INTRODUCTION
For several centuries, the global energy supply has depended primarily on fossil fuels. However, the ever-increasing release of greenhouse gases such as CO2, SOx, NOx, and CO caused by the combustion of fossil fuels has raised severe concerns about global warming, climate change, and related environmental issues [1–5]. Renewable energy sources have attracted considerable attention for environmental protection in economically sustainable manners. Renewable energy is energy collected from alternative energy sources such as solar light, biomass, wind, geothermal heat, and waves. These sources, used to supply electricity to grids, exist over a wide geographical area and provide approximately 50% of the total energy supply. However, to utilize renewable energy sources, grid-scale electricity storage systems are required because energy production is not constant. For example, the energy generated by solar light is produced only diurnally, whereas the energy generated by wind depends on the season. In this view, hydrogen (H2), another clean energy source, is currently of interest because it can be used not only as an energy source but also as an energy carrier [6–9]. Although H2 can be produced using diverse renewable sources (e.g., wind, solar, or hydroelectric), 95% of the required annual H2 production is still derived from fossil fuels [7,10,11]. Therefore, H2 production methods, such as thermochemical cycling, photocatalysis, and water electrolysis, that do not involve fossil fuels are required to achieve net zero [5–15]. Among these methods, water electrolysis is not only simple but also a green process that produces high-purity H2 using renewable energy source, making it “green H2” [13]. A water electrolysis unit is an electrochemical cell comprising two electrodes (anode and cathode), pure water or an electrolyte, and a power supply, as illustrated in Fig. 1. During water electrolysis using an alkaline electrolyte, the application of a direct current drives electrons to the cathode, where they react with water molecules, resulting in the generation of hydrogen gas (H₂) and hydroxide ions (OH⁻) through the reduction of water. The hydroxide ions migrate through a separator (diaphragm) to the anode, where they undergo oxidation to produce oxygen gas (O₂) and water (H₂O), while releasing electrons to maintain charge balance within the system [14].
Although water electrolysis can produce green H2, the intermittent nature of renewable energy requires battery energy storage to compensate for the variability in generation and imbalance between supply and demand [16–18]. Battery energy storage systems have gained widespread recognition as promising solutions to mitigate these variabilities, offering advantages such as rapid response, continuous power supply, and flexibility in terms of location [19]. To combine the advantages of both systems, redox flow battery based H2 production system was introduced in 2014 [20]. This approach offers spatial advantages by eliminating the need for separate energy storage and water electrolysis systems, and it overcomes the low energy density of batteries by allowing continuous H₂ production during battery charging. Building on these developments, Mulder et al. introduced a new concept of a hybrid system in 2017 that combined energy generation and storage within a single unit, coining the term “battolyser” for the first time [21]. This innovative system is designed to generate H₂ and O₂ from renewable energy sources while simultaneously storing energy within a battery during the charging process. The operating mechanism of this system is unique in that it can store energy while simultaneously producing H₂. During the charging phase, excess power is converted into H₂ through water splitting, which supports long-term energy storage. This enables energy conversion and reuse, providing a level of energy storage beyond what standalone battery systems can achieve. By managing battery charge-discharge cycles and water-splitting reactions simultaneously, the battolyser significantly enhances energy conversion efficiency, setting it apart from conventional battery and electrolysis systems. Additionally, combining a battery and an electrolyzer into a single device, the battolyser alleviates strain on grid infrastructure and enhances power management by adapting to fluctuating energy demand [18,21]. During peak demand, it supplies stored electricity, while in low-demand periods, it converts excess power into H₂ for storage. This dual functionality not only provides spatial advantages but also improves grid stability, reducing the need for long-distance power transmission and minimizing associated losses. Additionally, the integrated design of battolyser enables near-continuous operation, ensuring consistent energy generation and storage availability. This continuous functionality maximizes system utilization and stability, making the battolyser particularly valuable for efficient power management and renewable energy integration [18,21]. Although this approach is in its initial stages of development, with limited research since its inception in 2017, the underlying concept is important for carbon neutral renewable energy production. For instance, Jenkins et al. [22] studied the economic value of a battolyser connected to offshore wind turbines. A cost comparison between the battolyser and the electrolyzer based on the results of their operating scenarios is presented in Table 1. The levelized cost of electricity (LCOE) represents the average cost per unit of electricity generated over the lifetime of an energy system, accounting for the total costs to build, operate, and maintain the system divided by the total energy output it produces throughout its lifespan. This finding suggests that using a battolyser instead of an electrolyzer with offshore wind power is economically advantageous because electrolyzers alone are too costly to be economically viable.
In this review, we elucidate the concept, mechanism, construction design, and performance of a new hybrid energy storage system known as a battolyser. The integration of the battery and water electrolysis is explained in two parts: one in which the additional water electrolysis systems are externally integrated, and the other in which the water electrolysis systems are integrated within the battery as a single unit. For battolysers with externally integrated water electrolysis systems, we describe V-based redox flow batteries and Mn-based batteries. Subsequently, we then discuss single-unit batteries utilizing Ni–Fe and lead acid batteries.
BATTERY WITH EXTERNALLY INTEGRATED WATER ELECTROLYSIS SYSTEM
This section describes the external integration of all water electrolysis systems, with the electrodes for H2 and O2 located separately. Decoupled water electrolysis was first proposed by Symes and Cronin [23] to address the challenges posed by intermittent renewable energy. In conventional water electrolysis, H2 and O2 reactions occur simultaneously in a single device and cannot be effectively adapted to the intermittency of renewable energy sources. Moreover, decoupling the water electrolysis electrodes prevents electrode degradation and recombination of H2 and O2 [23–26]. Thus, decoupled water electrolysis, when externally integrated with a battery system, can convert renewable and/or surplus energy into H2, thereby providing an efficient and adaptable solution as a battolyser.
V-based redox flow battolyser
Currently, renewable energy, such as wind and solar energy, is being extensively studied worldwide; however, their associated power generators have output fluctuation problems [27]. Therefore, various energy-storage technologies have been explored to solve this problem. Among storage devices, batteries, particularly redox flow batteries (RFBs), have attracted considerable attention. RFBs are large-scale energy storage devices with independent output power and storage sections [28]. The primary advantages of RFBs are that they (i) provide a secondary stage and (ii) store surplus electricity in the form of H2 when the capacity of the charged electrolyte is exceeded. Among the various types of RFBs, such as Fe–Cr, Zn–Br, V–Ce, all-V, and V–O2 RFBs, V-based RFBs have been used by numerous research groups to produce H2. These scholars have formed a dual circuit by integrating an additional catalytic reactor bath to perform indirect water electrolysis reactions. A schematic of a V-based RFB with an externally integrated water electrolysis reactor is shown in Fig. 2. The red line represents the conventional electrochemical reactions of the RFB, and the green line represents the integrated dual-circuit system, which facilitates catalytic chemical reactions by discharging the RFB. A flow battery operates using two separate external electrolyte reservoirs, with the overall energy capacity depending on the electrolyte [29]. The anolyte and catholyte participate in redox reactions as they pass through the anode and cathode, respectively, and the ions migrate across the membrane to support the reaction. After the reaction, the electrolytes are cycled back into external tanks. The entire process is facilitated by a pump that ensures continuous electrolyte flow through the system. In particular, dual-circuit RFBs feature unique valves that allow the electrolyte to flow through external catalytic reactors for water electrolysis. In these reactors, the electrolyte can be chemically discharged as required to generate H2 and O2 via proton reduction and water oxidation, respectively, on each electrocatalyst. Because the RFB used in the battolyser is limited to V-based systems, the negative electrode (anode) is fixed as V. The general reactions that occur during charging in an RFB are as follows [28]:
The discharging reactions occur as the reverse of these equations. An electron is generated through an oxidation process during discharge at the anode, which has a high chemical potential, and the electrons travel through an external circuit [29]. Electrons are then accepted via a reduction process at the cathode, which has a lower chemical potential. During charging, the flow of electrons and corresponding chemical reactions are reversed. The specific reactions during charge and discharge for each V-based RFB, accompanied by the water electrolysis reactions, are explained in detail in the following subsections.
V–Ce redox flow battolyser
Amustutz et al. [20] first designed a dual-circuit V–Ce RFB with electrocatalysts in an externally integrated tank that could produce H2 and O2. They designed separate external circuits parallel to the V–Ce RFB containing a catalytic reactor for H2 generation (water splitting reaction). This system can alternatively discharge the charged redox species of trivalent V and Ce by producing H2 and O2 in a separate catalytic reactor instead of remaining in a charged state until it is connected to an electrical load for discharge. When surplus energy is available, H2 and O2 are produced during chemical discharge of the anolyte and catholyte, respectively. The equations can be written as [20]
Standard potentials were measured versus the standard hydrogen electrode (SHE). The reactions shown in Eq. 4 and 5 occur in the catholyte and anolyte during charging, respectively. Eq. 6 and 7 show the reactions that occur during H2 and O2 evolution through Mo2C and RuO2, respectively, in separate catalytic reactors. An energy diagram illustrating the redox catalytic reactions for the H2 evolution reaction (HER) using Mo2C and the O2 evolution reaction (OER) using Ru2O is shown in Fig. 3. For the HER, Mo2C is used as the electrocatalyst because of its stability and catalytic activity [30,31] and V2+ ions are utilized as electron donors. On the other hand, RuO2 is used as a catalyst, and Ce4+ ions are employed as electron acceptors in the OER. The overall cycle of the system is illustrated in (Fig. 2): 1) charged V2+ and Ce4+ ions pass through the external circuit to the external catalytic reactors consisting of RuO2 and Mo2C, respectively; 2) H2 and O2 are generated; 3) the electrolytes are chemically discharged, and V2+ and Ce4+ ions are simultaneously regenerated as V3+ and Ce3+ ions (initial redox species); and 4) the charging process is repeated.
The discharging and charging Coulombic efficiencies of the single-cell V–Ce RFB without an external catalytic reactor are 96% and 94%, respectively, in 1 M sulfuric acid. The charge and discharge process are conducted for 1 h and 10 min at a current density of 60 mA cm–2, and no H2 or O2 is observed during these processes. In fact, using a Ce mediator as the cathode in an RFB introduces various constraints into the system. The key issues include the deterioration of C-based electrodes owing to the strong oxidative properties of Ce(IV) and the necessity for high anodic potentials. Additionally, trivalent and tetravalent Ce compounds exhibit low solubility and high sensitivity to the solution pH and nature, resulting in the formation of complex precipitates. This presents significant challenges for the chemistry of trivalent and tetravalent Ce ions. However, the V–Ce RFB is particularly well suited for demonstrating indirect water splitting because the discharge process in this system is chemical rather than electrochemical [20].
An externally integrated reactor for water electrolysis was constructed using a glass tube with microporous fritted glass. This reactor ensured filtration of the electrolyte and prevented the catalytic particles from moving toward the main battery circuit. The energy efficiency of the entire system (ηoverall), including RFB and H2 generation, is 48%, calculated using the following equations [20]:
where E (= 1280 J) represents the energy required to charge the RFB fully, Uch is the mean charging voltage (2.5 V), j is the current density during charging (60 mA cm–2), and A and t are the electrode area (2 cm2) and full charging time (1 h and 10 min), respectively, in Eq. 8 [20]. In Eq. 9, EH2 (= 603 J) denotes the energy stored in the generated hydrogen, LHV represents the lower heating value (241 kJ mo1–1), C is the concentration of trivalent Ce and V species (0.1 mol L–1), and V is the volume of the solution (50 mL) [20]. In this calculation, the yield of HER is assumed to be 100%. The overall efficiency can be explained by the fact that the ratio of the energy required for full charging to the energy stored in the generated H2 is nearly half. The H2 evolution yield is 96 ± 4% in 1 M sulfuric acid containing an electrolyte using 1 mg of Mo2C catalyst, indicating almost maximum conversion efficiency without any side reactions. The O2 evolution yield is 78 ± 8% in 1 M sulfuric acid containing an electrolyte using 0.5 mg of RuO2 catalyst, revealing the existence of side reactions. As shown in Eq. 11, when RuO2 is partially hydrated, tetravalent Ce cations corrode RuO2 [32]; consequently, Ce ions are consumed during the oxidation reaction.
All-V redox flow battolyser
In another study, Peljo et al. [33] scaled up a H2 production reactor to the kilowatt scale and proposed an allV dual-circuit RFB for both H2 generation and desulfurization. All-V RFBs are widely regarded as the most successful commercialized systems among the various types of RFBs [34]. Unlike other RFBs, those that employ V as the sole electroactive species in the system prevent cross-contamination [34,35]. The mechanistic reactions of all-V RFBs are as follows [36]:
Equation 12 represents the reactions occurring in the cell catholyte during charging, whereas Eq. 13 describes the reactions in the catalytic reactor on the cathode side, leading to O2 generation with the discharge of V5+. The anolyte and H2 generation reactions in the catalytic reactor on the anode side occur similarly to Eq. 5 and 6 in Section 2.1.1, respectively. The fundamental mechanism of this all-V dual-circuit RFB is identical to that of a V–Ce dual-circuit RFB, as explained in Section 2.1.1.
In a previous study, Amstutz et al. [20] focused on the discharge of the positive electrolyte in the V–Ce RFB case. However, the water oxidation reaction with V5+ solution in an all-V RFB is thermodynamically not possible, as the standard potential for the VO2+/VO2+ redox couple is 0.99 V vs. SHE, which is lower than the standard potential required for O2 generation at 1.23 V vs. SHE [33]. Peljo et al. first considered charging a negative electrolyte via H2 production. Subsequently, several options were considered for indirectly discharging positive electrolytes. To maintain the charge balance between the catholyte and anolyte, hydrazine hydrate was utilized with an SO2 fuel cell. For HER, Mo2C with a ceramic support was used. The Mo2C/ceramic catalyst in the reactor was operated for more than six months without any decrease in activity. The all-V RFB for H2 production on a pilot scale utilized a CellCube 10 kW/40 kWh system (Gildemeister Energy Solutions) [33]. The electrolyte (1000 L) comprised 2 M sulfuric acid and 1.6 M of V [33]. This battery was chemically discharged over 17 h and required an electrolysis current of 520 A to achieve the highest H2 evolution rate (assuming 100% H2 evolution efficiency). Notably, this experimental setup allows 0.5 kg of H2 production per day [33].
V–Mn redox flow battolyser
Because the all-V RFB utilized as a battolyser is concerned with the water oxidation reaction with a V5+ solution and requires the addition of an SO2 fuel cell system, further development is necessary. Reynard and Girault introduced a V–Mn RFB for use as a battolyser in 2021. The fundamental reactions are identical to those of the V–Ce RFB dual circuit, with the only difference being the substitution of Ce with Mn (Fig. 4). Thus, the redox reactions of the catholyte and electrocatalyst for the OER are as follows:
The anolyte and HER in the catalytic reactor on the anode side occur similarly to Eq. 5 and 6 in Section 2.1.1, respectively. In separate catalytic reactors, Mo2C/graphene (Fig. 5a) and Zn2+-doped RuO2/C cloth (Fig. 5b) were used as electrocatalysts for the HER and OER, respectively. Unlike that of an all-V RFB, the standard potential of the Mn3+/Mn2+ redox couple is 1.51 V vs. SHE in 3 M H2SO4, which eliminates concerns regarding the discharge of the positive electrolyte. In addition, as described in Table 2 [37], the energy density of a V–Mn RFB in a H2SO4 electrolyte is higher than that of an all-V RFB. The energy densities in Table 2 were obtained experimentally by Reynard et al. [37]. However, trivalent Mn forms manganese oxide owing to its instability, as shown in Eq. 16 [37]:
The formation of MnO2 leads to performance degradation and reduced cell lifetime because of the mass transport resistance and passivation of the electrode and redox catalyst [38]. To overcome this problem, VO2+ was added to the positive electrolyte as a stabilizing agent [39]. With this design of the V–Mn redox flow battolyser, the maximum amount of H2 generated was 280 mL after 200 min at a pressure of 7 bar. Moreover, the Faradaic efficiency for H2 production was 100% as V2+ was fully converted, which is consistent with the total amount of generated H2. The maximum amount of generated O2 was approximately 15 mL after 150 min of discharge in the 0.1 M Mn3+/Mn2+ + VO2+ electrolyte. Thus, the production yield of HER and OER was close to 100%, and the maximum energy efficiency of the water electrolysis in the V–Mn dual-circuit RFB was calculated to be 66.1% at a current density of 30 mA cm–2.
Mn–Zn battolyser
In 2021, Huang et al. [40] reported the integration of decoupled amphoteric water electrolysis electrodes with an Mn–Zn battery in a proof-of-concept stage. Before discussing the integration, the basic concept of amphoteric water electrolysis is introduced. Amphoteric water electrolysis involves the HER and OER occurring in acidic and alkaline electrolytes, respectively, with a bipolar membrane between the two different electrolytes [41–43]. This approach has attracted attention because in acidic media, the kinetics of the HER are fast, whereas the OER exhibits sluggish kinetics [44]. Conversely, in alkaline media, the OER is more favorable, whereas the HER has high kinetic energy barriers [45–47]. Consequently, this amphoteric water electrolysis system allowed the HER and OER to occur independently in their respective favorable media.
Returning to our main consideration, the integration of Mn–Cu batteries and amphoteric water electrolysis systems was first proposed by Huang et al. in 2019 [48]. They introduced the concept of integrating a Mn–Cu battery with a H2 generation system without further demonstration. After two years, the same group proposed an amphoteric water electrolysis system integrated with an Mn–Zn battery. In this study, MnO2/C felt was used as the anode of a Mn–Zn battery, and a Pt-coated Ti mesh (Pt/Ti-mesh) and RuO2/IrO2-coated Timesh (RuO2/IrO2/Ti-mesh) were used as the HER and OER electrodes, respectively. However, the material used for the Zn electrode was not specified. A 1 M MnSO4 + 1 M H2SO4 electrolyte was used for the HER cell, 4 M KOH with a saturated ZnO electrolyte was utilized for the OER cell, and the bipolar membrane was purchased from Huamo Tech. Co., Ltd. The working principle and experimental configuration of the Mn–Zn battolyser are illustrated in Fig. 6. The Mn–Zn battolyser operates in three steps (Fig. 6a). In the first step, in which the HER occurs in a 1 M MnSO4 + 1 M H2SO4 solution (pink), divalent Mn is oxidized to MnO2, as shown in Eq. 17 and 18. This process also corresponds to the charging process for MnO2/C felt, as described in Eq. 21 [49]. In the second step, in which the OER occurs in a 4 M KOH solution saturated with ZnO (blue), divalent Zn is reduced to metallic Zn, as shown in Eq. 19 and 20. This process corresponds to the charging process of the Zn electrode, as described in Eq. 22 [49]. Subsequently, the MnO2 and Zn electrodes, once charged, constitute a Mn–Zn battery capable of delivering electricity on demand.
Step 1 (HER cell)
Step 2 (OER cell)
Step 3 (Mn–Zn battery)
In the experimental setup (Fig. 6b), the working, counter, and reference electrodes in the HER cell were MnO2/C felt, Pt/Ti mesh, and Ag/AgCl, respectively. In the OER cell, the working, counter, and reference electrodes were RuO2/IrO2/Ti-mesh, a Zn electrode, and Hg/HgO, respectively. In the Mn–Zn battery, in which the MnO2/C felt and Zn electrodes were placed in different electrolytes, the MnO2/C felt, Zn electrode, and Ag/ AgCl served as the working, counter, and reference electrodes, respectively. Finally, the Mn–Zn battolyser was operated by applying a charging voltage for the HER and OER processes and a discharging voltage to the Mn– Zn cell. Notably, the charged Mn–Zn battery could apply an operating voltage of 1.72 V at a current density of 10 mA cm–2 [40]. However, this study did not fully demonstrate the performance of the Mn–Zn battolyser, and further investigation is required to verify its potential and substantiation.
BATTERY INTEGRATED WITH WATER ELECTROLYSIS SYSTEM IN ONE DEVICE
Ni–Fe battolyser
Since the first Ni-Fe battery was introduced by Jungner in 1988, numerous research groups have attempted to use batteries in various fields. Among the numerous reported electrochemical redox couples for alkaline rechargeable Ni-metal batteries, such as Ni–Cd [50], Ni–Zn [51], Ni–Co [52], and Ni–Fe [53], the Ni-Fe battery is the most favorable couple because of the insolubility of its active materials in alkaline solution, abundance of constituent elements, low corrosiveness, and non-toxic characteristics [53–56]. Rechargeable Ni–Fe batteries are composed of two electrodes: an Fe anode and a NiOOH cathode with alkaline electrolytes. Ni and Fe are widely used materials in water electrolysis because of their optimal adsorption–desorption energies and cost-effectiveness [57]. Alkaline media are commonly employed because acidic electrolytes can cause significant corrosion [15]. Hence, the Ni–Fe battery facilitates the H2 and O2 evolution reactions when overcharged. In this context, Mulder et al. [21] introduced the concept of a “battolyser”, a system that combines a battery and a water electrolysis system into a single device. It stores and delivers electricity as a Ni–Fe battery (short-term storage) and splits water into H2 and O2 as an electrolyzer (long-term energy). When a battery is overcharged to its nominal capacity, H2 is generated from surplus electricity. The battolyser can operate continuously, providing a high degree of utilization by storing excess electricity in the form of H2. The theoretical energy density of a Ni–Fe-based battolyser is 1031 W h L–1 at an open circuit voltage (OCV) of 1.37 V, calculated using only the densities of materials such as Ni(OH)2 and Fe(OH)2. The practical battolyser cell showed an energy density of 100 W h L–1, which is comparable to those of other batteries, such as lead acid batteries (50–80 W h L–1), liquid Na–S batteries (150–250 W h L–1), and V redox-flow batteries (16–33 W h L–1) [21,58]. However, with the exception of battery capacities, the battolyser capacity includes electrolyzer capacities, providing unlimited additional W h from H2-based fuels.
A schematic of the Ni–Fe and battolyser systems is shown in Fig. 7. In the Ni–Fe battery, the two electrodes are positioned on either side of the separator, as illustrated in Fig. 7a. The schematic demonstrates the working mechanisms of the cell, including the electrochemical reactions occurring at each electrode in the charge– discharge state.
In the discharge stage, hydroxide ions are produced by reduction reactions at the NiOOH electrode, with electrons passing through the electric circuit (Eq. 24). These hydroxide ions reach the anode surface, pass through the porous separator, and react with Fe to generate electrons (Eq. 23). During overcharging, the HER and OER occur at the negative and positive electrodes, respectively, as shown in Fig. 7b. O2 and H2 gases are generated at each electrode by splitting the water supplied on each side. However, the system is only slightly modified from that of a Ni–Fe battery and ultimately becomes a hybrid system that generates and stores energy.
The OCV of the Ni–Fe battery is 1.37 V, calculated by combining + 0.877 V (vs. SHE, Eq. 24) and –0.49 V (vs. SHE, Eq. 23). This potential is significantly higher than the minimum thermodynamic potential required for water electrolysis (1.23 V vs. SHE), which is derived from the HER (Eq. 25, E0 = –0.059 V pH vs. SHE) and OER (Eq. 26, E0 = 1.171 V pH vs. SHE).
Mulder et al. [21] reported that an increase in the electrical current insertion resulted in increased charging of the battery electrode, which subsequently led to higher electrolytic gas production. When the battery was fully charged, the Fe and NiOOH electrodes became active and efficiently produced H2 and O2, respectively. The electrolysis efficiency of the battery improved with an increasing number of cycles. However, the gas generation was not constant during electrolysis. The authors increased the device temperature to promote electrolysis, resulting in an increased gas yield at a low battery charging rate. They also reported 80‒90% overall stability of the energy efficiency. Moreover, the battery capacity or electrolysis capacity did not degrade after different types of cycles, such as deep discharges, fast current reversals, and battery overcharging by up to 10 times its nominal capacity. The authors calculated the energy efficiency by considering the water consumed in electrolysis; however, gas chromatography or gas output data are required to calculate the H2 and O2 generation efficiencies precisely.
Two years later, Weninger and Mulder [59] modified the battolyser by introducing two additional gas production electrodes, specifically, the O2 evolution electrode (OEE) and H2 evolution electrode (HEE). An assembly of Ni(OH)2 and OEE electrodes and Fe(OH)2 and HEE electrodes were combined in parallel on the positive and negative sides, respectively, in a single device. The OEE and HEE were wired through a sideline from the main circuit, allowing independent control of the electrode currents. This approach enables the simultaneous operation of two or more working electrodes in an electrochemical cell and permits four reactions (half-discharging/charging and the OER/HER) to occur in various combinations. Introducing OEE and HEE electrodes into the battolyser provides flexibility for electrical provision as a battery while meeting the demands for O2 and H2. Nevertheless, the concept of operating two or more working electrodes in electrochemical storage and conversion systems has not yet been widely adopted, whereas it has been utilized broadly in scientific research, such as in bipotentiostats. As this concept is still in the proof-of-concept stage, further research is needed to determine its specific efficiency.
In 2021, Kortlever et al. collaborated with Mulder to model the battolyser and optimize the cell parameters [60]. They developed a COMSOL Multiphysics model to quantify the energy efficiency and optimize a battolyser prototype [60]. Initially, the developed model, which included electrochemical reactions, H2 and O2 generation, and the effects of components such as electrodes, electrolytes, and membranes, was validated against experimental results. The validity of the model was tested using a 1D cell, as shown in Fig. 8a. Following the simulation, the authors confirmed that this model could achieve results similar to those observed experimentally in terms of the battery charging onset potential, H2/O2 generation onset potential, and discharging capacity (Fig. 8b). Finally, they proposed the optimal cell parameters, as listed in Table 3.
To enhance the functionality of a Ni–Fe battolyser, modified Ni(OH)2 with various amounts of Fe was investigated, such as in the form of Ni hydroxide and α-Ni1–xFex(OH)2 (where x = 7, 15, and 20; labelled to NiFe7, NiFe15, and NiFe20) [61]. The interlayer distance was adjusted based on the Fe amount in such Ni–Fe materials and set to values such as 4.7, 6.88, 8.64, and 8.25 Å for Ni hydroxide, NiFe7, NiFe15, and NiFe20, respectively. The authors concluded that intercalated anions were not sufficiently present to fill the interlayer uniformly when the Fe concentration was below 20% in the Ni–Fe hydroxide materials. The effects caused different behaviors in the charge–discharge reaction; for example, NiFe15 has two different plateaus in the charge–discharge curves representing alpha and beta phases, whereas these characteristics are less visible in other curves because NiFe7 and NiFe20 include only a single phase of alpha or beta, as shown in Fig. 9a. Pure Ni hydroxide offers a higher capacity than the other samples, which is due to the higher Ni concentration per gram of the compound and the lower number of electrons exchanged per atom of Ni (NEE). Interestingly, NiFe20 also offers higher power density and capacity than NiFe7 and NiFe15 because the reduced Ni amount in the compounds lowers the Faradaic efficiency of the sample, leading to a higher number of exchanged electrons. NiFe20 was further characterized to evaluate its stability through a life-cycle experiment and demonstrated the best capacity performance than other samples. The NiFe20 electrode was subjected to 1000 cycles, including periods of reactivation and repression, to maintain contact with the current collector. In particular, it showed significant long-term stability, retaining 90% of its initial capacity (Fig. 9b) and maintaining its structure (Fig. 9c, d) after 1000 cycles. Moreover, NiFe20 demonstrated excellent OER catalytic performance over 200 h at 4 C and maintained a stable overpotential of approximately 200 mV. Even after 1000 cycles, it performed well with a significant electron exchange rate.
Barton et al. [62] designed a battolyser for application in mini-grids, making them the only other group to study battolysers apart from the Mulder group. Mulder et al. focused on the simultaneous use of a Ni–Fe battolyser as a battery and electrolyzer. Barton et al. attempted to operate a battolyser device in the real world by testing its performance over wide ranges of temperatures, charge and discharge rates, and other design parameters. The tests were performed using a commercial Ni–Fe battery cell (Sichuan Changhong Battery Co., nominal capacity: 10 Ah, 1.2 V). The authors suggested an optimum separation gap between each electrode and the membrane (1.25 mm) in a Ni–Fe battery when used as an electrolyzer for long-term gas generation (Fig. 10a). Fig. 10b shows the total gas production rates of electrolysis with 1.25 mm spacers. The gas production rates were found to depend on the cell voltage, with higher production rates observed when the cell was fully charged. Additionally, a Ni–Fe battery cell can hold 25% more than its nominal charge, but this excess charge dissipates at 60°C after short-term cycling. Furthermore, the self-discharge rate of the Ni–Fe batteries increased at elevated temperatures. However, as the temperature increased, the measured cell capacity and voltage decreased simultaneously, whereas the electrolysis efficiency improved. This phenomenon was consistent with the results reported by Mulder et al. Barton et al. measured the gas production during integrated charging-electrolysis tests and assessed the charge and energy efficiencies (Fig. 10c and d). The measured charge efficiency varied between 73% and 107%, with an average of 87%. This variability in the charge efficiency can be attributed to factors such as gas leakage, measurement errors, leakage currents, and temporary overcharging of the cell. The energy efficiency of H2 gas generation was compared with the electrical energy input and calculated over time using the higher heating value of the chemical energy. The calculated energy efficiency was 70% at a current of 2 A and a cell voltage of 1.9 V. By contrast, the energy efficiency was less than 40% when the current and voltage were as high as 10 A and 3 V, respectively. The average energy efficiency was 50%, which is relatively low for an electrolysis system.
Fig. 11 shows the expected performance of an ideal grid-stabilization energy storage system (GSESS) [61]. The GSESS system is based on stationary energy storage applications; thus, it must have very high energy efficiency, an extraordinarily long cycle life, high power discharge and charge for minutes, and low lifetime and capital costs. Comparison of Fig. 11a, b indicates that Ni–Fe batteries have a much lower full cell energy efficiency (65%–70%) and energy density (50 W h g–1) compared to Ni–Fe battolysers. The Ni–Fe battolyser, the combined system, allows long-term storage (H2 production) after battery function; thus, the energy efficiency is increased up to 80%–90%. Moreover, the Ni–Fe battolyser consisting of ɑ-Ni1–xFex(OH)2 has higher material availability, a superior energy efficiency, and a rate. Therefore, although the operational hours of the battery or electrolyzer and the method of storing H2 must be optimized to achieve a higher energy density, as indicated by the arrow, the shape of the radar chart for the Ni–Fe battolyser is very similar to that of the ideal GSES.
Lead acid battolyser
In 2022, Breton et al. [18] explored the use of lead acid batteries in battolyser systems to prevent overreliance on a single resource, addressing the concerns associated with the exclusive application of Ni–Fe batteries in single-device battolysers. Unlike Ni–Fe batteries, lead acid batteries operate in acidic media. Therefore, the authors aimed to establish the feasibility of an acidic battolyser and to develop a prototype system at the proof-of-concept stage [18].
Similar to Ni–Fe batteries, lead acid batteries also produce H2 during charging and discharging, resulting in electrolyte loss. In lead acid batteries, the electrodes consist of Pb as the anode, PbO2 as the cathode, and sulfuric acid as the electrolyte. The discharge/charge, H2 production, and O2 evolution reactions are as follows [18,63,64]:
During discharge, the Pb anode reacts with sulfate ions to form PbSO4, as described in Eq. 27. Conversely, at the cathode, PbO2 reacts with H2 and sulfate ions, resulting in the formation of PbSO4 and water, as described in Eq. 28. When the lead acid battery is overcharged, the HER and OER occur, as outlined in Eq. 29 and 30. Although the attempt to utilize a lead acid battery as a battolyser yielded promising results, the study findings indicated that both the battery and electrolyzer functions performed significantly below their theoretical potentials. This underperformance was attributed to factors such as the loss of electrodes; corrosion of the electrode frame caused by high voltages exceeding 2.6 V, which led to additional cell degradation; and formation of lead sulfate owing to the absence of a separator. In fact, the reliability of lead acid batteries is low owing to the necessity of using a separator as an electrolyzer, their performance being lower than that of Ni–Fe batteries, and the presence of hazardous materials, especially Pb. Nevertheless, this lead acid battolyser proof of concept offers an alternative approach for reusing or recycling waste lead acid batteries and opens up the possibility of employing other batteries with HER side reactions, such as an Al–air battery [65], as a single-device battolyser.
CONCLUSIONS
By summarizing the various battolyser metrics, this review provides insights into the efficiency and viability of battolysers for future energy systems. V-based RFBs for large grids and Ni–Fe batteries for small grids have demonstrated significant potential for use in electricity storage in off-grid power systems powered by renewable energy. In RFB-based battolysers, the water electrolysis system is externally separated into a catalytic reactor, and redox species play a crucial role in battolyser applications. The concept of decoupled electrodes in water electrolysis has also been applied in Mn–Zn batteries; however, they are externally separated between amphoteric separators with different electrolytes. In Ni–Fe batteries, the water electrolysis system is integrated into a single device, eliminating the need for additional electrodes for the HER and OER. Among the various types of battolysers, Ni–Fe battolysers have been extensively researched. However, despite the promise of these systems, the number of related studies including RFB-based battolysers still have limitation, and significant work remains to be done to optimize these systems for practical, large-scale deployment. First, the reported efficiencies of water electrolysis systems of battolyser relatively low, making it essential to improve electrolysis efficiency while maintaining battery performance to fully unlock the potential of energy storage. Second, current research primarily focuses on lab-scale battery cells, so solutions are needed to adapt this system for industrial applications Third, improvements in long-term performance and cost-efficiency are necessary. For instance, ongoing studies should evaluate the impact of prolonged storage and frequent charge–discharge cycles on performance. Additionally, although the LCOE for battolyser is lower than that of electrolyser, technological advancements are required to reduce initial installation and maintenance costs, which are critical steps toward the commercialization and widespread adoption of battolysers. To fully unlock the potential of energy storage, enhancing electrolysis efficiency while maintaining battery performance will be essential. Looking ahead, these improvements will position integrated energy storage and H₂ production systems as foundational elements of renewable energy infrastructure, advancing both battery and hydrogen storage technologies.
PDF Links
PubReader
ePub Link
Full text via DOI
Download Citation
