Self-powered electrochemical energy systems to produce fuels

Abstract

In the pursuit of efficient fuel production, the challenges posed by the requirement of an external power source have prompted the need for self-powered energy systems by obtaining energy from the environment. Until now, significant progress on developing self-powered energy systems has been made. However, a more basic and in-depth study on their configuration is required for industrial applications. In this review, we outline the latest advancements of self-powered electrochemical energy systems constructed with solar energy, rechargeable batteries/fuel cells and triboelectric nanogenerators. Critical evaluations of the electrochemistry are highlighted to address the issues in elevating the efficiency of fuel production. In addition, the existing challenges and future prospects are also discussed, aiming to develop highly-efficient self-powered energy systems for green fuel production in the future.

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Hui Zhao

Hui Zhao received her PhD degree in Physical Chemistry from Nankai University in 2017. She is currently working at Liaocheng University as an associate professor. Her research focuses on the advanced energy materials and electrodes for fuel cells, water splitting and metal–air batteries.

Zhong-Yong Yuan

Zhong-Yong Yuan received his PhD degree in Physical Chemistry from Nankai University in 1999. He worked as a postdoctoral fellow at the Institute of Physics, Chinese Academy of Sciences from 1999 to 2001. He then moved to Belgium, working as a research fellow at the University of Namur from 2001 to 2005, prior to joining Nankai University as a full professor. In 2016, he was elected as a fellow of the Royal Society of Chemistry (FRSC). His research interests are mainly on the self-assembly of hierarchically nanoporous and nanostructured materials for energy and environmental applications.

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Wider impact

This review highlights transformative advancements in self-powered electrochemical systems for sustainable fuel production, focusing on solar energy integration, rechargeable batteries/fuel cells, and triboelectric nanogenerators (TENGs). Key developments include high-efficiency electrocatalysts enabling solar-driven water splitting with >11% solar-to-hydrogen efficiency, thermodynamically favorable reactions (e.g., urea/hydrazine oxidation) reducing energy consumption by 30%, and seawater electrolysis systems addressing freshwater scarcity. Innovations like TENG-powered CO₂ reduction and integrated PV-battery-electrolyzer systems demonstrate uninterrupted hydrogen production, even under fluctuating solar conditions. These technologies directly align with global decarbonization goals by replacing fossil fuel-dependent processes and advancing renewable energy utilization. The field's significance lies in its potential to revolutionize green fuel synthesis, enhance energy security, and support UN Sustainable Development Goals (e.g., clean water, affordable energy). Future progress hinges on scalable catalyst synthesis, AI-driven system optimization, and robust integration strategies. By bridging materials innovation (e.g., heterostructured catalysts, multifunctional electrodes) with system-level engineering, this work provides a roadmap for translating lab-scale breakthroughs into industrial applications, ultimately accelerating the transition to a carbon-neutral energy economy.

1. Introduction

The industrial production of fuels such as hydrogen, carbon monoxide and hydrocarbons often involves complex, hazardous and energy consuming processes.^1–3 Exploring low-power-consumption and eco-friendly routes to produce sustainable fuels in the absence of an external power source is a pressing need. Self-powered energy systems can obtain energy from the environment, and have attracted much attention in recent years. They allow fuel production for off-grid use in remote locations and reduce energy losses from current conversion.^4–6 Current self-powered energy systems can mainly be divided into various types including self-powered electrochemical energy systems constructed with solar energy, rechargeable batteries/fuel cells, and triboelectric nanogenerators (TENG) according to diverse energy sources.

Solar energy is clean and has abundant reserves, making it one of the key new energy sources developed by countries around the world. Converting solar energy into fuels through solar-driven water and carbon dioxide electrolysis can satisfy the requirement of replacing traditional fossil fuels. Integrating solar photovoltaic panels with electrocatalytic equipment (PV–EC) can achieve the continuous conversion and efficient utilization of solar energy.^7–9 Current research in this area is focused on the exploration of catalysts that can improve the reaction kinetics in electrolyzers, the system stability under day–night cycle operation and the solar-to-fuel efficiency.¹⁰

In self-powered electrochemical energy systems constructed with rechargeable batteries/fuel cells, rechargeable batteries such as metal–air batteries or fuel cells can be used as power supplies to generate electricity for driving the desired electrochemical reactions. The redox reactions occurring at the cathode and anode should have sufficiently large equilibrium potential difference to surmount the energy barriers for the integrated system.¹¹ Some novel batteries such as Zn-hydrazine batteries can achieve efficient and separate hydrogen generation. Metal–NO₂⁻/NO₃⁻ batteries can simultaneously generate NH₃ and electricity. Moreover, due to the low stability and lower energy collection efficiency of solar energy systems compared with traditional energy sources, the integrated energy device of “photovoltaic panel-rechargeable battery-electrolytic cell (PV-RB-EC)” can be constructed to achieve continuous and self-powered fuel production.¹² When the sunlight is insufficient, the battery releases electrical energy to ensure the normal operation of the system.

TENGs can collect low-quality energy from ambient environments such as water waves, wind, sound, human walking and mechanical vibration to produce electrical power.^13–15 Integrating TENG technology with an electrochemical reaction process can lead to high energy efficiency and sustainable development of electrochemical systems without an external power supply. Besides, TENGs can also be integrated with various energy storage devices such as lithium-ion batteries and supercapacitors to form a self-charging power system.^16–19 As a consequence, the environmental energy can be concurrently collected and stored as a sustainable power supply. Thus far, self-powered energy systems constructed with TENGs have been highlighted as a potential technique for green fuel generation. Many literature reports have provided detailed reviews of the three self-powered energy systems mentioned above.^20–22 However, a comprehensive examination and classification of self-powered systems for fuel generation from both theoretical and practical perspectives is still scarce.

Herein, as shown in Scheme 1, this review discusses the state-of-the-art research on self-powered electrochemical energy systems to produce fuels, critically estimating the obstacles still hampering large-scale application in this field. The recent advances in self-powered electrochemical energy systems focused on various fuel production processes, including water electrolysis, NH₃ synthesis and CO₂ reduction reaction (CO₂ RR), will be discussed and the factors that affect the efficiency of these systems will be investigated. We expect that this review will lay the foundation for self-powered energy systems and bring their practical application into a reality.

Scheme 1 Illustration of self-powered electrochemical energy systems.

2. Self-powered electrochemical energy systems constructed with solar energy

The configuration of direct solar energy-powered energy systems includes a photoelectrochemical (PEC) system and photovoltaic–electrochemical (PV–EC) system.²³ For the PEC system, the direct contact between PV and electrolyte often leads to the instability and short lifetime of the system. Besides, a higher solar-to-fuel efficiency has been reported in the PV–EC system in comparison with the PEC system. This review covers the PV–EC system as the main content.

2.1 Basic configuration

A PV–EC system consists of PV cells and electrocatalysts where electrochemical reactions occur. PV cells generate photovoltage via the separation of electron–hole pairs under sunlight. The output voltage must exceed the thermodynamic potential of the target electrochemical reaction. It can be divided into two types, namely, coupled PV–EC and de-coupled PV–EC systems.^24,25 The coupled PV–EC system refers to when the PV components and EC devices are connected in series, wherein the output photovoltage of PV cells should match the voltage of the EC device without external bias. In the de-coupled PV–EC system, a dc–dc convertor is present between the PV and EC components, which signifies the PV cells can offer the maximum output power and optimum voltage and current to the EC components. Although an additional cost for the de-coupled PV–EC system is required due to the presence of the dc–dc convertor, it has been reported to possess higher solar-to-hydrogen efficiency compared with the coupled PV–EC system.²⁶

For the PV–EC systems, the higher power conversion efficiency of the solar cells and the lower overpotential of electrocatalysts is beneficial for reducing energy loss and improving the overall efficiency. In addition, the compact EC configuration featured with low electrolyte resistance and directional transport and collection of electrolytic product microbubbles within a confined space are required to optimize the system efficiency.²⁷ The currently reported PV cells in the system include inorganic, organic and dye-sensitized solar cells.²⁸ For example, halide perovskite solar cell-based PV–EC devices have attracted much attention due to the low-cost and high-efficiency of halide perovskite solar cells.²⁹ The lower overpotential of electrocatalysts depends on the highly efficient electrocatalysts. Current research in this area is focused on the exploration of catalysts that can improve the reaction kinetics in electrolyzers and the solar-to-fuel efficiency.

2.2 The exploration of highly efficient electrocatalysts

Various electrocatalysts have been developed, including noble metals (such as Pt, Ru and Pd) and non-noble metals (such as Co, Ni and Fe), signifying their promising applications. Phosphidated cobalt oxide with a nanoscale heterointerface (PCO-nHI) was prepared via a phosphorization process of cobalt oxide-coated nickel foam to serve as an alkaline water splitting electrocatalyst.³⁰ The presence of heterojunction interfaces between CoP and CoO facilitates the superior HER and OER activities. Theoretical simulations confirmed that the presence of CoO could promote water dissociation, while CoP presents high activity in OH* removal. The catalysts were connected and integrated with a Si PV cell to form the PV–EC system (Fig. 1(a)). Intersection of the J–V curves of PCO-nHI||PCO-nHI and Si PV cells occurred at 9.9 mA cm⁻² and 1.7 V (Fig. 1(b)). It can achieve a solar-to-hydrogen efficiency of 11.5% for 100 h (Fig. 1(c)). Consequently, various water splitting electrocatalysts have been developed to generate hydrogen, such as NiFeO_x(OH)_y,³¹ P,O-incorporated cobalt sulfide,³² MoNi₄/MoO₂,³³ and metal–organic frameworks.³⁴

Fig. 1 (a) Schematic illustration of the all-back-contact-structured four-cell Si PV module. (b) J–V curves of series-connected Si PV cells and PCO-nHI| PCO-nHI electrodes in a two-electrode setup, under AM 1.5G 100 mW cm⁻² illumination. The blue dot indicates their intersecting points. The dotted lines represent the J–V curves after stability measurements. (c) Long-term unassisted water splitting J–t curves of the PV–EC system under AM 1.5G 100 mW cm⁻² illumination and their corresponding STH (%) values. Reproduced with permission.³⁰ Copyright 2024, Wiley VCH. (d) LSV of OD-Co in the presence and absence of NO₃⁻. (e) Schematic of a PV-electrolyzer system for solar-driven NH₃ synthesis. A GaInP/GaAs/Ge triple-junction solar cell powers the electrochemical cell consisting of Ni foam for the OER and OD-Co for the NiRR in 1 M KNO₃ electrolyte of pH 14. The illuminated area of the solar cell is 16 cm² and the area of the electrodes is 8 cm². (f) Stable operating current, FE of NH₃, and STF efficiency over 3 hours. Reproduced with permission.³⁵ Copyright 2021, Elsevier.

The PV–EC system can produce NH₃ through electrocatalytic reduction of nitrogen (NRR) or nitrate (NIRR).^36,37 It has been reported that the NRR powered by solar cells showed 30% energy efficiency, environmentally outperforming the fossil fueled Haber–Bosch NH₃ synthesis process.³⁸ The thermodynamically more favorable NIRR compared with NRR has also been developed due to the lower dissociation energy of the N=O bond compared to the N≡N bond. Singh et al. investigated various metal catalysts for the NIRR in 1 M KNO₃ electrolyte.³⁵ The results showed that the oxide-derived Co catalyst (OD-Co) possessed the optimal activity with a maximum NH₃ current density of ∼565 mA cm⁻² and FE ∼92%. The high activity of the NIRR over the HER can be achieved (Fig. 1(d)). The activity is limited by protonation of adsorbed NO₂ to form NO₂H and can be improved by increasing the surface roughness. The PV–EC system constructed with the EC and GaInP/GaAs/Ge solar cell demonstrated a solar to NH₃ efficiency of 11%, FE of 95% and a stable current of ∼300 mA at 1 sun over 3 h (Fig. 1(e) and (f)).

The PV–EC system for CO₂ reduction facilitates the production of C₁, C₂ and other multi-carbon fuels.^39–41 Cu₂O/In(OH)₃ with a heterojunction structure was prepared as an CO₂RR electrocatalyst.⁴² The partial Cu⁺ was converted to Cu while In(OH)₃ remained stable during the CO₂RR process. The stable production of H₂/CO can be achieved by using the solar cells (at the output voltage of −1.2 V) as the energy supply. Several electrocatalysts including Cu–Ag,⁴³ Au,⁴⁴ Cu–Sn,⁴⁵ Cu,⁴⁶ Bi,^47,48 Bi₂O₂CO₃⁴⁹ and Cs⁺ anchored defective carbon,⁵⁰ have been reported to connect with various solar cells to construct PV–EC systems for CO₂ reduction.

Using low-cost and large-area electrocatalysts is essential to the development of PV–EC systems towards commercialization.⁵¹ In this respect, choosing a suitable supporting substrate is of great importance. Many conductive substrates including metal foam/foil,⁵² stainless steel,⁵³ conductive glass,⁵⁴ carbon cloth/paper,^55,56 and fabric⁵⁷ have been reported. For instance, the Ni/SnS₂/aramid electrode was synthesized by the solvothermal synthesis of SnS₂ and electrodeposition of Ni using aramid as the substrate.⁵⁷ When two 10 cm² Ni/SnS₂/aramid electrodes were combined with a Si PV cell, the system demonstrated a solar-to-hydrogen efficiency of over 13.5% for 120 h in 1 M NaOH electrolyte.

2.3 Replacing with thermodynamically favorable reactions

With the assistance of the electrooxidation of small organic molecules such as urea,^58,59 hydrazine,^60,61 methanol,⁶² and 5-hydroxymethylfurfural,⁶³ the overpotential and cost of PV–EC systems can be greatly reduced. Ni–Fe phosphide/P-doped Fe-(oxy)hydroxide (P-FeOOH) was synthesized by the phosphating process of NiFeO_xH_y precursor.⁶⁴ NiFeP can offer high electronic conductivity and abundant active sites, while the P-FeOOH can provide the suitable adsorption energy of O-containing species. Besides, the superhydrophilic and super-aerophobic surface of the catalyst contributes to the efficient mass transfer. The material exhibited multifunctional catalytic activity towards the OER, HER and urea oxidation reaction (UOR) in 1 M KOH. When the catalyst was used as both the anode and cathode, a cell voltage of 1.537 V and 1.37 V was required to deliver 10 mA cm⁻² for water and urea electrolysis, respectively (Fig. 2(a)). The solar-driven urea electrolysis can be achieved with superior stability over 30 h (Fig. 2(b)).

Fig. 2 (a) Polarization curves for water splitting with and without 0.33 M urea in 1 M KOH. (b) J–t curve under chopped illumination; the inset is a schematic diagram of simulated solar-driven urea hydrolysis. Reproduced with permission.⁶⁴ Copyright 2023, Elsevier. (c) LSV curves for WP/NF-WP/NF in a two-electrode configuration with NIRR-OER, NIRR-UOR, and NIRR-HzOR. (d) Schematic illustration of NIRR-HzOR. (e) I–V curves of the WP/NF(NIRR)-WP/NF(HzOR) in a two-electrode configuration and perovskite solar cell, where the junction point indicates the ideal working current. (f) Chrono-potentiometric of the unbiased PV–EC system combining WP/NF(NIRR)-WP/NF(HzOR) with perovskite solar cells. Reproduced with permission.⁶⁵ Copyright 2023, Wiley VCH.

The combination of NIRR with the urea oxidation reaction (UOR) or hydrazine oxidation reaction (HzOR) is an energy-efficient NH₃ production method.⁶⁶ Tungsten phosphide (WP) nanowires were prepared via thermal evaporation and subsequent chemical vapor deposition with NaH₂PO₂·H₂O,⁶⁵ which exhibited bifunctional catalytic performance towards both NIRR and HzOR in alkaline media. When WP/nickel foam was used as both the cathode and anode, the NIRR-HzOR required a potential of 0.24 V to offer 10 mA cm⁻², which is much lower than that required for NIRR-OER (1.53 V) (Fig. 2(c)). The PV–EC system was constructed by the NIRR-HzOR with a perovskite solar cell (Fig. 2(d)). The operation point of the system was calculated at 0.36 V and 23.8 mA cm⁻² (Fig. 2(e)). It was demonstrated that the NH₃ production rate was 1.44 mg cm⁻² h⁻¹ at 23 mA cm⁻² (Fig. 2(f)).

In conventional CO₂RR configurations, the OER occurs at the anode which limits the practical viability. Though, replacing the OER with some novel reactions can reduce the voltage of PV–EC systems and improve the economic profitability.^67–70 The N-doped hollow carbon sphere supported Ni nanoparticles and Ni single atoms were prepared by calcining the mixture of melamine, nickel salts and hollow carbon sphere.⁷¹ The isolated Ni nanoparticles enhanced the electron density and reduced the electrocatalytic barrier for the *COOH formation, thus improving the catalytic activity of the CO₂RR to CO. The material showed high activity and selectivity in the CO₂RR to CO. When it was used as both the cathode and anode of the CO₂RR-HzOR electrolyzer, the voltage can be reduced about 960 mV compared to the CO₂RR-OER system at 10 mA cm⁻² in alkaline media.

Ammonia splitting to generate H₂ is an alternative approach due to the lower thermodynamic requirements to oxidize ammonia compared with water. In this case, the required potential of solar cells can be reduced. It has been reported that the electrochemical cell can be constructed by using Ru-based molecular catalysts as the anode, Pt mesh electrode as the counter electrode and propylene carbonate as the solvent and tetrabutylammonium hexafluorophosphate as the electrolyte.⁷² A single perovskite solar cell can drive the device to produce N₂ and H₂ for more than 30 min with a faradaic efficiency of 89% with no external applied bias.

2.4 Solar-to-fuels in seawater

Seawater splitting (SWS) to produce H₂ is a promising way to solve the problem of global freshwater scarcity. It can be achieved by direct SWS and electrolysis after seawater desalination.^73,74 A solar-powered SWS system allows for fuel production from seawater without an external energy supply.

3D PdCo–Co₃S₄ heterostructure nanocages were prepared using zeolitic imidazolate framework-67 (ZIF-67) as the precursor and a subsequent cation exchange reaction between Co²⁺ and Pd²⁺.⁷⁵ The material exhibited a hierarchical nanocage–nanocluster structure and abundant amorphous–crystalline heterointerfaces, contributing to the superior catalytic performance and high stability (Fig. 3(a)). It is shown that the amorphous–crystalline heterointerfaces can reduce the Gibbs free energy and provide abundant active sites for the HER in alkaline seawater. The SWS system can be powered by commercial silicon solar cells, showing high HER activity and long-term stability for 100 h (Fig. 3(b)).

Fig. 3 (a) Polarization curves of PdCo–Co₃S₄ in varied electrolytes during the HER. (b) Solar-driven seawater electrolyzer. Reproduced with permission.⁷⁵ Copyright 2024, Wiley VCH. (c) Seawater electrolyzer driven by a solar cell. (d) Working mechanism of the multi-functional SWCNT-based hybrid film for seawater evaporation and collection. (e) Functions of the multi-functional SWCNT-based hybrid films. Reproduced with permission.⁷⁶ Copyright 2024, Elsevier.

Seawater is favorable for the dissolution of a high-concentration of CO₂ and the presence of Cl⁻ can inhibit the HER. Direct CO₂ electrolysis in seawater allows the simultaneous conversion of CO₂ into fuels and the chlorine ions into Cl₂.^77–80 Cobalt phthalocyanine-implanted graphitic carbon nitride nanosheet (CoPc/g-C₃N₄) catalysts were prepared via a mechanochemistry method to drive the CO₂ electrolysis in seawater.⁸¹ The catalyst is enabled with a Na⁺ preferential adsorption in electrolyte, thus is favorable for inhibiting HERs. The CoPc/g-C₃N₄ catalyst demonstrated a FE of 89.5% for CO with 16 mA cm⁻² in natural seawater and operated stably for 25 h in simulated seawater. Besides, the CO production rate was increasing under the promotion of the chlorine evolution reaction with fast reaction kinetics, thus achieving the co-production of CO and Cl₂.

However, direct SWS often suffers from several issues such as competition between the OER and chlorine evolution reaction (ClER) and cathode deactivation brought about by electrolyzer corrosion and metal precipitation.^82,83 To solve this issue, integrating thermodynamic favorable reactions such as HzOR into direct SWS could reduce the required voltages and prevent interference from the ClER.⁸⁴ Besides, a solar-driven SWS system without external energy has also been reported. The single-wall carbon nanotube (SWCNT)-based hybrid films composed of a partially oxidized single-wall carbon nanotube (POSWCNT) layer featuring hydrophilicity and a PtNiCoFeMo high-entropy alloy nanowire (HEAN)/SWCNT hybrid layer with hydrophobicity were used as both the interfacial water evaporation membrane and the electrode for water-splitting (Fig. 3(c)).⁷⁶ The seawater was first distilled into pure water and then electrolyzed to generate H₂ (Fig. 3(d)). The POSWCNT layer accelerates seawater absorption and transport and the (HEAN)/SWCNT layer has good photothermal conversion and salt resistance for evaporation (Fig. 3(e)). The as-fabricated solar-driven SWS system exhibits a hydrogen productivity of 1.04 × 10⁴ L day⁻¹ m⁻² and works continuously for 100 h without any external energy consumption.

2.5 PV-RB-EC systems

Implementation of rechargeable batteries into the PV–EC system to construct PV-RB-EC systems can smoothen the PV power fluctuation, stabilize the performance of the EC component and improve the solar-to-fuel efficiency.⁸⁵ In this case, PV cells are directly connected to a battery and an EC component in parallel. The smooth uninterrupted operation with high stability of EC can be achieved with the assistance of the battery.

It is reported that the solar-to-hydrogen efficiency in the PV-RB-EC system can be higher than that of PV–EC systems due to the spreading of the PV energy over the whole operation cycle.⁸⁶ The suitable battery voltage and capacity determine the efficient operation of the PV-RB-EC system. Batteries such as metal-ion batteries,⁸⁷ Zn–air batteries,^88,89 and NiZn batteries⁹⁰ have been introduced into the system.

For instance, FeNiP nanoparticles within a N, P-doped porous carbon nanofiber layer on carbon cloth (FeNiP@p-NPCF/CC) were prepared via a supermolecular assembly-assisted electrodeposition method.⁹¹ Benefiting from a 3D hierarchically porous structure and the hetero-engineering between FeNiP and carbon, the FeNiP@p-NPCF/CC possessed abundant active sites, good mass transfer characters and suitable adsorption energies of the reaction intermediates. Uninterrupted H₂ production could be obtained from an EC powered by a silicon photovoltaic cell in the day and aqueous zinc–air batteries in the night, and the integrated system showed superior stability in 15 day tests. Meanwhile, cobalt selenide particles distributed on N-doped carbon nanosheets (Co_0.85Se/NC) were synthesized via a selenylation-carbonization method.⁹² Profiting from the large surface area, rapid mass transfer process and abundant high oxidation state Co species, the Co_0.85Se/NC catalyst exhibited high trifunctional activity towards the OER, ORR and HER in alkaline media, and could be used as efficient electrodes for a two-electrode water electrolyzer and Zn–air battery. Subsequently, the self-powered system was constructed by connecting silicon solar cells, Zn–air batteries and water electrolyzers (Fig. 4). The Zn–air battery showed an open circuit voltage of 1.4 3 V after being charged by solar energy and subsequently offered a stable output voltage for over 260 min at 10 mA cm⁻². At night, the water electrolyzer can be powered by two Zn–air batteries and the calculated FE was 96.49–99.42%. The efficiency of the solar-to-hydrogen production was 4.34%.

Fig. 4 (a) Polarization curves of Pt/C//IrO₂ and Co_0.85Se/NC//Co_0.85Se/NC cells in a 1 M KOH electrolyte. (b) Charge–discharge curves and the power density curve for the ZAB based on Co_0.85Se/NC and Pt/C + IrO₂ catalysts. (c) The overall concept of a self-powered H₂ production system. Reproduced with permission.⁹² Copyright 2023, the Royal Society of Chemistry.

Notably, solar cells and electrocatalysts can degrade under prolonged exposure to moisture, heat, or corrosive electrolytes. In seawater electrolysis, chloride-induced corrosion of catalysts and anode materials remains a critical issue, leading to metal dissolution and performance decay. Researchers have developed numerous high-efficiency electrocatalysts based on metals and non-metals, focusing on optimizing their composition, electronic structure, interface structure and structure–performance relationship. Replacement of the OER by other anodic reactions with lower thermodynamic potentials can greatly reduce cell voltage and energy consumption. Furthermore, using the electro-oxidation of polluting molecules to replace the OER can accomplish both efficient fuel production and pollutant degradation. This strategy can also be applied to overcome the competition between the OER and chlorine evolution reaction (ClER) and cathode deactivation brought about by electrolyzer corrosion and metal precipitation in the seawater system. Besides, the development of rechargeable batteries with high capacity and high safety allows a steady output voltage for the electrolyzer at night, thus achieving an uninterrupted solar-to-fuel system.

3. Self-powered electrochemical energy systems constructed with rechargeable batteries/fuel cells

Rechargeable batteries and fuel cells are generally used as power suppliers. They provide the electricity required to drive the electrochemical reactions in an electrolyzer for fuel production. The continuous supply of cost-effective, stable and renewable electricity should be achieved by the batteries/fuel cells to satisfy the requirements of practical application.

3.1 Fuel cell-powered energy systems

Currently, most of the reported fuel cells for driving fuel production are hydrazine fuel cells (DHzFCs). Hydrazine hydrate has strong reducibility and is usually used as a reducing agent in the preparation of micro/nano materials, which generates a large amount of hydrazine wastewater in industrial production. DHzFCs have high theoretical energy density, high theoretical potential, low operating temperature, and environmentally friendly products.^93,94 Constructing a DHzFC-powered energy system can thus not only overcome the theoretical voltage limitation of fuel production, but also achieve the utilization of hydrazine wastewater. In a DHzFC, hydrazine is used as the fuel and O₂/air is used as an oxidant to generate electricity through the electrochemical reactions in alkaline solutions (cathode: N₂H₄ + 4OH⁻ → N₂ + 4H₂O + 4e⁻; anode: O₂ + 2H₂O + 4e⁻ → 4OH⁻).⁹⁵

For hydrazine-assisted H₂ production, the development of bifunctional HER/HzOR electrocatalysts is of paramount importance.⁹⁶ The design of catalysts can be achieved through the regulation of composition, electronic structure and lattice strain, etc.^97–99 The Ru single atoms on NiCoP nanowire arrays (Ru–NiCoP) were fabricated by a hydrothermal–phosphidation–pyrolysis procedure.¹⁰⁰ Benefiting from the atomically dispersed Ni(Co)–Ru–P interface sites on NiCoP immobilized with Ru single atoms, the prepared catalysts showed excellent HzOR and HER performance in alkaline electrolyte. DFT calculations indicated that the Ru single atom optimized H* adsorption and modulated the d band center, leading to a decrease in the energy barrier for the HzOR and optimization of the activity of HzOR. As a result, the overall hydrazine splitting (OHzS) using Ru–NiCoP as both anode and cathode catalysts exhibited a working potential of −60 mV and overpotential of 32 mV for 10 mA cm⁻² (Fig. 5(a)). The maximal power density of 176 mW cm⁻² for DHzFC can be attained (Fig. 5(b)). Furthermore, the H₂ production system utilizing the OHzS device driven by a DHzFC displayed a H₂ production rate of 24 mol h⁻¹ m⁻² (Fig. 5(c)).

Fig. 5 (a) Schematic illustration of a self-powered hydrogen production system integrated by driving the OHzS electrolyzer with DHzFC as the power supply. (b) Discharge polarization curve and power density plot of the DHzFC with a Ru₁–NiCoP anode and Pt/C cathode. (c) The amounts of H₂ generated from the self-powered H₂ production system. Reproduced with permission.¹⁰⁰ Copyright 2023, Wiley VCH. (d) Illustration of a self-powered H₂ production system integrating a fuel cell and bipolar hydrogen production unit. (e) Current density–voltage and power density plots for the DFFC. Reproduced with permission.¹⁰¹ Copyright 2024, Wiley VCH.

Despite the high cost of hydrazine, the OHzS electrolyzer simultaneously generates N₂ at the anode and H₂ at the cathode, and the subsequent gas separation procedure leads to an increase in the overall cost. Other fuel cells such as direct furfural fuel cells (DFFCs) were explored to drive H₂ production. A notable example is the construction of a self-powered H₂ production system by combining a DFFC with a furfural oxidation reaction (FOR)-assisted bipolar H₂ production electrolyzer (Fig. 5(d)).¹⁰¹ The RuCu/CF catalyst was prepared by dispersing Ru atoms into Cu nanowire arrays in situ grown on copper foam. The HER-FOR coupling system using RuCu/CF as the HER catalyst and Cu/CF as the FOR catalyst can reach 100 mA cm⁻² at a cell voltage of 0.43 V while producing H₂ with 200% faradaic efficiency. A maximal power density of 193 mW cm⁻² for DFFCs can be achieved (Fig. 5(e)). The DFFC powered HER-FOR unit can generate pure H₂ at a rate of 6.1 mmol h⁻¹ m⁻².

3.2 Metal–air battery-powered energy systems

Metal–air batteries have attracted much interest in recent years due to their high energy density. The sluggish gas reduction/evolution reaction rate and rapid catalysis decay restrict their industrial applications.¹⁰² For instance, the rechargeable ZAB uses the oxidation reaction of zinc and the ORR reaction to generate electricity to drive devices. During the charging process, the zinc oxide formed during the discharge process is reduced to metallic zinc at the anode and O₂ is released through the OER at the cathode. Both the ORR and OER are sluggish and complex reactions involving multiple electron and proton transfer processes are required.¹⁰³ Thus, the utilization of highly-efficient bifunctional catalysts for both the ORR and OER is essential for improving energy efficiency.

For a ZAB-powered H₂ production system, it is necessary to explore a trifunctional electrocatalyst that can simultaneously promote the kinetics of the OER, ORR and HER. Wang et al. fabricated Co_5.47N and Co₇Fe₃ embedded in a N-doped carbon nanotube modified cruciform carbon matrix (CoFeN-NCNTs//CCM).¹⁰⁴ This catalyst exhibited trifunctional activity with a half-wave potential (E_1/2) for the ORR of 0.84 V vs. RHE, an overpotential at 10 mA cm⁻² (η₁₀) for the OER of 325 mV, and η₁₀ of 151 mV for the HER. When applied as the cathode catalyst in a ZAB, the catalyst exhibited a charging–discharging voltage gap of 0.76 V with a stable working period over 445 h. Furthermore, when used as both the cathode and anode for overall water splitting, the catalyst needed a cell voltage of 1.63 V at 10 mA cm⁻². The CoFeN-NCNTs//CCM-based ZABs can drive the overall water splitting with 100% faradaic efficiency for H₂ and O₂. Another case is a trifunctional Ni–P/Fe–P collaborated electrocatalyst for the HER/OER/ORR in a solar cell and ZAB coupled water splitting devices.¹⁰⁵ As depicted in Fig. 6(a), ZABs were charged by the solar cell and the electricity from the solar cell split water to produce H₂ under a light reaction. The measured volumes of H₂ and O₂ were matched the theoretical values (Fig. 6(b)). The charged ZABs released electric energy to drive water splitting for H₂ production under unassisted light reactions. The conversion efficiency of the solar-to-hydrogen and solar-to-water splitting devices were 4.6% and 5.9%, respectively.

Fig. 6 (a) The overall concept of a self-powered energy system. (b) The corresponding measured and calculated gas volumes of H₂ and O₂. Reproduced with permission.¹⁰⁵ Copyright 2022, Wiley VCH. (c) Schematic illustration of the self-powered CO₂-to-CO conversion enabled by NFCNT as a tri-functional electrocatalyst. (d) FEs of CO and H₂, and the total EE of the ZAB-driven CO₂ RR. Reproduced with permission.¹⁰⁶ Copyright 2024, Wiley VCH.

Moreover, it is reported that replacing the anodic OER by oxidation reactions with more favorable kinetics can significantly decrease the charging voltage and improve the utilization efficiency of ZABs. A CoNi@NCNTs-LDH/CC catalyst was synthesized by using CoNi(CO₃)_0.5OH/CC as the precursor. The CoNi@NCNTs and ultra-thin nanosheet CoNi-LDH serve as the main active sites for the ORR and UOR, respectively.¹⁰⁷ The catalyst exhibits superior bifunctional catalytic activity towards both the ORR and UOR. The urea-assisted ZABs possess a high energy conversion efficiency of 74.6%, a stable operation over 350 h and a urea elimination rate of 78.9 g m⁻² h⁻¹.

The self-powered CO₂RR driven via one catalyst by ZABs has been investigated.¹⁰⁸ A nitrogen and fluorine co-doped carbon nanotube (NFCNT) was synthesized by pyrolysis of carboxylated CNT and 2,3,5,6-tetrafluoro-1,4-benzenedicarbonitrile.¹⁰⁶ The NFCNT exhibited trifunctional catalytic activities toward the ORR, OER and CO₂RR benefiting from charge redistribution induced by the synergistic effect of N, F-co-doping (Fig. 6(c)). The ZAB using the NFCNT as the air electrode offers a power density of 230 mW cm⁻² and good durability over 100 cycles. The NFCNT obtained CO₂-to-CO conversion with a high CO Faraday efficiency of 94%. Notably, the CO₂ electrolysis powered by two in-series connected ZABs achieved 80% CO Faraday efficiency and 60% total energy efficiency (Fig. 6(d)).

3.3 Other battery-powered energy systems

Recently, it was reported that some novel batteries could simultaneously achieve efficient fuel production and electricity generation. These batteries with integrated fuel production and electricity generation are more favorable for the demands of energy applications compared with conventional batteries.

For H₂ production, the metal–H₂ batteries with a cathodic HER and anodic metal oxidation have been assembled. In a study, an alkali-acid Zn–H₂ battery was constructed by using Ru nanoparticles/3D porous N-doped carbons as the cathode to catalyze the HER in an acid, along with the oxidation of a Zn anode in an alkaline.¹⁰⁹ A bipolar membrane was utilized to separate the anolyte of 4.0 M NaOH and catholyte of 2.0 M H₂SO₄. The Zn–H₂ battery exhibited good performance for H₂ generation with a power density of 126.5 mW cm⁻² and an energy density of 966 W h kg⁻¹. In addition, the OER can be replaced by the polysulfide oxidation reaction (SOR, S_x²⁻ + S²⁻ → S_x+1²⁻ + 2e⁻) which possesses a lower theoretical potential (−0.48 V vs. RHE) compared with that of the OER.¹¹⁰ The Yuan group synthesized a FeCoNiCuMn high-entropy alloy and Mo₂C nanoparticles on 3D porous carbon (HEA-Mo₂C/HPC).¹¹¹ The catalyst exhibited superior catalytic performance towards the HER (41.2 mV at 10 mA cm⁻²) and SOR (0.382 V at 100 mA cm⁻²) in alkaline electrolyte. The zinc–polysulfide battery was assembled by using HEA-Mo₂C/HPC as the cathode and a zinc plate as the anode. With the assistance of unassisted light reaction by solar cells, the batteries can continuously drive H₂ production at night. Recently, a Zn–NH₃ battery based on ammonia splitting achieved NH₃-to-H₂ conversion,¹¹² in which, an efficient bifunctional catalyst towards the ammonia oxidation reaction and HER was employed. Other batteries such as Zn–hydrazine batteries, nickel hydride batteries and Li-ion batteries have also been developed to drive H₂ production.^99,113,114

For NH₃ production, metal–N₂ batteries featuring high faradaic efficiency and high energy density were explored to produce NH₃ without electricity consumption.^115,116 The cathode reaction is the NRR reaction and the anode reaction is the metal oxidation reaction during the discharge process. Hollow molybdenum phosphate microspheres (MoPi/HSNPC) were fabricated as a bifunctional catalyst for the NRR and simultaneous power generation.¹¹⁷ Benefiting from the structural advantages and MoPi active components, superior NRR performance can be achieved with a NH₃ yield rate of 18.66 μg h⁻¹ mg_cat⁻¹ and a faradaic efficiency of 9.04% at −0.2 V vs. RHE in alkaline electrolyte. When used in Al–N₂ batteries as the cathode, the assembled battery demonstrated a peak power density of 2.37 mW cm⁻² and long-term energy output. Besides, the Zn–nitrate/nitrite batteries were reported to enable NH₃ production.^118–120 Fe-doped nickel phosphide (Fe/Ni₂P) was prepared via the phosphidation of NiFe layered double hydroxide.¹²¹ The doping of Fe can tune the electronic structure of the catalyst. As a result, Fe/Ni₂P exhibited good catalytic performance towards the NO₃⁻RR with a NH₃ production rate of 4.17 mg h⁻¹ cm⁻² and FE of 94.3% at −0.4 V. A Zn–NO₃⁻ battery formed by integrating Fe/Ni₂P as the cathode and zinc foil as the anode delivered a power density of 3.25 mW cm⁻² with an FE of 85% for NH₃ production.

Rechargeable metal–CO₂ batteries can integrate the CO₂RR with battery technology to achieve the conversion of CO₂ into value-added fuels/chemicals and green electricity at once. Among various metal–CO₂ batteries, the Zn–CO₂ battery stands out due to its good stability, large theoretical capacity, safety and environment friendliness.¹²² The cathodic reactions are the CO₂RR and OER, respectively, during discharging and charging processes. In a study, B and N-containing carbon (C-BN@600) was obtained using an ionic liquid and zinc imidazole borate framework composite.¹²³ The bifunctional catalytic activity of C-BN@600 towards both the CO₂RR and OER was observed due to the synergy between an electron-withdrawing B atom and an electron-donating N atom. The as-assembled Zn–CO₂ battery exhibited a maximum power density of 5.42 mW cm⁻² and a maximum methanol yield rate of 1491.36 μg h⁻¹ mg_cat⁻¹.

Fuel production via battery/fuel cell-powered electrochemical pathways has gained significant attention due to their ease of operation under ambient conditions using renewable energy source derived electric power supplies. The application of batteries such as DHzFCs could be a “killing three birds with one stone” strategy that is capable of pollutant removal, energy supply and fuel production. The utilization of metal–air batteries such as ZABs is a potential strategy because of its high-power density, low-cost and environmental friendliness. Besides, they can facilitate the construction of the integrated energy device “photovoltaic panel-rechargeable battery-electrolytic cell (PV-RB-EC)” to achieve continuous and self-powered fuel production. Novel batteries such as metal–H₂/N₂/CO₂ batteries, metal–hydrazine batteries and metal–nitrate/nitrite batteries could simultaneously accomplish fuel production and electricity generation, and deserve to be further developed. When the sunlight is insufficient, the battery releases electrical energy to ensure the normal operation of the system.

4. Self-powered electrochemical energy systems constructed with a triboelectric nanogenerator

TENG can harness small-scale mechanical energy such as water, wind and ocean energy from the environment and convert it into electrical energy. TENG-based self-powered electrochemical systems can achieve the high energy efficiency and sustainable development of electrochemical systems without requiring an external power supply. TENG can also be integrated with energy storage devices to form self-charging power systems. Thus, environmental energy can be concurrently collected and stored as a sustainable power supply.

4.1 TENG-powered water electrolysis

TENG-powered water electrolysis is a promising method for green hydrogen production.^124–129 A system that combines a TENG with a water electrolyzer can be self-powered without using an external energy source. Mechanical energy induces contact electrification between triboelectric materials, generating an alternating current (AC). A rectifier converts AC to direct current (DC) for electrochemical reactions.

A contact-separation mode TENG with MXene incorporated polydimethylsiloxane film as a negative triboelectric material and a magnetic covalent organic framework composite as a positive triboelectric material was designed (Fig. 7(a)).¹³⁰ As shown in Fig. 7(b), the TENG can harvest mechanical energy from human actions and generate a peak-to-peak open-circuit potential of 146 V. The energy was applied for water electrolysis to produce hydrogen. A half-wave rectifying water flow-driven TENG (HRWF-TENG) was fabricated.¹³¹ When the HRWF-TENG was coupled to the water splitting unit, it could drive the RuO_x/CNT catalyst for electrolytic hydrogen production with a hydrogen production rate of 12.32 μL min⁻¹ and a conversion efficiency from blue energy to hydrogen energy of 2.38%.

Fig. 7 (a) Schematic diagram of the contact-separation mode triboelectric nanogenerator (CS-TENG). (b) Schematic diagram of the conversion of biomechanical energy into green hydrogen fuel. Reproduced with permission.¹³⁰ Copyright 2023, Wiley VCH. (c) Schematic diagram showing the design of the self-powered electrochemical system from the blue energy to green resources. Floating TENGs, electrolyzers, and chemical storage units are included in this system. Reproduced with permission.¹³² Copyright 2022, Wiley VCH. (d) Diagram of the W-TENG module. Reproduced with permission.¹³³ Copyright 2022, American Chemical Society.

Electrochemical seawater splitting to produce hydrogen has also been investigated.¹³⁴ For instance, a system consisting of soft-contact mode TENG devices and a seawater-containing electrolyzer was designed, wherein the TENG devices serve as ocean wave energy harvesters to generate electricity and the electrolyzer is used for hydrogen production (Fig. 7(c)).¹³² The system can generate hydrogen at a rate of 814.8 μL m⁻² d⁻¹ with a Faraday efficiency of 69.1%. Furthermore, the development of highly efficient catalysts for seawater splitting is crucial due to the low conductivity of seawater and the large overpotential of the chlorine evolution reaction. A seawater electrolysis system comprising wind-driven TENG (W-TENG), transformers (Fig. 7(d)) and a seawater splitting unit was fabricated. Benefiting from the high catalytic activity of a carbon paper supported NiCoP-MOF catalyst, the system achieved a H₂ production rate of 1273.9 μL min⁻¹ m⁻² with a conversion efficiency of 78.9%.¹³³

4.2 TENG-powered electrocatalytic synthesis of NH₃

TENG-powered electrocatalytic synthesis of NH₃ is a green NH₃ synthesis method.¹³⁵ The high voltage of TENG contributes to the generation of a strong electric field, thus is favorable for breaking the chemical bonds of N₂. Tang et al. reported NH₃ synthesis via N₂ discharge driven by Tesla turbine TENGs.¹³⁶ The gas kinetic energy can be transformed into high-speed rotation energy to drive the TENG for power generation by using the bladeless turbine. The high-energy plasma produced from N₂ discharge can react with water to give NH₃ products. The NH₃ yield of 2.14 μg h⁻¹ was achieved by the self-powered system. Besides, they also reported a self-powered electrocatalytic NH₃ synthesis method directly from air as driven by dual TENGs.¹³⁷ By air discharge induced by a TENG device, N₂ was fixed into NO_x. Meanwhile, driven by the other TENG, electrocatalytic reduction is carried out to synthesize NH₃ using TiO₂ as the catalyst in an electrochemical cell. As a result, the NH₃ yield was 2.4 μg h⁻¹ under ambient conditions.

The electrocatalytic nitrate reduction reaction to NH₃ is a promising approach to NH₃ production. Gao et al. reported a self-powered electrocatalytic nitrate to NH₃ process by lightweight TENGs for wind energy harvesting.¹³⁸ The lightweight TENG can achieve energy harvesting in a broad wind speed band. A capacitor-based power management circuit was used to reduce the voltage and stabilize the power output. A polycrystalline Cu catalyst was synthesized by an electrodeposition method to accelerate the reaction kinetics of the electrocatalytic reactions. The NH₃ yield of the system is 11.48 μg cm⁻² h⁻¹ driven by the roller-shaped TENG.

4.3 TENG-powered CO₂ reduction reaction

A wave-energy-driven CO₂RR system comprising of a spring-assisted TENG, a power management circuit and an electrochemical cell for the CO₂RR and OER was designed.¹³⁹ The TENGs can convert wave energy to electricity stored in a capacitor and then the stored energy can be used to convert CO₂ into formic acid through electrochemical reduction. It is shown that the CO₂RR system can produce 2.798 μmol of formic acid per day with the wave energy collected from an area of 0.04 m².

The Du group reported a self-powered system consisting of mechanical energy driven triboelectric plasma and an In₂O₃ catalyst containing O vacancies to obtain CO₂RR to CO.¹⁴⁰ The triboelectric plasma was powered by the high output voltage of the TENG that harvests mechanical energy. The triboelectric plasma contains various reactive ions that can achieve the pre-activation of CO₂ molecules. The CO evaluation rate of 0.2 mmol g⁻¹ h⁻¹ and an energy conversion efficiency of 10.8% from electrical to chemical energy was achieved. Furthermore, to promote the energy efficiency of TENG, they constructed a large-area and high efficiency freestanding rotating TENG (FR-TENG).¹⁴¹ The conversion efficiency of FR-TENG from mechanical to electrical energy was 16.7%. When it was used to drive the triboelectric plasma CO₂RR, the generation rates of CO and O₂ were 8.5 and 4.33 μmol h⁻¹, respectively, and the conversion efficiency from mechanical to chemical energy was 1.84%. The field experiments demonstrated that the generation rates of CO and O₂ were 5.06 and 2.33 μmol h⁻¹, respectively, and the conversion efficiency from mechanical to chemical energy was 0.72% under a wind speed of 2.3 m s⁻¹.

The application of TENG-powered electrochemical systems offers a promising strategy of utilizing renewable energy for diverse electrochemical processes. Numerous studies have been devoted to the design of TENG-based self-powered electrochemical systems. Current research studies are mainly focused on the rational design of TENGs and the development of high-efficiency electrocatalysts. On one hand, it is crucial to improve the efficiency and stability of TENGs. On the other hand, the electrode materials play a key role in the electrochemical reaction, therefore, high-efficiency electrocatalysts are imperative for accelerating the kinetic process of the electrochemical reaction.

5. Conclusions and prospects

Self-powered electrochemical energy systems constructed with solar energy, rechargeable batteries/fuel cells and triboelectric nanogenerators provide a potent approach of employing renewable energy for fuel production. Numerous researchers have leveraged advanced techniques to develop a diverse range of self-powered electrochemical systems. This review offers a thorough overview of the most recent progress in self-powered electrochemical energy systems. Notwithstanding the significant progress achieved in the field of self-powered electrochemical systems, inherent challenges concurrently exist and need to be addressed for further development.

Firstly, researchers have developed numerous highly efficient electrocatalysts for self-powered electrochemical energy systems. The exploration of electrocatalytic reaction mechanisms usually requires theoretical calculations and the use of some in situ characterization techniques. To achieve commercial applications, there is an urgent need to explore low-cost and large-area preparation methods and technologies for electrocatalysts. Designing catalysts with atomic precision (e.g., single-atom catalysts, high-entropy alloys) and hybrid architectures could unlock unprecedented activity and durability. With the assistance of the electrooxidation of small organic molecules, the overpotential and cost of the energy systems can be greatly reduced. However, the catalyst used lacks universality and needs further exploration. Currently, seawater electrolysis is a promising solution with financial and technical merits. Efficient and stable catalysts for seawater electrolysis are imperative. Furthermore, the capital cost and device complexity still need to be addressed.

Secondly, solar-driven systems hold great promise due to their compatibility with existing infrastructure and high energy conversion efficiencies, but their reliance on intermittent sunlight necessitates hybrid designs integrating rechargeable batteries or TENGs for uninterrupted operation. Fuel cells, metal–gas batteries and other novel batteries have served as energy sources utilized in self-powered electrochemical energy systems. Most of the reported fuel cells to drive fuel production are hydrazine fuel cells. However, the high cost of hydrazine limits their wide application for large-scale fuel production. Metal–air batteries are not suitable for large current industrial applications owing to their high internal resistance. In this regard, more novel batteries/fuel cells with high capacity and high safety should be explored to overcome the limitations associated with current self-powered energy systems. The integration of a TENG device with an electrochemical device for improving the output power and stability of TENGs is of paramount significance. Besides, the design of efficient power management circuits is highly desirable due to the impedance mismatch between TENGs and electrochemical devices. Moreover, it is necessary to construct an integrated electrocatalysis system to alleviate the voltage requirements of the electrolyzer, thus promoting the energy utilization efficiency of the self-powered system.

Thirdly, the construction of self-powered electrochemical energy systems generally consists of three modules, namely the energy harvesting module, energy conversion and storage module and electrochemical reaction module. More efforts are required to optimize system design considering safety and durability concerns as well as the complexity of the integration process. Moreover, self-powered energy systems can achieve more efficient energy management and optimization through artificial intelligence (AI) assistance. AI can monitor and analyze the generation, storage, and use of energy in real-time, predict energy demand, and adjust energy supply strategies. For example, in solar energy-powered devices, AI can optimize the angle of solar panels and the charging and discharging strategies of energy storage systems based on weather conditions and energy demand. Herein, AI assistance could improve the reliability and stability of self-powered systems, providing strong support for sustainable energy development.

Fuel production through self-powered electrochemical energy systems is a highly competitive green solution for industrial applications. Although significant progress has been made in this field, it is still in the laboratory stage. The challenges are associated with the technical viability and the production cost for diverse scales of application. Further studies can assist in the improvement of the energy utilization efficiency and the integration strategies and widen the application scope of self-powered electrochemical energy systems.

Data availability

No new data were created or analyzed in this study.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by the Natural Science Foundation of Shandong Province (grant no. ZR2023MB090).

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