Hierarchical porous activated carbon in OER with high efficiency

Abstract

The oxygen evolution reaction (OER) has attracted much attention as it can be used to produce clean energy. Hierarchical porous activated carbon (HPAC) shows potential applications for the OER. HPAC derived from fallen leaves (Fraxinus chinensis) is prepared using a facile method, in which KOH and/or K₂CO₃ are used as activators. Characterization reveals that HPAC in the system of KOH/K₂CO₃ has a high surface area and possesses abundant hierarchical pores, which are favorable for mass transfer and charge transfer in the oxygen evolution reaction (OER). Acid washed HPAC displays relatively low activity and electrochemical tests demonstrate that ash composition may contribute to the OER. HPAC is a potential carbon-based catalyst that has prominent OER activity and durability.

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Introduction

Environmental issues and the global energy crisis are attracting considerable attention, promoting researchers to develop a variety of energy conversion and storage systems.¹ Electrochemical processes, including the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), are reactions that involve the electrochemical splitting of water, which can generate valuable hydrogen and oxygen² for fuel cells and other devices. The OER in alkaline solution involves a half reaction of 4OH⁻ → 2H₂O + O₂ + 4e⁻,³ which is a vital procedure for achieving beneficial reaction dynamics.¹ Oxygen molecules can be produced and transported within electrocatalysts and electrodes. This is the efficiency-limiting factor for the water splitting process, because the over-potential and catalyst for this reaction are considerable and expensive.⁴ In consideration of the sluggish OER reaction, various catalysts have been fabricated, such as noble metal (Ir, Ru based materials)⁵ and transition-metal catalysts.^6–8 However, high costs are a driving force to make efforts in non-precious metal or metal oxide catalysts. Carbon nanotubes (CNTs)⁹ and N-doped graphene¹⁰ with good conductivity promote the charge transfer of OERs at their surface. Their high activity can be ascribed to the large surface area with a great number of micropores, the high content of pyridine and pyrrolic-like nitrogen atoms within the materials, and the 3D interpenetrated network structure produced by many carbon nanosheets.¹⁰

Activated carbon materials (ACs), derived from biomass, have gained great interest due to their unique properties such as low price, reproducibility and environmentally friendly nature, and due to the fact that they exist in nature universally.^11,12 The existence of oxygen functional groups on the surface of ACs provides numerous active sites for catalytic processes. These groups might give adjacent carbon atoms a relatively high positive charge density because of the electron withdrawing ability of oxygen atoms.^1,13 The carbon atoms with more electrophilicity can absorb OH⁻ or H₂O via electrostatic forces, and consequently promote electron transfer between the catalyst surface and reactive intermediate O²⁻ ions, which expedite oxygen evolution and result in a high OER activity.¹⁴

In addition, the hydrophilic oxygen groups on the surface of ACs can interact with water molecules via hydrogen bonding and favor the adsorption of a large amount of water. The active species in the catalysts with higher electrophilicity can facilitate the adsorption of OH⁻ ions, promote transportation of electrons and assure easy and rapid recombination of two adsorbed oxygen atoms for O₂ evolution.^15,16 What’s more, the porosity of the ACs can directly determine transportation in the catalytic processes and is crucial for achieving optimal performances. Specifically, the hierarchical porous ACs can facilitate the easy infiltration of electrolytes, effective transfer of OH⁻ and the fast emission of O₂, conducing to a high performance in the OER.^4,13

Herein, we give a discussion of hierarchical porous activated carbon (HPAC) as an OER catalyst ​in alkaline media. It is prepared with a low cost and facile method. Fallen leaves are used as the carbon source and are activated with KOH and K₂CO₃ for the preparation of AC (denoted as MAC). A number of techniques are carried out to further investigate the morphology, structure, and composition of the catalyst. The related HCl treated sample (denoted as PAC) reveals the influence of ash, surface area and porosity to the OER efficiency.

Results and discussion

Physicochemical properties of materials

XRD patterns reveal the phase of the MAC, PAC and CAC (CAC is the sample activated by K₂CO₃), as shown in Fig. 1a. All samples demonstrate two broad diffraction peaks at 21–27° and 41–43°, originating from the (002) and (100) plane of graphene layers spaced at a distance of d₍₀₀₂₎ = 0.34 nm and d₍₁₀₀₎ = 0.21 nm.¹⁷ The broad diffraction peaks suggest the presence of amorphous or disordered carbon, that is, there are no fragments of oriented structures in this material. It can be seen clearly that all of the samples have considerable intensity in the low-angle scatter, implying the existence of mesopores.¹⁸

Fig. 1 (a) XRD patterns of the samples. SEM images of (b) FLC, (c) CAC, (d) MAC and (e) PAC, and (f) the TEM image of MAC. The inset in (c) is the corresponding low magnification image.

SEM and TEM are applied to investigate the microstructures of the carbon materials activated by different activators. FLC consists of a large block of carbon and has less obvious pores (Fig. 1b). When K₂CO₃ is used as the activator alone, the carbon material displays a fluffy and rough surface (Fig. 1c). The irregular patch is in whole area when observing at low magnification (Fig. 1c (inset)). With the activation of KOH and K₂CO₃, the structure of the as-obtained MAC possesses a porous morphology and fragments with different sizes exist randomly. We can clearly observe this material contains certain macropores bigger than 50 nm (Fig. 1d). As for PAC, the particle size and shape are almost the same as the sample of MAC (Fig. 1e). The rough surface with aggregated pores can be observed in a random distribution. Looking at the TEM image (Fig. 1f), the MAC is made up of hierarchical pores, which interconnect with each other and enter into the core of the carbon structure.

In order to illuminate the porous structure of prepared sample, the nitrogen adsorption and desorption isotherms are analyzed. The surface area and the calculated pore size distribution are shown in Fig. S1–4 and Table S1.† According to the IUPAC classification, isotherm curves all of the samples exhibit type I and type IV, due to the coexistence of micropores and mesopores.^19,20 The surface areas of MAC, CAC, PAC and AC are 1078, 1003, 824 and 1912 m² g⁻¹, respectively. Note that the surface area of PAC is lower than that of MAC, which may be attributed to some chlorine remaining chemisorbed onto the surface of the material, decreasing the value of the surface area.²¹ The insets of Fig. S1–4† show the corresponding pore size distribution obtained by DFT model. The pore size of MAC concentrates at around 1.2–4.3 nm and PAC displays a pore size of 1.1–1.9 and 2.4–5.0 nm. CAC and AC show pore sizes under 5 nm. Meanwhile, the ratio of S_micro/S_meso is 2.59 (CAC), 1.76 (AC), 1.46 (MAC) and 1.06 (PAC).

Raman spectra of the various as-obtained materials are shown in Fig. 2a. The discernible peaks centered at ∼1330 cm⁻¹ (D-band), and ∼1590 cm⁻¹ (G-band) demonstrate the disordered microstructure and the sp² vibration of the carbon atom. The G-band can enhance the electrical conductivity for carbon materials.^22,23 The disorder degree and graphitization of the carbon structure could be measured by the relative intensity between the D and G band.²⁴ The relative intensity ratios of I_D/I_G for MAC, PAC and CAC are 0.85, 0.92 and 0.98, respectively. The differences in the values may be due to the degree of etching by different activators. MAC possesses the highest G-band intensity, suggesting the successful conversion of disordered microstructures to sp² states. Meanwhile, the high level of sp² type carbon atoms promotes higher electrical conductivity in comparison with the amorphous AC.²²

Fig. 2 (a) Raman spectra and (b) TGA curves of the samples in air.

TGA analysis can be seen in Fig. 2b. The composites show a little weight loss below 150 °C because of the evaporation of adsorbed water and gases.²⁵ Significant weight loss can be observed from 300 to 560 °C, 350 to 580 °C and 350 to 570 °C for CAC, MAC and PAC, corresponding to a remaining weight of 8.28%, 14.42% and 3.25%, respectively. This can be attributed to the elimination of the surface functional groups and the decomposition of carbon due to the oxidation of carbon.^26,27 CAC shows relatively lower thermal stable properties, suggesting the existence of more organic component before the activation. The PAC remains at its lowest weight due to the removal of the ash and trace elements. The XRF spectra collected for MAC and PAC are represented in Fig. S5.† The ash content is obviously decreased after washing FLC in 1 mol L⁻¹ of HCl. The element peaks of PAC decrease with trace elements of Si, K, Ca and Ti left as acid treated ginkgo shells.²⁸ As for the sample of MAC and CAC, elements of Al, Si, S, K, Ca, Ti, Fe and Mn can be observed in a small variation. Obviously, the element peaks of AC are negligible, except for minor K and Cr.

The FT-IR spectra of CAC, MAC and PAC are shown in Fig. S6.† It is clear that all of the samples possess similar spectra. The peaks located at 3420 cm⁻¹ are due to typical O–H stretching vibrations, and the peaks at 1564 cm⁻¹ correspond to the stretching vibration of C=O groups.^29,30 Another peak at 1086 cm⁻¹ is assigned to C–O stretching of esters.³¹ These oxygen-containing functional groups existing in the prepared AC can have a certain effect on the electrocatalytic behaviours.

Electrochemical tests for carbon electrodes

To explore the advantages of the MAC material and its potential application as an electrocatalyst for the OER, the electrochemical performance of all of the samples is measured in 1 M KOH aqueous electrolyte in a conventional three-electrode system. The OER electrocatalytic activities of the obtained materials are evaluated by LSV measurements at a scan rate of 5 mV s⁻¹. As shown in Fig. 3a, the weak anodic waves near to 1.4 V belong to Ni foam, corresponding to the redox potential of Ni(II)/Ni(III).³² MAC gives a sharp potential over 1.6 V and PAC affords a lower potential than that of MAC. FLC, CAC and AC show negligible OER responses compared to MAC. The operating potentials to deliver a 10 mA cm⁻² mg⁻¹ current density are compared, as this is the value expected for a 10% efficient solar water-splitting device.³³ The corresponding OER potential of MAC is 1.596 V, which is comparable to those of the Co₃O₄/N-graphene (1.63 V)³⁴ and flavin derivatives.³⁵ Compared with CAC and PAC, MAC exhibits the greatest OER activity but the lowest defect sites, indicating that structural defects have a negligible impact on the electrochemical activity in those materials.³⁶

Fig. 3 (a) LSV curves in 1.0 M KOH solution (scan rate of 5 mV s⁻¹), (b) Tafel plots derived from the LSV curves.

The Tafel slope b is a significant kinetic parameter that can be used to estimate the OER performance. The slope of the Tafel plot is determined by fitting LSV data to the Tafel equation: η = a + b log j, where η is the over potential, b is the Tafel slope, and j is the current density. A small Tafel slope means a greatly enhanced OER rate with a moderate increasing potential.³⁷ The enhanced kinetics of MAC, shown in Fig. 3b, are approved with a Tafel slope of 60.0 mV dec⁻¹. This value is lower than those for the as-obtained materials: FLC (123.2 mV dec⁻¹), CAC (122.5 mV dec⁻¹), PAC (89.0 mV dec⁻¹) and AC (108.2 mV dec⁻¹). The Tafel slope value of MAC is also comparable to that of the previously reported highly active OER catalysts, such as N-doped graphene–CoO (71 mV dec⁻¹)³⁸ and Ni_xCo_3−xO₄ nanowire array (59–64 mV dec⁻¹),³⁹ suggesting its favorable reaction kinetics. The primary reaction can be proposed according to the interaction of the catalyst and OH⁻ groups: M + OH⁻ → MOH* + e⁻; MOH* → MOH.^40,41 The later step is the elemental step that controls the overall rate of the OER, which stems from surface chemical rearrangements (spillover effects). After that, the OER proceeds via the following steps: MOH + OH⁻ → H₂O + MO + e⁻; MO + OH⁻ → MOOH + e⁻; MOOH + OH⁻ → MO₂ + e⁻; MO₂ → M + O₂. Here M represents the surface active sites, MOH and MO, MOOH and MO₂ represent the adsorbed hydroxyl and adsorbed reaction intermediate species, respectively. The high surface area of the catalysts can provide a large number of electrochemically active sites, increasing the OER activity.⁴² On the other hand, the abundant hierarchical pores behave like an interconnected highway for mass and charge transport among the electrolyte, active surface and conductive substrate, leading to better OER electrochemical activity.⁴³ Specifically, the macropores can accelerate mass transport and electrolyte infiltration, and the mesopores and micropores can facilitate the efficient transfer of reactants, decreasing the diffusion rate and enhancing the reaction efficiency.¹³ Last but by no means least, the oxygen-rich functional groups on the surface of carbon provide a catalytic center.¹¹ These groups can play dual-roles, i.e. rendering the material as having somewhat hydrophilic properties via interacting with water molecules and serving as the active center to expedite the water dissociation reaction. By facilitating the adsorption of OH⁻ or H₂O on the carbon atoms, the electron transfer can be accelerated and oxygen evolution is promoted ultimately. Meanwhile, the well-developed hierarchical pore structure promotes rapid mass transport, efficient infiltration of electrolytes and the effective transfer of reactants, leading to better OER activity.

The electrochemical double-layer capacitance (C_dl) is used to explore the electrochemically active surface area of all of the samples, and is measured in a small potential range of cyclic voltammograms (CVs).⁴⁴ The working electrodes are scanned for several potential cycles until the signals are stabilized. As shown in Fig. 4a, the CVs of MAC are measured in the potential range of 0.93–0.99 V vs. RHE, at scan rates of 2, 4, 6, 8 and 10 mV s⁻¹. It is obvious that all of the CV curves exhibit the typical rectangular shape and have no apparent electrochemical features corresponding to a faradic current. The other samples display the same tendency as MAC (Fig. S7–10†). Fig. 4b is the plot of current against potential scan rate and the slope is the double-layer capacitance, according to the equation: i_c = vC_dl, where i_c is the current density (mA cm⁻² mg⁻¹, recorded at 0.967 V vs. RHE) measured from CVs at multiple scan rates, v is the scan rate (mV s⁻¹) and C_dl is the electrochemical double-layer capacitance (F cm⁻² mg⁻¹). Thus, a plot of i_c as a function of v yields a straight line with a slope equal to C_dl, which is proportional to the electrochemically active surface area of the catalysts.^37,45,46 Compared with other counterparts, the MAC exhibits the highest C_dl value (28.14 mF cm⁻² mg⁻¹), which is about 11.87, 3.77, 3.40 and 1.80 times that of the FLC (2.37 mF cm⁻² mg⁻¹), CAC (7.47 mF cm⁻² mg⁻¹), AC (8.28 mF cm⁻² mg⁻¹) and PAC (15.64 mF cm⁻² mg⁻¹). The results demonstrate that a large active surface area facilitates the utilization of the active sites and certainly contributes to a boost in the catalytic performance.

Fig. 4 (a) CV curves of MAC recorded at 0.93–0.99 V vs. RHE at various scan rates (2–10 mV s⁻¹) and (b) the plots of current density at 0.967 V vs. RHE against scan rate.

Fig. 5a displays the electrochemical impedance spectra (EIS) of all of the catalysts tested at 1.6 V vs. RHE in 1 mol L⁻¹ KOH, expressed as a Nyquist plot for the carbon electrode. All of the samples show a semicircle-shaped EIS profile representing the contact and charge transfer impedance. MAC possesses the highest electrical conductivity and the electrical conductivity decreases in the order of MAC (3.64 Ω), PAC (9.27 Ω), CAC (12.04 Ω), FLC (32.80 Ω) and AC (49.18 Ω). AC exhibits the maximum resistance due to the micropores that impede the ion diffusion from the electrolyte into the electrode materials.⁴⁷ The high electrochemical surface area is in favor of the exposure of electrochemical active sites and leads to a higher catalytic activity than that of CAC and FLC. The contact and charge resistance for MAC and PAC are lower than that of AC, demonstrating that the hierarchical pores can lower the resistance of the carbon materials and increase the electrocatalytic activity, ultimately. The E_OER, overpotential, Tafel slope and C_dl for other materials are summarized in Table 1 for comparison. It is clear that MAC exhibits the optimal Tafel slope and C_dl, and the lowest E_OER (with an overpotential of 564 mV), revealing an excellent OER performance. Although PAC represents the hierarchical pore structures, the overpotential reaches 576 mV. The acid-wash process can decrease the ash content of the activated carbon and affect the evolution of CO and CO₂.⁴⁸ We propose that a certain amount of ash and various trace elements exert an influence on the conductivity, leading to the improved OER performance.

Fig. 5 (a) EIS of the samples recorded at 1.6 V vs. RHE and (b) LSV curves of MAC before and after 1000 cycles.

Table 1 Surface area, pore size distribution, OER performance derived from LSV measured at 5 mV s⁻¹ and C_dl values of the activated carbons

Samples	SBET (m g^−1)	D (nm)	EOER^a (V)	Over-potential^b (η)	Tafel slope (mV dec^−1)	Cdl (mF cm^−2)
a EOER is calculated at 10 mA cm^−2 mg^−1 (vs. RHE).b Overpotential is calculated using the formula η = EOER − 1.23 (vs. RHE).
FLC	—	—	1.680	0.450	123.2	2.37
MAC	1078	3.14	1.596	0.366	60.0	28.14
CAC	1003	2.60	1.670	0.440	122.5	7.47
PAC	824	2.97	1.608	0.378	89.0	15.64
AC	1912	2.50	1.651	0.421	108.2	8.28

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In addition, the high durability of catalysts towards the OER is significant for an energy conversion system, which can be evaluated using the cycle performance from polarization curves. Fig. 5b shows the polarization curves before and after 1000 cycles at a scan rate of 100 mV s⁻¹. The MAC exhibits almost the same polarization curves with a faint reduction of anodic current density. We believe that the high durability of MAC is ascribed to its hierarchical porous structure, allowing efficient charge and mass transport during the catalytic process. The remarkable features of high activity, favorable kinetics and strong durability support the fact that MAC is a promising candidate catalyst of the OER for water splitting.

Experimental section

Sample preparation

Typically, fallen leaves (Fraxinus chinensis) were thoroughly washed with distilled water and any intrinsic moisture was removed by placing in an oven at 60 °C overnight. After cutting into debris, the leaves were carbonized in a tube furnace from room temperature to 500 °C for 2 h, with a heating rate of 10 °C min⁻¹, to obtain fallen leaf coke (FLC). After that, the FLC (1 g) was activated by KOH (1 g) and K₂CO₃ (2 g) under N₂ from room temperature to 700 °C for 1 h (10 °C min⁻¹). After being cooled to room temperature, the as-prepared material was neutralized with 1 mol L⁻¹ of HCl and distilled water. Finally, the obtained material was dried in an oven overnight. The final products that were activated by the mixed activators were named as MAC. CAC was the name of the carbon product when K₂CO₃ was used as the only activator. In order to know the effect of the ash composition, the FLC was pretreated in 1 mol L⁻¹ of HCl before being activated and the product was named as PAC. The commercial activated carbon purchased from Shanghai Heda Co., Ltd was named AC for comparison. All of the chemicals were used directly without any treatment.

Physicochemical characterization

Powder X-ray diffraction (XRD) was performed by employing a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 0.154 nm). Scans were run from 3 to 80° with a step width of 0.02°. Raman spectra were collected by using a Renishaw InVia confocal Raman spectrometer with a 514 nm wavelength laser. The morphologies of the obtained samples were observed using a scanning electron microscope (SEM, JSM-6490LV), with the sample being dispersed on a conductive resin. The transmission electron microscope (TEM, JEM 1010) was used to record the microstructure of the sample by dispersing the sample on the copper grid covered with a holey carbon film. The surface area and the porous texture of the as-obtained product were characterized by nitrogen adsorption with a Quantachrome Autosorb-1MP sorption Analyzer at liquid-N₂ temperature (−196 °C). All samples were outgassed at 200 °C for 12 h before the adsorption experiments were carried out. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method. The density function theory (DFT) model was utilized to determine the pore size distribution (PSD) of the samples. Thermogravimetric analysis (TGA) was characterized on a Rigaku thermogravimetry (TG) analyser with a heating rate of 10 °C min⁻¹ from room temperature to 800 °C. X-ray fluorescence (XRF) was carried out by using a CIT-3000SL X-ray fluorescence instrument composed of an X-ray tube working at 25 kV and 0.1 mA. The functional groups were observed using a Fourier transform infrared spectrophotometer (FTIR, Nicolet Fourier spectrophotometer) using the KBr pellet technique.

Preparation of electrodes and their electrochemical characterization

Electrochemical measurements were performed using a conventional three-electrode electrochemical cell using an IM6 & ZENNIUM electrochemical workstation, in which Pt wire and a saturated Ag/AgCl KCl electrode were used as the counter electrode and reference electrode, respectively. The working electrode with surface area of 1 cm² was prepared as follows: 4 mg of the synthesized catalyst and 1 mg of acetylene black were ultrasonically dispersed in a mixture of ethanol (550 μL) and Nafion (5%, 50 μL). Then the mixture was dispersed into the Ni foam electrode via a controlled drop casting approach with a mass loading of about 1 mg. After that the electrode was dried in air until the solvent had evaporated. 1.0 mol L⁻¹ of KOH solution was used as the electrolyte. Before the oxygen evolution reaction, a flow of N₂ was bubbled into the electrolyte for 20 min continuously to remove the air atmosphere.

Liner sweep voltammetry (LSV) was performed at a scan rate of 5 mV s⁻¹ and all the potentials were corrected for iR drop. Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying an alternating current oscillation of 5 mV in a frequency range of 0.01 to 100 000 Hz. A stability test for the OER was performed on the catalysts by cycling the electrode potential between 1.0 and 2.0 V at 100 mV s⁻¹ for 1000 cycles, after which LSV for the OER was measured. The electrical double layer capacitance (C_dl) was measured within a small potential range of 0.93–0.96 V from double-layer charging curves using CVs. In order to normalize the data scientifically, the current density was unified to mA cm⁻² mg⁻¹ and all of the potentials measured in this study were converted to the reversible hydrogen electrode (RHE) according to the equation: E_(RHE) = E_(Ag/AgCl) + 0.059 × pH + 0.1976. Moreover, all measurements were carried out at room temperature.

Conclusions

The hierarchical porous carbon materials serve as electrocatalysts for the OER. Owing to its hierarchical porous structure and highly exposed surface active sites, the obtained catalyst exhibits excellent activity, favorable kinetics and a strong stability for OER performance. The impurities from the carbon source promote the OER, as shown by a comparison of the activity of the sample after removing the ash composition. The material reported here is expected to pave the way to various new types of electrode materials based on the costless activated carbon derived from biomass. Further effort is currently underway towards enhancing their electrocatalytic activity via modifying the surface functional groups and porous structure.

Acknowledgements

This work was supported by the Scientific Research Foundation for the returned overseas Chinese Scholars, State Education Ministry and Hebei Provincial Key Lab of Green Chemical Technology & High Efficient Energy Saving, School of Chemical Engineering Technology, Hebei University of Technology and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19333a

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