Rehydrogenation of Sodium Borates to Close the NaBH₄-H₂ Cycle: A Review

Abstract

In 2007, the US Department of Energy recommended a no-go on NaBH₄ hydrolysis for onboard applications; however, the concept of a NaBH₄-H₂-PEMFC system has the potential to become a primary source for on-demand power supply. Despite the many efforts to study this technology, most of the published papers focus on catalytic performance. Nevertheless, the development of a practical reaction system to close the NaBH₄-H₂ cycle remains a critical issue. Therefore, this work provides an overview of the research progress on the solutions for the by-product rehydrogenation leading to the regeneration of NaBH₄ with economic potential. It is the first to compare and analyze the main types of processes to regenerate NaBH₄: thermo-, mechano-, and electrochemical. Moreover, it considers the report by Demirci et al. on the main by-product of sodium borohydride hydrolysis. The published literature already reported efficient NaBH₄ regeneration; however, the processes still need more improvements. Moreover, it is noteworthy that a transition to clean methods, through the years, was observed.

1. Introduction

The negative impact of the excessive use of fossil fuels has been a major concern since the beginning of the century, mainly in the current decade [1]. Thus, the need to reduce greenhouse gas concentrations to sustainable levels is mandatory to allow the future generations to live on an Earth at least as habitable as today, which has resulted in a demand for clean and abundant energy technologies [2]. In this context, hydrogen (H₂) arises as one of the most promising fuels of the future. Hydrogen can be efficiently oxidized in Polymer Electrolyte Membrane Fuel Cells (PEMFC) to provide electricity to a wide range of applications (e.g., portable and maritime applications) [3,4], involving an electrode (anode and cathode)/membrane electrolyte system. The energy stored in the H₂ fuel is converted into electricity and heat, producing only water as a by-product [5,6]. Thus, hydrogen is supplied in the anode and splits into electrons and protons, while oxygen is reduced at the cathode. The protons are carried through a polymer electrolyte membrane (PEM) to recombine with oxygen, whereas the electrons proceed to the cathode through an external circuit, providing the electrical output [6].

Hydrogen is an energy vector with a strong potential for energy storage due to its high gravimetric density (142 MJ.kg⁻¹) and lightness. Nevertheless, it is lighter than air, flammable, and is usually combined with other elements [3,7]. Due to its abundance, hydrogen can be produced by several methods using a wide range of sources, through fossil fuels to renewable energy resources. Currently, 96% of worldwide hydrogen is generated from fossil fuels; nevertheless, to be a low-carbon energy carrier, new generation methods must be developed [2,7,8,9].

Chemical hydrides, such as metal–boron hydrides and ammonia borane, have received strong attention, since they exhibit an impressive volumetric hydrogen storage density on a material-only basis and simultaneously allow the generation and storage of hydrogen [10,11]. Among chemical hydrides, sodium borohydride (NaBH₄) is a promising hydrogen carrier due to its high hydrogen storage capacity (10.8 wt %), safe handling, and fast kinetics of hydrogen release [12,13,14]. NaBH₄ releases molecular hydrogen via hydrolysis, at room pressure and temperature, as described by the chemical reaction presented in Equation (1) [3,15].

$${\mathrm{NaBH}}_4{+2\mathrm{H}}_2\mathrm{O}\to {\mathrm{NaBO}}_2{+4\mathrm{H}}_2$$

(1)

The reaction is spontaneous and exothermic (ΔH = −217 kJ.mol⁻¹), and the release of hydrogen can be accelerated by a catalyst [16,17]. The only by-product that forms upon hydrolysis usually retains two or four molecules of water, which decreases the hydrogen yield and generation rate. Therefore, an excess of water is needed, and the effective hydrolysis reaction of NaBH₄ is observed according to Equation (2), where × represents the hydration factor [18,19,20].

$${\mathrm{NaBH}}_4{+(2+\mathrm{x})\mathrm{H}}_2\mathrm{O}\to {\mathrm{NaBO}}_2{•\mathrm{x}\mathrm{H}}_2{\mathrm{O}+4\mathrm{H}}_2$$

(2)

Since the early 2000s, many efforts have been dedicated to the hydrolysis of NaBH₄; however, there is no consensus about the chemical structure of the by-product [21,22]. Although the majority of the published studies described sodium metaborate (NaBO₂) as the by-product of the reaction, several studies reported that the only by-product that forms is sodium tetrahydroxyborate (NaB(OH)₄) [21,22,23,24,25,26,27,28,29,30,31,32,33], since the solution is stabilized with pH beyond 11 [3]. Therefore, the appropriate equation would be written as follows:

$${\mathrm{NaBH}}_4{+(2+\mathrm{x})\mathrm{H}}_2\mathrm{O}\to {\mathrm{NaB}\left(\mathrm{OH}\right)}_4{+4\mathrm{H}}_2.$$

(3)

Although the hydrolysis of NaBH₄ combines the best properties for hydrogen generation and storage, including solubility in water, rapid controllable hydrolysis, overall stability, and moderate exothermicity, in 2007, the US Department of Energy (DOE) recommended a no-go on NaBH₄ hydrolysis for onboard applications [34]. This decision was based on three key issues: storage capacity targets, regeneration of NaBH₄, and costs [35]. In order to address these issues, the research dedicated to the synthesis and regeneration of NaBH₄ improved [36].

Bearing in mind the research articles concerning the NaBH₄ hydrolysis by-product recycling that have been published in peer-reviewed international journals since the early 2000s, only three studies were dedicated to this topic [37,38,39] prior to the US DOE report in 2007. About 96% of the papers on this topic [20,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68] were published after the DOE no-go recommendation (Figure 1). Despite the efforts, since 2009, to develop an efficient and reliable method to regenerate NaBH₄, the reported research is low when compared with the total number of published studies on NaBH₄ hydrolysis.

In the past 20 years, 12 review-type articles were published regarding the hydrogen generation through boron-based hydrides [21,22,35,36,69,70,71,72,73,74,75,76]. Although 10 of these articles focused on NaBH₄ hydrolysis, only five overviews highlight the importance to recycling the by-product in order to close the NaBH₄-H₂ cycle [21,22,35,36,71].

In 2015, Demirci et al. [21] analyzed 260 research articles published between 2000 and 2013 that were dedicated to “the hydrogen cycle with hydrolysis of sodium borohydride” and reported that only 8.9% of the papers were related with NaBH₄ regeneration. According to the authors, more than 60% of the research articles focused on catalysis. There is a noteworthy lack of research in recycling, which is a key issue to close the NaBH₄-H₂ cycle. Therefore, 217 research papers [11,17,20,23,24,28,29,30,31,50,52,53,54,55,56,57,58,59,60,61,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273], published from 2014 up to the publication of this paper, were selected to understand the evolution of the research on “hydrogen from NaBH₄ hydrolysis” after the report by Demirci et al. [21]. The selected articles were published in peer-reviewed international journals, and all the review-type articles and conference proceedings were discarded. According to their main focus, the research articles were divided in four topics: “catalysis”, “fundamentals”, “others”, and “regeneration” (Figure 2). The “catalysis” topic highlights the performance and characterization of catalysts; “fundamentals” focuses on reaction mechanisms, kinetics, and thermodynamics; the topic “other” emphasizes the experimental and computational study of reactors, prototypes, and developed systems; and the “regeneration” topic is dedicated to the synthesis of NaBH₄ from the reaction by-products. It is noteworthy that the high majority of the studies, about 80%, continue to be dedicated to the topic “catalysis”. Despite the need to close the hydrogen cycle from NaBH₄ hydrolysis, which is strongly dependent on the development and implementation of an efficient, low-cost, and reliable recycling method [21,36,61], only 5.5% of the published articles highlight the topic “regeneration”. Although important advances have been made through the years, there is still a low impact of this topic.

The usual path to synthesize NaBH₄ is the Brown–Schlesinger process, which was developed in the early 1950s [15,22]. This process is expensive, not clean, and inefficient. It consists of seven steps, which require high pressures and temperatures and produce various by-products [36,58]. The main reaction is presented in Equation (4), where sodium hydride (NaH) and trimethylborate (B(OCH₃)₃) react at 225 to 275 °C [15,42].

$${\mathrm{B}(\mathrm{OCH}}_3{)}_3+\mathrm{NaH}\to {\mathrm{NaBH}}_4{+\mathrm{NaOCH}}_3$$

(4)

An alternative method to produce NaBH₄ at industrial scale is the Bayer process, which is represented in Equation (5) [33,37]. Na₂B₄O₇•7SiO₂ reacts with Na at 400–500 °C under a hydrogen atmosphere. It is noteworthy that this process exhibits high potential risks due to the high temperature and hydrogen pressure operations. Moreover, it produces Na₂SiO₃ as a by-product, which is a residue with low commercial value that is difficult to discard [33,58].

$${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7{•7\mathrm{SiO}}_2{+16\mathrm{Na}+8\mathrm{H}}_2\to {4\mathrm{NaBH}}_4{+7\mathrm{Na}}_2{\mathrm{SiO}}_3$$

(5)

Although the synthesis of NaBH₄ from hydrolysis by-products remains a great challenge, the studies published to date present promising advances [25,33]. Nevertheless, a low-cost fully reliable alternative method to synthesize NaBH₄ has not yet been developed. Closing the NaBH₄-H₂ cycle is essential to implement a NaBH₄-H₂-PEMFC system as a primary energy source to provide on-demand power for portable and maritime applications [4,11]. Moreover, NaBH₄-H₂-PEMFC systems emerge as a promising off-grid technology capable of providing a fully reliable electricity supply in emerging countries and locations without a reliable grid.

Among the many possible hydrogen carriers, liquid ammonia and LOHC (liquid organic hydrogen carriers) have been gaining increased importance [22,70,274]. Ammonia has a very high hydrogen content (17.8% in weight) and simultaneously an impressive volumetric hydrogen density (108 kg H₂/m³ in liquid ammonia at 20 °C and 8.6 bar). However, ultra-pure hydrogen recovery from the ammonia cracker in the reconversion to hydrogen process is still a great challenge [28,275]. LOHC exhibit moderate H₂ content, reversibility, moderate dehydrogenation temperature, commercial availability, and compatibility with the existing gasoline infrastructure, but the reactor systems, process heat integration, and concerns such as catalyst recovery and regeneration, separation, and capture of CO₂ from the gaseous products formed need further developments [274]. In comparison, chemical hydrides present, as referred above, a high hydrogen content and low to moderate dehydrogenation temperatures (the hydrolysis reaction is an easy and controllable process and allows simultaneously hydrogen storage), but they suffer from irreversibility and energy-consuming regeneration. Therefore, a major challenge for the effective use of these hydrogen carriers is the development of alternative or improved routes to rehydrogenate the by-products formed. The specific advantages of the sodium borohydride carrier were already enlightened. Thus, this overview focuses on the published alternative methods to rehydrogenating the borates leading to the NaBH₄ regeneration, since this is a critical issue that needs to be overcome in order to close the NaBH₄-H₂ cycle and allow the implementation of this technology [36,59,61].

2. Regeneration of NaBH₄

The regeneration of sodium borohydride can be organized into three major groups of processes based on the energy source: thermochemical, mechano-chemical, and electrochemical. Thermochemical processes are based on reactions that involve high pressure and/or temperature, and they comprise most of the reactions to effectively regenerate NaBH₄. The mechano-chemical processes are similar to the thermochemical ones, but the source of energy used in this type of process relies on mechanical forces. Lastly, the electrochemical processes use electric energy to produce sodium borohydride by reducing or oxidizing other borates. Until 2015, the regeneration of sodium borohydride was mainly achieved through thermochemical processes [41,42] (Figure 3). However, since 2016, there has been a transition to cleaner processes to regenerate this compound, with mechano-chemical processes being the most used ones [20,43,45,50,52,53,54,55,56,57,58,59,60,61,68]. The lack of efficient NaBH₄ electrosynthesis also contributed to this “mechano-chemical processes boom” to regenerate NaBH₄.

An overview of the NaBH₄ regeneration yields through thermochemical and mechano-chemical processes is presented in Figure 4 and Figure 5, respectively. Regeneration yields based on thermochemical processes are presented according to the temperature (closed circles) and pressure (open circles) of the reaction (Figure 4). Almost all experiments were performed using pressure values between 2 and 4 MPa and temperature values between 800 and 900 K. For mechano-chemical processes, the regeneration yields are presented along with the time of milling (closed circles) and the year of publication of the articles (open circles) (Figure 5). As observed, higher yields are achieved when using higher milling times; nevertheless, regenerating NaBH₄ for more than 35 h (2000 min) is a considerable energy-consuming process. These higher yield values have been obtained since 2017, which may reflect a gradual improvement of these processes. It is noteworthy that the same color points in both figures correspond to the same first author, indicating that a wider variety of groups has progressively been focusing on mechano-chemical sodium borohydride regeneration.

Regarding the electrochemical synthesis of NaBH₄, studies conducted at Penn State University (PSU), in combination with DOW Jones Industrial Average and Los Alamos National Lab (LANL), indicated that no NaBH₄ was electrochemically obtained, and it would not be possible to effectively regenerate it from NaBO₂. This statement contributed to a lack of papers on the subsequent years using electrochemical methods to regenerate NaBH₄. Moreover, although several patents have claimed that the electrochemical regeneration was achieved, there are no published studies reproducing the results. It is noteworthy to highlight some papers that synthesize NaBH₄ through electrochemical processes as an intermediate step for desulfurization experiments [276,277,278,279].

The following sub-sections describe the results reported so far regarding the regeneration of NaBH₄. It is noteworthy that most of these results were obtained using commercially available boron compounds. Thus, there is an emergent need for developing synthesized boron compounds to regenerate NaBH₄, as the ultimate goal is to use the by-products of its hydrolysis for the regeneration process, which can react differently compared to commercial boron compounds.

2.1. Thermochemical Processes

Thermochemical processes are chemical reactions that occur at high pressure and/or temperature. As previously referred, NaBH₄ is usually synthesized through two thermochemical processes: (i) the Brown and Schlesinger process and (ii) the Bayer process.

The main disadvantage of the Brown and Schlesinger process is the formation of several by-products. To overcome this issue, Liu et al. [280] developed a modified closed system where NaBO₂ and ammonia borate ((NH₄)₃(BO₃) are simultaneously produced and recycled from NaBH₄. According to the authors, the metallic Na is obtained by the electrolysis of sodium chloride (NaCl) in a seawater treatment. Although this study presents a major improvement to the Brown and Schlesinger process, it continues to be a multi-step process that needs to use metallic Na. In contrast, the Bayer process allows a direct production of NaBH₄. Nevertheless, it has some associated issues: (a) the need to use metallic Na, (b) the disposal of the by-product (Na₂SiO₃), and (c) the risk of explosion due to high temperature (873 K) and hydrogen pressure [33,58].

Therefore, several authors attempted to synthetize NaBH₄ through different methods. Since the early 2000s, only 12 papers were published reporting the NaBH₄ hydrolysis by-product recycling through thermochemical processes (

Table 1. Summary of thermochemical processes for NaBH₄ regeneration.

Author	Boron Compound	Other Reactants	Yield/%	Ref.
Kojima	${\mathrm{NaBO}}_2$	${\mathrm{MgH}}_2$	97.0	[38]
		Mg, Si	98.0
Liu	${\mathrm{Na}}_4{\mathrm{B}}_2{\mathrm{O}}_5$	Al, ${\mathrm{H}}_2$, ${\mathrm{Na}}_2\mathrm{O}$	65.8	[39]
	${\mathrm{Na}}_3{\mathrm{BO}}_3$	Si	47.0	[40]
	${\mathrm{NaBO}}_2$	Al	69.0	[64]
		Mg	63.5	[66]
	${\mathrm{NaBO}}_2•0{.5\mathrm{H}}_2\mathrm{O}$	Mg	28.8
	${\mathrm{NaBO}}_2{•\mathrm{H}}_2\mathrm{O}$	Mg	12.3
	${\mathrm{NaBO}}_2$	Mg + 23.5% Ni	80.0	[65]
Kemmitt	${\mathrm{NaBO}}_2$	${\mathrm{CH}}_3{\mathrm{OH},\mathrm{NaAlH}}_4$	80.0	[62]
Kayacan	${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7$	$\mathrm{Mg},{\mathrm{H}}_2$	93.0	[44]
Ou	${\mathrm{NaBO}}_2$	${\mathrm{MgH}}_2$	90.4	[48]
		${\mathrm{MgH}}_2$	90.0	[49]
Figen		${\mathrm{MgH}}_2$	93.0	[47]
Eom		Mg	69.0	[46]

) [38,39,40,44,46,47,48,49,62,64,65,66]. All reported studies were carried out using either magnesium hydride (MgH₂) or magnesium under a hydrogen atmosphere. The thermochemical studies using MgH₂ reported higher conversion yields; however, this method also presents high costs due to the commercial value of MgH₂ [47]. On the other hand, although the NaBH₄ regeneration using Mg under a hydrogen atmosphere presented lower costs, this method is not suitable due to the need to use hydrogen as a reactant. Since the basis of these studies is to generate hydrogen as an energy carrier, this method does not seem to be profitable to close the NaBH₄-H₂ cycle.

2.1.1. Effect of Temperature, Pressure, and Time of Reaction

The thermochemical reactions are highly affected by the parameters considered, as observed in Figure 4. The published articles analyzed in this overview focus on two main parameters: the reaction temperature and pressure. Moreover, some articles also studied the influence of time and the benefit of specific additives.

In 2003, Kojima et al. [38] studied the mixture of NaBO₂ with MgH₂ (Equation (6)), at 7 MPa H₂ and 823 K, after 2 h of reaction, and they obtained a conversion yield of 97.0%. This is the higher yield reported for the NaBH₄ regeneration using a thermochemical process without additives.

$${\mathrm{NaBO}}_2{+2\mathrm{MgH}}_2\to {\mathrm{NaBH}}_4+\mathrm{MgO}$$

(6)

Ten years later, Ou et al. [48] studied the influence of temperature variation in Equation (6) at 3.1 MPa H₂. The authors increased the reaction temperature from 293 to 884 K followed by a consequent decrease to 293 K and reported a NaBH₄ regeneration of 90.4% yield. The calculus of the conversion yield was based on H₂ pressure variation, and 85.2% of the obtained value had been associated to the formation of NaBH₄ during the isothermal period between T = 873 K and T = 884 K. Therefore, Ou et al. [49] added two more parameters to the study: pressure and time of reaction (tr). In order to promote the hydrogen adsorption for NaBH₄ formation, the authors set the H₂ pressure between 2.1 and 3.1 MPa and observed that after 1.5 h, at 907 K, they achieved the maximum borate conversion. Nevertheless, at 857 K, the authors observed that it took 8 h to obtain the same NaBH₄ conversion.

Eom et al. [46] also studied Equation (6), in turn, at 6 MPa. After 1 h of reaction, the authors observed that the conversion yield increased with temperature (Figure 6a). The results obtained stand out, since the yield is considerably lower than the ones previously reported [38,48,49]. However, the authors justified the results with the tough in MgH₂ decomposition promoted by the high H₂ pressure.

According to these results, it is possible to conclude that the temperature presents a main role in thermochemical processes.

Eom et al. [46] also studied, at 873 K, the influence of H₂ pressure (Figure 6b) as well as other reducing agents. Thermodynamically, Ca is more suitable than Mg as a reducing agent. Nevertheless, the authors concluded that Ca does not act as an effective catalyst in the H₂-H^- conversion, since only a small amount of NaBH₄ was formed.

2.1.2. Effect of Additives and the Hydration of the Boron Compound

In addition to the effect of temperature, Kojima et al. [38] also studied the influence of additives in the reaction. The authors performed experiments with silicon (Equations (7) and (8)) at 7 MPa H₂ and 823 K, and they obtained NaBH₄ with a 98% yield after 2 h of reaction.

$${\mathrm{NaBO}}_2{+2\mathrm{Mg}+\mathrm{Si}+2\mathrm{H}}_2\to {\mathrm{NaBO}}_2{+\mathrm{Mg}}_2{\mathrm{Si}+2\mathrm{H}}_2$$

(7)

$${\mathrm{NaBO}}_2{+\mathrm{Mg}}_2{\mathrm{Si}+2\mathrm{H}}_2\to {\mathrm{NaBH}}_4+2\mathrm{MgO}+2\mathrm{Si}$$

(8)

In 2007, Liu et al. [39] studied the influence of Al as a reducing agent and observed that the formation of NaBH₄ did not occur. However, the authors reported a maximum NaBH₄ conversion of 65.8% when the compounds tetrasodium diborate (Na₄B₂O₅) and sodium oxide (Na₂O) were combined with Al (Equation (9)).

$${4\mathrm{Al}+6\mathrm{H}}_2{+\mathrm{Na}}_4{\mathrm{B}}_2{\mathrm{O}}_5{+\mathrm{NaBO}}_2{+\mathrm{Na}}_2\mathrm{O}\to {3\mathrm{NaBH}}_4{+4\mathrm{NaAlO}}_2$$

(9)

The studies were carried out at 873 K with a NaBO₂/Na₂O molar ratio between 1.5 and 4 (Figure 7). The authors concluded that although the presence of Na₂O has a positive impact on the regeneration of NaBH₄, the NaBO₂/Na₂O molar ratio must be optimized [64].

Furthermore, Liu et al. [40] studied the influence of several additives in the reaction of NaBO₂ with Mg and H₂, such as Fe, Co, Ni, and Cu. The authors reported that Fe and Ni affect the reaction similarly. The addition of 5 wt % of these metals not only increased the NaBH₄ yield (Fe: 88%; Ni: 82%) but also decreased the reaction temperature (Fe: ≈283 K; Ni: ≈313 K). Moreover, increasing the Fe concentration up to 20 wt % promotes the NaBH₄ generation rate with similar conversion yields. The authors also observed that the addition of 5 wt % of Co converted NaBH₄ with 85% yield, and the addition of the same amount of Cu did not show any significant effect in the reaction. Additionally, the authors concluded that none of the additives decreased the activation energy of the reaction (156.3 kJ.mol⁻¹)

A year later, Liu et al. [65] reported a eutectic alloy of Mg with 23.5% Ni in the recycling of NaBO₂. The studies were performed at 673 K and 3.0 MPa H₂, and a slight increase of 3 wt % H₂ absorbed on the Mg surface was observed, which promoted an increase of 8% in NaBH₄ conversion yield. The authors concluded that the Mg alloy induced the hydrogen dissociation and diffusion during the reaction.

Furthermore, Liu et al. [66] also studied the NaBH₄ by-product recycling without performing its initial dehydration. Bearing in mind Equation (2), the authors studied the influence of the by-product hydration between x = 0 and x = 2 and observed that while the hydration factor increases, the conversion yield considerably decreases. Moreover, the authors reported that for x = 4, no NaBH₄ was formed. Therefore, the authors realized that although it is a high-energy consumption step, the dehydration of the by-product is necessary.

2.2. Mechano-Chemical Processes

The mechano-chemical processes use mechanical movement instead of heat to generate the required energy to form the products of the reaction. These reactions are typically performed in a high-energy ball mill, at room temperature, with the reactants in the solid state (powder). The impact of the milling balls on the reactants reduces the size of the particles, which increases the contact between them and, consequently, promotes the reaction. It is noteworthy that although these reactions occur at room temperature, there is an increase of temperature inside the mill during the reaction that promotes the formation of NaBH₄. Similar to thermochemical processes, the mechano-chemical processes are carried out using MgH₂ or Mg under hydrogen atmosphere as reactants. Although mechano-chemical processes are more environmentally friendly, since they are low-energy consumption processes, the high times of milling are a considerable limitation. Moreover, the implementation of the ball milling process to synthesize NaBH₄ in a larger scale is not feasible mainly due to cost issues [58].

Since 2003, several authors reported the regeneration of NaBH₄ through ball milling (

Table 2. Summary of mechano-chemical processes for NaBH₄ regeneration.

Author	Borate	Other Reactants	Excess of Reactant/%	Additive	t/min	Yield/%	Ref.
Li	${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7$	${\mathrm{MgH}}_2$	100	-	60	43.0	[37]
			2.70	${\mathrm{Na}}_2{\mathrm{O}}_2$		67.0
			-	NaOH		64.0
			15.50	${\mathrm{Na}}_2{\mathrm{CO}}_3$		78.0
Hsueh	${\mathrm{NaBO}}_2$	${\mathrm{MgH}}_2$	40	-	120	76.0	[51]
Kong	${\mathrm{NaBO}}_2$	${\mathrm{MgH}}_2$	3.50	-	120	71.0	[63]
Zhang	${\mathrm{NaBO}}_2$	$\mathrm{NaH},{\mathrm{SiO}}_2$	-	-	360	32.0	[67]
Çakanyldirim	${\mathrm{NaBO}}_2$	${\mathrm{MgH}}_2$	27	-	700	73.9	[68]
	${\mathrm{B}}_2{\mathrm{O}}_3$		30	Na	800	25.0	[43]
	${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7$		10	-	400	84.0	[45]
Ouyang	${\mathrm{NaBO}}_2$	Mg	37.50	-	900	67.7	[54]
Lang	${\mathrm{NaBO}}_2$	${\mathrm{MgH}}_2$	35	${\mathrm{CH}}_3\mathrm{OH}$	480	89.0	[53]
Chen	${\mathrm{NaBO}}_2{•2\mathrm{H}}_2\mathrm{O}$	${\mathrm{MgH}}_2$	25	-	900	89.8	[20]
	${\mathrm{NaBO}}_2{•4\mathrm{H}}_2\mathrm{O}$		37.50	-	1 200	88.3
	${\mathrm{NaBO}}_2{•2\mathrm{H}}_2\mathrm{O},$${\mathrm{NaBO}}_2{•4\mathrm{H}}_2\mathrm{O}$		40	-	360	82.0
Huang	${\mathrm{NaBO}}_2{•2\mathrm{H}}_2\mathrm{O}$	Mg, ${\mathrm{Mg}}_2\mathrm{Si}$	25	-	1 200	86.0	[52]
Zhong	${\mathrm{NaBO}}_2{•2\mathrm{H}}_2\mathrm{O}$	${\mathrm{Mg}}_2\mathrm{Si}$	50	-	1 200	78.0	[55]
	${\mathrm{NaBO}}_2{•4\mathrm{H}}_2\mathrm{O}$		100	-	1 200	74.0	[57]
Ar	${\mathrm{B}}_2{\mathrm{O}}_3$	${\mathrm{NaNH}}_2$$,{\mathrm{MgH}}_2$	30	-	500	53.0	[56]
Qin	${\mathrm{NaBO}}_2{•2\mathrm{H}}_2\mathrm{O}$	${\mathrm{Mg}}_{17}{\mathrm{Al}}_{12}$	-	NaH	1 200	57.0	[59]
Le	${\mathrm{NaBO}}_2$	Mg, Al	-	-	2 160	99.7	[58]
	${\mathrm{NaBO}}_2{•4\mathrm{H}}_2\mathrm{O}$		-	-	2 160	99.5
Zhu	${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7{•10\mathrm{H}}_2\mathrm{O}$	Mg, ${\mathrm{Na}}_2{\mathrm{CO}}_3$	23.75	-	1 800	78.9	[61]

). In addition to MgH₂, some authors studied the reaction with Si compounds in order to find a low-cost alternative. Moreover, the influence of several additives was studied, such as sodium peroxide (Na₂O₂), sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), sodium hydride (NaH), and methanol (CH₃OH). The influence of the hydration of sodium borates was also studied, and it is noteworthy that NaBH₄ was regenerated with high yield without an initial dehydration step.

Nevertheless, the NaBH₄ regeneration through ball milling is a multi-step process, since after the ball mill reaction, it is necessary to proceed to NaBH₄ extraction. In this additional step, it is necessary to use a solvent (e.g., isopropyl amine) to dissolve NaBH₄ from the final reaction mixture and then filter and dry to obtain the final NaBH₄.

The information regarding the conditions inside the mill, mainly the inside temperature achieved and the inside pressure atmosphere, are rarely provided. However, it is possible to conclude from the reported articles that the inside temperature is strongly influenced by the time of milling, the speed of milling, and the ball mill material composition. Moreover, despite the poor information, researchers clearly observed a preference for H₂ or Ar atmospheres inside the mill.

2.2.1. Effect of Time, Ball to Powder Ratio, and Excess of Reactant

Kong et al. [63] were one of the first authors to report the ball mill reaction of NaBO₂ with MgH₂ (Equation (10)). The influence of the following parameters was studied and optimized: MgH₂/NaBO₂ molar ratio, ball to powder ratio (BPR), and time of milling (t).

$${\mathrm{NaBO}}_2{+2\mathrm{MgH}}_2\to {\mathrm{NaBH}}_4+2\mathrm{MgO}$$

(10)

According to the authors, it took 1 h of ball milling to start to synthesize NaBH₄ and an extra hour for the conversion to be complete, with a yield of 71%. The authors also concluded that BPR considerably influences the conversion yield, which is higher with the increase of BPR [63].

In 2010, Varin et al. [281] studied the milling of MgH₂ up to 100 h in H₂ and Ar atmospheres. Although it was concluded that both atmospheres did not influence MgH₂ molecules, the formation of Mg(OH)₂ was observed. However, in the H₂ atmosphere studies, the formed Mg(OH)₂ was naturally reduced to MgH₂ and water during the reaction, which allowed the authors to conclude that a H₂ atmosphere is more suitable to perform this process.

Çakanyldirim et al. [68] analyzed the effect of the milling time and the excess of MgH₂ in the synthesis of NaBH₄ through ball milling (Figure 8). The higher conversion yield reported (73.90%) was achieved after about 12 h of milling, with 27% excess of MgH₂. Lang et al. [53] studied the optimization of the amount of MgH₂ and reported similar results: a 71% yield of NaBH₄ after 12 h, with 35% of MgH₂ in excess.

In 2017, Ouyang et al. [54] also studied the influence of the same parameters (milling time, excess of Mg and BPR) in the ball milling reaction of NaBO₂ with Mg, expecting to achieve higher yields by increasing those parameters. Nevertheless, the obtained conversion was below the expectations, and a maximum yield of 67.7% was reported after 15 h of milling, with 37.5% excess of Mg and a BPR of 30:1.

Furthermore, bearing in mind the borates structure presented in the NaBH₄ regeneration process, Ar et al. [56] reported ball mill studies where B₂O₃ reacts with NaNH₂ and MgH₂ (Equation (11)).

$${\mathrm{NaNH}}_2{+3\mathrm{MgH}}_2{+\mathrm{B}}_2{\mathrm{O}}_3\to {\mathrm{NaBH}}_4{+3\mathrm{MgO}+\mathrm{NH}}_4{\mathrm{BH}}_4$$

(11)

Several experiments were carried out with time of milling between 400 and 800 min. According to the authors, the optimal value for the time parameter is 500 min of milling, since above this value, researchers observed the formation of MgO instead of the desirable NaBH₄. Moreover, Ar et al. reported that the excess of MgH₂ is necessary since, at the end of the reaction, it was observed that a portion of this compound was fixed to the reactor walls and did not react. On the other hand, if the amount of MgH₂ is too high, it may decrease the contact between the reactants and, consequently, the NaBH₄ yields will be lower. Thus, the authors concluded that the optimal value for the amount of MgH₂ is 35% excess, which is in agreement with the results published by Çakanyldirim et al. [68].

In 2019, Le et al. [58] reacted NaBO₂ with a Mg-Al based alloy at 7 MPa H₂ with a BPR of 20:1. The reported results showed a conversion close to 100% after 36 h of milling. Moreover, the authors stated that the milling time could be reduced to 12 h without any considerable loss since, at that point, the NaBH₄ conversion was about 97%.

2.2.2. Effect of Additives and the Hydration of the Boron Compound

In addition to the influence of the parameters mentioned before, Çakanyldirim et al. [68] also studied the influence of several additives (Al, Na, and Na₂CO₃) and observed that the all the studied additives promoted a negative effect in the conversion yield (Figure 9).

Lang et al. [53] studied the influence of methanol (CH₃OH) as an additive to the NaBO₂-MgH₂ mixture and reported a slight increase of NaBH₄ conversion yield when the time of milling was superior to 10 h (Figure 10).

Furthermore, several authors researched the possibility to regenerate NaBH₄ through ball milling using the borates in its hydrated form. Since the by-product of NaBH₄ hydrolysis is usually obtained in its hydrated form, and its dehydration is a high-energy consumption step, the possibility of ball milling the hydrated borates with high conversion yields makes this method more feasible. Thus, Huang et al. [52] studied the ball milling of the hydrated NaBO₂ with Mg and Mg₂Si (Equation (12)).

$${\mathrm{NaBO}}_2{•2\mathrm{H}}_2{\mathrm{O}+2\mathrm{Mg}+\mathrm{Mg}}_2\mathrm{Si}\to {\mathrm{NaBH}}_4+4\mathrm{MgO}+\mathrm{Si}$$

(12)

The reaction was performed in a vibrational ball mill under Ar atmosphere, and the time of milling as well as the reactants molar ratio were analyzed. The results presented a conversion yield of 86%, after 20 h, with a Mg:Mg₂Si:NaBO₂ molar ratio of 2.5:1:1.

Zhong et al. [57] also studied the recycling of the dehydrated sodium metaborate at room temperature in an Ar atmosphere; however, it was ball milled with Mg₂Si (Equation (13)). The process regenerates NaBH₄ with 78% yield, after 20 h of milling, with a Mg₂Si/NaBO₂•2H₂O molar ratio of 3:1.

$${\mathrm{NaBO}}_2{•2\mathrm{H}}_2{\mathrm{O}+2\mathrm{Mg}}_2\mathrm{Si}\to {\mathrm{NaBH}}_4+4\mathrm{MgO}+2\mathrm{Si}$$

(13)

Therefore, Zhong et al. [55] studied the recycling of tetrahydrated sodium metaborate in the same conditions and tried to optimize the reaction parameters. The results indicated a higher NaBH₄ yield of 74%, after 20 h, with a Mg₂Si/NaBO₂•4H₂O molar ratio of 4:1 [57].

Chen et al. [20] and Ouyang et al. [54] also studied the regeneration of NaBH₄ through the ball milling of NaBO₂•2H₂O and NaBO₂•4H₂O. In the work published by Chen et al. [20], the borates were mixed with MgH₂. The experiments with NaBO₂•2H₂O were performed with 25% excess of MgH₂, and after 5 h, 84.5% yield was obtained. This value increased to 90% after 15 h. NaBO₂•4H₂O experiments were performed with 37.5% excess of MgH₂, and a maximum yield of 88.3%, achieved after 20 h, was reported.

In the work of Ouyang et al. [54], the experiments were carried out by reacting the borates with Mg (Equations (14) and (15)) in a high-energy ball mill at room temperature and pressure during 15 h. Both borates were mixed in a 1:12.5 molar ratio and 68.6% and 64.1% of NaBH₄ were obtained for the NaBO₂•2H₂O and NaBO₂•4H₂O, respectively.

$${\mathrm{NaBO}}_2{•2\mathrm{H}}_2\mathrm{O}+4\mathrm{Mg}\to {\mathrm{NaBH}}_4+4\mathrm{MgO}$$

(14)

$${\mathrm{NaBO}}_2{•4\mathrm{H}}_2\mathrm{O}+6\mathrm{Mg}\to {\mathrm{NaBH}}_4{+6\mathrm{MgO}+2\mathrm{H}}_2$$

(15)

The reaction of NaBO₂•2H₂O with Mg₁₇Al₁₂ in the presence of NaH, as additive, was studied by Qin et al. [59]. The authors optimized the parameters to 20 h of milling and 5 wt % of NaH, and the results showed that NaH had a considerable impact, since the reaction yield has doubled. Nevertheless, the obtained yield of 57.0% is considerably lower when compared with other values published in the literature.

The conversion of NaBO₂•4H₂O into NaBH₄ was also studied by Le et al. [58]. The borate was ball milled with a Mg-Al-based alloy, up to 36 h, at 0.1 MPa in an Ar atmosphere, with a BPR of 20:1, and it resulted in a conversion close to 100%.

Moreover, Zhu et al. [61] developed a low-cost system to close the NaBH₄-H₂ cycle based on the recycling of borax (Na₂B₄O₇•10H₂O) with Na₂CO₃ and Mg (Equation (16)).

$${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7{•10\mathrm{H}}_2{\mathrm{O}+\mathrm{Na}}_2{\mathrm{CO}}_3+20\mathrm{Mg}\to {4\mathrm{NaBH}}_4{+20\mathrm{MgO}+\mathrm{CH}}_4$$

(16)

According to the authors, borax presents more similarities with NaBH₄ and has a reactivity higher than NaBO₂, which makes the regeneration of NaBH₄ easier. A maximum yield of 78.9% was obtained after 30 h of milling and 23.75% excess of Mg.

The NaBH₄ regeneration from Na₂B₄O₇ with MgH₂ was previously studied by Li et al. [37] in 2003. The experiments were carried out during 1 h, and the influence of three additives was studied: Na₂O₂, NaOH, and Na₂CO₃. The higher NaBH₄ conversion (78%) was achieved with the use of Na₂CO₃ as an additive.

2.2.3. Effect of the Mill

As referred above, there are different types of ball mills, which can also influence the reaction. The work published by Hsueh et al. [51] compares the results of NaBH₄ regeneration, which are presented in Equation (10) and were performed in a planetary ball mill and a shaker mill. Initially, the reactant mixture was ground for 6 h. Then, the experiments were performed by maintaining the amount of borate constant and increasing the excess of MgH₂ in both mills (Figure 11). Hsueh concluded that the shaker mill was more suitable than the planetary mill due to the difficulty of decomposing the reaction and promoting the NaBH₄ formation.

2.2.4. NaBH₄ Formation and Energy Efficiency

To better optimize the NaBH₄ synthesis, it is essential to understand NaBH₄ formation.

Ouyang et al. [54] studied the NaBO₂•2H₂O conversion into NaBH₄ and reported the structural rearrangement of the borate to NaB(OH)₄. The authors analyzed and described the ball mill reaction between NaBO₂•2H₂O and Mg and observed the formation of magnesium hydroxide (Mg(OH)₂), after 10 min of milling, as an intermediate in the reaction. This compound was also consumed after 1 h, as shown in Equation (17). The formation of Mg(OH)₂ plays an important role in this reaction, since its consumption during the process is responsible for its appearance of molecular hydrogen in the reaction medium. Those observations were also reported by Chen et al. [20], Huang et al. [52], and Qin et al. [59].

$${2\mathrm{Mg}\left(\mathrm{OH}\right)}_2+2\mathrm{Mg}\to {4\mathrm{MgO}+2\mathrm{H}}_2$$

(17)

Zhong et al. [55] studied the reaction mechanisms of NaBO₂•4H₂O with Mg₂Si₂ and observed that NaBO₂•2H₂O and molecular hydrogen are formed in an intermediate step (Equation (18)). The authors reported that this step is important to promote the NaBH₄ regeneration yield.

$${\mathrm{NaBO}}_2{•4\mathrm{H}}_2{\mathrm{O}+\mathrm{Mg}}_2\mathrm{Si}\to {\mathrm{NaBO}}_2{•2\mathrm{H}}_2{\mathrm{O}+2\mathrm{MgO}+\mathrm{Si}+\mathrm{H}}_2$$

(18)

Furthermore, in 2014, Ouyang et al. [50] calculated the energy efficiency of the consumption/regeneration of NaBH₄ based on the recycling of NaBO₂ with MgH₂ in a high-speed vibrating mill. The results showed an energy efficiency of about 50%, which led to the conclusion that the system is feasible. Nevertheless, it is important to find novel techniques to improve the energy efficiency of this process, for example by reusing the heat released from the reactions.

2.3. Electrochemical Processes

As previously referred, the regeneration of NaBH₄ can occur through the input of electric energy. When it is applied in the right conditions, it can reduce or oxidize compounds. This process takes place inside an electrochemical cell, which consists of an anode (where oxidation reaction occurs) and a cathode (where reduction reaction occurs). The electrolyte is essential to warrant the correct means to promote the reduction and/or oxidation of any intended compound. Moreover, the electrochemical cell can also be used to convert the chemical energy of a compound in electric energy. These processes are desired due to its low energy consumption, cleanliness, and reduced cost compared to the thermochemical processes.

The regeneration of sodium borohydride from sodium metaborate can theoretically be obtained mainly through the direct electroreduction, as shown in Equations (19) and (20).

Cathode:

$${\mathrm{BO}}_2^{-}{+6\mathrm{H}}_2{\mathrm{O}+8\mathrm{e}}^{-}\to {\mathrm{BH}}_4^{-}{+8\mathrm{OH}}^{-}$$

(19)

Anode:

$${8\mathrm{OH}}^{-}\to {4\mathrm{H}}_2{\mathrm{O}+2\mathrm{O}}_2{+8\mathrm{e}}^{-}.$$

(20)

Although there have been claims of sodium borohydride synthesis, those results have not been replicated. This fact is associated with the formation of other compounds at the potentials applied at the cathode: for example, the water decomposition into ${\mathrm{H}}_2$ and ${\mathrm{OH}}^{-}$. Other electrochemical processes have been referred in the literature; however, the results obtained were similar to the direct electroreduction process. From what the authors could find, the first paper reported in the literature that confirms the ${\mathrm{NaBH}}_4$ electrosynthesis was that of Zhu et al. [280,282] The authors reported a 15.1% current efficiency for the ${\mathrm{NaBH}}_4$ electrosynthesis at atmospheric pressure and room temperature. The electrochemical cell was composed of a copper working electrode and a lead counter-electrode. Then, 1 mol/L ${\mathrm{H}}_2{\mathrm{SO}}_4$ solution was added to the anode and 1 mol/L NaOH and 0.2 mol/L ${\mathrm{NaBO}}_2$ solution was added to the cathode. A cathodic peak was observed at −1.17 V, which is indicative of the ${\mathrm{BO}}_2^{-}$ reduction. Moreover, the increase of NaOH favors the electrosynthesis; however, excessive NaOH hardens the products’ adsorption and conduction. Similar observation was made for the ${\mathrm{NaBO}}_2$ concentration. Nevertheless, the process’ efficiency is too low.

Efficient electrosynthesis of ${\mathrm{NaBH}}_4$ was achieved by Shen et al. [276]. Given the use of ${\mathrm{NaBH}}_4$ as an effective reducing agent and its high cost, the authors considered its electrosynthesis as an intermediate step in a desulfurization experiment of a coal water slurry at ambient pressure and room temperature. The electrochemical cell consisted of cathode and anode reservoirs separated by a KCl bridge. On the cathode side, a solution containing 0.1 to 0.45 mol/L ${\mathrm{NaBO}}_2$ and 0.025 mol/L NaOH was initially added. Then, the coal was added, and the mix was maintained under stirring. The anode side was composed of an iron sulfide (${\mathrm{FeS}}_2$) and NaOH solution. The cathode was Pb and the anode was graphite (C). After the synthesis of ${\mathrm{BH}}_4^{-}$ in the cathode, its hydrolysis promoted the formation of hydrogen that reduced the ${\mathrm{FeS}}_2$ to hydrogen sulfide (${\mathrm{H}}_2\mathrm{S}$). Then, in the anode, the reaction with NaOH allowed the separation and removal of the sulfur. Equations (19) and (21)–(23) represent the primary reactions of the experiment.

$${2\mathrm{H}}_2{\mathrm{O}+\mathrm{BH}}_4^{-}{+\mathrm{FeS}}_2\to {\mathrm{Fe}+\mathrm{H}}_2{\mathrm{S}+\mathrm{S}+\mathrm{BO}}_2^{-}{+3\mathrm{H}}_2$$

(21)

$${\mathrm{H}}_2\mathrm{S}+2\mathrm{NaOH}\to {\mathrm{Na}}_2{\mathrm{S}+2\mathrm{H}}_2\mathrm{O}$$

(22)

$${2\mathrm{H}}_2{\mathrm{O}+2\mathrm{Na}}_2{\mathrm{S}+\mathrm{O}}_2\to 2\mathrm{S}+4\mathrm{NaOH}$$

(23)

The ${\mathrm{BO}}_2^{-}$ concentration, electrolytic time, and voltage were studied between 6 and 30 g/L, 1 and 5 h, and 2.5 and 3.3 V, respectively. An optimal total sulfur removal (TSR) of 36.6% was achieved with 12 g/L, 4 h, and 3.0 V, respectively. Considering the ${\mathrm{BO}}_2^{-}$ as an intermediate essential step of the process, these parameters also benefited the ${\mathrm{BH}}_4^{-}$ formation.

The group continued the experiments in the years forward, and the electrochemical cell for the process was also optimized, promoting the TSR and the ${\mathrm{BH}}_4^{-}$ formation. Considering similar electrochemical cells—however, with a combined anode and cathode reservoir [277]—the group studied again the ${\mathrm{BO}}_2^{-}$ concentration, electrolytic time, and voltage. However, the optimal values did not suffer a considerable change (0.2 mol/L, 4 h and 3.5 V). Moreover, the NaOH concentration and electrodes were analyzed. The NaOH concentration was studied up to 0.375 mol/L. It was observed that 0.05 mol/L was the optimal value to promote the TSR, since further increase favored the pH and reduced the hydrogen generation from the ${\mathrm{BH}}_4^{-}$ hydrolysis. Regarding the electrodes, five anode/cathode combinations were considered: Pb/Pb, C/Pb, Ti/Pb, C/Cu, and C/Zn. The results were in favor of the last. In this way, graphite not only better promotes the TSR but also presents significant advantages to consider, such as easy to process, reduced waste and price, and good thermostability and conductivity.

Following that, the group studied the desulfurization of gasoline [278]. The electrochemical cell was composed of a BDD electrode in the cathode (working electrode) and a C electrode in the anode (counter electrode) and the cathode and anode were separated by a cation exchange membrane. After ${\mathrm{BH}}_4^{-}$ formation, a gasoline model was added to the cathode, and after the reaction, ${\mathrm{NiCl}}_2{•6\mathrm{H}}_2\mathrm{O}$ was added to the last. Moreover, pulse voltage was studied during the ${\mathrm{BH}}_4^{-}$ electrosynthesis, and the researchers observed a considerable increase in the TSR (almost twice higher). Cyclic voltammetry tests were performed using 0.15 mol/L ${\mathrm{NaBO}}_2$, and a reduction peak was observed between −1.2 and −1.8 V, while the hydrogen and oxygen evolution reactions were initiated at voltage below −1.8 V and above 0.6 V, respectively. Forward and reverse pulse voltages were optimized considering these limits. Overall, the best TSR achieved was with a forward and reverse pulse of −1.5 V (applied after 1.5 s) and 0.3 V (applied after 0.5 s), respectively, which were explained considering that it avoided at maximum the hydrogen evolution reaction (happening simultaneously with the ${\mathrm{BH}}_4^{-}$ electrosynthesis), and the reverse pulse voltage was enough to attract the ${\mathrm{BO}}_2^{-}$ ions to the BDD electrode while avoiding the oxygen evolution reaction. Regarding the ${\mathrm{BH}}_4^{-}$ electrosynthesis step, other cell parameters were also studied: ${\mathrm{NaBO}}_2$ and NaOH concentrations, electrolytic time, and stirring rate. First, 0.15 mol/L of ${\mathrm{NaBO}}_2$ was maintained as an optimized value, since higher concentrations did not contribute to further increasing the TSR. The optimal NaOH concentration was 0.1 mol/L, since although it is important to maintain the cell as alkaline to avoid too fast ${\mathrm{BH}}_4^{-}$ hydrolysis, its excess reduces the oxygen evolution voltage, promoting it and making the ${\mathrm{BO}}_2^{-}$ transfer to the BDD electrode difficult. Excess stirring rate was also observed to affect the ${\mathrm{BO}}_2^{-}$ transfer, so 400 rpm was the best value defined. The higher efficiency of the desulfurization process was 97%.

More recently, the group studied the addition of ionic liquids (IL) to the desulfurization process of gasoline. The use of ILs as electrolyte has been increasingly reported in the literature due to their characteristics, which make them more suitable than water and other electrolytes. In the ${\mathrm{BH}}_4^{-}$ electrosynthesis, they avoid the need to control the hydrogen and oxygen evolution reactions and present a wider electrochemical window. Moreover, they can be recovered at the end of the experiment. Similar electrochemical cells to previous study were considered by the group. In this experiment, the working electrode was a glassy carbon electrode, and the counter-electrode was a Pt wire. The IL was added to both the electrodes reservoirs in the cell, while water was only present due to the ${\mathrm{NaBO}}_2$ solution and the need of hydrogen for the ${\mathrm{BH}}_4^{-}$ electrosynthesis. Cyclic voltammetry tests were performed, which indicated a reduction peak between −2.2 and −3.0 V, which was associated to the ${\mathrm{BH}}_4^{-}$. Moreover, the hydrogen and oxygen evolution reactions were observed at −3.0 and 2.0 V, respectively. As expected, the electrochemical window using IL is wider than that of water. Forward and reverse pulse voltage was equally applied, and their parameters were optimized. The optimal values to promote the ${\mathrm{BH}}_4^{-}$ electrosynthesis and the TSR (97%) were a forward pulse voltage of −2.6 V (0.6 s forward pulse time), reverse pulse voltage of 0.5 V (0.5 s reverse pulse time), and pull-off time of 0.8 s. The times that were associated to the ones referred and applied were the forward and reverse pulse voltage and turn-off. If they are too long, the reactions do not benefit; if they are too short, the reactions do not occur.

Based on these results of the group and assuming that an efficient electrosynthesis of ${\mathrm{BH}}_4^{-}$ is essential for the desulfurization of the coal water slurry, it is possible to affirm that ${\mathrm{NaBH}}_4$ can be obtained through an electrochemical process and that it is an alternative to the thermo- and mechano-chemical processes. Nevertheless, it is essential to promote the ${\mathrm{NaBH}}_4$ regeneration as the main aim instead of an intermediate step in another process. In addition, as noted in the thermo and mechano-chemical regeneration processes section, the study of hydrated ${\mathrm{NaBO}}_2$ is crucial, since it is the main by-product of the ${\mathrm{NaBH}}_4$ hydrolysis and it is demonstrated that its recycling is not as easy. Considering the practicability and possibly low cost of these processes, the electrochemical regeneration of ${\mathrm{NaBH}}_4$ must be considered. In that order, both the use of ILs and the pulse voltage are improvements to the process and must be studied.

3. Conclusions

Sodium borohydride stands as a promising hydrogen carrier for on-demand power supply. Despite the DOE no-go recommendation in 2007 for NaBH₄ hydrolysis for onboard applications, over 13 years later, several findings and improvements have been made in this area. Therefore, NaBH₄-H₂-PEMFC systems have emerged as a feasible candidate to supply energy for portable applications, maritime applications, as well as an off-grid technology capable of providing an electricity supply in emerging countries and locations without a reliable grid. Nevertheless, closing the NaBH₄-H₂ cycle is strongly dependent on the regeneration of NaBH₄. In this overview, a survey of alternative methods to recycle NaBH₄ hydrolysis by-products was made, and three types of methods stands out: thermo-, mechano-, and electrochemical. Although the first published studies were mostly dedicated to thermochemical processes, since they present high conversion yields, over the years, they have been replaced by mechano-chemical processes. This transition shows a concern for the development of environmentally friendly methods, since mechano-chemical processes use a mechanical source of energy instead of heat and, consequently, do not require high temperatures and pressures. Nevertheless, although it is possible to obtain NaBH₄ conversion yields as high as in thermochemical processes, they require high milling times to achieve those same yields. Moreover, both methods typically use expensive hydrides, such as MgH₂, to promote the NaBH₄ synthesis. Thus, electrochemical processes arise as a promising low-cost and clean alternative. Although the regeneration of NaBH₄ from NaBO₂ has been confirmed, as an intermediate step in a desulfurization experiment, the study of this process is very limited, and published articles dedicated to electrochemical methods to regenerate NaBH₄ are rare. There are a few issues in this process that need to be overcome: mainly, the water decomposition and the formation of other by-products. Thus, further studies to find alternative electrochemical reactions are needed in order to take advantage of the potential of this process. Therefore, the rehydrogenation of the borates formed in the NaBH₄ regeneration is a technology that needs urgent improvement, and it is essential to close the NaBH₄-H₂ cycle and, consequently, implement NaBH₄-H₂-PEMFC on-demand systems. After all, these systems may be the future off-grid technology that would address the increasing renewable energy supply.