Hybrid solar energy harvesting and storage devices: The promises and challenges

Materials Today Energy 13 (2019) 22e44

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Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/

Hybrid solar energy harvesting and storage devices: The promises and challenges D. Lau a, N. Song a, C. Hall a, Y. Jiang a, S. Lim b, I. Perez-Wurfl a, Z. Ouyang a, A. Lennon a, * a b

School of Photovoltaic and Renewable Energy Engineering, UNSW Sydney, Sydney, 2052, Australia The Electron Microscope Unit, UNSW Sydney, Sydney, 2052, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 February 2019 Received in revised form 4 April 2019 Accepted 5 April 2019

Hybrid devices that can harvest solar energy and store that energy electrochemically to provide a source of power are increasingly attracting attention due to their potential to provide autonomous power sources. Of particular interest is their ability to support sensors for the Internet of Things (IoT), wearable electronics and autonomous medical monitoring. Many such hybrid devices have been reported, however challenges exist with respect to electrode arrangements and operating modes, form factors, material compatibility and durability. In this perspective, we review both the application potential and design/ fabrication challenges for this class of device. It is proposed that device architecture and material choices need to be carefully selected according to the specific intended application to ensure adequate durability and offer practical outcomes over alternative solutions comprising individual solar harvesting and energy storage devices. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Hybrid energy harvesting storage device Solar cell Energy storage Electrochemical Solar-powered devices Sensors Internet of things

1. Introduction Photovoltaics (PV) allows for abundantly-available solar energy to be utilized as a source of electrical power. Since the early 2000's, terrestrial Si PV has been harnessed in an increasing scale as a renewable source of electricity that provides a viable alternative to burning fossil fuels and a pathway to reducing global warming [1]. The transition to using renewable energy, which can be harvested where electrical power is required, is causing a re-think of not only how energy is generated, but also how it is stored, distributed, and used. Historically, power generation has been centralized and then transmitted over long distances via electricity networks to local areas. Nowadays, solar or wind energy can be generated and distributed locally, which can reduce the need to maintain expensive transmission lines and provide improved energy security [2e8]. Harvesting solar energy for local uses can be realized at different scales. Utility scale PV can be used in conjunction with other sources of energy (e.g., wind, chemical fuels, hydropower) and energy storage systems [9e12] to enable micro-grids that can

* Corresponding author. E-mail address: [email protected] (A. Lennon). https://doi.org/10.1016/j.mtener.2019.04.003 2468-6069/© 2019 Elsevier Ltd. All rights reserved.

provide dispatchable power to local communities [4,5,8,13,14]. This approach can be a successful strategy, especially for remote communities due to the high cost of maintaining transmission infrastructure [3,8,13e16]. Alternatively, households can install PV modules on their rooftops and use batteries to store energy for later use when solar generation is no longer available (i.e., load shifting of daytime generation to night-time) [17]. However, another tantalizing possibility is the development of solar-powered appliances or devices which can: (i) directly harvest solar energy; (ii) electrochemically store that energy; and (iii) provide electrical power to drive a load as required. Seamless integration of solarpowered devices may present new opportunities including low power electronics applications [18e21], remotely-powered sensors [18,22,23], electrochromic smart-windows [24,25], remote antenna tracking devices [26e28] and self-charging wearable electronic devices [29e37]. These integrated solar energy conversion and storage systems have been variously referred to as photorechargeable energy storage systems [38e40], hybrid solar energy conversion/harvesting and storage systems [41], and solar batteries [42,43]. Reports of hybrid devices that can both harvest solar energy and electrochemically store the energy to power electronics date from the early 1970's. Early devices typically employed

D. Lau et al. / Materials Today Energy 13 (2019) 22e44

photoelectrochemical systems with semiconductor electrodes immersed in redox electrolytes [44e49], however, these devices usually required an aerobic environment for operation, were complex and had limited stability and durability. Consequently, they were quickly replaced with PV solar energy harvesting devices with examples being reported for a range of solar cell technologies including: organic solar cells (OSCs) [19,50e57]; quantum dot solar cells [58]; dye-sensitized solar cells (DSSCs) [18,32,33,55,59e100]; perovskite solar cells (PSCs) [32,35,101e106]; thin film solar cells [47,48,107,108]; polymer solar cells [20,29,109,110] and Si solar cells [21,44,46,49,111e116]. Photo-charging can also be achieved using photocatalytic systems coupled with redox flow batteries (RFB) in which the energy is generated and stored by photo-induced redox reactions [117,118]. The harvested energy can be stored as double layer capacitance at surfaces or through Faradaic reactions either at the surface or in the active material. In the former case, typically the stored energy is referred to as a capacitance (units of F per unit area) and in the latter case, a capacity is usually reported (mAh per unit area). Hybrid devices have been fabricated using different PV technologies with capacitances as high as 572 mF cm
2 [102] and capacities of up to 1.6 mAh cm
2 having been reported [107] (see Table 1). The use of areal capacitances/capacities, where the area typically refers to the solar cell's illuminated area, makes it difficult to directly compare the energy stored, in terms of specific capacitance or capacity (i.e., mF g
1, mAh g
1), with other devices. Although this area places limits on the amount of energy that can be stored and delivered, photo-charging can provide an adequate level of energy for continued operation of low power (mW) microelectronic devices and sensors [23,119]. Although hybrid solar energy harvesting and storage devices and functionality have been the subject of a number of reviews [38e40,66], an analysis that considers the promises of this class of device with a realistic assessment of the technical challenges associated with their fabrication and durable operation is lacking. In this perspective we: (i) propose potential applications that can derive a technical and/or application benefit in the integration of solar harvesting and energy storage functionality in a single device; (ii) review the different electrode configurations used in the fabrication of hybrid devices and assess their limitations; (iii) analyze the technical challenges facing the realization of devices, with a focus on limitations related to stored energy, dispatchable power, device configuration, material compatibility and the implications of electrolytes for durable devices; and (iv) introduce a discussion on reporting standardization that could support the future development of these new localized power sources. We assume a definition of a hybrid device as a single fabricated structure/ device that harvests solar energy and electrochemically stores that energy to provide power for a device function. 2. The potential The ‘Achilles heel’ of electricity generation from solar energy is that generation is directly linked to the availability of photons. When illumination is absent, such as at night or when a passing cloud interrupts the incident solar irradiation, a back-up energy store is required for continuous device operation. Batteries connected to a PV module or array via power electronics and wired circuits are a common solution to provide this storage functionality [33,120e122], however there are applications which may benefit from the solar energy conversion and storage functionality being more closely coupled. Combining both functionalities into single all-in-one compact devices has been highlighted to offer flexible form factor [35,50,123], reduced cost [28,34], less power loss due to reduced external wiring [41,124] and/or reduced device volume

23

[26,125] where surface area, volume and weight are all potential limitations. Solar-powered standalone devices can become especially attractive when wired electricity networks are not available. Such situations can include regions where electricity networks are not accessible due to power blackouts, grid infrastructure failures, economic reasons, remote geographical regions or in space (e.g., satellite applications) [126,127]. In each of these scenarios, hybrid devices that provide power by harvesting sunlight can allow equipment to continue to operate in the absence of solar energy. The solar harvesting functionality can be implemented to complement existing connected electrical operation and enable constant operation of standalone portable devices and/or sensors. For example, the combination of energy harvesting and storage hybrid devices may find applications in wireless sensors [23] utilized for monitoring temperature of perishable food [128,129], wildlife conservation [130e132] and air quality [133,134]. Additionally, the rapidly-evolving Internet of Things (IoT) [135e137] will require wireless self-powered remote sensing [119,138,139] for indicators of building climate, civil structure stresses [140e144], manufacturing monitoring systems [145,146], agricultural performance [147,148] and the monitoring of endangered species (e.g., bees where solar-powering of acoustic sensors can enable remote operation) [149e152]. Solar-powered antennas are expected to find applications in space satellites [126] where they can reduce the need to launch heavy batteries and allow autonomous operation (e.g., continuing data transfer when solar energy is not available). Hybrid devices have advantages when limitations exist with respect to component geometry, size and/or mass [153]. Specific form factors which require devices to be flexible or conformal to a shape can be difficult to integrate with rigid bulky batteries [154]. For example, the integration of solar-powering functionality in fabrics [155e158] (see Fig. 1A) can be used for wearable electronics without sacrificing a fabric's aesthetics, comfort and functionality [21,26,29,30,32e34,37,66,123,157e161]. Such smart wearable textiles could be designed to respond to external stimuli [162], releasing medication or moisturizer to the skin [162], measuring the vibration of muscles during athletic activities [162,163] or changing the color of the fabric for aesthetic or for thermoregulation purposes [162,164]. The planar nature of these textile applications is well-suited to providing relatively large areas for solar energy collection. Many sensing, monitoring and tracking applications also present limitations in terms of portability and form factor. Fig. 1B and C depict hybrid harvesting/ storage devices designed to power light-emitting diodes (LEDs) which can be used for emergency medical kits or toolkits [20,54]. Similarly, Fig. 1D shows an example of surface-mounted solarpowered RFID tags designed to transmit package identification signals for logistic tracking [27]. Integrated hybrid sensors can also be used to improve the autonomy and intensive care of medical patients [164,165], with implanted solar-powered wireless sensors potentially being used to monitor biological factors such as blood pressure and glucose levels [154,166,167]. Fig. 1E depicts a sub-dermal implant powered by harvested infrared energy that can provide a source of power for a pacemaker [168e172] or improve diagnosis and prognosis of illnesses [164e166,173]. Typically these solar-powered implants need to generate power in the mW to mW range and so a large solar cell surface area may not be necessary for viable operation [119,123,168e171]. Autonomously-powered bio-implants can present a practical benefit to patients by eliminating the need for the patient to undergo further operations to replace a battery. Not only are the additional surgical operations costly, but they also increase the risk of medical complications [174]. In the case of sub-dermal implants, the planar form factor that is required for patient

24

Table 1 List of reported hybrid solar energy harvesting and storage systems. Device configuration

PV technology

Storage electrode

2-Electrode - monolithic

Photoelectrochemical system Sn/SnS DSSC AgxV2O5/Ag6WO4/Agx-V2O5 DSSC WO3 c-Si solar cell Activated carbon nanoparticles DSSC NiOx or WOx oxide films Photoelectrochemical system Cyanine chromophore DSSC DSSC DSSC

Ferroelectric membrane PVDF PEDOT/CNT PVDF/ZnO nanowire array

Photoelectrochemical system Ag2O/Ag LiS LieO2 battery Poly(3,4-ethylenedioxypyrrole)/V2O5 AC with PEDOT:PSS

Photoelectrochemical system (KFe[Fe(CN)6] with TiN DSSC Pb and I

DSSC DSSC DSSC DSSC Organic solar cell DSSC

¡ LiI redox with I¡ 3 /I
LiI redox with I
3 /I Redox flow V3þ/V2þ Cell WO3.H2O/CNTs/PVDF Carbon-black electrode LiFePO4/CNTs

Photoelectrochemical system Bismuth oxyiodide/ZnO nanorod array 3-Electrode - interdigitated DSSC Organic solar cell c-Si solar cell

3-Electrode - monolithic

®

Polypyrrole Pt with Nafion Graphene Ink/PEDOT:PSS Laser scribed graphene oxide

DSSC Single walled CNT/buckypaper Photoelectrochemical system NiSO4 c-Si solar cell AB5 MH battery and NIOOH DSSC Li PPy electrode DSSC AC DSSC

PEDOT

DSSC

PProDOT-Et2 film

DSSC

Nanocrystaline-WO3

Organic solar cell

CNT

VOC of Capacity/ solar cell Capacitance

1.8 molal CsHS: 1.8 molal CsOH Ag6I4WO4 solid electrolyte 0.5 M LiI þ 0.5 M I2 w/PC C9H13N (CH3CH2)4NBF4 1 M KOH Cyanine solution with iodide and hexafluorophosphate 1 M LiPF6 EC:DMC 1:1 (v/v) LiCF3SO3/PC LiI:I2:guanidinium thiocyanate: 3dimethylimidazolium iodide: TBP in 3-methoxyproionitrile in AcN 1.0 M NaOH with 0.85 M ethanol 1 M LiClO4 EC:DMC 1:1 (v/v)
I
3 /I electrolyte 1 M LiClO4/PC 1 mM methyl viologen in 0.1 M Tris buffer solution Na2S2O8 Leadeorganohalide electrolyte CH3NH3I$PbCl2 w/N,Ndimethylformamide ¡ LiI redox with I¡ 3 /I Quasi-solid electrolyte H2SO4 LiI N-methyl-2-pyrrolidone gel LiPF6 EC:DEC: vinylene carbonate 0.1 M phosphate buffered saline with glucose 0.5 M LiI 0.05 M I2 in AcN Et4NBF4 - PC Ionogel - silica-1-butyl-3methylimidazolium bis (trifluor-omethylsulfonyl) imide PVA KOH K3[Fe(CN6]eK4[Fe(CN)6 12 M KOH 0.5 M LiClO4 in PC I¡/I3¡ electrolyte

0.51 V ~0.56 V 0.55 V

96.5 mW cm
2 e 1.8 C cm¡2

1987, Licht [49] 1990, Kanbara [83] 2002, Hauch [59]

0.45 V N/A 0.35 V

0.69 mF cm
2, 9.3 mWh m
2 e 8.22 mC cm
2

2004, Miyasaka [67] 2006, Niklasson [24] 2010, Pandey [53]

0.47 V 0.2 V ~0.9 V

37.62 mC cm
2 95 mF g
1 1.4 mWh kg¡1, 2.14 C g¡1

2011, Lo [80] 2011, Skunik [87] 2013, Zhang [18]

1.1 V

0.94 mW cm
2

2014, Han [189]

N/A 1.9 V ~1.9 V 0.43 V

199 mAh g
1 e 224 F g
1 1.04 mF

2015, 2015, 2015, 2015,

~0.9 V 0.73 V

77.8 mAh g
1 6.16 mC cm
2

2015, Thimmappa [191] 2015, Wang [68]

3.3 V ~0.6 V 1.3 V 0.62 V 0.92 V 3.75 V

32.6 Ah L¡1 0.653 C g
1 e 1.38 C cm
2 130 mF cm
2 104 mAh g
1

2015, Yu [85] 2015, Zhang [192] 2016, Azevedo [185] 2016, Huang [92] ^ne [57] 2016, Leche 2017, Paolella [82]

0.48 V

155 mW cm
2

2018, Wang [22]

0.64 V 2.0 V 0.38 V


2

37.8 mC cm 2.5 mF cm
2 4.6 W cm
2, 1 mF cm
2

2010, Saito [193] 2015, Chien [54] 2015, Thekkekara [111]

~1 V 0.75 V 1.28 V 0.6 V 0.8 V

95.25 F g
1 e 135.5 mW cm¡2 8.5 mC cm¡2 47 mWh cm¡2, 75 mC cm¡2, 0.65 F cm¡2 0.52 F cm¡2

2016, Lee [194] 1983, Yonezawa [46] 1999, Licht [114] 2004, Nagai [62] 2005, Murakami [70]

0.5 M LiClO4 in 3methoxypropionitrile 0.5 M LiClO4 in 3methoxypropionitrile 0.5 M LiI 0.05 M I2 0.5 M C9H13N in CH3OCH2CH2CN PVA with 1 M H3PO4

0.71 V

Reference


2

, 22 mWh cm

Li [190] Liu [110] Reddy [91] Takshi [55]

2010, Chen [61]
2

0.75 V

0.48 F cm

2010, Hsu [81]

~0.59 V

0.51 C cm
2

2010, Saito [99]

0.60 V

28 F g¡1, 17.5 C g¡1

2011, Wee [50]

D. Lau et al. / Materials Today Energy 13 (2019) 22e44

Hydrogen generation Polymer c-Si solar cell DSSC DSSC

Electrolyte (storage)

DSSC DSSC

3-Electrode - monolithic

DSSC DSSC DSSC DSSC DSSC DSSC Polymer c-Si solar cell DSSC c-Si solar cell DSSC DSSC DSSC DSSC DSSC DSSC

Thin film DSSC Perovskite solar cell DSSC Perovskite solar cell Perovskite solar cell Perovskite solar cell c-Si solar cell DSSC c-Si solar cell DSSC DSSC 4-Electrode e integrated RFB DSSC

c-Si solar cell DSSC

Electrically-connected 4 Electrode

DSSC Photoelectrochemical system Photoelectrochemical system Photoelectrochemical system Photoelectrochemical system DSSC

DSSC Polymer solar cell DSSC Photoelectrochemical system DSSC

1 M LiPF6 in EC:DMC 1:1 (v/v) 0.2 M LiClO4 0.2 M TBP with PC/ PPy LiI/I in ethylene glycol TiO2 nanotubes TiN/Ti with WO3/CNT counter PC:LiI:I2:LiClO4 RuOx(OH)y/RuOx(OH)y Nafion membrane RuOx(OH)y/RuO2 Nafion 117 MWCNT PVA with H3PO4 MWCNT-PANi PVA with H3PO4 Ti nanotubes and MWCNTs PVA with H3PO4 Ni(Co)Ox/AC 1 M KOH Porous Si PEO-EMIBF4:PC in 1:1:8 (w/w/ w) 0.1 M Li2WO4 0.2 M LiClO4 w/ Li2WO4 w/LiI & LISICON separator H2O & 0.1 M LiI 0.5 M TBP in PC 1 M LiClO4 and 5 mM I2 in LiO2 dimethylsulfoxide Porous Si PEO-EMIBF4 1:3 (w/w) LiFPO4 1 M Li2SO4 and 0.1 M LiI FePO 1 M Li2SO4 and 0.1 M LiI MWCNT LiCF3SO3/PC/ poly(methylmethacrylate) 0.8 M Na2S, 0.8M S, and 2M KCl Carbon Mesh with Cu2S film Polyviologen/Pt Mesh TEMPOL solution w/NaCl and H2O2 1 M LiPF6 EC:DMC 1:1 (v/v) LiFePO4/C with Li4Ti5O12 Na2S4 anolyte & NaI catholyte Na2S4 anolyte and NaI catholyte CuOH nanotubes PVA with KOH Ni foam 1 M Na2S4, 1 M NaClO4 in AN/ THF 2:1 (v/v) PVA with H3PO4 WO3 Reduced graphene oxide PVA with H3PO4 and EC PANi/CNT PVA with H2SO4 a-MoOx 0.1 M Na2SO4 MWCNT 2 M NaCl LIB LiCoO2/Li4Ti5O12 1 M LiPF6 EC:PC (1:1) (v/v) LIB/Ti foil/TiO2 nanotube arrays PVA with H3PO3 Fe2(MoO4)3 microspheres (NH4)6Mo7O24 NaI
þ I
10 mM I2 þ 2 mM LiI þ 0.2 M 3 /I and Fe(C10H15)2 /Fe(C10H15)2 LiClO4 with PC and 3methoxypropylnitrile AQDS/AQDSH2 and Br
AQDS/AQDSH2 w/Br
3 /Br 3 /Br Ferrocyanide and NaOH AQDS Na2/AQDS/Fe(CN)64
/ Fe(CN)36
KBr/LiH2PO4 and KI/I2/LiH2PO4 Br2/Br
and I
3 /I CdSe/S/Ag2S e CdSe0.65Te0.35/Cs2Sx/SnS OH
and S2
aqueous electrolyte Br
/Br2(MoSe2), S2
/S2en(CdSe) IeI2, Se2
/Se2e2 and Cd and Se2
/Se2e2(GaAs) KCl solution BaTiO3jCe4þ/3þjjFe3þ/2þjPt CNT fiber PVA/H3PO4 with 0.1 M LiI 0.05 M I2 0.6 M C8H15IN2 0.5 M C9H13N PANi-stainless steel 1 M H2SO4 Ni yarn with composite 1 M LiPF6 EC:DMC 1:1 (v/v) TiO2 nanotubes PVA with H3PO4 TiO2/vanadium (IV) with Pt/vanadium (III) VOSO4$xH2O w/H2SO4 b-cyclodextrin/poly-N-vinyl-2-pyrrolidon/MnCO3 [BMI][TFSI] electrolyte

3.39 V 0.76 V

38.89 mAh 8.3 mAh g
1

2012, Guo [71] 2012, Liu [60]

0.42 V ~0.70 V 0.9 V 0.90 V 0.75 V 0.75 V 0.40 V 0.80 V 0.56 V

140 mF cm
2 0.231 mAh cm
2 0.51 mWh cm
2 3.26 F cm
2, 0.17 mWh cm
2 26 F g¡1 83 F g¡1 0.161 mWh cm¡2 32 F g
1, 2.3 Wh kg
1 0.14 F m
2, 7 Wh kg
1, 86 J m
2

2012, Mini [75] 2012, Yan [100] 2013, Kulesza [88] 2013, Skunik-Nuckowska [73] 2013, Yang [195] 2013, Yang [195] 2013, Zhang [109] 2014, Bagheri [74] 2014, Westover [113]

~0.80 V

0.0207 mAh mL
1

2014, Yan [63]

0.73 V

-

2014, Yu [77]

0.64 V 2.88 V 2.88 V 0.5 V

0.17 mWh cm¡2, 22 mW/cm¡2 149 mAh g
1 138 mAh g
1 150 F g
1

2015, Cohn [196] 2015, Li [72] 2015, Li [72] 2015, Narayanan [197]

0.60 V 0.80 V

56.4 mJ 74 mC

2015, Shi [58] 2015, Suzuka [76]

2.09 V ~1.2 V ~0.8 V ~0.78 V

1.6 mAh cm
2 110 mAh g
1 1.15 mWh cm¡3 180 mF cm-2

2016, Agbo [107] 2016, Li [84] 2016, Li [35] 2016, Mahmoudzadeh [118]

0.61 V 0.91 V 0.7 V 0.6 V 0.67 V 5.4 V 0.64 V ~0.64 V 1V

430.7 F m¡2 144 F g
1 103.4 F g
1 34 mF cm
2 1.08 mWh cm¡2 0.5 mAh cm
2 86.67 mAh cm
2 60 mAh g
1 108 mAh L
1

2016, Zhou [104] 2017, Kim [105] 2017, Liu [106] 2017, Ouyang [112] 2017, Scalia [86] 2017, Um [116] 2017, Zhang [198] 2018, Gui [186] 2013, Liu [64]

0.80 V 0.74 V

730 mAh L
1 e

2016, Liao [115] 2016, Wedege [187]

~0.60 V ~0.15 V 0.5 V

0.8 Ah L
1 e e

2018, Zhang [79] 1976, Hodes [48] 1977, Manassen [47]

e

e

1980, Ang [45]

0.60 V 0.68 V

75 mW cm
2 2 mF cm¡2

1982, Sharon [44] 2012, Chen [89]

2.34 V 2.4 V 0.64 V e 0.72 V

41 mF cm¡2 85 mAh 3.32 mF cm¡2 0.299 mAh g
1 202 F g
1

2013, Fu [98] 2013, Lee [29] 2014, Chen [97] 2015, Liu [69] 2015, Selvam [90] 25

(continued on next page)

D. Lau et al. / Materials Today Energy 13 (2019) 22e44

Quantum dot solar cell DSSC

Li-ion TiO2 nanotubes PEDOT

Abbreviations: AC: activated carbon; AcN: acetonitrile; AQDS/AQDSH2: 9,10-anthraquinone-2,7-disulphonic sodium/1,8-dihydroxy-9,10-anthraquinone-2,7-disulphonic sodium); c-Si: crystalline Si; EC: ethylene carbonate; DMC: dimethyl carbonate; CNT: carbon nanotubes; MWCNT: multi-walled carbon nanotube; PANi: polyaniline; PC: propylene carbonate PEDOT: poly(3,4-ethylenedioxythiophene); PEDOT:PSS: poly(3,4ethylenedioxythiophene) polystyrene sulfonate; PEO-EMIBF4: polyethylene oxide with 1-ethyl-3-methyl imidazolium tetrafluoroborate; PPy: Polypyrrole; PVA: polyvinyl alcohol; PVDF: polyvinylidene difluoride; TBP: 4tertbutyl pyridine. Devices highlighted in bold are reports which have been cited more than 50 times.

a

1.74 mF cm
1 and 18.51 mF cm
2 2017, Liang [94] 22.6 mAh g
1 2017, Lei [95] Carbon fiber with TiO2 and MoS2 AB5 type-alloy hydrogen storage with LiI DSSC DSSC

0.74 V 1.46 V

2017, Gurung [103] 153.3 mAh g
1 Li4Ti5O12eLiCoO2 DSSC

0.68 V

2015, Xu [102] 2016, Chai [156] 2016, Gao [21] 2016, Lv [93] 2016, Wen [188] 2017, Gurung [103] 572 mF cm 0.36 mF cm¡1 55.04 Wh kg¡1 128 mW 1.9 mF cm¡1 151.3 mAh g
1 Bacterial Cellulose/PPy Ti/TiN Activated cotton textiles/graphene TiO2-coated CNT fiber PDMS RuO2·xH2O/Cu Li4Ti5O12eLiCoO2 Perovskite solar cell DSSC a-Si solar cell DSSC DSSC Perovskite solar cell

1.45 V 1.2 V 1.6 V 0.7 V 1.2 V 0.96 V

¡2

111.6 mAh g¡1 3.84 V

LiPF6 in a 1:1:1 EC:DMC:DC (v/ v/v) Nanofibers/MWCNTs PVA with KOH PVA with KOH I
/I3
electrolyte PVA/H3PO4 Gel 1 M LiPF6 EC:DMC:DC 1:1:1 (v/ v/v) 1 M LiPF6 EC:DMC:DC 1:1:1 (v/ v/v) PVA with H3PO3 0.1 M LiI and 0.5 M TBP in PC LiFePO4/Li4Ti5O12 Perovskite solar cell

VOC of Capacity/ solar cell Capacitance Electrolyte (storage) Storage electrode PV technology Device configuration

Table 1 (continued )

2015, Xu [101]

D. Lau et al. / Materials Today Energy 13 (2019) 22e44

Reference

26

comfort and ease of insertion are also conducive to flexible solar energy harvesting devices. Hybrid energy harvesting and storage devices where energy is harvested from mechanical forces using, for example, piezoelectric membranes [141,175e178] or thermal sources [179e181] have also been reported. Future hybrid devices may also harvest energy from magnetic levitation for remote rail sensors [182]. Depending on the particular application, these non-solar-charging hybrid devices may present advantages in terms of design and operation. For example, if the device is in constant motion, then a lower energy storage capacity may be required thus reducing overall device cost. Similarly, for applications where solar power may be limited (e.g., train tunnels), non-solar hybrid devices may present a more effective solution. However, in general, these other hybrid devices are expected to experience similar integration challenges to the solar-powered hybrid devices. For this perspective, we are interested in the development of solar-charging and integrated storage functionalities that can enable a reliable power source in the temporary absence of solar energy. Faradaic charge storage within a hybrid device provides a potential advantage in terms of increased energy density compared to energy stored via double layer capacitance, however it can do so at the expense of power capability [183,184]. Electrochemical capacitors (ECs) have been employed for the Airbus 380 emergency exit doors for its high power density characteristics [183]. Solarpowered emergency exit doors may find applications on mass transport vehicles where they can be used to allow passengers to safely exit in the absence of wired electrical power following an accident. The integration of solar harvesting functionality for these emergency power applications allows for continuous operation after the initial stored energy is used or lost via self-discharge, and this can advantageously reduce the required capacity (and hence cost) of the energy storage. The above discussion highlights the broad range of potential applications that exist for hybrid devices. This range necessitates a careful selection of technologies and materials for each of the energy harvesting and storage components when designing a hybrid device for the device to provide an advantage over solutions that comprise individual PV and energy storage devices and to be compatible with its operating environment (e.g., the biological compatibility of medical implants and exposure of remote sensors to harsh external environments). 3. Device architectures A number of different integration strategies can be employed to directly harvest solar energy and store the captured energy within a hybrid device. One way of grouping the different devices is by classifying device architectures according to the number of electrodes employed. Using this approach, we have classified devices according to whether they are: (i) two-electrode (see Fig. 2A) [55,59,67,83,185]; (ii) three-electrode (see Fig. 2B [48,112e114,186] and Fig. 2C [111]); and (iii) four-electrode devices (see Fig. 2D [63,64,115,187] and Fig. 2E [64,79,115]). In the following discussion we use the generic term ‘energy storage electrode’ to refer to the electrode at or in which charge is stored. In actual devices, this electrode can be realized as an EC or battery electrode depending on the charge storage mechanism employed. Of the different device architectures, the three-electrode option comprising of a monolithic structure, with a common electrode being shared between the solar cell and the energy storage component (see Fig. 2B), has been most commonly-reported (see Table 1). This back-to-back arrangement allows the solar cell and energy storage component to operate semi-independently (see Section 4.3 for a discussion of the operating mechanism). To

D. Lau et al. / Materials Today Energy 13 (2019) 22e44

27

Fig. 1. Potential applications for solar-powered hybrid devices: (A) knitted fabric with an integrated solar-powered sensor (reproduced with permission from The Royal Chemistry Society, 2014) [33,157]; (B) a solar cell with associated storage used to power LEDs (e.g., use in emergency kits) (reproduced with permission from J. Mater. Chem., 2010) [20]; (C) a DSSC-powered LED sensor with storage (reproduced with permission from Small Nano Micro, 2015) [54]; (D) a solar-powered RFID tag affixed to the surface of a box for parcel tracking (reproduced with permission from IEEE, 2011) [27]; and (E) sub-dermal near flexible solar cell infra-red harvester and storage device for powering of medical implants (reproduced with permission from IEEE, 2017) [168].

increase the effective area of the energy storage electrode exposed to the electrolyte, the common electrode can also be enhanced with micro- and/or nanostructures as shown in Fig. S1 to increase capacitance and/or surface Faradaic charge storage, however this strategy typically results in a more complicated fabrication process as explained in Section S1 in the Supporting Information. Fig. 2C shows an alternative three-electrode configuration where the energy storage and counter electrodes are arranged over the common electrode in an interdigitated pattern [111]. In this arrangement, the solar-charging current density is increased as a result of the reduced contact area of the common electrode and so care must be taken to ensure that the active material can be charged without degradation in the event of intense illumination. Although the interdigitated arrangement can allow for a more compact device and eliminates the need for a separator (due to the rigid engineered separation of the energy storage and counter electrodes), the complexity of the device fabrication is increased by the need for electrode patterning (e.g., printing [50,54] or laser

scribing [111]). Thekkekara et al. reported a three-electrode interdigitated device formed on a Si solar cell and suggested that their hybrid device provided benefits in terms of ready integration with electronics (i.e., on-chip solar powered devices) [111]. Solar charging and discharging can also be efficiently separated and individually optimized using four-electrode systems (such as depicted in Fig. 2D and E). Monolithic four-electrode devices have been reported with the use of RFBs for energy storage and photo-electrochemical cells for solar energy harvesting [63,64,115,187]. If the solar cell and energy storage component are connected by a wired connection (i.e., Fig. 2E), then the functionally of the system is very similar to the case of two separate devices and there is expected to be limited value to integration in a hybrid device given the typical large size of a RFB. However, in some cases where form factor is important for the application, four-electrode hybrid devices can provide benefits in terms of compactness. For example, wearable energy hybrid devices supported in fibers or wire may be more suited to

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D. Lau et al. / Materials Today Energy 13 (2019) 22e44

Fig. 2. Schematics depicting reported hybrid device configurations (see Table 1 for a list of reported devices): (A) two-electrode monolithic [18]; (B) three-electrode monolithic (adapted from Ouyang et al., 2017) [112]; (C) three-electrode interdigitated; (D) four-electrode integrated RFB system (adapted from Liao et al., 2016) [115]; and (E) electricallyconnected four-electrode (adapted from Chen et al., 2012) [89].

integration using a four-electrode system (i.e., Fig. 2E) and cannot be achieved using bulky batteries due to space or volume restrictions in fabric's integration [35,36,153,188]. Consequently, for this report, we focus on hybrid devices which present advantages in terms of compactness, cost or flexible form

factor. Different device architectures may be more suitable for different applications. For this reason, no single architecture can necessarily be considered superior to the others. Section 4.3 contains a discussion on the challenges arising from the use of the different device configurations.

D. Lau et al. / Materials Today Energy 13 (2019) 22e44

4. The challenges 4.1. Limits to stored energy The energy stored in a hybrid device is limited by the capacity (or capacitance) and the voltage at which the energy storage electrode is charged. The areal energy density of the storage electrode ES is given by:


 ðI V c ES J cm
2 ¼ dt Asc
 ð ES Wh cm
2 ¼

IC V dt 3600 Asc

(1)

(2)

where V is the potential difference over the storage electrode, Ic is the charging current and Asc is the area of the solar cell (i.e., solar energy capture area). If the energy is stored as capacitance then we can write:


 ES Wh cm
2 ¼

CV 2 2  3600  Asc

(3)

where C is the differential capacitance of the storage electrode [199e201]. Irrespective of whether charge is stored capacitively or through Faradaic reactions, ES depends on the charging voltage that the solar cell can generate with respect to a counter electrode, provided that this voltage is within the safe cycling limits of the electrolyte used. For Faradaic charge storage, the solar-charging voltage needs to exceed the electrochemical potential for the oxidation/reduction reaction. The maximum voltage that a solar cell can generate is its open-circuit voltage (Voc). This value depends on the material properties of the absorber material (e.g., bandgap, carrier lifetimes) and can vary from values < 1.0 V (e.g., for Si single-junction devices) [202] to values as high as 4.767 V for more complex multi-junction cells [203]. A solar cell's Voc can also be limited by carrier recombination at the electrodes' surfaces, and so any common electrodes need to be integrated with the energy storage electrode in such a way that carrier recombination is minimized at the solar cell absorber interface. For example, whilst introduction of the porous Si surface on a Si solar cell described by Westover et al. [113] may be beneficial for increasing charge storage by providing more active material's surface area to the electrolyte, it would be expected to reduce the solar cell's efficiency and charging voltage due to carriers recombining at the rear surface of the Si absorber material. Table 2 lists the maximum recorded Voc for selected PV technologies. Clearly, higher Voc values can be obtained by using monolithically-integrated multi-junction solar cells [107] and

29

hybrid tandem DSSC devices [71,114,185] which harvest solar energy from a much wider wavelength range. However, the latter devices involve far more complex and costly fabrication processes and consequently may only be justified for high-value applications such as solar concentrators or space applications. To-date, most reported hybrid devices have used DSSCs and perovskite devices (see Table 1) due to their reasonably high Voc (for single junction solar cells), relatively simple fabrication process [32], flexibility [35,215] and low-cost potential compared to other PV technologies [29,30,33,97,216]. Perovskite on Si tandem solar cells may represent a promising option in the future due to their larger voltage and recent rapid advances in energy conversion efficiencies [203,206]. Use of a Si solar cell for solar charging [44,46,49,67,111e114,116] has the potential advantage of leveraging the manufacturing experience for Si PV where module costs have fallen to < USD2018 $0.34/W [217]. Although maximum Voc values of 0.74 V have been achieved for laboratory-fabricated Si solar cells [204], typical industriallyproduced Si solar cells have Voc values of 0.65e0.66 V [e.g., for passivated emitter and rear cells (PERC)] [218] and this limits the charging voltage of the hybrid device. The reduced voltage of industrially-produced cells can result from the use of lower quality Si wafers and limitations arising from manufacturing, in particular from undesirable bulk recombination losses, induced degradation and surface recombination [219,220]. Additional (voltage) losses can occur when a solar cell is integrated with an energy storage electrode. Edge recombination losses [221] can contribute to additional voltage losses and are especially significant for smaller devices (e.g., as required for medical implants) and for indirect bandgap solar cell absorber materials, such as Si, where photon absorption and emission are less efficient than for a direct bandgap material like GaAs [222]. For Si wafer-based solar cells, the photovoltage can be increased by interconnecting a series of individual solar cells through either conventional tabbing methods [32,50,98,107] (see Fig. 3A) or shingled interconnection [101] (see Fig. 3B). In former option, solar cells are interconnected via metal ribbons that are soldered to metal busbars formed on the adjacent solar cells in a string. Shingled interconnection removes the need to use interconnection ribbon and series connection between individual solar cells is achieved by aligning front-surface busbars with the rear electrode of the adjacent cell in a string. With both methods, the voltage over the terminals of the string of solar cells is the sum of the individual cell voltages, however the PV-generated current density delivered to the energy storage electrode is determined by the area of the individual solar cells in the string. If the energy storage electrode is directly coupled to a solar cell, then typically its area is constrained by the area of the cell (as all cells in the string need to have a similar area to generate a similar current). The concept of serially connecting individual solar

Table 2 Maximum open-circuit voltage Voc values recorded for selected PV technologies. Solar Cell Technology c-Si (n-type rear IBC) III-V (GaAs) Thin film e copper indium gallium selenide (CIGS) Thin film (CdTe) Thin film e copper zinc tin sulfide (CZTS) Dye-sensitized solar cell (Co2þ/Co3þ Redox) Perovskite (FAPbI3 PSC) Organic solar cell (PC70BM) Amorphous Si Multi-junction with c-Si (GaInP/GaAs/Si) Perovskite/Si III-V 5 junction cells (bonded InGaP/GaAs/InGaAs/GaInP/GaAs)

Maximum Voc (V)

Reference

0.7403 1.1272 0.744 1.096 0.7306 0.91 1.125 0.79 0.896 3.125 1.813 4.767

[202,204] [202,205] [206,207] [208] [202,209] [210] [206,211] [212] [213] [214] [202] [202,203]

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D. Lau et al. / Materials Today Energy 13 (2019) 22e44

Fig. 3. Series connection of individual solar cells to achieve a larger charging voltage with: (A) showing interconnection via conventional tabbing ribbon; and (B) showing shingled cell interconnection. The schematic (C) is an electrically-connected four-electrode hybrid system made up of four perovskite solar cell connected in series to charge a Li-ion battery. Reproduced with permission from Nature, 2015 [101].

cells to obtain a sufficiently high voltage to charge a Li ion battery was demonstrated by Xu et al. (see Fig. 3C). The electricallyconnected four-electrode hybrid device achieved a Voc of 3.84 V which is sufficient to solar charge a Li-titanate electrode of the Liion battery [101]. 4.2. Limits to discharge power For many solar-powered devices, bursts of power may be required to either initiate a process or periodically transmit a signal. For example, a power of 10 mW is generally sufficient to measure

and transmit temperature readings from a sensor every 5 s [119]. When charge is stored in an electrical double layer then high power discharge is generally possible. However, if charge is stored through Faradaic reactions, then the discharge current can be limited by the rate of diffusion and/or the charge transfer kinetics in the storage electrode. This suggests that the charge storage mechanism needs to be engineered with an understanding of the power required for the device. Discharge power also depends on the voltage of the discharged pulse of power. The maximum specific power of a voltage source under matched impedance conditions (i.e., when the load resistance is equal to the

D. Lau et al. / Materials Today Energy 13 (2019) 22e44

31

equivalent series resistance (ESR) experienced during discharge), Pmi (W), is given by [223e225].

Pmi ¼

V 20 4 ESR

(4)

where V0 is the rated voltage of the source (e.g., battery or capacitor). The discharge efficiency (hD ) under these conditions is only 50% as half the energy is lost as heat. However, for practical devices a higher hD is desirable and consequently it has been proposed that Equation (4) is of limited use for batteries and ECs [223]. For batteries the maximum pulse power, Pmax,bat, can be calculated from the Voc of the battery using (see Supporting Information Section S2 for derivation):


 V2 oc Pmax;bat ¼ hD;Bat 1
hD;Bat ESR

(5)

where hD;Bat is the discharge efficiency defined as Vpulse/Voc, Vpulse being the voltage at which the current is discharged from the battery [226]. It should be noted from Equation (5) that high hD;Bat values reduce Pmax,bat and Pmi is obtained when hD;Bat is just 50% [223]. For electrodes where charge is stored as capacitance, the discharge efficiency is defined differently because the discharge voltage decreases linearly with discharge time from a fully charged capacitor at an open circuit potential Vrated. Assuming discharging from Vrated to ½ Vrated, then Vpulse can be assumed as ¾ Vrated, and the maximum pulse power, Pmax,ECs, is given by (see Supporting Information Section S2 for derivation):

Pmax;ECs ¼

9  2
V rated 16 1
hD;ECs ESR

(6)

where hD;ECs is the discharge efficiency reduced from 100% by the power lost in the capacitor due to internal resistance. [225]. For both Faradaic and capacitive energy storage, high hD results in a lower Pmax (i.e., a trade-off exists between discharge power and efficiency) [223]. Consequently, engineering strategies that require a device to operate at high power may also need to ensure that heat (generated through use of a lower efficiency) is appropriately dissipated. Additionally, both Pmax,bat and Pmax,ECs are proportional to the V2rated/ESR and so the discharge power is also limited by the solar cell's voltage. The ESR is an aggregate resistance of all the components (i.e., electrodes, electrolyte and wires/switches) in the current path during the discharge of the storage electrode. To achieve high pulse power, all contributions to ESR need to be minimized [227]. The value of ESR for the purposes of calculating device peak power can be estimated from the initial DV drop in a galvanostatic discharge of the storage element (e.g., see Fig. 4B). Alternatively, electrochemical impedance spectroscopy (EIS) [228e230] measurements can be performed on the hybrid device's energy storage electrode to estimate the ESR contribution. An alternative approach to quantifying peak power has been proposed by the United States Advanced Battery Consortium (USABC) [231]. The purpose of this more experimental test, which can be applied to both batteries and capacitors, is to measure the actual capability of a battery to deliver sustained power for fixed time interval (e.g., 30 s for electric vehicles) over a max/min voltage range. With this test, the peak power is calculated as the product of the sustained current and the time-averaged voltage from a maximum initial value to a minimum value. Although this method has been developed specifically for electric vehicles, it may also be

Fig. 4. (A) Solar charging and discharging of hybrid device comprising a commerciallyproduced screen-printed c-Si solar cell with a common rear electrode comprising screen-printed Al coated with an anodic MoOx storage electrode; and (B) a single solar charge/discharge cycle showing how the voltage over the capacitive storage electrode is limited to ~ 0.61 V, the approximate Voc of the c-Si solar cell. The voltage-drop (DV) experienced on initial discharge can be used to approximate the ESR of the storage electrode as described in Section 4.2 (adapted with permission from Ouyang et al.). [112].

more appropriate then Equation (5) and Equation (6) for hybrid devices which have particular power burst requirements. Another figure of merit when considering the pulse power capability of a device is the roundtrip efficiency resulting from a sequence of charge and discharge pulses over an extended time period. The roundtrip efficiency is defined as the ratio of the energy transferred during discharge pulses to that transferred during charge pulses and, in general, it decreases as the power of pulses increases [225]. It is a useful figure of merit for devices because it takes into account the effects of all the different chemical and electrical non-idealities in a device. 4.3. Limitations arising from device architecture The performance and utility of hybrid devices can be limited by factors that are inherent in the device architectures adopted. As mentioned in Section 3, some device architectures can provide advantageous properties but at the expense of fabrication cost. For applications such as remote sensors, low-cost is paramount and so trade-offs in terms of functionality to achieve cost targets are typically required. In this section we identify limitations that may arise due to device architecture. 4.3.1. Two-electrode devices Although the two-electrode configuration (Fig. 2A; see Fig. S2 for an equivalent circuit) is conceptually simple and easy to

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D. Lau et al. / Materials Today Energy 13 (2019) 22e44

fabricate, it suffers from some critical limitations. Fig. 5 shows a cross-sectional schematic of a monolithically-integrated two-electrode hybrid device based on an n-type solar cell with a p-type emitter in: (A) solar-charging; (B) fully-charged; and (C) discharging states. The first and, perhaps most important limitation, of twoelectrode devices is that once the device is fully-charged, current ceases to flow through the circuit and so power cannot be provided to the load. Hence, a two-electrode device may be more appropriately considered a solar-charged energy storage device rather than a hybrid device providing a continuous source of power to drive a load. A second limitation of two-electrode devices arises when discharging the device. Since the device only has two terminals, the current must flow back through the solar cell on discharge. This can result in a high ESR, the magnitude of which depends on the properties of the solar cell (e.g., Schottky barriers may need to be overcome) [32,67,70]. The use of a three-electrode configuration (as shown in Fig. 2B and C) can address these limitations with the electricallyconnected common electrode (shared by the solar cell and the energy storage electrode) allowing an alternate current path for discharging of the device. Murakami et al. reported that transitioning from a two-electrode to a three-electrode configuration reduced the ESR from 2600 to 330 U for their DSSC activated carbon hybrid device (see Fig. 6A) [70]. The reduced ESR resulted in a smaller DV on the onset of discharge for their three-electrode compared to the two-electrode system, meaning more useable energy was available from the storage element. 4.3.2. Three-electrode monolithic devices Although the three-electrode devices can enable continuous power generation and low-resistance discharging, these attributes require the use of switches to alternate between the different operation modes of the hybrid device (see Fig. 7 and Fig. S3 for an equivalent circuit). A three-electrode hybrid device can continuously provide current to drive a resistive load with the switches S1 and S2 closed (see Fig. 7A) even once the storage component is

fully-charged (see Fig. 7B). The continued photovoltage under this operation state serves to minimize self-discharge from the common electrode. When the solar hybrid device is no longer illuminated, S1 can be opened and S2 closed to prevent the current flowing back through the solar cell during discharge to a load as shown in Fig. 7C. This can diminish the ESR on discharge, reduce energy loss and thereby increase the power available to the load compared to a two-electrode device. Fast charging of the storage electrode can also be achieved by opening S2 and closing S1 as shown in Fig. 7D. Practical devices will require automated switching, however most reported three-electrode devices have employed manual switching for simplicity and have focused on the material used for the devices [112,118,186]. Autonomous switching can potentially be achieved by using micro-controllers that operate based on the measured potentials over the solar cell and the energy storage electrodes. For hybrid devices to be practical, the solar charging and energy storage elements need to be durable and able to sustain long autonomous operations to ensure high cost effectiveness. Like secondary batteries, cycling of hybrid devices can result in corrosion of electrodes and device degradation due to parasitic reactions involving electrolyte breakdown. The latter can result in the formation of solid electrolyte interphases [232e234] that increase the ESR as well as gas evolution which has implications in reducing the durability of encapsulated devices [235e237]. Durability for hybrid devices may require more attention than most batteries or ECs because the electrode materials need to be stable in the presence of light (i.e., UV) and be able to endure a wide range of operating environments (i.e., temperature, humidity), especially for the case of outdoor solar-powered sensors and antenna. Corrosion of the energy storage electrode can impact the performance of two and three-electrode systems and is expected to be more problematic for devices employing aqueous electrolytes. Ouyang et al. reported a capacity of 34 mF cm
2 for a monolithically-integrated three-electrode hybrid device using an

Fig. 5. Schematics of a two-electrode device based on an n-type solar cell with a p-type emitter in a charging state (A), fully charged (B) and when discharging (C). The arrows depict the direction of positive ion flow in the electrolyte, with the symbols 4 and . representing cations and anions, respectively. An equivalent circuit of a two-electrode device is provided in Fig. S2 in the Supporting Information.

D. Lau et al. / Materials Today Energy 13 (2019) 22e44

33

Fig. 6. (A) and (B) depict the DSSC two-electrode and three electrode devices reported by Miyasaka et al. [67] and Murakami et al. [70], respectively. (C) Shows galvanostatic discharge profiles obtained from the reported devices shown schematically in (A) and (B) at a discharge current of 47 mA cm
2 (no illumination). A large DV is evident at the onset of the discharge for the two-electrode device indicating a larger ESR than for the three-electrode device. The improvement of the Voc for the three-electrode device was due to the use of a more conductive electrolyte (I
/I3
) for that device. A, B and C are reproduced with permission from Miyasaka et al. [67], and Murakami et al. [70].

Fig. 7. Schematic diagrams depicting a three-electrode hybrid device (comprising an n-type solar cell with a p-type emitter) operating under different modes by closing or opening a series of switches. When the device is solar charging (A), current flows through both the resistive load and the energy storage circuit element in a parallel configuration positioned on the rear of electrode of the solar cell. With electrical connection to the common electrode, energy can be continuously provided to the resistive load even when the energy storage electrode is fully charged (B). When discharging in (C), current flows through the inner circuit with switch S2 closed and switch S1 opened to prevent discharging through the solar cell. (D) Depicts the case when fast charging of the energy storage electrode is required (i.e., switch S2 is opened to prevent the solar-generated current from passing through the load). The symbols 4 and . represent cations and anions in the electrolyte, respectively, and the arrows show the direction of positive ion flow in the electrolyte.

industrially-produced screen-printed p-type solar cell with a symmetrical MoOx storage electrode using aqueous electrolyte (see Fig. 8A) [112]. Although this device was able to be solar-charged and discharged for 100 cycles without any measurable change in capacity under a symmetrical MoOx structure, it was later found that side reactions involving the use of an aqueous electrolyte resulted in the corrosion of the common electrode leading to coulombic

efficiencies exceeding 100%. (see Fig. 8B). Electron microscopy with energy-dispersive X-ray (TEM-EDX) and inductively-coupled plasma mass spectroscopy (ICP-MS) (see Section S3 and Table S6 in Supporting Information for more detail) were used to show that the anodic MoOx electrode surface had corroded with cycling, exposing the Al electrode of the solar cell to the electrolyte. Electrode corrosion consumes electrolyte in side-reactions and

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D. Lau et al. / Materials Today Energy 13 (2019) 22e44

Fig. 8. (A) Monolithically-integrated three-electrode hybrid device fabricated using an industrially-produced p-type c-Si solar cell with an anodic MoOx electrode formed on the common screen-printed Al electrode using light-induced anodisation (adapted from Ouyang et al.) [112]. The included cross sectional TEM image with elemental tracing using EDX shows voids exposing the Al (green) within the layer of MoOx (blue) at the site of potential corrosion. The samples were coated with Pt prior to imaging to prevent milling damage to the sample. (B) Capacity and coulombic efficiency for a MoOx hybrid device from electrically charging and discharging at a constant current of 320 mA/cm2. The common electrode of the hybrid device was charged against a Pt counter electrode between 0 and
0.6 V (reference to Ag/AgCl in 3 M KCl) in an aqueous electrolyte of 1 M Li2SO4. Corrosion of the common electrode comprising screen-printed Al coated with MoOx during discharging contributed to an additional corrosion current leading to a coulombic efficiency of >100% (see Section S3 in the Supporting Information).

suppression of these side reaction(s) is essential to ensure device durability. This example demonstrates that side-reactions may not only affect device operating lifetimes via the degradation of the electrodes, but may also lead to erroneous capacities and capacitances during measurements. The probability of electrode corrosion can be reduced by: (i) ensuring that materials that are prone to corrosion are prevented from coming into contact with the electrolyte; and (ii) selecting a voltage cycling limit that maintains the electrolyte within its thermodynamic stability window (as discussed further in Section 4.5).

4.3.3. Four-electrode systems and devices Separated four-electrode (Fig. 2D) and electrically-connected four-electrode (Fig. 2E) devices may have larger-scale applications and, for this reason, may be more appropriately referred to as systems rather than devices. Integrated four-electrode systems [64,79,115,187] based on RFBs have potential advantages in terms of cyclability and scalability due to their energy capacity being correlated with the volume of electrolyte [238,239]. Consequently, provided that no space constraints exist, very high capacity fourelectrode monolithic systems can be technically constructed. However, smaller four-electrode architectures may also be used by devices that are integrated into fabrics or wearable materials (as proposed in Section 2). In these devices, form factor and the requirement to be flexible, durable and able to withstand rigorous physical bending/stress without cracking may limit the choice of electrode material and electrolyte. For example, energy storage electrodes can be wound coaxially with binders to achieve high flexibility [89,97,98,155], elasticity [66,123], or integrated into

different weave patterns of a fabric (see Fig. 9) to improve electrical contact, mass loading of active material, and hence energy density [29,158,240]. Although most batteries and ECs use liquid electrolytes, the requirement of fibers to be twisted and bent (e.g., while the fabric is being worn) makes gel (e.g., PVA with H3PO4) [89,97] or solid polymer [241] electrolytes potentially more suitable for these devices due to their reduced risk of leaking (see also Section 4.5), however this durability advantage is typically at the expense of electrolyte conductivity. Another restriction with regards to wearables is toxicity (discussed further in Section 4.4) since device components in wearables may come into contact with skin.

4.3.4. Device architecture lessons learned Although many earlier hybrid devices were two-electrode devices, this configuration cannot provide a continuous source of power to drive a load and suffers from high ESR on discharge. Consequently, most studies have focused on either three or fourelectrode devices/systems. Use of the four-electrode configuration allows complete decoupling of the solar harvesting and energy storage functionality and this increases the range of applications, especially when space/volume is not limited, or form factor requires looser coupling. However, three-electrode systems may find applications where compact devices are required (e.g., solarpowered sensors) and, for this reason, the majority of the reported hybrid devices are three-electrode devices. As highlighted above, the common electrode of the three-electrode configuration presents challenges with regard to corrosion and electrical switching to allow for continuous operation. Additionally, if high energy densities are desirable, then higher voltage solar cells are

D. Lau et al. / Materials Today Energy 13 (2019) 22e44

35

Fig. 9. Scanning electron microscope images showing the physical differences between plane weave (A) versus knitted (B) textiles containing a Ni coated C composite fibrous storage element (reproduced with permission from American Chemical Society, 2013) [29].

preferably adopted as use of lower voltage solar cells requires that cells are connected in series and then the case for the common electrode is less convincing if it can only be formed on a single cell in the series string. 4.4. Material compatibility Material selections are limited for hybrid devices that are either in contact with the skin (e.g., wearables) or are implanted. Many of the solar cell technologies listed in Table 1 contain elements or compounds that are harmful and/or toxic to humans and animals. Although Si solar cells are limited by their lower Voc values which reduce the energy and power density of hybrid devices, Si is nonhazardous and therefore may be more suitable for devices that are in contact with humans and animals. Similarly, chemicals used in electrolytes also need to be non-toxic, especially in the event of device leaks or punctures. Many of the hybrid devices listed in Table 1 employ volatile solvents (e.g., PC and EC) which may pose flammability as well as toxicity concerns. In addition, electrolytes such as LiClO4 and LiPF6 can be harmful to living tissues [242,243]. For example, in the presence of H2O, LiPF6 can release hydrofluoric acid which is extremely lethal to humans [244]. Therefore, the intended application of the hybrid device will place critical restrictions on the selection of materials for all components of the device. 4.5. Electrolyte selection For standalone energy storage and harvesting hybrid devices to be practically safe for the applications proposed in Section 2 and cost-effective, the devices need to have sufficiently long operating lifetimes and be able to function under different environmental conditions (e.g., high temperature regions for outdoor operation, exposed to irradiation in space, chemical compatibility in living tissues). In addition, implants or wearable devices may need to continue to operate in the presence of constant motion, twisting and bending. The performance and durability of hybrid devices depends on the properties of the electrolyte (e.g., conductivity and stable operating voltage) used both for the solar cell (if not solid state such as DSSC) [245,246] and the storage electrode. Table 3 lists a few of the common electrolytes that have been used in hybrid devices and their corresponding conductivity. Aqueous electrolytes have been extensively used for ECs due to their high conductivity which results in low ESR [227], low-cost and safer operation compared to organic liquid electrolytes which are less conductive and present flammability risks

[236,237,243,248,254]. However, devices with aqueous electrolytes are sensitive to H2O electrolysis and corrosion as demonstrated earlier in Section 4.3.2. The H2 evolution reaction (HER) and O2 evolution reaction (OER) occur at around
0.9 V and 1.3 V respectively, with respect to the standard hydrogen electrode potential (SHE), defining a lower and upper ceiling for the operation of a storage device with an aqueous electrolyte [255e257]. The decomposition limit of aqueous electrolytes can be tuned by changing the pH as shown in Fig. 10 though the magnitude of the available operating voltage window remains limited. While adherence to these stability limits can protect electrodes from being overcharged [257], the main disadvantage of aqueous electrolytes is that their operating voltage window is capped at ~ 1.2 V [243,248,257,258]. Additionally, O2 and H2 evolution present device durability problems which include electrode corrosion, gas evolution leading to device swelling, pressurized cells that are vulnerable to punctures, reduced capacity and cyclability [248]. Organic electrolytes can provide a wider operating voltage window of ~3.5 V due to their increased stability to decomposition and reduced side reactions due to the absence of H2O [243]. Subsequently, organic electrolytes are better suited to higher voltage devices and can result in devices with higher energy density compared to devices using aqueous electrolytes [236,237,255]. However, the use of organic electrolytes introduces higher fabrication costs due to the need to assemble devices in moisture and O2 free environments. Organic electrolytes also have the disadvantages of higher toxicity and flammability which preclude their safe use in devices that may be in contact with humans or living organisms (i.e., implants) [227,242,243]. Organic electrolytes typically have lower conductivity (usually < 100 mS cm
1) compared to aqueous electrolytes (up to ~ 1 S cm
1) [248,259]. Higher electrolyte conductivity allows faster ion diffusion and hence reduces the ESR which limits the rates [242] or capacity [248,258] at which batteries/capacitors can be charged and discharged. Hybrid devices that use liquid electrolytes can present complications due to leakage arising from liquid expansion and volatilization of solvents [98,216,260]. Electrolyte leakage can also occur as a result of bending/twisting stresses (e.g., in fibrous hybrid devices) [97,123,261e264] and is a particular problem for devices in contact with human skin or flesh such as subdermal energy harvesting/ storage hybrid devices [168e171]. Operation in climates with highly variable climates incur the risk of the electrolyte freezing at low temperatures and expanding at higher temperatures. This can cause seals, which are designed to constrain electrolyte, to fail [193].

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D. Lau et al. / Materials Today Energy 13 (2019) 22e44

Table 3 Common electrolytes used for hybrid devices and their conductivities.

a

Category

Electrolyte

Aqueous Aqueous Aqueous Organic Organic Organic Organic Organic Organic Polymer gel Polymer gel Polymer gel Polymer gel

H2SO4 (30 wt %) KOH (29 wt %) NH4Cl (25 wt%) Et4NBF4 (1 M) in AcN Et4NBF4 (1 M) in PC LiPF6 (1 M) EC:DME 1:1 (v/v) LiCLO4 (1 M) EC:DMC 1:1 (v/v) Propylene carbonate (PC) Acetonitrile (AcN) PEO-LiBF4 PMMA-LiClO4 in PC PVAeH2SO4eARS PVA-H3PO4 1:1.5 (w/w)

Conductivity s (mS cm
1)

Reference

730 540 400 60 11 16.6 9 10.6 49.6 0.7 5 33.3 3.4

[247] [247] [247] [247] [248] [247] [242,249] [227] [227] [250] [250,251] [252] [253]

Abbreviations: DME: dimethoxyethane; DMC: dimethyl ethylene; ARS: alizarin red S; PMMA: poly(methyl methacrylate).

To circumvent the leakage problems, liquid electrolytes can be substituted with quasi-solid-state electrolyte such as aqueous gels [123,253], thermoplastic gels [216,265] and conducting gel polymers [242]. While these solid gel electrolytes may reduce the risks of leakage, they typically have poorer interface wetting [264], lower conductivity (e.g. < 0.1 mS cm
1) [242,250] and reduced redoxactive species mobility compared to aqueous solutions [266]. They do, however, present viable solutions for hybrid devices that are being constantly flexed or bent or devices for which leakage cannot be risked due to safety and toxicity concerns. 4.6. Electronic integration To-date, most devices/sensors that use ambient energy for power are implemented as separate sub-systems and not as an integrated hybrid device as described in the above sections. However, solar-powered hybrid devices are expected to experience similar electronic integration challenges as less closely coupled power harvesting sensors with respect to electronic integration with wireless communication electronics. As discussed in Section 4.2, sensor applications typically only need power in the order of mW to transmit wirelessly sensor data [119,168]. However, further improvements in wireless communication protocols for low power devices can aid in this regard. A number of low-power

communication protocols have been proposed for wireless sensor applications, including Zigbee, Z-Wave, EnOcean, KNX-RF (see reviews in Refs. [267e269]). These protocols have typically been designed with specific applications in mind. For example, the ZigBee protocol (an IEEE 802.15.4-based specification) has been designed for low-cost and low-power transmissions for transportation [182], whereas smart grid monitoring and home automation products have tended to use EnOcean [270] due to its low energy use [269]. For the solar-powered hybrid devices discussed in this review, it is desirable to match the solar charging voltage to that of the storage element as discussed in Section 4.7. However, it is also desirable to minimize the need to step-up the voltage for a wireless communication protocol as power is lost with each DC to DC conversion step. Another integration option for solar-powered hybrid devices is to implement each of the solar harvesting, storage and communications electronics on a single chip [271,272]. This close integration can offer greater miniaturization, with the closer on-chip coupling potentially eliminating the need to compromise on the use of a common electrode. Furthermore, the use of Si CMOS technology for these devices may render them sufficiently durable for outdoor applications and safe for medical implantation devices.

4.7. Reporting standardization

Fig. 10. Graph showing potential (vs. the SHE) at which various side-reactions will occur as a function of pH in an aqueous electrolyte with a [Liþ] ¼ 1.0 M. The grey zone represents a limiting operating window of an aqueous electrolyte as a function of pH (adapted from Li et al., 1994) [255,256].

Since there are no established standards for reporting the performance of hybrid devices, the measured metrics listed in Table 1 vary greatly. As mentioned in Section 1, areal metrics (capacities and capacitances) are typically reported due to the dependence of charge storage on the solar charging area. It can also be useful to report specific or volumetric capacities or capacitances (mAh g
1 or mF g
1 and mAh cm
3 or F cm
3, respectively) to permit material comparisons between devices [273]. However, gravimetric metrics require knowledge of the loading mass/volume of the active material which is not always straightforward to estimate. Furthermore, use of gravimetric and/or volumetric metrics can be confusing as it is not always clear whether the mass/volume of binders, current collectors and electrolyte are included [274]. Volumetric measures may be more appropriate for the larger RFB based systems [64,79,115,187]. However for fibrous devices [66,89,97,98], area and volume can be difficult to define. Therefore, a gravimetric or 1D quantification (e.g., capacity cm
1; see Fig. 11) may be more appropriate for these four-electrode systems. Table 4 lists the suggested measurement units based on the type of hybrid device.

D. Lau et al. / Materials Today Energy 13 (2019) 22e44

37

Fig. 11. Schematics illustrating how electrodes can be combined in either a coaxial or twisted manner. In these situations, quantifying areal capacity/capacitance is complex and of limited use. A gravimetric or volumetric unit may be more appropriate (reproduced with permission from Nature, 2017) [66].

Table 4 Recommended performance reporting metrics and units for different electrode configurations. Electrode configuration Two-electrode monolithic Three-electrode monolithic Three-electrode interdigitated Four-electrode monolithic (i.e. RFB) Electrically-connected four-electrode

Performance metric Areal Areal Areal Volumetric Gravimetric or Volumetric or 1D

Another performance metric that is variously reported for hybrid devices is the overall photo-electric conversion/storage device efficiency (hdevice). This metric can be calculated using [40,98,101,103].

hdevice ð%Þ ¼ hPV  hstorage ¼ hPV  ¼

Edischarge  100 Pin  APV  tcharge  nPV

Edischarge  100 Pin  APV  tcharge

(7)

where hPV is the efficiency of the PV device and hstorage is the coulombic efficiency of the storage element of the hybrid system. The hstorage is obtained from the discharged energy (Edischarge) divided by the input energy from the PV system which is defined by the incident light power density (Pin) multiplied by the effective solar capture area (APV) under a defined illumination duration (tcharge). Complications arise when attempting to compute hdevice as the product of hPV and hstorage when the latter values are measured in a non-integrated fashion. For example, the coulombic efficiencies of storage electrode hstorage is typically measured using a galvanostatic charge discharge (GCD) experiment with a constant and fixed discharge current [201,275]. However, when the energy storage electrode is directly charged by the solar cell in the integrated device, the charging current is determined by the current-voltage (IeV) curve of the solar cell (see Fig. 12). As the resistive load across the electrodes of the storage device increases, then the charging current decreases rather than remaining constant as it would do in a GCD measurement. An overall efficiency is only reported for a few of the devices in Table 1. However, since each of the tabulated hybrid devices were characterized differently and under non-identical illumination intensities, it is difficult to directly compare efficiencies between hybrid devices. Whilst it is possible to stipulate that all device performances should be measured under a standard illumination intensity (e.g., 1 sun intensity or 1000 W/m2), this stipulation would not take into account the fact that different devices may perform optimally when charged at different rates (i.e., when the solar cell is illuminated at different intensities). For example, the use of a low

Recommended units Energy Energy Energy Energy Energy

unit unit unit unit unit

per per per per per

cm
2 cm
2 cm
2 cm
3 or mL
1 g
1 or cm
3 or cm
1

light intensity may result in optimal charging of an electrode where charge storage may involve a slower charge transfer or diffusionlimited process. In this case, use of higher illumination intensities may either degrade the hybrid device prematurely or result in a reduced hdevice through a lower stored capacity and hence hstorage. Illumination intensity also typically influences hPV, with the degree to which the efficiency changes depending on the solar cell materials and fabrication process. For example, lower light intensity is expected to reduce series resistance losses in the solar cell thereby increasing hPV. The different optimal current densities for each of the PV and storage devices, and the widely ranging applications for hybrid devices, make reporting hdevice values of limited value unless when considering a common either PV or storage sub-device, or a specific application.

Fig. 12. Graph showing an I-V curve (black) and power-V (red) curve of a c-Si p-type commercial solar cell. When the storage electrode is incorporated at the rear of the solar cell (i.e., via a common electrode), the charging current will reduce from Point A to B as storage electrode approaches its maximum capacity. Once the potential difference across the common electrode and the counter electrode exceed the Voc of the solar cell then current will cease to flow across the storage electrodes. If electrical switches are used as shown in Fig. 7, then the solar cell can continue to provide power to an external load.

38

D. Lau et al. / Materials Today Energy 13 (2019) 22e44

5. Conclusions Hybrid solar energy harvesting and storage devices have the potential to find applications in micro-electronics when wired electricity networks are not available or when compact devices with specific form factors are required, especially for low power (mW to mW) applications where autonomous operation is desirable. This review has highlighted the potential of these devices to exploit freely available solar energy with the ‘on-board’ storage enabling buffering of energy/power for autonomous and continued operation in the field. Example applications include IoT sensors operating in the absence of wired electricity networks, wearable electronics and medical device implants. To-date, a multitude of different devices have been reported that use solar cell technologies ranging from DSSC, perovskite cells to c-Si p-n junction solar cells and charge storage based on double layer capacitance and/or Faradaic reactions. Given the ubiquity of solar power, there are many opportunities for solar-powered hybrid devices in a digital era whether they are deployed in fabrics, medical implants, environmental sensors for IoT or for a range of other applications which have not yet been conceived. This review has summarized the research efforts that have aimed to explore the potential of such hybrid devices and discussed the challenges of real-world applications. Our report has identified and discussed several challenges facing the realization, fabrication, and autonomous operation of this class of device. First, challenges arise due to the way in which the energy harvesting and storage functionality are integrated in a hybrid device. Three-electrode configurations are preferred over two-electrode configurations as they remove the need to discharge through the solar cell; however, they do this at the expense of requiring switches to control the current flow during charging and discharging. For cases where the required form factor places strict requirements on the hybrid device design (e.g., for wearables), then four-electrode systems may be more appropriate. Second, material choices for both the solar capture and energy harvesting functionality of hybrid devices can be limited by the applications. This is a particularly pertinent challenge for medical implants or wearable devices where contact with skin may be possible or inevitable. Many solar cell technologies that yield high charging voltages contain toxic materials, consequently the viable technology alternatives are significantly reduced and may be practically limited to Si solar cells. Finally, the use of liquid electrolytes raises concerns with regards to possible device leakage. Consequently, solid or polymer electrolytes may be required for safe operation. Practical devices in the future will need to consider designs that are appropriate for the intended application. This requires a consideration of the device configuration, materials and component technologies as well as the power requirements for solarpowered operation. For these hybrid devices to offer practical outcomes, durability will be critical as devices with short lifetimes may offer few benefits when compared with devices powered by low-cost rechargeable batteries. Further innovations and improvements in fabrication techniques are required to develop and demonstrate new hybrid devices that present real operating or cost advantages over alternative solutions comprising independent solar harvest and energy storage capability.

Acknowledgement The authors acknowledge the support of the NSW Node of Australian National Fabrication Utility, the UNSW Electron Microscope Unit and the Australian Research Council through Discovery Grant DP170103219 and Future Fellowship FT170100447 (Alison

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