using chronoamperotiometry (CA). The data shownin Figure 4d reveal a stable current for HER, confirming theexcellent durability of N−Ti3C2Tx@600 under acidic electro-lyte. The preservation of morphology and chemical bonding ofthe material after the long-term stability test was confirmed byTEM and XPS. The XPS result indicated that there is only asmall increase of surface oxidation in the Ti 2p high-resolutionspectra (Figure S7). This is consistent with the commonphenomena observed in the HER process in an aqueoussolution. In particular, during the HER long-term experiment,the surface can be converted into oxide or hydroxide in the firststage and become steady afterward.39 Furthermore, the TEMimage further indicated the well-preserved microstructure forlong-term stability test (Figure S8).The second main aim of this work was to determine whether

the larger involvement of dopant concentration can lead tohigher HER performance. Therefore, we employed the DFTfor a theoretical understanding. Specifically, the relationshipbetween the N doping amounts relating to the ammonia heat-treatment temperature and its effect on the HER electro-catalytic activity was investigated by DFT calculations. TableS2 shows the side view of five configurations of N incorporatedinto the MXene lattice including the nitrogen-substituted and/or -terminated MXene (i.e., N-terminated (25%) Ti3C2, N-terminated (50%) Ti3C2, N-terminated (75%) Ti3C2, N-terminated (50%) Ti3NC, and N-terminated (75%) Ti3NC).The Gibbs free energy for hydrogen adsorption (ΔGH*) iswidely considered as the descriptor of HER activity. In detail,neither a negative value nor a positive value of ΔGH* will lead

to show kinetic of desorption or adsorption of intermediate Hatoms, respectively, thus resulting in poor HER activity. Hence,the three-state diagram describes step-by-step HER from theinitial H+, intermediate H-adsorbed atoms to the final H2evolution. Commonly, the Pt exhibits the ideal reactivitytoward HER since its intrinsic ΔGH* value is close to 0 eV.The ΔGH* diagram of a series of N−Ti3C2Tx with differentpercentages of concentrations of the N dopant and Pt as areference is displayed in Figure 5. As can be seen, the ΔGH* of

Pt is about −0.09 eV. The bare Ti3C2Tx exhibits the mostnegative ΔGH*, referring to the strong adsorption of H, thusresulting in the difficulty in the desorption of intermediate Hatoms. After the introduction of N, the ΔGH* approaches 0 eV,closing to that of Pt. Interestingly, the calculated results revealthat at certain concentrations, the increase of nitrogenincorporation indeed results in the more negative ΔGH*,finally leading to poor HER performance due to too stronghydrogen adsorption binding strength. Furthermore, the freeenergy (ΔGH*) of H adsorption on N-terminated (50%)Ti3NCTx at H coverage of 3/8 (−0.029eV) and N-terminated(75%) Ti3NCTx at H coverage of 4/8 (−0.058eV) are muchcloser to zero ΔGH* than that of the N-terminated Ti3C2Tx,which means that nitrogen atoms simultaneously terminatedon the surface functional group (N−H) and substituted at Catoms (Ti−N and O−Ti−N) in Ti3C2Tx with a sufficientamount should be more active in the electrocatalytic activityfor HER than that of the higher or lower loading of N intoTi3C2Tx. In other words, the sufficient amount of terminatedand substituted N into Ti3O2Tx (N−Ti3C2Tx@600) showsmuch better HER performance than the N−Ti3C2Tx@200,N−Ti3C2Tx@400, or N−Ti3C2Tx@800, which is wellconsistent with the experimental observation.Exfoliated derived N−Ti3C2Tx ultrathin nanosheets may

show further improved HER catalysis efficiency because of anincrease in the number of exposed active sites. We synthesizeddelaminated MXene by ultrasonication to evaluate thispossibility. Figure 6a shows that the overpotential needed toreach a current density of 10 mA cm−2 when usingdelaminated N−Ti3C2Tx@600 was smaller than that requiredwhen using nondelaminated N−Ti3C2Tx@600, indicatinghigher activity of d-Ti3C2Tx@600 after increasing the surfacearea by a delamination step. The obvious difference in double-layered capacitance (Cdl) between delaminated N−Ti3C2Tx@600 and the nondelaminated form indicated that the formerhad a larger electrochemical surface area than the latter (Figure6b,c). More exposed active centers with better wettability ofelectrode and eased accessibility to ions in the electrolyte

Figure 5. ΔGH* diagram of different H adsorption statescorresponding to H coverage of 3/8 on N-terminated (50%)Ti3NC, 4/8 on N-terminated (75%) Ti3NC, 2/8 on N-terminated(25%) Ti3C2, 3/8 on N-terminated (50%) Ti3C2, and 4/8 on N-terminated Ti3C2.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b04470ACS Sustainable Chem. Eng. 2019, 7, 16879−16888

16885

resulted in better ion-/charge-transfer kinetics and was theprimary reason for the improved HER activity of thedelaminated N−Ti3C2Tx@600 catalyst.

■ CONCLUSIONS

In summary, we successfully prepared a series of N−Ti3C2Txelectrocatalysts that showed enhanced activity and stability forHER, which we attributed to the synergistic effects of Ndoping that increased the reactivity and electrical conductivityof as-fabricated electrodes. The results indicated that theactivity and stability of as-prepared N−Ti3C2Tx weresignificantly improved compared to those of the recentlyreported MXene-based catalysts (Table S3). Specifically,systematic investigation revealed that the N−Ti3C2Tx@600sample showed the highest performance among synthesizedelectrocatalysts with low overpotential and long-term dura-bility, suggesting that all nitrogen-related species indeedcontributed to the HER enhancement rather than only Ti−Nx motifs, as reported in the literature. In other words, theoptimized annealing temperature can afford reasonable andstable nitrogen incorporation with a proper degree of nitrogendoping for optimizing catalytic behavior. Consequently, the as-prepared samples show excellent cooperative effects in terms ofcatalyzing the HER reaction and stabilizing the catalyst duringoperation. Nevertheless, heavy doping of nitrogen at hightemperature resulted in partial titanium nitrides derived fromtitanium carbide MXenes and poor HER activity. Moreover,we performed DFT calculations to obtain a theoreticalunderstanding of the effects of a controlled level of N dopingby simply adjusting calcined temperature. We anticipate thatour facile strategy to increase the reactivity of transition-metal-based electrocatalysts toward HER by tailoring the electronicstructure of materials can be used to directly moderate the

electrical conductivity and intrinsic activity of these types ofcatalysts.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acssusche-meng.9b04470.

Morphological analysis of SEM−EDX spectra of N−Ti3C2Tx in the N−Ti3C2Tx sample; TEM image of theN−Ti3C2Tx@600 sample; comparison of high-resolu-tion O 1s, Ti 2p spectra of bare MXene and MXeneannealed at 600 and 800 °C; electrochemical perform-ance of controlled titanium carbide MXenes annealedunder Ar; corresponding specific component of nitrogenin N−H, O−Ti−N, and Ti−N by XPS N 1s; ECSA andcorresponding Cdl by electrochemical measurement ofthe N−Ti3C2Tx-based catalysts at 200, 400, 600, and800 °C; cyclic voltammetry (CV) curves in thenonfaradic capacitance current of a series of N-dopedMXene in the scan rate range of 20−180 mV s−1 in thevoltage window of 0.15−0.25 vs RHE; comparison ofdifferent N-terminated models with different atomic Hcoverages and the corresponding energy, performance ofrecently reported MXene-based HER electrocatalysts inacid electrolyte; TEM morphology and chemical state byXPS after long-term stability test (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: hyoyoung@skku.edu.ORCIDHyoyoung Lee: 0000-0002-8031-0791

Figure 6. (a) HER polarization curves of commercial 20% Pt/C, multilayered N−Ti3C2Tx@600, and delaminated N−Ti3C2Tx @600; (b) cyclicvoltammetry (CV) curves in nonfaradic capacitance current at various scan rates (20−180 mV s −1); (c) corresponding calculated electrochemicaldouble-layer capacitance (Cdl).

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b04470ACS Sustainable Chem. Eng. 2019, 7, 16879−16888

16886

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTST.A.L. and H.L. acknowledge the support from the Institute forBasic Science (IBS-R011-D1). T.A.L. thanks T. A. La for hissupport on drawing the schemes and useful comments onwriting of this work. Q.V.B. is grateful to the high-performancecomputing support from the Institute for Materials Research(Tohoku University, Japan).

■ REFERENCES(1) Tran, N. Q.; Bui, V. Q.; Le, H. M.; Kawazoe, Y.; Lee, H. Anion−Cation Double Substitution in Transition Metal Dichalcogenide toAccelerate Water Dissociation Kinetic for Electrocatalysis. Adv. EnergyMater. 2018, 8, No. 170213.(2) Dresselhaus, M.; Thomas, I. Alternative Energy Technologies.Nature. 2001, 414, 332−337.(3) Koper, M. T. Hydrogen Electrocatalysis: A Basic Solution. Nat.Chem. 2013, 5, 255−256.(4) Handoko, A. D.; Steinmann, S. N.; She, Z. W. Theory-GuidedMaterials Design: Two-Dimensional MXenes in Electro- andPhotocatalysis. Nanoscale Horiz. 2019, 4, 809−827.(5) Di, J.; Yan, C.; Handoko, A. D.; Sei, Z. W.; Li, H.; Liu, Z.Ultrathin Two-Dimensional Materials for Photo- and ElectrocatalyticHydrogen Evolution. Mater. Today 2018, 21, 749−770.(6) Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K. C.;Uchimura, M.; Paulikas, A. P.; Stamenkovic, V.; Markovic, N. M.Enhancing Hydrogen Evolution Activity in Water Splitting byTailoring Li+-Ni(OH)2-Pt Interfaces. Science 2011, 334, 1256−1260.(7) Yin, H.; Zhao, S.; Zhao, K.; Muqsit, A.; Tang, H.; Chang, L.;Zhao, H.; Gao, Y.; Tang, Z. Platinum Single-Atom and ClusterCatalysis of the Hydrogen Evolution Reaction. Nat. Commun. 2016, 7,No. 13638.(8) Cheng, N.; Stambula, S.; Wang, D.; Banis, M. N.; Liu, J.; Riese,A.; Xiao, B.; Li, R.; Sham, T. K.; Liu, L. M. Pt Monolayer Coating onComplex Network Substrate with High Catalytic Activity for theHydrogen Evolution Reaction. Sci. Adv. 2015, 1, No. e1400268.(9) Popczun, E. J.; Mckone, J. R.; Read, C. G.; Biacchi, A. J.;Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured NickelPhosphide as an Electrocatalyst for the Hydrogen Evolution Reaction.J. Am. Chem. Soc. 2013, 135, 9267−9270.(10) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak,R. E. Highly Active Electrocatalysis of the Hydrogen EvolutionReaction by Cobalt Phosphide Nanoparticles. Angew. Chem., Int. Ed.2014, 126, 5531−5534.(11) Xing, Z.; Liu, Q.; Asiri, A. M.; Sun, X. Closely InterconnectedNetwork of Molybdenum Phosphide Nanoparticles: A HighlyEfficient Electrocatalyst for Generating Hydrogen From Water. Adv.Mater. 2014, 26, 5702−5707.(12) Zhu, C.; Gao, D.; Ding, J.; Chao, D.; Wang, J. TMD-BasedHighly Efficient Electrocatalysts Developed by Combined Computa-tional and Experimental Approaches. Chem. Soc. Rev. 2018, 47, 4332−4356.(13) Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y. 25thAnniversary Article: MXenes: A New Family of Two-DimensionalMaterials. Adv. Mater. 2014, 26, 992−1005.(14) Liang, X.; Garsuch, A.; Nazar, L. F. Sulfur Cathodes Based onConductive MXene Nanosheets for High-Performance Lithium−Sulfur Batteries. Angew. Chem. 2015, 127, 3979−3983.(15) Lukatskaya, M. R.; Mashtalir, O.; Ren, C. E.; Dall’Agnese, Y.;Rozier, P.; Taberna, P. L.; Naguib, M.; Simon, P.; Barsoum, M. W.;Gogotsi, Y. Cation Intercalation and High Volumetric Capacitance ofTwo-Dimensional Titanium Carbide. Science 2013, 341, 1502−1505.(16) Yoon, Y.; Le, T. A.; Tiwari, A. P.; Kim, I.; Barsoum, M. W.; Lee,H. Low Temperature Solution Synthesis of Reduced Two Dimen-sional Ti3C2 MXenes with Paramagnetic Behavior. Nanoscale 2018,10, 22429−22438.

(17) Ling, Z.; Ren, C. E.; Zhao, M. Q.; Yang, J.; Giammarco, J. M.;Qiu, J.; Barsoum, M. W.; Gogotsi, Y. Flexible and Conductive MXeneFilms and Nanocomposites with High Capacitance. Proc. Natl. Acad.Sci. U.S.A. 2014, 111, 16676−16681.(18) Le, T. A.; Tran, N. Q.; Hong, Y.; Lee, H. Intertwined TitaniumCarbide MXene within a 3D Tangled Polypyrrole Nanowires Matrixfor Enhanced Supercapacitor Performances. Chem. - Eur. J. 2019, 25,1037−1043.(19) Seh, Z.; Fredrickson, K.; Anasori, B.; Kibsgaard, J.; Strickler, A.;Lukatskaya, M.; Gogotsi, Y.; Jaramillo, T.; Vojvodic, A.; et al. Two-Dimensional Molybdenum Carbide (MXene) as an Efficient Electro-catalyst for Hydrogen Evolution. ACS Energy Lett. 2016, 1, 589−594.(20) Handoko, A. D.; Fredrickson, K. D.; Anasori, B.; Convey, K.W.; Johnson, L. R.; Gogotsi, Y.; Vojvodic, A.; She, Z. W. Tuning theBasal Plane Functionalization of Two-Dimensional Metal Carbides(MXenes) To Control Hydrogen Evolution Activity. ACS Appl.Energy Mater. 2018, 1, 173−180.(21) Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang,R.; Lei, Y.; Pan, B.; Xie, Y. Controllable Disorder Engineering inOxygen-Incorporated MoS2 Ultrathin Nanosheets for EfficientHydrogen Evolution. J. Am. Chem. Soc. 2013, 135, 17881−17888.(22) Bao, J.; Zhang, X. F.; Zhang, B. J.; Zhou, M.; Yang, W.; Hu, X.;Wang, H.; Pan, B.; Xie, Y.; et al. Ultrathin Spinel-StructuredNanosheets Rich in Oxygen Deficiencies for Enhanced Electro-catalytic Water Oxidation. Angew. Chem., Int. Ed. 2015, 54, 7399−7404.(23) Gao, G.; O’Mullane, A. P.; Du, A. 2D MXenes: A New Familyof Promising Catalysts for the Hydrogen Evolution Reaction. ACSCatal. 2017, 7, 494−500.(24) Rakhi, R. B.; Ahmed, B.; Hedhili, M. N.; Anjum, D. H.;Alshareef, H. N. C. J. Effect of Postetch Annealing Gas Compositionon the Structural and Electrochemical Properties of Ti2CTx MXeneElectrodes for Supercapacitor Applications. Chem. Mater. 2015, 27,5314−5323.(25) Yuan, W.; Cheng, L.; An, Y.; Wu, H.; Yao, N.; Fan, X.; Guo, X.MXene Nanofibers as Highly Active Catalysts for HydrogenEvolution Reaction. ACS Sustainable Chem. Eng. 2018, 6, 8976−8982.(26) Li, P.; Zhu, J.; Handoko, A.; Zhang, R.; Wang, H.; Legut, D.;Wen, X.; Fu, Z.; She, Z.; Zhang, Q. High-throughput theoreticaloptimization of the hydrogen evolution reaction on MXenes bytransition metal modification. J. Mater. Chem. A 2018, 6, 4271−4278.(27) Li, L. Lattice Dynamics and Electronic Structures of Ti3C2O2

and Mo2TiC2O2 (MXenes): The Effect of Mo Substitution. Comput.Mater. Sci. 2016, 124, 8−14.(28) Zhou, G.; Pei, S.; Li, L.; Wang, D. W. S.; Huang, K.; Yin, L. C.;Li, F.; Cheng, H. M.; et al. A Graphene-Pure-Sulfur SandwichStructure for Ultrafast, Long-Life Lithium-Sulfur Batteries. Adv. Mater.2014, 26, 625−631.(29) Ji, X.; Lee, K. T.; Nazar, L. F. A Highly OrderedNanostructured Carbon−Sulphur Cathode for Lithium−SulphurBatteries. Nat. Mater. 2009, 8, No. 500.(30) Li, W.; Liu, J.; Zhao, D. Mesoporous Materials For energyConversion and Storage Devices. Nat. Rev. Mater. 2016, 1, No. 16023.(31) Shui, J.; Wang, M.; Du, F.; Dai, L. N-doped CarbonNanomaterials Are Durable Catalysts for Oxygen Reduction Reactionin Acidic Fuel Cells. Sci. Adv. 2015, 1, No. e1400129.(32) J. jia, T.; Xiong, L.; Zhao, W.; Hong, Liu.; Hu, R.; Zhou, J.;Zhou, W.; Chen, S. Ultrathin N-Doped Mo2C Nanosheets withExposed Active Sites as Efficient Electrocatalyst for HydrogenEvolution Reactions. ACS Nano 2017, 11, 12509−12518.(33) Zhang, L.; Liu, W.; Dou, Y.; Du, Z.; Shao, M. The Role ofTransition Metal and Nitrogen in Metal−N−C Composites forHydrogen Evolution Reaction at Universal pHs. J. Phys. Chem. C2016, 120, 29047−29053.(34) Zhu, Y.; Zhang, B.; Liu, X.; Wang, D.-W.; Su, D. S. Unravellingthe Structure of Electrocatalytically Active Fe−N Complexes inCarbon for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed.2014, 53, 10673−10677.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b04470ACS Sustainable Chem. Eng. 2019, 7, 16879−16888

16887

(35) Ren, J.; Antonietti, M.; Fellinger, T. P. Efficient Water SplittingUsing a Simple Ni/N/C Paper Electrocatalyst. Adv. Energy Mater2015, 5, No. 1401660.(36) Ma, T.; Cao, J.; Jaroniec, M.; Qiao, S. Interacting CarbonNitride and Titanium Carbide Nanosheets for High-PerformanceOxygen Evolution. Angew. Chem., Int. Ed. 2016, 55, 1138−1142.(37) Yoon, Y.; Tiwari, A.; Lee, M.; Choi, M.; Song, W.; Im, J.;Zyung, T.; Jung, H.; Lee, S.; Jeon, S.; An, K. Enhanced Electro-catalytic Activity by Chemical Nitridation of Two-DimensionalTitanium Carbide MXene for Hydrogen Evolution. J. Mater. Chem.A 2018, 6, 20869−20877.(38) Wen, Y.; Rufford, E. T.; Chen, X.; Li, N.; Lyu, M.; Dai, L.;Wang, L. Nitrogen-doped Ti3C2Tx MXene Electrodes for High-Performance Supercapacitors. Nano Energy 2017, 38, 368−376.(39) Tran, N. Q.; Kang, B.; Woo, M.; Yoon, D. Enrichment ofPyrrolic Nitrogen by Hole Defects in Nitrogen and Sulfur Co-DopedGraphene Hydrogel for Flexible Supercapacitors. ChemSusChem2016, 9, 2261−2268.(40) Tian, Y.; Que, W.; Luo, Y.; Yang, C.; Yin, X.; Kong, L. SurfaceNitrogen-Modified 2D Titanium Carbide (MXene) with High energyDensity for Aqueous Supercapacitor Applications. J. Mater. Chem. A2019, 7, No. 5416.(41) Pang, S.-Y.; Wong, Y.-T.; Yuan, S.; Liu, Y.; Tsang, M.-K.; Yang,Z.; Huang, H.; Wong, W.-T.; Hao, J. Universal Strategy for HF-FreeFacile and Rapid Synthesis of Two-Dimensional MXenes asMultifunctional Energy Materials. J. Am. Chem. Soc. 2019, 141,9610−9616.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b04470ACS Sustainable Chem. Eng. 2019, 7, 16879−16888

16888