Marta Nunesa,
Inês M. Rocha
b,
Diana M. Fernandes
a,
Ana S. Mestre
ac,
Cosme N. Mourad,
Ana P. Carvalhoc,
Manuel F. R. Pereirab and
Cristina Freire*a
aREQUIMTE/LAQV, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal. E-mail: acfreire@fc.up.pt; Fax: +351 22 0402 695; Tel: +351 22 04020590
bLaboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE-LCM, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, 4200-465 Porto, Portugal
cCentro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal
dCIQ, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal
First published on 26th November 2015
The development of carbon-based metal-free electrocatalysts for the oxygen reduction reaction (ORR) is one of the most attractive topics in fuel cell field. Herein, we report the application of two sustainable sucrose-based activated carbons (ACs), denominated SC800 and SH800, as ORR electrocatalysts. In alkaline medium the ACs showed similar onset potentials at Eonset ≈ −0.20 V vs. Ag/AgCl (0.76 V vs. ERHE), which are 0.06 V more negative than that observed for 20 wt% Pt/C used as a reference. Higher diffusion-limiting current densities (jL(−1.0 V, 1600 rpm) = −3.44 mA cm−2) were obtained for the SH800 electrocatalyst, in contrast to SC800 (jL(−1.0 V, 1600 rpm) = −3.04 mA cm−2). These differences can be related with their different textural properties. The SH800 electrocatalyst revealed a higher specific surface area (ABET ≈ 2500 m2 g−1), larger micropores (widths between 0.7 and 2 nm) and sponge-like morphology. Conversely, SC800 showed a spherical shape, ABET ≈ 1400 m2 g−1 and narrow micropores with pore width <0.7 nm. Both ACs were neither selective to 2- or 4-electron ORR processes, opposing Pt/C which showed selectivity towards direct O2 reduction to water. SH800 and SC800 showed very similar Tafel plots, but with SH800 showing in both low and high current density regions, the lowest slopes values 53/171 mV dec−1 vs. 68/217 mV dec−1. Furthermore, the ACs presented excellent tolerance to methanol, with the SH800 electrocatalyst also showing greater long-term electrochemical stability than the Pt/C electrocatalyst which are very important advantages. The ACs-based electrocatalysts also showed ORR catalytic activity in acidic media, which makes them promising candidates for applications with acidic electrolytes (e.g. proton exchange fuel cells). In this case, Eonset = 0.06 V vs. Ag/AgCl (0.41 V vs. ERHE) for SC800 and Eonset = −0.01 V vs. Ag/AgCl (0.34 V vs. ERHE) for SH800, and the diffusion-limiting current densities are very similar for both ACs (jL = −2.59/−2.76 mA cm−2 at −1.3 V vs. Ag/AgCl, at 1600 rpm). SH800 and SC800 Tafel plots also showed two different slopes, but with higher values in both low and high current density regions, when compared with those obtained in an alkaline medium; still SH800 continues to show the lowest slopes.
Fuel cells are the most promising clean energy generation devices for which electrocatalysts play a key role.3 This technology appears as an alternative to counteract the depletion of fossil fuels and the growing threat of environmental pollution4 and, nowadays, is recognized as an excellent power source due to its high efficiency and negligible pollutant emission.5 Generally, the fuel cells devices are based on the electrocatalytic oxidation of a fuel (e.g. hydrogen and methanol) at the anode and the oxygen reduction reaction (ORR) at the cathode.6 The ORR plays a crucial role in controlling the overall performance of fuel cells due to its slow kinetics,4,7 requiring the use of an electrocatalyst in a high loading. Pt nanoparticles supported on carbon materials (Pt/C) are the most effective known ORR catalyst, leading to low ORR overpotential and large current densities, with selectivity toward a direct four-electron pathway.8 Nevertheless, important drawbacks associated with the high prices, scarcity, poor long-term durability and possible Pt-deactivation by methanol crossover, have limited the large-scale application of fuel cells with Pt-based electrocatalysts.9–11 To overcome these critical issues, research efforts have been made to developing alternative Pt-free ORR catalysts, with competitive ORR performance. In this context, carbon-based materials, with their versatile properties, appeared as ideal alternatives for ORR electrocatalysts and have been widely applied.
Several works have reported the application of graphite,12 carbon nanotubes,13–15 graphene,16,17 ordered mesoporous carbons9,18,19 and carbon nanoparticles8 as metal-free ORR catalysts. Activated carbons, an amorphous carbonaceous material, also have been applied with this purpose. These materials take advantage of their high surface areas and well developed pore structure,20 which are favourable conditions for a high ORR electrocatalytic activity, once increase the number of active sites exposed to the electrolyte.4,5,21 However, a great number of commercial carbons are prepared from coal, a non-renewable raw material.22 In this context, biomass emerged as a class of ideal starting materials for the green synthesis of carbon materials, once it is earth-abundant, readily available, cheap and environmental friendly.3,23 The hydrothermal carbonization of biomass, via dehydration reactions in aqueous medium under mild conditions,24 allows its conversion into valuable materials with fine tuning chemical structure and morphology.21 The use of hydrothermal carbonization followed by activation was recently evaluated,25 allowing to prepare superactivated carbons. The materials obtained are carbon- and oxygen-rich, due to the nature of biomass precursors,22 which can constitute an advantage, since the ORR electrocatalytic activity and H2O/H2O2 selectivity, could be influenced by the oxygen-containing groups.14 Nevertheless, usually, the ORR conducted on carbon catalysts without doping, functionalization or substitution exhibits the two-plateau peroxide pathway.1 Numerous studies have been published, reporting the application in ORR of carbon-based materials (undoped and doped) derived from several biomass sources, such as carbohydrates/polysaccharides (e.g. glucose24,26–28 and chitin29,30), proteins from silk,31,32 egg33 and blood,10 animal wastes,34,35 plants,3,5,21,36,37 fungus23 and marine algae.4,38 Some of these materials showed excellent ORR performance (assigned though Eonset and jL values), with catalytic activities competitive with the Pt/C electrocatalyst, but greater stability and tolerance to methanol poisoning is still a prior demand.4,21,38 Nevertheless, these results constitute a tremendous advantage and encourage further studies with this type of biomass-derived materials.
This work reports the application of two sustainable activated carbons (AC) prepared from a sucrose-derived hydrochar,25,39 denoted by SC800 and SH800, as ORR electrocatalysts. Their ORR electrocatalytic activities were explored in alkaline and acidic media using cyclic and linear sweep voltammetry, and their stabilities and tolerances to methanol poisoning effects in alkaline media were also evaluated by chronoamperometry and cyclic voltammetry, respectively. Complementary characterization, included ACs surface chemical composition by X-ray photoelectron spectroscopy and electron transfer properties of the AC-based modified electrodes in the presence of selected redox probes, [Fe(CN)6]3−/4−, [Ru(NH3)6]3+/2+ and Eu3+/2+. Dependency between the electron transfer properties and materials C/O ratios with multiple concurrent effects, have already been observed in studies involving graphite oxides,40 graphene-type materials41,42 and carbon quantum dots,43 evaluating the electrochemical response of materials in the presence of selected redox probes.
To the best of our knowledge, this is the first paper reporting the application of ACs metal-free ORR electrocatalysts derived from sucrose. Di Noto et al. already presented ORR electrocatalysts whose preparation included the use of sucrose;44–46 nonetheless, the resulting materials included metallic nanoparticles (Pt, Ni and Rh) deposited in carbon nitrides.
The X-ray photoelectron spectroscopy (XPS) measurements were performed at CEMUP (Porto, Portugal), in a Kratos AXIS Ultra HSA spectrometer using a monochromatic Al Kα radiation (1486.7 eV). The XPS spectra were deconvoluted with the XPSPEAK 4.1 software, using non-linear least squares fitting routine after a Shirley-type background subtraction. To correct possible deviations caused by electric charge of the samples, the C 1s band at 284.6 eV was taken as the internal standard. The surface atomic percentages were calculated from the corresponding peak areas, using the sensitivity factors provided by the manufacturer.
The electrochemical tests were accomplished in N2- or O2-saturated (purged for 30 min before the measurements) 0.1 mol dm−3 KOH or H2SO4/Na2SO4 buffer solution with pH = 2.5 (prepared by mixing appropriate amounts of a 0.2 mol dm−3 H2SO4 solution with a 0.5 mol dm−3 Na2SO4 solution). CV experiments were conducted at the scan rate of 0.005 V s−1 and the LSV at 0.005 V s−1 for different rotation speeds from 400 to 3000 rpm; chronoamperometry measurements were performed at E = −0.6 V vs. Ag/AgCl and 1600 rpm during 20000 s. The methanol-tolerance evaluation was performed by CV at 0.010 V s−1. The ORR current was obtained by subtracting the current measured in N2-saturated electrolytes from the current measured in O2-saturated electrolyte.
The onset potential (Eonset), defined as the potential at which the O2 reduction reaction starts, was calculated as described in literature.6 In order to facilitate the comparison with the literature, the Eonset values determined vs. Ag/AgCl were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation:
ERHE = EAg/AgCl + 0.059pH + EoAg/AgCl | (1) |
LSV data was analysed though Koutecky–Levich (K–L) eqn (2). The number of electrons transferred per O2 molecule (nO2) in the ORR process was calculated from the slopes of the K–L plot:21,38
![]() | (2) |
B = 0.2nO2F(DO2)2/3v−1/6CO2 | (3) |
To prepare the modified electrodes, the activated carbons (SC800 or SH800, 1 mg) were dispersed in N,N′-dimethylformamide (DMF, 1 mL) and sonicated for 20 min. Then, a 3 μL drop of the selected material dispersion was deposited onto the GCE surface, followed by solvent evaporation under an air flux. Prior to modification, the GCE electrode was conditioned as described above.
CV of the redox probes K3[Fe(CN)6], [Ru(NH3)6]Cl3 or EuCl3·6H2O were performed using solutions 1.0 × 10−3 mol dm−3 in KCl 1.0 mol dm−3. All solutions were prepared using ultra-pure water (resistivity 18.2 MΩ cm at 25 °C, Millipore). According with the electrochemical probe used, the potential was cycled between 0.75 and −0.25 V, 0.20 and −0.50 V or −0.10 and −1.00 V, at several scan rates from 0.010 to 0.500 V s−1. Each experiment was repeated until obtain concordant results.
The electroactive surface areas were determined using the Randles–Sevcik equation, eqn (4), assuming that the electrode process is controlled by diffusion:
ipc = 2.69 × 105n3/2ADx1/2Cv1/2 | (4) |
The heterogeneous electron transfer (HET) rates were evaluated by Nicholson's method, that relates the ΔEp to a dimensionless charge transfer parameter Ψ and, consequently, to kHET (HET rate constant), through eqn (5)57,58
![]() | (5) |
![]() | ||
Fig. 1 (a) N2 adsorption–desorption isotherms at −196 °C, closed symbols are the desorption points; (b) micropore size distributions of the activated carbons (adapted from ref. 39). |
Sample | ABET (m2 g−1) | Vtotala (cm3 g−1) | Vmesob (cm3 g−1) | αs methodc,d,e | ||
---|---|---|---|---|---|---|
Vα total (cm3 g−1) | Vα ultra (cm3 g−1) | Vα super (cm3 g−1) | ||||
a Evaluated at p/p0 = 0.975 in the N2 adsorption isotherms at −196 °C.b Difference between Vtotal and Vα total.c Vα total – obtained by back extrapolation of the high relative pressure region (αs < 1).d Vα ultra – intercept of the linear range defined the region p/p0 ≥ 0.02 (ϕ < 0.7 nm).e Vα super – difference between Vα total and Vα ultra (0.7 nm < ϕ < 2 nm). | ||||||
SH800 | 2431 | 1.14 | 0.06 | 1.08 | 0.00 | 1.08 |
SC800 | 1375 | 0.63 | 0.01 | 0.62 | 0.35 | 0.27 |
The SH800 and SC800 sucrose-derived activated carbons were further characterized by XPS; the obtained survey spectra for both materials are depicted in Fig. S1 in ESI.† The deconvoluted high resolution XPS spectra of SH800 in C 1s and O 1s regions are showed in Fig. 2 with the indication of the binding energies and the equivalent spectra for SC800 are depicted in Fig. S2 in ESI.†
In the XPS survey spectra, both activated carbons do not show any peak due to any catalytic active metal; besides C and O, the spectra show residual traces of Al 2p, Si 2p and K 2p. Consequently, the ORR electrocatalytic activity described below for both SC800 and SH800 activated carbons relates to metal-free electrocatalysts.
For both samples, the C 1s high-resolution spectra were deconvoluted with five peaks: a main peak at 284.6 eV assigned to graphitic carbon (sp2), a peak at 285.9/286.0 eV attributed to sp3 hybridized carbon, a peak at 287.1/287.3 eV assigned to C–O, a peak at 288.6/288.8 eV related to CO and a peak at 289.8/290.2 eV ascribed to –COO bonds.60 The O 1s spectra were fitted with two peaks, at 532.0 and 533.4 eV, assigned to C
O and C–O environments, respectively.20,61
The surface atomic percentages of each element in both materials are summarized in Table 2. The results showed that both materials have very similar C% ≈ 82 (≈65 mmol g−1) and O% ≈ 18 (≈14 mmol g−1), indicating that they are surface enriched in oxygen-containing groups. For SC800, C% is similar to the compositional data provided by the elemental analysis previously published (81.2 wt% of C, 67.6 mmol g−1)25 while, for SH800, the C% obtained by XPS is higher than the reported by elemental analysis (74.5 wt%, 62.0 mmol g−1),25 suggesting for the latter material inhomogeneity between the carbon and oxygen bulk and surface compositions.
Fig. 3(a) presents the CVs obtained for SC800 and SH800 modified electrodes in N2- and O2-saturated solutions. In N2-saturated solution no electrochemical processes are observed, while in O2-saturated solution all materials exhibited an irreversible electrochemical process indicative of their electrocatalytic activity for ORR. The SC800 modified electrode showed two cathodic peaks, at Epc = −0.26 and Epc = −0.44 V vs. Ag/AgCl (0.70 and 0.52 V vs. ERHE, respectively), and the SH800 showed only one defined cathodic peak, at Epc = −0.28 V vs. Ag/AgCl (0.68 V vs. ERHE).
Fig. 3(b) shows the ORR polarization plots of the prepared catalysts, the 20 wt% Pt/C and the bare GCE. The RDE voltammograms for the ORR on individual bare GCE and SC800, SH800 and Pt/C modified electrodes, at rotation rates from 400 to 3000 rpm, are depicted in Fig. S3.† Both activated carbons-based electrocatalysts showed similar onset potentials (Eonset = −0.19 and −0.20 V vs. Ag/AgCl (0.77 and 0.76 V vs. ERHE) for SC800 and SH800, respectively), but higher diffusion-limiting current densities (jL(−1.0 V, 1600 rpm) = −3.44 mA cm−2) were obtained for SH800 electrocatalyst, in contrast to SC800 (jL(−1.0 V, 1600 rpm) = −3.04 mA cm−2). These differences can be related to the distinct morphologies, surface area and micropore size distributions of the two materials. The material SH800 revealed a higher specific surface area, ABET, around 2500 m2 g−1, presenting larger micropores and widths between 0.7 and 2 nm (Table 1), and sponge-like morphologies; on the other hand, SC800 showed a spherical shape, with ABET ≈ 1400 m2 g−1 and narrow micropores with pore width <0.7 nm.25 Several works4,5,21 have reported that high surface areas and large pore structures are favourable conditions for ORR, since it would favour mass transport of the electrolyte, allowing for a higher catalytic current density, assuming mass transfer limited currents. In this context, the better performance (higher current densities) exhibited by SH800 is a consequence of its larger pores that enable the electrolyte solution to flow into/out of the catalyst more easily.
The results indicated a superior ORR performance of the activated carbons modified electrodes in comparison to the bare GCE (Eonset = −0.33 V vs. Ag/AgCl (0.63 V vs. ERHE), jL(−1.0 V, 1600 rpm) = −2.18 mA cm−2), with onset potential at less negative values and higher current densities, which demonstrate the advantage of the electrode modification. On the other hand, the results obtained with activated carbons are near to those obtained with 20 wt% Pt/C (Eonset = −0.14 V vs. Ag/AgCl (0.82 V vs. ERHE), jL(−0.9 V, 1600 rpm) = −3.91 mA cm−2), with a difference of ΔEonset = 0.06 V between the onset potentials of Pt/C and ACs-based modified electrodes.
In the context of other biomass-based carbon materials (usually, highly porous materials) for ORR,4,5,26–28 the majority refers to heteroatom doped materials (N and S), with the undoped analogues being equivalent to those prepared in this work. The Eonset values obtained for SH800 and SC800 (−0.20 V vs. Ag/AgCl) compare well or are less negative to those obtained for undoped and N-doped glucose-derived carbon aerogels (Eonset = −0.1, −0.20, −0.30 V vs. Ag/AgCl) when they are used as electrocatalysts in alkaline medium.5,26–28
The ORR kinetic parameters were analysed by the Koutecky–Levich (K–L) plots (j−1 vs. ω−1/2) at various potentials, using the RDE voltammograms. The slopes of their linear fit lines were used to estimate the number of electrons transferred per oxygen molecule (nO2), although it is recognised that the high surface areas and high degree of porosity may interfere in the transport properties of the active species (O2 and reaction intermediates) in the electrolyte. From the corresponding K–L plots (in the range −0.5 to −1.0 V vs. Ag/AgCl), Fig. S4,† it can be seen that the data exhibited good linearity, although the different straight lines exhibited different slopes, anticipating a variation in the nO2 values with potential. From Fig. 3(c) it is possible to verify that the estimated number of electrons transferred is very similar between the SC800 and SH800 catalysts, increasing as the potential become more negative, revealing that the electrocatalysts are not selective for either the 2 (indirect reduction through peroxide pathway) or 4 electron processes (direct O2 reduction): at E = −0.5 V vs. Ag/AgCl, the electrocatalysts are involved in a nO2 = 1.85–1.93 electrons process, shifting to nO2 = 3.03–3.12 electrons at E = −1.0 V vs. Ag/AgCl. These values are bigger than that displayed by GCE (nO2 = 1.70 electrons, at E = −1.0 V vs. Ag/AgCl), being closer to the ñO2 = 3.94 electrons estimated for 20 wt% Pt/C, for which a 4 electron processes is observed. The obtained nO2 values and its dependence with potential are also very similar to what is observed when glucose-derived carbon materials are used as ORR electrocatalysts in alkaline medium.26–28
In alkaline medium, oxygen indirect reduction through peroxide pathway envolves a two reaction steps mechanism of two electrons: in the first reaction, O2 is reduced to HO2− (O2 + H2O + 2e− → HO2− + HO−) and, in the second reaction, the intermediates are reduced to H2O/HO− (HO2− + H2O + 2e− → 3HO−). On opposite, the direct O2 reduction reaction corresponds to a single reaction involving four electrons, where O2 is directly reduced to H2O/HO− (O2 + 2H2O + 4e− → 4HO−).1
Further information on ORR kinetics and mechanism can be obtained from the Tafel plots for both ACs and Pt/C, Fig. 4. Both ACs showed very similar Tafel plots with two different slopes with similar values: 53/68 mV dec−1 in low current density region and 171/217 mV dec−1 in high current density region, for SH800 and SC800, respectively. As expected, the highest performance electrocatalyst SH800 showed lower slopes. The Pt/C electrocatalyst showed a different Tafel plot with different slope values (85/192 mV dec−1), suggesting a different mechanism in which Pt is the active site.62 The ORR mechanism in undoped carbon materials that present oxygen groups is still a matter of debate and only few papers address the topic, for pyrolytic graphite, CNTs and graphene oxide.1 The identified oxygen containing groups with ORR activity are the so-called quinone-like groups, that are capable of O2 adsorbing and efficiently mediate the first 2e− reduction of O2 to HO2− and subsequent reduction of the intermediates to HO−.63–65 The ACs-based electrocatalysts have very similar oxygen contents and although XPS is not able to discriminate the oxygen-type groups and respective quantities, the similarity of the Tafel slopes and profiles suggests that the different electrocatalytic activities may be not directly related to the type of oxygen groups, but to the different textural properties, namely the higher surface area and larger microporous of SH800 comparative to SC800.
![]() | ||
Fig. 4 ORR Tafel plots for SC800 and SH800 activated carbons and 20 wt% Pt/C, obtained from LSV data in Fig. 3(b); current intensities normalized to the mass of each electrocatalyst deposited on electrode (see Experimental section). |
An important parameter in fuel-cells where methanol is used as fuel, as in direct methanol fuel-cells, is the tolerance of the catalyst to fuel molecules, that may pass across the membrane from the anode to the cathode and to poison the catalyst.21,38 The platinum-based electrocatalysts have the disadvantage of selectivity to methanol oxidation, which completely subdues the ORR and reduces the current output.4 Therefore, methanol tolerance tests were performed by cyclic voltammetry, with SC800, SH800 modified electrodes and standard Pt/C for comparison. The CVs obtained in O2-saturated 0.1 mol dm−3 KOH solutions, in the presence and absence of methanol (1.0 mol dm−3), for SC800, SH800 and 20 wt% Pt/C modified electrodes are depicted in Fig. 5(a)–(c), respectively. In the presence of methanol, the Pt/C electrocatalyst showed an anodic peak at Epa = −0.17 V vs. Ag/AgCl attributed to methanol oxidation, that overlaps the ORR and reflects the low tolerance of Pt to methanol, as expected. On the contrary, there were no significant changes in the electrochemical response of SC800 and SH800 electrocatalysts after the introduction of methanol, demonstrating their high selectivity for oxygen reduction against the electro-oxidation of methanol and, consequently, the excellent tolerance towards crossover methanol effect. These results make SC800 and SH800 materials promising electrocatalysts to be applied in fuel cells, especially in direct methanol fuel cells.
Another important parameter is the stability/durability of the ORR electrocatalysts. Thus, the long-term stability of the SC800, SH800 and Pt/C electrocatalysts were evaluated by chronoamperometric measurements at E = −0.60 V vs. Ag/AgCl in O2-saturated 0.1 mol dm−3 KOH solution at 1600 rpm, and the results are present in Fig. 5(d). After 20000 s, the 20 wt% Pt/C chronoamperogram showed a current decay to 68.0%. At the end of the same period of time, the SC800 current declined to 53.9%, while the SH800 showed a current decay only to 72.9%. These data revealed that the activated carbons have different electrochemical stabilities, with the SH800 being the most stable ORR electrocatalysts. These differences can be related to the combination of distinct morphologies, surface areas and porosity: the SH800 electrocatalyst showing higher surface area and larger pores, will have higher number of active sites (quinone-like groups) that will be easily accessed by the O2/HO2−/electrolyte, leading to higher durability.25 Furthermore, SH800 also showed to be more stable than the standard Pt/C catalysts, which is a promisor result. This could be a consequence of the migration and aggregation of Pt particles caused by continuous potential, contrasting to the higher stability carbon materials structure.
Fig. 6 shows the results obtained in acidic media (H2SO4/Na2SO4 buffer solution, pH 2.5). The CVs obtained in N2- and O2-saturated solutions are depicted in Fig. 6(a). The activated carbons-based electrocatalysts showed ORR electrocatalytic activity also in acidic media, exhibiting cathodic peaks at Epc = −0.16 V and Epc = −0.11 V vs. Ag/AgCl (0.18 V and 0.24 V vs. ERHE), for SC800 and SH800, respectively. This is a key result because usually carbon materials are known to be inactive or present lower ORR activity in acidic media.21 The ORR catalytic activity in acidic media is also very important for proton exchange fuel cells which required acid electrolytes.66 Interestingly, other electrochemical processes (at E = −0.72/0.28 V for SC800 and E = −0.73/0.06 or 0.20 V for SH800 vs. Ag/AgCl) were also observed in both N2- and O2-saturated solutions, which can be related to other redox processes inherent of the activated carbon materials in acidic media and not directly related to ORR.
The ORR polarization plots for all materials are depicted in Fig. 6(b) and the individual RDE voltammograms can be observed in Fig. S5 in ESI.† Similarly to the results obtained in alkaline media, also in acidic media, the electrodes modified with SC800 and SH800 showed higher ORR performance than the bare GCE, exhibiting onset potentials at more positive values (Eonset = 0.06 V vs. Ag/AgCl (0.41 V vs. ERHE) for SC800 and Eonset = −0.01 V vs. Ag/AgCl (0.34 V vs. ERHE) for SH800). These onset potential values are considerably more negative than those obtained with the Pt/C modified electrode (Eonset = 0.41 V vs. Ag/AgCl (0.76 V vs. ERHE)), resulting in a ΔEonset = 0.35–0.42 V. The declining of the carbon materials performance in acidic media, when compared with Pt/C, can be associated to some active sites deactivation, for example protonation of oxygen ORR active groups which become inactive and non-accessible for O2 adsorption.6 Furthermore, all modified electrodes displayed identical ORR diffusion-limiting current densities (jL = −2.68, −2.59 and −2.76 mA cm−2 for bare GCE, SC800 and SH800, respectively (at E = −1.3 V vs. Ag/AgCl), and jL = −2.78 for 20 wt% Pt/C (at E = −0.3 V vs. Ag/AgCl)), at 1600 rpm. In comparison with other biomass-based carbon materials (N- and S-doped) reported as ORR electrocatalysts,5,26 the Eonset values obtained for SC800 and SH800 (0.06 and −0.01 V vs. Ag/AgCl, respectively) are less positive than those obtained for N-doped glucose-based carbons materials (0.20–0.50 V vs. Ag/AgCl).5,26 K–L plots are presented in Fig. S6 in ESI† and the number of electrons transferred per O2 molecule and its variation according with the applied potential can be observed in Fig. 6(c) for all electrocatalysts. The SC800 and SH800 electrocatalyst continue to present similar nO2 values, but with some variation with the applied potential: at E = −1.3 V, nO2 = 2.26 for SC800, nO2 = 2.31 for SH800 and nO2 = 1.80 for the bare GCE. At this experimental conditions, Pt/C electrocatalyst presented nO2 = 3.27 (at −0.3 V).
Tafel plots for ACs, depicted in Fig. S7 in ESI,† also presented two different slopes: 227/1001 mV dec−1 for SC800 and 145/561 mV dec−1 for SH800, in low and high current density regions, respectively. Globally, the results suggest lower electrocatalytic activity and different ORR mechanisms compared to the equivalent data in alkaline medium; nevertheless SH800 still showed the lowest slopes and consequently highest ORR performance. Moreover, Pt/C electrocatalyst showed lower Tafel slopes (81/504 mV dec−1) than for ACs, indicating highest catalytic activity.62
The electrochemical properties of activated carbons were explored in the presence of the redox probes [Fe(CN)6]3−/4−, [Ru(NH3)6]3+/2+ and Eu3+/2+, Fig. 7(b)–(d), respectively. The CVs for [Ru(NH3)6]3+/2+ using the SC800 and SH800 modified electrodes, Fig. 7(c), showed one pair of redox peaks at Epc = −0.20 V/Epa = −0.13 V vs. Ag/AgCl that are assigned to Ru3+/Ru2+ electronic transfer; furthermore, the CVs profiles are very similar to that observed when using GCE, indicating that the redox probe does not interact significantly with the modified electrodes. The anodic to cathodic peak-to-peak separations (ΔE) obtained for both modified electrodes are very similar (ΔEp = 0.072 V for SC800 and ΔE = 0.071 V for SH800 vs. Ag/AgCl), which are in agreement with the insensitivity of the [Ru(NH3)6]3+/2+ probe to surface defects and oxygen-containing groups.42 With the [Fe(CN)6]3−/4− redox probe (Fig. 7(b)), the CVs profiles for both SC800 and SH800 modified electrodes are also similar to the bare GCE, with a cathodic peak at Epc ≈ 0.24 V and an anodic peak at Epa ≈ 0.32 V (Fe3+/Fe2+ electron transfer), and with ΔE = 0.083 and 0.080 V vs. Ag/AgCl for SC800 and SH800, respectively. The similarity between peak potentials and ΔE values for the modified and bare electrodes are also indicative of a non-significant interaction between the redox probe and the modified electrodes surface.
The CV of Eu3+/2+ probe using the bare GCE, Fig. 7(d), showed the electrochemical process associated with Eu3+/Eu2+ electron transfer, with Epc = −0.67 V and Epa = −0.50 V and ΔE = 0.172 V vs. Ag/AgCl. In the CVs of activated carbons-modified electrodes it is also possible to observe the redox processes due to Eu3+/Eu2+, but the peak potentials are shifted: Epc values are shifted to less negative potentials whereas the Epa values are shifted to more negative potentials (Epc = −0.64 V/Epa = −0.54 V for SC800 and Epc = −0.63 V/Epa = −0.55 V for SH800 vs. Ag/AgCl). These variations are reflected in a decrease in ΔEp values, compared to the GCE: ΔE = 0.097 V for SC800 and ΔE = 0.080 V for SH800 vs. Ag/AgCl. These results indicate a different interaction of the redox probe with each modified electrode through the oxygen groups, since the probe can, in this case, bind directly to the carbon–oxygen groups.42 Thus, in the CV of redox probe using GCE the electrochemical process due to Eu3+/Eu2+ is dominated by the europium species in solution. On the other hand, in CVs of the redox probe using the activated carbons-modified electrodes, the observed electrochemical process due to Eu3+/Eu2+ may also have contributions from the europium species attached to the oxygen groups in carbon materials. Although the activated carbons showed similar surface oxygen content (O% from XPS), the material SH800 have a higher bulk oxygen content associated with a higher surface area and well developed porosity. In this context, the lower ΔE value compared to SC800-modified electrode is compatible with a higher % of adsorbed Eu3+/Eu2+ due to the highest bulk oxygen content.
In the experimental timescale employed (scan rates in the range 0.01 to 0.5 V s−1) both Epc and Epa varied less than 0.010 V for [Fe(CN)6]3−/4−, 0.010 V (SC800) and 0.005 V (SH800) for [Ru(NH3)6]3+/2+ and 0.030 V (SC800) and 0.009 (SH800) for Eu2+/3+ vs. Ag/AgCl. Fig. 8 shows the plots of logip versus log
v for bare GCE and SC800 and SH800 modified electrodes. All plots have slopes around 0.50, indicating predominantly diffusion-controlled processes.56 In addition, the ratios ipa/ipc are close to one in all cases (see Table S1 in ESI†).
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Fig. 8 Plots of log![]() ![]() |
The electroactive surface area of bare GCE and SC800 and SH800 modified electrodes were determined from CVs of 1 × 10−3 mol dm−3 K3[Fe(CN)6], [Ru(NH3)6]Cl3 and EuCl3·6H2O in KCl 1.0 mol dm−3 (Fig. S9–S11 in ESI,† respectively), using the eqn (4); the A values are summarized in Table 3.
Sample | [Fe(CN)6]3−/4− | [Ru(NH3)6]3+/2+ | Eu2+/3+ |
---|---|---|---|
GCE | 0.0679 (±0.0017) | 0.0678 (±0.0004) | 0.0447 (±0.0015) |
SC800 | 0.0776 (±0.0054) | 0.0848 (±0.0009) | 0.0597 (±0.0010) |
SH800 | 0.0807 (±0.0013) | 0.0840 (±0.0047) | 0.0601 (±0.0022) |
For all probes, the electroactive areas determined for SC800 and SH800 are larger than for bare electrode, indicating that the modification with these materials provide a more conductive pathway for the electron transfer of tested probes. The areas determined using the [Fe(CN)6]3−/4− and [Ru(NH3)6]3+/2+ probes are very similar for each modified electrode and, even, identical between the SC800 and SH800 materials. With the Eu2+/3+ probe, the areas determined for activated carbon materials remains very similar between them, but are smaller than the areas calculated with the other two probes.
The HET rate constants, kHET, were determined through eqn (5) for SC800 and SH800 modified electrodes for all redox probes solutions. The higher kHET values were obtained for [Ru(NH3)6]3+/2+, kHET = 1.23 × 10−2 cm2 s−1, for both modified electrodes, reflecting the lowest ΔEp values. For [Fe(CN)6]3−/4−, kHET are similar for both modified electrodes, kHET = 6.20 × 10−3 and 6.82 × 10−3 cm2 s−1 for SC800 and SH800 modified electrodes, respectively. In the case of Eu2+/3+ probe, the SC800 modified electrode showed a kHET = 4.72 × 10−3 cm2 s−1, which is different from that of SH800 (kHET = 6.30 × 10−3 cm2 s−1).
Both activated carbons showed ORR electrocatalytic activity in alkaline media, with similar onset potentials (Eonset ≈ −0.20 V vs. Ag/AgCl) but higher current densities were obtained for SH800, which was explained by its more well-developed porosity and higher surface area. Both electrocatalysts revealed excellent tolerance to methanol, with SH800 presenting inclusive, greater long-term electrochemical stability than the state-of-the-art Pt/C electrocatalyst. The sucrose-derived activated carbons also showed ORR catalytic activity in acidic media, although with medium ORR performances, but making them promising for applications in proton exchange fuel cells.
All of these results, allied with the fact of both electrocatalysts are made from a renewable biomass, highlight the potential of these porous activated carbons (mainly SH800, due to its superior stability in alkaline media) towards electrocatalysts for energy conversion. Moreover, although the 4-electrons mechanism (selectivity to H2O) is known to be favourable for ORR in order to have higher efficiencies, a process with H2O2 selectivity (2-electrons mechanism), can also be useful for the co-generation of hydrogen peroxide and electricity under the same electrochemical reaction conditions; consequently, the sustainable approach used offers a versatile protocol for carbon-based electrocatalysts application.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra20874b |
This journal is © The Royal Society of Chemistry 2015 |