3D printing electrodes for energy conversion

Additive manufacturing (AM) or 3D printing is an emerging technology that allows the fabrication of three-dimensional objects by depositing the build material layer-by-layer, with precise digital coordination and customizable design.^1,2  Compared to subtractive manufacturing, where the fabricated object is obtained by removing excess of material, 3D printing minimizes waste production and allows for fast prototyping.¹  Furthermore, when compared to traditional fabrication processes, which usually require a predetermined mold for the fabrication of several identical parts, additive manufacturing provides great flexibility because it allows the design of tailored objects for specific applications. In a context where industrial processes are increasingly more interconnected and technology rapidly shifts to intelligent and flexible production systems, additive manufacturing offers great potential to reduce waste and greatly reduce prototyping time and cost.

3D printing has been proving to be a useful technology in scientific research, where it has appeared in many research areas. In electrochemistry, additive manufacturing allows the easy fabrication of electrochemical cells and electrodes with the advantage of low cost and reduced production time.^1,3  3D printed electrodes are usually obtained from conductive filaments based on carbon materials, which offer great stability and a broad potential window to the fabricated electrode.^1,4 

Another advantage of 3D printing technology is the decentralization of the productive process, in other words, small production in different sites or the personalization of the production. Offshore oil platforms have been using 3D printing to produce spare parts and some electronic components, for example. As consequence, the productive chain might be closer to the raw material and natural resources or even in strategic regions. This strategy, in another example, can contribute to energy production on-demand and on-site. Thus, the 3D printing process can contribute to increased use of the electrochemical processes for energy conversion. In this chapter, we have highlighted some strategies and examples of the use of 3D printing in energy conversion processes.

The contribution of additive manufacturing to scientific research has increased in recent years, and, as a result, we see its application in several areas, such as aerospace, biomedical industry, sensing, construction, and the automotive industry.^5–7  The broad and increasing applications of 3D printing reflect the main advantages presented by this technology: the optimization of production time, costs, reduction of waste, production of parts with complex geometries and functionality. Three important points must be considered when talking about the production of objects by 3D printing: the 3D printing process, the technique used, and the material used during the printing process.⁸  These three main points represent what makes many 3D printing technologies exist, opening possibilities for different applications in the most diverse areas.

3D printing processes can be organized in terms of the mechanism by which they are used during printing. Among these processes, the most widespread technologies are light-curing, powder-based, and extrusion. In the photopolymerization process, ultraviolet (UV) light is used to photo cure layers of a polymer/resin that is sensitive to light. As for the powder-based process, the material to be used during printing is evenly distributed layer by layer across the printing platform followed by laser melting, for example. In the extrusion process, thermoplastic polymers are melted at a certain temperature, in order to be used in the manufacture of parts.^8–10  It is important to highlight that in each one of these processes different techniques can be applied, as well as different materials. In the photopolymerization process, the most used techniques are Stereolithography (SLA) and Digital Light Processing (DLP), and these two techniques use liquid photopolymers. Selective Laser Melting (SLM) and Selective Laser Sintering (SLS) are techniques that can be applied for the powder-based process, where the material in powder form is used, usually a thermoplastic or metal.^3,11,12  Finally, the Fused Deposition Modeling (FDM) technique is the most common when we consider the extrusion process.¹³  In FDM, a filament-shaped material is melted and deposited layer by layer on a heated surface. The materials used in this technique are filaments of thermoplastic polymers, which are polymers that lose their mechanical strength with the increase in temperature.

As previously stated, the 3D printing technique FDM is one of the most common when discussing extrusion processes, however, in addition to being a prominent technique, it is also the most well known compared to other 3D printing techniques. This can be attributed to its low cost, the easiness of use, and also the possibility of using a great variety of filaments (depending on the type of thermoplastic and additives that compose them).¹⁴ 

The most general use of filaments in additive manufacturing refers to printing individual parts or repairing equipment. There are many types of thermoplastics used in the manufacture of filaments for 3D printing and the choice of thermoplastic is directly associated with the requirements of the part to be printed or the properties to be obtained. In general, the main polymers used as filaments in FDM are Acrylonitrile Butadiene Styrene (ABS) and Polylactic Acid (PLA).^15,16  These materials are used because of their properties and characteristics: ABS, for example, presents good mechanical strength and toughness compared to PLA itself and it is also resistant to corrosive chemicals;¹⁷  PLA, in turn, is a biodegradable, low-cost, and versatile polymer that can still be recycled even after becoming waste from processes involving 3D printing.¹⁸ 

Due to the characteristics and properties of the polymers mentioned above, their use in many areas of research has been reported.^19,20  However, a deeper analysis about the most used types of filaments reveals that they are usually insulators. Therefore, since they are not capable of conducting electric current, their direct use in applications involving electrochemistry is unfeasible.

To circumvent this situation, the addition of conductive charges to the filaments is necessary, providing the insulator material the necessary property for application in electrochemical systems. Regarding the types of filaments that can and are used in research related to electrochemical systems for energy conversion, we can observe that those with PLA are present in all works reviewed in the literature. To illustrate this, Table 1 was elaborated from data found in the literature. It presents the final application, composition, and manufacturer of the filament in each work verified.

As it can be seen in Table 1, all the filaments presented are composite materials prepared with the thermoplastic polymer and a conductive charge, generally derived from carbon or metals (Cu/Pt). Another highlight is that the minority of the filaments found are produced in the laboratory, during the process of experimental procedures developed in scientific research. This reveals the preference of researchers to work with commercial filaments for this type of application, as it will be explored further on. In addition, there is a predominance of the use of commercial filament known as BlackMagic3D™ (PLA with graphene as conductive filler). This great use can be attributed to the good performance of these filaments during the 3D electrode printing process. However, it is important to mention that the performance of the conductive filaments depends on their aging, as described by Kalinke et al.³⁴ 

One of the main conveniences provided by commercial conductive filaments is the simplicity of use. This implies that fewer steps in the experimental procedure are necessary, optimizing research time. Another important point is related to the flexibility and mechanical strength of these conductive filaments, while the lab-prepared filament is reported as fragile.²⁵  Commercial conductive filaments contain additives that help flexibility, making the filaments more malleable, thus greatly optimizing the step of printing the object or part of interest. In addition, conductive filaments are very accessible, as they are inexpensive, further benefiting their dissemination in research, an effect that can be noted in Table 1.

Despite the advantages mentioned above, commercially conductive filaments also present some limitations that must be discussed. One of them is about the processing of the filament after printing the part for application in photo and electrochemical systems for energy conversion. Some researchers^26,28–32  have reported in their work that these filaments do not present good conductivity and therefore require activation or pre-treatment in order to expose the functional groups responsible for the conductivity of the filaments, that is, to obtain a more electrochemically active surface. Another aspect that we can consider is also related to the composition of the commercial conductive filaments.^35–38  To preserve the originality and uniqueness of the commercial filaments, manufacturers do not disclose the technical sheet of these materials in their entirety. As a result, they preserve the originality of their filaments but end up providing limitations for researchers, as they do not know the clear composition of the material that is being used. This was evident in the study presented by Browne et al.,²⁹  where they found metal “impurities” in the studied filament (BlackMagic3D), such as iron and titanium. They also verified that these impurities affected the performance of electrocatalysis, revealing that researchers in this field must be cautious with the presence of impurities not described by the manufacturer.

Faced with the presence of impurities in the commercially available filaments, researchers started to study the production of these filaments in laboratories. Filament production is quite simple when we scale this for electrochemical systems for small-scale energy conversion. In general, conductive filaments produced in the laboratory are used to manufacture electrodes, which are mostly very small in size. As a result, the amount of material used is greatly reduced, and this is also a positive point considering the waste produced by the process is minimum. In general, the thermoplastic polymer along with the additives is previously mixed and only then processed, thus forming filaments to be used later. The conductive filaments produced in the laboratory and identified in Table 1 used the same basic principle.^21,23,25  Initially, the thermoplastic polymer was solubilized in an organic solvent, followed by the addition of the necessary additives for a formulation with the desired properties (see Fig. 1A). After that, the system is left under agitation for a specified period of time (see Fig. 1B). This step aims to provide homogeneity and favor interaction between additives and polymer. Next, the material is left to dry (see Fig. 1C), only then to be processed in an extruder at a determined temperature (referring to the melting temperature of the polymer used) (see Fig. 1D). After extrusion, the filament is ready to be used for printing objects or electrodes (see Fig. 1E).

Additivation of thermoplastic polymers is the process in which the physical or chemical properties of a polymer are improved by adding a material, the additive, to the polymeric matrix. The additive must present a determined property that will be attributed to the polymer in the new composite material obtained after additivation. A great example of additivation is the incorporation of graphite into a PLA matrix: PLA by itself is not able to conduct electric current, but when graphite was used as an additive in the aforementioned study,²¹  the fabricated PLA filament after additivation and extrusion presented characteristics of a conductive material. In addition to the conductive property, the fluidity of the additive polymer during processing in the extruder is another important property for additivation. In general, during the extrusion process for the manufacture of conductive filaments, there is some resistance when extruding the filament through the extruder nozzle. But this can be modified by additivation with a plasticizer, among which we can mention 2D-MoSe₂ and polyethylene glycol (PEG).^23,25  Hughes et al.²³  comments that after the addition of 2D-MoSe₂, the fluidity of the composite (PLA and conductive charge) improved, thus enabling the process of manufacturing conductive filament without impairing the electrochemical activity.

Further reported strategies to improve the electrochemical performance of filaments used for printing parts for electrochemical applications, especially for printed electrodes, refer to the pre-treatment, activation, or functionalization of the printed object. These strategies, which will be explored in the next topic, also aim to improve the electrochemically active area of the printed object by either exposing the conductive component or adding an active catalyst to the surface of the electrode.

Hydrogen (H₂) presents promising perspectives to be applied as a renewable energy carrier, due to its cleanliness and low greenhouse gas emission compared to fossil fuels.³⁹  Furthermore, the energy yield of hydrogen is 2.75 greater than hydrocarbon fuels, making it a clean alternative to power fuel cells, for example.⁴⁰  However, even though hydrogen does not generate carbon emissions at the end-use point, the production pathway and the energy used to produce it are major points to determine the level of cleanliness of the energy obtained from H₂.⁴¹ 

Hydrogen can be obtained from different feedstock and production pathways, and, currently, the most significant production comes from processes that use nonrenewable sources, such as cracking or reform of fossil fuels. On the other hand, H₂ can also be obtained from water as the raw material through the water splitting (WS) process, also referred to as electrolysis of water, and from alternative sources, such as biomass and waste materials.^39,42  Among these, the H₂ production through the electrolysis of water fits the zero-emission carbon approach, only requires water as the raw material, and does not generate toxic by-products, being considered a clean process.

The electrolysis of water is essentially a process where an electric current passes through two electrodes to induce the breakdown of water into hydrogen (H₂) and oxygen (O₂).^39,42  Two half-reactions can be described from this process: the oxygen evolution reaction (OER), which takes place at the anode, and the hydrogen evolution reaction (HER), which takes place at the cathode:

The source of the external electric energy (input energy) necessary to promote the water-splitting reaction also impacts the cleanliness of the hydrogen production. The cleanest level of hydrogen usually referred to as “green hydrogen”, is obtained when a renewable energy source, such as solar or wind energy, is used to produce H₂ through the electrolysis of water.^39,40  Compared to other H₂ production pathways, green hydrogen production presents the clear advantage of being clean and renewable, matching the current demand for decreasing greenhouse gases emission to face the matter of climate change. However, the production of hydrogen through water splitting still faces challenges regarding the efficiency of H₂ generation, besides, it still presents a high generation cost.

Although the cathodic reaction for the water-splitting process presents its challenges, the main drawback for green H₂ production is found in the anodic reaction. This drawback is related to the fact that this reaction involves a complex four-step proton-coupled electron transfer (PCET), making it energy-intensive and kinetically slow. Thus, a huge variety of efficient and stable catalysts and electrode materials have been studied aiming to decrease the overpotential of this reaction. However, state of art materials still relies on rare and expensive metals, bringing problems for scale-up applications and preventing the wide use of water splitting for hydrogen production.^39,43 

Additive manufacturing is an important tool to help us achieve green hydrogen production considering the versatility of this processing technique, as it can be used to obtain electrochemical cell components, structured systems to support catalysts, and devices with multiple designs. The use of 3D printing to obtain electrodes allows the production of structured systems that can be applied as platforms for different electrochemical devices, including setups for the production of H₂ and O₂ through water splitting.³  For electrodes, the geometry and exposed area are essential factors to assess the efficiency of the electrochemical process. In this sense, additive manufacturing is a tool that enables us to tune the design of the electrode, as well as explore alternative materials and surface treatments to improve the performance of electrodes. Furthermore, as previously discussed, additive manufacturing provides a faster and cheaper way to fabricate devices, which can be the key to achieving a low-cost platform to produce electrodes that will make the production of green H₂ via water splitting an economically viable process.

For the fabrication of 3D-printed electrodes, researchers usually apply composite materials, combining non-conductive and conductive components. In this sense, carbon-based materials, such as graphene, carbon nanotubes, and carbon black are commonly employed as conductive fillers. Additionally, in order to achieve the activity and performance required for highly efficient electrodes, the 3D electrodes can further go through activation processes using a solvent, electrochemical treatment, or even a combination of both, to increase the active surface area and enhance electrocatalytic features.^36,44,45  They can also be modified with metal catalysts to improve the catalytic properties. However, the use of cheap and easily prepared materials is essential to make these electrodes viable for scale-up applications.

Among the different catalyst materials, the first-row transition metals (Earth-abundant elements) (oxy)hydroxides are a promising alternative to noble and expensive metal-based electrocatalysts and can have a key role in decreasing the cost of the overall water splitting alongside with 3D-printing. In 2019, Dos Santos et al.²⁶  reported the use of cost-effective NiFe (oxy)hydroxides modified graphene additive manufactured electrodes (Gr/AMEs) as highly efficient electrocatalysts for the OER in alkaline medium (see Fig. 2A and B). In this work, the authors compared the material activity with different Fe content (5, 10, 20, and 40%) and the optimal performance was observed for the electrode with 10% content of Fe, which displayed an OER onset overpotential of 519 mV (vs. RHE). This catalyst activity and performance were comparable with polycrystalline Iridium (413 mV) and presented a significantly less electropositive overpotential than the bare/unmodified 3D printed electrode (see Fig. 2C and D).

As 3D printing facilitates the study of electrodes with different geometries, Ahn et al.⁴⁶  more recently developed a new 3D printing strategy to produce a highly active 3D pyramid electrode for OER. Their strategy consisted of fabricating the electrode in a three-step method, where firstly an ink-based 3D printing approach using functional graphene ink was used to print the 3D graphene pyramids array, followed by successive electrodeposition of the Cu conductive layer, and finally, the NiFe-layered double hydroxide (LDH) electrocatalyst layer was grown on this electrode surface (see Fig. 2E). This strategy contributed to an increase in the number of active sites on the electrode with a relatively larger surface area when compared to fat electrodes with the same base area. As a result, η and exchange current density (i₀) of the NiFe-LDH pyramid electrode in a 0.1 M KOH solution were 258 mV at 10 mA cm⁻² and 0.818 µA cm⁻², presenting a better activity when compared to an IrO₂ pyramid electrode (see Fig. 2F and G), without potential decay at both 10 and 100 mA cm⁻² for 60 h. This 3D printing strategy provides an effective approach for the fabrication of highly active, stable, and low-cost OER electrocatalyst electrodes, which can be crucial for water splitting scale-up applications.

Another important material that has been studied for applications with 3D printing is Prussian blue and its analogues (PBAs). Besides presenting earth-abundant metals in their composition, which leads to low cost and an easy preparation, one of the greatest advantages of using PBAs is the possibility of working under mild conditions, facilitating scale-up applications. In 2020, Zambiazi et al.³³  proposed the modification of commercially conductive filaments with CoCoPBA and CoFePBA, followed by the printing of 3D electrodes using the modified filament. After the printing process, the electrodes went through an activation process and were applied for water oxidation under mild conditions (see Fig. 3A). The results showed that the electrodes modified with CoCoPBA presented a better performance toward Water Oxidation Reaction (WOR), exhibiting an overpotential of 325 mV at 1 mA cm⁻² (see Fig. 3B). These results indicate that the development of 3D-printed electrodes modified with PBA catalysts is a promising field for studies of high-performance and low-cost systems for water oxidation under mild conditions.

As a recent advance, Hughes et al.²³  outlined a facile technique for the fabrication of highly reproducible 2D-MoSe₂–carbon/PLA and Pt/C additive manufactured filaments to produce electrodes that would work for both WOR and HER. The authors explored the incorporation of varying percentages of the additives into the PLA matrix and reported that a 25% mass incorporation is the ideal compromise between electroactivity and printability. The fabricated electrodes were denoted as M_X%C_X% and Pt/C_X% where M represents 2D-MoSe₂, C is the electro-conductive carbon, Pt/C is the commercially sourced carbon with a 20% mass loading of Pt and X_% is the percentage incorporated within the PLA filament/AME. When using the optimized M_10%–C_15%-AME and Pt/C_25%-AME as the cathode and anode, respectively, whilst a commercially available alkaline battery applied a potential of 1.5 V, water splitting was achieved with obvious effervescence occurring at each electrode (see Fig. 3C–E). The process has demonstrated the potential to reduce the quantities of expensive electrocatalytic materials, allowing researchers, industry, and any interested individuals to rapidly go from ‘desktop designs’ to workable electrochemical prototype devices, with a great performance.

The fabrication of 3D-printed electrodes for the HER has also been investigated in recent years. Different from the anodic reaction (OER), where the complex electron transfer process is the main drawback of the process, the H₂ evolution presents challenges regarding the catalyst design, especially on means of hydrogen adsorption energies on the electrode surface. Taking this into consideration, the focus of the research on 3D-printed electrodes is related to the functionalization of the electrodes. In 2017, Foster et al.⁴⁷  reported for the first time the utilization of 3D-printed electrodes manufactured from a commercial PLA/graphene filament to produce hydrogen via HER as an alternative to commonly used platinum-based electrodes. Since then, different surface functionalization techniques, as well as different conductive materials, have been employed to improve the electrochemical activity of 3D-printed electrodes towards the HER.

Dos Santos et al.³⁵  presented pre-treatment procedures to improve the electron transfer kinetics of 3D-printed PLA/graphene electrodes, and Browne et al.³⁷  reported the activation of 3D-printed PLA/graphene electrodes through both solvent and electrochemical treatments and evaluated their performance towards the HER. These results have shown that the pure electrochemical treatment was able to expose graphene sheets by decreasing the amount of insulating PLA on the electrode's surface, thus increasing their electrocatalytic activity, and the combination of solvent followed by electrochemical treatment was able to produce electrodes with higher electrocatalytic activity towards the HER. More recent reports present the functionalization of 3D-printed electrodes with the deposition of active catalysts for HER. In this sense, many studies have explored transition metal dichalcogenides catalysts, which are known for their great activity towards the HER, as well as bimetallic catalysts, as alternatives for the expensive platinum-based catalysts.

Several functionalization strategies have been employed to add active catalysts to 3D-printed electrodes and improve their electrochemical activity towards HER. Gusmão et al.,³¹  for example, investigated the functionalization of 3D-printed PLA/graphene electrodes with spray-coated MoS₂. In this work, the authors reported that the functionalization with spray coating was appropriate for 3D-printed electrodes as it was able to increase the catalyst loading and improved the electrochemically active area, the performance towards HER, and stability when compared to the as-printed 3D electrode. Additionally, the electrodeposition of materials,^22,24,26,48  nano architectonic immobilization by immersion in an organic solvent,²⁸  atomic layer deposition,³²  and dip-coating⁴⁹  have also been reported as efficient methods to functionalize PLA/graphene 3D-printed electrodes with transition metal dichalcogenides. Fig. 4 shows a scheme of employed strategies to functionalize 3D-printed electrodes and improve their electrochemical activity.

Another reported strategy to functionalize 3D-printed electrodes consists in manipulating the filament to be used for 3D printing, as previously discussed. Hughes et al.²³  demonstrated the possibility of incorporating MoSe₂ catalyst into a PLA matrix to produce conductive filaments that were further processed to obtain 3D-printed electrodes. More recently, Ghosh et al.²⁵  reported the preparation of MoS₂/carbon/PLA filaments and further 3D-printed electrodes from this material aiming for application on the HER.

In addition to metal dichalcogenides, bimetallic catalysts have been studied on 3D-printed supporting electrodes. Most recently, Hüner et al.²⁴  have reported the electrodeposition of different volume ratios of Ni and Cu metals on 3D-printed PLA/carbon black electrodes and investigated the activity of the prepared electrodes towards the HER in alkaline medium. The study demonstrated that the kinetic performance of NiCu coated electrodes significantly improved compared to bare electrodes, with Ni_xCu_3x presenting almost 80% higher current density than the uncoated electrode in alkaline medium.

Besides carbon and polymer-based 3D-printed electrodes for HER, there have been reports in the literature where the electrode is fabricated using 3D metal printing. In this sense, Ambrosi et al.⁵⁰  reported the use of metal 3D printing to produce helical-shaped stainless-steel electrodes and demonstrated the applicability of this technique to fabricate custom-made electrodes for diverse electrochemical applications. In 2017, Ambrosi and Pumera⁵¹  have demonstrated that, despite the versatility of 3D-printed stainless-steel electrodes, when used as printed they presented poor catalytic properties towards OER and HER, however the surface modification of these electrodes using electrochemical procedures could grant and/or tune the desired properties. More recently, Sullivan et al.⁵²  have reported a 3D-printed and hierarchically porous NiMo-based electrocatalyst, with a high surface area and significantly increased electrochemically active surface area, which was able to lower the overpotentials for HER.

In addition to presenting numerous advantages in the design and fabrication of electrodes for both OER and HER, as previously discussed, decreasing the material and production cost, achieving high activities, contributing to a scaling-up process, additive manufacturing and 3D printing can be used to design and fabricate electrolyzer devices. It is important to highlight that for large-scale production of H₂ and O₂, in addition to studying the production of highly active electrodes, it is also equally important to develop innovative fabrication methodologies that enable easy and cost-effective production of all water/gas handling electrolyzer components. Given that, researchers have been studying the use, combination, and integration of different additive manufacturing technologies driven by a common structural design, believing this could facilitate the fabrication of functional devices at lower costs in comparison to traditional methods.

Focusing on this goal, in 2018, Ambrosi et al.⁵³  proposed the potential combination of selective laser melting for metal printing (metallic components – electrodes) and fused deposition modeling for plastic material printing (liquid/gas handling components – cells) to design, fabricate and assemble a water electrolyzer prototype (see Fig. 5). The metal electrodes were primarily fabricated in stainless steel and further modified with highly active earth-abundant electrocatalysts for improved HER and OER, MoS₂–Ni composite for H₂ evolution, and Ni/Fe hydroxide for O₂ evolution. After the prototype assembly, electrolysis was carried out in a 1 M KOH solution, and using modified electrodes, a 10 mA cm⁻² current was obtained at an overall potential difference of about 1.72 V vs. RHE. This work shows it is possible to design and fabricate a fully functional water electrolyzer device by using AM technologies, in a cheap, easy, and functional way.

Lee et al.⁵⁴  demonstrated in 2019 the impact of surface patterning of the electrode topography toward enhanced electrocatalytic performance. For this, they reported the feasibility of designing electrodes with a concave base, to increase the surface area, facilitating enhanced contact with the reactants for both reactions (OER and HER), besides, the distribution of products, oxygen, and hydrogen, is also facilitated by this design. As expected, the conical arrays exhibited enhanced peak current in comparison to the flat electrode, with a 90 mV lower overpotential to achieve a current density of 10 mA cm⁻². Furthermore, they tested its integration to the full water splitting cell by combining the metallic and polymeric-based 3D printing approaches that allowed the fabrication of a 3D printable compartmental electrochemical water splitting device. As one of the advantages, when necessary, the cell can be converted to a photoelectrochemical water splitting cell simply by placing quartz glass at the entrance port for light illumination on the electrode. The produced gases, O₂ and H₂, can be easily collected in their respective anodic and cathodic compartments. The cell showed excellent performance and using the nickel-modified Ti conical array electrode, there was about 80% of current retention after 1 h electrolysis. Although the authors report there is still room for improvement concerning the electrode's performance and stability, the assembled cell showed a great performance and opens opportunities for a cheap and easy design and assembly for water splitting electrolyzers.

For validation purposes and to show how important 3D printing can be for water splitting devices studies and applications, in 2021, Browne et al.⁵⁵  compared a range of metal oxides as catalysts for the OER under acidic/PEM conditions in a conventional RDE (rotating disk electrode) three-electrode cell and in a 3D printed electrolyzer device with 3 electrodes. The results of their work indicated that the performance of the metal oxides in conventional RDE three-electrode cells does not completely correlate with those measured in an electrolyzer device, leading to an erroneous characterization of the electrodes during the overall WS. To overcome this issue and provide a more accurate comparison, the authors proposed the design and construction of a low-cost electrolyzer device using various additive manufacturing technologies (fused deposition modeling (FDM) and stereolithography (SLA) to evaluate these catalysts under more realistic conditions, thus, providing a better catalyst evaluation and proposal for the development of future and scale-up electrolyzers. Their work shows how important 3D printed electrolyzers can be for the study and application of catalysts and devices for water splitting, helping with a better understanding of their performance and providing a better overview for future easy and cheap scale-up applications.

As it has been discussed in this section, the search for devices with a low cost and high performance is essential for a viable scale-up of water-splitting technology. In this sense, 3D printing can play a crucial role in the development and study of industrial water electrolyzers, decreasing the cost and making the production of green H₂ from water a competitive process.

As humanity grows and technology becomes more advanced, the tendency is to move far beyond in the exploration of new places and areas, leading to an increase in space exploration in the near future, researching beyond earth in different aspects, such as low-Earth orbit (LEO), high-Earth orbit (HEO), near-Earth asteroids (NEAs), Moon, Mars, and even other planets and galaxies set in a further distance from Earth.^39,56  Nonetheless, the main drawback of space exploration is the huge amount of energy required, thus, it is essential to develop a system to harvest and store energy in a fast and viable way, with high energy efficiency.^57,58  However, the development of these systems is limited by the difficulty of finding materials that are cheap, light, durable and operate well under the unconventional conditions found in space.^39,56,59  Therefore, scientists that are focused in this area are researching new materials for energy harvesting and storage to be adaptable and help spacecraft, launch vehicles, landers, rovers, spacesuits, tools, communication networks, and anything that requires power and energy.

Currently, the main energy sources in space, which have been used and applied during the main space missions until this day, are based on solar energy and power, using photovoltaic (PV) panels to convert this widely available source into electricity and power the machines and equipment already mentioned. Although PV panels are a good alternative in space, they are still limited by their low energy conversion efficiency, especially in deep space missions, where solar radiation is low compared to the irradiation levels on Earth. Another alternative that is used in space missions is nuclear power, nonetheless, this is not a renewable energy source, producing nuclear residues, and it is not a completely safe alternative, bringing additional risks to the mission.^60–63  Thus, other renewable alternatives with greater efficiency have been researched to improve energy harvesting in space.

In this way, the use of Fuel Cells (FCs) is an interesting alternative to harvest energy from oxygen and hydrogen has been receiving more attention in the last decade, since it stands out for its high efficiency, durability, low cost, and for presenting only water as a side product.^39,60  One of the great advantages of electrolysis and FCs utilization is that they can easily be adapted to scale and be used for in situ resource utilization (ISRU), where the cells are directly adapted in spacecraft, robots, launch vehicles, landers, rovers, and spacesuits, producing electricity directly into them during the missions. Hence, the materials used to design and assemble these FCs need to be easily adaptable into different sizes, present great durability, and operate at high efficiency throughout the whole mission, which could take from days to months and even years.^64–66 

Forthwith, The European Space Agency (ESA), alongside the Canadian Space Agency (CA) and the Japanese Aerospace Exploration Agency (IAXA), are already working on hydrogen regenerative fuel cells that could be sent to space in the next few years. This system will be capable of generating heat and electricity that will be necessary to help the craft withstand the long and bitterly cold nights.^39,62 

Although FCs are a great alternative, they only present a good advantage if the material used for its design and assembly meets the requirements aforementioned for great efficiency in space. In this way, the use of 3D modeling and printing can be essential and bring enormous advantages for energy harvesting and storage in space, contributing to the production of cheap, light, versatile, and robust materials.^45,67  Besides, the fast prototyping process of 3D printed devices makes it possible to design, print, and assemble the parts required direct in space, either at the International Space Station (ISS), a spacecraft, or even at a different planet, moon, or asteroid being explored at any moment. Just as an estimate, according to Leach, it is estimated that the cost of transporting one brick to the moon can reach around two million dollars, which is completely unfeasible; nonetheless, the agencies could take a 3D printer into space and easily print as many bricks as they need.⁶⁷  In addition, for FCs design, 3D printing can be used to print and produce electrodes to work as both the anode and cathode, with a great efficiency towards electricity generation, at a very low price, being extremely beneficial to regenerative FC design.

Currently, there are already several projects that have been testing whether creating parts in a lunar base using scrap metal or existing surface materials would be feasible. Starting with ICON's Project Olympus, which aims to test and develop prototypes for a possible future full-scale additive construction system that could print infrastructure on the Moon. Redwire has a similar idea as they have sent supplies for the Redwire Regolith Print (RRP) study to the ISS to determine whether it would be possible to 3D print with lunar regolith, loose rock, and soil, to create on-demand habitats on other planets and the moon.^56,68,69 

Moreover, space exploration and research agencies have been studying the development of 3D printed satellites, rovers, rocked engines, whole spacecraft, and even space food using bioprinting. Aleph Farms announced a project in 2019 aiming to 3D print meat on the board the ISS, and so far, they managed to create cell-based meat that retains the texture, flavor, and structure of a classic steak. Furthermore, they aim to develop new space foods to help astronauts thrive in this field.⁵⁶ 

Although the perspectives presented in this section still require massive investment in research to be developed in the best possible way, the highlighted points here demonstrate how 3D printing is important and is already playing an essential role to make space exploration viable, providing a better scientific understanding of outer planets and deep space. Therefore, it is possible to affirm that 3D prototyping will enable the design and development of uncountable essential parts for space missions, since energy harvesting and generation, to spacecraft prototypes and even food 3D bioprinted.

As discussed above, additive manufacturing is a powerful processing technique that has already conquered its space in different research areas. There are several 3D-printing techniques and precursor materials being explored, and it is a research field that has been rapidly growing, mainly due to its versatility. When considering energy conversion, especially the electrolysis of water, additive manufacturing offers an alternative to fabricating different parts with a customized and adaptable design. For the water-splitting reaction, additive manufacturing can potentially assist with reducing the cost of the process and make green hydrogen production economically viable, as it offers the possibility of producing inexpensive electrodes in a short period of time and with a tailored design.

Despite the great potential of additive manufacturing to produce green hydrogen, there are still a lot of challenges to face. Currently, there are only a few available commercial filaments, and their composition is not completely known, which can introduce impurities into the fabricated electrodes. Furthermore, fabricating and processing lab-made filaments is still a challenging task with only a few reported examples. The filaments’ fabrication process described in the literature does not favor the mechanical properties of the final material, and as previously discussed, these filaments are usually presented as fragile and brittle. Additionally, electrodes produced both from commercial and lab-prepared filaments require, in general, a pre-treatment step to expose the conductive material.

Another point that calls to attention is the number of reports on 3D-printed electrodes for the electrolysis of water compared to the number of studies involving 3D printing that have been reported in the last few years. Although the number of studies in different areas that involve 3D printing has been increasing considerably, only a few research groups have reported the fabrication and application of 3D printed electrodes for the OER and HER. This can be associated with the necessity of additivation, and functionalization found in electrodes produced from polymeric filaments combined with conductive additives.

Ultimately, it is important to highlight that there is still room to explore the potential of additive manufacturing in the production of 3D-printed electrodes. Improving the fabrication of filaments to overcome the challenges still found in the process, as well as the functionalization methods to add active catalysts to the electrodes are two strategies that can greatly enhance the activity of 3D-printed electrodes in energy conversion.

The authors of this chapter are grateful for the financial support of Brazilian Funding Agencies. This study was supported in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001, Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq (grant#308203/2021-6) and Fundação de Amparo à Pesquisa do Estado de São Paulo, FAPESP (grant#2017/11986-5, grant#2018/25092-9, grant#2020/14769-8, grant#2021/05976-2).