Giuseppe Boniello

Christophe Tribet

Emmanuelle Marie

Vincent Croquette

Draºen Zanchi

Tunable and switchable soft adsorption of polymer-coated microparticles on a at substrate

Keywords: Colloids, temperature responsive polymers, PNIPAM, colloidal sorting, adsorption, desorption

Soft adhesion to a horizontal at substrate of micron-sized colloids coated by a controlled molar fraction f of PLL-g-PNIPAM is tuned and triggered by controlling the temperature of the sample. This is because PNIPAM is a thermoresponsive polymer, which undergoes a phase transition at a LCST (lower critical solution temperature) T c = 32 ± 1 • C. In order to capture the very nal events before the immobilization of colloids the T-ramp protocol is designed: the particles suspension is injected in cell at room temperature, the temperature is increased at constant rate up to 38 • C> T c , and kept constant until the end of the acquisition. Attraction between beads and the at substrate is thereby triggered when crossing the critical temperature T c . Both ascending and descending ramp experiments are done in order to access both adsorption and desorption kinetics. 3-D beads motion is real-time tracked using slightly defocused microscopy in parallel illumination. We use track records to have access at pre-adsorption diusion coecient and to characterize adsorption and desorption dynamics. Present results corroborate that adsorption is controlled by PNIPAM doping, and indeed dominated by rolling and memory (aging) eect

Introduction

Functionalized colloids are micrometric or nanometric particles whose surface has been modied to introduce selected properties. For example, continuous or discrete sticky regions can be added to induce attractive interactions with other particles [START_REF] Grzybowski | Self-assembly: from crystals to cells[END_REF][START_REF] Risbud | Dynamic self-assembly of colloids through periodic variation of inter-particle potentials[END_REF][START_REF] Boniello | Reversible and dynamical control of aggregation and soft adhesion of t-responsive polymer-coated colloids[END_REF] and/or with a substrate [START_REF] Duadar | Dynamic adhesion behavior of micrometerscale particles owing over patchy surfaces with nanoscale electrostatic heterogeneity[END_REF][START_REF] Santore | Micrometer scale adhesion on nanometer-scale patchy surfaces: adhesion rates, adhesion thresholds, and curvature-based selectivity[END_REF]. Self-assembling of particles attracts increasing interest for the possibility of bottom-up designed composite materials [START_REF] Largo | Eective nonadditive pair potential for lock-and-key interacting particles: The role of the limited valence[END_REF][START_REF] Sciortino | Reversible gels of patchy particles[END_REF]. Assembling dynamics in modulated external elds (temperature, light, pH, magnetic elds) enables to trigger the interaction resulting in interesting new out-of-equilibrium structures and patterns [START_REF] Straube | Reversible clustering under the inuence of a periodically modulated binding rate[END_REF][START_REF] Bishop | Localized chemical wave emission and mode switching in a patterned excitable medium[END_REF]. On the other hand, comprehension of adsorption properties of sticky colloids on surfaces are relevant pre-requisite for their potential use in biology, as bacterial adhesion [START_REF] Gon | How bacteria adhere to brushy peg surfaces: clinging to aws and compressing the brush[END_REF][START_REF] Vissers | Bacteria as living patchy colloids: Phenotypic heterogeneity in surface adhesion[END_REF] and the design of biofouling resistant devices [START_REF] Gon | Interaction of cationic proteins and polypeptides with biocompatible cationically-anchored peg brushes[END_REF]. Controlled interactions with the solid phase can be also involved in a eld-ow approach [START_REF] Giddings | Field-ow fractionation: analysis of macromolecular, colloidal, and particulate materials[END_REF][START_REF] Martin | Size selectivity in eld-ow fractionation: Lift mode of retention with near-wall lift force[END_REF] as separation technique for purication of colloidal suspensions (e.g., to sort particles as a function of surface composition or patches architecture). A very common tool for reversibly adhesive colloids are DNA strands [START_REF] Tkachenko | Morphological diversity of dna-colloidal self-assembly[END_REF][START_REF] Valignat | Reversible self-assembly and directed assembly of dna-linked micrometersized colloids[END_REF]. Beyond self-assembly, dynamics of adsorption and desorption processes on at substrate have been addressed [START_REF] Licata | Dynamics of particles with "key-lock" interactions[END_REF][START_REF] Licata | Kinetic limitations of cooperativity-based drug delivery systems[END_REF][START_REF] Xu | Subdiusion of a sticky particle on a surface[END_REF]. Brownian dynamics of colloids sticking on a at surface have been studied in a number of approaches [START_REF] Xu | Subdiusion of a sticky particle on a surface[END_REF][START_REF] Fang | Using ow to switch the valency of bacterial capture on engineered surfaces containing immobilized nanoparticles[END_REF]. Observations by optical microscopy in a nely tuned tempera-ture range show thermo-sensitive behaviors around DNA melting temperature.

Here we propose a dierent approach for both surface functionalization and temperature-dependent study. It is characterized by high experimental accessibility and eciency, ideal for aordable devices in large scaled applications.

Grafted PLL-g-PNIPAM [START_REF] Luzinov | Responsive brush layers: from tailored gradients to reversibly assembled nanoparticles[END_REF] functionalizes reversibly T-responsive particles.

Employing dierent optical video microscopy methods we study pre-adsorption dynamics, adsorption and desorption kinetics for particle populations having dierent PLL-g-PNIPAM surface coverage. Unlike the classical approach [START_REF] Xu | Subdiusion of a sticky particle on a surface[END_REF] in which the cell is nely thermostated and the dynamics is recorded for each temperature separately, we adopt a "T-ramp" technique where the particle motion is recorded for an increasing (or decreasing) temperature at constant rate.

These results conrm and give strength to the "rolling-aging model" that we previously introduced in a recent paper [START_REF] Boniello | Rolling and aging in temperature-ramp soft adhesion[END_REF].

Materials and methods

Sample preparation. Polymer-coated particles are obtained following the protocol proposed in ref. [START_REF] Malinge | Tailored stimuli-responsive interaction between particles adjusted by straightforward adsorption of mixed layers of poly (lysine)-g-peg and poly (lysine)g-pnipam on anionic beads[END_REF].

Commercial silica beads (Bangs Laboratories, SS03N) with diameter of 0.96µm are dispersed in 1M sodium hydroxide (NaOH) solution and dialysed against water (Slide-A-Lyzer, M W cuto 3500 kDa, Thermo Scientic). The solution is diluted in 0.01 M Phosphate-buered Saline (PBS) solution. Particle are coated by mixing 22µL of colloidal suspension in PBS and 100µL of polymer solution (PLL-g-PEG + PLL-g-PNIPAM) at a concentration of 10g.L -1 . (PLL: M w = 15000-30000g.mol -1 , grafting density 20%; PEG: M w = 20000g.mol -1 , R g = 6.9 nm; PNIPAM: M w = 7000g.mol -1 , R g = 3.8 nm). Surface coating is simply achieved because of the electrostatic interaction between the anionic silica surface (after treatment in NaOH) and the cationic PLL backbone of the polymers [START_REF] Malinge | Tailored stimuli-responsive interaction between particles adjusted by straightforward adsorption of mixed layers of poly (lysine)-g-peg and poly (lysine)g-pnipam on anionic beads[END_REF]. Polymer chains adsorb on the particle surface up to saturation density. The surface coverage f % of the thermo-responsive PLL-g-PNIPAM (which, for the sake of this work, mediated the particle-substrate interaction) is controlled by the concentration of poly-mer solution. (100 -f )% of PLL-g-PEG is added to ensure steric repulsion and dilute active polymer. Such a nal suspension is incubated for 30 min.

Polymer excess is removed by 5 centrifugation cycles (1,800g for 5 min) replacing the supernatant by deionized water. DLS measurements [START_REF] Boniello | Reversible and dynamical control of aggregation and soft adhesion of t-responsive polymer-coated colloids[END_REF] conrmed that colloidal suspensions at any f are stable up to PNIPAM cloud temperature (T c = 32 M w = 15000 -30000g.mol -1 , grafting density 20%; PEG: M w = 20000g.mol -1 , R g = 6.9 nm, 1g.L -1 in 0.01M Phosphate-buered Saline solution) in the channel. After 30 min incubation the cell is rinsed with deionized water to remove polymer solution and polymer excess, and dried with compressed air. Zetapotential of PLL-g-PEG coated glass slides are of the order of -10mV [START_REF] Gon | Manipulating protein adsorption using a patchy protein-resistant brush[END_REF][START_REF] Kalasin | Near-surface motion and dynamic adhesion during silica microparticle capture on a polymer (solvated peg) brush via hydrogen bonding[END_REF].

A colloidal suspension of particle in PBS (∼ 20µL, particle concentration 0.1 We calculate:

G(t) = x,y I(t)I(t f ) x,y I 2 (t f ) (1)
where x,y stands for the sum of all the pixel values on the same resulting image. The correlation function increases when more particles achieve their nal position (Fig. 2b). Getting rid of small uctuation, G(t) give us the 115 cumulative distributions of adsorption times (Fig. 2c), i.e. the fraction P a (t) of particles attached at the substrate as a function of time [START_REF] Boniello | Rolling and aging in temperature-ramp soft adhesion[END_REF].

iii) Desorption kinetics. The same principle as above is used for the inverse experiments. Initial temperature is set at T i = 36 • C. As initial condition, all the particles in the frame of view are adsorbed on the substrate and have 120 as particles desorb from the substrate. The analysis is here limited to three signicant coatings: low (f = 2%), medium (f = 20%) and high (f = 90%) amounts of PNIPAM on the surface.

Experimental results

Pre-adsorption dynamics. Particle tracking techniques provide the full particle trajectory in the 3 directions (horizontal plane x, y and vertical particlesubstrate distance z) along all the acquisition in the temperature ramp. We already used these information, for instance, to track particle vertical position before adsorption [START_REF] Boniello | Rolling and aging in temperature-ramp soft adhesion[END_REF] or rocking dynamics after adhesion [START_REF] Boniello | Reversible and dynamical control of aggregation and soft adhesion of t-responsive polymer-coated colloids[END_REF]. Here we integrate previous results observing pre-adsorption dynamics in dierent temporal windows ∆t, but all ending at the adsorption time (t = t 0 ). Fig. 3 shows the Mean Square Displacements (MSDs) for a representative particle (f = 50%, in the middle of the experimental range of surface coverage). When the MSD is sampled over a long ∆t (∆t = 33.3s, green points) we observe a linear curve, characteristic of Brownian diusion. If we reduce the analysis to times closer to adsorption time (∆t = 16.7s, blue points) MSD is still linear, but with a lower slope (i.e., a lower diusion coecient). Diusion is hindered by the presence of a solid boundary [START_REF] Leach | Comparison of faxén's correction for a microsphere translating or rotating near a surface[END_REF], and by more frequent particle-substrate interactions.

Slopes decreases when we reduce ∆t (∆t = 10.0s, red points). In the very last few seconds before adhesion (∆t = 3.3s, black points) MSD shows a plateau. Adsorption and desorption kinetics. In our previously reported work we an-alyzed and modeled the curves of adsorbed particles in the full surface coverage range (2% < f < 100%) in a temperature ramp at a constant rate [START_REF] Boniello | Rolling and aging in temperature-ramp soft adhesion[END_REF]. We want now to measure the eect of the PNIPAM collapse phase transition kinetics on the adsorption and desorption dynamics within a given T-ramp. Namely, in our previous work, it was assumed that in all cases the PNIPAM conformational changes occur instantaneously in regard to rolling and sticking processes during the T-ramp. Here, we provide new insights on the eect of dierent ramps.

For the sake of discussion, we limit the analysis at two representative values of f (f = 90%, f = 2%) and two ramps: fast (10 • C/min) and slow (1 • C/min).

Results are reported in Fig. 4a and show dierent behaviors for the two surface coverage. At f = 90% we have a quick adsorption after PNIPAM transition, since sticky patches cover almost completely the colloid. Instantaneous adsorption is clearly observed with the fast ramp ( 10• C/min, black curve in Fig. 4a).

A slower ramp (1 • C/min, gray curve in Fig. 4a) doesn't change the interac- tion, but only smeared PNIPAM transition in a wider time frame. Full particle adsorption is delayed by approximately 1 minute when the sample temperature is increased of 1 • C. On the contrary, particle with lower surface coverage don't

show such a delay. The two curves at dierent temperature rates overlap within experimental error (dark and light red curves in Fig. 4a), indicating much slower smearing of PNIPAM transition temperature than in the case of high coverage f .

A decreasing T-ramp allows to explore desorption kinetics (Fig. 4b). We use three particle populations with dierent f : 90% (black points), 20% (light blue points), 2% (red points). The acquisition starts at T = 38 • C with all the particle adsorbed on the substrate (fraction of adsorbed particle P a (t 0 ) = 1)

and ends when all the accessible particles are released (P a (t f ) = 0). No relevant dierences are found between dierent polymer coverage. All the particle populations desorb with the same kinetics. This result tell us that desorption only depends on PNIPAM state: when the transition from collapsed to coil states occurs, colloids are not sticky anymore and leave the substrate. Adsorption was tuned by rolling of the particle, looking for sticky patches (for larger f , patched are more frequent and adhesion faster). In contrast, the fast and uniform release of particles suggests that rolling or rocking dynamics are not involved prior to desorption.

Discussion

Temperature-sensitive polymer (PLL-g-PNIPAM) induces reversible particle adsorption and desorption as a function of the sample temperature. The PLL-g-PNIPAM surface coverage f is able to tune adsortion kinetics (but not desorption one): larger is f , faster is the adhesion. We recently provided a model to describe this phenomenon and t experimental curves [START_REF] Boniello | Rolling and aging in temperature-ramp soft adhesion[END_REF]. The model accounts for rolling and aging eect in soft adhesion: at lower f the colloids spend more time in proximity of the substrate before nding a sticky polymer patch. We divide the particles into three populations, corresponding to the three stages of a particle's life. The rst is the population of untethered colloids, represented by the probability P b (t). The second population represents the rolling colloids, P r (t), while the third population of colloids is in arrest, P a (t). Thus, the rate equations are written:

Ṗb = -κP b + k o ρ(t/τ * )P r Ṗr = κP b -[k o ρ(t/τ * ) + ϕ(T ) k g(t/τ * )] P r P a = 1 -P b -P r , (2)
where κ is the free sedimentation rate and corresponds to the fraction of particles absorbed per unit time by a totally absorbing sink [START_REF] Brenner | The slow motion of a sphere through a viscous uid towards a 355 plane surface[END_REF]. The term k of f is the re-dispersion rate of rolling colloids, and k is the irreversible stopping rate of rolling colloids. The typical time scale for contact aging is τ * , and allows us to write aging functions ρ(x) = exp(-x) and g(x) = 1 -ρ(x). Initial conditions correspond to all the colloids dispersed in the bulk: P b (0) = 1, P r (0) = P a (0) = 0. The nal state is P a (t f ) = 1. See ref. [START_REF] Boniello | Rolling and aging in temperature-ramp soft adhesion[END_REF] for full details.

This translates in dierent adsorption proles for fast and slow T-ramps for f = 90% (Fig. 4a, black/gray lines).

Desorption doesn't show a dependence of PNIPAM coverage f on colloids surface. Rolling and soft contact are not involved, but particles are released when PNIPAM tethers become non-sticky (Fig. 4b). This depends on sample temperature, but not on tethers density. For desorption a simple rate equation describes the phenomenon at any f :

Ṗa = -κ T P a , (4)
where κ T is the re-dispersion rate dictated by tethers conditions.

Application: sorting in microuidic channel. Strong dependence of adsorption rate on f , reversibility of the eect in temperature, together with fast and f -independent desorption rate are precious ingredients for designing a separation device for patchy colloids.

The principle is depicted in Fig. 5: a suspension containing a mixture of colloids with dierent patchy patterns is injected at constant ow into a separation device containing PEG coated silica horizontal substrate, subject to sedimentation. At the beginning, the temperature is below T c . At some time, the temperature is rapidly raised above T c . Colloids with dierent patchiness will take dierent times to adsorb. After a sticky time τ s the temperature is dropped below T c . All particles are released and continue the ow during the free time τ f . The process is repeated as long as the device requires (depending on its length and the ow speed). The crucial point is that τ s and τ f can be tuned in order to get the best separation. For example, if we want to separate particles heaving f = 50% from the ones with f = 2%, the sticking time must be chosen in a way that most of 50s get stuck but most of 2s continue to ow.

The same separation technique can be used for resolving particles with same f , but with dierent patch patterns. For this purpose, we use the fact that particles mostly roll while they are sticky, and merely jump while they are free.

Fig. 6a-d illustrates an extreme case in which two particles with same coverage f but dierent coverage patterns (N random patches vs complete de-mixing, N=1). The trail (red) that the contact domain draws on the colloid surface is continuous in the case of rolling (crawling) (Fig. 6b-e) and is composed of discrete drops in the case of jumping (Fig. 6c-f ). It is well known that the rst passage times are radically dierent in the two cases [START_REF] Lee | Ligand-receptor interactions in chains of colloids: When reactions are limited by rotational diusion[END_REF][START_REF] Zhou | Brownian dynamics study of the inuences of electrostatic interaction and diusion on protein-protein association kinetics[END_REF]. In our separation device, we can tune between the two: if we want continuous trails to dominate, we will make longer τ s . On the contrary, if we want discontinuous searching, we prefer short τ s . The free time should be adjusted in both cases to allow most of particles to desorb, otherwise no material can be detected at the end.

Conclusions

The present work reports further experimental results on colloid-substrate adhesion for microparticles coated by a controlled amount f of temperatureresponsive polymer chains (PLL-g-PNIPAM). We used 3-D real time tracking within an ascending and descending temperature ramp around the polymer collapse temperature, triggering the stickiness of the particles. Soft adhesion is reversible and can be nely tuned by f . The combination of new observations (pre-adsorption diusion, adsorption smearing at dierent T-ramps, desorption) strengths our model, conrming that soft adhesion is controlled by rolling and aging eects. Furthermore, on the basis of our results, we propose a simple strategy for separation of colloids with soft sticky patches. Separation with respect to both the overall surface patch density and the patch pattern is discussed.

Figure 1 :

 1 Figure 1: Principles of PICOUEYE tracking analisys. (a) Typical image of a slightly-out-offocus particle, with characteristic interference rings. (b) Calibration prole giving interference rings width at dierent vertical positions (relative distance sample-objective). Red line highlights vertical position corresponds to picture in (a).

Figure 2 :

 2 Figure 2: Adsorption kinetics tracking. (a) Extract of video tracking under optical microscopy before (left) and after (right) adsorption. Trajectories in (x,y) plane are recorded. (b) Detail of the algorithm that, among all the particles present in the visible frame (150×120px 2 ), identies only the ones adsorbed (white dots) at each time t via image correlation. (c) Intensity prole (blue line) vs. time extracted from pictures as in (b), normalized by intensity at the end of the acquisition. G(t) (Eq. 1 estimates the fraction Pa(t) of adsorbed particles with respect to the total number of tracked ones. Light blue region indicates a condence interval for the measurement.

Figure 3 :

 3 Figure 3: MSDs of a PLL-g-PNIPAM coated particle (f = 50%) before adsorption (at time t = t 0 ). Trajectories are extracted in dierent time windows ∆t (∆t = 3.3, 10, 16.7, 33.3 s), all ending at t 0 .

Figure 4 :

 4 Figure 4: (a) Fraction of adsorbed particles as a function of time in increasing T-ramps at dierent rates, 10 • C/min (dark colors), 1 • C/min (light colors), for two representative surface coverage f : 90% (black/gray) and 2% (red). (b) Fraction of adsorbed particle as a function of time in decreasing T-ramp (rate 10 • C/min) for three particle populations: high (f = 90%, black), medium (f = 20%, light blue) and low (f = 2%, red) surface coverage.

  The results in this work go in the same direction. They conrm previous observation and strengthen the rolling-aging model. The model has been designed under the assumption of an instantaneous trigger of PNIPAM repulsion/attraction at the PNIPAM cloud temperature (T c = 32 • C). It can be accounted by a step function for PNIPAM response: ϕ(T ) = 0 for T < T c , and ϕ(T ) = 1 for T > T c . This approximation works when the temperature transition around T c is much faster than adsorption (temperature transition time t T emp τ * ), i.e. for slow adsorption (low f ) or fast T-ramp (10 • C/min). As a result, adsorption proles at f = 2% are independent of T-ramp (Fig. 4a, red lines). Otherwise, when t T emp ∼ τ * , the response function becomes smeared:

Figure 5 :Figure 6 :

 56 Figure 5: Application of soft adhesion in particle sorting/separation. (a) The separation device is a microuidic channel with horizontal PEG-coated glass substrate, placed in a temperaturecontrolled environment, rapidly tunable and programmable. The ow v 0 is controlled and kept constant during separation. (b) The separation procedure is achieved by a sequence of T-jump/T-drop cycles with tunable sticky time τs and free time τ f .

Acknowledgments

This work was supported by ANR DAPPlePur 13-BS08-0001-01 and program "investissement d'avenir" ANR-11-LABX-0011-01.


Jean-Fabien Petit

Umit B Demirci
email: umit.demirci@umontpellier.fr

Discrepancy in the thermal decomposition/dehydrogenation of ammonia borane screened by thermogravimetric analysis

Keywords: Ammonia borane, Hydrogen storage, Thermal decomposition, Thermogravimetric analysis, Thermolysis

Ammonia borane NH 3 BH 3 (AB), in pristine or destabilized state, has been much investigated as a solid-state chemical hydrogen storage material. The potential of such compound for thermolytic dehydrogenation (and decomposition) can be fast and reliably screened and assessed by thermogravimetric (TG) analysis via descriptors like both the onset temperature of dehydrogenation and the overall weight loss over the temperature range 100-200°C. However, comparisons to results reported in the open literature need to be done with caution. The objective of the present article is thus to show that the thermolytic properties of AB determined by TG analysis are much dependent on the operation conditions. We have observed that much different results are obtained by changing the analyzer, and especially the crucible (comparable to a semi-closed reactor in one case or similar to an open reactor in the second case). For example, a stable AB sample is able to suffer a weight loss of 18.4 wt% starting from 116.5°C when a semiclosed crucible is used, versus a weight loss of 51.9 wt% starting from 91°C with the utilization of an open crucible. Our main results are reported herein in the form of a short technical communication: it aims at sharing our experimental observations in order to avoid misinterpretation and misunderstanding as for the TG analyses of AB and of any kind of derivatives. The present work should help in comparing the data available in the open literature with care and, from now on, should be regarded as a

Introduction

Ammonia borane (or borazane, NH 3 BH 3 ; denoted AB) is a key material in the field of solidstate chemical hydrogen storage owing to its solid state at ambient conditions, stability under inert atmosphere, high hydrogen density (19.5 wt% H and 146 g H L -1 ) and ability to dehydrogenate/decompose from about 100°C [START_REF] Zhang | Progress and thermolytic dehydrogenation of ammonia borane for mobile applications[END_REF][START_REF] Li | Ammonia borane, past as prolog[END_REF][START_REF] Akbayrak | Ammonia borane as hydrogen storage materials[END_REF]. It is however thermodynamically unrealistic to target the recovery of all of the hydrogen (because of the formation of the highly stable boron nitride [START_REF] Frueh | Pyrolytic decomposition of ammonia borane to boron nitride[END_REF]). In ideal thermolytic conditions it may be expected an effective gravimetric hydrogen storage capacity of at best 13 wt% (corresponding to 4 out of 6 H of NH 3 BH 3 ) [START_REF] Al-Kukhun | Mechanistic studies of ammonia borane dehydrogenation[END_REF][START_REF] Demirci | Ammonia borane, a material with exceptional properties for chemical hydrogen storage[END_REF].

Under heating, the transformation of pristine AB starts with melting occurring between 97 and 119°C:

NH 3 BH 3 (s) → NH 3 BH 3 (l) (1)
The discrepancy in the melting temperature is due to different factors like the heating rate (1-10°C min -1 ), the sample purity and the pre-dimerization of the borane [START_REF] Zhang | Progress and thermolytic dehydrogenation of ammonia borane for mobile applications[END_REF]. The melting is concomitant with the first decomposition [7,[START_REF] Baitalow | Thermal decomposition of ammonia-borane under pressures up to 600 bar[END_REF] whose weight loss measured by thermogravimetric (TG) analysis is roughly comparable to 6.5 wt% [START_REF] Petit | Ammonia borane H 3 N-BH 3 for solid-state chemical hydrogen storage: Different samples with different thermal behaviors[END_REF][START_REF] Neiner | Promotion of hydrogen release from ammonia borane with mechanically activated hexagonal boron nitride[END_REF], suggesting that AB mainly dehydrogenates:

NH 3 BH 3 (l) → 1/n [-NH 2 BH 2 -] n (s) + H 2 (g) (2)
Decomposition into e.g. aminoborane NH 2 BH 2 , ammonia NH 3 and diborane B 2 H 6 also occurs in some extent [START_REF] Jepsen | Investigations of the thermal decomposition of MBH4-2NH3BH3, M = Na, K[END_REF]. From about 130°C (and up to about 200°C), the second decomposition takes place, being characterized by an important weight loss (>20 wt%) [START_REF] Geanangel | A TG-DSC study of the thermal dissociation of (NH 2 BH 2 ) x[END_REF]. AB decomposes into the undesired gaseous products NH 3 , B 2 H 6 and borazine B 3 N 3 H 6 [START_REF] Wolf | Calorimetric process monitoring of thermal decomposition of B-N-H compounds[END_REF][START_REF] Biliškov | High influence of potassium bromide on thermal decomposition of ammonia borane[END_REF].

Dehydrogenation also takes place such as [7]:

[-NH 2 BH 2 -] n → [-NHBH-] n (s) + H 2 (g) (3)
Such thermolytic properties do not match the requirements for solid-state chemical hydrogen storage in terms of hydrogen purity and operation temperatures. Thereupon, innovative strategies (i.e. homogeneous catalysis [START_REF] Erickson | Dehydrocoupling of amine boranes via tin(IV) and tin(II) catalysts[END_REF][START_REF] Glüer | Highly active iron catalyst for ammonia borane dehydrocoupling at room temperature[END_REF], doping [START_REF] Zhang | Thermal decomposition of alkaline-earth metal hydride and ammonia borane composites[END_REF][START_REF] Ploszajski | Freeze-dried ammonia borane-polyethylene oxide composites: phase behaviour and hydrogen release[END_REF], nanoconfinement [START_REF] Moussa | Room-temperature hydrogen release from activated carbon-confined ammonia borane[END_REF][START_REF] Diaz | Ammonia borane based nanocomposites as solid-state hydrogen stores for portable power applications[END_REF],

nanosizing [START_REF] Lai | Nanosizing ammonia borane with nickel: a path toward the direct hydrogen release and uptake of B-N-H systems[END_REF][START_REF] Lai | Nanosizing ammonia borane with nickel -An all-solid and all-in-one approach for H 2 generation by hydrolysis[END_REF], and chemical modification [START_REF] Nakagawa | Synthesis, structural characterization, and hydrogen desorption properties of Na[Al(NH 2 BH 3 ) 4[END_REF][START_REF] Møller | Synthesis and thermal decomposition of potassium tetraamidoboranealuminate, K[Al(NH 2 BH 3 ) 4[END_REF]) have been developed to mitigate, even suppress, the decomposition of AB and concomitantly to improve the dehydrogenation properties. The benefits of such destabilization strategies can be evaluated by several techniques, including TG analysis.

Thus far, we have studied four of the aforementioned strategies and a number of AB-based materials and derivatives had been prepared. In our laboratory the thermolytic properties of each any AB-based material is systematically evaluated by TG analysis, which is a simple and effective way for screening fast and reliably the aforementioned compounds. Despite the complexity of AB thermal decomposition [START_REF] Al-Kukhun | Mechanistic studies of ammonia borane dehydrogenation[END_REF], the TG results allow to select or discard the screened materials on the basis of the onset temperature of decomposition and the weight losses occurring between 80 and 200°C (Figure 1). However, comparisons to results

reported in the open literature need to be considered with caution. Indeed, in the present work, it is reported that much different results can be collected by changing the analyzer and the crucible (comparable to a semi-closed reactor or to an open reactor). For a same AB sample it is for instance possible to see much different weight losses at 200°C (differential of more than 30 wt%) and incomparable onset temperatures of dehydrogenation (e.g. 91°C vs.

116.5°C). This is a critical situation, and to our knowledge, it has not been reported and discussed before. With the present technical communication we aim at sharing our experimental observations about the aforementioned discrepancies, that is, about the differences observed in the thermolytic dehydrogenation properties of AB by just changing the TG analyzer. Such a report should be of importance to avoid, in the future, misinterpretation and misunderstanding as for the TG analyses of AB and of any kind of AB derivatives.

Materials and methods

Four different samples of AB were synthesized. The samples denoted 1, 2 and 3 were prepared by metathesis-dehydrogenation using borohydride and ammonium salts (all from Sigma-Aldrich) and an anhydrous organic solvent. The reactions were performed at 35°C for The screening of the different AB samples (i.e. thermal stability and thermogravimetric features) was done by using TG analysis. In our laboratory, a Texas Instruments Q500 analyzer (denoted A), located in a room, is available. It works under nitrogen flow (50 mL min -1 ). We use aluminum crucibles because they can be sealed in the argon-filled glove box.

Typically, a crucible and a lid are put in the glove box to be loaded with the AB sample to be analyzed. The crucible is loaded with a maximum of 2-3 mg (because of the foaming of the borane under heating) of AB, is sealed and is taken out the glove box. Just before starting the TG analysis, the lid is pierced with a needle to allow the evolving gas to escape through A TG analysis of AB can be described through five descriptors (Figure 1): the onset temperature of the first decomposition (T 1 ), the weight loss for the first decomposition (∆m 1 ), the onset temperature of the second decomposition (T 2 ), the weight loss for the second decomposition (∆m 2 ), and the overall weight loss at 200°C (∆m). Herein, for clarity, the TG curves will be discussed of terms in T 1 and ∆m mainly.

Results and discussion

The first reports about the thermal stability of pristine AB studied by TG analysis point out the effect of the heating rate on the results [START_REF] Wolf | Calorimetric process monitoring of thermal decomposition of B-N-H compounds[END_REF][START_REF] Sit | The thermal dissociation of NH 3 BH 3[END_REF][START_REF] Baitalow | Thermal decomposition of B-N-H compounds investigated by using combined thermoanalytical methods[END_REF][START_REF] Baumann | Thermal decomposition of polymeric aminoborane (H 2 BNH 2 ) x under hydrogen release[END_REF]. Generally, T1 and T2 increase when the heating rate is increased (e.g. from 1 to 5°C min -1 [START_REF] Baitalow | Thermal decomposition of B-N-H compounds investigated by using combined thermoanalytical methods[END_REF]). Such a change may be explained by the low decomposition kinetics of AB [START_REF] He | Nanosized Co-and Nicatalyzed ammonia borane for hydrogen storage[END_REF][START_REF] Miranda | Ab initio investigation of ammonia-borane complexes for hydrogen storage[END_REF]. Also, with rising heating rate, the overall weight loss ∆m generally increases. Weismiller et al. [START_REF] Weismiller | Confined rapid thermolysis of ammonia borane[END_REF] have plotted ∆m as a function of the heating rate (range 2 to 50°C min -1 ) and have found a nearly logarithmic evolution.

According to Baumann et al. [START_REF] Baumann | Thermal decomposition of polymeric aminoborane (H 2 BNH 2 ) x under hydrogen release[END_REF], the amount of evolved H 2 is independent of the heating rate whereas the amount of the volatile by-products strongly increases. This is confirmed by Weismiller et al. [START_REF] Weismiller | Confined rapid thermolysis of ammonia borane[END_REF]. A similar conclusion comes from Hélary et al. who, besides, have suggested variations in the degradation pathways according to the heating rate [START_REF] Hélary | A physicochemical study of an NH 3 BH 3 -based reactive composition for hydrogen generation[END_REF].

The impact of the heating rate (1 vs. 5°C min -1 ) is verified in our experimental conditions. For that the analyzer A was used. The TG curves for our four samples are shown in Figure 2.

On the one hand, we confirm that the increase of the heating rate results in a shift of T 1 (as well as of T 2 ) to higher values. With 1, T 1 is equal to 107°C at 1°C min -1 while it is 116.5°C at 5°C min -1 (∆T 1 of 9.5°C) The difference is even more important for 3 and 4 as T 1 increases by 16 and 21°C respectively. On the other hand, the weight losses are impacted by the change of the heating rate. The decomposition is favored with the increase of the heating rate. For example, ∆m for the sample 1 is about 18 wt% at 1°C min -1 and 42 wt% at 5°C min -1 . For 2, 3 and 4, the weight losses at 1 and 5°C min -1 are as follows: 31.9 and 48.3 wt%; 14.8 and 19.3 wt%; 20.3 and 39.8 wt%. This is in agreement with the higher decomposition extent (i.e. more of the unwanted by-product borazine) as reported elsewhere [START_REF] Wolf | Calorimetric process monitoring of thermal decomposition of B-N-H compounds[END_REF][START_REF] Baitalow | Thermal decomposition of B-N-H compounds investigated by using combined thermoanalytical methods[END_REF].

We elsewhere have discussed the interest of investigating various AB samples [START_REF] Petit | Ammonia borane H 3 N-BH 3 for solid-state chemical hydrogen storage: Different samples with different thermal behaviors[END_REF]. Our objective was to find the suitable synthesis method for getting pure AB with improved decomposition properties (i.e. showing lower T 1 ). The sample 1 has for example been found to be the most stable AB, with T 1 as high as 116.5°C (at 5°C min -1 ). It has been directly compared to 2, considered as the least stable pure AB (T 1 = 104°C). With respect to 3, it has shown the lowest T 1 (93.5°C) because of traces of impurities (LiCl). With respect to 4, it has been found to be less stable (T 1 = 98°C) than 1, which is explained by differences at the molecular scale. For more details about the purity of AB and its impact on the TG results, the reader is invited to scrutinize ref. [START_REF] Petit | Ammonia borane H 3 N-BH 3 for solid-state chemical hydrogen storage: Different samples with different thermal behaviors[END_REF] where in fact twelve different AB samples have been compared. In the present work, 1, 2, 3 and 4 are further studied but with another TG analyzer, the apparatus B. Our objective is initially to confirm the differences between the samples we observed with the apparatus A. Two major observations stand out (Figure 3). At first sight, the TG curves obtained with the analyzer B and those collected with the other analyzer are similar in their allure. However a further exploration shows that the curves cannot be superimposed: there are three main differences. First, the TG curves obtained with B confirm that the most stable AB is still 1 (T 1 = 91°C).

The other samples are less stable. For example, T 1 for 3 is about 65°C. In other words, the Second, the temperatures T 1 are significantly decreased when the analyzer B is considered.

In comparison to the data collected with the analyzer A, there is a gap of about 15, 20, 27 and 24°C for 1, 2, 3 and 4 respectively. With 3 and 4, T 1 is even slightly lower than 65°C.

Such onset temperatures are comparable to those found with a destabilized AB, that is, AB doped with a metal chloride [START_REF] Nakagawa | A systematic study of the effects of metal chloride additives on H 2 desorption properties of ammonia borane[END_REF] or for an AB nanoconfined into the open porosity of mesoporous silica [START_REF] Sullivan | Towards an understanding of the beneficial effect of mesoporous materials on the dehydrogenation characteristics of NH 3 BH 3[END_REF]. This observation may challenge the reliability and relevancy of crosscomparisons of TG results.

Third, the weight losses are affected (Table 1). With the analyzers A and B, ∆m is found to be respectively 18.4 and 51.9 wt% for 1, 31.9 and 66.1 wt% for 2, 14.8 and 63 wt% for 3, and 20.3 and 62.6 wt% for 4. The weight losses are clearly much higher with B. This demonstrates that the decomposition extent of an AB sample is strongly dependent on the analyzer and that TG results obtained with different systems must be compared with care.

Table 1. Weight losses found at 200°C for the samples 1, 2, 3 and 4 with the analyzers A and B (based on the curves in Figure 3). The weight losses are given in wt% as well as in equivalent of H2 (such as 13 wt% = 2 equivalent of H2).

Weight loss in wt% / Weight loss in equiv. H2 At the scale of our experiments using two different TG analyzers, the crucible can be seen as the main difference. With A, the aluminum crucible is sealed and the gas evacuation is done through a pinhole (ø < 1 mm) made to the lid. With B, the alumina crucible is not sealed. The former may thus be seen as a semi-closed "reactor" and the latter may be an open "reactor", which makes a difference.

With the open crucible of B, the evacuation of the forming gases is favored because the TG flowing gas leaches the surface of the sample. The gases have a short residence time in the crucible "cloud". Hence, a dehydrogenation by-product like NH 2 BH 2 and a decomposition byproduct like NH 3 or B 2 H 6 , both forming during the first weight loss, are favorably passed into the TG exhaust (Figure 4). This is also true for the trimeric gaseous by-product B 3 N 3 H 6 mainly forming during the second weight loss. In contrast, with the semi-closed crucible of A, the evacuation of the forming gases is more difficult. Their residence time in the crucible "cloud" is much longer than for the previous analyzer. During the first weight loss, equilibria between the gaseous by-products (mainly NH 2 BH 2 , NH 3 , B 2 H 6 ) and the decomposing AB are likely to take place (Figure 4). The former gases may then be involved in further reactions with the remaining solid towards the formation polymeric solid residues [START_REF] Al-Kukhun | Mechanistic studies of ammonia borane dehydrogenation[END_REF]. This is also true for B 3 N 3 H 6 , mainly evolving during the second weight loss, a fraction of it being then able to polymerize into polyborazylene (a solid thermolytic residue).

In our conditions, the use of both types of crucibles has led to significantly different TG profiles despite the use of the same AB samples. Though TG analysis is an easy-to-use and fast tool for screening chemical hydrogen storage materials, care must be taken with the collected data. Direct comparisons of TG results are possible and reliable if, and only if, the conditions of the TG experiments are equivalent. Otherwise any attempt to make direct comparison will result in misinterpretations and erroneous conclusions. For example, the low weight loss recorded for 1 (18.4 wt%) with the analyzer A could let us imagine that the sample would be suitable for boron nitride formation with high ceramic yield (<81%).

However when the results obtained with B are considered it is obvious that the ceramic yield will be much lower (<48%). One way to think about this is that the semi-closed "reactor" would allow higher ceramic yields.

In the field dedicated to thermolytic decomposition of AB, the present findings add complexity to the interpretations of the reported results. Up to now, the complexity lied in the heating rate of the TG experiment [START_REF] Wolf | Calorimetric process monitoring of thermal decomposition of B-N-H compounds[END_REF][START_REF] Sit | The thermal dissociation of NH 3 BH 3[END_REF][START_REF] Baitalow | Thermal decomposition of B-N-H compounds investigated by using combined thermoanalytical methods[END_REF][START_REF] Baumann | Thermal decomposition of polymeric aminoborane (H 2 BNH 2 ) x under hydrogen release[END_REF][START_REF] He | Nanosized Co-and Nicatalyzed ammonia borane for hydrogen storage[END_REF][START_REF] Miranda | Ab initio investigation of ammonia-borane complexes for hydrogen storage[END_REF][START_REF] Weismiller | Confined rapid thermolysis of ammonia borane[END_REF][START_REF] Hélary | A physicochemical study of an NH 3 BH 3 -based reactive composition for hydrogen generation[END_REF] and/or in the purity (molecular composition) of the AB sample. For example, the ionic dimer of AB, i.e. diammoniate of diborane [(NH 3 ) 2 BH 2 ] + [BH 4 ] -, that may form during the synthesis of AB is known to be less stable than AB and to release much less volatile by-products [START_REF] Fang | Facile solid-phase synthesis of the diammoniate of diborane and its thermal decomposition behavior[END_REF]. A mixture of AB and DADB has then a different thermal behavior in comparison to pure AB [START_REF] Petit | 11 B MAS NMR study of the thermolytic dehydrocoupling of two ammonia boranes upon the release of one equivalent of H 2 at isothermal conditions[END_REF]. From now, the aforementioned complexity lies, also, in the operating/use conditions of TG analysis.

Conclusion

For screening different AB samples as well as any chemical hydrogen storage materials, TG analysis is an easy-to-use, efficient and reliable tool. However, the results are much dependent on the operating conditions. In the present work we have shown that significantly different TG profiles can be obtained for a given AB sample by changing the analyzer and the way of preparing the crucible. For example, an AB sample, considered as being stable, is able to suffer a weight loss of 18.4 wt% starting from 116.5°C with one TG analyzer, versus a weight loss of 51.9 wt% starting from 91°C with another analyzer. On the one hand (for the former analyzer), a sealed aluminum crucible (with a pinhole) has been used; it may be compared to a semi-closed "reactor". On the other hand (for the latter analyzer), an alumina crucible (not sealed) has been handled; it may be compared to an open "reactor". This makes a difference that makes big differences in TG results. We have thus shown, for the first time, that one given AB sample is able to show much different thermolytic behaviors depending on the TG analyzer and the crucible that is used. In consequence, care must be taken when TG results in the open literature are compared and when comparative discussions are done about both AB stability and thermolytic dehydrogenation. This also applies to any boron-and/or nitrogen-based materials investigated for chemical hydrogen storage. Direct comparisons can be possible if the operating conditions are much comparable. This is the best approach to avoid misinterpretations and erroneous conclusions.

Figure 1 .

 1 Figure 1. Typical TG analysis curve for AB: focus on the descriptors T1 (the onset temperature of the first main decomposition), ∆m1 (the weight loss of the first main decomposition), T2 (the onset temperature of the second main decomposition), ∆m2 (the weight loss of the second main decomposition) and ∆m (the overall weight loss from 80 to 200°C).

2 days. For 1

 1 (purity ≥99%), lithium borohydride LiBH 4 , ammonium carbonate (NH 4 ) 2 CO 3 and 1,4-dioxane (Sigma-Aldrich) were used. For 2 (purity ≥99%), sodium borohydride NaBH 4 , (NH 4 ) 2 CO 3 and tetrahydrofuran (Acros) were used. For 3 (purity ≥97%), LiBH 4 , ammonium chloride NH 4 Cl and 1,4-dioxane were used. The sample denoted 4 (purity 100%) was prepared by base displacement reaction involving dimethylsulfide borane (CH 3 ) 2 S⋅BH 3 (2 M solution in toluene; Sigma-Aldrich), ammonia NH 3 (Alpha Gaz) and anhydrous toluene (Acros). The bubbling of NH 3 in the borane solution was performed at 0°C for 15 min. Details about the syntheses, the samples purity and the respective characterizations are available in reference [9]. The samples were stored in an argon-filled glove box (MBraun M200B, O 2 <0.1 ppm, H 2 O <0.1 ppm).

  the as-obtained pinhole. The experiments are carried out from 25 to 200°C at a heating rate of 1 or 5°C min -1 . During the progress of our work, we had an access to another TG analyzer, a Rigaku thermoplus TG 8120 (denoted B). It is located in an argon-filled glove-box (MIWA MDB-1B, O 2 <0.1 ppm, H 2 O <0.1 ppm). It works under a helium flow (50 mL min -1 ).With this apparatus, we used an alumina crucible that is unnecessary to seal. The sample mass was again limited to 2-3 mg. All of the steps for an experiment with this analyzer are realized in the glove box. The experiments are carried out from 25 to 200°C at a heating rate of 1°C min -1 .

Figure 2 .

 2 Figure 2. TG curves of the AB samples 1, 2, 3 and 4 studied with the analyzer A at two heating rates: 1 and 5°C min -1 .

Figure 3 .

 3 Figure 3. TG curves of the AB samples 1, 2, 3 and 4 studied with the analyzers A and B at 1°C min -1 .

Figure 4 .

 4 Figure 4. Schematic representation of a so-called (a) open "reactor" and of a (b) semi-closed "reactor".

Acknowledgments

The authors acknowledge SAFRAN Herakles (Vert-Le-Petit, France) for financial support and the CNRS for administrative support (project n°073852). The authors thank Prof. Takayuki Ichikawa (Hiroshima University) for giving access to the TG analyzer B.


Umit Pierre Tignol

Umit B Demirci
email: umit.demirci@umontpellier.fr

Nickel-based catalysts for hydrogen evolution by hydrolysis of sodium borohydride: from structured nickel hydrazine nitrate complexes to reduced counterparts

Keywords: Hydrogen storage, Hydrolysis, Nickel, Mesostructure, Sodium borohydride

est destinée au dépôt

Introduction

Hydrolysis of sodium borohydride NaBH 4 is a simple approach for the generation of high amounts of hydrogen H 2 at 10-80°C and under atmospheric pressure [START_REF] Demirci | About the technological readiness of the H 2 generation by hydrolysis of B(-N)-H compounds[END_REF]:

NaBH 4 (aq) + 4H 2 O (l) → NaB(OH) 4 (aq) + 4H 2 (g)

The attractive features of the reaction have been widely discussed in the open literature within the past two decades. One may cite three of them: half of the generated H 2 comes from water H 2 O [START_REF] Schlesinger | Sodium borohydride, its hydrolysis and its use as a reducing agent and in the generation of hydrogen[END_REF]; the couple NaBH 4 -H 2 O is able to achieve higher effective gravimetric hydrogen storage capacities (e.g. 6.5 wt%) than the other material-based solutions investigated so far for chemical H storage [START_REF] Gislon | Hydrogen production from solid sodium borohydride[END_REF]; and, the H 2 generation rates are readily tuned by changing the catalyst and/or the operating conditions [START_REF] Brack | Heterogeneous and homogenous catalysts for hydrogen generation by hydrolysis of aqueous sodium borohydride (NaBH 4 ) solutions[END_REF]. This however should not hide the fact that the reaction has also drawbacks, the main three being: the storage irreversibility implying a difficult regeneration of the spent fuel NaB(OH) 4 [START_REF] Liu | The concept about the regeneration of spent borohydrides and used catalysts from green electricity[END_REF]; the recurring catalyst deactivation [START_REF] Arzac | Deactivation, reactivation and memory effect on Co-B catalyst for sodium borohydride hydrolysis operating in high conversion conditions[END_REF]; and the presence of borates in the H 2 stream [START_REF] Petit | By-product carrying humidified hydrogen: an underestimated issue in the field of hydrolysis of sodium borohydride NaBH 4[END_REF]. Nonetheless hydrolysis of NaBH 4 has shown to still have real industrial and application prospects [START_REF] Lapeña-Rey | A fuel cell powered unmanned aerial vehicle for low altitude surveillance missions[END_REF].

Accelerating the H 2 generation has been one of the most investigated aspects. Numerous homogenous and heterogeneous accelerators have been considered so far but the most prominent ones have been non-noble metal-based heterogeneous catalysts [START_REF] Brack | Heterogeneous and homogenous catalysts for hydrogen generation by hydrolysis of aqueous sodium borohydride (NaBH 4 ) solutions[END_REF]. Cobalt is the leading catalytic metal. It is a promising option owing to low cost and high catalytic activity [START_REF] Patel | Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: a review and the perspectives to substitute noble metals[END_REF]. Yet its development is somehow hindered by the fact that it faces a recurring deactivation problem [START_REF] Arzac | Deactivation, reactivation and memory effect on Co-B catalyst for sodium borohydride hydrolysis operating in high conversion conditions[END_REF]. Another potential catalytic metal is nickel. The first works date back the first half of the 2000s. In 2003, Hua et al. reported a nickel boride Ni x B catalyst being active for the release of H 2 at a rate of about 240 mL(H 2 ) min -1 g NixB -1 [START_REF] Hua | Hydrogen production from catalytic hydrolysis of sodium borohydride solution using nickel boride catalyst[END_REF]. In 2004, Kim et al. described pasted filamentary nickel as an active catalyst that however gradually loses activity with cycling [START_REF] Kim | Study on degradation of filamentary Ni catalyst on hydrolysis of sodium borohydride[END_REF]. The catalytic activity degradation was explained by an agglomeration of the catalyst as well as the formation of a film on the pasted catalyst. The film was found to be mainly composed of borate species. Thereafter, a number of alternative nickel-based catalysts emerged. Examples are as follows: nickel (II) salts [START_REF] Cento | Hydrogen generation by hydrolysis of NaBH 4[END_REF]; nickel nanoclusters [START_REF] Metin | Hydrogen generation from the hydrolysis of sodium borohydride by using water dispersible, hydrogenphosphate-stabilized nickel(0) nanoclusters as catalyst[END_REF]; bimetallic catalysts like NiRu [START_REF] Ferreira | Alkali free hydrolysis of sodium borohydride for hydrogen generation under pressure[END_REF]; trimetallic nanoparticles like NiAuCo [START_REF] Jiao | Synthesis of Ni/Au/Co trimetallic nanoparticles and their catalytic activity for hydrogen generation from alkaline sodium borohydride aqueous solution[END_REF]; nanocomposites with polymers [START_REF] Cai | Robust nickel-polymer nanocomposite particles for hydrogen generation from sodium borohydride[END_REF] or silica [START_REF] Umegaki | Fabrication of hollow silica-nickel particles for the hydrolytic dehydrogenation of ammonia borane using rape pollen templates[END_REF]; and nickel nanoparticles supported on Fe 3 O 4 [START_REF] Miao | Nickel-decorated Fe 3 O 4 nanoparticles as recyclable magnetic self-stirring nanocatalysts for microreactions[END_REF] or on reduced graphene oxide-branched polyethyleneimine aerogel [START_REF] Nabid | One pot synthesis of nickel nanoparticles stabilized on rGO/polyethyleneimine aerogel for the catalytic hydrogen generation[END_REF]. Recently new materials have been suggested as potential nickel-based catalysts. Kılınç et al. focused on coordination chemistry to propose a complex based on the interaction of the empty orbital of a nickel cation and the lone pair electrons of ligands with nitrogen and oxygen atoms [START_REF] Kılınç | Investigation on salisylaldimine-Ni complex catalyst as an alternative to increasing the performance of catalytic hydrolysis of sodium borohydride[END_REF]. The nickel-5-amino-2,4dichlorophenol-3,5-di-tert-butylsalisylaldehyde complex was prepared and used as catalysts in hydrolysis of NaBH 4 . It was found to accelerate the reaction and then to retain 72% of its initial activity after a sixth use. Better performance in terms of activity and stability was afterwards achieved when the complex was supported on alumina [START_REF] Kılınç | Salisylaldimine-Ni complex supported on Al 2 O 3 : highly efficient catalyst for hydrogen production from hydrolysis of sodium borohydride[END_REF].

Recently, we focused on the nickel hydrazine nitrate complex [Ni(N 2 H 4 ) 3 ][NO 3 ] 2 (Figure S1).

It is a coordination compound where the hexacoordinated metallic center is complexed by three N 2 H 4 bidentate ligands. The overall charge of [Ni(N 2 H 4 ) 3 ] 2+ is neutralized by two NO 3 anions. Since 1997, it has been mainly considered for its energetic properties [START_REF] Zhu | Evaluation of a new primary explosive: nickel hydrazine nitrate (NHN) complex[END_REF][START_REF] Hariharanath | Detonator using nickel hydrazine nitrate as primary explosive[END_REF][START_REF] Wojewódka | Hydrazine complexes of transition metals as perspective explosives[END_REF]. In the present work, the complex was considered as a potential catalyst for hydrolysis of NaBH 4 . We had three scientific objectives: (i) synthesizing morphology-controlled complexes via a simple experimental procedure and studying the effect of the morphology on the catalytic activity;

(ii) investigating the evolution of the morphology as well as that of the chemical state of nickel after the hydrolysis reaction, knowing that the reaction takes place in a reducing medium; (iii) and assessing the potential of such catalysts in terms of activity and stability (i.e. catalyst reusability) over cycles. Herein are presented our main results.

Experimental

The morphology-controlled nickel hydrazine nitrate complexes were prepared according to a procedure described elsewhere [START_REF] Wang | A facile sonochemical route to morphology controlled nickel complex mesostructures[END_REF]. Three different morphologies were targeted (Table S1).

Hexagonal plate-like morphology (denoted hNHN) was obtained at room conditions and as follows. In a 250-mL beaker, 0.

Results and discussion

Characterization of the complexes

The nickel hydrazine nitrate (denoted also NHN) complexes were targeted in three morphologies: hexagonal plates, clew-like objects, and discs. They are denoted hNHN, cNHN and dNHN respectively. The successful synthesis of each of these morphologies was first visually verified by optical microscopy (Figure S3). The presence of hexagonal plates and discs are confirmed for hNHN and dNHN. Spherical shapes similar to clew-like morphologies can be seen for cNHN. A precise observation was performed by SEM (Figure 1). hNHN mainly consists of hexagonal plate-like objects of about 3 µm. Clew-like morphologies of equivalent size are also seen. With cNHN, shapes that can be compared to clews were observed. They are big, with sizes of 6-7 µm. However there are other objects of smaller size and of different shapes. The SEM image of dNHN shows a binary mixture of discs and clew-like morphologies. The mixture is homogeneous in terms of size, with an average of 2 µm. Our observations are in quite good agreement with those reported in the seminal work [START_REF] Wang | A facile sonochemical route to morphology controlled nickel complex mesostructures[END_REF].

The samples hNHN, cNHN and dNHN were analyzed by FTIR spectroscopy (Figure 2). The spectra are identical for the three morphologies. They show bands in the stretching and deformation regions of both of the N-H and N-O bonds [START_REF] Wojewódka | Hydrazine complexes of transition metals as perspective explosives[END_REF][START_REF] Miller | Infrared spectra and characteristic frequencies of inorganic ions[END_REF][START_REF] Patil | Synthesis and characterisation of metal hydrazine nitrate, azide and perchlorate complexes[END_REF][START_REF] Chhabra | Synthesis, characterization and thermal studies of (Ni/Co) metal salts of hydrazine: potential initiatory compounds[END_REF]. The spectra were also compared to the spectrum of another nickel hydrazine complex, i.e. [Ni(N 2 H 4 ) 3 ][Cl] 2 [START_REF] Park | Preparation of fine Ni powders from nickel hydrazine complex[END_REF].

Parallels were found for the N-H and N-N vibrational bands. Our spectroscopic observations are in accordance with the formation of the complex

[Ni(N 2 H 4 ) 3 ][NO 3 ] 2 .
The XRD patterns (Figure S4) are identical for hNHN, cNHN and dNHN. They match the pattern of [Ni(N 2 H 4 ) 3 ][NO 3 ] 2 reported elsewhere [START_REF] Bushuyev | Ionic polymers as a new structural motif for high-energy-density materials[END_REF] where it revealed a trigonal structure, with a space group P-3c1.

XPS analyses were performed and similar spectra were obtained for hNHN, cNHN and dNHN (Figure S5). The binding energies (BE) were ascribed with the help of the NIST XPS database [START_REF]NIST X-ray Photoelectron Spectroscopy Database[END_REF]. For Ni 2p1/2 (e.g. 873.6 eV for hNHN) and Ni 2p3/2 (e.g. 855.9 eV for hNHN), the BEs (Figure 3) are consistent with values reported for Ni 2+ (in e.g. complexes).

For N 1s, there are two signals, at 400.1-400.3 eV and 406.3-406.8 eV (Figure 4). They may be respectively attributed to amine/hydrazine and nitrate groups found in metal complexes.

The XPS data are thus in good agreement with the molecular structure [Ni(N 2 H 4 ) 3 ][NO 3 ] 2 .

Catalytic activity of the complexes

The catalytic activity of hNHN, cNHN and dNHN was assessed for the generation of H 2 by hydrolysis of NaBH 4 at 80°C. The H 2 evolution curves are shown in Figure 5. They look sigmoidal and the curve can be divided into three parts.

The first part corresponds to the first minutes of the reaction during which the H 2 evolution gradually increases. It resembles to the induction period generally reported for cobalt-based catalysts [START_REF] Arzac | Deactivation, reactivation and memory effect on Co-B catalyst for sodium borohydride hydrolysis operating in high conversion conditions[END_REF][START_REF] Patel | Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: a review and the perspectives to substitute noble metals[END_REF]: during this period, the catalyst surface is transformed (often reduced) into a catalytically active form.

The second part of the curves is the linear part. It is due to the proper hydrolysis of NaBH 4 [START_REF] Garron | New insights into the mechanism of H 2 generation through NaBH 4 hydrolysis on Co-based nanocatalysts studied by differential reaction calorimetry[END_REF], where the H 2 generation rate is constant. It is calculated to be 10.2, 14.2 and 17 mL(H 2 ) min -1 for hNHN, dNHN and cNHN respectively. Expressed in L(H 2 ) min -1 g NHN -1 , the rates are 0.93, 1.29 and 1.55 respectively. The better catalytic activity of cNHN is explained by a higher number of active sites owing to more surface defects (Figure 1). With respect to dNHN, it is more active than hNHN. This can be due to the presence of clew-like particles (Figure 1). In comparison to previously reported catalysts [START_REF] Brack | Heterogeneous and homogenous catalysts for hydrogen generation by hydrolysis of aqueous sodium borohydride (NaBH 4 ) solutions[END_REF], the catalytic activity of hNHN, cNHN and dNHN is average. With respect to the salisylaldimine-nickel complex reported by Kılınç et al. [START_REF] Kılınç | Investigation on salisylaldimine-Ni complex catalyst as an alternative to increasing the performance of catalytic hydrolysis of sodium borohydride[END_REF], a rate of 2.24 L(H 2 ) min -1 g complex -1 was found at 30°C, such a performance being explained by the relatively high specific surface area of 48.5 m 2 g -1 ; the rate was even improved by a factor of 28 by supporting the complex onto alumina [START_REF] Kılınç | Salisylaldimine-Ni complex supported on Al 2 O 3 : highly efficient catalyst for hydrogen production from hydrolysis of sodium borohydride[END_REF].

The third part of the H 2 evolution curves is characterized by rates dropping to 0 mL(H 2 ) min -1 because of the depletion of NaBH 4 in the slurry (end of the reaction).

In order to calculate the apparent activation energy, the temperature of the hydrolysis experiment was varied from 50 to 80°C. Predictably, the H 2 generation rates (determined via the linear part of the curves) were found to drop with the decrease of the temperature (Figures S6 to S8). They were injected in the Arrhenius equation to plot their Napierian logarithm as a function of the reverse of the temperature in Kelvin (Figure S9). The slope of the as-obtained line was multiplied by R = 8.314 J mol -1 K -1 to get the apparent activation energy, namely, 38.7 ± 5, 48.7 ± 4 and 49.5 ± 4 kJ mol -1 for hNHN, cNHN and dNHN respectively. They are consistent with most of the values reported for metal-catalyzed hydrolysis of NaBH 4 [START_REF] Retnamma | Kinetics of hydrolysis of sodium borohydride for hydrogen production in fuel cell applications: A review[END_REF].

After hydrolysis, the spent fuel was recovered for analyses (Figure 6). Prior to any analysis, the aforementioned liquid-state spent fuel was first separated from the catalyst by centrifugation (6000 rpm for 10 min). We systematically observed that, during the hydrolysis experiments, the solids changed color from purple violet to black (Figure S2),

indicating the occurrence of a transformation as also suggested by the occurrence of an induction period (Figure 5) [START_REF] Arzac | Deactivation, reactivation and memory effect on Co-B catalyst for sodium borohydride hydrolysis operating in high conversion conditions[END_REF][START_REF] Patel | Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: a review and the perspectives to substitute noble metals[END_REF]. Similar color change upon reduction was observed for the complex [Ni(N 2 H 4 ) 3 ][Cl] 2 [START_REF] Park | Preparation of fine Ni powders from nickel hydrazine complex[END_REF]. The black solids were washed with water twice, with ethanol once, and dried at 80°C overnight. For each solid, a fine powder was recovered for analyses.

Characterization of the black "reduced" complexes

The complexes after hydrolysis were supposed to be "reduced" [START_REF] Arzac | Deactivation, reactivation and memory effect on Co-B catalyst for sodium borohydride hydrolysis operating in high conversion conditions[END_REF][START_REF] Patel | Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: a review and the perspectives to substitute noble metals[END_REF] because of the reducing character of NaBH 4 . Accordingly the "reduced" counterparts of hNHN, cNHN and dNHN (i.e. after hydrolysis) are denoted RhNHN, RcNHN and RdNHN (with R for reduced).

The fine powders RhNHN, RcNHN and RdNHN were scrutinized by optical microscopy.

The initial morphologies were not found (Figure S11). They were then observed by SEM (Figure 7). Each of the complexes evolved from a morphological point of view. Spherical particles are observed for RhNHN, RcNHN and RdNHN. There are however few differences. The spherical shape is not perfect with RhNHN and the particles (with an average size of 120 nm) are much agglomerated (Figure S12). One may even distinguish the presence of very few hexagonal plates (Figure S13) and which surface is much rougher than for hNHN. With respect to RcNHN, the particles are spherical and agglomerated (Figure S14). Their size is quite uniform with 100 nm. The sample RdNHN has a more heterogeneous composition, with spherical particles of 120-160 nm as well as clew-like morphologies (Figure S15). They look like the spherical particles of 150-180 nm reported elsewhere and that they were found to form from the complex [Ni(N 2 H 4 ) 3 ][Cl] 2 by hydrazine reduction [START_REF] Park | Preparation of fine Ni powders from nickel hydrazine complex[END_REF][START_REF] Huang | Preparation of fine nickel powders via reduction of nickel hydrazine complex precursors[END_REF].

The samples RhNHN, RcNHN and RdNHN were analyzed by FTIR spectroscopy (Figure S16). The spectra are comparable for the three compounds, but they are much different from the spectra collected with the starting complexes (Figure 2). These results are in accordance with an evolution of the complexes. The bands are wider and for most they do not have the same wavenumber. The observed vibrational bands were favorably compared to the bands generally reported for borate species [START_REF] Miller | Infrared spectra and characteristic frequencies of inorganic ions[END_REF][START_REF] Jun | FT-IR and Raman spectroscopic study of hydrated borates[END_REF][START_REF] Kanturk | Synthesis, crystal structure and dehydration kinetics of NaB(OH) 4 ⋅2H 2 O[END_REF]. The presence of hydrazine cannot be discarded [START_REF] Wojewódka | Hydrazine complexes of transition metals as perspective explosives[END_REF][START_REF] Miller | Infrared spectra and characteristic frequencies of inorganic ions[END_REF][START_REF] Patil | Synthesis and characterisation of metal hydrazine nitrate, azide and perchlorate complexes[END_REF][START_REF] Chhabra | Synthesis, characterization and thermal studies of (Ni/Co) metal salts of hydrazine: potential initiatory compounds[END_REF][START_REF] Park | Preparation of fine Ni powders from nickel hydrazine complex[END_REF], at least for RdNHN. The spectra over the wavenumber range 3600-1600 cm -1 resemble the one reported for the complex [Ni(N 2 H 4 ) 3 ][Cl] 2 after reduction by hydrazine [START_REF] Park | Preparation of fine Ni powders from nickel hydrazine complex[END_REF]; in that study, the formation of Ni(OH) 2 was evidenced.

The samples RhNHN, RcNHN and RdNHN were analyzed by XRD (Figure S17). They did not diffract, suggesting amorphous solids. Similar observations were made for various cobaltbased catalysts or precursors of in situ forming catalysts; upon reduction by NaBH 4 , cobalt was found to be amorphous, resulting in a debate about the nature of the catalytically active phase [START_REF] Patel | Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: a review and the perspectives to substitute noble metals[END_REF][START_REF] Demirci | Cobalt-based catalysts in hydrolysis of NaBH 4 and NH 3 BH 3[END_REF]. Here, the XRD patterns of RhNHN, RcNHN and RdNHN (Figure S17 The samples RhNHN, RcNHN and RdNHN were then analyzed by XPS (Figure S18). The binding energies (BE) were ascribed with the help of the NIST XPS database [START_REF]NIST X-ray Photoelectron Spectroscopy Database[END_REF]. The BEs measured for Ni 2p (Figure 8) are much comparable to the values reported for the fresh samples (Figure 3), suggesting Ni 2+ -based surfaces. The formation of nickel hydroxide Ni(OH) 2 cannot be discarded [START_REF] Park | Preparation of fine Ni powders from nickel hydrazine complex[END_REF]. Boron was found on the superficial layers of RhNHN, RcNHN and RdNHN (Figure 9). The B 1s BE is 191.8 ± 0.1 eV. This is a typical value for borates and other boron oxides. No nitrogen was detected on the surface of both RhNHN and RcNHN. One N 1s signal was distinguished for RdNHN (Figure S19). The BE of is 399.7

eV indicating the presence of residual hydrazine groups. According to the XPS results, the surface of the spherical particles observed for RhNHN, RcNHN and RdNHN are composed of Ni 2+ and borate species. It is worth mentioning that similar observations for in situ forming cobalt-based catalysts were interpreted as the formation of surface species of formulae Co II @B x O y (OH) z [START_REF] Patel | Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: a review and the perspectives to substitute noble metals[END_REF][START_REF] Demirci | Cobalt-based catalysts in hydrolysis of NaBH 4 and NH 3 BH 3[END_REF]. In the present conditions, the formation of surface Ni II @B x O y (OH) z might be assumed.

Catalytic activity of the black "reduced" complexes

RhNHN, RcNHN and RdNHN were tested for their catalytic ability in hydrolysis of NaBH 4

(Figures S20 to S22). They were compared to hNHN, cNHN and dNHN respectively. The results are summarized in Figure 10, where the cycle 0 refers to the H 2 generation rates of hNHN, cNHN and dNHN and the cycle 1 refers to H 2 generation rates of RhNHN, RcNHN and RdNHN. Two observations stand out. First, the reduced forms of the complexes are more active than the fresh counterparts. For example, the H 2 generation rate of RhNHN is almost thrice the H 2 generation rate of hNHN. The explanation of that may be the smaller size of the particles observed for RhNHN, RcNHN and RdNHN (Figure 7 vs. Figure 1). Second, the H 2 evolution curves of RhNHN, RcNHN and RdNHN does not show an induction period.

This is typical of a catalyst which surface consisted of the catalytically active sites [START_REF] Arzac | Deactivation, reactivation and memory effect on Co-B catalyst for sodium borohydride hydrolysis operating in high conversion conditions[END_REF][START_REF] Patel | Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: a review and the perspectives to substitute noble metals[END_REF]. This confirms that the surface of hNHN, cNHN and dNHN evolved during the cycle 0 into the active form found for RhNHN, RcNHN and RdNHN.

When the catalytic activities of RhNHN, RcNHN and RdNHN are compared to each other (Figure 10), it stands out that the H 2 generation rates are comparable: 28.7 mL(H 2 ) min -1 for RhNHN, 28.8 mL(H 2 ) min -1 for RcNHN, and 29.4 mL(H 2 ) min -1 for RdNHN. Expressed in

L(H 2 ) min -1 g NHN -1
, the rates are between 2.6 and 2.7 respectively. This is consistent with the microcopy observations (Figure 7) and the performed characterizations (Figures 8 and9); indeed there is almost no difference between the three samples RhNHN, RcNHN and RdNHN. In other words, the initial morphology of hNHN, cNHN and dNHN does not have a remarkable impact on the morphology, structure and catalytic activity of RhNHN, RcNHN and RdNHN. Note that the aforementioned rates are slightly better than the rate (2.24 L(H 2 )

min -1 g complex -1 ) reported for another nickel complex [START_REF] Kılınç | Investigation on salisylaldimine-Ni complex catalyst as an alternative to increasing the performance of catalytic hydrolysis of sodium borohydride[END_REF].

Reusability tests were carried out with RhNHN, RcNHN and RdNHN (Figure 10). The cycle 2 to the cycle 5 refer to successive uses of these samples. After each cycle, the catalyst was extracted from the reaction slurry by centrifugation, washed twice with water, washed once with ethanol and dried at 80°C to be re-used. The reusability tests show a decrease of the H 2 generation rates, specifically of 19-28% at the cycle 2 and of 43-47% at the cycle 5 when the rates are compared to those of the cycle 1. In our conditions, these "reduced" catalysts deactivate over cycles. They behave as the in situ forming cobalt-based catalysts do [START_REF] Patel | Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: a review and the perspectives to substitute noble metals[END_REF][START_REF] Demirci | Cobalt-based catalysts in hydrolysis of NaBH 4 and NH 3 BH 3[END_REF] and, similarly, poisoning due to strong borates adsorption is much likely [START_REF] Kim | Study on degradation of filamentary Ni catalyst on hydrolysis of sodium borohydride[END_REF].

The apparent activation energy for each of the samples RhNHN, RcNHN and RdNHN was determined (Figures S23 to S26). It was found: 48.1 ± 5, 50.3 ± 5 and 41.6 ± 5 kJ mol -1 respectively. They are comparable to the energies found for the fresh complexes. They are besides consistent with most of the values reported for metal-catalyzed hydrolysis of NaBH 4 [START_REF] Retnamma | Kinetics of hydrolysis of sodium borohydride for hydrogen production in fuel cell applications: A review[END_REF].

Putting in perspective

Nickel complexes have been recently proposed as being potential catalysts for the evolution of H 2 by hydrolysis of NaBH 4 [START_REF] Kılınç | Investigation on salisylaldimine-Ni complex catalyst as an alternative to increasing the performance of catalytic hydrolysis of sodium borohydride[END_REF][START_REF] Kılınç | Salisylaldimine-Ni complex supported on Al 2 O 3 : highly efficient catalyst for hydrogen production from hydrolysis of sodium borohydride[END_REF]. Our first objective was thus to gain more insight.

Considering besides that the morphology of nickel complexes can be quite easily controlled by tuning the synthesis conditions [START_REF] Wang | A facile sonochemical route to morphology controlled nickel complex mesostructures[END_REF], the objective was expanded to the study of the effect of the morphology on the catalytic activity of the samples. Three morphologies (hexagonal plate-like, clew-like and disc-like) of the nickel hydrazine nitrate complex were successfully targeted. They showed slightly different catalytic performances. All of them allowed getting a total conversion of NaBH 4 into H 2 and NaB(OH) 4 , but this was achieved at different H 2 generation rates. The most efficient complex was found to be the clew-like morphology because it has more defects (particularly via the numerous edges and corners it has); this means more active sites.

The aqueous alkaline solution of NaBH 4 is reductive [START_REF] Demirci | About the technological readiness of the H 2 generation by hydrolysis of B(-N)-H compounds[END_REF][START_REF] Schlesinger | Sodium borohydride, its hydrolysis and its use as a reducing agent and in the generation of hydrogen[END_REF][START_REF] Gislon | Hydrogen production from solid sodium borohydride[END_REF][START_REF] Brack | Heterogeneous and homogenous catalysts for hydrogen generation by hydrolysis of aqueous sodium borohydride (NaBH 4 ) solutions[END_REF]. With metal cations used as precursors of in situ-forming catalysts, the evolution takes place during the first seconds/minutes of the reaction during the so-called induction period [START_REF] Arzac | Deactivation, reactivation and memory effect on Co-B catalyst for sodium borohydride hydrolysis operating in high conversion conditions[END_REF][START_REF] Patel | Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: a review and the perspectives to substitute noble metals[END_REF][START_REF] Demirci | Cobalt-based catalysts in hydrolysis of NaBH 4 and NH 3 BH 3[END_REF]]. An induction period was observed with our nickel hydrazine nitrate complexes, suggesting an evolution towards a catalytically active form. The first evidence was a change of the color of the powder (from purple violet to black). The second evidence was provided by a set of characterization results. The nature of the "reduced" samples is not identified yet. Like for the muchinvestigated cobalt [START_REF] Patel | Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: a review and the perspectives to substitute noble metals[END_REF][START_REF] Demirci | Cobalt-based catalysts in hydrolysis of NaBH 4 and NH 3 BH 3[END_REF], the results are difficult to exploit and are open to speculation. Like for some of the cobalt-based catalysts, it was tentatively suggested the formation of surface M II @B x O y (OH) z (with M = Ni or Co). The evolution of the nickel hydrazine nitrate complexes during the hydrolysis reaction has two additional consequences. Morphologically, the samples change towards aggregated spherical particles of much smaller size (100-160 nm). Catalytically, the "reduced" samples are more active in hydrolysis. The H 2 generation rates were found to be, at best, thrice of the rates determined with the fresh complexes. It may be concluded that the fresh nickel hydrazine nitrate complexes are not appropriate as catalysts for the hydrolysis of NaBH 4 . In contrast, they may be seen as precursors of catalytic fine powders since the "reduced" forms have better catalytic potential.

Unless the catalyst is intended to be used once (one-shot approach) [START_REF] Demirci | About the technological readiness of the H 2 generation by hydrolysis of B(-N)-H compounds[END_REF], it is important to assess the stability (or reusability) of the catalyst over cycles. This was our third objective.

The nickel hydrazine nitrate complex evolves in a form that is more efficient. This "reduced" form was thus re-used in five cycles. The H 2 generation rates were found to decrease cycle after cycle. At best, 57% of the initial catalytic activity was retained after the fifth cycles.

However, the total conversion of NaBH 4 was not affected, being constant at 100%. These results are also comparable to what was reported for cobalt-based catalysts so far [START_REF] Demirci | About the technological readiness of the H 2 generation by hydrolysis of B(-N)-H compounds[END_REF][START_REF] Brack | Heterogeneous and homogenous catalysts for hydrogen generation by hydrolysis of aqueous sodium borohydride (NaBH 4 ) solutions[END_REF][START_REF] Arzac | Deactivation, reactivation and memory effect on Co-B catalyst for sodium borohydride hydrolysis operating in high conversion conditions[END_REF][START_REF] Patel | Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: a review and the perspectives to substitute noble metals[END_REF][START_REF] Demirci | Cobalt-based catalysts in hydrolysis of NaBH 4 and NH 3 BH 3[END_REF].

It may be concluded that the active surface of in situ-forming Ni 2+ -and Co 2+ -based catalysts have more than one common feature.

Conclusion

Morphology

  ) display one broad shoulder of low intensity at about 2θ 45°. In comparison to the most intense diffraction peak that can be observed in the referenced pattern of various nickel-based compounds, several species might be suggested. Examples are cubic nickel (ref. 00-004-0850), tetragonal Ni 2 B (ref. 00-048-1222), orthorhombic Ni 3 B (00-048-1223), and cubic NiO (ref. 00047-1049). A further discussion at this point would be, as for the aforementioned cobalt-based catalysts, speculative.

Figure 1 .

 1 Figure 1. SEM images of the nickel hydrazine nitrate complexes denoted hNHN (hexagonal plate-like morphology), cNHN (clew-like morphology) and dNHN (disc-like morphology).

Figure 2 .

 2 Figure 2. FTIR spectra of the nickel hydrazine nitrate complexes denoted hNHN (hexagonal plate-like morphology), cNHN (clew-like morphology) and dNHN (disc-like morphology).

Figure 3 .

 3 Figure 3. XPS spectra of the nickel hydrazine nitrate complexes denoted hNHN (hexagonal plate-like morphology), cNHN (clew-like morphology) and dNHN (disc-like morphology): focus on the Ni 2p signals.

Figure 4 .

 4 Figure 4. XPS spectra of the nickel hydrazine nitrate complexes denoted hNHN (hexagonal plate-like morphology), cNHN (clew-like morphology) and dNHN (disc-like morphology): focus on the N 1s signals.

Figure 5 .

 5 Figure 5. H 2 evolution curves obtained with hNHN, cNHN and dNHN. The hydrolysis conditions were as follows: 80°C; 11 mg of catalyst; 2 mL of alkaline water (0.5 NaOH); 100 mg NaBH 4 .

Figure 6 .

 6 Figure 6. Analyses results of the spent fuel in either liquid-state ( 11 B NMR) or solid-state ( 11 B MAS NMR, FTIR and XRD). The chemical shifts of the NMR signals are given. The FTIR bands are ascribed to e.g. the B-O stretching and deformation modes. Some of the most intense XRD peaks are indexed (reference pattern 01-081-1512).

Figure 7 .

 7 Figure 7. SEM images of the "reduced" nickel hydrazine nitrate complexes (recovered after the NaBH 4 hydrolysis experiments) denoted RhNHN, RcNHN and RdNHN.

Figure 8 .

 8 Figure 8. XPS spectra of the "reduced" nickel hydrazine nitrate complexes (recovered after the NaBH 4 hydrolysis experiments) denoted RhNHN, RcNHN and RdNHN: focus on the Ni 2p signals.

Figure 9 .

 9 Figure 9. XPS spectra of the "reduced" nickel hydrazine nitrate complexes (recovered after the NaBH 4 hydrolysis experiments) denoted RhNHN, RcNHN and RdNHN: focus on the B 1s signals.

Figure 10 .

 10 Figure 10. Comparison of the H 2 generation rates (denoted cycle 1) of RhNHN, RcNHN and RdNHN to the rates (denoted cycle 0) found with hNHN, cNHN and dNHN, and evolution of the H 2 generation rates of RhNHN, RcNHN and RdNHN over five cycles (cycle 1 to cycle 5; i.e. reusability experiments). The rates were calculated from the H 2 evolution curves shown in Figures S20 to S22.

  determination of the apparent activation energies with the help of the Arrhenius equation. The variation of the volume of H 2 was systematically video-recorded to be computationally exploited and to plot the time-dependent H 2 evolution curve. The H 2 generation rate was determined over the linear part of the H 2 evolution curve (i.e. slope of the curves).

	dried at 60°C for 12 h. Clew-like morphology (denoted cNHN) was obtained according to a
	similar procedure except that two parameters changed. The volume of hydrazine hydrate was
	decreased to 0.52 mL and the ultrasonication time was increased to 1 h. With respect to the
	disc-like morphology (denoted dNHN), the PVP solution was substituted by an aqueous
	solution of polyethylene glycol (PEG; H(OCH 2 CH 2 ) 90 OH; 4000 g mol -1 ; Sigma-Aldrich) at 1
	mM; the volume of hydrazine hydrate was 0.52 mL; and the ultrasonication time was 15 min.
	Visual inspection of the morphology-controlled nickel hydrazine nitrate complexes were
	carried out by optical microscopy (Leica ICC50W) and scanning electron microscopy (SEM;
	Hitachi S4800). Structural analyses were performed by Fourier-transform infrared
	spectroscopy (FTIR; Thermo Fisher Nexus; attenuated total reflection mode (ATR;
	DuraSamplIR); 4000-700 cm -1 ; 32 scans), 11 B nuclear magnetic resonance (NMR; Bruker
	AVANCE-300; probe head BBO10 operated at 96.29 MHz; D 2 O as reference), and 11 B magic
	angle spinning (MAS) NMR (Varian VNMR 400 spectrometer; 128.4 MHz). Crystallographic
	analyses were done by X-ray diffraction (XRD; PANalytical X'PERT Pro; Cu-Kα radiation λ
	= 1.5406 Å; working voltage 40 kV and working current 30 mA). For pattern matching,
	PANalytical X'Pert HighScore software and the available crystallographic databases (PDF-4+
	v. 4.1403) were used. Surface analyses of the morphology-controlled nickel hydrazine nitrate
	complexes were performed by X-ray photoelectron spectroscopy (XPS; ESCALAB 250 from
	Thermo Electron; monochromatic source Al Kα ray 1486.6 eV; analyzed surface of 400 µm
	diameter; binding energies (BE) of all core levels referred to the C-C of C1s carbon at 284.8
	eV). Thermal analyses of the hydrolysis by-product were carried out by thermogravimetric
	analysis (TGA; TA instruments Q500) and differential scanning calorimetry (DSC; TA
	Instruments Q20). For both techniques, ~10 mg of sample was analyzed under the following
	conditions: 25-400°C, heating rate of 5 ºC min -1 , and N 2 flow of 60 mL min -1 .
	green solution. Then, 2.1 mL of hydrazine hydrate (N 2 H 4 ⋅H 2 O; 64-65 wt%, reagent grade The hydrolysis experiments were performed on our catalytic set-up based on the inverted-
	98%; Sigma-Aldrich) was added dropwise. After the first drop the solution turned blue. Upon burette method. It is made up of a reactor (Schlenk-type tube), a cold trap, and an inverted
	the addition of hydrazine, the mixture was ultrasonicated for 15 min, resulting in the
	formation of a purple violet precipitate, in good agreement with the color of the nickel
	hydrazine nitrate complex (Figure S2) [22-25]. The solid was separated by centrifugation
	(6000 rpm; 10 min), the process being interspersed by two washings with deionized water
	(Millipore milli-Q, resistivity >18.2 MΩ cm) and one with ethanol. The solid was finally

17 g of nickel (II) nitrate hexahydrate (Ni(NO 3 ) 2 ⋅6H 2 O; Sigma-Aldrich) was weighted. 15.6 mL of an aqueous solution of polyvinylpyrrolidone (PVP;

(C 6 H 9 NO) 360 ; 40000 g mol -1 ; Sigma-Aldrich) at 1 mM was added under stirring, resulting in a burette filled with blue-colored water. In the present work, the hydrolysis conditions were as follows: 80°C; 11 mg of catalyst (10 wt% of the couple catalyst-borohydride); 2 mL of an aqueous alkaline (NaOH at 0.5 M) solution of NaBH 4 (100 mg, corresponding to 90 wt% of the couple catalyst-borohydride). The molar ratio H 2 O/NaBH 4 was 42, implying an excess of water in relation to the stoichiometric reaction shown by Eq. 1. Lower temperatures were applied for the

  The liquid-state spent fuel was first analyzed by NMR of the nucleus 11 B. There is no signal at negative shifts, indicating a total hydrolysis of NaBH 4 . One singlet is viewed at 2.55 ppm. It is typical of the tetrahydroxyborate anion B(OH) 4 The weight loss at 400°C is 34.2 wt% while the water content in NaB(OH) 4 is 35.3 wt%. Over this temperature range, NaBO 2 ⋅2H 2 O loses two equivalents of H 2 O, much likely resulting in the formation of sodium metaborate NaBO 2 .

-

[START_REF] Sinton | Complexation chemistry of sodium borate with poly(vinyl alcohol) and small diols. A 11 B NMR study[END_REF]

. The water of the liquid-state spent fuel was extracted at 60°C for 72 h and the as-obtained solid spent fuel was kept at this temperature. It was analyzed by

11

B MAS NMR. There is only one signal peaking at 2.04 ppm. This is in good agreement with tetravalent B found in boron oxides and borates like NaB(OH) 4

[START_REF] Zhou | Correlations between 11 B NMR parameters and structural characters in borate and borosilicate minerals investigated by high-resolution MAS NMR and ab initio calculations[END_REF]

. The fingerprint of a borate like NaB(OH) 4 was obtained by FTIR spectroscopy

[START_REF] Jun | FT-IR and Raman spectroscopic study of hydrated borates[END_REF]

. Another evidence of the formation of NaB(OH) 4 is the XRD pattern. It well matches the reference pattern 01-081-1512 (score of 78) that suggests a crystalline NaB(OH) 4 compound with a monoclinic structure and the space group P21/a. A last confirmation of the nature of the borate (also denoted NaBO 2 ⋅2H 2 O) comes from the TGA and DSC results (Figure

S10

). It is observed a stepwise evolution from 100°C to 400°C that may be attributed to H 2 O desorption

[START_REF] Kanturk | Synthesis, crystal structure and dehydration kinetics of NaB(OH) 4 ⋅2H 2 O[END_REF]

.

  -controlled nickel complexes are possible catalytic accelerators for H 2 evolution by hydrolysis of NaBH 4 . In the present work, the nickel hydrazine nitrate complex [Ni(N 2 H 4 ) 3 ][NO 3 ] 2 has been studied as microscale hexagonal plate-, clew-and disc-like structures. They are indeed able to catalyze the hydrolysis of NaBH 4 , with H 2 generation rates being in the average of what has been reported hitherto in the field. In our conditions, the "reduced" sample is more active for H 2 evolution by hydrolysis of NaBH 4 . In the best case, the H 2 generation rate is increased threefold, making it more attractive than the starting complex. Reusability tests performed with the "reduced" sample shows that the total conversion of NaBH 4 is constant but the H 2 generation rates drop cycle after cycle. More than 50% of the initial catalytic activity is retained after the fifth cycles. This result also makes our in situ-forming Ni 2+ -based black fine powder resemble the in situ-forming Co 2+ -based black solid.In closing, there are opportunities for the nickel-based complexes as catalysts and/or catalyst precursors for hydrolysis of NaBH 4 . Using nickel makes sense as it is much cheaper than cobalt.

	The clew-like sample is more active owing to more defects and more active sites. The H 2
	evolution curves however show an induction period that suggests an evolution of the
	complex. A catalytically active material (called "reduced" sample) forms and this has been
	confirmed by visual inspection as well as by characterizations. The main observations are as
	follows. The color of the solids changes from purple violet to black. SEM observations of the
	as-obtained fine black powders reveal a change in the morphology; whatever the initial shape
	of the complex, aggregated nanoscale spherical particles (100-160 nm) form. The XRD
	pattern shows the transition from a crystalline material (trigonal, space group P-3c1) to an
	amorphous compound. According to the XPS scans, the nickel element still has an oxidation
	state of +2 but the hydrazine ligand and the nitrate anion have been mostly removed; instead
	boron is found.

Such observations are alike what has been reported and debated at length for Co 2+ within the past decade. By analogy, it is tentatively suggested that Ni 2+ and Co 2+ behave in a similar way and that surface species such as M II @B x O y (OH) z (with M = Ni or Co) might form.

Acknowledgements

The authors thank Maria-José Valero-Pedraza (Univ Montpellier) and Salem Ould-Amara (Univ Montpellier) for occasional assistance in XRD and NMR.