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Zhang, J.; Azari, R.; Poerschke, U.; Hall, D.M. Electrochemical Applications in Buildings. Encyclopedia. Available online: https://encyclopedia.pub/entry/52945 (accessed on 18 May 2024).
Zhang J, Azari R, Poerschke U, Hall DM. Electrochemical Applications in Buildings. Encyclopedia. Available at: https://encyclopedia.pub/entry/52945. Accessed May 18, 2024.
Zhang, Jingshi, Rahman Azari, Ute Poerschke, Derek M. Hall. "Electrochemical Applications in Buildings" Encyclopedia, https://encyclopedia.pub/entry/52945 (accessed May 18, 2024).
Zhang, J., Azari, R., Poerschke, U., & Hall, D.M. (2023, December 19). Electrochemical Applications in Buildings. In Encyclopedia. https://encyclopedia.pub/entry/52945
Zhang, Jingshi, et al. "Electrochemical Applications in Buildings." Encyclopedia. Web. 19 December, 2023.
Electrochemical Applications in Buildings
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This entry is adapted from the peer-reviewed paper 10.3390/mi14122203

The integration of distributed renewable energy technologies (such as building-integrated photovoltaics (BIPV)) into buildings, especially in space-constrained urban areas, offers sustainable energy and helps offset fossil-fuel-related carbon emissions. However, the intermittent nature of these distributed renewable energy sources can negatively impact the larger power grids. Efficient onsite energy storage solutions capable of providing energy continuously can address this challenge. Traditional large-scale energy storage methods like pumped hydro and compressed air energy have limitations due to geography and the need for significant space to be economically viable. In contrast, electrochemical storage methods like batteries offer more space-efficient options, making them well suited for urban contexts.

electrochemical energy harvesting electrochemical energy storage building skins

1. Building-Integrated Photovoltaic (BIPV) Technologies

Photovoltaic (PV) technology, which converts solar energy into electricity, is a good example of distributed renewable energy. By making use of the unique properties of semiconductors, solar radiation can be converted into a direct current through crystalline silicon [1].
Building-integrated solar panels have gained increasing popularity, as cities utilize solar energy to reduce the net load demand and promote onsite energy generation [2]. Today, solar energy accounts for over 5% of the electricity generated in the United States, which is nearly 11 times its share a decade ago [3]. The orientation of a photovoltaic (PV) installation significantly influences energy gain. An optimized PV installation incorporates a rotatable system to consistently capture maximal solar energy. Moreover, system design considerations such as PV sizing, energy storage, and the electrical and mechanical balance of systems play crucial roles in ensuring effective energy utilization [4].
The integration of PV technology can extend to different building elements, such as shading systems, rainscreen systems, curtain walls, double-skin facades, atria, and canopies [5]. PV systems in buildings can be designed as either grid-connected or stand-alone systems. Grid-connected systems can incorporate storage or operate without it, while stand-alone systems heavily rely on battery storage. Currently, batteries are the most common and commercially available technology for PV with electricity-storage systems in buildings, but future electrochemical technologies may also be employed [5].
The residential sector in the U.S. consumes 21% of the total energy (including end-use consumption and the energy losses within the electrical system related to retail electricity sales across various sectors), and the commercial sector consumes 18%, together making up the majority of energy consumption in all U.S. buildings [6]. However, with the increasing affordability of solar energy solutions and their adaptability to various climates, there is a promising opportunity for occupants to save on their energy costs in the future. By leveraging the potential of solar energy, occupants can not only contribute to a greener environment but also enjoy economic benefits by reducing their reliance on traditional energy sources [7][8]. The adoption of small-scale solar systems has been consistently rising, indicating a growing trend toward sustainability. According to the Independent Statistics and Analysis from the U.S. Energy Information Administration, by the year 2020, approximately 3.7% of single-family homes in the U.S. had already implemented such solar installations [9].
In this project, PV technology serves as the key component for collecting solar energy and converting it into electricity. Electrochemical components play a crucial role in storing energy to mitigate the intermittent problem of solar energy in buildings.

2. Electrochemical Applications in Buildings

Electrochemistry is a discipline that studies the relationship between electrical energy and chemical change. In plain language, it can be understood as an energy conversion between electrical energy and chemical energy [10]. For instance, hydrogen and oxygen can react and produce electricity through electrochemical energy conversion devices known as fuel cells. Another common example is the battery that can provide electricity from stored chemical energy. Some fundamentals are needed to understand an electrochemical device’s behavior and performance, such as charge transport, electrochemical thermodynamics, and electrochemical kinetics. When these electrochemical devices are integrated into buildings, they have the potential to alter the characteristics of building skins, impacting factors such as thermal performance and moisture control. Vice versa, electrochemical devices integrated into buildings always deal with real-world operating conditions, which may be different from ideal conditions.
The electricity storage patches up the mismatch between load demand and renewable energy resources generation profile [11]. Lund et al. quantified the mismatch of renewable sources at the building level. This mismatch is attributed to hourly variations in energy production and consumption within buildings. The findings indicate a 63% mismatch for PV and a 39% mismatch for wind [12]. Electrochemical energy storage is versatile. Not only being used for grid-scale energy storage and automobiles but electrochemical applications such as batteries have been already used in buildings to support intermittent renewable energy [11]. Nowadays, with the emergence of many electrochemical technologies, how to choose a suitable energy storage method for buildings is a very important topic. Energy storage density plays a critical role in determining the storage capacity of a system [13], whereas power density is focused on how quickly the energy can be delivered. The measure of energy density can take the form of volumetric energy density, commonly expressed in units such as watt-hours per liter (Wh/L). This metric signifies the amount of energy that can be stored within a specific volume. Gravimetric energy density, on the other hand, pertains to the quantity of energy that can be stored in a given mass and is typically quantified in units like watt-hours per kilogram (Wh/kg). Volumetric power density is often assessed in units like watts per cubic meter (W/m3) or watts per square meter (W/m2), denoting the rate at which power can be delivered in relation to volume or area, respectively. When it comes to buildings, it is essential to have flexible energy storage capacity without a substantial increase in costs. For example, it is meaningful to figure out how much space inside buildings is needed to achieve a particular storage capacity and to identify the maximum storage capacity possible without making significant changes to the building structure. Additionally, considering power density is crucial, as buildings may require electricity promptly, although not necessarily as rapidly as vehicles do. Right now, batteries’ energy and power density performance is in between fuel cells and capacitors [14]. They are relatively reliable and technically mature. However, the widespread implementation of batteries does not mean other technologies are not possible in the future. Many researchers have been working on fuel cells and supercapacitor applications in built environments in cities. When implementing energy storage technology in buildings, both gravimetric energy density and volumetric energy density are important. Designers weigh trade-offs when creating architectural plans. Volumetric energy density becomes crucial when the storage system occupies specific spaces within the building, such as the basement, where higher volumetric energy density allows for space-saving storage solutions. On the other hand, gravimetric energy density will be a factor in situations where the buildings require lighter components for efficient energy storage.
When reviewing electrochemical devices, several parameters need to be considered. The common parameters include energy storage and power density capacities, response time, operational conditions (temperature), round-trip efficiency, and lifetime [15]. However, if these devices are intended for building applications, additional special considerations come into play. Safety becomes a major concern since buildings are designed for occupancy activities. Factors such as fire safety, strength safety, and overall safety measures should be prioritized. Durability is also crucial for building skins, as they are often subjected to various weather conditions. Therefore, materials chosen for electrochemical devices in building skins should be weather-resistant, water-resistant, and fire-resistant where possible. Efficiency, cost, and maintenance are other practical aspects that should be taken into account for building applications [16].
Research on electrochemical energy storage methods, including batteries, reversible fuel cells, and supercapacitors, has gained considerable attention in building applications. Among these methods, batteries currently dominate the field, particularly when paired with renewable energy sources like solar or wind power. For instance, lithium-ion batteries have been extensively studied due to their favorable combination of gravimetric and volumetric energy densities [17][18][19]. Reversible fuel cells present an alternative option and have been compared to batteries in terms of their levelized cost of storage for buildings [20][21]. These fuel cells are also utilized as cogeneration systems in buildings, offering additional benefits [22][23]. Supercapacitors, on the other hand, have been integrated into building components and materials to enhance energy efficiency [24][25][26]. It is important to note that they work just as well as batteries, but right now, they might not be affordable for some building projects [27]. As technology gets better and cheaper, these different ways of storing energy might become more popular for buildings. The upcoming sections explain foundational concepts and illustrate them with contemporary examples. These examples showcase recently completed projects, with some drawn from the latest literature. The criteria for selection are rooted in the following considerations: 1. Pertinence to architecture or communities; 2. Recent research discoveries or actual projects.

2.1. Traditional Batteries and Flow Batteries

Batteries have been categorized into two groups. Primary batteries are batteries that cannot be recharged. Common examples are zinc–manganese dioxide alkaline cells and lithium metal batteries [28]. Secondary batteries, which are rechargeable, are mainly discussed. Common examples of secondary batteries are lead acid batteries and lithium-ion batteries [29]. Batteries are usually designed as sealed systems that require less operation and maintenance, but this design also causes other problems, such as degradation and self-discharging [30][31]. Lithium-ion batteries stand out due to their extended life cycle and high energy density, making them a favorable option for renewable energy storage [32]. Their suitability for Building Integrated Photovoltaic (BIPV) systems is particularly promising. Tervo et al. [18] demonstrated that their lithium-ion system competes effectively with the grid in terms of long-term costs. Their model reveals a cost of USD 0.11 per kilowatt-hour (kWh) for the lithium-ion system, whereas the average electricity cost in California, for instance, is approximately USD 0.3 per kWh.
Though less common, redox flow batteries are another interesting option for building applications. Redox flow batteries differ from traditional batteries by using two external electrolyte tanks, which are circulated using pumps to facilitate redox and oxidation reactions for energy storage and release [33]. As these are also secondary batteries, redox flow batteries can be recharged. For building applications, this design offers several advantages over other batteries. Firstly, they are scalable by adding more electrolyte solutions without affecting power output. Additionally, they have long lifespans due to the absence of permanent self-discharge, as reported in the study by Weber et al. [34]. Furthermore, flow batteries are safer than traditional batteries since they use non-flammable and less toxic electrolyte solutions [35]. Their efficiency and response times are also noteworthy, as flow batteries can quickly react to provide electricity. These advantages, combined with its distinctive design and traits, make flow batteries a promising technology for storing energy within buildings.
Despite numerous studies demonstrating the high efficiency of redox flow batteries (RFBs), their widespread application in the energy and building sectors is still limited. There are some examples of RFB usage in grid-scaled infrastructure, such as the world’s largest RFB power station—Dalian Constant Current Energy Storage Power Station Co., Ltd. [36], in Dalian, China. Other projects in the USA are undertaken by Largo Clean Energy and partners in Wilmington, Massachusetts, and Quino Energy, Inc. in Menlo Park, CA, USA. These initiatives, supported by the DOE, aim to reduce the cost of energy storage by 90% within 10 years [37]. There are residential vanadium flow battery applications from companies like StorEn based in South Carolina [38] and instances of flow batteries being installed in communities as micro-grid power plants, such as the one in San Diego that powers 1000 homes. These projects emphasize the long lifespan and cost-saving benefits of redox flow batteries in the long run.
The integration of redox flow batteries into buildings has not been extensively studied, although there are some theoretical analyses available at the pack level. Nguyen group [39][40] analyzed the performance of a photovoltaic–vanadium redox battery microgrid system under various loads and temperatures. They provided detailed studies on the microgrid system and the VRB schematic diagram, which could be applicable on a building scale. Parameters such as solar insolation, temperature, voltage, and current were monitored and recorded at 5-second intervals. State of charge, open-circuit voltage, discharge, and charge performance were analyzed. Two operational modes were designed for performance studies: “renewable mode”, where a small building was powered by the VRB and PV, and “grid mode”, where the building was powered by the grid while the VRB was charged using PV. A schedule was created to control the modes to ensure the building was fully powered by the grid, VRB, or PV. In their 2015 study, the same group, led by Qiu, optimized the size of the PV-VRB system to maximize efficiency. Qiu et al. [41] conducted another study on a standalone system, where the PV supplied energy during the day and excess energy was converted to VFB, allowing the building to be powered solely by the VRB during the night. However, the specific location of the energy-storage devices in the building was not explicitly mentioned. It was noted that the temperature of these storage devices was regulated by an HVAC system, suggesting their placement within the building itself. Zhang et al. [42] proposed a computational method to assess the cost and efficiency of PV-RFB systems in residential buildings, addressing concerns regarding installation costs. The simulation indicated a payback period of 3–5 years for the system. Other hybrid systems have also been developed, such as a multi-story building system comprising PV, hybrid gravity power, and vanadium redox flow batteries [43]. These hybrid systems housed in walls and basements cater to the needs of more complex and demanding buildings.

2.2. Supercapacitors

Compared to fuel cells and flow batteries, supercapacitors have high power density, which means the charging and discharging speed is higher than that of reversible fuel cells and flow batteries. However, the energy density is limited by the special structure of supercapacitors [44]. The capacitance of supercapacitors is positively proportional to the surface area and negatively proportional to the distance between the double layers. The capacitance is also related to the electric constant and dielectric constant, which depend on the property of the material between the plates [45]. Supercapacitors achieve energy storage by utilizing the electric double layer (EDL). The theory of the EDL was originally proposed by Hermann von Helmholtz in 1853. Further developed supercapacitors were first commercially available in 1971, and the low internal resistance supercapacitor was invented in 1982 for military purposes [46].
Supercapacitor applications in buildings are relatively new, with studies emerging over the past five years. The Fraunhofer Institute for Solar Energy Systems (ISE) has undertaken projects that focus on integrating solar cells with supercapacitors at the device level, resulting in a device known as a photo-supercapacitor. One group, led by Delgado Andres [47], introduced a novel solar charging system. Their device features a three-electrode configuration comprising high-performance organic solar cells (OSCs) combined with nitrogen-doped carbon supercapacitors. The highest energy density of 1.6 × 10−4 Wh cm−2 was achieved at 0.2 mA cm−2. Although the energy conversion efficiency of this system tested at 2% is lower than the 17% efficiency achieved by a standard approach from the literature, it represents a new device capable of simultaneous energy harvesting and storage. Another group, Berestock et al. [48], also from the Fraunhofer ISE, pursued the concept of combining PV cells with capacitors using a different technology at the device level as well. They integrated a perovskite solar cell with a mesoporous carbon capacitor, achieving a peak energy conversion efficiency of 11.5% with an energy density of 4.27 μ Wh cm−2. These two studies share a similar design for photo-supercapacitor devices: the PV cell on top harvests solar energy, while the supercapacitor beneath it stores the converted energy as chemical components. The overall efficiency of the system is defined as the product of conversion efficiency and storage efficiency. It is worth noting that both studies face limitations, such as the high cost of supercapacitors and small-scale testing without application to an entire building. Both innovative devices hold the potential for utilization similar to PV panels within buildings. They can be installed on rooftops or integrated into building envelopes, enabling them to effectively harvest and store energy.
Another research publication [49] demonstrated the integration of 3D-printed electrochemical devices with bricks for energy storage, also at the device level. They developed a novel brick design by incorporating printed supercapacitors into the voids of brick insulation. The use of a Ti3C2@PPy-coated electrode exhibited favorable conductive and capacitive performance. The most favorable energy density outcomes are achieved at 2.64 Wh kg−1. If this “smart brick” can seamlessly integrate with the entire building system, it could present a promising and novel option for energy storage.
While these studies showcase promising developments in integrating supercapacitors into building applications, challenges such as cost and scalability remain to be addressed. Nonetheless, these innovations highlight the potential for utilizing supercapacitors for energy storage in building skins.

2.3. Fuel Cells

Fuel cells are devices that convert the chemical energy of a fuel and an oxidant into electrical energy [50]. The key components of a fuel cell are an ion conductor, electrodes, and a catalyst. The anode and cathode are the two electrodes that facilitate two different reactions—oxidation and reduction. Oxidation generates electrons, while reduction accepts electrons [51]. The potential difference between these two electrodes creates an electric current. The ion conductor is the component that conducts ions between the two electrodes [44]. The catalyst is usually added to the cell to accelerate the reaction [52]. In many cases, gas-diffusion layers are also necessary to diffuse gas to the key components [53]. Typically, a fuel cell is not a single “sandwich” structure. Like batteries, many fuel cells are stacked together to produce higher energy and power [51].
DOE describes six common types of fuel cells, namely polymer electrolyte membrane fuel cells (PEMFCs), alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), and direct methanol fuel cells (DMFCs) [52][54]. Each of them works in different conditions but has similar energy efficiencies ranging from 40% to 60% [55]. In addition to the six above-mentioned types, a microbial fuel cell (MFC) is another type that is relatively uncommon.

2.3.1. Solid Oxide Fuel Cells (SOFCs)

Solid oxide fuel cells (SOFCs) use non-porous ceramics as ion conductors. These cells can achieve an impressive efficiency of 80–85 % if a waste heat recycling system is in place [56]. However, SOFCs require high-temperature operational conditions, usually around 900 °C [57]. One major advantage of these cells is that they are tolerant to sulfur and carbon monoxide, allowing them to use some fossil fuels directly from coal as an energy source [52][58]. The ion conducted through an ion-conductive ceramic is O2−. The cathode typically uses La0.8Sr0.2MnO3 (LSM) and the anode is mixed with Ni-YSZ [59]. Solid oxide fuel cells can be regenerative cells like PEM regenerative fuel cells. Therefore, the same device can be used for either galvanic or electrolytic purposes [60]. SOFC is well developed and has a low operating cost, but the start-up time is longer because of the high operating temperature [52][54].
One key advantage of SOFCs is their high efficiency, making them a potential cogeneration system for buildings [61]. Several studies have explored the use of SOFCs in buildings at the pack level, including work by [55][62][63]. Ref. [64]’s computational model investigated the parameters that influence SOFC performance and simulated different system configurations for small- and large-scale buildings in various weather conditions. The results showed that efficiency depends on system configurations, building types, and weather conditions. Meanwhile, Ref. [63] demonstrated that SOFCs can be integrated with cooling and heating systems in buildings, reaching an impressive energy efficiency of 60% in Aspen Plus. These research studies demonstrate that SOFC systems can generate electricity, and buildings can make use of the heat produced as a byproduct of SOFCs. When integrated with cooling and heating systems in building designs, they can improve overall energy efficiency, but there is no study that indicates SOFCs can be used as building skin components.

2.3.2. Alkaline Fuel Cells (AFCs)

Alkaline fuel cells display cost effectiveness and quick reaction times [65]. The electrolyte in it is made up of a solution containing potassium hydroxide (KOH), and this can be sensitive to CO2 in the air, leading to potential maintenance problems [63]. The operational temperature can be below 100 °C [52][66]. A range of catalysts, including nickel (Ni) or silver (Ag), can be used, and OH- is conducted through electrolytes [67].
The recent launch of a micro-combined heat and power (CHP) alkaline fuel cell prototype by PWWR Alkaline Fuel Cell Power Corp named Jupiter 1.0 offers a new possibility for small buildings to generate electricity with a high efficiency of 90% [57] at the pack level. Hydrogen is converted into both heat and electricity. This device is potentially stored in buildings or onsite to serve as a backup energy production system. Behling [58] developed a simulation model to evaluate pack-level AFC-based CHP systems and compared them with other CHP technologies such as gas engines, Stirling engines, PEM, or SOFC-based fuel cell systems, in terms of their electricity efficiency defined as HHV and thermal efficiency. While the AFC-based CHP system showed similar total efficiency as the other technologies, its thermal efficiency was not as good. Since a building skin component will be created in the upcoming study, its sensitivity to CO2 [68] makes it less than ideal for use in environments exposed to unavoidable contamination.

2.3.3. Phosphoric Acid Fuel Cells (PAFCs)

The phosphoric acid fuel cell (PAFC) is named after its electrolyte, which consists of phosphoric acid soaked in a matrix [69]. It can be operated at temperatures ranging from 150 °C to 200 °C [70][71][72]. H+ ions are conducted through electrolytes, and concentrated phosphoric acid offers relatively stable thermal and electrochemical performance. However, the use of platinum as a catalyst increases the cost of PAFCs [63].
PAFCs were the first fuel cell technology to be welcomed into residential and commercial buildings, with UTC Power being one of the biggest North American companies producing PAFC products. Their stationary fuel cells have been successfully proven to provide electricity to buildings such as hotels, and educational institutions with lower energy costs and high system efficiency [73]. While PAFCs have several advantages, including their ability to operate at relatively low temperatures with a range of 150 °C to 200 °C and their tolerance of CO2, the use of platinum as a catalyst increases their cost [70]. Ruan et al. [74] assessed onsite stationary cogeneration systems by comparing gas turbines, gas engines, and PAFC with heating and cooling capabilities. Despite the PAFC system having a longer payback period, the research demonstrated that it offers greater energy savings in specific building types, such as hospitals. PAFCs can be used as building energy cogeneration systems but are not ideal for flexible building skin-cladding components.

2.3.4. Molten Carbonate Fuel Cells (MCFCs)

Molten carbonate fuel cells require a high operating temperature, usually around 600 °C to 700 °C [75]. In MCFC, CO32− is the ion conducted in the electrolyte [76]. Solutions of lithium, sodium, or potassium carbonates soaked in the matrix are common electrolytes [52].
Santa Rita Jail made use of MCFC as an additional power source to the grid and solar panels. As mentioned in their energy report, the energy from MCFC accounted for 50% of total electricity usage. Heat as the byproduct of chemical reaction could provide 18% of heating for the jail. It is a stationary power plant located onsite [77]. The relatively high operating temperature makes it not ideal for building skins.

2.3.5. Direct Methanol Fuel Cells (DMFCs)

Many fuel cells primarily rely on hydrogen as their fuel source. However, direct methanol fuel cells offer the advantage of using readily available methanol as their fuel [78]. They can operate at temperatures ranging from 60 °C to 100 °C [52]. Nonetheless, one drawback of this fuel cell type is the production of CO2 as a byproduct [78].

2.3.6. Proton Exchange Membrane Fuel Cells (PEMFCs)

The proton exchange membrane fuel cell (PEMFC), also known as the polymer electrolyte membrane fuel cell, utilizes a perfluoro sulfonated acid polymer layer as the electrolyte, which can only conduct protons (H+) [79]. This cell typically employs platinum as the catalyst, allowing it to operate at a relatively low temperature (below 80 °C), making it easier to integrate with building skin components [80].
Regenerative fuel cells can be either discrete or unitized. The unitized regenerative fuel cell (RFC) is receiving more attention due to its multifunctional properties [81]. However, one of the most challenging problems in making a highly efficient reversible PEMFC is ensuring that the oxygen-side catalyst layer works effectively in both the galvanic and electrolysis modes while also providing anti-corrosion and long-lasting cells. Further research is needed to address this challenge [82].
PEM fuel cells have been studied as part of the cogeneration system of buildings at the pack level. In a study by Ashari et al. [83], a PEM fuel cell system was designed to cover electrical, hot water, heating, and cooling loads in a residential building. They estimated the residential building’s load and then designed eight stacks of fuel cells with 8.4 kW power capable of providing enough energy for the building. Natural gas was used as fuel, and the electricity cost was 5.41 USD/kWh. Similar studies, such as those conducted by Ham et al. [84] and Chahartaghi et al. [85], have also developed energy models for building cogeneration applications. The efficiency of these systems ranges from 45% to 82%, which is optimistic for further development.
Chadly et al. [17] conducted a cost simulation for energy storage systems using PV as the original renewable energy source. The electricity generated by the PV system was stored in Li-ion batteries, reversible SOFCs, or PEM reversible fuel cells. Although the cost of PEM-based RFC (39.17 USD/kWh) is not competitive with the other two options (5.49 USD/kWh for Li ions, 26.45 USD/kWh for SOFCs), it has other merits such as low operating temperature and totally clean byproducts, which make it a potential player in the future building skin market.

2.3.7. Microbial Fuel Cells (MFCs)

Microbial fuel cells (MFCs) are a promising technology because they use microorganisms as their fuel source and can potentially generate electricity from a variety of organic materials [86]. They are named for their fuel, not their electrolyte, and typically use a proton-exchange membrane to conduct H+ ions [87]. It is worth noting that microbial electrolysis cells use a different design and system and as a result are not regenerative like PEMFC [88].
In 2019, You et al. [89] published a device-level study on the use of microbial fuel cells (MFCs) in buildings. The study validated the feasibility of converting buildings into micro-power stations by using MFCs. The idea is that bricks could be used as MFC reactors, with microorganisms generating electricity through chemical reactions. The researchers tested two common bricks used in the UK, as well as one handmade brick, and designed each brick differently. The MFC bricks were tested in ambient conditions and fed with either municipal wastewater or human urine. The output of the MFC bricks was recorded in electrical potential (volts), and the experimental results indicate that MFCs can be integrated with commercially available building bricks, with a maximum power output of 1.2 mW per brick. While the output was relatively lower than existing literature due to the non-optimized configuration and the use of cheap, low-efficiency catalysts, it is still considered to be a promising alternative for onsite power generation. Further studies of such fuel cell applications are worth exploring.

2.4. Two Electrochemical Energy Storage Applications for Building Skins in This Research

For the majority of electrochemical applications discussed earlier, it is evident that they are primarily employed as stationary power plants rather than for energy-storage purposes. In simpler terms, some applications only serve as co-generation systems to supplement grid electricity. Their potential as contributors to distributed energy resources is significant, particularly when they utilize clean energy sources. As mentioned previously, the researchers goal is to create a building skin component capable of harvesting renewable energy and storing and regenerating energy. Hence, a reversible system that can store energy is preferred for future design and development.
When thinking about using these technologies in building skins, there are additional aspects to be considered. In this research, a redox flow battery (RFB) and reversible proton exchange membrane fuel cell (RPEMFC) have been chosen as two options for integration into building skins after looking at different electrochemical technologies. The reasons are as follows. 1. Scalability: Energy storage options like lithium-ion batteries and supercapacitors cannot scale up as well as flow batteries and fuel cells. 2.
Both RPEMFCs and RFBs possess a comparable assembly structure, and our goal is to explore their individual strengths and weaknesses when they are employed in the integration of building skins.
The subsequent phase of this research involves proposing and evaluating new building skin solutions that function as decentralized energy sources. This is achieved through BIPV and electrochemical energy storage technologies, notably reversible PEM fuel cells and redox flow batteries. The advantages of incorporating these devices into building skins encompass the facilitation of multifunctional building skins and streamlined installation of energy components. These advantages are especially useful when renovating existing buildings, as they eliminate the need for extra space to fit energy systems and offer a more space-efficient option for building designs.
Redox flow batteries offer extended life cycles and cost–benefit considerations for long-term applications. Moreover, they are relatively safer due to the absence of flammable materials and can be conveniently scaled up. Given these advantages of flow batteries, exploring their implementation in building skins holds significant promise [90].
Reversible PEM fuel cells allow for increased energy-storage potential by expanding the size of the storage system, like the way RFBs store energy. RPEMFCs are promising energy storage technology that can support the grid [91]. Hydrogen is the stored chemical substance, which is a clean source of energy with zero CO2 emissions [92]. The U.S. Department of Energy is encouraging the use of hydrogen in various applications [93]. Out of the six types of fuel cells, considering their reversibility and operational temperature, reversible PEM fuel cells appear to be the most suitable choice for building skins.

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Jingshi Zhang
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