Photo‐rechargeable batteries and supercapacitors: Critical roles of carbon‐based functional materials

Abstract

As a clean and renewable energy source, solar energy is a competitive alternative to replace conventional fossil fuels. Nevertheless, its serious fluctuating nature usually leads to a poor alignment with the actual energy demand. To solve this problem, the direct solar‐to‐electrochemical energy conversion and storage have been regarded as a feasible strategy. In this context, the development of high‐performance integrated devices based on solar energy conversion parts (i.e., solar cells or photoelectrodes) and electrochemical energy storage units (i.e., rechargeable batteries or supercapacitors [SCs]) has become increasingly necessary and urgent, in which carbon and carbon‐based functional materials play a fundamental role in determining their energy conversion/storage performances. Herein, we summarize the latest progress on these integrated devices for solar electricity energy conversion and storage, with special emphasis on the critical role of carbon‐based functional materials. First, principles of integrated devices are introduced, especially roles of carbon‐based materials in these hybrid energy devices. Then, two major types of important integrated devices, including photovoltaic and photoelectrochemical‐rechargeable batteries or SCs, are discussed in detail. Finally, key challenges and opportunities in the future development are also discussed. By this review, we hope to pave an avenue toward the development of stable and efficient devices for solar energy conversion and storage.

INTRODUCTION

Richard Errett Smalley, the Nobel Prize winner in chemistry, said: “Energy is the single most important problem facing humanity today and energy is the largest enterprise on Earth.” This means that the energy source is one of the fundamental elements for human society and plays very critical roles in almost all fields of modern society, including industry, agriculture, transportation, and so on. With the rapid industrialization of society and expansion of the population, however, a huge amount of energy has been excessively consumed in the last decades. To make things even worse, the global energy demand is still rapidly increasing every year. Specifically, it is estimated that the global energy demand will grow by nearly 60% in the next decade and then double by 2050. ^1–3 Unfortunately, both the energy generation and the energy consumption in the world still strongly depend on fossil fuels, including coal, oil, and natural gas, which are nonrenewable and will eventually lead to an energy crisis. Besides, the combustion of fossil fuels also releases a serial of toxic/greenhouse gases (e.g., CO, CO₂, SO _x , NO _x , etc.), thus causing serious environmental problems, such as global warming, El Niño phenomenon, water body deterioration, and so forth. ^4,5

To achieve the goal of sustainable development, harvesting energy from renewable and clean sources has been regarded as the most promising solution for simultaneously tackling energy crisis and environmental pollutions. With the rapid development of energy and materials science, until now, an increasing number of sustainable energy sources have been intensively investigated, including wind energy, ⁶ solar energy, ^7–9 ocean energy, ¹⁰ biomass energy, ¹¹ geothermal energy, ¹² and hydropower (Figure 1A). ¹³

Figure 1. (A) Various clean and renewable energy sources. (B) Advantages and shortcomings of solar energy. (C) A schematic illustration of functions and applications of integrated devices

Among these sustainable energy sources, solar energy is a very competitive candidate, which can be regarded as a fundamental source of other types of renewable energy, to replace those conventional fossil fuels due to its multiple merits, including high abundance, environment friendliness, facile harvesting, and high accessibility. ^14–17 First, the irradiation energy of sunlight reaching the Earth's surface is ~5.4 × 10²¹ kWh per year, approximately equaling 5000 times of the global annual energy consumption, meaning that 2‱ portion of which would satisfy the annual demand of the human society. Second, solar energy has almost no emission during its consumption, which is highly desirable to protect the environment. Third, unlike traditional fossil fuels, solar energy is equally accessible around the globe, thus leading to a great convenience for the energy demand in remote areas (Figure 1B).

Despite these advantages, solar energy also exhibits intrinsic drawbacks, which significantly impede its large-scale and long-term application. ^18,19 On the contrary, solar energy shows a low energy density. The average irradiation power of solar energy on the Earth's surface is only 1 kW·m⁻², which is much lower than that of fossil fuels or many other sustainable energy sources (e.g., wind energy and geothermal energy). On the contrary, solar energy also suffers from high fluctuation. Specifically, affected by the weather conditions and the day–night transition, solar energy is highly intermittent and unstable (Figure 1B). Such unsatisfactory energy density and poor stability directly pose a significant challenge for the practical application of solar energy. In particular, this fluctuating nature usually leads to a poor match with the actual energy demand of society, which has been considered as the biggest stumbling block on the road toward commercial applications of solar energy. ^20–22 Therefore, to achieve a stable energy supply, searching feasible storage technologies of solar energy has become a research hotspot.

The direct storage of solar energy is very difficult and even unrealistic. Generally, solar energy is first converted to other energy forms and then stored. ^23–25 By far, there have been some prevailing methods for the conversion and storage of solar energy, such as solar-to-thermal, ^26,27 solar-to-biomass, ²⁸ solar-to-chemical, ^29–31 and solar-to-electrochemical energy conversion and storage. ^32–34 Among these strategies, the solar-to-electrochemical one shows a number of unique advantages. For instance, compared with the first two ones, the solar-to-electrochemical energy conversion and storage is a more facile and low-cost procedure for energy storage. Compared with the third technology (e.g., solar-driven hydrogen generation, CO₂ reduction, nitrogen fixation, etc.), ^35–38 the solar-to-electrochemical energy conversion and storage exhibits higher energy conversion efficiency, because it does not involve the formation of new chemical bonds. ^20,39,40 In addition, this solar-to-electrochemical route can also be easily integrated with various lately emerged devices, such as wearable electronic devices and equipments for the “Internet of Things.” ^41,42 Obviously, the development on solar-to-electrochemical energy conversion and storage has become one of the most potential technologies for the large-scale, long-term, and multipurpose applications of solar energy.

To realize the solar-to-electrochemical energy conversion and storage, integration of solar cells with electrochemical energy storage (EES) devices is a general strategy. ^43–45 Specifically, an integrated solar energy conversion and storage device includes two major parts: a solar cell as the energy harvesting unit and an EES device (e.g., a rechargeable battery or a supercapacitor [SC]) as the energy storage unit. In this hybrid device, the solar cell is responsible for harvesting solar energy and then converting it into electricity, which is then electrochemically stored in the EES unit. The combination of these steps is collaboratively named as the “photo-charging” process. Reversibly, the stored electrochemical energy can be further released to power other electrical appliances in the subsequent discharge process of the EES unit (Figure 1C).

Due to their efficiency and convenience of utilizing solar energy, these integrated solar energy conversion and storage devices (i.e., photo-rechargeable batteries and SCs) have shown great potentials in many rising realms. For example, miniaturized and flexible integrated devices can be combined with wearable/implanted sensors, thus realizing the self-powered and portable electronic devices. Furthermore, integrated devices can also be used in the equipment for the “Internet of Things,” such as unmanned electric vehicles (EVs).

From the materials' point of view, the introduction of novel materials with high performance into these integrated devices is of great significance to improving their performance. Among a large number of new materials, novel carbon-based materials can play an essential role in achieving efficient and stable hybrid solar energy systems. As one of the most earth-abundant elements, carbon and carbon-based materials own many outstanding merits, such as abundance, low cost, nontoxicity, high conductivity, and versatile allotropes, resulting in a significant application potential in the development road of integrated devices toward commercialization.

Specifically, the role of carbon materials and their analogs in integrated devices shows the following aspects. On the one hand, solar cells, as the solar energy harvest parts in the integrated devices, directly determine the final performance of the whole system. For them, introduction of appropriate carbon-based functional materials can greatly enhance their photoelectric properties, thus laying a solid foundation for high-performance hybrid devices. For example, the incorporation of carbon nanotubes (CNTs) into the photoelectrode (PE) materials of solar cells can greatly improve their photovoltaic conversion efficiency due to the reduced exciton recombination and the rapid transport of photogenerated electrons. ⁴⁶ Besides, graphene has been reported to be capable of suppressing the back-transport reaction of solar cells, thus leading to a high energy conversion efficiency. ⁴⁷

On the other hand, as mentioned above, the EES units (i.e., rechargeable batteries and SCs) in the integrated devices are responsible for electrochemically storing the electricity generated from solar energy and releasing it during subsequent discharge processes. Thus, for the integrated devices, EES units are equally important as the solar cells. Similar to solar cells, the carbon-based functional materials can also enhance the performance of rechargeable batteries and SCs. ^48–51 As is well known, carbon black can greatly increase the conductivity of electrode materials (e.g., Li₄Ti₅O₁₂, LiCoO₂, and LiFePO₄), thus bringing enhanced charging/discharging features to batteries. Carbon coatings on the surface of electrode materials can also enhance the cyclic stability of the EES units in integrated devices. As for CNTs and graphene, they can be directly used as the electrode active materials of batteries and SCs due to their high electronic conduction, large surface area, and adjustable electrochemical activity. ^3,52

Apart from the above functions, nanocarbon assemblies (e.g., CNT- and graphene-based fibers and membranes) can act as ideal flexible substrate materials for integrated devices to achieve flexibility, portability, and wearability. ^53,54 In brief, it is clear that carbon materials and their analogs play very critical roles in the configuration design and the performance improvement of integrated devices. ⁵⁵

By far, a series of original works have been reported on the design and fabrication of such integrated devices; meanwhile, a few review articles about this hotspot topic have also been published. However, to the best of authors' knowledge, there have not been any comprehensive reviews provided, which pay attention to the important role of carbon-based materials in the integrated devices. In fact, as mentioned above, carbon-based functional materials play very important roles in integrated devices from device design to performance enhancement. Considering this, we herein introduce elementary principles and recent research progresses of these advanced integrated devices, especially roles of carbon materials in these hybrid solar energy systems. First, the principles of the integrated devices will be introduced, including the integration modes and the processes of solar energy conversion and storage. Subsequently, two major types of integrated devices, including the photovoltaic (PV) and photoelectrochemical (PEC)-rechargeable batteries and SCs, will be categorized on the basis of their device configurations and summarized in detail. In particular, the essential roles of carbon-based functional components employed in these integrated devices will be highlighted. Lastly, we will conclude with the key challenges and opportunities for the future development of these state-of-the-art integrated devices for solar energy conversion and storage. We hope that this review can offer insights into this very promising field and shed light on its future progress.

PRINCIPLES OF PHOTO-RECHARGEABLE INTEGRATED DEVICES Integration modes between solar cells and EES units

The integration modes between solar cells and EES units in photo-rechargeable batteries and SCs can be classified into two main categories, as follows ⁴⁴ :

• (i)

  Type I: The solar cell and the EES unit are simply connected via an external circuit (Figure 2A). That is to say, the solar cell and its corresponding EES unit are relatively independent in this configuration. For the hybrid devices with this integration mode, the processes of solar energy conversion and storage follow two independent steps: solar energy is first harvested and converted into photocurrent (i.e., electric energy) by the solar cell, and then the electric energy is stored in the EES unit by charging it.

• (ii)

  Type II: The solar cell and the EES unit are integrated into one single device by means of some novel configurations, such as shared electrodes (Figure 2B). Different from the Type I, the Type II configuration successfully achieves a deep incorporation between the solar light conversion part and the EES unit, resulting in a simultaneous conversion and storage of solar energy without any intermediate processes.

Figure 2. A schematic illustration of integration modes of photo-charging hybrid devices: (A) Type I and (B) Type II. Photo-charging and discharging processes of (C) PV and (D) PEC-rechargeable integrated devices. EES, electrochemical energy storage; PEC, photoelectrochemical; PV, photovoltaic

For the aforesaid two modes, the Type I integration is the simplest connection method and can be applied to almost all kinds of solar cells and EES devices. However, the energy loss caused by the resistance of the external circuit may become the main bottleneck for achieving an efficient conversion and storage of solar energy. Besides, the long distance between the solar cell and the EES unit usually leads to large volumes of the integrated devices, thus reducing their volume energy density and portability.

To solve this problem, designing highly integrated hybrid devices has become more and more imperative for the development of new-generation photo-rechargeable batteries and SCs. Notably, emerging Type II integration mode exhibits a significant potential for high-performance integrated devices. Specifically, the shortened distance between the solar cell and the EES unit in this integration modes can bring a high space efficiency, as well as a volume energy density. ^20,44 Besides, many additional advantages, such as lightweight, portability, and flexibility, make such Type II integrated devices capable of being deployed for versatile applications for miniaturized, wearable/implantable, and self-powered devices. ^56,57

Solar conversion and storage in integrated devices

Solar cells in photo-rechargeable integrated devices are the key components, which are responsible for absorbing solar radiation energy and then converting it into electricity. Up to now, various solar cells have been successfully combined with EES units to construct state-of-the-art integrated devices, including silicon-based solar cells, ^58–60 organic solar cells (OSCs), ⁶¹ perovskite solar cells (PSCs), ⁶² dye-sensitized solar cells (DSSCs), ^63,64 and quantum dot (QD)/dye co-sensitized solar cells. ⁶⁵ Furthermore, these solar cells can be divided into two categories: PV and PEC ones. A main difference between PV and PEC cells lies in whether or not the redox reactions occur in solar energy conversion process. Specifically, PEC cells involve the redox reactions in their working processes, but PV cells do not. Silicon-based solar cells, OSCs, and PSCs belong to the PV cells, whereas DSSCs and QD/dye co-sensitized solar cells fall into the group of PEC cells.

According to the types of solar cells used in integrated devices, photo-rechargeable batteries and SCs can also be divided into two classes: PV and PEC-rechargeable devices. For the former one, the solar cells are usually PV cells, such as silicon-based solar cells, OSCs, and PSCs. For the latter one, however, the hybrid devices are commonly based on PEC cells, such as DSSCs and QD/dye co-sensitized solar cells. Besides, some single PEs, such as CdS and α-Fe₂O₃, also can act as the solar harvest and conversion parts. These PE-based integrated systems also belong to the PEC-rechargeable devices.

More importantly, the different types of solar cells in the integrated devices will lead to significant differences in their respective solar energy conversion and storage behaviors. For a PV-rechargeable battery or SC, photogenerated electrons and holes are produced due to the PV effect of semiconductors when the PV cell is irradiated by incident light. Subsequently, photogenerated electrons are collected by the photoanode of the PV cell and then further conducted to the anode of the EES unit. In the meantime, holes move to the cathode of EES unit through the counter electrode of PV cell. By this way, the PV-rechargeable integrated device completes its photo-charging process. Obviously, there are not any photo-induced redox reactions in this process. In the subsequent discharge process, the electrochemical energy stored in EES unit will be released in the form of electric current, and electrons will be conducted from the anode to cathode of EES unit (Figure 2C). ²⁰

In contrast, photo-induced redox reactions take place during the working processes of PEC-rechargeable integrated devices. Specifically, photosensitive materials in the photoanode of a PEC cell (e.g., dye molecules in DSSCs) are excited under solar light illumination. The excited photosensitive materials are unstable and they instantly inject the photogenerated electrons into the photoanode of the PEC cell. Afterward, the photogenerated electrons on the photoanode are further conducted to the anode of EES unit. The left holes will be reduced by the redox pairs of the electrolyte. Meanwhile, the cathode of the EES unit will release electrons, and then these electrons are conducted to the counter electrode of the PEC cell to neutralize the electrolyte. By this way, solar energy can be successfully converted into electrochemical energy and stored in the EES units. In the subsequent discharge process, the anode of EES unit will release electrons, and these electrons will come back to the cathode of the EES unit, thus generating a discharge current (Figure 2D). ²⁰

As mentioned above, carbon plays an essential role in controlling the actual performance of these integrated devices. In the following sections, we will introduce the recent progress on photo-rechargeable EES devices according to their different solar energy conversion manners (i.e., PV and PEC procedure), and the essential carbon-based functional materials employed in these devices will be paid special attention. Moreover, we will also provide a prospective for technology trends on these advanced hybrid systems for solar energy harvest, conversion, and storage.

PV-RECHARGEABLE INTEGRATED DEVICES

As mentioned above, the PV-rechargeable batteries and SCs are based on PV cells as the energy conversion units to harvest solar energy. So far, different types of PV cells have been utilized as the solar energy conversion units to construct hybrid systems for solar energy conversion and storage, including silicon-based solar cells, OSCs, and PSCs. In this section, we will review in detail the representative works in this field as well as the application of essential carbon-based materials in them.

Integrated devices with silicon-based solar cells

Silicon-based solar cells, which are mainly composed of silicon materials, are the early-developed solar cells and have been widely used in many fields, including satellites, space stations, street lamps, grid-connected PV power stations, and so on. According to the different crystalline structures of silicon materials, silicon-based solar cells can be classified into three main groups: crystalline silicon (c-Si) solar cells, ^66,67 polycrystalline silicon (poly-Si) solar cells, ^68,69 and amorphous silicon (a-Si) solar cells. ^70,71

In the early stage, the silicon-based solar cells were simply connected with rechargeable batteries through wires to achieve the photo-charging process (i.e., Type I integration). For instance, as early as mid-1990s, a commercial poly-Si solar cell module was parallelly connected with a lithium metal battery (LMB) with a capacity of 500 mAh and a constant resistance load to fabricate a PV-rechargeable battery. More importantly, this integrated device exhibited a good stability during 20 cycles, indicating the feasibility of photo-charging the batteries by silicon-based solar cells. ⁷² Later on, an a-Si solar cell module (Sanyo HIP-190BA3) was also integrated with a lithium-ion battery (LIB) pack to realize the PV charging ability. In this hybrid solar energy system, commercial graphite was utilized as the anode material of LIB for Li⁺ storage. Notably, a high overall energy conversion and storage efficiency (ŋ _overall) of 14.5% were achieved at the low photo-charging voltage (~3.3 V). ⁷³

For some practical applications (i.e., EVs), high-voltage batteries are more suitable than low-voltage ones due to their high energy densities. However, most silicon-based solar cells can only provide a low voltage output, which significantly limits the application of PV-rechargeable integrated devices. To solve this problem, a voltage transformer of direct current was utilized to boost the low voltage obtained from four a-Si solar cell arrays (every array containing 10 Sanyo HIP-190BA3 modules) to the high voltage (>300 V) for photo-charging a high-voltage nickel metal (NiMH) battery pack. ⁷⁴ This technology provided a potential for the application of PV-rechargeable integrated devices in EVs.

All of the above integrated devices are based on the Type I mode (i.e., wire connection), thus leading to bulky and heavy device configurations. To realize better miniaturization and portability, highly integrated devices based on Type II method have been designed and prepared. For example, a miniaturized c-Si solar cell was integrated with a printed solid-state LIB to fabricate a photo-rechargeable monolithic hybrid device, in which carbon black was used as the conductive additives in both Li₄Ti₅O₁₂ anode and LiCoO₂ cathode of LIB (Figure 3A). ⁷⁵ Under incident light illumination, this hybrid device showed a fast photo-charging process (<2 min), as well as ŋ _overall of ~7.61%. In the subsequent discharging process, a high current density of 28 C was generated. Besides, this novel c-Si/LIB device can be embedded into a smartcard to light a light-emitting diode (LED) lamp (Figure 3B) or to charge a mobile phone under sunlight illumination (Figure 3C). In brief, this important breakthrough opened an easy and scalable strategy to develop single-unit, PV-rechargeable integrated devices.

Figure 3. (A) Digital photographs of the crystalline silicon (c-Si) solar cell/lithium-ion battery (LIB) integrated device. Inset shows the backside of integrated device. (B) A digital photograph of embedding the integrated device into a smartcard. Inset shows the thickness of integrated device/smartcard. (C) A digital photograph of a mobile phone that is being charged by the c-Si solar cell/LIB integrated device under sunlight illumination. Reproduced with permission: Copyright 2017, The Royal Society of Chemistry.75 (D) A schematic illustration and wearable application demonstration of the integrated device of polycrystalline silicon solar cell/Mg-ion supercapacitor. (E) ŋoverall at different light intensities. (F) Cycling stability of this flexible integrated device. Reproduced with permission: Copyright 2019, Nature Publishing Group76

Apart from LIBs, SCs can also be integrated with silicon-based solar cells. For example, an Mg-ion asymmetric SC (MnO₂@C as the cathode, vanadium nitride [VN] nanowires as the anode, and MgSO₄-polyacrylamide gel as the electrolyte, respectively) was incorporated with a commercial poly-Si solar cell to design a PV-rechargeable integrated device (Figure 3D). ⁷⁶ In this system, carbon black was utilized as the conductive additive to greatly improve the conductivity of the MnO₂ active materials. This hybrid solar energy device of an a-Si solar cell/Mg-ion SC exhibited an excellent ŋ _overall of 17.57% and a high photo-charging cycling stability (98.7% capacitance retention after 100 cycles; Figure 3E,F). In addition, this integrated device also showed an excellent flexibility, meaning that it was suitable for wearable self-powered devices.

Integrated devices with OSCs

Recently, OSCs have attracted an increasing amount of attention due to their unique advantages, including lightweight, tunable optical and electronic characteristics, low cost, and so forth. ^77–79  More importantly, compared with other PV cells (i.e., silicon-based solar cells and PSCs), OSCs are easier to be fabricated into flexible substrates by various wet-coating processes. ^20,80,81 Consequently, OSCs have been integrated with various EES units to develop flexible and wearable integrated devices.

For instance, an OSC and an LIB were directly integrated to produce a wearable PV-rechargeable integrated device. ⁸² Specifically, the flexible LIB was composed of an Li₄Ti₅O₁₂ anode and an LiFePO₄/conductive carbon cathode; meanwhile, an Ni-coated polyester yarn was used as the charge collector and the stress releaser, and a polyurethane binder acted as the adhesive and the separator, respectively. As a result, this LIB delivered a satisfactory flexibility and was embedded in clothes or watch band (Figure 4A). Furthermore, it was combined with a flexible and lightweight OSC with an energy conversion efficiency of 5.49% for achieving a PV charging ability (Figure 4B). The stored electrochemical energy in the LIB was able to be released for powering nine LED lamps during the discharging process (Figure 4C). Notably, this wearable integrated device showed a good durability for repeated folding/unfolding cycles (Figure 4D).

Figure 4. (A) A digital photograph and structure diagram of the textile lithium-ion battery (LIB) embedded in clothes and watch band. (B) A schematic diagram and digital photograph of the textile LIB integrated with an organic solar cell. (C) This hybrid device is capable of lighting up nine light-emitting diode (LED) bulbs during its discharging process. (D) Potential–time curves of the textile battery during photo-charging in the presence and absence of the repeated folding/unfolding cycles. Reproduced with permission: Copyright 2013, American Chemical Society82

Except for LIBs, SCs were also integrated with OSCs to construct the hybrid solar conversion and storage system. For example, a laser-scribed graphene SC was integrated with a silicon nanowire array-modified OSC consisting of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) to assemble a PV-rechargeable power pack (Figure 5A). ⁸³ This hybrid device exhibited a maximum photo-charging voltage of 0.5 V under the simulated sunlight irradiation. Subsequently, it was able to discharge at different current densities (3.84, 5.48, and 10.96 A·g⁻¹) under a dark condition (Figure 5B). Furthermore, six such power packs in series were capable to light up an LED lamp, indicating its feasibility for energy-saving lighting applications (Figure 5C).

Figure 5. (A) A structural illustration (left) and a cross-sectional SEM image (right) of an OSC/SC power pack. (B) Photo-charging under simulated sunlight illumination and discharging in the dark at fixed current densities of 3.84, 5.48, and 10.96 A·g−1, respectively. (C) A digital photograph of six power packs connected in series to power an LED. Reproduced with permission: Copyright 2018, American Chemical Society.83 (D) A structural illustration of the all-solid-state, coaxial, and self-charged “energy fiber” based on OSC/SC. (E) Circuit diagrams of the fibrous integrated hybrid device at photo-charging and discharging processes. (F) Photo-charging and discharging curve. The current during discharging process is 0.1 μA. Reproduced with permission: Copyright 2014, Wiley84 CHI, Shanghai Chenhua electrochemical analyzer; ES, energy storage; LED, light-emitting diode; MWCNT, multiwalled carbon nanotube; OSC, organic solar cell; P3HT, poly(3-hexylthiophene); PC, photovoltaic conversion; PCBM, phenyl-C61-butyric acid methyl ester; PEDOT:PSS, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate; PET, polyethylene terephthalate; PVA, polyvinyl alcohol; SC, supercapacitor; SEM, scanning electron microscopy

The aforesaid OSC-based and PV-rechargeable devices usually have a planar configuration. With the rapid development of multifunctional textiles, however, there have been stronger demands for fiber-shaped devices. Consequently, great efforts have been made to obtain the high-performance, fiber-shaped, and integrated devices for solar energy conversion and storage. In this respect, OSCs show their unique advantages for achieving such fibrous configurations. For instance, an all-solid-state and integrated fiber was designed and fabricated based on OSC/SC. ⁸⁴ This fibrous hybrid device was composed of a TiO₂ nanotube arrays (TNAs)-modified Ti wire as a core electrode and an aligned multiwalled carbon nanotube (MWCNT) sheet as a shell electrode, respectively (Figure 5D,E). It is worth mentioning that the flexible and transparent MWCNT sheet plays a key role in allowing the incident light transmission to the photoanode and charge storage in the hybrid device. As a result, this fiber-shaped hybrid OSC/SC device successfully realized a photo-charging/discharging capability (Figure 5F), and its ŋ _overall achieved 0.82%. Despite the relatively low efficiency, this novel “energy fiber” shows a great potential for application in smart textiles.

Integrated devices with PSCs

To fabricate high-performance integrated devices, it is critical to increase the photoelectric conversion efficiency of the solar cells. PSCs exhibit not only excellent energy conversion efficiency but also high voltage output, which makes them one of the most studied new-generation solar cells. ^85–87 Furthermore, coupling PSCs and various rechargeable battery chemistries and SCs can provide a broad space for the development of efficient and stable integrated devices. ^22,88

Up to now, many efforts have been devoted to the design and manufacture of the integrated devices using PSCs as the solar energy conversion units. A representative work in this field was reported in 2015. ⁸⁹ In this study, a pack containing four serial-connected CH₃NH₃PbI₃-based PSCs was utilized as the solar energy conversion unit, whereas an LIB composed of an Li₄Ti₅O₁₂ anode and an LiFePO₄ cathode was responsible for EES. In this device, acetylene black was utilized as conductive additives and blended with two above electrode materials to enhance their electronic conductivity. In the photo-charging process, the photogenerated electrons and holes in the PSCs were migrated to the LIB to carry on oxidation reactions in anode and reduction reactions in the cathode, respectively. In the discharging process, the electrochemical energy stored in the LIB was released by switching off S1 and switching on S2 (Figure 6A). More importantly, this hybrid energy system of PSC/LIB delivered a good cycling stability, a high ŋ _overall of 7.80%, and an excellent energy storage efficiency of ~60%, respectively (Figure 6B–D).

Figure 6. (A) A structural and operation illustration of the perovskite solar cell/lithium-ion battery (PSC/LIB) integrated device. (B) Voltage–time curves of the PSC/LIB integrated device. (C) ŋoverall and (D) energy storage efficiency as a function of the cycle number for the PSC/LIB integrated device. ŋ2 in (C) and ŋ3 in (D) refer to ŋoverall of integrated device and energy storage efficiency of LIB, respectively. Reproduced with permission: Copyright 2015, Nature Publishing Group.89 (E) A schematic diagram of the PSC/Li–S hybrid energy device. (F) Discharge capacity and ŋoverall of different cut-off photo-charging voltages. Reproduced with permission: Copyright 2019, Wiley90

Superior to the frequently used LIBs, lithium–sulfur (Li–S) batteries show higher theoretical gravimetric and volumetric energy densities. ^91–94 Consequently, a combination of Li–S batteries and PSCs can bring a great performance enhancement for the hybrid devices. In this context, a solid-state PSC was integrated with an Li–S battery using a shared carbon-based electrode, which was constituted by a composite film containing consecutive layers of carbon paste as the counter electrode of the PSC, S-loaded CNTs as the cathode of the Li–S battery, and the carbon paper as the bridge to electrically connect two parts (Figure 6E). ⁹⁰ Interestingly, at the beginning of the photo-charging process, the gradually increased photocurrent generated from PSC matched well with the sluggish dynamics of the redox reactions of the Li–S battery, thus leading to the more effective charge storage. As a result, the ŋ _overall and specific capacity reached high values of 5.14% and 762.4 mAh·g⁻¹, respectively (Figure 6F).

Another kind of EES units, SCs, have also been combined with PSCs to prepare PSC-based integrated devices. For instance, a PV-rechargeable power pack combining a CH₃NH₃PbI₃-based PSC and a polypyrrole-based SC was reported in 2015 (Figure 7A,B). ⁹⁵ In 2017, an MnO₂-based asymmetric solid-state SC and a PSC with a carbon counter electrode were also integrated by a common carbon electrode (Figure 7C,D). ⁹⁶ The ŋ _overall reported in the former achieved 10%, whereas the value was 5.26% in the latter research.

Figure 7. (A) A digital photograph and (B) structural illustration of an integrated device composed of four CH3NH3PbI3-based PSCs and a PPC. Reproduced with permission: Copyright 2015, American Chemical Society.95 (C) A schematic and structural illustration of a PSC/SC integrated device with carbon electrodes connected in parallel (left) and a cross-sectional SEM image of the integrated device (right). Inset is the close-up of the PSC part. (D) A schematic and structural illustration of this integrated device connected in series. Reproduced with permission: Copyright 2017, American Chemical Society.96 (E) A schematic illustration of a flexible PSC/LIC/sensor system. (F) Voltage–time profiles and (G) ŋoverall and energy storage efficiency as a function of the cycle number of PSC/LIC integrated device. (H) A digital photograph of the flexible PSC/LIC hybrid device being directly attached to the clothing (upper), and a concept diagram of this integrated device powering for a wearable sensor. Reproduced with permission: Copyright 2019, Elsevier Science B. V.97 FTO, fluorine-doped tin oxide; LIC, lithium-ion hybrid capacitor; LTO, lithium–titanate oxide; PPC, polypyrrole-based supercapacitor; PSC, perovskite solar cell; SC, supercapacitor; SEM, scanning electron microscopy

Compared with the conventional SCs, the lithium-ion hybrid capacitors (LICs), which are constituted by a typical LIB anode and a capacitive cathode, exhibit the combined advantage of high energy density and good cycling stability, thus providing a great potential as high-performance EES. ^98–101 The development of PSC/LIC integrated devices has been considered as one of the most feasible strategies toward efficient PSC-based solar conversion and storage. Very recently, a pioneering study on PSC/LIC integrated device was carried out in 2019, in which Li₄Ti₅O₁₂/reduced graphene oxide (rGO) and activated carbon were used as the anode and the cathode of LIC, respectively. ⁹⁷ Notably, the addition of rGO into the anode can significantly improve the rate capability and cycling stability of LIC. As a result, the PV-rechargeable hybrid power pack achieved continuous and stable photo-charging and discharging processes, and delivered a ŋ _overall of 8.41% and a charge storage efficiency of ~80%, respectively (Figure 7E–G). More importantly, this PSC/LIC integrated device showed high flexibility, and it was even able to be attached to clothing or embedded in a wearable strain sensor to realize the self-powered and portable features (Figure 7H).

In this section, we have introduced the PV-rechargeable integrated devices in detail. In such devices, the PV units include silicon-based solar cells, OSCs, and PSCs, whereas the EES units involve LIBs, SCs, LICs, and so on. ^102–106 To better illustrate the features of these devices, a summary of the PV-rechargeable batteries and SCs is listed in Table 1. As shown, the integrated devices with the silicon-based solar cell usually exhibit a high ŋ _overall, which is caused by the outstanding photoelectric conversion efficiency of silicon-based solar cells. For the OSC/EES unit hybrid energy systems, they seem more suitable for flexible and wearable applications. As for the PSC-based integrated devices, they own a high potential to be used for developing high-performance PV-rechargeable batteries and SCs. More importantly, carbon materials play critical roles in these PV-rechargeable integrated devices, such as conductive additives (carbon black and pyrocarbon), counter electrodes of solar cells (carbon film), electrode materials of batteries and SCs (graphite, graphene, and CNTs), and shared electrodes of integrated devices (carbon paper).

Table 1 Summary of PV-rechargeable batteries and SCs

[Table omitted. See PDF]

Abbreviations: a-Si, amorphous silicon; c-Si, crystalline silicon; CNT, carbon nanotube; EES, electrochemical energy storage; LIB, lithium-ion battery; LMB, lithium metal battery; MWCNT, multiwalled carbon nanotube; OSC, organic solar cell; PEDOT/PSS, poly(3,4-ethylenedioxythiophene)/polystyrenesulfonate; poly-Si, polycrystalline silicon; PSC, perovskite solar cell; PV, photovoltaic; rGO, reduced graphene oxide; SC, supercapacitor; TNA, TiO₂ nanotube array; VN, vanadium nitride.

PEC-RECHARGEABLE INTEGRATED DEVICES

All of the photo-rechargeable integrated devices discussed in section 3 are based on PV cells (e.g., silicon-based solar cells, OSCs, and PSCs) as the solar energy harvesting units. Apart from them, there are many PEC-rechargeable integrated devices, in which the PEC cells are utilized to carry out the solar-to-electric energy conversion. Different from the PV-rechargeable hybrid systems, the PEC-based integrated devices usually involve PEC reactions during their photo-charging processes. The so-called PEC reactions refer to photo-induced redox reactions occurring on the surfaces of PEs, which have been widely used in energy and environment fields, such as DSSCs, ^107,108 QD-sensitized solar cells, ^109,110 solar fuel production, ^111,112 pollutant degradation, ¹¹³ and so forth. In this section, we will introduce the recent advances in the PEC-rechargeable integrated devices, especially the important roles of carbon and carbon-based functional materials.

Integrated devices with DSSCs or QD/dye co-sensitized solar cells

As a typical representative of PEC cells, DSSCs have been intensively studied due to their outstanding advantages of high efficiency, low cost, and facile fabrication. ^114,115 In particular, due to the similar architecture and electrochemical characteristics shared by the DSSCs and EES units, great efforts have been put into integrating them into one solar energy conversion and storage device. In these hybrid energy systems, DSSCs play the role of converting incident photons to photogenerated electrons and holes. Subsequently, charges are stored in EES units by different mechanisms: as the double-layer charges at the electrode/electrolyte interface, through the redox reactions of electrode materials, as redox couples in the electrolyte, or a combination of above-mentioned forms. ^20,44

In the early research, some immature technologies were applied to the integration of DSSCs with EES units. For example, a double-electrode system composed of BaTiO₃|Pt (Ce^4+/3+ and Fe^3+/2+ as redox pairs in electrolyte) was developed in 1982. Later on, a similar structure using n-Cd(Te, Se)|SnS as the electrodes and aqueous Cs₂S _x as the electrolyte was also investigated. Unfortunately, the low efficiency and poor stability of these systems seriously limited their practical applications. ^116–118

With the rapid development of Li-based secondary battery technology, integration of DSSCs with LIBs provides a feasible method for the DSSC-based integrated devices. For instance, a photo-rechargeable DSSC/LIB power pack was designed, which was based on a Ti sheet with double-sided TNAs as the shared electrode. Specifically, one side of the double-sided TNA/Ti sheet acted as the photoanode of the DSSC and the other side was used as the anode of the LIB; meanwhile, the cathode materials of LIB were LiCoO₂ with conductive carbon additives (Figure 8A). ¹¹⁹ During the photo-charging process, the photogenerated electrons moved into the conduction band of TiO₂ from dye molecules and then migrated through the double-sided TNA/Ti electrode to the LIB anode. At the same time, the de-lithiation reaction of the LiCoO₂ cathode released free electrons, which were transferred to the Pt-based counter electrode of DSSC to achieve the charge balance (Figure 8B). This novel DSSC/LIB integrated device exhibited a ŋ _overall of about 0.82%.

Figure 8. (A) Structure description and (B) principle illustration of a DSSC/LIB power pack based on a Ti sheet with double-sided TNAs as the shared electrode. Reproduced with permission: Copyright 2012, American Chemical Society.119 (C) Principle illustration of a DSSC/LIB integrated devices with three electrodes: a PE, an SE, and an auxiliary DE. (D) A TEM image of graphene-coated LiMn2O4 particles. (E) The stability of ŋoverall for DSSC/LIB integrated device under dim-light illumination (light source: a fluorescent lamp, Pin = 0.24 mW·cm−2). Reproduced with permission: Copyright 2020, The Royal Society of Chemistry.120 DE, discharge electrode; DSSC, dye-sensitized solar cell; LIB, lithium-ion battery; PE, photoelectrode; SE, storage electrode; TEM, transmission electron microscopy; TNA, TiO2 nanotube array

Very recently, a DSSC/LIB integrated device was designed and fabricated to harvest the energy of indoor light (i.e., dim light) and then convert and store it in an EES unit. ¹²⁰ This hybrid energy system consisted of three electrodes: a dye-sensitized TiO₂ mesoscopic film as the PE, a film composed of LiMn₂O₄ particles wrapped with few-layered graphene as the storage electrode (SE), and an auxiliary discharge electrode made by depositing Pt on the PE side of the Li⁺-conductive separator in a stripe pattern, respectively (Figure 8C,D). In particular, in the SE, the few-layered graphene can significantly improve the reversibility of the active materials (i.e., LiMn₂O₄). Unlike most situations, this novel DSSC/LIB integrated device delivered a unique and excellent photo-charging performance under dim-light illumination. Specifically, when the thermodynamically favorable, but kinetically slow, Cu⁺/²⁺ (dmp)₂ (dmp is referred as 2,9-dimethyl-1,10-phenanthroline) was utilized as the mediator of the integrated device, a high ŋ _overall of 11.5% under the dim-light illumination (0.24 mW·cm⁻²) was achieved, because the light harvesting capacity was not limited by the kinetics (Figure 8E). The work opened a window for realizing indoor light conversion and storage, which is essential for energy-saving buildings.

Integration of DSSC with SC is also a feasible way for building DSSC-based hybrid systems toward the simultaneous solar conversion and storage. For example, a DSSC/SC integrated device was developed using free-standing and aligned MWCNT films as the shared electrode (Figure 9A,B). ¹²¹ Specifically, MWCNT films simultaneously played two important roles in this hybrid device: the counter electrode of DSSC and the active material of SC. Owning to the excellent electronic property of MWCNT films, the integrated device exhibited a ŋ _overall of ~5.12%. In addition, the new DSSC/SC integrated device showed a great flexibility, thus providing the feasibility for flexible and portable electronic applications. In another work, a dual-sided porous silicon wafer, which was modified by a few-layered graphene-like carbon coating, was utilized as the bifunctional electrode, working as both the top side for the counter electrode DSSC and the bottom side for the cathode of SC (Figure 9C). ¹²² In this device, the few-layered graphene-like carbon coating can greatly improve the stability of porous silicon, as well as enhance its electrochemical activity. As a result, this hybrid device delivered a photo-charging and subsequent discharging ability, and its ŋ _overall achieved 2.1% (Figure 9D).

Figure 9. (A) A digital photograph and (B) schematic illustration of a DSSC/SC integrated device based on aligned MWCNT films as the shared electrode. Inset in (A) is an SEM image of the aligned bare MWCNT film. Reproduced with permission: Copyright 2013, The Royal Society of Chemistry.121 (C) Structure diagram and (D) photo-charging and discharging potential versus time profile of a DSSC/SC integrated device using a dual-sided porous silicon wafer as shared electrode. Reproduced with permission: Copyright 2015, American Chemical Society.122 (E) A digital photograph and (F) schematic illustration of a hybrid device composed of a CdS QD/hb dye co-sensitized solar cell and a symmetric SC of PEDOP/MnO2. Reproduced with permission: Copyright 2018, American Chemical Society.65 DSSC, dye-sensitized solar cell; FTO, fluorine-doped tin oxide; hb, hibiscus; MWCNT, multiwalled carbon nanotube; PEDOP, poly(3,4-ethylenedioxypyrrole); QD, quantum dot; SC, supercapacitor; SEM, scanning electron microscopy

Despite the many advantages with DSSCs, the difficulty in the synthesis and the high cost of the commonly used Ru-based organic dyes (e.g., N719) significantly limit their further development. In addition, the serious photo-bleaching of organic dyes also compromises the durability of DSSCs. As an alternative to the organic dyes, the inorganic semiconductor nanocrystals, also known as QDs, also can be used as sensitizers to achieve solar light harvest and conversion. Furthermore, QDs not only can overcome the shortcomings of those Ru-based dyes but also show lots of unique superiorities, such as easily tunable band gap, higher extinction coefficient than dyes, and possibility of utilizing hot electrons. ^123–125 Therefore, addition of QDs into DSSCs to build QD/dye co-sensitized solar cells can significantly enhance their photoelectric conversion efficiency. ¹²⁶ To obtain an efficient integrated device, a CdS QD/hibiscus dye co-sensitized solar cell was used as the solar energy conversion unit to couple with a symmetric SC composed of poly(3,4-ethylenedioxypyrrole)/MnO₂ (Figure 9E,F). ⁶⁵ Due to its effective light absorption and accelerated exciton separation, this novel photo-rechargeable hybrid solar energy system exhibited a high power conversion efficiencies of 6.11%.

Integrated devices with semiconductor PEs and catalysts

Compared with the integrated devices constituted by solar cells and EES units, coupling of a single semiconductor PE with an EES unit can result in a more stable photo-charging process due to avoidance of the fluctuations of electricity output from solar cell. ²⁰ Recently, semiconductor-based PEs have been combined with rechargeable redox flow batteries (RFBs) with a series of merits, including high reliability, long cycling life, and low maintenance cost. ^127,128 For example, a CdS photoanode was used to optically charge a V-based RFB. The photovoltage generated by CdS photoanode was enough for charging the RFB up to 75% with no external bias (Figure 10A). ¹²⁹ Later on, a hematite (i.e., α-Fe₂O₃) photoanode was also integrated with an RFB (Figure 10B). ¹³⁰ Notably, the hematite photoanode was connected in series with the DSSC and provided a photovoltage of ~1.6 V, which was sufficient to fully charge the RFB. In these two systems, carbon felts were utilized as the current collector of RFBs due to their good mechanical strength, large specific area, high conductivity, excellent chemical stability, and full contact with electrolytes.

Figure 10. (A) A structural diagram of a CdS photoanode/V-based redox flow battery (RFB) integrated device. Reproduced with permission: Copyright 2016, Elsevier Science B. V.129 (B) A schematic illustration of a hematite photoanode and a dye-sensitized solar cell (DSSC) for unbiased photoelectrochemical-rechargeable RFB. Reproduced with permission: Copyright 2019, Elsevier Science B. V.130 (C) A schematic illustration of photo-charging and discharging redox reactions for an Li–S battery with Pt/CdS photocatalysts. (D) The initial discharge curve and re-discharge curve after 2 h photocharging of the Li–S battery with Pt/CdS photocatalysts. Middle photographs show changes in the color of the catholyte before (left) and after (right) 2-h irradiation. Reproduced with permission: Copyright 2015, Wiley131

As discussed in section 3.3, Li–S batteries are ideal EES units for integrated devices due to their higher specific capacity than LIBs. Recently, Pt-modified CdS nanoparticles were used as the photocatalysts and added into Li–S battery system for obtaining the PEC charging ability of the Li–S battery (Figure 10C). This integrated device delivered a specific capacity of 792 mAh·g⁻¹ after a PEC charging process of 2 h (Figure 10D). Interestingly, an accompanied hydrogen generation (1.02 mmol g⁻¹·h⁻¹) was simultaneously carried out during the PEC charging process. Besides, even after a short photo-charging process (10 min), this PE/Li–S hybrid system still exhibited a specific capacity of 199 mAh·g⁻¹, located at the level of conventional LIBs, implying its development potential for rapid charging/discharging. ¹³¹

Fibrous PEC-rechargeable integrated devices

As is well known, fiber-shaped power supplies can be easily woven into clothes or packages to power portable and wearable electronic devices. ^132–134 In particular, the integrated energy wires can achieve the direct utilization of solar energy for smart textiles. In section 3.2, we have discussed a fibrous OSC/SC integrated device. In more cases, however, DSSCs were integrated with various EES units to construct integrated solar energy conversion and storage wires.

For example, an integrated power fiber was designed by incorporating a fibrous DSSC and a fibrous SC. ¹³⁵ Specifically, in this hybrid solar system, an N719 dye-sensitized TiO₂ mesoporous film on the surface of Ti wire was used as the photoanode of fibrous DSSC, whereas a polyaniline-coated stainless steel wire acted as the shared electrode of this integrated power wire: the counter electrode of DSSC and the charge SE of fibrous SC (Figure 11A). More importantly, this DSSC/SC hybrid fiber successfully delivered a photo-charging ability, and its ŋ _overall achieved 2.1% (Figure 11B). In another research, a Ti wire modified by different layers of vertically aligned TNAs and MWCNT was used as two electrodes to fabricate a fiber-shaped DSSC/SC integrated device (Figure 11C). ¹³⁶ For this smart “energy fiber,” the photoelectric conversion and charge storage efficiency were 2.73% and 75.7%, respectively, thus leading to a ŋ _overall of about 2.07%. Besides, a DSSC wire and a flexible fibrous ZnBr₂ battery (Pt wire or carbon fiber as cathodes) were combined for obtaining a PEC charging “energy wire.” The ŋ _overall of hybrid energy system reached a promising value of 3.4%. ¹³⁹

Figure 11. (A) A structural schematic and digital photograph of an integrated power fiber consisting of a fibrous DSSC and SC. (B) Photo-charging curve of the integrated device. Inset is the photograph of the integrated device lighting an LED. Reproduced with permission: Copyright 2013, The Royal Society of Chemistry.135 (C) A schematic illustration and digital photograph of an integrated device coaxially composed of a DSSC and an SC. Reproduced with permission: Copyright 2014, The Royal Society of Chemistry.136 (D) A schematic illustration of ternary hybrid wire of MoS2/TiO2/carbon fiber. (E) A schematic illustration and operational mechanism of a photo-powering integrated energy fiber constructed by a fibrous DSSC and a fibrous SC based on the fiber-shaped electrode of MoS2/TiO2/carbon fiber. Reproduced with permission: Copyright 2017, Wiley.137 (F) A schematic illustration of all-solid tailorable “energy textile” integrated by a DSSC and an SC. Reproduced with permission: Copyright 2016, American Chemical Society.138 CF, carbon fiber; CNT, carbon nanotube; DSSC, dye-sensitized solar cell; ES, energy storage; HER, hydrogen evolution reaction; LED, light-emitting diode; LIB, lithium-ion battery; PC, photovoltaic conversion

Recently, a coaxial fiber was designed, which was composed of MoS₂ nanosheets, TiO₂ nanoparticles, and carbon fiber. The good conductivity of carbon fiber and the high electrochemical activity of MoS₂ synergistically lead to the remarkable electrochemical and PEC performances of this ternary wire, which has versatile applications in energy conversion and storage fields, including fiber-shaped solar cells and EES units (e.g., SCs and LIBs) and flexible electrodes for hydrogen evolution reaction (Figure 11D). ¹³⁷ Moreover, the ternary wire was utilized as a fibrous electrode to fabricate a flexible fiber-shaped DSSC/SC device. Under simulated sunlight illumination, this solar-powering energy fiber showed a ŋ _overall of 1.8% (Figure 11E).

Besides, an all-solid-state and tailorable “energy textile” was developed using a ZnO-based DSSC as the solar harvesting part and a TiN-based symmetric SC as the energy storage part. It is worth noting that an ultrathin amorphous carbon shell was deposited on the surface of the TiN nanowires to enhance their cycling stability. As a result, the integrated textile for solar energy application could be fully charged to 1.2 V in 17 s by solar light illumination and fully discharged in 78 s at a discharge current density of 0.1 mA. More importantly, it exhibited an outstanding tailorability and knittability, resulting in a great potential to be used in functional textiles (Figure 11F). ¹³⁸

In this section, we have discussed the PEC-rechargeable integrated devices, including DSSC/LIB, DSSC/SC, co-sensitized solar cell/SC, and some semiconductor PE-based hybrid systems. The device configurations involve planar type, fibrous shape, and textile. A list of these integrated devices is presented for better understanding their principles and performances (Table 2). As shown, the QSSC-based integrated devices are the most widely studied PEC-rechargeable integrated devices. This could be attributed to two aspects. On the one hand, as the PEC cells have the longest development history, the fabrication technology of DSSCs is very mature, and their photoelectric conversion efficiencies are generally higher compared with other PEC cells, which provide a solid foundation for obtaining high-performance PEC-rechargeable integrated devices. On the other hand, the similar electrochemical behaviors of the DSSCs and EES units are also favorable for their integration. At present, the major development goal of QSSC-based integrated devices is to further improve their efficiencies. As for the integrated devices that are based on QD/dye co-sensitized solar cells or PEs, more fundamental research should be carried out to help us better understand their mechanisms and design the device structures.

Table 2 Summary of PEC-rechargeable batteries and SCs

[Table omitted. See PDF]

Abbreviations: AQDS, anthraquinone-2,7-disulfonate disodium; CNT, carbon nanotube; DSSC, dye-sensitized solar cell; LIB, lithium-ion battery; MWCNT, multiwalled carbon nanotube; PEC, photoelectrochemical; PEDOT, poly(3,4-ethylenedioxythiophene); QD, quantum dot; RFB, redox flow battery; SC, supercapacitor; TNA, TiO₂ nanotube array.

In addition, similar to those PV-rechargeable integrated devices, a variety of carbon-based materials also play key roles in PEC-rechargeable ones. For example, pyrocarbon can serve as the conductive additives to increase the conductivity of LIB electrodes, thus enhancing the ŋ _overall of integrated devices. Carbon felts can be used as the high-performance current collector for PE-charged RFBs. As for other nanoscale carbon function materials, such as graphene and CNTs, they can act as electrodes or modifiers for efficient and stable PEC integrated devices.

SUMMARY AND PERSPECTIVE

As a feasible strategy of solar-to-electrochemical energy conversion and storage, photo-rechargeable integrated devices consisting of solar cells/photoanodes and EES units have attracted worldwide attention due to their outstanding advantages, including potential high efficiency, versatile applications, portability, and wearability. So far, many efforts have been devoted to designing and fabricating these advanced integrated devices for hybridizing solar energy harvest and storage processes.

In this review, we have summarized the latest progress in various photo-rechargeable integrated devices, including PV and PEC-rechargeable batteries and SCs. These important works have successfully realized the solar-to-electrochemical energy conversion and storage, and further brought miniaturized, portable, and wearable merits for integrated devices. In particular, carbon-based functional materials play very key roles in state-of-the-art hybrid solar energy systems, such as conductive additives, active materials, electrode stabilizer, flexible substrates, shared electrodes, and so on.

Despite many achievements, the research in this field is still in the infant stage. Most of the current works pay too much attention to the design and demonstration of the prototypes of integrated devices, whereas they ignore the efficiency improvement and practicability enhancement, which are the most important steps toward the real-world application.

To pave the way from the laboratory to commercialization, two aspects of endeavors should be simultaneously made to further improve the efficiency and enhance the practicability. Thus, we make the following six suggestions for the future development of photo-rechargeable integrated devices (Figure 12).

• (i)

  Device coupling: The low ŋ _overall of current integrated devices is mainly caused by parameter mismatching between solar energy conversion parts and EES units. For example, the photocurrents and the photovoltages generated by solar cells or PEs could be nonoptimal working conditions for EES units during the photo-charging process, resulting in the underutilization of EES performance. Therefore, performance coupling between solar energy parts and EES units should be the first priority in future research. Adjusting the outputs of solar cell/PEs by intelligent circuit designing could be a promising way for meeting the electrochemical kinetic process of EES units.

• (ii)

  Novel materials: For efficiency improvement, more novel materials should be introduced into integrated device systems. For example, single-atom catalysts (SACs) show an ultrahigh catalytic activity for various redox reactions. ^140,141 If SACs are introduced into DSSCs or LIBs, ŋ _overall of the whole hybrid system will be significantly enhanced due to optimized electrochemical reactions. Phosphorene, a promising 2D nanomaterial exceeding graphene, can bring a significant efficiency improvement for integrated devices due to the adjustable bandgap with layer numbers and the high electron mobility. ¹⁴² Metal–organic frameworks (MOFs), the crystalline and porous materials composed of a three-dimensional network of metal ions held in place by multidentate organic ligands, have been widely applied in many energy-related fields, including secondary batteries, water splitting, CO₂ reduction, and so on. ^143–145 More importantly, MOFs can significantly improve the rate capability and the stability of integrated devices when they are introduced into electrodes of EES units, due to their abundant active sites and porous structures. ¹⁴⁶ Besides, more kinds of carbon nanomaterials should be introduced into these integrated devices. For instance, carbon QDs can greatly improve not only the photoelectric conversion efficiency of solar cells but also rate capability of batteries. ^147,148 Graphdiyne owns an excellent hole-transfer property, resulting in the accelerated separation rate of photogenerated electrons/holes. ¹⁴⁹ All of these are beneficial to the enhancement of the performance of integrated devices.

• (iii)

  New methods: As it is well known, advanced fabrication methods can greatly increase device performance. For example, laser-scribing strategy can fabricate the high-performance graphene-based SCs due to the precise micromorphology control. ¹⁵⁰ The atomic/molecular layer deposition is an ideal way for overcoming the interfacial issues in solid-state batteries. ¹⁵¹ As for ion implantation technology, it has been regarded as a promising doping method for photoelectric conversion materials, because it does not involve any undesired elements other than dopants. ^152–154 Consequently, besides other technologies, we suggest that the above three new technologies should also be considered in the preparation processes of integrated devices for further performance improvement.

• (iv)

  Suitable application: For different types of integrated devices, there exist unique application scenarios. For instance, some OSC, PSC, or DSSC-based batteries and SCs can realize miniaturization and lightweight, and even be made in the flexible planar or the fibrous configuration. Therefore, they are suitable for applications in portable and wearable devices, including self-powered mobile phones and wearable/implantable biosensors. However, silicon-based solar cell/LIB hybrid systems own relatively high efficiency and safety. So, they can be deeply embedded into the “Internet of Things,” such as driving driverless EVs or industrial robots. As for photo-rechargeable RFBs, it seems more suitable to adjust the output fluctuation of PV power station to meet the grid-connected demands as they can store electrochemical energy at a large scale. In short, more attention should be paid to practical applications and finding the appropriate positions for each type of integrated devices.

• (v)

  Life-cycle stability: The robustness of devices, especially the stability in full life cycle, is very critical for their commercialization. In this context, the stability of individual counterparts in integrated devices should continue to be enhanced. In addition, reliable encapsulation technologies are also important for the stability of the whole integrated device.

• (vi)

  Multi-integration: On the basis of the piezoelectric or triboelectric effects, mechanical energy can be converted into electric energy and then stored in EES units. ^155–157 Besides, industrial waste heat can also be used as a type of energy source for thermal–electric–electrochemical energy conversion and storage. ^158,159 Obviously, the multi-integrated devices for synergistic utilization of solar, mechanical, and industrial waste energies will result in a profound energy revolution.

Figure 12. Proposed improvements for the future development on photo-rechargeable integrated devices. PEC, photoelectrochemical; PV, photovoltaic

Last but not least, except for the above points, other emerging technologies should also be paid equal attention, to obtain high-performance integrated devices. For example, tandem solar cells (e.g., perovskite/silicon tandem solar cells) have a higher photoelectric conversion efficiency than conventional ones ³⁹ ; meanwhile, Zn-ion batteries have been considered as an ideal candidate for the next-generation EES units due to their unique advantages, including high safety, availability, environment friendliness, and so forth. ¹⁶⁰ Therefore, the integration of the above devices can exhibit a great potential to be utilized in advanced solar energy hybrid systems. In addition, the development of single PE-based integrated devices has been regarded as a very promising strategy for achieving efficient and stable solar energy conversion, storage, and utiization. ^161,162 Specifically, the electrode materials in these devices can directly harvest the incident solar light without the help of solar cells or PEs, thus avoiding the mismatching issue between the solar cells/PEs and the EES parts. Generally, the V₂O₅-based cathode can possess a great potential to be applied in this type of hybrid solar energy systems due to its capabilities of visible light absorption (∼560 nm) and reversible insertion/desertion of Zn²⁺ ions during the photo-charging/discharging cycling. ^163,164

To summarize, photo-rechargeable integrated devices exhibit great potentials for practical solar energy application. ¹⁶⁵ In spite of many important obstacles that hinder further development, we do believe that these state-of-the-art hybrid solar energy systems will help us to open a door toward the new solar energy era. Lastly, we think that carbon-based functional materials will play a key role in the development road of photo-rechargeable integrated devices.

ACKNOWLEDGMENTS

This study was supported by the Natural Science Foundation of China (No. 51072130, 51502045, and 21905202), Innovative Research in the University of Tianjin (TD13-5077), Developed and Applied Funding of Tianjin Normal University (135202XK1702), the Australian Research Council (ARC) through the Discovery Project (No. DP200100365), and Discovery Early Career Researcher Award (DECRA, DE170100871) program.

CONFLICT OF INTERESTS

The authors declare that there are no conflict of interests.