Green synthesis of Si–GQD nanocomposites as cost-effective catalysts for oxygen reduction reaction

Abstract

Hybrid silicon nanosheets (NSs)–graphene quantum dot nanocomposites (Si–GQD NCs) were prepared from a mixture of GQDs and Si NSs in ethanol at 25 °C for 2 h and used as a catalyst for oxygen reduction reactions (ORR) in direct methanol fuel cells (DMFCs). GQDs were prepared from fenugreek seed extracts (300 °C, 8 h) and wrinkled Si NSs were obtained from pyrolysis of rice husks (700 °C, 2 h). The Si–GQD NCs fabricated glassy carbon electrode (GCE) has greater electrocatalytic activity for ORR in comparison to Si NSs and GQDs modified GCEs, showing the synergistic effect provided by Si NSs and GQDs. The GQDs enhance O₂ adsorption and ORR activity, while Si NSs function as a support to increase charge transfer. Additionally, high surface area and the wrinkled structure of Si NSs allow efficient mass transfer, leading to greater ORR activity. The onset potential of the Si–GQD NC electrode is −0.33 V (versus Ag/AgCl) with a current density of 2.61 ± 0.27 mA cm⁻², showing greater electrocatalytic activity. Furthermore, the Si–GQD NC electrode exhibits greater tolerance against methanol and carbon monoxide poisoning than the Pt/C electrode. The environmentally-friendly, active, stable and inexpensive Si–GQD NCs hold great potential for DMFCs.

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Introduction

The role played by catalysts for the cathodic oxygen reduction reaction (ORR) is vital in determining the performance of power output and the open circuit potential of fuel cells.^1,2 Platinum (Pt) and its alloys are well known as active and efficient catalysts for ORR, but their high cost and limited lifetime hinder their applicability to broad commercialization.³ Therefore, development of non-precious catalysts with high ORR activity and durability at much lower cost is desirable.^4,5 Numerous less-expensive catalysts such as inorganic/nanocarbon hybrids⁶ and nanocarbon materials (carbon nanotubes, graphene, carbon nanofibers, carbon quantum dots (CQDs)) doped with heteroatoms (N, B, S and P)^4,7–11 have been developed for ORR, showing advantages of high catalytic activity, cost-effectiveness and great surface area. Nonetheless, development of inexpensive, efficient and scalable electrocatalysts remains elusive.

Graphene quantum dots (GQDs) have risen as an outstanding material due to their unique physico-chemical properties of quantum confinement, edge effects, excellent electron transport and optical properties.^12–14 GQDs containing different functional groups on their surfaces can be functionalized easily with other chemicals/nanomaterials for various applications such as fuel cells and bio-imaging.^15,16 GQDs and metal oxides can be used to prepare nanocomposites (NCs) such as TiO₂/CQDs, SiO₂/CQDs, Cu₂O/GQDs and Fe₂O₃/CQDs, which provide high photocatalytic activities for many chemical reactions (e.g. oxidation of methanol and degradation of benzene).¹⁷ Additionally, GQDs supported graphene nanoribbons¹⁸ and GQDs/multiwalled carbon nanotubes¹⁹ have been used as inexpensive metal-free catalysts for ORR. However toxic organic precursors (e.g. methylbenzene and hexabromobenzene) are used in the preparation of graphene nanoribbons, while additional surface treatment (500 °C for 10 h) for the removal of amorphous carbon from multiwalled carbon nanotubes increases the preparation time and cost.

Previously, we showed that silicon–carbon dots/silicon dioxide (SiC-dots/SiO₂) NCs prepared from 3-aminopropyl trimethoxysilane (APTS) have high electrocatalytic activity for ORR.²⁰ The result shows that carbon based NCs may be used instead of metal based catalysts in fuel cells. In this study, we prepared Si–GQD NCs from GQDs and Si nanosheets (NSs) under magnetic stirring. The GQDs were synthesized from raw plant seeds of Fenugreek (Trigonella foenum-graecum) via hydrothermal treatment at 300 °C for 8 h.^21–25 The Si NSs were prepared through pyrolysis of rice husks at 700 °C for 2 h. The environmentally-friendly Si–GQD NCs were used as cathode catalysts for ORR, showing electrocatalytic activity for ORR as a result of excellent electron transport properties and the synergistic effect provided by Si NSs and GQDs.

Experimental

Chemicals

Fenugreek seeds and rice husks were bought from local markets in India and Taiwan, respectively. Sulfuric acid (99.9%), potassium nitrate (≥99.0%) and hydrochloric acid (37%) were bought from Sigma Aldrich (Milwaukee, WI, USA). Ethanol (99%) was purchased from Shimakyu's Pure Chemicals (Osaka, Japan). Nafion 117 (5 wt%) and KOH (≥99.8 wt%) were purchased from Fluka (Buchs, Switzerland). Pt/C (40 wt%) was purchased from Alfa Aesar (Ward Hill, MA, USA). Oxygen (≥99.5 wt%) and nitrogen (≥99.9 wt%) gases were purchased from Ciao Chong gaseous corporation (Taipei, Taiwan). Monosodium phosphate monohydrate (99%) and disodium phosphate heptahydrate (99%) were purchased from J. T. Baker (Phillipsburg, NJ, USA) and Janssen Chimica (Geel, Belgium), respectively. Ultrapure water was obtained using a Milli-Q ultrapure (18.2 MΩ cm) system. All other chemicals and reagents used in this study were of reagent grade.

Synthesis of Si NSs

The raw rice husks were boiled in HCl aqueous solution (10 wt%) for 2 h, which was then rinsed with deionized water and dried at 100 °C for 24 h. The dried rice husks were then pyrolyzed for 2 h in a muffle furnace that was preheated at 700 °C to prepare Si NSs. A control sample was prepared under the same pyrolysis condition but using water rinsed rice husks instead of HCl treated ones. The as-prepared Si NSs were then ultrasonicated in KNO₃ aqueous solution (0.20 M), followed by stirring for 1 h. The samples were filtered and finally dried at 100 °C for 4 h.

Synthesis of GQDs and Si–GQD NCs

GQDs were synthesized using our previously reported approach.²¹ Typically, hydrothermal treatment of Fenugreek seed extract (250 mg, 20 mL ultrapure water) was carried out in an autoclave at 300 °C for 8 h. The obtained solution was centrifuged at a relative centrifugation force (RCF) of 25 000g for 20 min to remove the larger particles. The GQDs solution (10 mL) was then dialyzed in ultrapure water (2 L) through a dialysis membrane (MWCO = 3.5–5 kD, Float-A-Lyzer G2, Spectrum Laboratories, Rancho Dominguez, CA, USA) for 24 h. The yield of the purified GQDs was about 48.4%; ∼121 mg of GQDs were produced from 250 mg of powdered fenugreek seeds.²² Aliquots (10 mL) of the as-prepared solution containing purified GQDs were dried overnight at 60 °C in an oven to obtain pure GQDs. Si–GQD_0.5 NC, Si–GQD_1.0 and Si–GQD_2.0 NCs were prepared by mixing 100, 75, and 50 mg of Si NSs with 50, 75, and 100 mg of GQDs, respectively, in ethanol (10 mL) for 2 h under stirring at ambient temperature (25 °C). The mixed solutions were then kept for drying at 100 °C overnight to obtain the Si–GQD NC powders.

Characterization of Si NSs, GQDs and Si–GQD_1.0 NCs

JEOL JSM-1230 and FEI Tecnai-G2-F20 transmission electron microscopes (TEM) were used to measure the sizes and shapes of the as-prepared Si NSs, GQDs and Si–GQD_1.0 NCs. The re-dispersed Si NSs, GQDs and Si–GQD_1.0 NCs were separately placed on formvar/carbon film Cu grids (200 mesh; Agar Scientific) and dried at ambient temperature. X-ray diffraction (XRD) patterns of the samples were measured using a PANalytical X'Pert PRO diffractometer (PANalytical B.V., EA Almelo, Netherlands) and Cu-Ka radiation (λ = 0.15418 nm). An energy dispersive X-ray (EDAX) system (Inca Energy 200, Oxford) was used to determine the composition of the as-prepared Si NSs, GQDs and Si–GQD_1.0 NCs. X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5000 VersaProbe (Physical Electronics, Eden Prairie, MN, USA). UV-vis absorption spectra of Si NSs, GQD and Si–GQD_1.0 NCs were recorded using a Cintra 10e double-beam UV-vis spectrophotometer (GBC Scientific Equipment, Victoria, Australia).

Electrode fabrication and electrocatalytic analysis

A rotating disk electrode (RDE) with a geometrical area of 0.196 cm⁻² was polished on a clean Buehler polishing cloth using 0.05 μm alumina slurry. The as-prepared Si NSs, GQDs and Si–GQD_1.0 NCs (optimal mass loading: 1.04 mg cm⁻²) were individually placed onto the glassy carbon surface of each RDE (diameter: 5 mm). Subsequently, the electrodes were air-dried for 1 h at ambient temperature followed by placing Nafion solution (0.5%, 1 μL) onto each of the electrodes. Control electrodes were prepared in a similar manner using a commercial Pt/C solution with the same mass loading (1.04 mg cm⁻²). Each of the three-electrode electrochemical cells was built by using one of the modified RDEs as the working electrode, a Pt wire as the auxiliary electrode, and a saturated Ag/AgCl as the reference electrode. A CHI 760D electrochemical workstation (Austin, TX, USA) was used to measure the electrocatalytic activities of Si NSs, GQDs and Si–GQD_1.0 NCs. Prior to each electrochemical measurement, pure O₂ gas was purged into 0.1 M KOH for 45 min. The cyclic voltammetry (CV) measurements of the Si–GQD_1.0 NCs RDEs in O₂ and N₂-saturated 0.1 M KOH were carried out over a potential range from 0 to −0.6 V at a scan rate of 20 mV s⁻¹. Linear sweep voltammetry (LSV) measurements of the as-prepared RDEs in O₂-saturated 0.1 M KOH were recorded over a potential range of 0 to −0.6 V at a scan rate of 5 mV s⁻¹ with different rotation speeds. Before the Tafel plots were plotted, kinetic currents in the mixed activation–diffusion region in LSV curves were calculated from eqn (1):in which J, J_K and J_L are the measured current density, kinetic current density and limiting current density, respectively, while [J_L/(J_L − J)] is the mass transport correction factor.²⁶ All CV and LSV polarization curves were recorded after achieving 8 reproducible cycles. Prior to each measurement, O₂ or N₂ gas was purged into 0.1 M KOH solutions for 45 min.

Methylene blue (MB) adsorption method for specific surface area (SSA) measurement

Specific surface area (SSA) of the prepared Si–GQD_1.0 NCs was determined using the methylene blue (MB) adsorption method that is a viable alternative to Brunauer–Emmett–Teller (BET) measurements.²⁷ A known mass of Si–GQD_1.0 NCs was added into a known volume of standard MB solution (10 μM). The mixed suspension was sonicated for 2–3 h and stirred continuously for 24 h to reach the adsorption–desorption equilibrium. The mixture (5 mL) was collected and then centrifuged to remove the pellet. The MB concentration in the supernatant was determined by the absorbance at a wavelength of 665 nm. The SSA of the Si–GQD_1.0 NCs was then calculated using the following equation:where N_A represents Avogadro number (6.02 × 10²³ mol⁻¹), A_MB is the covered area per MB molecule (1.35 nm²), C₀ and C_e are the initial and equilibrium concentrations of MB, respectively, V is the volume of MB solution, M_MB is the relative molecular mass (373.9 g mol⁻¹) of MB, and m_s is the mass of the sample.

Result and discussion

Characterization of Si NSs, GQD and Si–GQD_1.0 NCs

Fig. 1A displays the TEM image of Si–GQD_1.0 NCs, showing the distribution of GQDs on the surfaces of Si NSs. The Si NSs possess a wrinkled and folded sheet-like morphology similar to that of graphene sheets. The TEM and HRTEM images of the GQDs are displayed in Fig. S1,† showing GQDs with an average size of 5.6 (±0.3) nm (50 counts). The TEM image (Fig. S2A†) and XPS spectrum (Fig. S2B†) show wrinkles on the Si NSs, and a sharp peak of Si2p at 102.7 eV, respectively. The XPS spectrum of the Si–GQD_1.0 NCs in Fig. 1B exhibits a typical peak for Si2p at 102.6 eV. The presence of carbonyl and carboxylic groups are apparent from the high-resolution C1s spectrum, showing C=C (284.4 eV), C–O (285.8 eV), C–N (286 eV) and C=O (288 eV) peaks. Fig. S3† shows the UV-vis absorption spectra of Si NSs, GQD and Si–GQD_1.0 NCs. The absorption peak at 305 nm is for Si NCs, while that around 305 nm is for the GQDs that is assigned to the π–π* transition of the aromatic sp² domains. The high-resolution N1s spectrum of the GQDs reported in our previous studies showed that they have characteristic peaks for both pyridine-like (398.5 eV) and pyrrolic (401 eV) N atoms.²¹ Raman spectrum of GQDs also shows the D and G bands at 1316 cm⁻¹ (disordered structures of carbon) and 1580 cm⁻¹ (graphitic structures), respectively.

Fig. 1 (A) TEM image (inset: high magnification), (B) XPS, (C) XRD and (D) EDX spectra of Si–GQD_1.0 NCs.

The presence of pyridinic and pyrrolic nitrogen in the GQDs can facilitate the electron transfer from the electronic bands of carbon to the antibonding orbital of oxygen, suggesting the potential of Si–GQD_1.0 NCs as cathode catalysts for ORR.²⁰ The XRD spectrum of Si–GQD_1.0 in Fig. 1C shows a broad (002) peak at 22.5 degree corresponding to the graphitic carbon (JCPDS no. 75-1621).²⁰ The characteristic Si (111) peak at around 22.7 degree overlapped with that of carbon, which is broader than that of Si NSs as shown in Fig. S2C.† Additionally, minor Si peaks can be observed at 42.2 and 60.1 degrees, which correspond to Si (220) and Si (311), respectively, which are in good agreement with reference patterns.²⁸ The elemental composition of the raw rice husk before pyrolysis is provided in Table S1.† The EDX spectra of Si–GQD NCs and Si NSs displayed in Fig. 1D and S2D† separately confirm the characterization by displaying the peaks for Si, C, N and O elements in Si–GQD_1.0 NCs and the presence of Si in Si NSs respectively. These results also reveal that most of the metal elements in the rice husk were removed by the sample pretreatment (boiled in hot HCl aqueous solution and washed with water).

Electrocatalytic activities of Si–GQD_1.0 NCs-modified electrodes

We evaluated the ORR activity of Si–GQD_1.0 NCs, Si NSs and GQD electrodes in O₂ saturated 0.1 M KOH solution. As displayed in Fig. 2A, the Si–GQDs electrode exhibited an enhanced cathodic peak at −0.33 V corresponding to ORR, which is 140 and 110 mV more positive than that for the Si NSs and GQD electrodes, respectively. The current density of the Si–GQD_1.0 NCs electrode at −0.33 V is 1.8- and 1.2-folds higher than that of the Si NSs and GQD electrodes at −0.47 and −0.44 V, respectively (Fig. 2B). The onset potential of Si–GQD_1.0 electrode is ca. −0.18 V, which is more positive than that (−0.33 and −0.25 V) of the Si and GQD electrodes, respectively. As displayed in Fig. 2C, Si–GQD_1.0 NCs electrode exhibited enhanced peak at −0.33 V in O₂ than in N₂, showing its high oxygen reducing ability that is attributed to the synergistic effect, pyridinic and pyrrolic nitrogen atoms doped in GQDs, improved kinetics and enhanced mass transport. The enhanced catalytic activity of Si–GQD_1.0 NCs electrode for ORR over that of individual GQDs or Si NSs is attributed to the significant synergistic effect provided by GQDs and Si NSs. The high-resolution O1s spectra of Si–GQD_1.0 NCs and GQDs reveal the co-existence of both lattice oxygen (LO) (≤531 eV) and adsorbed oxygen (AO) (≥532 eV), with a greater ratio of LO in the NCs (Fig. S4†). LO in comparison to AO withdraws more electrons from carbon atoms of GQDs, leading to greater charged sites in GQDs for O₂ adsorption and enhancement of the ORR activity.¹⁹ On the other hand, the Si NSs in Si–GQD_1.0 NCs functions as a support to increase the charge transfer of the electrodes. Having a structural framework, the Si NSs allows highly dispersed GQDs on their surfaces.

Fig. 2 Comparison of electrochemical performance of Si NSs, GQDs, and Si–GQD_1.0 NCs-modified electrodes. (A) CV and (B) current density. (C) CV curves of Si–GQD_1.0 NCs-modified electrodes under O₂ and N₂ saturated solutions. Conditions: 0.1 M KOH at a scan rate of 20 mV s⁻¹. Data shown in (B) were obtained from triplicate measurements.

In addition to the synergistic effect, a greater surface area (high specific surface area of 144.7 ± 3.9 m² g⁻¹) due to the 3-D framework structure of the Si–GQD_1.0 NCs is also responsible for their greater ORR activity. Such a high surface area provided greater diffusion of O₂ through the surface and an enhanced mass transfer. We further investigated the effect of electrode compositions on the ORR activity by varying the molar ratio of Si–GQD, with a constant total amount (1.04 mg cm⁻²). Fig. 3A displayed that the electrode fabricated from the NCs prepared from Si NSs and GQDs at a mass ratio of 1.0 (Si–GQD_1.0 NC) had the highest limiting current density of 2.61 ± 0.27 mA cm⁻², which is higher than that prepared from the mass ratios of Si–GQD_0.5 NC (2.14 ± 0.11 mA cm⁻²) and Si–GQD_2.0 NC (1.96 ± 0.18 mA cm⁻²). At a lower Si/GQD mass ratio, most of the surfaces of Si NSs were occupied by GQDs, leading to a decreased mass transfer. On the other hand, at a higher mass ratio, insufficient GQDs were present on the surfaces of Si NSs, leading to lower ORR activity. Thus, Si–GQD_1.0 NCs prepared from equal amount of Si NSs and GQDs were used for further study. The optimal mass loading was then determined by loading different amounts (0.26, 0.52, 0.78, 1.04 and 1.30 mg cm⁻²) of Si–GQD_1.0 NCs on RDEs. Their corresponding current densities were determined to be 1.29 ± 0.11, 1.78 ± 0.08, 2.02 ± 0.13, 2.61 ± 0.27 and 2.39 ± 0.16 mA cm⁻², respectively (Fig. 3B). A t-test was performed (95% confidence) and we found the ORR activities of Si NS, GQD, Si–GQD_1.0 to be significantly different. The current density steadily increased upon increasing the loading of Si–GQD_1.0 NCs up to 1.04 mg cm⁻². The current density decreased slightly when the mass loading is greater than 1.04 mg cm⁻², mainly due to slow electron transfer from the bulk solution to the surface of the glassy carbon. Fig. 4A shows the ORR activities of Si–GQD_1.0 NCs electrode under O₂ and N₂ saturated conditions, providing limiting current densities of 2.61 ± 0.27 and 0.28 ± 0.04 mA cm⁻², respectively. The CV onset potential of Si–GQD_1.0 NCs for ORR was found to be −0.18 V vs. Ag/AgCl, which is slightly lower than that of other reported modified electrodes, supporting their greater activity for ORR.¹⁸ The limiting current density of the Si–GQD_1.0 NCs modified electrode at −0.44 V (vs. Ag/AgCl) increased from 2.21 ± 0.13 to 3.43 ± 0.18 mA cm⁻² with increasing the rotational speed from 400 to 3600 rpm (Fig. 4B). The result reveals that the overpotential of the four electron reaction of oxygen reduction is related to the proton and electron transfer.^29,30

Fig. 3 Effect of the Si–GQD (A) mass ratio and (B) mass loading on the limiting current densities of Si–GQD NCs electrodes. (A) Total mass loading: 1.04 mg cm⁻² and (B) Si–GQD mass ratio of 1.0.

Fig. 4 (A) LSVs of the Si–GQD_1.0 NC electrode in N₂ and O₂-saturated 0.1 M KOH at a constant rotational rate of 1600 rpm with a scan rate of 5 mV s⁻¹. (B) LSVs recorded from a Si–GQD_1.0 NC electrode in 0.1 M KOH at different rotation rates. (C) Koutecky–Levich plots obtained from the data shown in (B) at −0.40, −0.45, −0.50 and −0.55 V. (D) Electron transfer number (n) calculated from the data shown in (C) at different potentials.

To investigate the kinetics of the ORR occurring at the Si–GQD_1.0 NCs electrode, the Koutecky–Levich equation was used:

where J is the measured current density, J_K and J_L are the kinetic and diffusion limiting current densities, respectively. From the LSV curves (Fig. 4B), J_L values at different potentials were obtained. The LSV curves with eqn (1) allow calculation of J_K values at different potentials. An important parameter that influences the efficiency of the electrocatalyst is the number of electrons transferred in the ORR.³¹ According to eqn (4), n values at different potentials can be obtained from the plot of J⁻¹ versus ω^−1/2.

J_L = 0.62nFC₀D₀^2/3ν^−1/6ω^1/2 (4)

where n is the overall number of electrons transferred, F is the Faraday constant, C₀ is the saturated oxygen concentration in 0.1 M KOH (∼1.14 × 10⁻⁶ mol cm⁻³), D₀ is the diffusion coefficient of O₂ (∼1.73 × 10⁻⁵ cm² s⁻¹), ν is the kinematic viscosity of the solution (0.01 cm² s⁻¹), and ω is the rotation rate (rad s⁻¹).³¹ The linearity and parallel plots shown in Fig. 4C indicate the first-order reaction kinetics.³²

The results are further summarized in Fig. 4D, showing that the Si–GQD_1.0 NCs electrode provided n values of ca. 3.2, 3.7, 3.6 and 3.6 at −0.40, −0.45, −0.50 and −0.55 V, respectively, which are closer to 4 for the theoretical electron number for ORR. The n values obtained are close to that obtained using a commercially available Pt/C electrode or a N-doped GQD electrode.^18,33 According to eqn (5), the electron transfer rate constant (k) of an electrode can be determined.

J_K = nFkC₀ (5)

At a fixed potential of −0.55 V vs. Ag/AgCl, a k value of 2.04 × 10⁻² cm s⁻¹ for the Si–GQD_1.0 NCs electrode was obtained, which is higher than that of the reported N-doped GQD electrode, revealing higher activity of the Si–GQD_1.0 NCs electrode for ORR.^18,34

To further show the advantage of the Si–GQD_1.0 NCs electrodes over others, the LSV curves of Si NSs, GQDs, Si–GQD_1.0 NCs and Pt/C modified electrodes in 0.1 M KOH were recorded at a scan rate of 5 mV s⁻¹. Fig. 5A shows that the current density of the Si–GQD_1.0 NCs electrode (2.61 ± 0.27 mA cm⁻²) is higher than that (1.03 ± 0.12 and 1.91 ± 0.37 mA cm⁻², respectively) of the Si NSs and GQD electrodes, but it is lower than that of the Pt electrode (3.41 ± 0.17 mA cm⁻²) as obtained from Fig. S5.† In addition to lower cost, the major advantage of Si–GQD_1.0 NCs modified electrode over the Pt/C electrode is its significantly higher CO tolerance; there is no response to CO in the Si–GQD_1.0 NC electrode (Fig. 5B). In contrast, Pt/C displayed a significant response upon injecting CO instantly. These results show that the Si–GQD_1.0 NCs hold great potential as a viable alternative to commercial Pt/C for use as cathode catalysts in direct-methanol fuel cells (DMFCs).

Fig. 5 (A) LSVs of Si NSs, GQD, Si–GQD_1.0 and Pt/C electrodes in O₂-saturated 0.1 M KOH at a constant rotational rate of 1600 rpm with a scan rate of 5 mV s⁻¹. (B) Chronoamperometric responses of Si–GQD_1.0 NC and Pt/C electrodes upon injecting CO at 300 s at −0.4 V vs. Ag/AgCl; rotation rate: 1600 rpm. (C) Mass transport corrected Tafel plots of various electrocatalysts. Scan rate: 5 mV s⁻¹. (D) Stability of Si–GQD_1.0 NC and Pt/C electrodes in 0.1 M KOH at a constant potential of −0.4 V vs. Ag/AgCl under a rotation rate of 1600 rpm.

We then studied the effects of methanol poisoning, which curtails the lifetime of Pt-based electrocatalysts and is a severe limitation in the development of DMFCs.³⁵ It must be pointed out that the movement of methanol from the anode to the cathode in DMFCs is inevitable. Thereby, it is imperative for an ORR electrocatalyst to be resistant to methanol poisoning. Fig. S6† displays that the current densities of the Si–GQD_1.0 NCs in the absence and presence of 1 M methanol are almost the same, revealing its excellent methanol tolerance. The Pt/C electrode on the other hand suffers from severe methanol poisoning, with evidence of a sharp peak appeared at −0.13 V as shown in Fig. S6.†^2,36,37 The Tafel slope values of Si NSs, GQDs, Si–GQD_1.0 NCs and Pt/C electrodes (Fig. 5C) were determined to be 165 ± 2, 109 ± 3, 96 ± 2, and 93 ± 6 mV per decade, respectively. The Si–GQD_1.0 NC electrode provided a Tafel slope value very close to that of Pt/C while being smaller than its counterparts of Si NSs and GQDs, displaying its improved kinetics and superior ORR performance. GQD supported graphene nanoribbons, metal free N-doped carbon materials and Co₃O₄@SiO₂ have slopes of 87.7, 113 and 151.3 mV per decade respectively.^38–40 Furthermore, we evaluated the durability of Si–GQD_1.0 NCs and Pt/C electrodes. Fig. 5D displays chronoamperometry of the Si–GQD_1.0 NCs electrode in 0.1 M KOH under rotation, with a decrease (11%) in current occurring in the first 1000 s. The decrease is largely due to the loss of surface active sites as a result of adsorption of intermediate species on the catalyst surface.³⁷ Importantly, the current remained almost a constant from 3000 to 30 000 s, revealing the superior durability of the Si–GQD_1.0 NCs electrode. The Pt/C electrodes on the other hand show a decrease in current by 25% after 15 000 s, revealing their poor durability.

We further investigated the accelerated ageing or accelerated durability tests (ADTs) of the Si–GQD_1.0 NCs electrodes by conducting LSV sweeps at a scan rate of 5 mV s⁻¹ with a rotational rate of 1600 rpm after they were subjected to 2000 voltammetric cycles in the potential range from 0 to −0.6 V at a scan rate of 50 mV s⁻¹ (Fig. S7†). The Si–GQD_1.0 NCs electrode exhibits an insignificant shift in the E_1/2 value after 2000 cycles of ADT, revealing its superior stability. The ADT results of the Si–GQD_1.0 NCs electrode are comparable with reported Si-NC electrodes.^41,42 The reported Pt/C electrodes, on the other hand, exhibited greater shifts (30 mV and 35 mV) in the E_1/2 value after 2000 cycles of ADT.^36,42 The onset potential, peak potential and the stability of Si–GQD_1.0 NCs electrode are comparable to other carbon nanomaterials modified electrodes (see Table S2†). Having high tolerance to methanol and CO poisoning, it is thus our strong belief that the low-cost and durable Si–GQD_1.0 NC electrode hold great promise for use as cathode catalysts in DMFCs.

Conclusion

A simple wet chemical approach was developed for the preparation of low-cost Si–GQD NCs electrodes that provides high catalytic activity for ORR in alkaline media. The high electrocatalytic activity of Si–GQD_1.0 NC electrodes results mainly from the high activity of pyridinic and pyrrolic nitrogen containing GQDs and efficient mass transport and electron transfer provided by Si NSs. The graphene-like wrinkled structure on the Si NSs further increase the loading of GQDs, diffusion and mass transfer of O₂ to the electrode surface. When compared to expensive Pt/C electrodes, the Si–GQD_1.0 NCs electrodes provide a comparable catalytic activity for ORR, with higher durability and tolerance to CO and methanol poisoning. The highly active, low-cost and stable Si–GQD_1.0 NC electrodes are suitable as alternative cathode electrodes in DMFCs.

Acknowledgements

This study was supported by the Ministry of Science and Technology (MOST) of Taiwan under contracts NSC 101-2627-M-002-007 and 103-2923-M-002--MY3. P. R. and A. P. P. are grateful to the NSC and National Taiwan University for a postdoctoral fellowship under the contract number 103-2811-M-002--154 and 103-2811-M-002--153, respectively. We thank Ms Ching-Yen Lin and Ms Ya-Yun Yang for their assistance for TEM measurement in the Instrumentation Center at National Taiwan University.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental characterization of GQDs and Si NSs (TEM, XPS, XRD and EDX) and CV curves of Pt/C electrode in N₂- and O₂-saturated environment. Additionally, details regarding methanol poisoning, the durability of the nanocomposites and a comparative table has also been provided. See DOI: 10.1039/c6ra23892k

‡ These authors contributed equally.

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