Moist-electric generation

Jiaxin Bai ^ab, Yaxin Huang ^a, Huhu Cheng *^a and Liangti Qu *^abc
aKey Laboratory for Advanced Materials Processing Technology, Ministry of Education of China, State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China. E-mail: lqu@mail.tsinghua.edu.cn; huhucheng@tsinghua.edu.cn
^bSchool of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, PR China. E-mail: lqu@bit.edu.cn
^cDepartment of Chemistry, Tsinghua University, Beijing 100084, P. R. China

First published on 15th August 2019

---

---

Jiaxin Bai

Jiaxin Bai received her Master's degree in the School of Chemistry and Chemical Engineering in the Beijing Institute of Technology. She is currently a Ph.D. candidate in Prof. Liangti Qu's group at the Department of Mechanical Engineering, Tsinghua University. Her researches mainly focus on graphene-based materials and applications on photoelectric conversion devices.

Yaxin Huang

Yaxin Huang is currently a Ph.D. candidate in Prof. Liangti Qu's group at the Department of Mechanical Engineering, Tsinghua University. His researches mainly focus on graphene-based materials and applications on energy conversion/storage devices.

Huhu Cheng

Huhu Cheng received a Ph.D. in Chemistry from Beijing Institute of Technology. He is an Assistant Professor in the Department of Mechanical Engineering, Tsinghua University. His research interests include the modification and processing of carbon-based materials and construction of energy conversion devices.

Liangti Qu

Liangti Qu received a Ph.D. in Chemistry from Tsinghua University (Beijing, China) in 2004. His research interests in materials chemistry mainly focus on the synthesis, functionalization and application of nanomaterials with carbon–carbon conjugated structures, including carbon nanotubes, graphene and conducting polymers.

---

1. Introduction

The exploration of green and clean energy resources to produce electricity is regarded as a promising solution for the growing energy demand and increasingly severe pollution.¹ As a natural and useful resource, water is abundant and widely accessible in glaciers, rivers, lakes, oceans, and soils as well as in living things that constantly exchange energy with moist air by natural evaporation/condensation, transpiration or breathing processes (Fig. 1).² Besides, the water flow and moisture diffusion also act as huge energy carriers on Earth. Centuries ago, antiquated facilities (e.g. large dams and water wheels) were built by mankind to convert the kinetic energy of water flow into mechanical energy for agriculture and simple social production.^3,4 In modern times, many hydroelectric power engineering projects have converted energy from water flow into electricity of about 4.75 × 10¹¹ W in 2018 according to the BP statistical review of world energy.⁵ However, this is only a small portion of the tremendous amount of energy that is carried by water (10¹⁵ W) on the Earth every year.⁶

Recently, with the development of advanced nanomaterials, moist-electric generation (MEG) systems have been developed for electricity generation by the interactions of ubiquitous moisture and nanomaterials with ionized groups, providing new ways to harvest energy from water.^7–9 In MEG, water molecules in moisture air can be spontaneously absorbed by functional materials that will lead to the release and migration of positively/negatively charged ions from functional groups and through materials, respectively, inducing considerable electricity in external circuits.¹⁰ Unlike traditional hydroelectric technology and liquid water movement triggered electricity generation process,^11,12 MEG can produce electricity from ubiquitous air, avoiding the physical contact between water and materials that could induce the damage of materials as well as the geographic limitations for practical applications.¹³ In addition, gaseous water molecules in moisture always are in a higher energy state because liquid ice or solid water absorbs energy to transform into the gaseous state. Thus, moisture could supply much more power if exploited deeply. Related research has led to an upsurge in studies on advanced nanomaterials and MEG that can deliver a high level of electrical output of up to 1.5 V and are able to power electronic components by scale assembly. However, so far, there is no report on a systematic and timely discussion on the development of functional materials for MEG. In this minireview, we have briefly summarized the recent progress in advanced functional materials for MEG, including graphene and other carbon-based materials, polymers and metallic oxides, with an emphasis on their working mechanisms and practical applications. The challenges and potential trends in the development of MEG and related functional materials are also discussed.

2. Basic mechanisms

Similar to the photovoltaic effect that generates voltage or electric current in photovoltaic cells (PV cell) when exposed to sunlight,¹⁴ the basic mechanism of MEG could be described as a moisture-voltaic effect that generates voltage or electric current in nanomaterials upon exposure to moisture (Fig. 2). In the electricity generation process, materials bearing hydrophilic functional groups (e.g., –OH, –COOH, and –SO₃H) initially absorb water molecules (Fig. 2a).^15–17 Second, the aggregation of water molecules in materials leads to the dissociation of functional groups, which weakens and breaks the polar chemical bonds, releasing numerous positively and/or negatively charged ions, and accordingly resulting in a concentration difference in the specific region (Fig. 2b).^10,18 Third, electric potential occurs as a result of separated positively and negatively charged ions (Fig. 2c). The positively or negatively charged functional groups can be immobilized because of their large volume or by bonding with skeletons in materials, while the counter charged ions can freely move as carriers.^19,20 The mobile carriers transport from the high concentration region to the low concentration region, thus inducing charge separation in functional materials and producing voltage or an electric current.²¹ Finally, the chemical potential energy (ΔE) from water molecule absorption on materials is converted into electric potential power (Fig. 3).²²

The key factor is to construct a concentration difference of charged ions in functional materials that will apply a force for the directional diffusion of movable charged ions, which can be achieved via two strategies (Fig. 3). (1) The asymmetric moisturizing of functional materials can form high and low concentrations of charged ions on the two sides of a material. A high concentration of charged ions will be introduced on the side of material that contacts with high relative humidity (RH) moisture. On the other side of the functional materials, a low RH will form a lower concentration of charged ions.^9,13,16,23 (2) The regulation of chemical composition in functional materials can directly influence the inner ion concentration difference after absorbing moisture. A high concentration of charged ions will be formed on the side of materials with a high functional group content, while the side with low functional group content will have a low concentration of charged ions.^7,8,15,24

3. Graphene-based MEG

Known as an atom-thick material,²⁵ graphene has attracted widespread attention in energy conversion due to its high charge carrier mobility,²⁶ excellent tensile strength,²⁷ huge specific surface area,²² and superior thermal and electrical conductivities.²⁸

More importantly, the easily regulated functional groups and the abundant macroscopic assemblies of graphene provide favorable advantages for high-performance and diversified MEG systems as below.

3.1 Two-dimensional (2D) graphene-based film for MEG

For the sufficient absorption of water molecules from moisture in the electricity generation of MEG, the utilized graphene-based materials should first feature hydrophilic functional groups.²⁹ Considering the presence of numerous oxygen-containing functional groups (e.g., –OH and –COOH), graphene oxide (GO) film has been widely employed for electricity generation in MEG.³⁰ For example, Xu et al. reported a single and homogeneous GO film-based MEG material (Fig. 4a), which can generate an open-circuit voltage (V_oc) of 0.4–0.7 V (Fig. 4b) in an external circuit after treatment by asymmetric moisturizing.⁹ The proposed process is shown in Fig. 4c. With its abundant oxygen-containing functional groups, GO on the top side can actively absorb water molecules and release substantial number of free H⁺ ions as charge carriers under the dissociation effect. As water molecules gradually penetrate the GO film from the top to the bottom, an H⁺ ion concentration difference is formed, and the free H⁺ ions move along the direction of the diffused water molecules, while the negatively charged immobile groups (e.g., –COO⁻) remain anchored on the GO backbone. As a result, positive and negative charges separate in the GO film, inducing electric voltage and current in an external circuit. The electricity generation process correlated well with the directional movement of the spontaneously released H⁺ ions within the GO film. The number of mobile H⁺ ions decreased with water molecule desorption under moisture evacuation, leading to the recombination of free H⁺ ions with negatively charged groups and a decrease in the generating potential (Fig. 4c). This research showed that homogeneous GO film MEG can generate a maximum V_oc of 0.7 V and an output power of 180 mW m⁻² when exposed to moisture with an RH of about 100%. Subsequently, a screen-printing method was developed to achieve the large-scale production of GO film MEGs on a piece of flexible paper. The sizes and numbers of these printed GO film MEGs can be modulated for practical requirements. The output voltage was increased to 2 V by simply connecting GO film MEGs in series, which is sufficient to run an electrical calculator. In this GO film MEG, the direction of moisture diffusion is an important factor that determines the construction of ion concentration differences and final electrical performance.¹³

In 2015, our group regulated the distribution of functional groups in a GO film by a moisture-electric annealing (MeA) method, forming a gradient oxygen-group distributed the GO film (g-GOF, Fig. 5a).⁷ This as-prepared g-GOF when used could harvest energy by moisture absorption and produce a V_oc and current outputs of 35 mV and 10 μA cm⁻² respectively, with a change in RH (ΔRH = 55%), without the directional limitation of moisture diffusion (Fig. 5b and c). The oxygen-containing functional groups gradually increased in number from the top side to the bottom side of g-GOF. When exposed to moisture, the water absorption content in g-GOF also gradually increased, and thus the generated H⁺ ions developed a concentration gradient, resulting in H⁺ ions spontaneously migrating from the region of high to the region of low concentration. This g-GOF material was so sensitive to the RH changes that even human breathing (ΔRH = 21%) on g-GOF could be used to produce the V_oc and current outputs of ≈18 mV and ≈5.7 μA cm⁻² (Fig. 5d and e), respectively, which could serve as a self-powered monitor of respiratory frequency relating to the human heart rate.⁷

In GO or g-GOF films, the GO sheets stack together so that the transport of H⁺ ions must overcome the interlaminar barrier between sheets during electricity generation. Combining the MeA method and direct laser writing technology, we further regulated the functional group distribution to increase along the horizontal direction of the GO sheet, forming an in-plane GO film MEG (IPMEG, Fig. 6a).¹⁷ In the electricity generation process, the lamellar structure of the GO sheets provides unobstructed H⁺ channels along the horizontal direction, resulting in an appreciable electrical output (70 mV, 12 mA cm⁻²) in external circuits (Fig. 6b and c), which had been used to drive self-powered touchless devices (e.g., finger position annunciator, touchless switches and a handwriting panel, Fig. 6d and e). In addition, the planar configuration of IPMEG renders it highly flexible that enable MEGs with diversified functions. For example, we have designed MEGs with sophisticated architectures (rollable, stretchable, and even multidimensional transformation) by a versatile laser processing strategy.¹⁰ These MEGs could assemble and transform their geometries under highly deformative conditions, dramatically increasing the potential applications in the field of complex power supply systems.¹⁷

3.2 3D graphene-based foam for MEG

The porous structure of a 3D graphene-based foam can strongly facilitate the diffusion of water molecules and the dissociation of free H⁺ ions to produce higher electrical power.^31,32 By the freeze-drying and MeA treatments, the oxygen-containing group gradient 3D GO foam (g-3D-GO) was fabricated (Fig. 7a), in which free H⁺ ions released and distributed along a gradient after the foam absorbed moisture.⁸ Because of its very large specific surface area and porous interconnected skeletal structure,^33–35 g-3D-GO exhibited faster moisture permeation and accelerated ionic transfer for remarkable water molecules bound by capillarity action. Therefore, g-3D-GO MEG generated a V_oc of 0.26 V and a current density of 3.2 mA cm⁻² (Fig. 7b). Four series-connected g-3D-GO MEGs could light up a commercial light-emitting diode (LED, Fig. 7c).⁸ In another study, primary 3D GO was created using directionally controlled laser irradiation to engineer the integration of gradient-reduced GO layer and unreduced GO layer (Fig. 7d), constructing a heterogeneous 3D GO foam (h-3D-GO).¹⁵ The additional GO layers to h-3D-GO could supply more movable ions compared with a single gradient-reduced GO. Moreover, sandwiching h-3D-GO between a pair of Ag and Au electrodes with a rational work function, a space charged zone formed at the Ag/h-GO interface that restrained electrons flowing from the Ag electrode to the GO layer and thus inhibited charge recombination (Fig. 7d). With synergetic management of the heterogeneous structure and mediation of the electrode/material interfaces, the output performance of a h-3D-GO MEG was increased to the high value of 1.5 V (Fig. 7e). Besides, a pocket of ten h-3D-GO MEGs can generate a voltage of 18 V and charge a capacitor to 2 V (10 μF) within 5000 s under moisture, and can power white LEDs and eight-bit digital display tubes (Fig. 7f).¹⁵

3.3 1D graphene-based fiber for MEG

GO fibers with highly oriented arrangements of GO sheets were successfully fabricated. The fiber sheets possessed many merits, including excellent flexibility, a simple production process and good compatibility with textiles, which could be used to explore fibrous MEG for the power supplies of next-generation wearable electronic devices.^36–38 Liang et al. reported a GO fiber of alternating reduced GO (rGO) electrodes and gradient GO active regions with highly oriented GO sheets arranged inside the 1D space by MeA and laser processes, in which the fiber-based structure facilitated deformation into any direction for wearable applications (Fig. 8a).²⁰ One GO fiber MEG (80 μm diameter, 1 mm length) had an output voltage of ca. 0.4 V under a moist environment, and the output could be enhanced to 1.3 V by increasing the number of units along the fiber (Fig. 8b). Moreover, 136 GO fiber MEG units have been well integrated into flexible textiles for self-powered information storage from human breath (Fig. 8c).²⁰ Shao et al. also prepared a fibrous MEG (f-MEG) with a core/shell structure.³⁹ As shown in Fig. 8d, f-MEG is composed of a core silver wire electrode, a homogeneous GO layer shell and another entwined silver wire electrode. Under moist conditions, H⁺ ions release at the outer side of the homogeneous GO layer due to the water absorption and dissociation effects. Driven by differences in H⁺ ion concentration between the outer and inner GO layers, the H⁺ ions moved spontaneously, thus producing an induced potential and free electron flow in the external circuit. Hence, this GO-based coaxial f-MEG can provide an output voltage of 0.3 V and current density of 0.7 μA cm⁻¹ with a power density of 0.21 μW cm⁻¹ (Fig. 8e). In addition, the high mechanical flexibility and compatibility with textiles of f-MEG facilitated easy integration with textiles of various shapes to create portable and wearable electronic devices (Fig. 8f and g). Through simple series/parallel connections, the energy output enabled the integrated supply to run a calculator and support its normal operations.³⁹

4. Other carbon-based materials for MEG

The significant role of asymmetric oxygen-containing groups in graphene oxide is noticeable in MEG devices. It is expected that other functionalized carbon-based materials can produced similar results.³ Liu et al. reported a porous carbon film (PCF) MEG with a higher content of oxygen-containing groups on one half and reduced content on the other half (Fig. 9).⁴⁰ This PCF-MEG exhibited an output voltage of 68 mV, stimulated by RH of >95%. Conversely, no signal was detected from a homogeneous device with the same content of oxygen-containing groups. Therefore, a working mechanism for PCF-MEG that is similar to that for the graphene-based MEG was verified by ab initio molecular dynamics calculations and grand canonical Monte Carlo simulations. The absorbed water not only facilitated H⁺ release to form a concentration difference but also provided a bridge for H⁺ transportation in the electricity generation process. In addition to PCF, induced potential can be found in other porous carbon materials, including carbon dots,⁴¹ cellulose nanofibrils,⁴² silk cocoon,⁴³ multi-walled carbon nanotube films, and acetylene–carbon nanoparticle films with a functional group concentration gradient.⁴⁰

5. Polymer-based materials for MEG

Many polymers (e.g., polyvinyl alcohol, polyacrylic acid, hydroxyethyl cellulose, guar gum and sodium alginate) exhibit substantial numbers of hydrophilic groups that are also employed absorb moisture for electricity generation. For example, a free-standing 1 × 1 cm⁻² membrane casted by poly (4-styrensulfonic acid) (PSSA) solution without any treatment could produce a V_oc of 0.8 V with a corresponding power density of 170 mW m⁻² under an RH of 80% (Fig. 10a and b).¹⁶ As a polyelectrolyte rich in sulfonated functional groups, PSSA can release substantial quantities of free dissociated H⁺ into the aqueous solution.⁴⁴ The released H⁺ ions penetrated the PSSA film along with moisture, and gradually accumulated on the opposite side from the moisture source.⁴⁵ Six PSSA-MEG units connected in series can light and LED bulb when placed over a water container (Fig. 10c). In addition, the flexible PSSA-MEG composed of carbon-based electrodes and PSSA could be easily integrated into a wearable mask to generate power from human breath.³⁹ Many other polymer films (e.g. polyvinyl alcohol (PVA), polyacrylic acid (PAA), hydroxyethyl cellulose, Nafion, and natural polysaccharides such as guar gum, and sodium alginate) also presented effective electrical output when exposed to moist air, under the moist-voltaic effect (Fig. 10d).^16,46 In another study, a gradient doping strategy was exploited to form a gradient concentration of Na⁺ and dodecyl benzene sulfonate (DBS) ions by doping longitudinally along the polypyrrole nanowire (GDNw).^47,48 In electricity generation process, Na⁺ ions diffused from the high-concentration side to the low-concentration side spontaneously in GDNw, which an induced potential of 72 mV with the assistance of immobilized DBS ions. The current density could be scaled up from 0.45 nA to 0.14 μA by the incorporation of a nanowire array in GDNw.^47,48

6. Metallic oxides for MEG

The surface of a metallic oxide is normally hydrophilic. In 2009, Galembeck et al. reported the surface potential variation of aluminum and chrome-plated brass along with RH changes.⁴⁹ However, it is hard to realize the electricity generation on aluminum and chrome-plated brass due to the difficulty of forming a concentration difference.^49,50 Shen et al. developed hydrophilic titanium dioxide (TiO₂) nanowire network films with porous nanochannels, which facilitated the diffusion and penetration by water from moisture (Fig. 11a).⁵¹ Under the impact of negative charges on TiO₂ surface,⁵² H⁺ ions in the water molecules are attracted toward the TiO₂ surface along with water diffusion in the interstitial nanochannels. When water molecules arrive at the narrowest channels, small H⁺ ions can pass deeper into the channel while negative ions are excluded by negative charges on the TiO₂ surface.⁵³ As a result, the positive and negative charges separated and the TiO₂ based MEG (TiO₂-MEG) produced a V_oc of up to 0.5 V with a power density of 4 μW cm⁻² (Fig. 11b). This TiO₂-MEG was also used as a self-powered moisture sensing touch panel and artificial skin for consumer electronics applications.⁵¹

7. Conclusions and perspectives

In conclusion, we have reviewed a novel MEG technology from its discovery to recent development in its basic principle, MEG-related functional materials and their promising performance for harvesting energy from ubiquitous moisture in air. The developed functional materials for MEG ranged from carbon materials, polymers, metallic oxides, and so on, which exhibited special properties such as large surface areas, abundant functional groups, hydrophilicity and rational porous structures. In a humid environment, water molecules are attracted by these materials, and the resulting ion migration creates a charge imbalance, thereby inducing a potential and producing current flow in an external circuit. The electricity generation process is explained by the moisture-voltaic effect in MEG, similar to the photovoltaic effect in a PV cell. Benefiting from the regulation and modification of functional materials, the generated electric voltage of MEG devices has increased from about 35 mV in 2015 to a considerable value of 1.5 V in only four years (Fig. 12a and Table 1). In 2015, GO film MEG had a power density of about 4.2 mW cm⁻², and this value has increased to about 170 mW cm⁻² on PSS-MEG. Moreover, large-scale assemblies of MEGs (Fig. 12b) have shown adequate output voltage (28 V) and power for driving commercial electronics devices, such as calculators,³⁹ light-emitting diodes,⁵¹ digital display tubes,¹⁵ and so on, which opened new opportunities for natural resource transformation and utilization.

---

Although great efforts have been made, many challenges or issues with MEG still need to be overcome at this current stage. (1) The electricity generation process involves water absorption, dissociation, diffusion, and charge separation steps in functional materials, which require further and deeper understanding to improve the working mechanisms and to provide guidance for the exploration of high-performance MEG. (2) There is plenty of scope for optimizing power density of MEG. For example, the utilization of new functional materials in MEG and the structure and characterstic regulation of materials for high water absorption and efficient charge separation. (3) For practical applications, the assembling techniques should be well designed and developed to realize the power requirements of many electronics devices, and it is also important to ensure that interior functional materials have extensive access to moisture in air during operation. We believe that with the multidisciplinary efforts from materials, electronics, chemistry and engineering science, MEG devices and related functional materials will have a rapid development in natural energy conversion and future applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support from the National Key R&D Program of China (2017YFB1104300, 2016YFA0200200), Tsinghua University Initiative Scientific Research Program (2019Z08QCX08), NSFC (No. 51673026, 51433005, 21805160), NSFC-MAECI (51861135202), Tsinghua-Foshan Innovation Special Fund (2018THFS0412).

References

1. J. Feng, M. Graf, K. Liu, D. Ovchinnikov, D. Dumcenco, M. Heiranian, V. Nandigana, N. R. Aluru, A. Kis and A. Radenovic, Nature, 2016, 536, 197 CrossRef CAS PubMed .
2. M. Wild, D. Folini, M. Z. Hakuba, C. Schär, S. I. Seneviratne, S. Kato, D. Rutan, C. Ammann, E. F. Wood and G. König-Langlo, Clim. Dyn., 2015, 44, 3393 CrossRef .
3. Y. Xu, P. Chen and H. Peng, Chem. – Eur. J., 2018, 24, 6287 CAS .
4. N. Nouri, F. Balali, A. Nasiri, H. Seifoddini and W. Otienob, Appl. Energy, 2015, 248, 196 CrossRef .
5. B. Dudley, BP Statistical Review of World Energy, London, UK, 2019 Search PubMed .
6. G. L. Stephens, J. Li, M. Wild, C. A. Clayson, N. Loeb, S. Kato, T. L'ecuyer, P. W. Stackhouse Jr., M. Lebsock and T. Andrews, Nat. Geosci., 2012, 5, 691 CrossRef CAS .
7. F. Zhao, H. Cheng, Z. Zhang, L. Jiang and L. Qu, Adv. Mater., 2015, 27, 4351 CrossRef CAS PubMed .
8. F. Zhao, Y. Liang, H. Cheng, L. Jiang and L. Qu, Energy Environ. Sci., 2016, 9, 912 RSC .
9. T. Xu, X. Ding, C. Shao, L. Song, T. Lin, X. Gao, J. Xue, Z. Zhang and L. Qu, Small, 2018, 14, 1704473 CrossRef PubMed .
10. C. Yang, Y. Huang, H. Cheng, L. Jiang and L. Qu, Adv. Mater., 2019, 31, 1805705 CrossRef PubMed .
11. B. Ji, N. Chen, C. Shao, Q. Liu, J. Gao, T. Xu, H. Cheng and L. Qu, J. Mater. Chem. A, 2019, 7, 6766 RSC .
12. J. Yin, X. Li, J. Yu, Z. Zhang, J. Zhou and W. Guo, Nat. Nanotechnol., 2014, 9, 378 CrossRef CAS PubMed .
13. Y. Liang, F. Zhao, Z. Cheng, Y. Deng, Y. Xiao, H. Cheng, P. Zhang, Y. Huang, H. Shao and L. Qu, Energy Environ. Sci., 2018, 11, 1730 RSC .
14. Y. Cui, H. Yao, J. Zhang, T. Zhang, Y. Wang, L. Hong, K. Xian, B. Xu, S. Zhang and J. Peng, Nat. Commun., 2019, 10, 2515 CrossRef PubMed .
15. Y. Huang, H. Cheng, C. Yang, P. Zhang, Q. Liao, H. Yao, G. Shi and L. Qu, Nat. Commun., 2018, 9, 4166 Search PubMed .
16. T. Xu, X. Ding, Y. Huang, C. Shao, L. Song, X. Gao, Z. Zhang and L. Qu, Energy Environ. Sci., 2019, 12, 972 CAS .
17. H. Cheng, Y. Huang, L. Qu, Q. Cheng, G. Shi and J. Lan, Nano Energy, 2017, 45, 37 CrossRef .
18. F. Zhao, L. Wang, Y. Zhao, L. Qu and L. Dai, Adv. Mater., 2017, 29, 1604972 CrossRef PubMed .
19. H. Cheng, F. Zhao, J. Xue, G. Shi, L. Jiang and L. Qu, ACS Nano, 2016, 10, 9529 CrossRef CAS PubMed .
20. Y. Liang, F. Zhao, Z. Cheng, Q. Zhou, H. Shao, L. Jiang and L. Qu, Nano Energy, 2017, 32, 329 CrossRef CAS .
21. Y. Huang, H. Cheng, G. Shi and L. Qu, ACS Appl. Mater. Interfaces, 2017, 9, 38170 CrossRef CAS PubMed .
22. T. Xu, Q. Han, Z. Cheng, J. Zhang and L. Qu, Small Methods, 2018, 2, 1800108 CrossRef .
23. Y. Huang, H. Cheng, C. Yang, H. Yao, C. Li and L. Qu, Energy Environ. Sci., 2019, 9, 4166 Search PubMed .
24. L. Qu, H. Cheng, Y. Huang, F. Zhao, C. Yang, P. Zhang, L. Jiang and G. Shi, Energy Environ. Sci., 2018, 11, 2839 RSC .
25. Q. Tang and P. Yang, J. Mater. Chem. A, 2016, 4, 9730 RSC .
26. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666 CrossRef CAS PubMed .
27. Y. Zhang, S. Gong, Q. Zhang, P. Ming, S. Wan, J. Peng, L. Jiang and Q. Cheng, Chem. Soc. Rev., 2016, 45, 2378 RSC .
28. A. A. Balandin, Nat. Mater., 2011, 10, 569 CrossRef CAS PubMed .
29. K. Hatakeyama, M. R. Karim, C. Ogata, H. Tateishi, A. Funatsu, T. Taniguchi, M. Koinuma, S. Hayami and Y. Matsumoto, Angew. Chem., Int. Ed., 2014, 53, 6997 CrossRef CAS PubMed .
30. H. Cheng, J. Liu, Y. Zhao, C. Hu, Z. Zhang, N. Chen, L. Jiang and L. Qu, Angew. Chem., Int. Ed., 2013, 52, 10482 CrossRef CAS PubMed .
31. H. Cheng, Y. Huang, G. Shi, L. Jiang and L. Qu, Acc. Chem. Res., 2017, 50, 1663 CrossRef CAS PubMed .
32. N. Yousefi, X. Lu, M. Elimelech and N. Tufenkji, Nat. Nanotechnol., 2019, 14, 107 CrossRef CAS PubMed .
33. X. Yu, H. Cheng, M. Zhang, Y. Zhao, L. Qu and G. Shi, Nat. Rev. Mater., 2017, 2, 17046 CrossRef CAS .
34. J. Mao, J. Iocozzia, J. Huang, K. Meng, Y. Lai and Z. Lin, Energy Environ. Sci., 2018, 11, 772 RSC .
35. H. Cheng, Y. Hu, F. Zhao, Z. Dong, Y. Wang, N. Chen, Z. Zhang and L. Qu, Adv. Mater., 2014, 26, 2909 CrossRef CAS PubMed .
36. Y. Cheng, X. Lu, K. H. Chan, R. Wang, Z. Cao, J. Sun and G. W. Ho, Nano Energy, 2017, 41, 511 CrossRef CAS .
37. H. Cheng, C. Hu, Y. Zhao and L. Qu, NPG Asia Mater., 2014, 6, e113 CrossRef CAS .
38. F. Meng, W. Lu, Q. Li, J. H. Byun, Y. Oh and T. W. Chou, Adv. Mater., 2015, 27, 5113 CrossRef CAS PubMed .
39. C. Shao, J. Gao, T. Xu, B. Ji, Y. Xiao, C. Gao, Y. Zhao and L. Qu, Nano Energy, 2018, 53, 698 CrossRef CAS .
40. K. Liu, P. Yang, S. Li, J. Li, T. Ding, G. Xue, Q. Chen, G. Feng and J. Zhou, Angew. Chem., Int. Ed., 2016, 55, 8003 CrossRef CAS PubMed .
41. Q. Li, Z. Ming, M. Yang, Q. Yang, W. Qian, Z. Zhang and J. Yu, J. Mater. Chem. A, 2018, 6, 10639 RSC .
42. M. Li, L. Zong, W. Yang, X. Li, J. You, X. Wu, Z. Li and C. Li, Adv. Funct. Mater., 2019, 1901798 CrossRef .
43. B. Tulachan, S. K. Meena, R. K. Rai, C. Mallick, T. S. Kusurkar, A. K. Teotia, N. K. Sethy, K. Bhargava, S. Bhattacharya and A. Kumar, Sci. Rep., 2014, 4, 5434 CrossRef CAS PubMed .
44. M. Rikukawa and K. Sanui, Prog. Polym. Sci., 2000, 25, 1463 CrossRef CAS .
45. K. D. Kreuer, S. J. Paddison, E. Spohr and M. Schuster, Chem. Rev., 2004, 104, 4637 CrossRef CAS PubMed .
46. Z. Luo, C. Liu and S. Fan, Nano Energy, 2019, 60, 371 CrossRef CAS .
47. J. Xue, F. Zhao, C. Hu, Y. Zhao, H. Luo, L. Dai and L. Qu, Adv. Funct. Mater., 2016, 26, 8784 CrossRef CAS .
48. X. Nie, B. Ji, N. Chen, Y. Liang, Q. Han and L. Qu, Nano Energy, 2018, 46, 297 CrossRef CAS .
49. T. R. Ducati, L. H. Simões and F. Galembeck, Langmuir, 2010, 26, 13763 CrossRef CAS PubMed .
50. R. F. Gouveia and F. Galembeck, J. Am. Chem. Soc., 2009, 131, 11381 CrossRef CAS PubMed .
51. D. Shen, M. Xiao, G. Zou, L. Liu, W. W. Duley and Y. N. Zhou, Adv. Mater., 2018, 30, 1705925 CrossRef PubMed .
52. M. Kosmulski and E. Matijević, Colloids Surf., 1992, 64, 57 CrossRef CAS .
53. S. Y. Tee, K. Y. Win, W. S. Teo, L. D. Koh, S. Liu, C. P. Teng and M. Y. Han, Adv. Sci., 2017, 4, 1600337 CrossRef PubMed .

This journal is © The Royal Society of Chemistry 2019