A bi-functional catalyst for oxygen reduction and oxygen evolution reactions from used baby diapers: α-Fe₂O₃ wrapped in P and S dual doped graphitic carbon

Abstract

The development of energy conversion and storage devices and the disposal of solid waste represent two major challenges for environmental sustainability. The development of many key sustainable energy technologies relies on fast oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics. However, both reactions are sluggish. In the present work, we address these two issues synergistically by fabricating Fe₂O₃ wrapped in P and S dual-doped graphitized carbon (Fe₂O₃/P–S-CG) from soiled baby diapers, whose disposal and recycling are beneficial to the environment. Electrochemical tests revealed that Fe₂O₃/P–S-GC has a significant catalytic activity towards both the ORR and OER in 0.1 M KOH. In particular, the difference between the ORR potential at 3 mA cm⁻² and the OER potential at 10 mA cm⁻² is as small as ∼0.86 V. This value is comparable to that of commercial precious metal-based catalysts. In addition, Fe₂O₃/P–S-GC exhibits a favorable catalytic durability, making it a promising bi-functional non-precious catalyst for the ORR and OER.

---

1. Introduction

Traditional methods to generate electricity are based on fossil fuels, resulting in substantial emissions of carbon dioxide and severe pollution to the environment.^1,2 Due to the ever-increasing demand for sustainable energy, scientific research has focused on developing alternative energy conversion and storage systems.³ Many of these sustainable technologies, including fuel cells,⁴ direct-solar and electricity-driven electrolyzers,³ and metal-air batteries,⁵ critically rely on oxygen reduction reaction (ORR)⁶ and oxygen evolution reaction (OER)^7,8 processes. However, both ORR and OER undergo complicated reaction pathways, and face kinetic sluggishness.^9,10 Despite the tremendous research efforts, developing low-cost and high-activity oxygen catalysts still remains a great challenge. To date, Pt/C is the most common commercial ORR catalyst,¹¹ while Ir and Ru based catalysts are the mainstays for OER catalysis.¹² It is well known that such precious metals (Pt, Ir, and Ru) suffer from scarcity and high costs,¹³ which are the bottlenecks for large-scale commercialization of the sustainable technologies listed above. Hence, efficient and low-cost catalysts for ORR and OER are highly needed.^14,15

Recently, carbon-based ORR and OER catalysts have attracted considerable attention because they do not require precious metals. Particularly, B,^16,17 N,^18–23 P,^24–26 and S^27–29 doped carbon materials have shown tremendous potential as ORR or OER catalysts, since the introduced heteroatoms can cause the charge redistribution within the carbon structure and provide additional electrocatalytically active sites. In addition, there have been reports on the dual and ternary-doped graphitic carbon, which further exploits the aforementioned synergistic effects for enhanced ORR/OER catalytic activity.^30–32

It is also worth mentioning that the ever-increasing production of solid waste is a severe threat to the environment. In particular, thousands of single-use diapers are needed by an infant every year and their weight accounts for up to 2% of the total municipal solid waste.³³ Such disposable baby diapers are highly polluting because it is difficult to decompose their absorbent layers, which are made of superabsorbent polymers (e.g. sodium polyacrylate [–CH₂–CH(COONa)–]_n).³⁴

Based on the above considerations, the main idea of this article is to transform soiled diapers into electrochemical catalysts for energy conversion and storage devices. As shown schematically in Fig. 1, we synthesized Fe₂O₃ wrapped in P and S dual-doped graphitized carbon (Fe₂O₃/P–S-GC) with the method of high temperature pyrolysis,³⁵ where soiled diapers served as the precursor for the carbon material and Fe(NO₃)₃ was the Fe source. As is well known, the sodium polyacrylate [–CH₂–CH(COONa)–]_n present in the absorbent layers of the diapers has abundant tunnels with organic functional groups that can absorb the Fe present in Fe(NO₃)₃.^36–38 Furthermore, the absorbed Fe ions serve as catalysts for carbon graphitization, and the baby waste in the diapers could provide extra dopants, including P and S. Finally, the Fe used for catalyzing carbon graphitization is gradually oxidized at high temperature, forming the transition metal oxide Fe₂O₃,^39,40 and ultimately leading to Fe₂O₃/P–S-GC. The electrochemical tests suggest that the as-prepared Fe₂O₃/P–S-GC achieves an excellent bi-functional activity towards both ORR and OER.

Fig. 1 Trash to treasure: schematic depiction of the preparation of catalysts from environmental trash.

2. Experimental

2.1 Preparation of Fe₂O₃/P–S-GC

In a typical synthesis process, the absorbent layers were first taken out from the used baby diapers. After drying, the absorbent layers (7 g) were put into a solution of Fe(NO₃)₃ (0.5 M, 200 mL). The suspension was stirred for 12 h at room temperature. Finally, the mixture was separated by centrifugation. The lower sediment layer was put again into the Fe(NO₃)₃ solution to ensure that a sufficient amount of Fe³⁺ ions would be available in the absorbent layers. After additional vacuum drying, the resulting sediment was heat-treated at 1100 °C in N₂ atmosphere for 45 min. The obtained black Fe/P–S-GC composite was washed with deionized water and CH₃CH₂OH. Subsequently, the sample was vacuum dried for 1 h. Finally, the mixture was annealed in air at 300 °C for 2 h and Fe₂O₃/P–S-GC was obtained.⁴¹ For comparison, Fe₂O₃/GC was synthesized with new baby diapers rather than used ones. Additionally, P–S-GC was prepared by removing Fe₂O₃ from Fe₂O₃/P–S-GC with hydrochloric acid (2.0 M) for 6 h at 80 °C.

2.2 Catalyst ink fabrication

A glassy carbon electrode (GCE) (4 mm in diameter) was polished to a mirror finish by a 0.05 μm alumina suspension before each experiment and served as the underlying substrate for the working electrode. The catalyst layer was prepared by ultrasonically dispersing 10 mg of the sample in 1.9 mL of ethanol, where 0.1 mL of 5 wt% Nafion solution (Sigma Aldrich) was also added. The suspension was ultrasonically treated for 30 min to obtain a homogeneous solution. 10 μL (∼0.4 mg cm⁻² per disk) of the dispersion was pipetted on the top of the GCE and dried in air. It is important to note that commercial Pt/C (20% wt% Pt) from Sigma Aldrich was also tested for comparison.

2.3 Sample characterizations

The morphology of the samples was analyzed with scanning electron microscopy (SEM, JEOL 6300) and transmission electron microscopy (TEM, JEOL 2012F). Carbon-coated nickel grids were used as sample holders for the TEM analysis. The phase structures are examined by X-ray diffraction (XRD) using an Empyrean PANalytical diffractometer. The valence state of the various elements in the prepared samples was studied by X-ray photoelectron spectroscopy (XPS) using a Physical Electronics PHI 5600 multi-technique system with Al monochromatic X-rays at 350 W. Raman measurements were performed with a Jobin Yvon HR 800 micro-Raman spectrometer, where the laser beam with a wavelength of 514 nm was focused onto the sample.

2.4 Electrochemical tests

Electrochemical measurements were carried out using a rotating ring disk electrode device (RRDE-3A, ALS Co., Ltd, Japan). Linear sweeping voltammetry (LSV) was performed with a conventional three-electrode method with an ALS2325E (ALS Co., Ltd, Japan) potentiostat. The GCE was employed as the working electrode, Pt wire acted as the counter electrode and mercury/mercury oxide (MMO, filled with 1.0 M NaOH) was utilized as the reference electrode. The measurements were conducted in a 0.1 M KOH solution with a negative scanning rate of 5 mV s⁻¹. The potential difference between MMO electrode and reversible hydrogen electrode (RHE) in alkaline solution was calibrated to be 0.92 V, i.e., E(RHE) = E(MMO) + 0.92 V. The electrolyte resistance was measured to be 60 Ω by electrochemical impedance spectroscopy for IR correction.

3. Results and discussion

3.1 Structural characterizations

Sodium polyacrylate [–CH₂–CH(COONa)–]_n in the absorbent layers of the diapers, whose structure is shown in Fig. S1–S3 in the ESI,† is an anion-exchange resin containing –OH and C–C=O groups. The process leading to the Fe₂O₃/P–S-GC material is illustrated in Fig. 2. Firstly, the sodium polyacrylate [–CH₂–CH(COO)–]_n was mixed with Fe(NO₃)₃ aqueous solution to obtain ion-exchange resin–metal ions complexes.³⁶ The Fe³⁺ was then reduced into Fe metal by carbon at high temperature under N₂, merging into a bigger Fe core and acting as the substrate for the precipitation of C, P and S atoms.⁴⁰ The Fe wrapped in P and S dual-doped graphitized carbon was then formed. Afterwards, the as-obtained Fe/P–S-GC composite was washed with deionized water and CH₃CH₂OH. Finally, after being dried in vacuum for 1 h, the composite was annealed at high temperature in air.⁴¹ During the synthesis process, the Fe acts on one hand as a catalyst for carbon graphitization, and on the other hand, serves as the precursor of Fe₂O₃.

Fig. 2 Diagrams of the formation for Fe₂O₃/P–S-GC from the used baby diaper.

Fig. 3 displays the SEM and TEM images of Fe₂O₃/P–S-GC at different magnifications. It is clear that Fe₂O₃ is wrapped into carbon capsules, and that the carbon is characterized by multiple layers. In addition, the size of the Fe₂O₃ nanoparticles varies from 20 to 200 nm.

Fig. 3 (a) SEM and (b)–(d) TEM images of Fe₂O₃/P–S-GC at different magnifications.

Raman spectroscopy was undertaken to further evaluate the graphitization degree of the carbon layers in Fe₂O₃/P–S-GC. As shown in Fig. 4a, for P–S-GC only three peaks at 1582, 1343, and 2752 cm⁻¹ are observed. The peaks can be assigned to G-, D- and 2D-bands of the carbon, respectively. Instead for Fe₂O₃/P–S-GC and Fe₂O₃/GC, additional characteristic peaks at 206, 280, 393, and 571 cm⁻¹ are present, which can be ascribed to Fe₂O₃. It is clear that the intensity ratio between the D-band and G-band of the Fe₂O₃/P–S-GC (I_D/I_G = 0.75) is higher than that of Fe₂O₃/GC (ID/IG = 0.55). This may be attributed to P and S dopants, which influence the crystallographic structure of the GC. In addition, the increased defect concentration brought by the heteroatom doping can significantly influence the ORR and OER activities. XRD was performed in order to confirm that the structure of Fe₂O₃. As presented in Fig. 4b, the peaks of the iron oxide in Fe₂O₃/P–S-GC, locating at 24.5°, 33.3°, 35.9°, 41.5°, 50.0°, 54.2°, 58.1°, 62.8° and 64.6, are well indexed with α-Fe₂O₃, verified by comparison with JCPDS file no. 33-0664.

Fig. 4 (a) Raman spectra of Fe₂O₃/P–S-GC, Fe₂O₃/GC and P–S-GC; (b) XRD survey spectrum of Fe₂O₃/P–S-GC.

Furthermore, XPS shows that the atomic content of C, S, P, Fe, and O in the Fe₂O₃/P–S-GC sample are 36.20 at%, 3.06 at%, 1.26 at%, 20.24 at%, and 30.52 at%, respectively. The remaining impurities, including K⁺, Na⁺ and Mg²⁺, which account for about 8.72 at%, do not contribute to the ORR/OER activity. The XPS spectra of C 1s, Fe 2p, S 2p, P 2p, and O 1s core are further analyzed. As shown in Fig. 5a, a sharp peak at 284.5 eV with an asymmetric tailing toward high binding energy is observed for the C 1s of Fe₂O₃/P–S-GC. The fitted peaks locating at 284.5, 285.4 and 288.8 eV are assigned to the C=C, C–O–H and C–C=O, respectively.^42,43 The existing C–O–H and C–C=O likely comes from the sodium polyacrylate, which contains C–O–H and C–C=O groups. After the treatment, a few functional groups remained. In addition, it is observed that the ratio of the fitted peaks at high binding energies is increased in Fe₂O₃/P–S-GC (Fig. 5a) compared with that of Fe₂O₃/GC (Fig. 5b). This can be attributed to the influence of the introduced P and S dual doping into the carbon structure.^44,45 As shown in Fig. 5c, the peaks located at 711 eV, 724 eV, 719 eV, and 732.5 eV are in good agreement with the Fe 2p3/2, Fe 2p1/2, and the satellite peaks of Fe³⁺, respectively.^46–49 The binding energies for S 2p3/2 and S 2p1/2 are 163.7 eV and 165.3 eV, respectively, as shown in Fig. 5d, and these peaks are ascribed to the presence of sulfide groups (–C–S–C).^49,50 It should be noted that the –C–SOX–C peak (at 170.3 eV) is also observed, however it is an unlikely contributor to the ORR performance.²⁷ Fig. 5e displays the P 2p peak at approximately 132.0 eV, which may be assigned to the P–C bond.^26,51 Finally, the peaks at 530.2 and 532.5 eV in the O 1s spectrum (Fig. 5f) can be attributed to the ferric oxides (Fe₂O₃), oxygen bridge (–O–) on the interfaces and the residual –OH and –COOH in GC. These results are in accordance with the insights gained from the analysis of the C 1s core spectra.

Fig. 5 XPS survey spectra: C 1s of (a) Fe₂O₃/P–S-GC and (b) Fe₂O₃/GC; (c) the fitted Fe 2p core of Fe₂O₃/P–S-GC; (d) the fitted S 2p core of Fe₂O₃/P–S-GC; (e) P 2p core of Fe₂O₃/P–S-GC; and (f) the fitted O 1s core of Fe₂O₃/P–S-GC.

The Brunauer–Emmett–Teller (BET) specific surface area of the investigated samples are derived from the N₂ adsorption–desorption isotherm curves in shown Fig. S4.† From the results we observe that Fe₂O₃/P–S-GC (289.5 m² g⁻¹) has a similar specific surface area to Fe₂O₃/GC (294.5 m² g⁻¹), but smaller than that of P–S-GC (372.8 m² g⁻¹).

3.2 ORR activity

To assess the ORR performance of the Fe₂O₃/P–S-GC, linear sweep voltammetry (LSV) was carried out on a rotating disk electrode (RDE). The polarization curves were recorded in a 0.1 M KOH solution saturated with pure oxygen with a scanning rate of 5 mV s⁻¹ (Fig. 6a). A commercial catalyst, i.e., 20 wt% Pt on Vulcan XC72 (Aldrich), P–S-GC and Fe₂O₃/GC samples were also tested for comparison. The onset potential of Fe₂O₃/P–S-GC (∼0.97 V vs. RHE) is just slightly lower than that of commercial Pt/C (∼1.01 V vs. RHE), and their limiting current densities are virtually identical (∼5.4 mA cm⁻² for Fe₂O₃/P–S-GC and ∼5.5 mA cm⁻² for commercial Pt/C). In addition, it is observed that P–S-GC and Fe₂O₃/P–S-GC have virtually identical ORR activity, including the same onset potential (∼0.97 V vs. RHE) and close half-wave potential (∼0.790 V vs. RHE and 0.785 V vs. RHE). While, in comparison with Fe₂O₃/GC, the onset potential (∼0.97 V vs. RHE) and half-wave potential (0.79 V vs. RHE) of Fe₂O₃/P–S-GC are more positive than corresponding values (∼0.57 V vs. RHE and 0.43 V vs. RHE) of Fe₂O₃/GC. This suggests that P and S dual doping contributes to lowering the ORR overpotential and to enhancing the current density.^{25–27,32,52,53} The improved ORR catalytic performance is attributed to the doping-induced charge redistribution around the heteroatom elements, which changes the O₂ chemisorption mode and effectively weakens the O–O bond, providing more active sites and facilitating ORR rate at the heteroatom-doped carbon electrodes.^29,54 In addition, it is apparent that Fe₂O₃ does not significantly contribute to the ORR activity. In fact, it has been reported that only certain morphologies and loadings of Fe₂O₃ can considerably affect the ORR activity.⁵⁵ Even if Fe₂O₃ in Fe₂O₃/P–S-GC fails to enhance the ORR activity, it has an important impact on the OER performance, as will be discussed later in this article.

Fig. 6 (a) LSV curves of Fe₂O₃/P–S-GC, Fe₂O₃/GC, P–S-GC and Pt/C in the ORR potential range, the scanning is carried out at the rate of 2000 rpm; (b) LSV curves of Fe₂O₃/P–S-GC at different rotating speeds, the inset is corresponding Koutecky–Levich plots with the fitted electron transfer number n; (c) Tafel plots of Fe₂O₃/P–S-GC, Fe₂O₃/GC, P–S-GC and Pt/C; (d) ORR stability test for Fe₂O₃/P–S-GC with a rotation speed of 2000 rpm.

Further insights regarding the ORR activity of Fe₂O₃/P–S-GC can be gained by carrying out LSVs at different rotating rates. Fig. 6b shows the polarization curves of Fe₂O₃/P–S-GC in 0.1 M KOH solution at rotation rates from 1200 to 2500 rpm. The RDE data were analyzed using the Koutecky–Levich equation:^56,57

B = 0.62nFC₀D^2/3₀v^−1/6 (2)

J_K = nFkC₀ (3)

where J is the measured current density, J_K and J_L are the kinetic and diffusion limiting current densities, respectively, ω is the electrode rotation rate, n is the overall number of electron transferred, F is the Faraday constant (96 485 C mol⁻¹), C₀ is the bulk concentration of O₂ dissolved into the electrolyte (1.2 × 10⁻⁶ mol cm⁻³), D₀ is the O₂ diffusion coefficient (1.9 × 10⁻⁵ cm² s⁻¹), and ν is the kinematic viscosity of the electrolyte (0.01 cm² s⁻¹).⁵⁸ Based on the Koutecky–Levich equation, n for ORR can be extracted. On the basis of the results, it is evident that the ORR in Fe₂O₃/P–S-GC and P–S-GC preferentially occurs via a four-electron transfer process, in which O₂ are reduced directly to OH–, with n = 3.9 (see the inset in Fig. 6b) and n = 3.4 (see the polarization curves in Fig. S5a†), for Fe₂O₃/P–S-GC and P–S-GC respectively. It is worth noting that the ORR electron transfer number of Fe₂O₃/P–S-GC is close to the corresponding value of commercial Pt/C (n = 4.0, see Fig. S5b†).

The ORR Tafel plots for the investigated catalysts are displayed in Fig. 6c, from which the obtained Tafel slopes for Fe₂O₃/P–S-GC, P–S-GC, Fe₂O₃/GC and Pt/C are 91, 90, 123 and 87 mV dec⁻¹, respectively. In addition, the Fe₂O₃/P–S-GC and P–S-GC have similar Tafel slopes as Pt/C, which indicate that both Fe₂O₃/P–S-GC and P–S-GC have comparable kinetic activity to Pt/C.

Since durability is a major issue in practical applications, the stability of the Fe₂O₃/P–S-GC electrode was evaluated using Dai's method.⁶ We performed the accelerated durability tests (ADT) in 0.1 M KOH with a catalyst loading of ∼0.4 mg cm⁻² per disk for all samples. The cycles were scanned at the rate of 0.2 V s⁻¹ and the LSV were swept at 0.005 V s⁻¹. As shown in Fig. 6d, no obvious decline of activity for the Fe₂O₃/P–S-GC electrode after 3000 cycles was observed, suggesting its favorable catalytic stability.

3.3 OER and bi-functional activity

In addition to the ORR activity, we also investigated the OER performance of the Fe₂O₃/P–S-GC. From Fig. 7a we can see that Fe₂O₃/P–S-GC has the highest OER current density (∼33 mA cm⁻² at 1.75 V vs. RHE) and the lowest onset potential (∼1.43 V vs. RHE) among the investigated materials. Its OER performance is even comparable to that of Ir-based catalysts.⁵⁹ In addition, we also observe that both Fe₂O₃/P–S-GC and Fe₂O₃/GC have a higher OER activity than P–S-GC, indicating that Fe₂O₃ plays a significant role in facilitating OER, which is consistent with previous reports.^59,60 In fact, Fe₂O₃ is an OER active material.^61,62

Fig. 7 (a) LSV curves of Fe₂O₃/P–S-GC, Fe₂O₃/GC and P–S-GC in the OER potential range. The scanning was carried out at the rate of 0.005 V s⁻¹ with the rotation speed of 2000 rpm; (b) corresponding Tafel plots of Fe₂O₃/P–S-GC, Fe₂O₃/GC and P–S-GC with fitted Tafel slopes. (c) OER stability test for Fe₂O₃/P–S-GC with a scanning rate of 3000 rpm. IR corrections were applied to all the above LSV curves.

Furthermore, Fig. 7b shows that the Fe₂O₃/P–S-GC has a Tafel slope of 55 mV dec⁻¹, which is smaller than the corresponding value for Fe₂O₃/GC (67 mV dec⁻¹) and P–S-GC (110 mV dec⁻¹), supporting once again that an improved OER catalytic activity is achieved for the P and S dual doped GC coupled with Fe₂O₃.

In particular, the OER activity of Fe₂O₃/P–S-GC proves to be stable, since the polarization curves change little from the 1st and 3000th measurement, as shown in Fig. 7c. Its excellent OER performance can be attributed to the favorable wrapping of Fe₂O₃ into carbon layers, as shown in Fig. S6.† This configuration can fully utilize the Fe₂O₃ surface and reduce the transport resistance between electrons and oxygen,^55,63 thus significantly enhancing the OER activity.

It is worth mentioning that the potential difference between the ORR at 3 mA cm⁻² and the OER at 10 mA cm⁻² is calculated to be 0.86 V. As compared in Table S1,† this value is smaller than corresponding potential difference of most non-precious metal catalysts, and it is comparable to those of many precious metal catalysts.^64–70

In summary, the promising bi-functional catalytic activity of Fe₂O₃/P–S-GC towards ORR and OER can be ascribed to the following two factors: (i) the synergistic effect of P, S dual doping into carbon structure; (ii) the favorable configuration of Fe₂O₃ nanoparticles wrapped by carbon layers, enhancing the contact area between carbon and the metal oxide, which is also beneficial to the interfacial charge transfer on the catalyst surface.

Finally, we should also point out that the elemental content of individual Fe₂O₃/P–S-GC sample may be not fixed, owing to the variability of the waste precursors used. Here we compared the atomic contents for 5 Fe₂O₃/P–S-GC samples prepared from different used diapers, and the results are shown in Table S2.† Even though small deviations are present, there only exists a slight difference among the obtained activities, either in ORR or OER. From Fig. S7† it is observed that the average ORR limiting current density is ∼5.3 mA cm⁻² for 5 samples, and the maximum current deviation is smaller than 0.3 mA cm⁻². In addition, their onset potentials are almost identical. On the other hand, a similar small difference of OER activity is also verified by the same 5 samples, as shown in Fig. S8.†

4. Conclusions

In this work, we developed a cost-effective route to achieve a highly-active bi-functional OER/ORR catalyst. We prepared Fe₂O₃/P–S-GC using soiled disposable baby diapers as the source of C, P, and S. The as-obtained Fe₂O₃/P–S-GC exhibits a significant catalytic activity towards ORR, which is characterized by a quasi-four-electron ORR reaction route. In addition, the electrochemical tests suggest that the Fe₂O₃/P–S-GC also achieves a considerable OER activity. Furthermore, the Fe₂O₃/P–S-GC proves to be stable in alkaline electrolyte and a long-term catalytic durability is demonstrated, making it a promising oxygen catalyst. The bi-functional catalytic activity of Fe₂O₃/P–S-GC can be attributed to the synergistic enhancement brought by P–S dual doping and the wrapped Fe₂O₃. The results reported in this work provide a promising bi-functional oxygen catalyst and expand the options for developing energy materials from solid waste.

Acknowledgements

The authors gratefully acknowledge the Research Grants Council of Hong Kong for support through the project ECS 639713. M. S. acknowledges the support of the HKPFS.

References

1. U. Soytas and R. Sari, Ecol. Econ., 2009, 68, 1667–1675.
2. M. Dresselhaus and I. Thomas, Nature, 2001, 414, 332–337.
3. J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough and Y. Shao-Horn, Science, 2011, 334, 1383–1385.
4. D.-W. Wang and D. Su, Energy Environ. Sci., 2014, 7, 576–591.
5. L. Li, S.-H. Chai, S. Dai and A. Manthiram, Energy Environ. Sci., 2014, 7, 2630–2636.
6. Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H. Dai, Nat. Mater., 2011, 10, 780–786.
7. Y. Li, P. Hasin and Y. Wu, Adv. Mater., 2010, 22, 1926–1929.
8. D. Chen, C. Chen, Z. M. Baiyee, Z. Shao and F. Ciucci, Chem. Rev., 2015, 115, 9869–9921.
9. Y. Bing, H. Liu, L. Zhang, D. Ghosh and J. Zhang, Chem. Soc. Rev., 2010, 39, 2184–2202.
10. C. W. Bezerra, L. Zhang, H. Liu, K. Lee, A. L. Marques, E. P. Marques, H. Wang and J. Zhang, J. Power Sources, 2007, 173, 891–908.
11. B. Lim, M. Jiang, P. H. Camargo, E. C. Cho, J. Tao, X. Lu, Y. Zhu and Y. Xia, Science, 2009, 324, 1302–1305.
12. S. H. Oh, R. Black, E. Pomerantseva, J.-H. Lee and L. F. Nazar, Nat. Chem., 2012, 4, 1004–1010.
13. M. Jacquin, D. J. Jones, J. Rozière, S. Albertazzi, A. Vaccari, M. Lenarda, L. Storaro and R. Ganzerla, Appl. Catal., A, 2003, 251, 131–141.
14. F. Cheng, J. Shen, B. Peng, Y. Pan, Z. Tao and J. Chen, Nat. Chem., 2011, 3, 79–84.
15. H. Zhao, C. Chen, D. Chen, M. Saccoccio, J. Wang, Y. Gao, T. H. Wan and F. Ciucci, Carbon, 2015, 90, 122–129.
16. H. Wang, Y. Zhou, D. Wu, L. Liao, S. Zhao, H. Peng and Z. Liu, Small, 2013, 9, 1316–1320.
17. J. Wang, R. Zhao, Z. Liu and Z. Liu, Small, 2013, 9, 1373–1378.
18. D. Wei, Y. Liu, Y. Wang, H. Zhang, L. Huang and G. Yu, Nano Lett., 2009, 9, 1752–1758.
19. H. M. Jeong, J. W. Lee, W. H. Shin, Y. J. Choi, H. J. Shin, J. K. Kang and J. W. Choi, Nano Lett., 2011, 11, 2472–2477.
20. D. Geng, Y. Chen, Y. Chen, Y. Li, R. Li, X. Sun, S. Ye and S. Knights, Energy Environ. Sci., 2011, 4, 760–764.
21. H. Wang, T. Maiyalagan and X. Wang, ACS Catal., 2012, 2, 781–794.
22. J.-C. Li, P.-X. Hou, L. Zhang, C. Liu and H.-M. Cheng, Nanoscale, 2014, 6, 12065–12070.
23. J. Wang, H. Zhao, Y. Gao, D. Chen, C. Chen, M. Saccoccio and F. Ciucci, Int. J. Hydrogen Energy, 2016, 41, 10744–10754.
24. R. Li, Z. Wei, X. Gou and W. Xu, RSC Adv., 2013, 3, 9978–9984.
25. C. Zhang, N. Mahmood, H. Yin, F. Liu and Y. Hou, Adv. Mater., 2013, 25, 4932–4937.
26. D.-S. Yang, D. Bhattacharjya, S. Inamdar, J. Park and J.-S. Yu, J. Am. Chem. Soc., 2012, 134, 16127–16130.
27. Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. a. Chen and S. Huang, ACS Nano, 2011, 6, 205–211.
28. G. K. Pradhan, N. Sahu and K. Parida, RSC Adv., 2013, 3, 7912–7920.
29. J. Liang, Y. Jiao, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. Ed., 2012, 51, 11496–11500.
30. J.-i. Ozaki, N. Kimura, T. Anahara and A. Oya, Carbon, 2007, 45, 1847–1853.
31. C. H. Choi, S. H. Park and S. I. Woo, J. Mater. Chem., 2012, 22, 12107–12115.
32. J.-S. Li, S.-L. Li, Y.-J. Tang, K. Li, L. Zhou, N. Kong, Y.-Q. Lan, J.-C. Bao and Z.-H. Dai, Sci. Rep., 2014, 4, 5130.
33. C. Lehrburger, Diapers in the Waste Stream: A Review of Waste Management and Public Policy Issues, C. Lehrburger, 1989.
34. R. Kou, Y. Shao, D. Mei, Z. Nie, D. Wang, C. Wang, V. V. Viswanathan, S. Park, I. A. Aksay and Y. Lin, J. Am. Chem. Soc., 2011, 133, 2541–2547.
35. M.-M. Titirici, R. J. White, C. Falco and M. Sevilla, Energy Environ. Sci., 2012, 5, 6796–6822.
36. L. Wang, C. Tian, B. Wang, R. Wang, W. Zhou and H. Fu, Chem. Commun., 2008, 5411–5413.
37. H. Zhao, K. Hui and K. Hui, Carbon, 2014, 76, 1–9.
38. H. Zhao and T. Zhao, J. Mater. Chem. A, 2013, 1, 183–187.
39. Z. Mo, H. Peng, H. Liang and S. Liao, Electrochim. Acta, 2013, 99, 30–37.
40. T. Palaniselvam, H. B. Aiyappa and S. Kurungot, J. Mater. Chem., 2012, 22, 23799–23805.
41. J. Lin, A. R. O. Raji, K. Nan, Z. Peng, Z. Yan, E. L. Samuel, D. Natelson and J. M. Tour, Adv. Funct. Mater., 2014, 24, 2044–2048.
42. C.-C. Hu and C.-C. Wang, J. Power Sources, 2004, 125, 299–308.
43. D. Zhang, Y. Hao, L. Zheng, Y. Ma, H. Feng and H. Luo, J. Mater. Chem. A, 2013, 1, 584–7591.
44. D. Bhattacharjya, H.-Y. Park, M.-S. Kim, H.-S. Choi, S. N. Inamdar and J.-S. Yu, Langmuir, 2013, 30, 318–324.
45. C. Morant, J. Andrey, P. Prieto, D. Mendiola, J. Sanz and E. Elizalde, Phys. Status Solidi A, 2006, 203, 1069–1075.
46. J. Baltrusaitis, D. M. Cwiertny and V. H. Grassian, Phys. Chem. Chem. Phys., 2007, 9, 5542–5554.
47. J. Wang, M. Saccoccio, D. Chen, Y. Gao, C. Chen and F. Ciucci, J. Power Sources, 2015, 297, 511–518.
48. J. Wang, K. Y. Lam, M. Saccoccio, Y. Gao, D. Chen and F. Ciucci, J. Power Sources, 2016, 324, 224–232.
49. X. Wang, J. Wang, D. Wang, S. Dou, Z. Ma, J. Wu, L. Tao, A. Shen, C. Ouyang and Q. Liu, Chem. Commun., 2014, 50, 4839–4842.
50. S. Yang, L. Zhi, K. Tang, X. Feng, J. Maier and K. Müllen, Adv. Funct. Mater., 2012, 22, 3634–3640.
51. Z. W. Liu, F. Peng, H. J. Wang, H. Yu, W. X. Zheng and J. Yang, Angew. Chem., 2011, 123, 3315–3319.
52. Z. Yang, H. Nie, X. a. Chen, X. Chen and S. Huang, J. Power Sources, 2013, 236, 238–249.
53. J. P. Paraknowitsch and A. Thomas, Energy Environ. Sci., 2013, 6, 2839–2855.
54. Y. Zheng, Y. Jiao, M. Jaroniec, Y. Jin and S. Z. Qiao, Small, 2012, 8, 3550–3566.
55. M. Sun, Y. Dong, G. Zhang, J. Qu and J. Li, J. Mater. Chem. A, 2014, 2, 13635–13640.
56. R. Liu, D. Wu, X. Feng and K. Müllen, Angew. Chem., 2010, 122, 2619–2623.
57. W. Chen, D. Ny and S. Chen, J. Power Sources, 2010, 195, 412–418.
58. Z.-S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng and K. Müllen, J. Am. Chem. Soc., 2012, 134, 9082–9085.
59. J. I. Jung, H. Y. Jeong, J. S. Lee, M. G. Kim and J. Cho, Angew. Chem., 2014, 126, 4670–4674.
60. A. Kleiman-Shwarsctein, Y.-S. Hu, A. J. Forman, G. D. Stucky and E. W. McFarland, J. Phys. Chem. C, 2008, 112, 15900–15907.
61. K. Sivula, F. Le Formal and M. Grätzel, ChemSusChem, 2011, 4, 432–449.
62. D. K. Zhong, J. Sun, H. Inumaru and D. R. Gamelinv, J. Am. Chem. Soc., 2009, 131, 6086–6087.
63. X.-G. Wang, W. Weiss, S. K. Shaikhutdinov, M. Ritter, M. Petersen, F. Wagner, R. Schlögl and M. Scheffler, Phys. Rev. Lett., 1998, 81, 1038.
64. Y. Zhu, C. Su, X. Xu, W. Zhou, R. Ran and Z. Shao, Chem.–Eur. J., 2014, 20, 15533–15542.
65. Y. Gorlin and T. F. Jaramillo, J. Am. Chem. Soc., 2010, 132, 13612–13614.
66. D. Wang, X. Chen, D. G. Evans and W. Yang, Nanoscale, 2013, 5, 5312–5315.
67. X. Zhai, W. Yang, M. Li, G. Lv, J. Liu and X. Zhang, Carbon, 2013, 65, 277–286.
68. W. G. Hardin, D. A. Slanac, X. Wang, S. Dai, K. P. Johnston and K. J. Stevenson, J. Phys. Chem. Lett., 2013, 4, 1254–1259.
69. Q. Liu, J. Jin and J. Zhang, ACS Appl. Mater. Interfaces, 2013, 5, 5002–5008.
70. W. Zhou and J. Sunarso, J. Phys. Chem. Lett., 2013, 4, 2982–2988.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11352d

‡ These authors contributed equally to this work.

---