A variety of device designs have been reported in the literature with continuous improvements in energy conversion efficiency and short-term stability achieved by improved material properties of components directly involved in the PEC process namely the photo-absorber, electrocatalysts, electrolytes and electrode supports. Additionally, efforts have been made to reduce the material costs of such components in order to make PEC economically competitive with other forms of energy generation so as to accelerate entry into the energy market. Nevertheless, although the safety and reliable operation afforded by robust containment vessels will be important for the meaningful deployment of photo-electrochemical devices in the global energy system, the materials research for these components has been largely neglected in pertinent literature. For this purpose, it is important to identify which materials could potentially be used to design flow cells/containment vessels and their sealing, that are easily scalable and low cost and at the same time durable for tens of thousands of hours’ service in chemically reactive species under fluctuating temperature and fluid velocity conditions.
4.1. Compilation of Fully Functional PEC Systems
Here, we present a compilation of reported PEC designs, which have promise for scale-up beyond laboratory prototypes. The data set is limited because the majority of laboratory scale solar cells area ~ ≤5 cm2
are excluded from the scope of this review because since they are too small to warrant accurate quantification of gas production as previously observed [3
] without careful design of the product gas separation. The images of a selection of fully functional PEC prototypes are presented in Figure 1
a–f and for comparison, a square meter-sized demonstrator is presented in Figure 1
In all the devices shown, the electrical connection between the photo-absorber and the electrolysis process is contained within the encasement. There are generally three ways of sealing the joints, one is by using adhesives to glue the parts together (this work; [18
]), the second is to use compression seals that are clamped by screws [19
] and the third using adhesives supported by a metallic brace to maintain compression sealing [22
]. The details of the devices presented in Figure 1
, as well as those of a wider selection derived from the literature, are presented in Table 1
. The entries are arranged in ascending order of the solar collection area. While the laboratory size prototypes operate purely from solar energy and have provisions for separately collecting H2
, the demonstrators either require an additional external bias, or only consist of either a photo-anode or photocathode.
Some of the small photo-aborbers with a solar collection area of less than 25 cm2
when combined with solar concentration systems produce significantly high amounts of hydrogen despite the small size of the reactor [21
]. For these devices, the casing is comparatively highly advanced compared to the other listed laboratory prototypes of roughly the same size that were designed for operation under one sun [19
]. The design of intermediate sized prototypes for 1 sun operation of a solar collection area larger than 25 cm2
but smaller than 100 cm2
have full functionality, but lower cost casing materials were used. The larger size of these devices somewhat increases the confidence in the H2
production measurements as well as the scalability although leakages cannot be ruled out.
In the next category are somewhat larger prototypes measuring at least 200 cm2
but still smaller than 500 cm2
], this work). This size lies in between the laboratory prototypes and the demonstrators and the reported devices were designed for separate generation and collection of the product gases. However, since the prototypes are too large for typical continuous solar simulator, these devices could only be characterised using natural sunlight, as was the case for the demonstrators. The first one combining a PEC-PV hybrid (with a solar collection area of roughly 200 cm2
) with solar concentration was developed in the PECDEMO project [29
] based on the modular design (Figure 1
f) published by Villanova et al. [20
]. Another design, for which three different units are shown in Figure 1
a, has been developed at the Helmholtz Zentrum Berlin (HZB) with a solar collection area of 294 cm2
One of the demonstrators with a PEC collection area of 820 cm2
was used to generate H2
to feed an ammonia generator; however, it required an additional bias provided by a separate PV cell to power the full water splitting reaction [30
]. In that system, natural sunlight was split such that wavelengths from 280–700 nm were directed to the PEC and the rest to the PV device with only a fraction of the aperture area (200 cm2
) were illuminated by the concentrated sunlight. Another demonstrator also shown in Figure 1
g with a solar collection area of 910 cm2
comprised a particulate photocathode used with a hole scavenger to support the hydrogen evolution reaction (HER) but without provision for the oxygen evolution reaction (OER) [22
]. A later particulate system with a collection area of 1.0 m2
:Al with a co-catalyst in pure water and produced both H2
although no provisions were made for gas separation [31
]. In a further demonstrator, several BiVO4
photoanodes assisted by silicon PV modules were assembled to make a system with a collection area of 1.6 m2
for which the authors reported a hydrogen production rate of 3g/h, but did not disclose the details of the construction [32
]. The table clearly illustrates the gap in functionality between prototypes and demonstrators of the roughly square meter size and thus the challenge of designing commercially relevant PEC devices.
Poly methyl methacrylate (PMMA), also known as acrylic and by tradenames such as Plexiglas and Perspex, to name a few, appears to be the most widely used material for the PEC device casing. The PMMA in form of plates was used either for structural support [18
] or as an illumination window [19
] or both [30
]. PMMA is probably a material of choice because of its being easy to machine, its relatively high mechanical strength, low cost and ultra violet radiation resistance.
Additionally, a variety of polymers, mainly proprietary composites for 3-D printing have been used to fabricate PEC device encasements for example, Fullcure RGD720, a photopolymer [28
], polylactic acid based composite [19
] as well as several PMMA-based composites ([24
]; this work). 3-D printed flow plates and casements can be used for rapid prototyping, but despite the potential for low cost production, issues remain regarding the cost of the filaments, the longevity, especially of the polymeric components, and possible contaminants from the printing filaments since these are usually proprietary recipes. The three devices shown in Figure 1
a were made at the Helmholtz Zentrum Berlin, (HZB) Germany under the PECSYS project, with the electrolysis encasement made out of different 3-D printed PMMA based polymer composites (left) VeroClear, RGD810; (centre) Objet-RGD525-High-Temperature-White and (right) VeroBlackPlus, RGD875 [33
While the majority of 3-D printed polymer encasements in the literature are used with acidic and neutral electrolytes, in our laboratory, designs were made for alkaline (high pH) PEC operation with different levels of success. Depending on the type of non-platinum group catalyst used, the solar irradiance and ambient temperature, solar to hydrogen efficiency values lay between 4–10% relating to a photon collection area normalised hydrogen production rate of 1.0–2.0 g –H2/(h m2) (to be published elsewhere). Thus far, the left hand device made using VeroClear, RGD810 based on PMMA has shown the best thermo-mechanical stability. Contrarily, the casing of the centre device using Objet-RGD525-High-Temperature-White, cracked after several tens of hours of intermittent operation and the right hand device using VeroBlackPlus, RGD875 tended to soften when the PV module temperature rose above ~40 °C, causing a leakage of electrolyte at the joint between the PV module and the electrolysis casing.
Unlike polymers, metals have a high thermal conductivity allowing heat transfer between components thus avoiding over-heating; moreover, they retain their mechanical strength at high temperatures. Thus, for high solar concentration, metallic casement materials seem to be the preferred material in the form of machined plates [23
] or as 3-D printed Ti6
V alloy [21
]. In another design, aluminium reflector plates with a PTFE insert were used to protect the PMMA casing of a prototype from overheating during solar concentration ~ ×18 [30
Since the use of two different materials is unavoidable, the window glass for illumination and the rest of the casing with a less rigid material, attaining a hermetic seal is more challenging for these devices than for discrete electrolysers. Materials for the transparent window include in ascending order of cost, PMMA [19
], glass ([18
], this work]) and quartz [20
], to illuminate the photo-absorber. However, because these materials have poor compression strength, they tend to shatter easily and are thus incompatible with clamped seals that employ screws. The front glass of the PV module of left-hand PEC device in Figure 1
a was shattered by the compression force during clamping just after the adhesive was applied illustrating the incompatibility of glassy transparent components of the encasement with compression stress required for hermetic sealing. Furthermore, glass has the disadvantage of a rather low thermal expansion coefficient and low thermal conductivity and thus can easily shatter under high thermal loading in a PEC system [21
Epoxy resins have been used to seal against electrolyte leakage at the joint between PV and electrolysis cell ([21
], this work) and to fix the inlet and outlet flow ports into the casing [26
]. However, the seals made using epoxy between the PV cell and the electrolysis cell are prone to failure and may not be water tight as we experienced and as was reported by others [21
]. This is because the quality (inclusion of air pockets) of the applied adhesive varies from run to run if applied manually. Additionally, from our experience, the seals at the flow ports are prone to leaking probably as a result of vibrations from the flowing electrolyte.
The choice of materials for o-rings and/or gaskets is more varied and includes Viton, a fluoroelastomer [27
] and polyether ether ketone (PEEK) [29
] for both alkaline and acidic environments. For alkaline PEC devices fluorosilicone [25
] and ethylene propylene diene EPDM [this work] gaskets have been reported while silicone elastomer [28
] gaskets were used for PEC devices with PEM. These materials have already been engineered for more demanding applications than those typical of PEC devices and thus can be considered well developed. The remaining challenge is to find techniques of applying uniform and sufficient compression either by clamping or adhesion without damaging the brittle but essential illumination window.
There have been a few explicit accounts of challenges associated with choice of suitable materials for casements in functioning devices. For example, Walczak et al. reported the parasitic absorption of O2
(g) onto the internal surface of the chassis and the epoxy materials that were used to construct a fully integrated, acid stable and scalable louvered solar driven water splitting system [25
]. Becker et al. reported H2
crossover in a PEC device encased in metal supported PMMA with edge sealing using polyether ether ketone (PEEK) o-rings fixed in place with a commercial epoxy resin (Hysol 9483, Henkel) [18
]. They attributed the H2
crossover to possible leakage across the membrane or at its edges where the o-rings separated the two chambers from each other. In a follow-up paper, Welter et al., reporting on the same device set-up, acknowledged that fluctuating irradiance and temperature might affect the mechanical stability of the EC housing [34
]. Despite performing laboratory stability tests with simulated solar day and night cycles, no conclusive results on the effects of dynamic loading on mechanical stability were presented.
Additionally, Tembhurne et al. [21
] reported that a commercial thermally conductive epoxy resin used to glue the rear of the PV cell to the anode in their concentrated PEC design was not water tight. A commonality for the aforementioned reports is that when epoxy was used, either gas or liquid leaks happened. We observed the same effect in our laboratory, whereby the joint between the back of the PV module and the electrolyser casing developed an electrolyte leak after several hours of exposure to illumination that caused the PV module to heat to ~ 50 °C. This is likely caused by the different coefficients of linear thermal expansion of the PV cover glass, the electrolyser casing and the dried epoxy applied to the joint to fix the two together. It is also possible that the dried epoxy did not have sufficient viscoelasticity to compensate for the different coefficients of linear expansion of the more rigid parts. In contrast, joints sealed using gaskets or o-rings made out of soft thermoplastics, e.g., viton or PTFE or PEEK clamped by screws tend to be more leak tight as evidenced by their wide spread use in discrete electrolysers.
4.2. Critique of Candidate Materials for Hermetic Sealings
Candidate materials for hermetic sealing can be identified by examining the properties of materials used for the casing and seals of PEC devices reported in the literature. They can be also identified by surveying the established materials used in the technologically more mature but related fields of electrolysers, fuel cells and photovoltaic modules. Table 2
lists selected properties of materials that have been used in the past for PEC devices, electrolysers, fuel cells and photovoltaic modules.
Most designers of PEC device encasements borrow ideas from discrete electrolysers because of the close similarity in functionality. Metallic encasements are often used for discrete electrolysers because of their high mechanical strength, high temperature stability, high thermal conductivity and high electrical conductivity. Additionally, where weight should be kept to a minimum, such as for mobile applications or for siting on rooftops or integrating in building facades, metals may be undesirable. Typically, oxidation-resistant metals are required on the anode side, while materials resistance to hydrogen embrittlement are used on the cathode side. Austenitic stainless steel which have a relatively low carbon content, 18–25 wt.% Cr and 8–20 wt.% Ni, in particular the AISI 316L grade (Fe-17Cr-12Ni-2.5 Mo) with a high corrosion resistance [35
], could be a candidate material for PEC encasements.
Another option is titanium, because it is relatively lightweight, while maintaining sufficient mechanical strength for structural integrity. It also spontaneously forms a protective oxide preventing further deterioration under more positive potentials than that for oxygen evolution in water electrolysis, particularly in alkaline media. However, titanium when used on the cathode side is prone to hydrogen embrittlement, and although it forms a corrosion protective oxide layer as an anode, this may be undesirable if the casing is to provide electrical contact [38
]. Moreover, titanium can potentially self-ignite under oxygen enrichment [40
] and it is difficult to machine, making it relatively costly compared to stainless steel. These problems may be somewhat overcome by using titanium aluminium vanadium alloy Ti6Al4V, in which the aluminium increases the mechanical strength and decreases the weight of the alloy while the vanadium improves corrosion resistance [35
]. Ti-6Al-4V also has a high fatigue strength with good tensile strength and creep resistance at temperatures typical of low temperature electrolysis. It is used to replace titanium, which easily oxidises to form a corrosion resistant passive film. Other metals commonly for structural components used such as aluminium, iron and copper are unsuitable for PEC devices chiefly because of the poor resistance to corrosion. Although nickel does not suffer from hydrogen embrittlement and is resistant to corrosion in alkaline solutions, dilute acids readily attack it [35
]. Additionally, because of the relatively high cost, nickel is undesirable as an encasement material.
The EU Horizon 2020 funded NEXPEL project reported that, with the exception of platinum group metals and refractory metals and their alloys, which are costly, no other materials can provide both structural strength and corrosion resistance under high anodic bias in low pH conditions typical of proton exchange membranes (PEM) [40
]. Thus, much research has been directed at finding protective coatings for steel as an alternative. A platinum-titanium bilayer was used to protect steel anode in a discrete PEM electrolyser from corrosion by electrolyte [41
]; however, this would be costly to implement for large production volumes. The NEXPEL project consortium also tested a variety of protective coatings on stainless steel for resistance to degradation under high anodic bias in acidic conditions typical of PEM electrolysers [40
]. They observed that with the exception of tantalum and platinum group metals, most materials such as nitrides of titanium and chromium, which from previous studies were predicted to be stable, failed to provide the required corrosion protection [40
Polymers are organic composites made out of small molecular units that are crosslinked together either in an ordered or random way to form long chains. They are a versatile class of materials, which are generally low cost, easy to process and machine and have a reasonably high mechanical strength despite their very low weight, making them an interesting substitute for metals. They are typically grouped into thermoplastics and thermosets and occasionally, a third grouping called elastomers and rubbers may be categorised. The Young’s Modulus of polymers increases from elastomers to glassy polymers to polymer crystals. Another important material property is the glass transition point, which dictates the maximum temperature at which polymers can be used for structural strength because it is an indicator of the softening of the material.
Thermoplastics are polymers that can be melted when heated then cooled down to a hardened form, almost indefinitely, without chemically changing. They can be easily formed into different shapes by compression moulding and injection moulding and thus are suitable for high volume automated production. They are generally electrically insulating but if necessary can be mixed with conductive fillers [42
]. Thermoplastics such as acrylonitrile butadiene styrene (ABS) and polyvinyl chloride (PVC) are less interesting for PEC devices because of their low temperature stability, low chemical resistance and low mechanical strength.
Polystyrene has a relatively higher glass transition temperature of 100 °C, but also has a low chemical resistance. Similarly, polycarbonate can be used between −170 °C and +121 °C, has a high impact resistance, but is not stable at high and low pH levels. PMMA and PLA have a high enough mechanical strength to provide structural support for PEC encasements; however, on hot summer days, the device temperature can reach and exceed 60 °C, the maximum allowed operating temperature. Thus, despite their attractiveness for prototyping, these materials are unsuitable for PEC devices targeting several years of service.
Although perfluoroalkoxy alkanes (PFA) and poly(tetrafluoroethylene) (PTFE) are stable in hot concentrated KOH, they lack the mechanical stability required for withstanding high operating pressures of tens of bars [43
] and impacts that are likely in outdoor operation. Thus, they are only suitable as compression sealing gaskets for hot alkaline conditions [44
]. Additives can be used to tune the properties of a type of material within a certain limit; however, a compromise must be made between cost and added functionality. For example, relatively thick lower cost Teflon seals instead the more expensive Gylon, also PTFE based, despite its higher stability in hot alkaline conditions [44
]. Fallisch et al. reported that chlorinated polyvinyl chloride as endplates led to non-uniform compression in a PEM electrolyser cell probably because of insufficient mechanical hardness [23
]. Improvements in mechanical stability and distribution of compression force were realised by using a titanium anode endplate and either chlorinated polyvinyl chloride (CPVC) or polyphehylene sulfide (PPS) as the cathode endplate.
Thermosets are another type of polymers that do not melt when heated. They have a higher mechanical strength and better heat resistance than thermoplastics but tend to be more rigid. Most epoxy resins and glues used for PEC devices are thermosets. As already seen from the survey of PEC devices, epoxy resins are unlikely to be suitable for long-term durability of joint seals. Other groups also observed similar effects when using adhesives to fix separator in the casing of a discrete alkaline electrolyser because of complexity in implementation and difficulties in quality control [44
]. One possible reason is that the manufacturers rarely disclose the phyisico-chemical properties of their products. Indeed, the epoxies seem to be used as a last resort because there is no suitable product on the market for adhesive sealing of PEC devices. The data sheets of the LOCTITE®
EA9460, EA9492 and EA 9483 epoxies, which have been widely used as sealants for PEC devices as shown in Table 1
, actually discourage their use in oxygen rich systems [45
EA 9492 has a slightly better chemical compatibility with aqueous environments than EA9460 and EA9483 but this is probably insufficient for prolonged service at extreme pH levels.
Rubbers and elastomers may be either thermoset or thermoplastic or a composite of both. They are polymers that can recover their shape almost immediately after a stretching load has been removed and are thus more suitable for edge sealing in PEC devices. Rubbers and elastomers are characterised by a glass transition temperature below 25 °C, a low Young’s modulus and very high elongation at break, resulting in high flexibility. They are often used for sealing with compression by screws because of their softness. Care must be taken to operate elastomers below their maximum allowable temperature, as they may undergo irreversible chemical changes and lose their elasticity. Thus, most electrolysers stacks operating at high temperatures are made by a series of plates clamped together and sealed by compression of flat gaskets made of elastic chemically resistant material, e.g., polytetrafluoroethylene between two neighbouring plates [48
Ceramics are generally brittle and have a low fracture toughness and thus are the least suited class of materials for load bearing applications. Glassy ceramics are attractive as the illumination window in PEC devices because of their high wear resistance and high optical transparency. However, they are more susceptible to thermal shock than metals or heat resistant plastics because of their combination of a lower thermal conductivity and higher brittleness. They are also difficult to fabricate but have a higher wear resistance than some metals and most polymers. Common ceramics that have been used for PEC devices are quartz and soda lime glass. Other alternatives could be borosilicate glass with a relatively higher thermal shock resistance than ordinary glass and sapphire glass, which is very hard. Ordinary glass may also be toughened by tempering or coating with tough protective layers.
The choice of materials for hermetic sealing of PEC devices must consider risks related to the nature of hydrogen and oxygen, fluctuations in temperature and internal pressure, and in some cases, corrosive electrolytes. Polymers are generally easy to machine, mould or print, moreover, they are lightweight and, in most cases, of low cost. However, the thermal mechanical properties of polymers are generally inferior to those of metals. Moreover, the H2
permeability of polymers increases with temperature and the effects of pressure are not yet understood [49
]. Nevertheless, some polymers have excellent chemical compatibility in a wide range of pH in the range of operation for low temperature electrolysis. Additionally, the properties of final 3-D printed materials may differ from that of the base polymer depending on the density of the printed material, and post printing treatment. Also, the choice of polymer material for PEC encasement is not trivial because manufacturers put different additives to essentially the same material making comparisons difficult. Nevertheless, once a material with sufficient mechanical properties is identified, the next step is to check its stability in the expected pH of operation at the highest or typical expected temperature of operation of the PEC device. Figure 2
shows photographs of different candidate materials that we considered for use as components in a hermetically sealed PEC device encasement, before (left) and after a seven-day long immersion in 1.0 M KOH held at 60 °C.
The non-conductive adhesive, and most of the 3-D printed materials (Objet RGD525 High temperature white acrylic-based thermoplastic for high temperature application, Rigur polypropylene based; VeroBlack and VeroClear both PMMA derivatives) showed changes in appearance and can be deemed unsuitable at least for long service lifetime. On the hand, the bulk PMMA plate, the EPDMA piece and the PV backsheet, as well as the stainless steel, showed no visible change. The PV back sheet is in fact a laminate of different polymers namely Coveme Dymat®
a polyester protected by Tedlar®
, a polyvinyl fluoride, with good outdoor wearing properties and probably benefits from a combination of the advantages of both materials. These observations on a centimetre length scale were also confirmed by closer inspection of the surface using the laser microscopy at ×20 magnification. The resulting laser microscope images are presented in Figure 3
, with the samples presented in the same order as was used for Figure 2
. Significant the changes in the surface appearance were evident for the Loctite adhesive (Figure 3
a), which was removed in some places, while the surfaces of the 3-D printed polymers Rigur (Figure 3
d), RGD525 (Figure 3
e) and VeroClear (Figure 3
g) appeared to have roughened after the warm alkali exposure. The Veroblack (Figure 3
f) showed evidence of a polishing effect whereby, the surface smoothened after the warm alkali exposure. In contrast, the microscopic appearance surfaces of the remaining materials remained unchanged.
In conclusion, none the materials commonly available on the market today are able to individually achieve all the requirements for hermetic sealing of PEC devices. Moreover, such materials are expected to have multiple functionalities, which may not be reconcilable with the related added cost of processing. Thus, the next section seeks to draw inspiration from synergies with photovoltaic devices as well as related electrochemical devices to address the challenge of hermetic sealing in PEC devices.