Method for characterizing a catalyst

Abstract

Method for Characterizing a Catalyst. The present invention relates to a method for characterizing a catalyst A, comprising at least the steps of: - Contacting an aqueous solution of pH greater than or equal to 11 comprising at least one borohydride salt with the catalyst A; - Determining the normalized open-circuit potential Et(A); and - Comparing the normalized open-circuit potential Et(A) to a normalized open-circuit potential Et(B) pre-acquired at the same time t with a reference catalyst B under the same conditions as the normalized open-circuit potential Et(A), and / or comparing time t to a time t' at which a normalized open-circuit potential Et'(B) pre-acquired with a reference catalyst B under the same conditions as the normalized open-circuit potential Et(A) is equal to the normalized open-circuit potential Et(A). Figure for the abstract: None

Description

Title of the invention: Method for characterizing a catalyst technical field

[0001] The present invention relates to the field of hydrogen generation from borohydride salt-based solutions, and more particularly to the field of catalysts used to catalyze the hydrolysis reaction of borohydride salts. More specifically, the invention relates to a method for characterizing catalysts, particularly those intended for catalyzing the hydrolysis reaction of borohydride salts, notably for characterizing the catalytic performance of such catalysts. Prior art

[0002] Borohydride salts, particularly alkali or alkaline earth metal borohydrides, are known for their ability to store hydrogen. They can therefore be used to transport and / or store hydrogen, especially as an alternative to pressurized or liquefied hydrogen.

[0003] Following its transport and / or storage, hydrogen can be generated by hydrolysis from an aqueous solution of borohydride salts. For example, the hydrolysis of a borohydride salt with the general formula MBH4, where M is generally an alkali metal, occurs in contact with water according to the following exothermic reaction:

[0004] [Chem.l] MBIL t (2+x)H2O -> MBCh.xfhO 4- 4H2

[0005] This reaction is spontaneous at acidic or neutral pH, involving a spontaneous and uncontrolled release of hydrogen. Thus, the hydrolysis of borohydride salts is generally carried out using a solution with a pH greater than or equal to 11, or even greater than or equal to 12, which slows down the kinetics of borohydride salt hydrolysis. An aqueous solution of borohydride salts can therefore be stored at a high pH.

[0006] The generation of hydrogen by hydrolysis can then be controlled, at such a basic pH, according to the needs, by implementing a catalyst.

[0007] Hydrogen generated by the hydrolysis of borohydride salts can be used for various applications such as direct combustion or the generation of electrical energy, for example, powering a fuel cell. For these applications, and particularly in the context of fuel cells, it is advantageous to be able to generate hydrogen with a controlled flow rate that is sufficient and adapted to the power of the fuel cell used, to achieve a high hydrogen capacity, and to supply hydrogen of high purity.

[0008] Consequently, a catalyst is generally chosen based on its performance in the hydrolysis of borohydride salts in order to increase the hydrogen generation kinetics and thus the hydrogen release rate, but also based on its availability, cost, and / or its ability to adhere to a support. Therefore, there is a constant drive to improve the catalytic performance and thus the catalytic activity of a catalyst, particularly a catalyst based on non-critical metals such as metals not belonging to the platinum group (also called PGMs in Anglo-Saxon terminology). There is also a drive to optimize the catalyst synthesis process to improve its performance. Therefore, evaluating the catalytic performance of a catalyst is a key objective in the search for higher-performing catalysts.

[0009] Furthermore, for a known catalyst, it may happen that an isolated batch of catalysts does not exhibit the required performance after its synthesis. The catalyst may also undergo aging leading to a loss of performance, for example, following storage or use. In addition, the performance of a catalyst can depend on the specific environment in which it is used.

[0010] It is also sought to estimate the reproducibility of catalysts following their synthesis with a view to their future use in an end application, and to evaluate the catalytic performance of a catalyst as part of quality control before use.

[0011] The performance of a catalyst can be evaluated by monitoring the kinetics of the catalyzed hydrolysis of borohydride salts.

[0012] The kinetics of hydrolysis can be monitored via the volume of hydrogen released during hydrolysis, which requires a specific installation, complex to implement.

[0013] It can also be monitored by techniques involving sampling the reaction medium and then quantifying one of the reactants or products. For example, the borohydride content in the reaction medium can be assessed by measuring the volume of hydrogen released during the acid hydrolysis reaction of sodium borohydride. It is also possible to measure the NaBO2 content formed in the reaction medium by monitoring its reaction in acidic medium. Measuring the borohydride salt content in the reaction medium has also been considered, using a method based on the determination of diiodine (I2) formed by the action, in acidic medium, of potassium iodate (KIO3) added in excess to the medium and potassium iodide (Kl). Electrochemical methods (Cyclic Voltammetry (CV), Linear Scanning Voltammetry (LSV), and Square Wave Voltammetry (SWV)) have also been explored. were considered, as for example reported by McLafferty et al. (ECS Transactions, 2007), which proposes to oxidize the BH4 ion.

[0014] These methods involve sampling and / or modifying the reaction medium, and therefore several steps, which makes their implementation complex. Furthermore, these methods in most cases involve a so-called destructive analysis of the catalyst, meaning that the characterized catalyst can no longer be used subsequently.

[0015] Thus, there is a need for an inexpensive, simple-to-implement process for characterizing catalysts for the hydrolysis of borohydride salts.

[0016] There is also a need for an inexpensive, simple-to-implement process for comparing and / or evaluating the catalytic performance of different catalysts.

[0017] There is also a need for a process to control, in a simple, rapid and non-destructive way, the quality of catalysts intended for the hydrolysis of borohydride salts, in particular before their use to catalyze the hydrolysis of borohydride salts.

[0018] In particular, there is a need for a process allowing the "quality control" of a catalyst, especially after its synthesis.

[0019] There is also a need for an in situ and operando process for the characterization of a catalyst.

[0020] The invention aims to satisfy these needs. Description of the invention

[0021] Thus, the present invention relates to a method for characterizing a catalyst, referred to as catalyst A, in particular for characterizing the catalytic performance of catalyst A, comprising at least the steps of: (i) Have an aqueous solution with a pH greater than or equal to 11, preferably greater than or equal to 12, comprising at least one borohydride salt; ii) Contact, at an initial time t0, said aqueous solution from step i) with catalyst A to initiate the hydrolysis reaction of said borohydride salt; iii) Measure the open circuit potential E0Cvt(A) of catalyst A in said aqueous solution at a time t strictly greater than the initial time t0, the open circuit potential of catalyst A in said aqueous solution being measured between a reference electrode and a working electrode, the working electrode comprising catalyst A; iv) Determine the normalized open-circuit potential Et(A) at time t by normalizing the open-circuit potential E0Cvt(A); and (v) Compare said normalized open circuit potential Et(A) to a normalized open circuit potential Et(B) pre-acquired at the same time t with a reference catalyst B under the same conditions as the normalized open circuit potential Et(A) and / or compare time t to a time t' at which a normalized open circuit potential Ef(B) pre-acquired with a reference catalyst B under the same conditions as the normalized open circuit potential Et(A) is equal to the normalized open circuit potential Et(A).

[0022] It has already been proposed by Amendola et al. (Talanta, 1999) to measure the open circuit potential of metals immersed in aqueous alkaline solutions of borohydride ions saturated with borates.

[0023] To the inventors' knowledge, however, it had never been envisaged to compare, between several catalysts, the normalized open circuit potential during a catalyzed hydrolysis of borohydride salts.

[0024] In the present text, the term “hydrolysis” is simply used to refer to a hydrolysis reaction.

[0025] For the purposes of the invention, "potential" means an electrical potential.

[0026] In particular, the characterization of the catalyst may include the characterization of the catalytic performance of the catalyst, the characterization of the structure of the catalyst, the characterization of the surface state of the catalyst and / or the characterization of the specific surface area of ​​the catalyst, in particular the characterization of the catalytic performance of the catalyst.

[0027] Catalyst A and catalyst B are advantageously catalysts for the hydrolysis reaction of borohydride salts. Catalyst A and catalyst B can be catalysts for the hydrolysis reaction of the borohydride salt(s) present in the aqueous solution in step i).

[0028] Advantageously, the method according to the invention can be implemented to compare the catalytic performance of catalyst A with the catalytic performance of catalyst B.

[0029] As can be seen from the examples below, the evolution of the normalized open-circuit potential during the hydrolysis of an aqueous solution of borohydride salt(s), catalyzed by the catalyst present at the working electrode, provides information concerning the extent and kinetics of the hydrolysis reaction and therefore the performance of the catalyst. Thus, a low performance for a catalyst is characterized by slower hydrolysis kinetics; consequently, the evolution of the normalized open-circuit potential measured during the hydrolysis of borohydride salt(s) is also slower.

[0030] As will be shown in more detail later, step v) allows the catalytic performance of catalyst A to be characterized in that: - the normalized open-circuit potential Et(A) is less than the normalized open-circuit potential Et(B) when the catalytic performance of catalyst A is less than the catalytic performance of catalyst B, and the normalized open-circuit potential Et(A) is greater than the normalized open-circuit potential Et(B) when the catalytic performance of catalyst A is greater than the catalytic performance of catalyst B, and / or - time t is greater than time t' when the catalytic performance of catalyst A is less than the catalytic performance of catalyst B and time t is less than time t' when the catalytic performance of catalyst A is greater than the catalytic performance of catalyst B.

[0031] The process of the invention can also provide indications as to the surface condition, specific surface area, loading rate or porosity of a catalyst.

[0032] Advantageously, the process of the invention makes it possible to characterize catalysts in a specific environment without taking into account operating auxiliaries, the control system or other systems that may be implemented in the final use of the catalyst.

[0033] Advantageously, the process of the invention makes it possible to evaluate the performance of a catalyst in its form as implemented in its end use and / or at the level of a hydrolysis reaction carried out under conditions similar or even identical to those of its end use. Thus, the process of the invention makes it possible to evaluate the performance of a catalyst directly under the conditions of its implementation to generate hydrogen, in particular by catalyzing the hydrolysis reaction of at least one borohydride salt.

[0034] Advantageously, the process of the invention is non-destructive, both in terms of catalytic performance and mechanical integrity.

[0035] Thus, the invention also relates to the use of a catalyst characterized by the process according to the invention to generate hydrogen, in particular by catalyzing the hydrolysis reaction of at least one borohydride salt.

[0036] The invention also relates to the use of the process according to the invention to control the quality of a catalyst, in particular a catalyst for the hydrolysis reaction of at least one borohydride salt.

[0037] The present text also describes a method comprising at least the steps of:

[0038] - Have an aqueous solution with a pH greater than or equal to 11, preferably greater than or equal to 12, comprising at least one borohydride salt, in particular as described in this text; - To bring said aqueous solution into contact with a catalyst at an initial time to in order to initiate the hydrolysis reaction of said borohydride salt; - Measure the open-circuit potential Eocvt of said catalyst in said aqueous solution at a time t strictly greater than the initial time t0, the open-circuit potential of said catalyst in said aqueous solution being measured between a reference electrode and a working electrode, the working electrode comprising said catalyst; and - Determine the normalized open circuit potential Et at time t by normalizing the open circuit potential Eocvt.

[0039] In particular, the steps of the method described above can be carried out as steps i), ii), iii) and iv) described in this text.

[0040] The normalized open-circuit potential Et can be compared to a standard value. For example, the standard value can correlate the value of the normalized open-circuit potential at a time t' for a reference catalyst or to a conversion rate of the borohydride salt.

[0041] In particular, the method described above may further include a step of determining the conversion rate of the borohydride salt by comparison to a standard value correlating a conversion rate of the borohydride salt to the normalized open-circuit potential Et. In particular, the standard value may be previously measured for a catalyst identical to catalyst A.

[0042] In this text, unless otherwise indicated, the term "hydrogen" is used to refer to dihydrogen, unless it is specified that it refers to the hydrogen atom. Brief description of the drawings

[0043] [Fig.1] schematically illustrates an example of an electrochemical cell (100) enabling the process according to the invention to be implemented.

[0044] [Fig 2] represents the open circuit potential profile ([Fig.2a]) and the normalized open circuit potential profile ([Fig.2b]) for two reference catalysts comprising cobalt on a nickel foam, as described in Example 1.

[0045] [Fig 3] represents the normalized open circuit potential profile ([Fig.3a]) and the NaBH4 conversion rate as a function of time ([Fig.3b]) for 18 cobalt-on-nickel foam catalysts, synthesized separately according to the same synthesis protocol, as described in Example 2.

[0046] [Fig 4] represents the normalized open-circuit potential profile ([Fig.4a]) and the NaBH4 conversion rate as a function of time ([Fig.4b]) for three cobalt-on-nickel foam catalysts, synthesized simultaneously by the same synthesis protocol, after calcination at 450°C, after calcination at 600°C, and without calcination directly after synthesis, as described in example 3a.

[0047] [Fig 5] represents the normalized open-circuit potential profile during the hydrolysis reaction ([Fig.5a]) and at the beginning of the hydrolysis reaction ([Fig.5b]), and the NaBH4 conversion rate as a function of time ([Fig.5c]) for several cobalt-on-nickel foam catalysts synthesized simultaneously by the same synthesis protocol, after several cycles of chemical aging, or without aging directly after synthesis, as described in Example 3b.

[0048] [Fig 6] represents the normalized open-circuit potential profile ([Fig.6a]) and the NaBH4 conversion rate as a function of time ([Fig.6b]) for a catalyst containing cobalt on nickel foam and for a catalyst containing ruthenium on nickel foam, as described in Example 4. Detailed description Step i)

[0049] The aqueous solution in step i) comprises at least one borohydride salt, in particular selected from alkali or alkaline earth metal borohydrides and mixtures thereof.

[0050] A borohydride salt comprises at least one borohydride anion BH4 associated with at least one cation, namely a cation of an alkali or alkaline earth metal, for example Na+ or K+.

[0051] The borohydride salt(s) may be selected from sodium borohydride NaBH4, potassium borohydride KBH4, lithium borohydride LiBH4, magnesium borohydride Mg(BH4)2, calcium borohydride Ca(BH4)2, zinc borohydride Zn(BH4)2, ammonium borohydride NH4BH4 and mixtures thereof.

[0052] In particular, the borohydride salt(s) may be selected from alkali metal or alkaline earth borohydrides and mixtures thereof, especially from alkali metal borohydrides and mixtures thereof. Specifically, the borohydride salt(s) may be selected from sodium borohydride NaBH4, potassium borohydride KBH4, lithium borohydride LiBH4, magnesium borohydride Mg(BH4)2, calcium borohydride Ca(BH4)2 and mixtures thereof, especially from NaBH4, KBH4 and mixtures thereof, more particularly including at least NaBH4. Preferably, the borohydride salt is NaBH4.

[0053] The aqueous solution in step i) may have a concentration of borohydride anions greater than or equal to 0.01 mol / L, in particular greater than or equal to 0.1 mol / L, more particularly greater than or equal to 0.5 mol / L, or even greater than or equal to 0.8 mol / L, or even greater than or equal to 1 mol / L.

[0054] The aqueous solution in step i) may have a concentration of borohydride anions less than or equal to 3.3 mol / L, in particular less than or equal to 2 mol / L, or even less than or equal to 1.5 mol / L, the process being carried out in particular at a temperature ranging from 0°C to 40°C.

[0055] The aqueous solution in step i) may have a concentration of borohydride anions ranging from 0.01 mol / L to 3.3 mol / L, in particular from 0.1 mol / L to 2 mol / L, more particularly from 0.5 mol / L to 1.5 mol / L, or even from 1 mol / L to 1.5 mol / L.

[0056] Advantageously, the aqueous solution in step i) has a pH that prevents spontaneous hydrolysis of the borohydride salt(s) before contact in step ii). The aqueous solution in step i) can have a pH ranging from 11 to 14, in particular from 12 to 14, more particularly from 13 to 14.

[0057] The aqueous solution in step i) may comprise at least one alkali or alkaline earth metal hydroxide. In particular, the aqueous solution in step i) may comprise at least one alkali metal hydroxide, specifically selected from sodium hydroxide NaOH, potassium hydroxide KOH, lithium hydroxide LiOH, and mixtures thereof. More specifically, the aqueous solution in step i) may comprise at least NaOH.

[0058] An alkali or alkaline earth metal hydroxide comprises at least one hydroxide anion associated with at least one cation of an alkali or alkaline earth metal.

[0059] The aqueous solution in step i) may have a concentration of hydroxide anions greater than or equal to 3.103 mol / L, in particular greater than or equal to 0.03 mol / L, more particularly greater than or equal to 0.1 mol / L.

[0060] The aqueous solution in step i) may have a concentration of hydroxide anions less than or equal to 2.6 mol / L, in particular less than or equal to 1.3 mol / L, more particularly less than or equal to 0.3 mol / L.

[0061] The aqueous solution in step i) may have a concentration of hydroxide anions ranging from 3.103 mol / L to 2.6 mol / L, in particular from 0.03 mol / L to 1.3 mol / L, more particularly from 0.1 mol / L to 0.3 mol / L.

[0062] The aqueous solution in step i) may have BO2 ions in a concentration less than or equal to 0.8 mol / L, in particular less than or equal to 0.4 mol / L, more preferably less than or equal to 0.01 mol / L.

[0063] The volume of aqueous solution in step i) can be selected for immersing the catalyst in said aqueous solution. In particular, the aqueous solution in step i) can have a volume greater than or equal to 10 mL, in particular between 20 mL and 1 L, more particularly between 40 mL and 0.5 L.

[0064] In particular, the aqueous solution in step i) may consist of at least 90% by mass, in particular at least 95% by mass, more particularly at least 99% by mass, or even 100% by mass, relative to the total mass of said aqueous solution, of water, of at least one borohydride salt, and of at least one alkali or alkaline earth metal hydroxide.

[0065] In particular, the aqueous solution in step i) may be similar, or even identical, to that used to generate hydrogen by catalytic hydrolysis of at least one borohydride salt in a subsequent application of the catalyst. Step ii)

[0066] In step ii), the aqueous solution from step i) is brought into contact, at an initial time t0, with the catalyst A to initiate the hydrolysis reaction of the borohydride salt(s) present in the aqueous solution.

[0067] Catalyst A is a catalyst for the hydrolysis reaction of borohydride salts.

[0068] Catalyst A is advantageously a catalyst for the hydrolysis reaction of the or borohydride salts present in the aqueous solution in step i).

[0069] The catalyst A can be in a form suitable for its implementation in a hydrogen generation process, in particular by catalyzing the hydrolysis reaction of at least one borohydride salt.

[0070] In particular, catalyst A is in a form suitable for its implementation to generate hydrogen intended to power a fuel cell.

[0071] Known catalysts for the hydrolysis of borohydride salts include catalysts comprising at least one transition metal, in particular platinum, ruthenium, palladium, iridium, cobalt, nickel, iron, copper, and mixtures thereof. Specifically, a catalyst for the hydrolysis of borohydride salts may be in the form of a metal selected from platinum, ruthenium, palladium, iridium, cobalt, nickel, iron, copper, and mixtures thereof, supported by a porous substrate, in particular a porous metal substrate such as nickel, more particularly being in the form of cobalt supported by a porous nickel substrate.

[0072] Preferably, the catalyst A can be separate from a metal wire.

[0073] Steps i) and ii) may be simultaneous or consecutive.

[0074] The aqueous solution can be prepared prior to its contact with the catalyst A.

[0075] Preferably, the aqueous solution is brought into contact with the catalyst A in step ii) by immersing the catalyst A in the aqueous solution.

[0076] In particular, the aqueous solution and the catalyst A are brought into contact in a volume ratio aqueous solution / catalyst A ranging from 10 to 1000, in particular from 20 to 200, more particularly from 40 to 100.

[0077] Advantageously, the aqueous solution from step i) has not undergone any reaction before contact in step ii).

[0078] In particular, the aqueous solution of step i) can be prepared less than 24 h before contact in step ii), in particular less than 60 min before contact in step ii), more particularly less than 30 min before contact in step ii).

[0079] In step ii), the aqueous solution is brought into contact with the working electrode, in particular with the working electrode and the reference electrode. Preferably, step ii) comprises the formation of an electrochemical cell including the working electrode, the reference electrode, and the aqueous solution from step i), with the catalyst A in contact with said aqueous solution, in particular fully immersed in said aqueous solution. An example of an electrochemical cell is illustrated in [Fig. 1].

[0080] It is understood that the aqueous solution remains in contact with the catalyst A after the contacting in step ii) to catalyze the hydrolysis of the borohydride salt(s) present in the aqueous solution.

[0081] The solution can be used at any temperature suitable for the hydrolysis of borohydride salts. In particular, the aqueous solution can be at a temperature between 10°C and 90°C, especially between 20°C and 30°C, from the time of contact in step ii). Step iii)

[0082] In step iii), the open circuit potential E0Cvt(A) is measured at a time t strictly greater than the initial time t0, in particular at a time t during the hydrolysis reaction of the borohydride salt(s) catalyzed by catalyst A. Advantageously, the borohydride salt(s) react between time t0 and time t by hydrolysis reaction catalyzed by catalyst A.

[0083] For the purposes of this invention, "open circuit potential," also called OCV (Open Circuit Voltage), abbreviated Eocv, refers to the electrical potential difference between two electrodes when no current is applied between them. In other words, the open circuit potential can be considered the open-circuit voltage of a circuit when it is not connected to any load. The open circuit potential can be measured with a voltmeter connected to the two electrodes via conducting wires.

[0084] It is understood that the catalyst A is in contact with the aqueous solution from step ii) and at least until step iii), that is, from the initial time t0 and at least until time t. The catalyst A is in contact with the aqueous solution during the measurement of the open-circuit potential E0Cvt(A). Preferably, the catalyst A is fully immersed in the aqueous solution from the initial time to and at least until time t, more preferably the catalyst A is fully immersed in the aqueous solution throughout the hydrolysis reaction.

[0085] Advantageously, the time t is such that hydrolysis is in progress. In particular, the aqueous solution comprises at least one borohydride salt at time t. Advantageously, the open-circuit potential varies with time at time t, in particular by more than 5% of its value, more particularly by more than 10% of its value, with increasing times.

[0086] The time t can be such that the conversion rate of the borohydride salt(s) in the aqueous solution is greater than or equal to 5%, in particular greater than or equal to 10%, more particularly greater than or equal to 20%, especially greater than or equal to 30%.

[0087] The time t can be such that the conversion rate of the borohydride salt(s) in the aqueous solution is strictly less than 100%, in particular less than or equal to 95%, more particularly less than or equal to 90%, notably less than or equal to 80%, or even less than or equal to 70%.

[0088] In particular, the time t is such that the conversion rate of the borohydride salt(s) in the aqueous solution is greater than or equal to 5% and strictly less than 100%, in particular between 10% and 95%, more particularly between 20% and 90%, notably between 30% and 80%, or even between 30% and 70%.

[0089] For the purposes of the invention, the "conversion rate" of the borohydride salt(s) means the number of moles of the borohydride salt(s) that have reacted divided by the number of moles of the borohydride salt(s) initially present in the aqueous solution in step i). The conversion rate of the borohydride salt(s) can be determined by measuring the volume of dihydrogen generated by the hydrolysis of the borohydride salt(s) over time.

[0090] The time t can be such that the open circuit potential E0Cvt(B) is greater than or equal to Eocvmin(B)+0.05(Eocvmax(B)-Eocvmin(B)), in particular greater than or equal to Eocvmin(B)+0.1(Eocvmax(B)-Eocvmin(B)), more particularly greater than or equal to Eocvmin(B)+0.2(Eocvmax(B)-Eocvmin(B)), in particular greater than or equal to Eocvmin(B)+0.3(Eocvmax(B)-E0Cvmin(B)), or even greater than or equal to Eocvmin(B)+0.4(Eocvmax(B)-Eocvmin(B)).

[0091] The time t can be such that the open circuit potential E0Cvt(B) is strictly less than E0Cvmax(B), in particular less than or equal to Eocvmin(B)+0.95(Eocvmax(B)-E0Cvmin(B)), more particularly less than or equal to Eocvmin(B)+0.9(Eocvmax(B)-Eocvmin(B)), in particular less than or equal to Eocvmin(B)+0.8(Eocvmax(B)-Eocvmin(B)), or even less than or equal to Eocvmin(B)+0.7(Eocvmax(B)-Eocvmin(B)).

[0092] In particular, the time t is such that the open-circuit potential E0Cvt(B) is greater than or equal to Eocvmin(B)+0.1(Eocvmax(B)-Eocvmin(B)) and strictly less than E0Cvmax(B), in particular can be between Eocvmin(B)+0.2(Eocvmax(B)-Eocvmin(B)) and Eocvmin(B)+0.95(Eocvmax(B)-Eocvmin(B)), in particular can be between Eocvmin(B)+0.3(Eocvmax(B)-Eocvmin(B)) and Eocvmin(B)+0.9(Eocvmax(B)-Eocvmin(B)), more particularly between Eocvmin(B)+0.4(Eocvmax(B)-Eocvmin(B)) and Eocvmin(B)+0.8(Eocvmax(B)+0.95(Eocvmax(B)-Eocvmin(B)). (B)-E0Cvmin(B)).

[0093] E0Cvt(B) represents the pre-acquired open-circuit potential at the same time t with the reference catalyst B. E0Cvmin(B) represents the minimum pre-acquired open-circuit potential during the hydrolysis reaction with the reference catalyst B, and E0Cvmax(B) represents the maximum pre-acquired open-circuit potential at the end of the hydrolysis reaction with the reference catalyst B when the open-circuit potential reaches a constant value over time. It is understood that E0Cvt(B), E0Cvmin(B), and E0Cvmax(B) are pre-acquired with the reference catalyst B under the same conditions as the open-circuit potential E0Cvt(A). In particular, E0Cvt(B), Eocv min(B) and E0Cvmax(B) are obtained, with catalyst B instead of catalyst A, by the open circuit potential measurement method described in step iii), after implementation of steps i) and ii) under the same conditions as for catalyst A.

[0094] In the context of the invention, the open-circuit potential reaches a constant value over time if the open-circuit potential does not vary significantly, in particular by more than 5% of its value, and more particularly by more than 1% of its value, with increasing times. The open-circuit potential can be characterized as conforming to E0Cvmax(B) from the time when no increase of more than 5%, and in particular more than 1%, of its value is observed over time.

[0095] The duration between the initial time t0 and the time t, denoted t-to, can be greater than or equal to 30 s, in particular greater than or equal to 1 min, more particularly greater than or equal to 10 min, notably greater than or equal to 40 min.

[0096] The duration between the initial time t0 and the time t can be less than or equal to 12 h, in particular less than or equal to 6 h, more particularly less than or equal to 200 min.

[0097] In particular, the duration between the initial time t0 and the time t can be between 30 s and 12 h, in particular between 1 min and 6 h, more particularly between 10 min and 200 min, especially between 40 min and 200 min.

[0098] According to a particular embodiment, the time between the initial time t0 and time t is greater than or equal to a predetermined time on a reference curve representing the normalized open-circuit potential as a function of time for a reference catalyst, in particular catalyst B. The reference curve can be obtained by implementing steps i) and ii) under the same conditions as for catalyst A, the open-circuit potential being measured at different times according to the open-circuit potential measurement method described in step iii) and normalized according to the same normalization method as in step iv). In particular, the predetermined time is such that, on this reference curve, from the moment of contact, the normalized open-circuit potential has increased by 5%, in particular by 20%, more particularly by 30%, in particular by 40%, of the difference between the maximum normalized open-circuit potential and the minimum normalized open-circuit potential on the reference curve.

[0099] The open circuit potential of catalyst A in the aqueous solution is measured between the reference electrode and the working electrode.

[0100] The working electrode includes the catalyst A.

[0101] In particular, the working electrode may further comprise a conducting wire, with catalyst A in electrical contact with the conducting wire. Catalyst A may be brought into electrical contact with the conducting wire before step ii) or during step ii), in particular before step ii). The conducting wire may be a metallic wire, in particular any conductive metal such as, for example, nickel. Advantageously, the conducting wire, particularly with regard to its small specific surface area, does not interfere with the catalyst with respect to the hydrolysis reaction; that is to say, its catalytic activity is negligible compared to that of catalyst A.

[0102] In step iii), the catalyst A is in contact with the aqueous solution, in particular is fully immersed in the aqueous solution.

[0103] A reference electrode has a stable and known electrical potential. It maintains an invariable electrical potential, regardless of the chemical reaction occurring in the electrochemical system. The potential of a working electrode can thus be compared to this invariable electrical potential, and therefore measured.

[0104] In particular, the reference electrode can be chosen from among the reference electrodes known to the person skilled in the art, for example from a mercurous sulfate electrode Hg / Hg2SO4, a silver chloride electrode Ag / AgCl, a standard hydrogen electrode (SHE) and a saturated calomel electrode (SCE).

[0105] The open-circuit potential can be obtained by measuring the potential difference between the working electrode and the reference electrode. The potential difference between the working electrode and the reference electrode can be measured by methods well known to those skilled in the art, for example with a voltmeter electrically connected to the working electrode and the reference electrode, in particular via a conducting wire connecting the working electrode to the voltmeter and via a conducting wire connecting the reference electrode, in particular catalyst A, to the voltmeter.

[0106] In particular, the working electrode, the reference electrode, and the aqueous solution form an electrochemical cell in step iii) in which the catalyst A is in contact with the aqueous solution. The electrochemical cell can be formed in step ii) as described previously. In particular, the reference electrode and the working electrode are in contact with the aqueous solution. In particular, the electrochemical cell comprises only two electrodes, the working electrode and the reference electrode.

[0107] In particular, the electrochemical cell can be sealed.

[0108] An example of an electrochemical cell 100 for measuring the open circuit potential is shown in [Fig.1] and comprises an aqueous solution 20, a reference electrode 5, a conducting wire 10 and a catalyst 15, the catalyst 15 forming the working electrode. Step iv)

[0109] In step iv), the normalized open circuit potential Et(A) at time t is determined by normalization of the open circuit potential E0Cvt(A).

[0110] Normalizing the open-circuit potential allows for comparison of the open-circuit potential measured with different catalysts. In particular, the normalized open-circuit potential can be determined by a normalization method comprising adjusting the open-circuit potential based on a common reference scale among different catalysts.

[0111] Different normalization methods can be used.

[0112] According to a preferred normalization method, the normalized open-circuit potential at time t, denoted Et, can be determined, for a catalyst-catalyzed hydrolysis reaction, from the open-circuit potential Eocvt and the minimum open-circuit potential Eocvmin measured during the hydrolysis reaction, according to the formula Eocvt - Eocvmin. It is understood that Eocvmin is measured during the same hydrolysis reaction as the open-circuit potential Eocvt.

[0113] In particular, the normalized open-circuit potential Et(A) can be determined from the open-circuit potential E0Cvt(A) and the minimum open-circuit potential E0Cvmin(A) measured during the hydrolysis reaction, according to the formula E0Cvt(A) - Eocvmin(A). It is understood that E0Cvmin(A) is measured from the initial time t0, in particular between the initial time t0 and time t.

[0114] According to another normalization method, the normalized open-circuit potential at a time t, denoted Et, can be determined for a catalyzed hydrolysis reaction by a catalyst, from the open circuit potential Eocvt, the minimum open circuit potential Eocvmin measured during the hydrolysis reaction, and the maximum open circuit potential Eocvmax measured at the end of the hydrolysis reaction when the open circuit potential reaches a constant value over time, according to the formula a(E0Cvt- E0Cvmin) / (E0Cvmax- Eocvmin)+[3], where a and [3] are normalization coefficients, and a and [3] can be selected randomly. In particular, a can correspond to the difference between Eocvmin and Eocvmax for a reference curve and / or [3] can be equal to zero. It is understood that Eocvmin and Eocvmax are measured during the same hydrolysis reaction as the open circuit potential Eocvt.

[0115] Advantageously, the open circuit potential is normalized according to the same normalization method for different catalysts to be compared. Step v)

[0116] In order to characterize the catalyst A, in particular the catalytic performance of the catalyst A, the normalized open circuit potential Et(A) can be compared to a normalized open circuit potential Et(B) pre-acquired at the same time t with a reference catalyst B under the same conditions as the normalized open circuit potential Et(A) and / or the time t can be compared to a time t' at which a normalized open circuit potential Et(B) pre-acquired with a reference catalyst B under the same conditions as the normalized open circuit potential Et(A) is equal to the normalized open circuit potential Et(A).

[0117] The reference catalyst B is a catalyst for the hydrolysis reaction of borohydride salts. The reference catalyst B is advantageously a catalyst for the hydrolysis reaction of the borohydride salt(s) present in the aqueous solution in step i).

[0118] Advantageously, the reference catalyst B can be a catalyst whose catalytic performance is suitable for the generation of hydrogen, in particular for powering a fuel cell, in particular for the generation of hydrogen by catalysis of the hydrolysis reaction of at least one borohydride salt.

[0119] In particular, the reference catalyst B may be a catalyst whose catalytic performance has been previously evaluated by a standard characterization method, for example by measuring the volume of dihydrogen generated during the hydrolysis reaction of the borohydride salt(s).

[0120] Catalyst A and catalyst B may have the same chemical nature, structure and / or geometry. Catalyst A and catalyst B may also have different chemical nature, structure and / or geometry.

[0121] According to one embodiment, catalyst A may come from the same batch as catalyst B. Catalysts belonging to the same batch (also called a "lot" according to (Anglo-Saxon terminology) of catalysts were synthesized simultaneously under the same conditions, and stored together under the same conditions.

[0122] In particular, catalysts A and B may be the same catalyst sample, characterized at two distinct times, for example before and after storage.

[0123] According to another particular embodiment, catalyst A may be from a batch distinct from the batch from which catalyst B is derived. Catalysts are derived from distinct batches when they have been synthesized according to the same process, but have been synthesized separately and / or stored separately.

[0124] According to a particular embodiment, catalyst A can be synthesized by the same process as reference catalyst B but separately from reference catalyst B.

[0125] According to another particular embodiment, catalyst A can be synthesized by the same process as reference catalyst B but have been stored under different conditions than reference catalyst B.

[0126] According to another particular embodiment, catalyst A can be synthesized by a process distinct from the synthesis process of reference catalyst B and be of the same chemical nature as reference catalyst B.

[0127] According to another particular embodiment, catalyst A may have a chemical nature distinct from the chemical nature of the reference catalyst B.

[0128] A normalized open circuit potential, in particular Et(B) and / or Et(B), is pre-acquired with a reference catalyst, in particular the reference catalyst B, under the same conditions as the normalized open circuit potential Et(A) by implementing steps i), ii), iii) and iv) with said reference catalyst, in particular the reference catalyst B, under the same conditions as steps i), ii), iii) and iv) implemented with catalyst A.

[0129] Step i) is carried out under the same conditions for at least two catalysts by using the same aqueous solution. Step iii) is carried out under the same conditions for at least two catalysts by measuring the open-circuit potential using the same measurement method. Step iv) is carried out under the same conditions for at least two catalysts by normalizing the open-circuit potential using the same normalization method.

[0130] In particular, the normalized open circuit potential Et(B) and / or Et(B) is pre-acquired according to steps i), ii), iii) and iv) by replacing catalyst A with catalyst B in steps i), ii) and iii) of the process according to the invention.

[0131] According to a particular embodiment, step v) may include comparing the normalized open-circuit potential Et(A) to a normalized open-circuit potential Et(B) pre-acquired at the same time t with a reference catalyst B under the same conditions as the normalized open circuit potential Et(A).

[0132] The normalized open circuit potential Et(B) is advantageously pre-acquired at the same time t by determining the normalized open circuit potential for the catalyst B at a time such that the duration since the contact in step ii) of the aqueous solution with the catalyst B is equal to the duration between the initial time t0 and the time t.

[0133] In particular, the normalized open-circuit potential Et(B) is pre-acquired by: - ​​contacting catalyst B, at an initial time t0B, with an aqueous solution identical to the aqueous solution of step i) for catalyst A, to initiate the hydrolysis reaction of the borohydride salt(s),

[0134] - measurement of the open-circuit potential E0Cvt(B) of catalyst B in said solution aqueous solution at a time equal to t0B+(t-t0), the open-circuit potential of catalyst B in said aqueous solution being measured between a reference electrode and a working electrode, the working electrode comprising catalyst B, and - determination of the normalized open circuit potential Et(B) by normalization of the open circuit potential E0Cvt(B) according to the same normalization method as that implemented in step iv) for catalyst A.

[0135] According to a particular embodiment, the process according to the invention comprises at least the steps of:

[0136] i) Have an aqueous solution of pH greater than or equal to 11, preferably greater than or equal to 12, comprising at least one borohydride salt; ii) Contact, at an initial time to, said aqueous solution from step i) with catalyst A to initiate the hydrolysis reaction of said borohydride salt; iii) Measure the open circuit potential E0Cvt(A) of catalyst A in said aqueous solution at a time t strictly greater than the initial time to, the open circuit potential of catalyst A in said aqueous solution being measured between a reference electrode and a working electrode, the working electrode comprising catalyst A, ; iv) Determine the normalized open circuit potential Et(A) at time t by normalizing the open circuit potential E0Cvt(A); i') Have a second aqueous solution identical to the aqueous solution from step i); ii') Contact, at an initial time t0B, said second aqueous solution from step i') with the reference catalyst B to initiate the hydrolysis reaction of said borohydride salt under the same conditions as the contact in step ii); iü') Measure the open circuit potential E0Cvt(B) of catalyst B in said second aqueous solution at a time equal to t0B+(t-t0), the open circuit potential of catalyst B in said second aqueous solution being measured between a second reference electrode and a second working electrode, the second working electrode comprising catalyst B, the second reference electrode being in particular identical to the reference electrode implemented in step iü); iv') Determine the normalized open circuit potential Et(B) at time t by normalizing the open circuit potential E0Cvt(B) according to a normalization method identical to that implemented in step iv); v) Compare said normalized open circuit potential Et(A) to the normalized open circuit potential Et(B).

[0137] Steps i'), ii'), iü') and iv') can be carried out before step i).

[0138] As is apparent from the foregoing and the examples below, the higher the value of the The higher the normalized potential Et(A), the more efficient the catalyst A.

[0139] In particular, if the normalized open-circuit potential Et(A) is less than the normalized potential Et(B), the catalytic performance of catalyst A is less than the catalytic performance of the reference catalyst B.

[0140] In particular, if the normalized open-circuit potential Et(A) is greater than the normalized potential Et(B), the catalytic performance of catalyst A is greater than the catalytic performance of the reference catalyst B.

[0141] In particular, if the normalized open-circuit potential Et(A) is equal to the normalized potential Et(B), the catalytic performance of catalyst A is similar to the catalytic performance of the reference catalyst B.

[0142] According to a particular embodiment, step v) may include comparing time t to time t' at which a normalized open circuit potential Et (B) pre-acquired with a reference catalyst B under the same conditions as the normalized open circuit potential Et(A) is equal to the normalized open circuit potential Et (A).

[0143] In particular, the duration t-to is compared to the duration t'-t0B, with t0B being the time at which an aqueous solution identical to the solution of step i) is brought into contact with the catalyst B to initiate the hydrolysis reaction of the borohydride salt(s).

[0144] As can be seen from the above and the examples below, the lower the time t, the more efficient the catalyst A.

[0145] In particular, if time t is less than time t', the catalytic performance of catalyst A is greater than the catalytic performance of reference catalyst B.

[0146] In particular, if time t is greater than time t', the catalytic performance of catalyst A is less than the catalytic performance of reference catalyst B.

[0147] In particular, if time t is equal to time t', the catalytic performance of catalyst A is similar to the catalytic performance of reference catalyst B.

[0148] In particular, the method may include: - in step iii), the measurement of the open-circuit potential of catalyst A in said aqueous solution at several times f strictly greater than the initial time t0 to determine the open-circuit potential profile E0CV(A) as a function of time from the initial time to, - in step iv), the determination of the normalized open-circuit potential profile E(A) as a function of time from the initial time to by normalizing the open-circuit potential profile E0CV(A), and - in step v), the comparison of the normalized open circuit potential profile E(A) to a pre-acquired normalized open circuit potential profile E(B) with the reference catalyst B under the same conditions as the normalized open circuit potential profile E(A).

[0149] The open circuit potential profile, denoted Eocv, corresponds to the curve representing the open circuit potential as a function of time.

[0150] The open-circuit potential is measured at several times t, strictly greater than the initial time t0, over time, in particular at several successive times f, between the reference electrode and the working electrode. Advantageously, the catalyst A is completely immersed in the aqueous solution between the initial time t0 and the times f.

[0151] In particular, the open circuit potential is measured over time from the initial time to to determine the open circuit potential profile E0CV(A) as a function of time from the initial time to.

[0152] In particular, the open-circuit potential profile E0CV(A) is determined at least between the initial time t0 and the time at which the hydrolysis reaction is complete, in particular at least between the initial time t0 and the time at which the open-circuit potential reaches its maximum value, and more particularly at least between the initial time t0 and the time at which the normalized open-circuit potential reaches a constant value Econstant(A) with time. The normalized open-circuit potential can be characterized as conforming to Econstant(A) from the time at which no variation of more than 5%, and in particular of more than 1%, is observed in its value with increasing times.

[0153] In particular, the open-circuit potential profile E0CV(A) is determined at least between the initial time to and the time at which the conversion rate of the borohydride salt(s) in the aqueous solution reaches 10%, in particular 20%, plus particularly 30%, notably 50%, or even 70%, or even 90%, for example 100%.

[0154] The open circuit potential profile E0CV(A) can be determined from the initial time t0 for a duration greater than or equal to 30 s, in particular greater than or equal to 1 min, more particularly greater than or equal to 10 min, in particular greater than or equal to 40 min, or even greater than or equal to 80 min.

[0155] The open circuit potential profile E0CV(A) can be determined from the initial time t0 for a period less than or equal to 12 h, in particular less than or equal to 8 h, more particularly less than or equal to 6 h, in particular less than or equal to 300 min, or even less than or equal to 200 min.

[0156] In particular, the open circuit potential profile E0CV(A) can be determined from the initial time t0 for a duration between 30 s and 12 h, in particular between 1 min and 8 h, more particularly between 10 min and 6 h, notably between 40 min and 300 min, or even between 80 min and 200 min.

[0157] The times f can be spaced by a duration less than or equal to 25 min, in particular less than or equal to 5 min, more particularly less than or equal to 1 min, in particular less than or equal to 30 s, or even less than or equal to 10 s, or even less than or equal to 5 s, from the initial time t0. The first time f can be spaced from the initial time t0 by a duration less than or equal to 25 min, in particular less than or equal to 5 min, more particularly less than or equal to 1 min, in particular less than or equal to 30 s, or even less than or equal to 10 s, or even less than or equal to 5 s.

[0158] The normalized open circuit potential profile, denoted E, corresponds to the curve representing the normalized open circuit potential as a function of time.

[0159] The normalized open-circuit potential profile E(A) as a function of time is determined from the initial time t0 by normalizing the open-circuit potential profile E0CV(A), in particular by normalizing the open-circuit potential measured at times f. It is understood that the open-circuit potential is normalized according to the same normalization method at the different times t;.

[0160] In particular, the normalized open-circuit potential profile E(A) is determined during the same duration and / or at the same times as the open-circuit potential profile E0CV(A) in step iii).

[0161] The normalized open-circuit potential profile E(B) is pre-acquired with the reference catalyst B under the same conditions as the normalized open-circuit potential profile E(A) by implementing steps i), ii), iii) and iv) with the reference catalyst B, under the same conditions as steps i), ii), iii) and iv) implemented with catalyst A, in particular as detailed previously.

[0162] In particular, the pre-acquired normalized open circuit potential profile E(B) forms a reference curve, also called a nomogram, allowing one or more catalysts to be compared to a known reference catalyst B.

[0163] In particular, the normalized open-circuit potential profile E(B) is pre-acquired by: - contacting catalyst B, at an initial time t0B, with an aqueous solution identical to the aqueous solution of step i) for catalyst A, to initiate the hydrolysis reaction of the borohydride salt(s),

[0164] - measurement of the open-circuit potential of catalyst B in said aqueous solution at several times t;B strictly greater than the initial time t0B to determine the open circuit potential profile E0CV(B) as a function of time from the initial time t0B, the open circuit potential of catalyst B in said aqueous solution being measured between a second reference electrode and a second working electrode as described above, - determination of the normalised open circuit potential profile E(B) as a function of time from the initial time t0B by normalisation of the open circuit potential profile E0CV(B) according to the same normalisation method as that implemented in step iv) for catalyst A.

[0165] According to a particular embodiment, the process according to the invention comprises at least the steps of:

[0166] i) Have an aqueous solution of pH greater than or equal to 11, preferably greater than or equal to 12, comprising at least one borohydride salt; ii) Contact, at an initial time t0, said aqueous solution from step i) with catalyst A to initiate the hydrolysis reaction of said borohydride salt; iii) Measure the open circuit potential of catalyst A in said aqueous solution at several times h strictly greater than the initial time t0 to determine the open circuit potential profile E0CV(A) as a function of time from the initial time t0, the open circuit potential of catalyst A in said aqueous solution being measured between a reference electrode and a working electrode, the working electrode comprising catalyst A; iv) Determine the normalized open circuit potential profile E(A) as a function of time from the initial time t0 by normalizing the open circuit potential profile E0CV(A); i') Have a second aqueous solution identical to the aqueous solution from step i); ii') Contact, at an initial time t0B, said second aqueous solution from step i') with the reference catalyst B to initiate the hydrolysis reaction of said borohydride salt under the same conditions as the contact in step ii); iii' ) Measure the open-circuit potential of catalyst B in said second aqueous solution at several times t;B strictly greater than the initial time t0B to determine the open-circuit potential profile E0CV(B) as a function of time from the initial time t0B, the open circuit potential of catalyst B in said second aqueous solution being measured between a second reference electrode and a second working electrode as described previously; iv') Determine the normalized open circuit potential profile E(B) as a function of time from the initial time t0B by normalizing the open circuit potential profile E0CV(B) according to a normalization method identical to that implemented in step iv); v) Compare the normalized open circuit potential profile E(A) to the normalized open circuit potential profile E(B).

[0167] Steps i'), ii'), iii') and iv') can be carried out before step i).

[0168] Comparison of the normalized open-circuit potential profile E(A) to the profile of normalized open circuit potential E(B), may include comparison of the shape of the profiles, the value of the normalized open circuit potential at one or more times, or the time associated with one or more values ​​of the normalized open circuit potential.

[0169] As is apparent from the foregoing and the examples below:

[0170] - if the normalized open-circuit potential profile E(A) is substantially identical at normalized open circuit potential profile E(B), then the catalytic performance of catalyst A is similar to the catalytic performance of catalyst B;

[0171] - if the normalized open-circuit potential profile E(A) exhibits an evolution slower over time than the normalized open-circuit potential profile E(B), then the catalytic performance of catalyst A is lower than the catalytic performance of catalyst B;

[0172] - if the normalized open-circuit potential profile E(A) exhibits a more faster over time than the normalized open circuit potential profile E(B), the catalytic performance of catalyst A is superior to the catalytic performance of catalyst B.

[0173] In particular, the normalized open-circuit potential profile E(A) can be compared to the normalized open-circuit potential profile E(B) by comparing, at several times tj, the normalized open-circuit potential at time tj for the catalyst A, denoted Et j(A), with the normalized open circuit potential at the same time tj for catalyst B, denoted Et j(B).

[0174] Without being bound by any theory, the inventors consider that the normalized open circuit potential profile during hydrolysis makes it possible to characterize, under specific and known conditions, a given catalyst, and as such can be considered as an electrochemical signature of the catalyst.

[0175] Without being bound by any theory, the inventors attribute the normalized open circuit potential profile during hydrolysis to the progress of the hydrolysis reaction, but also to certain characteristics of the catalyst such as the surface state of the catalyst, in particular the presence of reactive species on the surface of the catalyst such as oxides or hydroxides, its chemical composition, its loading rate, its porosity or its specific surface area.

[0176] A catalyst can be characterized by the process according to the invention in order to determine its suitability for use in generating hydrogen in an end application, in particular for powering a fuel cell, in particular for generating hydrogen by catalysis of the hydrolysis reaction of at least one borohydride salt.

[0177] Advantageously, in the context of the invention, a catalyst A may be suitable for use in generating hydrogen in an end application if the normalized open-circuit potential profile E(A) is similar to the normalized open-circuit potential profile E(B), with B being a reference catalyst for hydrogen generation in said end application. In particular, the normalized open-circuit profile E(A) is similar to the normalized open-circuit profile E(B) if the normalized open-circuit potential values ​​E(A) do not vary by more than 20%, in particular by more than 10%, more particularly by more than 5%, or even by more than 1%, with respect to the normalized open-circuit potential values ​​E(B) at the same times h of said profiles.

[0178] According to a particular embodiment, the process uses in step i) 50 mL of an aqueous solution comprising 1.1 mol / L sodium borohydride and 0.1 mol / L sodium hydroxide, and catalyst A, which may be suitable for use in generating hydrogen to power a fuel cell if the normalized open-circuit potential Et(A) is greater than or equal to 0.05 V, in particular greater than or equal to 0.10 V, more particularly greater than or equal to 0.15 V, at a time t that is 120 min after time t0 on the normalized open-circuit potential profile E(A). Uses

[0179] A catalyst characterized by the process according to the invention can be used to generate hydrogen. The catalyst used can be the same sample as that directly characterized by the process according to the invention or a catalyst sample from the same batch. In particular, the catalyst used is suitable for use in generating hydrogen.

[0180] Hydrogen can be generated by catalysis of the hydrolysis reaction of at least one borohydride salt.

[0181] The process according to the invention can be implemented before the use of the catalyst.

[0182] The hydrogen generated can be used to power a fuel cell.

[0183] The hydrogen generated can be used in the field of transport, for example from maritime transport, in the field of stationary applications, for example in the building sector, in the field of massive hydrogen transport, particularly from country to country or even from continent to continent.

[0184] In particular, the invention relates to a method comprising at least the steps of: (i) Have an aqueous solution with a pH greater than or equal to 11, preferably greater than or equal to 12, comprising at least one borohydride salt; ii) Contact, at an initial time t0, said aqueous solution from step i) with catalyst A to initiate the hydrolysis reaction of said borohydride salt; iii) Measure the open circuit potential E0Cvt(A) of catalyst A in said aqueous solution at a time t strictly greater than the initial time t0, in particular measure the open circuit potential of catalyst A in said aqueous solution at several times h strictly greater than the initial time t0 to determine the open circuit potential profile E0CV(A) as a function of time from the initial time t0, the open circuit potential of catalyst A in said aqueous solution being measured between a reference electrode and a working electrode, the working electrode comprising catalyst A; iv) Determine the normalized open circuit potential Et(A) at time t by normalizing the open circuit potential E0Cvt(A), in particular determine the normalized open circuit potential profile E(A) as a function of time from the initial time t0 by normalizing the open circuit potential profile E0CV(A); (v) Compare said normalized open-circuit potential Et(A) to a normalized open-circuit potential Et(B) pre-acquired at the same time t with a reference catalyst B under the same conditions as the normalized open-circuit potential Et(A) and / or compare time t to a time t' at which a normalized open-circuit potential Et(B) pre-acquired with a reference catalyst B under the same conditions as the normalized open-circuit potential Et(A) is equal to the normalized open-circuit potential Et(A), in particular compare the normalized open-circuit potential profile E(A) to a normalized open-circuit potential profile E(B) pre-acquired with the reference catalyst B under the same conditions as the normalized open-circuit potential profile E(A); and

[0185] vi) if the normalized open-circuit potential Et(A) does not vary by more than 20%, in particular by more than 10%, in particular by more than 5%, or even by more than 1% with respect to the value of the normalized open-circuit potential Et(B) and / or if the time t does not vary by more than 20%, in particular by more than 10%, in particular by more than 5%, or even by more than 1% with respect to the time t', in particular if the values ​​of the normalized open-circuit potential Et;(A) do not vary by more than 20%, in particular by more than 10%, in particular by more than 5%, or even by more than 1% with respect to the values ​​of the normalized open-circuit potential Et;(B) at the same times f, generate hydrogen by catalysis of the hydrolysis reaction of at least one borohydride salt, the hydrolysis reaction being catalyzed by catalyst A or a catalyst from the same batch as catalyst A.

[0186] In particular, steps i) to v) can be carried out according to the method of the invention, in particular as described above.

[0187] The process according to the invention can be used to control the quality of a catalyst, in particular a catalyst for the hydrolysis reaction of at least one borohydride salt.

[0188] In particular, the method according to the invention can be used to control the quality of a catalyst against a reference catalyst for the intended application. Preferably, the method according to the invention can be used to control the quality of a catalyst against a catalyst of the same chemical nature, structure, and / or geometry.

[0189] In particular, the process according to the invention can be used to control the quality of a catalyst after its synthesis.

[0190] In particular, the process according to the invention can also be used to control the quality of a catalyst that may have degraded, in particular by aging or during a catalytic reaction.

[0191] The method according to the invention can also be used to compare two catalysts of distinct compositions.

[0192] The process according to the invention can also be used to compare two catalysts obtained by distinct processes.

[0193] In particular, the process according to the invention can be used to optimize the process of synthesizing a catalyst.

[0194] The process according to the invention can also be used to develop new catalysts.

[0195] The method according to the invention can also be used to characterize the physical and / or chemical properties of a catalyst, in particular its specific surface area and / or its surface state.

[0196] The invention will now be described by means of the following examples, given by way of illustration and not limitation of the invention. Examples

[0197] Protocol for measuring the normalized open-circuit potential for a catalyst

[0198] 50g of aqueous solution is prepared by dissolving 0.5% by mass of NaOH then 4% mass of NaBH4 in water. The theoretical volume of dihydrogen that can be generated by hydrolysis of NaBH4 from this solution is 5 IL.

[0199] The aqueous solution (50g) is introduced as an electrolyte 20 into a sealed electrochemical cell 100, illustrated in [Fig.1], comprising a saturated mercury sulfate electrode (Hg / Hg2SO4) as a reference electrode 5 and a catalyst 15 to be characterized as a working electrode connected to a nickel wire 10.

[0200] After introduction of the aqueous solution into the electrochemical cell, the catalyst is immersed in the aqueous solution, resulting in the hydrolysis of NaBH4.

[0201] The open circuit potential Eocv of the catalyst in the aqueous solution is measured between the reference electrode and the working electrode over time with a voltmeter following the introduction of the aqueous solution into the electrochemical cell.

[0202] The normalized open-circuit potential is calculated from the measured open-circuit potential Eocv and the minimum open-circuit potential measured during the hydrolysis reaction Eocvmin according to the formula Eocv-Eocvmin. The normalized open-circuit potential is plotted as a function of time, with t0=0 when the aqueous solution is introduced into the electrochemical cell, to form the normalized open-circuit potential profile.

[0203] The electrochemical cell is connected to a flow meter to monitor the hydrogen flow rate during hydrolysis, allowing the hydrolysis kinetics to be measured and compared with the evolution of the normalized open-circuit potential. A desiccant material is positioned between the electrochemical cell and the flow meter to trap any water that may be transported with the hydrogen flow generated during hydrolysis and to protect the flow meter. Example 1

[0204] The open-circuit potential, also called OCV, is measured during the hydrolysis of NaBH4, according to the protocol described above, for two identical reference catalysts from two separate batches containing cobalt deposited on nickel foam. Cobalt can be deposited on nickel foam to form a catalyst according to the processes described in Dai et al. (Journal of Power Sources, 2008) or in Gang et al. (International Journal of Hydrogen Energy, 2016).

[0205] The OCV is represented on [Fig 2] before normalisation, noted “OCV RAW” ([Fig.2a]) and after normalisation, noted “OCV NORMALISE” ([Fig.2b]).

[0206] As shown in [Fig. 2a], the OCV decreases during the first few minutes to reach a minimum OCV, which differs between the two reference catalysts. The difference in OCV between the minimum and maximum OCV is similar for both catalysts (on the order of 200 mV), which reflects an initial and a final state.

[0207] In order to compare the open-circuit potential during hydrolysis, the normalized OCV is calculated and shown in [Fig. 2b] as a function of time. It can be deduced that the two similar reference catalysts exhibit a similar normalized open-circuit potential profile. Example 2

[0208] The open circuit potential and the flow rate of hydrogen generated are measured during the hydrolysis of NaBH4, according to the protocol described above, for 18 catalysts comprising Cobalt deposited on a nickel foam, synthesized separately according to the same synthesis protocol.

[0209] The normalized OCV is shown in [Fig.3a] and compared to the NaBH4 conversion rate, calculated from the measured hydrogen flow rate, shown in [Fig.3b].

[0210] As can be seen from [Fig. 3a], the normalized OCV profile is similar for all 18 catalysts. [Fig. 3b] confirms that the hydrolysis reaction kinetics are similar for all 18 catalysts.

[0211] Therefore, the normalised OCV profile characterises the fact that the 18 catalysts exhibit similar performance and that the synthesis protocol makes it possible to obtain reproducible catalysts.

[0212] The process of the present invention thus makes it possible to estimate the reproducibility of the synthesized catalysts for their future integration into the final application system. It can therefore be implemented as a "quality control" of the catalyst resulting from the synthesis, this quality control being non-destructive, both in terms of catalytic performance and mechanical integrity. Example 3 Example 3a

[0213] Three catalysts comprising Cobalt deposited on a nickel foam were synthesized simultaneously by the same synthesis protocol as in Examples 1 and 2.

[0214] The first catalyst, for which the results are reported under the designation "calcined at 450°C", is calcined in air for 3 hours at a temperature of 450°C. The second catalyst, for which the results are reported under the designation " "Calcined at 600°C", is calcined under air for 3 hours at a temperature of 600°C. After calcination, the OCV and the hydrogen flow rate generated are measured during the hydrolysis of NaBH4 according to the protocol detailed previously.

[0215] The third catalyst, for which the results are reported under the designation "uncalcined," undergoes no treatment and is characterized directly after its synthesis. The OCV and hydrogen flow rate are measured during the hydrolysis of NaBH4 according to the protocol detailed above.

[0216] As can be seen from [Fig.4a], the normalised OCV profile obtained during hydrolysis for the catalyst calcined at 600°C evolves more slowly than the normalised OCV profile obtained during hydrolysis for the catalyst calcined at 450°C, and the normalised OCV profile obtained during hydrolysis for the catalyst calcined at 450°C evolves more slowly than the normalised OCV profile obtained for the non-calcined catalyst.

[0217] This evolution of the normalized OCV profiles is correlated with the kinetics of NaBH4 hydrolysis, as confirmed by [Fig. 4b], which represents the NaBH4 conversion rate calculated from the measured hydrogen flow rate. More specifically, NaBH4 hydrolysis is slower for calcined catalysts than for the uncalcined reference catalyst, and even slower at higher calcination temperatures.

[0218] The normalised OCV profile therefore makes it possible to characterise the degradation of the catalytic performance of a catalyst following its calcination. Example 3b

[0219] Several catalysts comprising Cobalt deposited on a nickel foam were synthesized simultaneously by the same synthesis protocol as in examples 1, 2 and 3a.

[0220] These catalysts were chemically aged by successive catalysis of the hydrolysis reaction of borohydride salts from aqueous solutions containing 2 wt% NaBH4 and 0.25 wt% NaOH, followed by a period of immersion in the solution obtained after hydrolysis. At the end of each hydrolysis reaction, the catalyst is immersed in a new aqueous solution containing 2 wt% NaBH4 and 0.25 wt% NaOH. Each hydrolysis reaction is called an aging run. Fourteen aging runs were carried out over a period of three months, corresponding to four months of aging in the end-use product.

[0221] Two catalysts were sampled at the end of 2 runs, 3 runs, 5 runs, 7 runs, 10 runs, 11 runs, 12 runs, 13 runs and 14 runs to measure the OCV and the hydrogen flow generated during the hydrolysis of NaBH4, according to the protocol described above. The OCV and hydrogen flow rate are similar for both catalysts sampled from the same run.

[0222] The OCV and hydrogen flow rate are also measured during the hydrolysis of NaBH4 carried out according to the protocol detailed above, for a reference catalyst which has not undergone any aging run, for which the results are reported under the designation "unaged".

[0223] As can be seen in [Fig. 5a], where the curves represent the average for the two sampled catalysts, the more aging runs a catalyst has undergone, the slower the normalized OCV profile obtained during the hydrolysis of NaBH4 evolves. Furthermore, the normalized OCV profiles obtained for the aged catalysts evolve more slowly than the normalized OCV profile obtained for the unaged catalyst.

[0224] This evolution of the normalized OCV profiles is correlated with the kinetics of NaBH4 hydrolysis, as confirmed by [Fig. 5c], which represents the NaBH4 conversion rate, calculated from the measured hydrogen flow rate, as a function of time. [Fig. 5c] indeed shows that NaBH4 hydrolysis slows down as a function of the number of catalyst aging runs.

[0225] The normalised OCV profile therefore makes it possible to characterise the degradation of the catalytic performance of a catalyst following chemical aging.

[0226] As can be seen from Figures 4a and 5a, the normalized OCV profile is correlated with the kinetics of the NaBH4 hydrolysis reaction.

[0227] It also appears from the comparison of the normalized OCV profiles and the dihydrogen flow rate over time that the OCV continues to evolve after the hydrolysis reaction, following the complete conversion of the NaBH4 initially present in solution, until it reaches an identical plateau for all catalysts. Furthermore, for the calcined catalysts, a plateau is observed in the normalized OCV profile at the beginning of the hydrolysis reaction. Figure 5b shows the normalized OCV profile at the beginning of the hydrolysis reaction after aging runs.

[0228] Without being bound by any theory, the inventors attribute these characteristics of the normalized OCV profile to the nature of the catalyst, in particular its surface state, its specific surface area or its chemical nature, and to its potential reactivity during the hydrolysis of NaBH4.

[0229] It may be noted that, in all the tests in Example 3, the total variation of the open circuit potential during hydrolysis, between the minimum of the open circuit potential and the maximum of the open circuit potential at the end of the reaction, is constant, of about 200mV. Example 4

[0230] A first catalyst comprising Cobalt deposited on a nickel foam was synthesized by the same synthesis protocol as in examples 1, 2, and 3.

[0231] A second catalyst comprising Ruthenium on a nickel foam has also been synthesized.

[0232] The OCV and hydrogen flow rate are measured during the hydrolysis of NaBH4 according to the protocol detailed above for each catalyst.

[0233] The evolution of the NaBH4 conversion rate obtained from the hydrogen flow rate, illustrated in [Fig.6b] and the normalised OCV profile illustrated in [Fig.6a] are correlated in that the hydrolysis reaction is faster with the Ruthenium catalyst, as well as the normalised OCV profile evolves faster with the Ruthenium catalyst.

[0234] The normalised OCV profile therefore makes it possible to characterise a better catalytic performance of the second catalyst compared to the first catalyst, and thus to compare the catalytic performance of catalysts of different chemical natures.

Claims

Demands

1. A method for characterizing a catalyst, said catalyst A, in particular for characterizing the catalytic performance of catalyst A, comprising at least the steps of: i) Having an aqueous solution of pH greater than or equal to 11, preferably greater than or equal to 12, comprising at least one borohydride salt; ii) Contacting said aqueous solution of step i) with catalyst A at an initial time t0 to initiate the hydrolysis reaction of said borohydride salt; iii) Measuring the open-circuit potential E0Cvt(A) of catalyst A in said aqueous solution at a time t strictly greater than the initial time t0, the open-circuit potential of catalyst A in said aqueous solution being measured between a reference electrode and a working electrode, the working electrode comprising catalyst A;(iv) Determine the normalized open-circuit potential Et(A) at time t by normalizing the open-circuit potential E0Cvt(A); and (v) Compare said normalized open-circuit potential Et(A) to a normalized open-circuit potential Et(B) pre-acquired at the same time t with a reference catalyst B under the same conditions as the normalized open-circuit potential Et(A) and / or compare time t to a time t' at which a normalized open-circuit potential Et(B) pre-acquired with a reference catalyst B under the same conditions as the normalized open-circuit potential Et(A) is equal to the normalized open-circuit potential Et(A).

2. The process according to claim 1, wherein the borohydride salt(s) are selected from alkali or alkaline earth metal borohydrides and mixtures thereof, in particular from sodium borohydride NaBH4, potassium borohydride KBH4, lithium borohydride LiBH4, magnesium borohydride Mg(BH4)2, calcium borohydride Ca(BH4)2 and mixtures thereof, more particularly from NaBH4, KBH4 and mixtures thereof, in particular comprising at least NaBH4, or even being NaBH4.

3. Process according to claim 1 or 2, the aqueous solution in step i) having a concentration of borohydride anions greater than or equal to 0.01 mol / L, in particular from 0.1 mol / L to 2 mol / L, more particularly from 0.5 mol / L to 1.5 mol / L, or even from 1 mol / L to 1.5 mol / L.

4. A process according to any one of the preceding claims, the aqueous solution in step i) having a pH from 11 to 14, in particular from 12 to 14, more particularly from 13 to 14.

5. A process according to any one of the preceding claims, the aqueous solution in step i) comprising at least one alkali or alkaline earth metal hydroxide, in particular at least one alkali metal hydroxide, more particularly selected from sodium hydroxide NaOH, potassium hydroxide KOH, lithium hydroxide LiOH and mixtures thereof, in particular comprising at least NaOH.

6. A process according to any one of the preceding claims, wherein the aqueous solution in step i) is composed of at least 90% by mass, in particular at least 95% by mass, more particularly at least 99% by mass, or even 100% by mass, relative to the total mass of said aqueous solution, of water, at least one borohydride salt, and at least one alkali or alkaline earth metal hydroxide.

7. A method according to any one of the preceding claims, the contacting in step ii) being carried out by immersing the catalyst A in said aqueous solution, in particular the catalyst A being fully immersed in the aqueous solution from the initial time t0 and at least until time t.

8. A method according to any one of the preceding claims, wherein time t is such that the pre-acquired open-circuit potential E0Cvt(B) at the same time t with the reference catalyst B is greater than or equal to E0Cvmin(B) + 0.1(Eocvmax(B) - Eocvmin(B)), in particular greater than or equal to Eocvmin(B) + 0.2(Eocvmax(B) - Eocvmin(B)), notably between Eocvmin(B) < 3(Eocvmin(B) - Eocvmin(B) and Eocvmin(B) + 0.9(Eocvmax(B) - E0Cvmin(B)), with E0Cvmin(B) representing the minimum pre-acquired open-circuit potential during the hydrolysis reaction with the reference catalyst B and E0Cvmax(B) representing the maximum pre-acquired open-circuit potential at the end of the reaction hydrolysis with the reference catalyst B when the open circuit potential reaches a constant value over time.

9. A method according to any one of the preceding claims, the duration between the initial time t0 and the time t, denoted t-t0, being greater than or equal to 30 s, in particular greater than or equal to 1 min, in particular between 1 min and 6 h, more particularly between 10 min and 200 min.

10. A method according to any one of the preceding claims, comprising: - in step iii), measuring the open-circuit potential of catalyst A in said aqueous solution at several times h strictly greater than the initial time t0 to determine the open-circuit potential profile E0CV(A) as a function of time from the initial time t0, - in step iv), determining the normalized open-circuit potential profile E(A) as a function of time from the initial time t0 by normalizing the open-circuit potential profile E0CV(A), and - in step v), comparing the normalized open-circuit potential profile E(A) to a pre-acquired normalized open-circuit potential profile E(B) with the reference catalyst B under the same conditions as the normalized open-circuit potential profile E(A).

11. Use of a catalyst characterized by the process according to any one of the preceding claims for generating hydrogen, in particular by catalyzing the hydrolysis reaction of at least one borohydride salt.

12. Use of the process according to any one of claims 1 to 10 to control the quality of a catalyst, in particular a catalyst for the hydrolysis reaction of at least one borohydride salt.

13. Use according to the preceding claim to control the quality of a catalyst at the end of its synthesis.

14. Use according to claim 12 to control the quality of a catalyst that may have degraded, in particular by aging or during a catalytic reaction.

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