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Article

Biowaste-Derived Heteroatom-Doped Porous Carbon as a Sustainable Electrocatalyst for Hydrogen Evolution Reaction

by
Raji Atchudan
1,*,†,
Suguna Perumal
2,†,
Thomas Nesakumar Jebakumar Immanuel Edison
3,†,
Ashok K. Sundramoorthy
4,†,
Namachivayam Karthik
5,
Sambasivam Sangaraju
6,
Seung Tae Choi
5 and
Yong Rok Lee
1,*
1
School of Chemical Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea
2
Department of Chemistry, Sejong University, Seoul 143747, Republic of Korea
3
Department of Chemistry, Sethu Institute of Technology, Kariapatti, Virudhunagar District 626115, Tamil Nadu, India
4
Department of Prosthodontics, Saveetha Dental College and Hospitals, Saveetha Institute of Medical and Technical Sciences, Poonamallee High Road, Velappanchavadi, Chennai 600077, Tamil Nadu, India
5
Functional Materials and Applied Mechanics Lab, School of Mechanical Engineering, Chung-Ang University, Seoul 06974, Republic of Korea
6
National Water and Energy Center, United Arab Emirates University, Al Ain 15551, United Arab Emirates
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2023, 13(3), 542; https://doi.org/10.3390/catal13030542
Submission received: 30 December 2022 / Revised: 23 February 2023 / Accepted: 2 March 2023 / Published: 8 March 2023
(This article belongs to the Special Issue Advanced Functional Materials for Photo/Electro-Catalysts for Environmental and Energy Applications)
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Abstract

:
Heteroatom-doped porous carbon material (H-PCM) was synthesized using Anacardium occidentale (cashew) nut’s skin by a simple pyrolysis route. The resulting H-PCM was thoroughly characterized by various analytical techniques such as field emission scanning electron microscopy (FE-SEM) with energy-dispersive X-ray (EDX) spectroscopy, high-resolution transmittance electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, nitrogen adsorption–desorption isotherms, X-ray photoelectron spectroscopy (XPS), and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The obtained results strongly demonstrated that the synthesized H-PCM exhibited a porous nature, continuous sponge-like and sheet-like smooth morphology, and a moderate degree of graphitization/crystallinity with oxygen-, nitrogen-, and sulfur-containing functionalities in the carbon matrix. After the structural confirmation, as-prepared H-PCM has used a sustainable electrocatalyst for hydrogen evolution reaction (HER) because the metal-free carbonaceous catalysts are one of the most promising candidates. The H-PCM showed excellent HER activities with a lowest Tafel slope of 75 mV dec−1 and durable stability in 0.5 M H2SO4 aqueous solution. Moreover, this work provides a versatile and effective strategy for designing excellent metal-free electrocatalysts from the cheapest biowaste/biomass for large-scale production of hydrogen gas through electrochemical water splitting.

Graphical Abstract

1. Introduction

Recently, nanomaterials with different architectures and supports have played a key role in industrial catalysis. Electrocatalysts are a variety of catalysts, which participate in a selective electrochemical reaction to increase the reaction rate of redox processes at the electrode/electrolyte interface. The electro-catalytic processes can occur at the electrode surface or electrode itself [1,2]. In recent days, various electro-catalytic reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and carbon dioxide reduction reactions are significantly important due to their involvement in green energy and environment-based applications [3,4]. Particularly, electro-catalytic HER is mainly focused on the bulk production of hydrogen gas through water splitting. Platinum and platinum-based composites are served as benchmark electrocatalysts for HER in all pH ranges. The scarceness and expense of platinum-based electrocatalysts hinder their applicability [5,6,7,8,9]. Hence, the replacement of noble platinum is necessary with cost-effective materials. Recent studies proved that nitrogen-doped carbon materials act as good electrocatalysts for HER and ORR reactions [10,11,12].
Carbon is capable of forming more than one crystalline form due to its valency, which is called allotropes of carbon. Graphite, diamond, and amorphous carbon are the most common allotropes of carbon. Depending on the arrangement of atoms the properties of allotropes can vary [13]. The crystal structure of diamond is an infinite three-dimensional array of carbon atoms, which makes less chemical reactivity, extreme hardness, and insulating properties of diamond, whereas the crystal structure of graphite amounts to a parallel stacking of layers of carbon atoms leads to the fascinating properties such as good electrical conductivity, softness, and lubricity of graphite [14,15]. The electrical conductivity, defects, and porosity of carbon materials can be easily tuned by the doping of electron-rich heteroatoms including nitrogen, oxygen, sulfur, and boron. Hence, the heteroatom-doped carbon materials received great attention from researchers owing to their excellent electrical conductivity, optical properties, and capacitive behavior which helps the applicability of heteroatom-doped carbon materials in catalysis, electrocatalysis, energy storage, and fluorescent sensors [16,17,18]. Particularly, nitrogen-doped carbon nanoparticles have competed with other heteroatom-doped carbon nanomaterials due to their applicability in energy-related applications including water-splitting reactions HER, OER, and ORR.
The nitrogen-doped carbon materials can be synthesized by pyrolyzation of green carbon sources such as glucose, citric acid, dried plant parts, and biowastes under the nitrogen atmosphere. This method provides various benefits including cost-effectiveness and a simple experimental setup [19,20]. Sun et al. reported the synthesis of nitrogen-doped porous carbon using orange peel by carbonization using a tube furnace under the nitrogen atmosphere [21]. Sekhon and Park conducted an excellent review on biomass-derived nitrogen-doped porous carbon nanosheets for energy technologies [22]. Moreover, Matsagar et al. reviewed the recent progress of biomass-derived nitrogen-doped porous carbon and its applications [23]. Chen et al. described that nitrogen/sulfur-doped porous graphene displayed enhanced electro-catalytic activity towards HER that was comparable to that of commercial electrocatalysts such as Pt-free MoS2 catalysts [24,25]. Many authors utilized natural green biowastes/biomass for the synthesis of carbon materials [26,27,28].
Here, porous carbon materials were synthesized using cost-effectiveness and a greener route. The present study involved the synthesis of heteroatom-doped porous carbon material (H-PCM) using Anacardium occidentale (cashew) nut’s skin (CNS) by a simple pyrolysis route under the nitrogen atmosphere. The CNS contains several phytoconstituents and is easily accessible at a low cost. The corresponding phytoconstituents can act as the source of carbon, nitrogen, oxygen, and sulfur for the formation of a carbon matrix during the pyrolysis process. The synthesized H-PCM was characterized by various surface- and structure-confirming analytical tools. Further, the synthesized H-PCM are coated on carbon cloth and used as electrocatalysts for HER in acid media. The HER reaction is monitored by linear sweep voltammetry, electrochemical impedance spectroscopy (EIS), and Tafel methods. Moreover, this investigation throws light on the mere future for more possibilities to transform solid waste into environmentally friendly energy conversion.

2. Results and Discussion

The morphologies investigation of the synthesized porous carbon material was thoroughly studied using FE-SEM with energy-dispersive X-ray (EDX) spectroscopy, selected area electron diffraction (SAED), and TEM measurements were performed. The FE-SEM images of H-PCM (Figure 1a–e) displayed the porous structure with interconnected channels that might be favored for the transport of electrolytes. In addition, the presented pores facilitate the easy release of bubbles that are produced during the HER. Figure 1f–k displays the FE-SEM images and their corresponding elemental mapping of H-PCM. Carbon, oxygen, nitrogen, and sulfur have been detected in the H-PCM from the elemental mapping measurements. The presented heteroatoms were distributed uniformly throughout the carbon matrix. Furthermore, as shown in Figure 1l, the elemental composition of the synthesized H-PCM was further confirmed from the EDX spectrum. The spectrum demonstrated that the H-PCM exhibits carbon, oxygen, nitrogen, and sulfur elements. Apart from these essential elements, silicon, and platinum are presented in the spectrum, silicon is from silicon wafers that are used as substrate, and platinum that used for coating.
Further, the overall morphology and porous structure of the synthesized H-PCM were revealed by TEM/HRTEM analysis. Figure 2a–c shows the TEM images of synthesized H-PCM with different magnifications. These images demonstrated that the H-PCM exhibits a mixture of continuous sponge-like and sheet-like architecture with many disordering interconnected porous textures [29,30]. The obviously observed pores are distributed in the range of approximately several nanometers, and this is in favor of the rapid transport of electrolytes which might be facilitated the electrochemical reaction toward HER. HRTEM image (Figure 2d) shows indistinct lattice fringes with a typical turbostratic structure which revealed the partial graphitization of synthesized H-PCM. The existence of abundant heteroatom-containing functional groups mitigates the degree of graphitization. It was expected that this outstanding nature of H-PCM will enhance the electric conductivity and will be a potential candidate for electrochemical reactions [31]. Furthermore, the analysis of SAED confirms the degree of crystallinity/graphitic nature of materials. The inset of Figure 2d depicts the dim narrow rings suggesting partial graphitization/crystallinity of H-PCM.
Figure S1 demonstrates the XRD pattern of H-PCM synthesized using CNS waste. The XRD peaks positioned at 2θ = 26 and 44° correspond to the reflections of the (002) and (100) planes for the characteristic graphitic carbon, respectively [32,33,34]. The absence of sharpness in the two diffraction peaks (2θ = 26 and 44°) indicates that the H-PCM partial graphitic state/disordered carbon structure, and also might have smaller carbon sheets. This result coincides well with the TEM results. Apart from these two distinguished peaks (2θ = 26 and 44°) that are responsible for graphitic carbon, many other minor peaks were observed. The minor peaks might be from the phytoconstituents and minerals from the biomass/biowaste materials. The quality of the carbon materials in respect of the graphitization of synthesized H-PCM was further evaluated by Raman spectroscopy. Raman spectra of H-PCM (Figure 3a) exhibit two major peaks positioned at 1345 and 1595 cm−1 are attributed to the D- and G-bands, respectively [35,36]. The D-band is correlated to the breathing mode of defective/disordered (sp3) carbon, whereas the G-band corresponds to the vibration mode of the graphitic/crystalline (sp2) carbon [37]. The slightly low intensity of the graphitic band (G-band) compared to the D-band can be due to the existence of functional groups. The relation between the intensity of the D-band and G-band (ID/IG) determined the graphitization/crystallinity and arrangement of the graphene planes in the synthesized H-PCM [38]. The ID/IG value of H-PCM was calculated as 1.03, which implies the partial graphitization/crystallinity and structural defects that might be due to the porosity as well as because of the doping of heteroatoms/heteroatoms-containing functionalities. Apart from these two major peaks, the low intense and broad Raman vibration is located around 2800 cm−1 and is denoted as a 2D-band which is a fingerprint of graphene order (number of graphene sheets) in the H-PCM [39,40]. In the case of unclear separation between the D-band and G-band, the determination of graphitization/crystallinity is complicated. Hence, the Raman spectrum of H-PCM was deconvoluted, and the graphitization/crystallinity was determined by the area of the D-band and G-band (AD and AG). The ratio of AD/AG was calculated as 0.95 (Figure 3b), which indicates the acceptable graphitic order of H-PCM. Overall results strongly revealed that the synthesized H-PCM exhibit graphitic order with partial structural defects.
ATR-FTIR spectroscopy is used to study the chemical composition and surface functional groups on the synthesized H-PCM. Figure S2 shows the ATR-FTIR spectrum of as-synthesized H-PCM. The characteristic absorption bands at 3375 cm–1 belong to the stretching vibration of O–H groups on the H-PCM surface [41]. The N–H stretching vibration of H-PCM appeared around 3100 cm−1 [42]. The absorption peaks of H-PCM around 2835 and 2700 cm−1 are overlapped and displayed as a broader band which belongs to the C–H and S–H stretching vibrations, respectively. The ATR-FTIR spectrum of H-PCM exhibits absorption bands at 2362 and 2332 cm−1, which can be accredited to the stretching vibration of atmospheric –CO2 and –C≡N functional groups, respectively. The absorption bands at 2106 and 1650–1850 cm−1 correspond to the stretching vibration of –C=N and C=O functional species. The absorption peak at 1580 cm−1 is assigned to the stretching vibration of sp2 carbon bonding (C=C) in the porous carbon matrix of the H-PCM [43]. The stretching vibration bands of C–N/C–S are located in a range of 1330–1420 cm−1. The absorption bands in the area between 1120 and 1000 cm−1 might be ascribed to the stretching vibrations of C–OH (alkoxy) and C–O–C (epoxy) functional groups, respectively [44]. The out plane aromatic ring stretching vibration of the –CH2 band emerged at 826 cm−1 in the ATR-FTIR spectrum of the H-PCM [45]. The absorption peaks located at 710 and 616 cm−1 correspond to the stretching vibrations of the C–S groups [46]. The absorbance peak at 470 cm−1 corresponds to the S–O stretching vibrations in the porous carbon framework. The ATR-FTIR spectral results show that elements such as carbon, oxygen, nitrogen, and sulfur are exhibited in the synthesized H-PCM which suggests the heteroatoms doping into the H-PCM.
The XPS is used to further investigate the chemical composition and elemental state of surface groups on H-PCM. As shown in Figure S3, the synthesized H-PCM mainly contains carbon, oxygen, nitrogen, and sulfur with atomic percentages of 67, 28, 3, and 2, respectively. The high-resolution XPS spectrum of C 1s (Figure 4a) exhibits five main peaks at 284.1 eV, corresponds to the carbon-bonded with hydrogen (C–H), 284.8 eV credited to the signal of disorder sp3-bonded carbon and graphitic sp2-bonded carbon, 285.8 eV confirms the C–O (epoxy and alkoxy), C–N, and C–S functionalities, 286.6 eV suggests the presence of carbonyl groups (C=O), and 287.5 eV, attributed to carboxyl species (H–OC=O) [6,47,48,49]. The existence of graphitic sp2 (C=C) bonds and epoxy/alkoxy (C–O–C/C–O–H) bonds might enhance expectations of the electric conductivity of electrocatalysts (H-PCM). The high-resolution O 1s spectrum can be deconvoluted (Figure 4b) into three distinct binding energy peaks at 531.3, 532.5, and 533.6 eV that are accredited to the C=O, C–O/S–O, and O=C–O functional species, respectively [39,50]. The high-resolution N 1s spectrum can be deconvoluted into three peaks positioned at binding energies of 398.9, 400.8, and 404.1 eV, which were credited to the C–N–C (pyridinic), C–N–H (pyrrolic), and C3–N (quaternary N atoms) functional moieties, respectively [51,52]. The presence of C–N proves that nitrogen was incorporated successfully into the porous carbon matrix of H-PCM. The high-resolution S 2p spectrum can be divided into four binding energy peaks by deconvolution. The peaks appeared at 164.1 and 165.3 eV, corresponding to S 2p3/2 and S 2p1/2, respectively, suggesting the possibility of aromatic C–S–C and C=S functionalities [53,54]. The C–S–C group profoundly demonstrates that the H-PCM is successfully doped with sulfur [55]. Other binding energy peaks positioned at 168.8 and 169.9 eV suggests the existence of oxidized sulfur (OS) in the form of OS3/2 and OS3/2, respectively; basically present C–SO–C (sulfoxide) and C–SO2–C (sulfone) in the H-PCM [56,57]. The aromatic C–S–C and C=S functionalities could modify the atmosphere of the porous carbon surface/edges and improve the total polarization of the medium [57]. The incidence of sulfur species can also enhance electrochemical reactions by increasing the conductibility of the carbon matrix. These findings of heteroatom functionalities are consistent with FT-IR results. Thus it was believed that the HER performance would be enhanced by the presence of heteroatoms [58].
The numerous number of micropores and mesopores is one of the key factors in determining the electrochemical activity by easily allowing electrolytes and strong holding of heteroatoms during the electrochemical reaction. Nitrogen sorption (adsorption–desorption) measurements were performed on the synthesized H-PCM under standard temperature and pressure. As shown in Figure S4, the synthesized H-PCM exhibited a combination of type I and type IV isotherm characteristics in nature with a type H4 hysteresis loop appearance that indicates acceptable pores with a layered structure in the carbon matrix of H-PCM. The characteristic type I isotherms were displayed in the lower relative pressure (P/P0 < 0.1) with high nitrogen adsorption quantity in a vertical design, suggesting that H-PCM possessed a higher number of micropores [59]. The nitrogen sorptions relative pressure (P/P0) within the range from 0.2 to 0.9 represents the type IV isotherms with an H4 hysteresis loop, indicating that the H-PCM had mesoporous adsorption characteristics nature [60]. The isotherms that appeared with higher relative pressure (P/P0 > 0.9) represent the macropores that originated from slit holes in the carbon structure and wide spacing within the layered structure of H-PCM. The combination of isotherms type I and IV indicated that the synthesized H-PCM contains adequate porous structures including micropores, mesopores, and macropores. Micropores and mesopores are responsible for the higher specific surface area, whereas macropores also help to transport chemical components. The presented porosity of synthesized H-PCM has been verified by the FESEM and TEM/HRTEM observations. The obtained Brunauer–Emmett–Teller (BET) surface area of synthesized H-PCM was 565 m2 g–1. These porous structures with high surface presented H-PCM is more beneficial for the electrons and ions which resulting enhanced electro-catalytic activity towards HER [61].
The HER performance was examined using synthesized H-PCM as a catalyst in 0.5 M H2SO4 aqueous solution under atmospheric conditions. The linear sweep voltammetry studies of H-PCM-adorned carbon cloth (H-PCM/CC) are performed at the scan rate of 10 mV s−1, and the corresponding polarization curves are presented in Figure 5a and Figure S5. As shown in Figure 5a and Figure S5, the cathodic current density (CCD) of the unadorned carbon cloth did not alter much upon increasing the negative potential, suggesting the lower catalytic activity of bare carbon cloth. Whereas, the CCD of the synthesized H-PCM/CC increased quickly by increasing the negative potential, indicating that the synthesized H-PCM possessed higher catalytic activity toward HER [59]. The initial potential of the bare platinum plate (standard catalyst) was nearing zero, and the delivered lowest overpotential (70 mVRHE) at a CCD of 10 mA cm−2, while unadorned carbon cloth has negligible HER activity (overpotential: 918 mVRHE). The synthesized H-PCM catalyst delivered an overpotential of 133 mVRHE at a CCD of 10 mA cm−2 that is comparable to that of the bare platinum catalyst. The synthesized H-PCM (H-PCM/CC) catalyst exhibits high catalytic activity toward HER compared to bare carbon cloth. The doping of heteroatoms including nitrogen and sulfur possibly enhanced the electro-catalytic activities of the synthesized H-PCM. The Tafel slope of the catalyst is one of the significant parameters for determining the HER performance. The Tafel slope of H-PCM/CC is 75 mV dec−1 in the HER process follows the Volmer–Heyrovsky mechanism (Figure 5b) [62]. The lowest Tafel slope of H-PCM-adorned carbon cloth implies fast reaction kinetics of synthesized H-PCM catalyst towards HER [63]. Moreover, the lower Tafel slope of H-PCM suggests their excellent catalytic performances. Even though the Tafel slope is greater than that of the bare platinum catalyst, that is comparable to or lower than that of earlier reports (Table 1).
EIS is one of the significant tools for studying electro-catalytic materials in terms of examining the electrode and electrolyte interface reaction including charge transfer resistance and coating effect. The EIS Nyquist plot of the synthesized H-PCM electrode (H-PCM/CC) was measured at open circuit potential in 0.5 M H2SO4 aqueous solution, and obtained results were fitted (Figure 5c). As shown in the EIS Nyquist plot, the charge transfer resistance of H-PCM/CC is around 1.9 Ω cm−2. The lowest charge transfer resistance for the H-PCM/CC working electrode demonstrated that the synthesized H-PCM exhibits faster reaction kinetics which can result in excellent HER. The highest HER performance might be due to the comparable degree of graphitization, high surface area with acceptable porosity, and heteroatom-containing rich functionalities. In addition, the EIS Nyquist plot is nearly perpendicular to the y-axis, suggesting the good conductance of the prepared electrocatalyst (H-PCM). The recorded EIS Nyquist plot is perfectly fitted with the proposed equivalent circuit as shown in the inset of Figure 5c [6]. Furthermore, the EIS Nyquist plots are recorded at different overpotentials (0.57, 0.47, 0.37, 0.27, 0.17, 0.07, −0.03, and −0.13 VRHE), which are shown in Figure 5d. The EIS Nyquist plots displayed that charge transfer resistance gradually decreased while increasing the overpotential, suggesting the kinetics of HER depends on the supplied potential. Notably, the higher HER performance occurred at the higher overpotential.
The stability of the synthesized electrocatalyst is one of the essential criteria for the applicability of real-time applications. Hence, the long-term durability measurement of synthesized H-PCM is performed by the chronoamperometry method at a controlled potential of −0.15 VRHE, as shown in Figure 6a. The durability test was conducted in 0.5 M H2SO4 aqueous solution for 24 h, and the retention rate of current density was maintained at nearly 100% compared to the initial current density, manifesting that the H-PCM was remarkably stable [64]. Nitrogen and sulfur-rich carbon materials showed remarkable catalytic activity toward HER with outstanding stability [65]. The linear sweep voltammetry polarization curve of the H-PCM/CC electrode was again recorded at the scan rate of 10 mV s−1 after the completion of the stability study. The obtained linear sweep voltammetry polarization curve was compared with the initial linear sweep voltammetry polarization curve of the H-PCM/CC electrode (Figure 6b). There is no significant difference in both linear sweep voltammetry polarization curves (before and after stability), authenticating that the synthesized H-PCM is greatly stable and could be a potential candidate as an electrocatalyst for HER in the acidic media. In addition, the EIS Nyquist plot of the synthesized H-PCM electrode measured at open circuit potential in 0.5 M H2SO4 aqueous solution after the prolonged stability and obtained results were compared to before the prolonging stability. Figure S6 shows the EIS Nyquist plots of synthesized H-PCM electrodes obtained before and after prolonged stability. The EIS Nyquist plots exhibit insignificant changes before and after, suggesting excellent stability in the acidic medium towards HER. Aforesaid, all the analytical results strongly imply that the synthesized H-PCM catalyst (metal-free electrocatalyst) had great application value toward HER in the acidic medium.
Table
Table 1. The HER performances of H-PCM were compared with the reported ones.
Table 1. The HER performances of H-PCM were compared with the reported ones.
ElectrocatalystsPreparation
Method		Overpotential at 10 mA cm−2 (mV)Tafel Slope
(mV dec–1)		Reference
SrTiO3@MoS2Calcination16581.41[66]
NA9Carbonization184164[67]
NPCFAnnealing248135[64]
S–MoS2/rGO/CNTsCarbonization15985[68]
Mo2C/CPyrolysis13371[69]
LS-PCPyrolysis13585[45]
PS/MoS2Pyrolysis15471[70]
Porous Mo2C/CSintering556123.9[59]
Ni/P-doped carbonPyrolysis297134.9[71]
BS-800Calcination41398[72]
H-PCMPyrolysis13375This work

3. Materials and Methods

Synthesis of Heteroatom-Doped Porous Carbon Material

CNS waste was dried in sunlight, then it was made into a fine powder using a commercial mixer grinder. The fine powder of CNS was carbonized at a mild temperature of 800 °C under the nitrogen atmosphere for 3 h in a tubular furnace. After the completion carbonization process, the furnace was allowed to cool down to room temperature, and the resulting fine black powder was collected for further analysis. The detailed synthesis process of H-PCM was described in Scheme 1.

4. Conclusions

Here, an easy and cost-effective metal-free electrocatalyst was established using waste biomass of CNS via a one-pot pyrolysis route and utilized for HER in an acidic medium. The heteroatoms such as oxygen, nitrogen, and sulfur species are uniformly distributed as well as chemically bonded in the biomass-derived porous carbon matrix. The obtained H-PCM exhibits a moderate degree of crystallinity with abundant functionalities. Notably, the synthesized H-PCM possessed a high BET surface area of 565 m2 g−1. The exhibited porous structures with interconnected channels not only support the transportation/ penetration of electrolytes but also the bubbles produced are rapidly released during the HER process. Furthermore, the nitrogen and sulfur doping in the porous carbon matrix played an essential role in the improved electrochemical conductivity during HER. Such outstanding specific characterization of H-PCM bestows the excellent electro-catalytic activity for HER in the acidic electrolyte (0.5 M H2SO4 aqueous solution), and the H-PCM displayed a lower Tafel slope (75 mV dec−1) and excellent long-lasting durability. This work demonstrates a facile strategy to design the waste biomass-based H-PCM electrocatalysts by simple pyrolysis for HER applications and will also be extended in essential energy-related applications.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal13030542/s1. Materials, instrumentation methods, and fabrication of working electrode and electrochemical measurements of the synthesized H-PCM, Figure S1: XRD pattern of synthesized H-PCM; Figure S2: ATR-FTIR spectrum of synthesized H-PCM; Figure S3: XPS survey scan spectrum (inset of Pie chart: presented elements with atomic %) of synthesized H-PCM; Figure S4: Nitrogen sorption isotherms of synthesized H-PCM under standard temperature and pressure; Figure S5: HER polarization (LSV) curves for bare platinum plate, H-PCM/CC, and bare carbon cloth electrodes in 0.5 M H2SO4 aqueous solution at a scan rate of 10 mV s−1; Figure S6: EIS Nyquist plots of synthesized H-PCM electrode obtained before and after prolonged stability.

Author Contributions

Conceptualization, methodology, formal analysis, investigation, data curation, and writing—original draft, R.A.; writing—review and editing and visualization, S.P.; formal analysis and investigation T.N.J.I.E.; investigation and visualization, A.K.S.; formal analysis and investigation, N.K.; visualization, S.S.; investigation, S.T.C.; project administration and supervision, Y.R.L. All authors equally contributed to this work. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a–e) FE-SEM images at different magnifications of synthesized H-PCM; (f) FE-SEM electron image of synthesized H-PCM and the corresponding EDX mapping of (g) carbon, (h) oxygen, (i) nitrogen, (j) sulfur, and (k) overlapping of all the presented elements; (l) EDX spectrum results of presented elements in the synthesized H-PCM.
Figure 1. (a–e) FE-SEM images at different magnifications of synthesized H-PCM; (f) FE-SEM electron image of synthesized H-PCM and the corresponding EDX mapping of (g) carbon, (h) oxygen, (i) nitrogen, (j) sulfur, and (k) overlapping of all the presented elements; (l) EDX spectrum results of presented elements in the synthesized H-PCM.
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Figure 2. (a–d) TEM/HRTEM images at different magnifications of synthesized H-PCM (inset (d): SAED pattern of H-PCM).
Figure 2. (a–d) TEM/HRTEM images at different magnifications of synthesized H-PCM (inset (d): SAED pattern of H-PCM).
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Figure 3. (a) Raman spectrum and (b) Raman spectrum with deconvolution of D-band/G-band of synthesized H-PCM.
Figure 3. (a) Raman spectrum and (b) Raman spectrum with deconvolution of D-band/G-band of synthesized H-PCM.
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Figure 4. XPS high-resolution spectra with deconvolution peaks of (a) C 1s, (b) O 1s, (c) N 1s, and (d) S 2p regions of synthesized H-PCM.
Figure 4. XPS high-resolution spectra with deconvolution peaks of (a) C 1s, (b) O 1s, (c) N 1s, and (d) S 2p regions of synthesized H-PCM.
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Figure 5. Electro-catalytic studies of synthesized H-PCM, bare platinum plate, and bare carbon cloth electrodes in 0.5 M H2SO4 aqueous solution towards HER under static atmosphere; (a) linear sweep voltammetry polarization curves at the scan rate of 10 mV s−1, (b) Tafel plots, (c) EIS Nyquist plot and suitable fitting with the equivalent circuit of H-PCM electrode recorded at open circuit potential, and (d) EIS Nyquist plot of H-PCM electrode at different overpotentials.
Figure 5. Electro-catalytic studies of synthesized H-PCM, bare platinum plate, and bare carbon cloth electrodes in 0.5 M H2SO4 aqueous solution towards HER under static atmosphere; (a) linear sweep voltammetry polarization curves at the scan rate of 10 mV s−1, (b) Tafel plots, (c) EIS Nyquist plot and suitable fitting with the equivalent circuit of H-PCM electrode recorded at open circuit potential, and (d) EIS Nyquist plot of H-PCM electrode at different overpotentials.
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Figure 6. (a) Stability curve of synthesized H-PCM electrode was recorded in 0.5 M H2SO4 aqueous solution towards HER for 24 h and (b) linear sweep voltammetry polarization curves of synthesized H-PCM electrode obtained before and after prolonged stability.
Figure 6. (a) Stability curve of synthesized H-PCM electrode was recorded in 0.5 M H2SO4 aqueous solution towards HER for 24 h and (b) linear sweep voltammetry polarization curves of synthesized H-PCM electrode obtained before and after prolonged stability.
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Scheme 1. Synthesis of biomass-derived heteroatom-doped porous carbon material by simple pyrolysis and their eco-friendly energy applications.
Scheme 1. Synthesis of biomass-derived heteroatom-doped porous carbon material by simple pyrolysis and their eco-friendly energy applications.
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Atchudan, R.; Perumal, S.; Jebakumar Immanuel Edison, T.N.; Sundramoorthy, A.K.; Karthik, N.; Sangaraju, S.; Choi, S.T.; Lee, Y.R. Biowaste-Derived Heteroatom-Doped Porous Carbon as a Sustainable Electrocatalyst for Hydrogen Evolution Reaction. Catalysts 2023, 13, 542. https://doi.org/10.3390/catal13030542

AMA Style

Atchudan R, Perumal S, Jebakumar Immanuel Edison TN, Sundramoorthy AK, Karthik N, Sangaraju S, Choi ST, Lee YR. Biowaste-Derived Heteroatom-Doped Porous Carbon as a Sustainable Electrocatalyst for Hydrogen Evolution Reaction. Catalysts. 2023; 13(3):542. https://doi.org/10.3390/catal13030542

Chicago/Turabian Style

Atchudan, Raji, Suguna Perumal, Thomas Nesakumar Jebakumar Immanuel Edison, Ashok K. Sundramoorthy, Namachivayam Karthik, Sambasivam Sangaraju, Seung Tae Choi, and Yong Rok Lee. 2023. "Biowaste-Derived Heteroatom-Doped Porous Carbon as a Sustainable Electrocatalyst for Hydrogen Evolution Reaction" Catalysts 13, no. 3: 542. https://doi.org/10.3390/catal13030542

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