Sodium Borohydride (NaBH₄) as a Maritime Transportation Fuel

Abstract

Hydrogen (H₂) storage is one of the most problematic issues regarding the widespread use of hydrogen, and solid-state hydrogen storage materials are promising in this regard. Hydrogen storage by sodium borohydride (NaBH₄) takes attention with its advantages and idiosyncratic properties. In this study, potentials and challenges of sodium borohydride are evaluated considering storage conditions, safety, hydrogen purity, storage capacity, efficiency, cost, and the maturity. Moreover, marine use of NaBH₄ is demonstrated, and the pros and cons of the NaBH₄ hydrogen storage method are stated. According to evaluations, whereas advantages can be sorted as fuel availability, fuel recyclability, mild storage conditions, exothermicity of reaction, pressure flexibility, and H₂ purity, challenges can be sorted as high costs, catalyst deactivation, regeneration, and practical/technical implementation issues. The great potential of NaBH₄ marine use (against road/aerial vehicles) is water availability, no need to carry all the required water for the entire journey, and reduced system weight/volume by this way.

1. Introduction

Shipping powered mainly by fossil-based fuels contributes global carbon dioxide emissions [1]. Carbon dioxide emission takes attention with its global warming [2] and ocean acidification effects [3]. To eliminate carbon dioxide emissions in maritime transportation, various short-, mid-, and long-term measures were considered [4] by different authorities. For example, the European Commission proposed “Fit for 55” (including shipping) to reduce greenhouse gas emissions 55% by 2030 against 1990 levels [5]. The International Maritime Organization (IMO) put forward 2050 greenhouse gases targets (GHG) considering step by step development [6]. To ensure emission reduction legislations, various methods were proposed, as well as technology transition, market based measures, regulatory needs, and fuel transition [7]. Fuel transition is one of the most promising methods to achieve GHG reduction targets [8].

Among the alternative fuels, hydrogen may be a sustainable solution among alternatives thanks to its versatility, compatibility, clean use, etc. Despite the advantages, its storage still seems to be the most problematic issue. Safe, efficient, environmental, economic, high capacity, quick loading, and de-loading durations are some of the desired factors about hydrogen storage.

Hydrogen storage methods can be listed as compression, liquefaction, adsorbtion, storage in metal hydrides or chemical hydrides, etc. [9]. These methods can be clustered into two sections as physical and chemical storage. Whereas compressed, liquified, and physically adsorbed hydrogen belong to the physical method, metal and complex hydrides and liquid organic hydrides belong to the chemical method [10]. Moreover, physical storage uses the physical properties and has physical bond structure, but chemical storage has a strong chemical bond [11]. Compression to the high pressures, such as 350 bar or 700 bar, was the simple and well-experienced method until now. It has short refilling time, but has low density, energy consumption, and explosion risk [12,13]. Moreover, tanks are heavy and require complex transportation infrastructure [14]. To increase volumetric density, hydrogen can be liquified below 20K [15]. High density is obtained by liquefaction, but high energy is consumed, tanks are complex and costly, and evaporation loss is inevitable [16]. The last storage method mentioned herein is physical adsorption. Adsorption is created by weak van der Vaals interaction at the solid surface. Reversibility of reaction [17] and its inexpensiveness [18] is advantageous. However, adsorption reaction is exothermic, and requires a significant amount of heat removal [19].

Instead of physical storage methods, hydrogen can also be stored by chemical method. Metal and complex hydrides store hydrogen mainly by absorption [20]. High volumetric energy density can be achieved safely, but desorption is endothermic and needs heat inlet [21]. The liquid organic hydrogen carrier (LOHC) gave gained attention recently with easy transportation and storage [13] and its compatibility with existing infrastructure [22]. However, its dehydrogenation is carried out by catalytic endothermic reaction [23]. The last chemical method is to store hydrogen in chemical hydrides, such as sodium borohydride (NaBH₄) or ammonia borane (NH₃BH₃). Sodium borohydride takes attention with 10.7% hydrogen content. It releases its hydrogen via the hydrolysis method in mild conditions. Reaction is exothermic but irreversible. Moreover, it has some challenges, as will be mentioned below.

Whereas some of the above-mentioned storage methods consume significant energy, some methods need specific temperature and heat inlet to release hydrogen. Some methods need extremely high pressures and low temperatures. Irreversibility of reaction, cost, efficiency, H₂ purity, hydrogen leakage, and social perspective are other issues. It can be said that today, there is no single storage method that everyone approves of and that is advantageous in all its features. Each storage method has its own advantages and disadvantages. In this study, the potential use of sodium borohydride for maritime transportation, which is not widely mentioned in the literature, will be discussed. Indeed, sodium borohydride was evaluated in various studies for unmanned aerial vehicles and for road vehicles [24,25,26,27]. However, water required for the reaction as the basis of the case is attractive for marine use because of the availability of water. For this reason, this study aims to introduce the sodium borohydride hydrogen storage method, evaluates the storage performance of NaBH₄, and is trying to make inferences for marine use of NaBH₄.

2. Hydrogen Storage by NaBH₄

Utilizing sodium borohydride for hydrogen storage can be divided into three stages for the end user: 1. storage of NaBH₄, 2. hydrogen release by “hydrolysis” reaction, and 3. storage of the waste of the reaction. Stored waste solution is discharged to regenerate it to produce NaBH₄ again. The regeneration and production of the NaBH₄ from sodium metaborate (NaBO₂) solution is carried out offboard. Regeneration is not reversible, but recyclable [28]. All steps are illustrated in Figure 1.

As an advantage, from the user perspective, hydrogen storage mechanism does not include or not cause any airborne emission. Moreover, recyclability in theory is a significant superiority.

2.1. NaBH₄ Storage

Storage of the NaBH₄ can be carried out as dry or wet. In case of dry storage, NaBH₄ powders should be stored under an inert atmosphere due to moisture sensitivity while keeping away from water as well. To release hydrogen from NaBH₄ with the hydrolysis mechanism, NaBH₄ and water must be brought later. In case of wet storage, controlling coupling takes place. Sodium borohydride is dissolved into the water in the presence of a stabilizer, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), etc. Without a stabilizer, a spontaneous reaction occurs between water and NaBH₄, and H₂ generation cannot be controlled. NaOH stabilizes the solution. The stabilized solution is contacted with a catalyst to release its hydrogen later. The schematic illustration of the commonly used dry and wet storages is presented in Figure 2.

2.2. Hydrogen Release by Hydrolysis

The next step is hydrogen evolution by hydrolysis reaction. The sodium borohydride hydrolysis reaction has some superiorities compared to thermolysis of the NaBH₄; while thermolysis requires high temperatures such as >400 °C [29], the hydrolysis reaction is spontaneous and exothermic (no need of any heat input or etc.), can be carried out at room temperatures, and last of all, half of the hydrogen comes from water (H₂O) [30]. Indeed, sodium borohydride decomposition can also be carried out by carbon dioxide (CO₂) and this system is attractive because hot exhaust gases of internal combustion engines can be used to obtain an exhaust (mainly CO₂) + H₂ mixture by hydrolysis and the obtained mixture can be used in combustion in internal combustion engines again [31]. Coşkuner et al. [32] produced formic acid from CO₂ in two steps and in the first part of the study, NaBH₄ solution is hydrolyzed by passing CO₂ bubbles through the solution. This application is gaining interest for capturing and converting CO₂ [32]). Further, release of H₂ from NaBH₄ can be carried out by water or steam hydrolysis [33] in the presence of (or not) solid catalyst (heterogenous catalyst), and in the presence of an acid accelerator (homogenous catalyst) [34], and can be carried out by alcohol as alcoholysis [35,36]. Nevertheless, the water availability from a marine ecosystem is attractive. For this reason, hydrolysis by water will be mentioned in this study, which is the most studied option. To produce controllable hydrogen, pure water-NaBH₄–NaOH solution and catalyst is contacted. The hydrogen and heat are released, and NaBO₂ solution as waste of the reaction is produced. Hydrolysis reaction is simulated in Figure 3.

The hydrogen releasing hydrolysis reaction of the NaBH₄ with water is given in Equation (1) [37].

NaBH₄ + 2H₂O → NaBO₂ + 4H₂+ Heat

(1)

The above reaction is an ideal (stoichiometric) equation. In reality, including solubility of the NaBH₄ and to prevent precipitation of borates, reaction is modified as [38,39,40]:

NaBH₄ + (2 + x) H₂O → NaBO₂ xH₂O + 4 H₂ + Heat.

(2)

In this equation, x is the “excess hydration factor”. NaBO₂xH₂O is denoted as hydrated borate as well. Hydrated borate as waste of the reaction is environmentally safe [41,42] and in processing the waste, NaBH₄ can be produced. The above formation of the hydrogen via the hydrolysis of the NaBH₄ can be obtained under relatively lower temperatures, such as <80 °C, by using suitable catalysts [43]. Without catalysts, the reaction rate depends on the pH and temperature (T) of the solution [44] for the wet storage as mentioned above.

log t_1/2 = pH − (0.034T − 1.92)

(3)

In the above formulation, t_1/2 is the time in minutes that it takes for half of a solution to decompose. It can be understood from the equation that whereas pH of the solution increases the stability, temperature stimulates the decomposition. The reaction without catalyst is self-inhibiting with increasing pH [45], and conversion of NaBH₄ is low as only 7–8% [46]. However, the desired reaction is fast and efficient. For this reason, various solid catalysts were investigated so far [47].

3. NaBH₄ and Maritime Transportation

Water is the basis of the reaction and maritime vehicles can easily access water. Therefore, it is inevitable that sodium borohydride and maritime transportation will meet at a common point. Sodium borohydride for hydrolysis is already mentioned as an ‘in-direct fuel’ for power generators [48] and is already mentioned as a “marine fuel” option in several studies [49,50,51,52,53]. Fresh water producing from seawater is a well-known and already practiced method from past to today with fresh water generators in ships. Fresh water is produced on board ships using the cooling water temperature of the ship’s main diesel engine, at approximately 40–50 °C in vacuum type or directly by reverse osmosis method. With water purifiers, requested pure water for a chemical reaction is purified. With the addition of the NaBH₄ powder and NaOH pellets, the fuel is ready. NaOH can be added to prevent uncontrolled H₂ formation in the pipes and thus, irregular supply challenges [54]. This liquid solution is pumped to a reactor to meet with the catalyst (there are also other methods to gather reactants and catalyst, such as dropping the NaBH₄ onto the water [55], feeding the water catalyst to NaBH₄ [56,57] and water to the NaBH₄-catalyst [58], but the issue is simplified here). Hydrogen is produced by exothermic hydrolysis reaction. Temperature of the produced hydrogen may be cooled down and pressure is maintained by a pressure regulator. Then, hydrogen is redirected to a fuel cell to produce work. Byproduct of the fuel cell, pure water, may be used to create a solution again. Last of all, NaBO₂–H₂O solution is accumulated and will be discharged at the port for regeneration. The main advantage of the system is that there is no need to carry all of the water for a reaction that requires a complete journey. Required water is produced just before the reaction. The simplified hydrogen generation is depicted in Figure 4 (adapted from [49,53]).

There are different critical points of the NaBH₄ considering marine use. While evaluating and comparing sodium borohydride with other alternatives, some topics should be prioritized, such as storage conditions, hydrogen storage capacity, cost, energy consumption, and energy efficiency, safety, maturity, etc.

4. Evaluation of NaBH₄ H₂ Storage Performance

4.1. Handling of NaBH₄ (Storage Conditions)

The initial priority emerges as storage precautions when starting with the storage of the sodium borohydride. First of all, dry storage needs to avoid low pH and also heat, moisture, and water since hygroscopic NaBH₄ decomposes and releases hydrogen when in contact with water (spontaneously) or moisture in the air [59]. Contact with air raises impurities of NaBH₄ [60,61] by means of moisture content and since CO₂ lowers the pH, and thus, accelerates the decomposition of NaBH₄ [60,62]. Contact with air causes polyborate and carbonate of sodium formation [60]. Impurity in NaBH₄ also causes the reduced reaction rates and low efficiency of the hydrolysis reaction [60]. Nevertheless, positive results are reported in the literature for dry atmosphere [63]. Prepared aluminium (Al)-NaBH₄ tablets were stored in a dry atmosphere for 3 months, and no degradation was observed with the obtained 100% recovered H₂ through hydrolysis reaction. These results show the beneficial effect of dry atmosphere and studies are needed to show the effect of humidity in the air. Moreover, the advantage of dry storage is that there is no need for NaOH use. NaBH₄ also can be stored as a solution with water and a stabilizer. With the increasing alkalinity of the solution via adding NaOH, stability of the solution is rising [64] and unintentionally releasing hydrogen can be reduced but not eliminated completely [65]. Practical implementation difficulty was reported for solution storage since “it loses reactivity with time and need to prepare freshly before operation” [66]. Whereas dry powder is industrially delivered in plastic bags in steel drums, for the harsh caustic storage conditions, stainless steel and alkaline-resistant plastics are suggested [67]. A detailed study for material interactions of NaBH₄ slurries was carried out over 8 months in the presence of alkaline stabilizer [63]. Whereas “stainless steel 304” and “polyethylene” were found as the most suitable (inert) materials with minimum degradation of NaBH₄; “polycarbonate”, “carbon steel”, “copper”, and “brass” were mentioned as incompatible since they react (polycarbonate) or catalyze (other materials). Both dry and solution of NaBH₄ are irritant, harmful if swallowed, inhalants, harmful when contact is made with skin and eyes, and personal protective equipment is needed, and also they should be handled in tightly sealed containers and in cool, low-humidity, and well-ventilated (critical for ships that sail in humid environment) closed spaces [67,68,69]. These operational/technical management needs can be summarized as ‘required storage care’ for NaBH₄.

4.2. Safety

Safety should be ranked as one of the important topics when the subject is hydrogen [70]. Hydrogen is volatile, compressible [71], diffusive, prone to leakage, and has more combustion heat [72]. Its fire is smokeless and not visible in small fires [73]. These properties all can cause major explosion accidents [74]. Moreover, the most used methods in the industry, such as compressed or cryogenic storage, need extreme conditions such as 700 bar and −140 °C [75], and all these weaken the safety aspect of hydrogen (it should be noted here that low pressure is advised to avoid embrittlement [76]). Moreover, use of high temperature fuel cells [77] and use of hydrogen internal combustion engines with vibrations induce safety risks and also global warming risk due to leakages of hydrogen as mentioned in the above sections. Many precautions are considered, such as the correct positioning of the storage tanks (that also affects ship stability), ventilation, insulations, detection and alarm systems, etc. [73]. However, all these precautions also increase complexity. The harsh conditions of shipping, such as vibration and corrosive environment, stimulate the probability of problems. In case of NaBH₄, conditions and priorities are completely different. First of all, the NaBH₄ flash point is 69 °C and storage temperature is mild at 15–25 °C for dry [78], and same for aqua solution [67]. Its autoignition temperature is 220 °C [79] (remember that diesel fuel’s autoignition temperature is 263 °C and flash point is 62 °C [80]). Ultra-high pressure or low temperature used in some methods (compressed, cryogenic, and liquified) or a high temperature requirement as a subject for dehydrogenation of some metal hydrides is eliminated. Storage may be carried out at room temperatures and pressures. However, dry NaBH₄ may liberate flammable hydrogen gas if contact with water, moisture, or acid, and this flammable hydrogen may be ignited by the evolved heat of the exothermic hydrolysis reaction [49]. In case of alkaline solution storage (wet storage), despite the relative stabilization ensured by NaOH, liberated flammable H₂ in time from the solution may endanger. Minimum 10% dead volume in the container is suggested for aqua solution handling [67]. Demirci mentioned this issue as 14.3 L (L) hydrogen is released in one week for 1 kg NaBH₄ solved in 10 L water while pH is 14 and temperature is 20 °C, and this creates a pressure increase [65]. The long-term stabilization issue is experimentally investigated by different studies. Whereas higher concentrations of NaOH and low temperatures slow down the hydrolysis rate, with the existence of 1 N NaOH and at room temperature conditions, hydrolysis degree resulted as 0.01% NaBH₄/h [81]. In the other study, up to 30 °C, hydrolysis rate does not exceed 0.02% NaBH₄/h, and higher than 5% alkali concentration is required for temperatures more than 30 °C [82]. At 50 °C, a lack of possibility of storing more than 1–2 days was mentioned [83]. However, Alligier et al. [48] performed a series of experiments between −15 and 60 °C with 13.14 wt% NaBH₄ and 4M NaOH and two-thirds of initial NaBH₄ was lost after 3 weeks at 60 °C. Furthermore, with cold conditions, such as −15 °C and 4 °C, losses are controlled since 0.2 wt% and 0.5 wt% loss was observed in 12 weeks, respectively. After one year storage at 25 °C, 76 wt% (for 25 wt% NaBH₄ and 1 wt% NaOH), and 66 wt% (for 15 wt% NaBH₄ and 5 wt% NaOH) hydrogen loss was observed [84]. With the help of these stability studies, following conclusions may be extracted: cooling of the closed spaces to low temperatures such as 4 °C for aqua solution of NaBH₄ can be carried out but energy consumption for this cooling is not preferred. Instead of this, keeping the temperatures at 20–25 °C and handling the NaBH₄ in solid state in sealed containers is favorable. Nevertheless, in both storage methods, technically well-designed ventilation and cooling systems should be working properly and the long-term failure of the cooling system is not tolerated in hot areas where the ship is sailing. This is especially important for marine vehicles since they sail on the sea surface in high humidity. In addition to storage, the conditions of the hydrolysis reaction should be mentioned as well. Hydrolysis reaction may occur even at room temperature and at atmospheric pressure, but higher temperatures and pressures are favorable for fast hydrogen release. Considering the studies in the literature for this, low temperatures such as, 25–30 °C [85], 40 °C [86], 65 °C [24], 80 °C [66], 60–95 °C [87], 80–95 °C [63], 82 °C [88], 75–100 °C [25], 140 °C [89], and 0.25–1.3 bar [24], 1 bar [63], 1–3 bar [87], 4–5.5 bar [66], 2–6 bar [88], and 5.5–6 bar [89] pressures were observed. These conditions are remarkably lower and favorable for safety. On the other hand, hydrolysis conditions forced up to 25 bar [90], between 30 and 80 °C and up to 100 bar [91], up to 30 bar [92], up to 12.6 bar [93], between 160 and 200 °C and 40 bar (acid accelerator) [46], and even above 1000 bar were demonstrated [94]. In particular, it was emphasized that the reached pressure is always lower than the expected value due to raised gas solubility with increased pressure [92]. As a result, pressurization of hydrogen by exothermic NaBH₄ hydrolysis reaction instead of mechanical compression is attractive. Hydrogen is stored and transported mostly at the gaseous phase for refueling stations today, and with hydrolysis of NaBH₄, high pressures are achievable [94]. As a consequence, temperatures and pressures are low for typical hydrolysis reaction of NaBH₄, but if desired, high-pressure hydrogen can also be obtained as chemical compression.

On the other hand, material stability is critical for safety. NaBH₄ handling and use in hydrolysis reaction need material precautions. In case of alkaline solution use, strength materials are needed against harsh alkaline conditions and exothermic reaction [95]. Hot temperatures and a caustic environment are also important for catalyst mechanical and chemical durability [96]. However, NaBH₄ can be decomposed via lowered pH using low acid solutions. Material should be selected properly according to the used method and depending on the acidic or basic environment. In the literature, used materials in the reactor were mentioned as stainless steel [97], polytetrafluoroethylene (PTFE) [98], acrylic [99], carbon fiber reinforced plastic (CFRP) [88], etc. To handle NaBH₄ slurries for a long time without degradation of NaBH₄, stainless steel 304 and polyethylene was suggested [63]. In addition to these, as mentioned above, against the corrosive and irritating structure of NaBH₄, protective equipment should be deployed by personnel. NaBH₄ fire requires the special extinguishers, such as dry and solid powders instead of water, CO₂, foam, etc. [100], since CO₂ (as low pH) and water (as a reactant of hydrolysis reaction) heat up the reaction.

4.3. Purity of H₂

Purity of the hydrogen is important, especially for fuel cells. Actually, H₂ purity is already studied for different processes such as hydrogen production by steam methane reforming, gasification [101], methanol reforming [102] or ammonia (NH₃) cracking [103], etc. Impurities may originate from fuel, air, or even system components [104] and may adversely affect the catalytic activity [105] lifetime of fuel cells (FCs) [106]. Moreover, the membrane is sensitive against the temperature, H₂O amount [107], or Na ions [108], etc. Purification/filtration adds an extra cost [109] and additional weight to the system and lowered gravimetric storage capacity [65]. In case of sodium borohydride hydrolysis, H₂ is released from the NaBH₄, NaOH, H₂O, and NaBO₂ including solution via exothermic reaction. The probable existence of these impurities in formed H₂ was mentioned by Liu and Li [110], especially with high H₂ rates, high temperatures [110], and high concentrations [111]. Different strategies can be used to eliminate impurities in H₂, such as washing with water [112], cooling [107], using silica gel [66], resin [26], etc. On the other hand, fuel cell performances from the reaction/reactor are also a matter of interest. In the different studies, similar [113,114,115,116], better [117], balanced in time [118], 6.1% lower [119], 3.73% higher [120], 3.13% higher [121], and 1.53% higher [112] performance were observed with generated hydrogen compared to industrial, pure, and compressed hydrogen gas. In addition to these, the initial and final pH of the water in the water tank through which the hydrogen passed was measured and no difference was observed [122]. High performance of the fuel cell can be obtained thanks to humidification of hydrogen. With the experimented anode potentials and fuel cell performances in the literature, releasing hydrogen from the “reactor” has adequate purity to feed fuel cells. Moreover, humid H₂ gas coming from the hydrolysis reaction is useful for the internal combustion engine as it prevents an autoignition problem that is experienced by compressed hydrogen use [27] (it should be noted that in this study, response time of NaBH₄ hydrolysis was found as comparable to compressed hydrogen also). Humidification is also beneficial for the internal combustion engines and is already a method to reduce nitrogen oxide (NO_x) emissions [123,124,125]. Despite the mentioned potential [27], to the best of the author’s knowledge, there is no performance and NO_x comparison study that uses the compressed hydrogen and hydrolysis reaction’s hydrogen in an internal combustion engine (ICE).

4.4. Storage Capacity

What is expected from energy storage systems is to store high amounts of energy (e.g., hydrogen in our case) with a low weight/volume system. Hydrogen storage density (wt%) and specific energy storage density (Wh/kg) are notable parameters for this reason. Sodium borohydride has a 10.7% gravimetric hydrogen capacity as a fuel itself. Gravimetric and volumetric H₂ capacities of different H₂ carriers including compressed H₂, liquid H₂, diesel (C₁₂H2₆), toluene (C₇H₈), methanol (CH₃OH), ammonia (NH₃), magnesium hydride (MgH₂), sodium aluminium hydride (NaAlH₄), ammonia borane (NH₃BH₃), and sodium borohydride (NaBH₄) were evaluated using known molecular mass and density values and are demonstrated in Figure 5.

According to Figure 5, ammonia borane and sodium borohydride take attention with their high gravimetric and volumetric H₂ contents. However, to ensure solubility of NaBH₄ and NaBO₂ (solubility of NaBH₄ is 55 g per 100 g water [126] and solubility of NaBO₂ is 28 g per 100 g water [30]), gravimetric H₂ content further decreases [38]. Moreover, as mentioned by Demirci and Miele, system capacity (including equipment) is more meaningful. All storage weights, including NaBH₄, water, NaOH, catalyst, cooler, tank weights, separator, heater, filter, pump, controller, sensors, cables, etc., should be accounted. To increase system energy density and hydrogen storage density, reduced weights (such as use of lightweight material instead of stainless steel [127]) and increase in NaBH₄ concentration in the solution are suggested [24,98]. Hydrogen capacity may be further increased with increasing NaBH₄ concentration, but this is limited by the solubility of byproducts [99,128]. This is crucial, especially at the low ambient temperature, and to prevent crystallization of byproducts, concentration of NaBH₄ may be lowered at low ambient temperatures (or a heater may be used to heat the waste tank, considering energy consumption and precipitation on the catalyst). Zhang mentioned this issue, as 10–15% NaBH₄ concentration may be the maximum limit for a reliable system, in their study [129]. In the literature, different energy densities are reported, such as 165 Wh/kg (including fuel cell) [24], 226 Wh/kg [98], 269 Wh/kg [107], 325 Wh/kg (including FC) [26], 463 Wh/kg (acid accelerator) [130], 540 Wh/kg [57], 600 Wh/kg [114], 713 Wh/kg [86], 739 Wh/kg (acid accelerator) [88], and 931 Wh/kg [85]. Moreover, hydrogen storage capacities were mentioned in the studies, as 5.1% (acid accelerator) [88], 3.5% (acid accelerator) [130], and 3.55% [114]. In comparison to other hydrogen storage methods, sodium borohydride takes attention with a low weight container, low volume, and low total weight [131].

When it comes to marine use, access to water stands out as a significant superiority. Nievelt [49] carried out a detailed study, and until 200 h, volume of the system was found as comparable with the diesel system. After the hydrolysis reaction, created and stored byproducts are disadvantageous compared to evaporated diesel fuel exhaust. Normally, conventional liquid fuels or compressed/liquified hydrogen are consumed in FCs or consumed in internal combustion engines (ICEs) and disappeared as weight and volume (hydrogen is consumed, and exhaust gases disperse into the atmosphere for ICE or electricity and water is produced in FCs as products). However, in the case of NaBH₄ use as a fuel, waste is accumulated and carried onboard, its weight increased in time until the discharge at port and this creates a disadvantage. On the other hand, producing water onboard, implementing a volume exchanging tank, and partly filtering the water out of the byproduct can affect compactness positively [49]. The volume exchange tank was demonstrated for the NaBH₄ system to reduce system size [98]. Nievelt [49] took this argument further, emphasizing the potential of the ballast tank-integrated volume exchange tank to reduce the effect of spent fuel weight. Water availability was also mentioned by Lensing [53], whom investigated NaBH₄ use for a Port of Amsterdam vessel that uses a 40 kW fuel cell. A 2.65% gravimetric hydrogen storage capacity of the onboard water storage increased to 4.38%, utilizing seawater to produce pure water. Moreover, superiority was mentioned about energy storage potential compared to compressed hydrogen and battery options, especially with the increased range by Lensing. Further, high volumetric energy density (including packing, spent fuel weight, energy losses, etc.) of NaBH₄ use was reported and this value can be further increased by removing the H₂O in the spent fuel, considering energy consumption for this processing [132].

In addition to these, marine application and water availability provide some extra advantages. Normally, pure water (distilled water) is used for hydrolysis reaction. For this reason, fresh water may be produced by fresh water generators and after that, a purifier may be used to produce pure water. However, direct seawater use may simplify the system, and may be cost effective, as well as reduce the complexity and weight [61,133]. In the literature, slower [133] and faster [63,134,135] hydrogen generation rates are reported for different catalysts in case of seawater use compared to distilled water for hydrolysis reaction. Obtained slow reaction with seawater was explained with formed complexes between transition metal and chloramine/dissolved organics and it was concluded as seawater cannot be used with a transition metal catalyst [133]. Becker and Wayne used an acid accelerator instead of a transition metal catalyst to prevent poisoning by the impurities of seawater, and faster hydrogen generation was obtained compared to distilled water [136]. Moreover, accelerated hydrogen generation rates were explained by a little alkalinity of seawater [135]. In addition to this, it was found that the presence of magnesium chloride (MgCl₂) instead of sodium chloride (NaCI) and sodium sulfate (Na₂SO₄) is effective for acceleration [63]. These results show us the potential of using seawater for the hydrolysis reaction and that lightweight, low-cost, simpler systems using seawater can be developed and used in marine vessels. This is a notable advantage for maritime transportation compared to other modes of transport (air, road, etc.). It should be noted here that other methods have individual advantages, such as aerial vehicles potentially purging byproducts from aircraft [66]. Nevertheless, more studies are needed for different water use as mentioned by Demirci [61], including catalyst durability.

Last of all, exothermic hydrolysis reaction creates heat, and the surplus heat should be removed. This heat load is valid for fuel cell stacks as well. Cooling of stacks or a hydrogen generator is carried out preferably by air at aerial vehicles [24,85,137]. However, water has high heat capacity and circulation of seawater is already used as a method in ships to cool heat loads. For this reason, seawater availability is also useful and may make cooling easy if the high heat loads are created in the reactor.

4.5. Efficiency and Cost

It is desired that the hydrogen storage system be efficient, and the costs and emissions are low. Compressed storage or liquification processes and heat leakages during liquified storage consume remarkable energy [138,139]. Moreover, high-quality insulation requires tanks that are expensive for liquified storage [140]. To dehydrogenate the metal hydrides, liquid organic hydrogen carriers, and ammonia, heat input is required in an endothermic reaction [132,141]. Instead, NaBH₄ hydrolysis is exothermic. Sodium borohydride can be stored at room temperature and atmospheric pressure and hydrolysis reaction is carried out at moderate conditions. Without extreme conditions, high cost of the forcible storage or utilizing conditions and extreme materials, as is the case with compressed, liquified, etc., are softened. Hydrolysis reaction as the foundation of the NaBH₄ storage option is efficient, since in most of the literature, over 90% and even 100% H2 conversion efficiencies were reported. Hydrogen conversion efficiency depends on a lot of factors such as geometry, design, catalyst type, NaBH₄ flow rate, temperature, pressure, etc. [129]. However, the regeneration step includes energy consumption, and beside emissions, well to tank and well to engine efficiencies of NaBH₄ are low as compared to other storage options [142].

Energy consumption sources in the hydrolysis reactor can be sorted as fresh and pure water production [53], feed pump, sensors, and controller, heating the waste tank or preheating the reactor, etc. Moreover, to prevent uncontrolled H₂ release in a long period if the solution (wet) storage is preferred, the solution should be cooled to low temperatures such as 4 °C [48], and energy consumption for this option should be considered (for this reason, dry storage is more preferable). To prevent crystallization of byproducts in a waste tank, NaBH₄ concentration of the initial solution can be regulated [129], or a heater can be used [66]. Moreover, catalyst deactivation is a significant problem. Durability of the catalyst reduces with time and after a certain point, it should be changed. Cost of this catalyst replacement should be evaluated, or a reactor design without catalyst may be a better solution. Another problem arises with regeneration of byproducts and price of the NaBH₄. After the thermodynamically irreversible hydrolysis reaction, created byproducts are transformed to NaBH₄ via energy-consuming steps. Cost of the NaBH₄ is related to the energy consumptions of its regeneration and the high cost of the NaBH₄ or NaBH₄ used system was mentioned as a disadvantage from past to today [39,49,61,65,66,89,143,144]. An efficient, low-cost regeneration step is needed.

4.6. Maturity

NaBH₄ has several promising potentials and technical implementation challenges. First, commercial availability of the NaBH₄ is significantly advantageous for bunkering as NaBH₄ is already a widely used bleaching or chemical reducing agent in the industry. Moreover, the existence of different ways to meet the reactants (NaBH₄, NaOH, catalyst, and H₂O) provides flexibility for reactor design. Normally, for the hydrolysis reaction, factors that affect the reaction may be sorted as temperature, pressure, reactor geometry, catalyst type, catalyst amount, NaBH₄ amount, mixing, NaOH amount, etc. In case of continuous flow reactions, things are getting more complex since in addition to these factors, startup management, flow rate of the solution, catalyst exposure time, reactor configuration, temperature management, waste management, durability of catalyst after cyclic use, etc., were added to the factors. Experimental application is a bit more difficult than theory. Different problems and challenges were reported so far by the studies, such as need of storage care [66], powder gumming and hydrogen leakage (for powder dropping method) [55], catalyst durability problems and deactivation [88] due to oxidation, agglomeration, alkalinity, accumulation of B-O, Na-based species [145], byproduct precipitation or crystallization (especially with decreased temperatures [99]) of metaborate in equipment (such as solenoid [26]), in piping or the tank due to solubility limits and washing or maintenance requirement for this reason [89,114], and high costs [146]. There is a need for methods used for a long time without any problems, as well as comprehensive and satisfactory prototype studies. Maturity is important for policymakers and managers for developing appropriate policies and understanding the challenges [147]. NaBH₄ is found as the most mature compound between boron-based materials [65,132,148]. The technological readiness level (TRL) of the NaBH₄/PEMFC was mentioned as 6 in 2001 [149] and 6–7 in 2007 [150] in studies investigating military use. NaBH₄ was experienced for mobile applications or aircraft systems [151] instead of marine vehicles if we ignore the studies conducted in recent years. In 2017, NaBH₄ use for unmanned aircraft (or aerial) vehicles (UAVs) was mentioned as promising, but readiness level was found to be insufficient in the study [66] in which deactivation of catalysts in the short time, degradation of the fuel, high cost, byproduct precipitation at low ambient temperatures, requirement of water flushing, and high maintenance need were reported as main drawbacks. For the marine use, NaBH₄ is defined as “in infancy stage” [50]. Nevertheless, in the study published in 2020, TRL of the NaBH₄ for marine use was referred to as 6 [53]. Today, there is a still need for studies showing that aforementioned problems are overcome and TRL is mentioned as <7 [61]. However, the results of the developing vessel that will be fueled with NaBH₄ for the “Port of Amsterdam” as a part of the “H2Ships” project will be an important threshold and will shed light on the future of this technology for maritime use. It will take time for the NaBH₄ system to mature [51], so it can be seen as a long-term solution.

4.7. Literature Survey

In

Table 1. Chemical hydride referred maritime studies.

Year	Authors	Scope	Results
2016	M. Nazin [63]	Power generator for unmanned undersea vehicles based on PEMFC and Aluminium (Al)-NaBH4 using generator was investigated.	Seawater shows catalytic effect on hydrolysis reaction. Stainless steel 304 and polyethylene were found as suitable materials for storage of NaBH4 slurries in the presence of alkaline stabilizer.
2019	F. M. Van Nievelt [49]	Technical design and transient behavior of the powertrain of sailing passenger vessel were analyzed for NaBH4 use.	Dry powder storage was found as suitable for compactness. Until 200 h of the trips, the NaBH4 system volume was found as comparable to the diesel fuel system volume.
2019	B. T. W. Mestemaker et al. [50]	Emission reduction methods were assessed according to shipbuilders’ perspective.	In a small part of the study, sodium borohydride was mentioned, and it was emphasized that it is in the infancy stage.
2020	T. J. van der Maas [51]	“What is the commercial and operational most promising alternative marine fuel for coastal and inland shipping in 2030 and beyond in order to decarbonise the Amsterdam port?” The question was attempted to be answered, among the seven alternatives.	The most promising alternative for 2030 is selected as biodiesel. Sodium borohydride was selected as 4th. NaBH4 will be the fuel of the Amsterdam port authority vessel. NaBH4 was selected as the safest fuel. NaBH4 will be mature and economical towards 2040 and beyond.
2020	B. Diesveld and E. De Maeyer [52]	Different fuels and fuel cell types were assessed based on storage type, density, safety, emissions, and maturity via decision-making.	Sodium borohydride was mentioned as safe and emission free fuel with its flash point is higher than 60 °C and CO2 free structure.
2020	D. Lensing [53]	Possibilities and limitations, and integration of NaBH4 system into maritime application, reactor design, correct sizing of components, energy management and comparison with other zero emission alternatives were evaluated for Port of Amsterdam pilot vessel.	Hydrogen storage capacity increased from 2.65% to 4.38% by weight with the utilization of seawater instead of onboard water storage. This is the superiority of the maritime transportation regarding NaBH4 hydrogen storage.
2021	L. Van Hoecke et al. [152]	Different aspects/challenges of hydrogen for maritime use are reviewed and discussed.	The major challenge of the hydrogen uses in maritime was mentioned as storage of hydrogen and the development of bunkering infrastructure. Proper reactor development and bunkering difficulties due to sensitivity of solid hydrogen carriers to moisture was referred.
2021	IEA [153]	A comprehensive report about hydrogen uses in maritime was submitted, including valuable information about sodium borohydride.	The NaBH4 powered ship project was introduced and it was mentioned that SolidHydrogen (a Dutch company) started to industrialize the concept.
2022	A. Düll et al. [154]	Performance evaluation and feasibility of potassium tetrahydroborate (KBH4) as a fuel for inland-waterway cargo vessel is evaluated.	Overall weight of the H2 release and power system is 1.4 wt% and overall volume is 0.8 vol% of the overall cargo capacity. With easy handling and GHG free application, potential of KBH4 for the maritime was highlighted, but efficient regeneration of spent fuel is required.
2023	A. Ibrahim et al. [94]	Export costs (including shipping) of different H2 carriers were analyzed.	Chemical compression of hydrogen above 1000 bar was demonstrated by hydrolysis of NaBH4. NaBH4 regeneration with renewable energy may be cost competitive with other hydrogen carriers.
2023	Rheenen et al. [132]	A total of 15 different hydrogen carriers were evaluated for maritime.	Potential of the borohydrides was mentioned with their safety, high TRL, and well-known dehydrogenation process. High volumetric energy density of NaBH4 system can be further increased by removing H2O in the spent fuel.

, studies in the literature that used chemical hydride are summarized with the scope and results. Literature may be commented as follows: Considering that the storage problem is the major drawback for hydrogen use in maritime [152], NaBH₄ is emerging as an important hydrogen storage option with its idiosyncratic rules and prominent features. Despite the technical challenges such as reactor development and fuel moisture sensitivity [152] and cost reduction requirement [51], NaBH₄ shows potential [132] and promises hope for the long term [51]. First of all, NaBH₄ is a safe fuel [51,52,132]. Moreover, water availability increases the gravimetric hydrogen storage capacity [53] and system volume is comparable against diesel up until a certain range [49]. Volumetric energy density of the NaBH₄ system is attractive [132]. The potential of direct seawater use for the reaction [63] should be more investigated. Furthermore, NaBH₄ hydrolysis reaction may be used to compress hydrogen (chemically) instead of mechanical compression that has mechanical maintenance, energy consumption, and embrittlement issues [94]. Moreover, utilizing the renewable energy for byproduct regeneration, NaBH₄ may be cost competitive for storage and transport of hydrogen compared to liquid organic hydrogen carriers, liquid hydrogen, and ammonia [94].

4.8. Final Evaluation

Table 2. Advantages and disadvantages of the NaBH₄ as a fuel, NaBH₄ hydrolysis reaction, and NaBH₄ used system.

Fuel		Reaction		System
Advantages	Disadvantages	Advantages	Disadvantages	Advantages	Disadvantages
Availability	Moisture sensitivity	Mild conditions	Catalyst deactivation	Mild conditions	Technical implementation challenges
Mild storage conditions	Cost	Safe	Alkaline/acidic media	Safe	Cost
Relatively stable		Humid H2	Byproduct crystallization	Storage capacity	Byproduct weight/volume
Safe		Environmental		Pure H2	Discharge of byproducts
Recyclability		Efficient			Regeneration of byproducts
		Exothermic
		Pressurized H2 availability

demonstrates the advantages and disadvantages of NaBH₄ fuel, its hydrolysis reaction, and hydrogen generator using NaBH₄. This table also summarizes the above discussions.

5. Conclusions

This study discusses the hydrogen storage performance of sodium borohydride and introduce its potentials for maritime transportation use. Findings are summarized below:

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Hydrogen coming from sodium borohydride hydrolysis is humid and this is favorable for fuel cell membrane and also favorable for internal combustion engines thanks to autoignition preventation and nitrogen oxide (NO_x) emission reduction potential. The amount of “NO_x emission reduction potential” of NaBH₄ should be revealed by future studies. Studies are needed that combine sodium borohydride and internal combustion engine.

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High (up to 1000 bar) and low-pressure hydrogen can be obtained via NaBH₄ hydrolysis. Flexibility of liberated hydrogen pressure is important. Low-pressure hydrogen may be fed to inlet a manifold of existing internal combustion engines. Low-pressure and low-temperature storage by NaBH₄ is critical and useful for safety and for global warming since it may reduce the leakage of hydrogen, which causes global warming. Instead, high-pressure hydrogen may be obtained as well.

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The sodium borohydride hydrogen storage method has its own characteristics. Whereas fuel availability, fuel recyclability, mild storage conditions, and thus relative safety, exothermicity of reaction, pressure flexibility of hydrolysis, and high H₂ purity from the reactor can be listed as advantages of NaBH₄, moisture sensitivity of NaBH₄, corrosive environment, and thus material issues, high costs of NaBH₄ and catalysts, catalyst deactivation, regeneration requirement, and practical/technical implementation issues can be listed as disadvantages/challenges.

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The major advantage of the marine environment for NaBH4 is water availability, contrary to aerial/road vehicles. The water required for the reactor does not need to be carried by the ship for the entire journey. Moreover, direct use of seawater in the reaction (with or without catalyst) and direct use of seawater in the cooling may improve and simplify the system and need further investigations. In addition to these, decomposition of NaBH₄-seawater solution with high temperature exhaust and reuse of obtained gas mixture in internal combustion engines may be attractive for the future.

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Major obstacles to the use of sodium borohydride as a marine fuel today are its regeneration problem, cost, and the lack of durable and commercialized reactor prototypes. Once these obstacles are resolved, it will take its place among marine fuels.