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Investigating the Factors Affecting the Ionic Conduction in Nanoconfined NaBH4

Xiaoxuan Luo
Aditya Rawal
2 and
Kondo-Francois Aguey-Zinsou
HERC and MERLin, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
NMR Facility, Mark Wainwright Analytical Centre, The University of New South Wales, Sydney, NSW 2052, Australia
Author to whom correspondence should be addressed.
Inorganics 2021, 9(1), 2;
Submission received: 28 November 2020 / Revised: 22 December 2020 / Accepted: 23 December 2020 / Published: 1 January 2021
(This article belongs to the Section Inorganic Solid-State Chemistry)


Nanoconfinement is an effective strategy to tune the properties of the metal hydrides. It has been extensively employed to modify the ionic conductivity of LiBH4 as an electrolyte for Li-ion batteries. However, the approach does not seem to be applicable to other borohydrides such as NaBH4, which is found to reach a limited improvement in ionic conductivity of 10−7 S cm−1 at 115 °C upon nanoconfinement in Mobil Composition of Matter No. 41 (MCM-41) instead of 10−8 S cm−1. In comparison, introducing large cage anions in the form of Na2B12H12 naturally formed upon the nanoconfinement of NaBH4 was found to be more effective in leading to higher ionic conductivities of 10−4 S cm−1 at 110 °C.

Graphical Abstract

1. Introduction

The development of solid-state electrolytes is critical for the advancement of all solid-state high-energy density batteries. In this respect, several families of inorganic solid-state electrolytes have been intensively investigated because of their non-flammability and capability in leading to a high Li+/Na+ transference number and thus safety with extended battery life-time [1]. However, current solid-state electrolytes still suffer from low ionic conductivity [2]. Current materials reported to lead to high ionic conductivity are oxide solid-state electrolytes like La0.52Li0.35TiO2.96 (ionic conductivity of 103 at room temperature) [3]. However, in this case, the poor contact between the electrode and the ceramic electrolyte causes a high interfacial resistance [4]. Sulfide-based solid-state electrolytes have also been found to lead to high ionic conductivity (e.g., Li9.54Si1.74P1.44S11.7Cl0.3 has an ionic conductivity of 2.5 × 10−2 S cm−1 at room temperature) [5], and this is comparable to the ionic conductivity of organic electrolytes [6]; however, sulfide electrolytes have a narrow electrochemical stability window and tend to decompose upon operation and release toxic H2S gas [7,8].
Recently, metal borohydrides have drawn considerable interest because of their chemical and electrochemical stability, as well as chemically compatibility with Li/Na metal [9], but modifications are needed to enable high ionic conductivity [10] in complex borohydrides at ambient and not high temperatures. To date, the most common strategies to tailor the ionic conductivity of complex borohydrides are based on anionic substitution [11,12,13] or nanoconfinement approaches [10,14]. For example, partial anionic substitution of BH4 by NH2 in NaBH4 leads to an increase in ionic conductivity from 1 × 109 to 2 × 106 S cm1 at 26 °C [11]. Nanoconfining LiBH4 (LiBH4@MCM-41) within the pores of ordered silica scaffolds such as MCM-41 was reported to lead to a high ionic conductivity of 0.2 mS cm−1 at 55 °C instead of 1 × 108 S cm1 [10] owing to the interface interaction between LiBH4 and MCM-41 [15,16]. Further testing of such a material showed that nanoconfined LiBH4 in an Li/S cell could deliver a high capacity of 1220 mAh g−1 at a working voltage of 2 V and at 0.03 C rates for 40 cycles, which is comparable to the sulfide electrolytes [17,18]. To date, such improvements have only been reported for LiBH4, with no report on the possibility of such an observation to other borohydrides and in particular NaBH4, which is of interest as a solid electrolyte for Na batteries. Except for borohydrides materials, nanoconfinement has been proven as an effective method to tailor the ionic conductivity of electrolyte materials including solid composite polymer electrolytes and ionic liquid electrolytes confined in Metal Organic Frameworks [19,20,21]. Herein, we report on the successful confinement of NaBH4 into the MCM-41, and the resulting limited improvement in ionic conductivity. As a reference, LiBH4 was also confined into the scaffold material MCM-41 and the existence of an oxide phase in LiBH4@MCM-41 further proved our hypothesis, i.e., the strong oxidation of NaBH4 upon nanoconfinement in MCM-41 and thus the formation of an extensive oxide phase at the borohydride/MCM-41 interface limits Na and Li conduction. Better improvement in ionic conductivity could be achieved by partially decomposing NaBH4 into a mixture of Na2B12H12 and NaBH4.

2. Results and Discussions

2.1. Infiltration of NaBH4 in MCM-41

To confirm the degree of infiltration of NaBH4 into MCM-41, Brunauer–Emmett–Teller (BET) measurements were carried out. As expected, melt infiltration of NaBH4 into MCM-41 resulted in a pore volume reduction from the original MCM-41 value of 1.02 to 0.02 cm3 g1, and this corresponded to a 78% pore filling. As reflected by BET, the melt infiltration of NaBH4 into MCM-41 also led to a significant decrease of the surface area of NaBH4@MCM-41 from 1110.91 m2 g1 for pristine MCM-41 to 3.5 m2 g1 (Table S1 and Figure S1).
Further analysis by transmission electron microscopy (TEM) showed that the ordered porous structure of MCM-41 was filled upon nanoconfinement of NaBH4 with the disappearance of a clear porous structure (Figure S2a). Additionally, elemental mapping showed a signal of Na and Si overlapping, and this was taken as additional evidence that NaBH4 was melt infiltrated within the porosity of MCM-41 (Figure S3).
X-ray diffraction (XRD) patterns of the pristine and nanoconfined NaBH4 are shown in Figure 1a. For NaBH4@MCM-41, all the peaks are assigned to NaBH4, which indicates that no detectable additional phase was formed during the melt infiltration process at 560 °C. However, all the diffraction peaks showed a significant boarding and shift to lower diffraction angles as compared with pristine NaBH4 (Figure 1b), which suggested some confinement of the borohydride [22,23,24]. Indeed, a shift has been reported to occur for confined borohdyrides owing to the lattice strain imposed by the MCM-41 scaffold [22,25], as further revealed by the small angle X-ray diffraction analysis of MCM-41 and NaBH4@MCM-41. As shown in Figure 1c, the main (100) diffraction peak of MCM-41 broadens. This peak also shifted to higher diffraction angles, further indicating that NaBH4 was located within the internal pore of MCM-41 [26]. From these results, it can thus be concluded that NaBH4 is infiltrated within the porosity of MCM-41. Another indirect evidence of the nanoconfinement is the shift in the dehydrogenation peak of NaBH4 upon infiltration in MCM-41, from 550 to 520 °C (Figure S4).
To further determine any amorphous phases that may have formed during the nanoconfinement of NaBH4 in MCM-41 at 560 °C, Fourier Transform Infrared Spectrometery (FTIR)and nuclear magnetic resonance (NMR) analyses were carried out. By FTIR (Figure 2), the typical BH stretching and bending vibrations corresponding to the BH4 anion in NaBH4 were observed in NaBH4@MCM-41 in the range from 2400 to 2200 cm−1 and at 1091 cm−1, in agreement with previous reports [27]. The broad peaks between 3800 and 3200 cm1 were assigned to OH stretching modes corresponding to a partial oxidation of NaBH4 in contact with the walls of the MCM-41. As a scaffold material, MCM-41 possesses silanol (Si–OH) and hydrogen-bonded terminal hydroxyl (Si–OH–O–Si) groups located within its internal structure [28]. Therefore, during the melt infiltration of NaBH4, it is not surprising that silanol and/or hydroxyl groups readily react with NaBH4 to lead to the formation of boron oxide phases in NaBH4@MCM-41, as evidenced by the peaks at 1626, 883, and 794 cm1 owing to the B–O vibrational modes (Figure 2) [29,30,31]. To eliminate the concern regarding the formation of other oxide compounds (e.g., NaO2), we carefully checked the FTIR spectrum of NaBH4@MCM-41 in the range of 800–400 cm1 (Figure S8), and the only peak located at 473 cm1 was assigned to NaBO4 instead of NaO2 [32]. A similar oxidation has previously been observed upon the infiltration of LiBH4 in SBA-15, and in this case, this led to the formation of LiBO2 [33]. Besides these oxidized phases, the peak observed at 2496 cm1 was attributed to the formation of Na2B12H12, which is commonly reported to occur upon a partial decomposition of NaBH4 [34,35]. These results are also in agreement with the 11B NMR spectrum (Figure 3) showing the typical resonance of the BH4 anion centered at −41.95 ppm [36]; a single sharp peak at −2 ppm, assigned to boron in tetrahedral BO4 environments [37]; and at −15.58 ppm, a peak corresponding to the dodecahedral [B12H12]2− anion in Na2B12H12 [38].

2.2. Ionic Conductivity of Nanoconfined NaBH4

The ionic conductivity of the pristine NaBH4 and NaBH4@MCM-41 was determined by electrochemical impedance spectroscopy (EIS) (Figure 3). The ionic conductivity of NaBH4@MCM-41 is 10 times higher (i.e., 7.4 × 1010 S cm1 at 20 °C) than that of pristine NaBH4 in the temperature range of 20–70 °C. However, above 70 °C, the ionic conductivity of NaBH4 and NaBH4@MCM-41 was found to be of the same magnitude. To our surprise, encapsulating NaBH4 into MCM-41 did not significantly enhance the ionic conductivity. This may be because of the extensive reaction of nanoconfined NaBH4 with the walls of MCM-41 during melt infiltration, which resulted in the formation of an insulating oxide phase [39]. Indeed, boron oxide phases (e.g., NaBO4) have been reported to be poor ionic conductors [40]. In previous reports, it has been suggested that the ionic conductivity of MCM-41 nanoconfined LiBH4 was the result of the LiBH4/MCM-41 interface promoting the reorientation of BH4 [10,25]. To verify this hypothesis, we reproduced the nanoconfinement of LiBH4 by melt infiltration in MCM-41 [10]. Successful nanoconfinement of LiBH4 was verified by BET and thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis (Figures S5 and S6). In this case, the ionic conductivity of LiBH4@MCM-41 was found to be 1.34 × 107 S cm1 at 110 °C (Figure S9), which is four times lower in magnitude as compared with the reported value of 103 S cm1 [10]. Through careful analysis by FTIR (Figure S7), strong B–O vibrations were observed in LiBH4@MCM-41 as compared with pristine LiBH4. These were assigned to lithium borates (Li2B4O7) [41], and indicated a strong oxidation of the borohydride phase during nanoconfinement. The discrepancy between the ionic conductivity of the current LiBH4@MCM-41 and reported data may thus be attributed to the extensive oxidation of the complex borohydride within the scaffold material. Here, we emphasize the inevitable reaction between the pore wall in MCM-41 and complex borohydrides and the resulting negative effect on ionic conduction.
One factor that may explain the increase in ionic conductivity observed for NaBH4@MCM-41 below 70 °C is the presence of Na2B12H12, which was reported to lead to improved ionic conductivity [42]. Through integrating the chemical shifts of B12H12 located at −15.6 ppm in NaBH4@MCM-41(Figure 4), 18% of amorphous Na2B12H12 appeared in NaBH4@MCM-41 owing to decomposition of NaBH4 (Figure 4). We believed that the appearance of the Na2B12H12 in the nanoconfined material is due to the interaction between the NaBH4 and scaffold materials, triggering a partial decomposition of NaBH4 upon its oxidation and the release of B2H6, further reacting with NaBH4 to lead to Na2B12H12 [43,44].
To investigate if the presence of Na2B12H12 within NaBH4 can effectively lead to improved ionic conductivities, we synthesised NaBH4 with higher amounts of Na2B12H12. After the reaction between B2H6 and NaBH4, 43% of the starting NaBH4 was converted to Na2B12H12 (Figure S10). Such a mixture of NaBH4 and Na2B12H12 is stable up to 300 °C (Figure S11) and exhibited a drastic enhancement in ionic conductivity, with an ionic conductivity of 106 S cm1 at 110 °C (Figure 5). This is two magnitudes higher than pristine NaBH4 [45]. Thus, the enhancement in ionic conductivity observed in nanoconfined NaBH4@MCM-41 can be assigned to the existence of the B12H122− anion. Anion rotation is believed to significantly enhance cation hopping [46,47], and B12H12 exhibits an intrinsically high dynamic motion [48]. Accordingly, the enhanced ionic conductivity in NaBH4@MCM-41 was attributed to the co-existence of B12H122− and BH4. We thus believe that fine tuning the ratio between Na2B12H12 and NaBH4 will further increase the ionic conductivity of NaBH4 and other complex borohydrides. Further investigations along this path are underway.

3. Materials and Methods

3.1. Synthesis

All the operations were carried out under an inert atmosphere in an argon-filled LC-Technology glove box (<1 ppm O2 and H2O, Salisbury, MA, USA). Sodium borohydride (NaBH4, 99%) was purchased from Sigma-Aldrich (Sydney, NSW, Australia) and further purified. ZnCl2 (≥98%) was purchased from Ajax Finechemn (Sydney, NSW, Australia), and dried at 120 °C overnight on a Schlenk line under vacuum (0.01 MPa) before use. Lithium borohydride (LiBH4, 95%) was purchased from Acros (Sydney, NSW, Australia). Prior to use, LiBH4 was purified following the reported procedures [49]. MCM-41 was purchased from ACS materials and dried under vacuum at 200 °C for 2 h before use to remove any water traces.

3.1.1. Synthesis of the Nanoconfined Complex Borohydrides

For the nanoconfinement of NaBH4 into MCM-41 (noted NaBH4@MCM-41), a mixture of 1.5 g NaBH4 grinded with 1.27 g MCM-41 was heated at 5 °C min1 and kept at 560 °C for 3.5 h under 8 MPa H2 pressure. This temperature was chosen because NaBH4 melts at 500 °C (Figure S4)
LiBH4@MCM-41 was synthesized as per previous report [10]. LiBH4 infiltration was carried out to fill 100% of the pore volume of MCM-41. This was achieved by mixing 0.85 g of LiBH4 with 0.127 g of MCM-41 in a mortar and pestle. The mixture was then placed in a stainless-steel sample holder and heated to 295 °C at 5 °C min1 under an H2 pressure of 10 MPa. Infiltration was done at this temperature for 30 min.

3.1.2. Synthesis of the Mixture of NaBH4 and Na2B12H12 Was via Solid–Gas Reaction

NaZn2(BH4)5 was synthesised by ball milling pristine NaBH4 and ZnCl2 with a molar ratio of 2:1 with a Retsch MM301 mill operated at a frequency of 20 Hz. The mixture of NaBH4 and ZnCl2 was milled in a stainless-steel vial (25 mL) containing a single stainless-steel ball (15 mm diameter) for 10 min. Sodium borohydride (120 mg) was ball-milled in a similar manner under high purity argon in a 15 mL stainless steel vial filled with a single stainless-steel ball (1.5 g and 15 mm diameter).
The synthesis of NaBH4/Na2B12H12 mixed compounds was undertaken in an in-house built Sievert apparatus with a customized sample holder, which had two compartments separated by a stainless-steel mesh (20 μm porosity). Then, 100 mg of ball milled NaBH4 was placed on the top of the mesh and 400 mg NaZn2(BH4)5 was placed at the bottom of the sample holder. The reaction was carried out at 150 °C for 4 h under 10 MPa H2. The materials synthesized by this solid–gas reaction were then ball milled in a 15 mL stainless steel vial filled with a single stainless steel ball (1.5 g and 15 mm diameter) at a frequency of 20 Hz for 10 min, and the previous synthesis route was repeated to maximise the Na2B12H12 yield.

3.2. Characterization

The crystalline nature of the materials was determined by X-ray diffraction (XRD) using a Philips X’pert Multipurpose XRD system (Bruker, Preston, Victoria, Australia)operated at 40 mA and 45 kV with a monochromatic Cu Kα radiation (λ = 1.541 Å)-step size = 0.01°, time per step = 10 s·step1. The materials were protected against oxidation from the air by a Kapton foil.
Infrared analysis was carried out on a Bruker Vertex 70 V (Bruker, Preston, Victoria, Australia) equipped with a Harrick diffuse reflectance Praying Mantis accessory. The materials were loaded in an air-tight chamber in the glovebox and the chamber was fitted on the Praying Mantis. Spectra were acquired with a 1 cm1 resolution with an MCT-detector.
Solid-state 11B magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments were carried out on a narrow-bore Bruker Biospin Avance III solids-700 MHz spectrometer (Bruker, Preston, Victoria, Australia) with a 16.4 Tesla superconducting magnet operating at a frequency of 224.7 MHz 11B nucleus. Approximately 3–10 mg of material was packed into 4 mm zirconia rotors fitted with Kel-f® caps or vespel caps, respectively. The 4 mm rotors were spun in a double resonance H-X probe head at 14 kHz at the magic angle. The 11B spectra were acquired with hard 1 to 3 μs radio frequency pulses corresponding to a 30° tip angle. The recycle delays of up to 10 s were used to ensure full relaxation of the signals of all nuclei, and up to 512 transients were co-added to ensure sufficient signal to noise. The spectra were obtained at room temperature and chemical shifts were referenced using a 1 M NaCl(aq) for 23Na, NaBH4(s) for 11B, and 1 M LiCl(aq) for 7Li. The spectral deconvolution was carried out using the Dmfit software [50].
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in conjunction with mass spectrometry (MS) were conducted at 10 °C min1 under an argon flow of 20 mL min1 using a Mettler Toledo TGA/DSC 3 (Mettler Toledo, Melbourne, Victoria, Australia) coupled with an Omnistar (Pfeiffer) MS. Masses between m/z = 2 and 100 were followed and 40 μL alumina crucibles were used.
The MCM-41 and nanoconfined NaBH4 morphology and elemental mapping were determined by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) using a Philips CM200 (Philips, Eindhoven, Netherlands) operated at 200 kV. For TEM analysis, the materials were ground and dispersed in cyclohexane, sonicated for a few seconds, and then dropped onto a carbon coated copper grid. Brunauer–Emmett–Teller (BET) was performed using a Micromeritics TriStar 3000 Analyzer from Micrometrics Instrument Corporation (Norcross, GA, USA).
The ionic conductivity of the materials was determined by electrochemical impedance spectroscopy (EIS) (Biologic, Sydney, NSW, Australia). Here, 30 mg of materials was placed in a 10 mm diameter die and uniaxially cold pressed at 9 MPa with a hydraulic press to make a pellet of 0.03 cm thickness and 0.712 cm2. Two polished sheets of stainless steel were used as electrodes. The pellet was then placed into a controlled environment sample holder (CESH) from BioLogic (Biologic, Sydney, NSW, Australia). The cell was assembled in the glove box under argon atmosphere. EIS was conducted using an alternating current impedance spectroscopy method with a VMP3 potentiostat from BioLogic. The AC impedance measurement was set from 100 mHz to 1 MHz. The measurement was conducted in the temperature range from 25 to 135 °C with an interval of 10 °C. Before each measurement, the sample dwelled for 20 min for temperature equilibration.

4. Conclusions

In conclusion, we encapsulated NaBH4 into MCM-41 and the resulting NaBH4@MCM-41 material was expected to lead to higher Na+ ionic conductivities. However, any enhancement in ionic conductivity was found to be hindered by the formation of oxide phases in NaBH4@MCM-41, and similar observations made with LiBH4@MCM-41 tend to prove this hypothesis. We thus propose that the inevitable oxidation between complex borohydrides and MCM-41 may have a negative effect on the ionic conductivity. However, the presence of the large cage-like anions (B12H12)2− formed upon the partial decomposition of the borohydrides during their nanoconfinement may lead to a more effective path to improve their ionic conductivity.

Supplementary Materials

The following are available online at, Figure S1: BET analysis for MCM-41 and NaBH4@MCM-41: (a) N2 physisorption and (b) pore size distribution; Figure S2: Typical TEM image of (a) the empty scaffold MCM-41 and (b) NaBH4@MCM-41; Figure S3: STEM elemental mapping of NaBH4@MCM-41; Figure S4: TGA-DSC-MS profiles of (a) pristine NaBH4 and (b) NaBH4@MCM-41; Figure S5: BET analysis for MCM-41 and LiBH4@MCM-41: (a) N2-physisorption and (b) pore size distribution; Figure S6: TGA-DSC-MS profiles of (a) pristine LiBH4 and (b) LiBH4@MCM-41; Figure S7: FTIR analysis of LiBH4@MCM-41 and pristine LiBH4; Figure S8: FTIR analysis of NaBH4@MCM-41 and pristine NaBH4; Figure S9: Arrhenius plot of LiBH4@MCM-41 and pristine LiBH4; Figure S10: 11B NMR of the NaBH4 + Na2B12H12 composite synthesised by exposing NaBH4 to B2H6; Figure S11: Arrhenius plot of NaBH4 and the NaBH4+Na2B12H12 composite synthesised by exposing NaBH4 to B2H6; Table S1: Summary of BET analysis for MCM-41, NaBH4@MCM-41, and LiBH4@MCM-41.

Author Contributions

X.L. performed the research and analyzed the data. A.R. and K.-F.A.-Z. conceived and supervised the work, and analyzed the data. All authors have read and agreed to the published version of the manuscript.


We acknowledge support from the UNSW Digital Grid Futures Institute, UNSW Sydney, under a cross-disciplinary fund scheme and the ARC Research Hub on Integrated Energy Storage solutions.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. (a) X-ray diffraction (XRD) of nanoconfined NaBH4 along with that of pristine NaBH4 and (b) magnification of (a) for 2θ = 23–42°; (c) small angle XRD of MCM-41 and NaBH4@MCM-41.
Figure 1. (a) X-ray diffraction (XRD) of nanoconfined NaBH4 along with that of pristine NaBH4 and (b) magnification of (a) for 2θ = 23–42°; (c) small angle XRD of MCM-41 and NaBH4@MCM-41.
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Figure 2. FTIR analysis of NaBH4@MCM-41 and pristine NaBH4.
Figure 2. FTIR analysis of NaBH4@MCM-41 and pristine NaBH4.
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Figure 3. Arrhenius plot of NaBH4@MCM-41 and pristine NaBH4. The cause for the small “jump” in ionic conductivity for NaBH4 is unknown as it does not correspond to any known phase transition.
Figure 3. Arrhenius plot of NaBH4@MCM-41 and pristine NaBH4. The cause for the small “jump” in ionic conductivity for NaBH4 is unknown as it does not correspond to any known phase transition.
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Figure 4. 11B nuclear magnetic resonance (NMR) of pristine NaBH4@MCM-41 and NaBH4.
Figure 4. 11B nuclear magnetic resonance (NMR) of pristine NaBH4@MCM-41 and NaBH4.
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Figure 5. Arrhenius plot of NaBH4 and the NaBH4+Na2B12H12 composite synthesised by exposing NaBH4 to B2H6.
Figure 5. Arrhenius plot of NaBH4 and the NaBH4+Na2B12H12 composite synthesised by exposing NaBH4 to B2H6.
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Luo, X.; Rawal, A.; Aguey-Zinsou, K.-F. Investigating the Factors Affecting the Ionic Conduction in Nanoconfined NaBH4. Inorganics 2021, 9, 2.

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Luo X, Rawal A, Aguey-Zinsou K-F. Investigating the Factors Affecting the Ionic Conduction in Nanoconfined NaBH4. Inorganics. 2021; 9(1):2.

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Luo, Xiaoxuan, Aditya Rawal, and Kondo-Francois Aguey-Zinsou. 2021. "Investigating the Factors Affecting the Ionic Conduction in Nanoconfined NaBH4" Inorganics 9, no. 1: 2.

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