CN111952609A

CN111952609A - Anode catalyst of direct borohydride fuel cell and preparation method thereof

Info

Publication number: CN111952609A
Application number: CN202010810630.0A
Authority: CN
Inventors: 田晓; 刘昕瑀; 杨艳春
Original assignee: Inner Mongolia Normal University
Current assignee: Inner Mongolia Normal University
Priority date: 2020-08-13
Filing date: 2020-08-13
Publication date: 2020-11-17

Abstract

The invention relates to an anode catalyst of a direct borohydride fuel cell, a preparation method thereof, the direct borohydride fuel cell and an anode catalyst of the direct borohydride fuel cell, wherein the anode catalyst is La-Mg-Ni A series₂B₇A hydrogen storage alloy of the type having the molecular formula: (Mm)_1‑xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21Where x is 0.1-0.5 and Mm is a misch metal consisting of 63.1 wt% La, 26.0 wt% Ce, 2.7 wt% Pr, and 8.2 wt% Nd. The anode catalyst is La-Mg-Ni system A₂B₇The hydrogen storage alloy not only has direct catalytic capability, but also can adsorb (or store) the generated hydrogen and release the hydrogen in the form of electric energy, improves the fuel utilization rate through double functions, and solves the problems that the noble metal serving as an anode catalyst in the prior art is high in price and the common hydrogen storage alloy has high storage capacityThe catalytic activity of the hydrogen alloy is not good.

Description

Anode catalyst of direct borohydride fuel cell and preparation method thereof

Technical Field

The invention relates to the field of fuel cells, in particular to an anode catalyst of a direct borohydride fuel cell, a preparation method of the anode catalyst and the direct borohydride fuel cell.

Background

Limited fossil energy source eliminatingThe consumption causes serious environmental pollution at the same time. The research on clean new energy and new energy technology capable of replacing fossil energy is an urgent issue to be solved urgently by all mankind. Hydrogen energy has attracted much attention as a clean energy with abundant reserves, wide sources and high energy density. Fuel cells are a new energy technology actively researched and developed by people due to the advantages of high energy conversion efficiency, high energy density, small environmental pollution, low noise, reliable performance, long service life and the like. A Direct Borohydride Fuel Cell (DBFC) is a Fuel Cell that will store Borohydride BH₄ ^-The chemical energy in the fuel cell is directly converted into electric energy under the action of an anode catalyst. Borohydrides are a class of compounds with a high hydrogen content (e.g., NaBH)₄Hydrogen content of 10.6 wt%), the utilization of the chemical energy of borohydride is essentially the utilization of hydrogen energy. DBFC is a new energy technology that combines hydrogen energy with fuel cells. The DBFC has higher output voltage and power density than a common fuel cell, can replace noble metal by using a common catalyst, can work at normal temperature, is nontoxic, is safe in transportation and the like, and simultaneously overcomes the bottleneck problem of hydrogen storage and transportation in the hydrogen energy utilization process. In recent years, DBFC has received great attention from researchers. However, the performance of DBFCs depends largely on the electrochemical reaction of borohydride on the anode, which is directly controlled by the anode catalyst.

Fuel cells generally use noble metals as catalysts, and the expensive price and scarce resources of noble metals become limitations to the practical applications of fuel cells. Therefore, the research and development of the non-noble metal catalyst with high stability and good catalytic activity have important theoretical and practical significance for promoting the industrial development of the fuel cell.

Disclosure of Invention

The technical problem to be solved by the present invention is to provide an anode catalyst for a direct borohydride fuel cell, which is La-Mg-Ni a system, for solving the above problems of the prior art₂B₇The hydrogen storage alloy not only has direct catalytic capability, but also can absorb (or store) the generated hydrogen) And then released in the form of electric energy, and the fuel utilization rate is improved through double functions, aiming at solving the technical problems that in the prior art, the noble metal used as the anode catalyst is high in price, and the catalytic activity of the common hydrogen storage alloy is poor.

The technical scheme adopted by the invention for solving the technical problem is as follows:

an anode catalyst of a direct borohydride fuel cell is characterized in that the anode catalyst is La-Mg-Ni A₂B₇A hydrogen storage alloy of the type having the molecular formula: (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21Where x is 0.1-0.5 and Mm is a misch metal consisting of 63.1 wt% La, 26.0 wt% Ce, 2.7 wt% Pr, and 8.2 wt% Nd.

Preferably, the purity of the Ni, the Co, the Mn and the Al is higher than 99.8 percent.

Preferably, x is 0.2.

Preferably, x is 0.3.

A preparation method of an anode catalyst of a direct borohydride fuel cell is characterized by comprising the following steps: (1) using metal Ni, Co, Mn, Al and mixed rare earth Mm as raw materials, wherein the Mm consists of 63.1 wt% of La, 26.0 wt% of Ce, 2.7 wt% of Pr and 8.2 wt% of Nd, adopting a non-consumable vacuum arc furnace to smelt, and the smelting temperature is higher than the melting point temperature of each metal to obtain the Mm_1-xNi_2.485Co_0.525Mn_0.28Al_0.21And x is 0.1-0.5 alloy. (2) Mm obtained by smelting_1- _xNi_2.485Co_0.525Mn_0.28Al_0.21Crushing the alloy and mixing with Mg powder according to the mixing ratio (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21Wherein x is 0.1-0.5, and mechanically milling the mixture by using a high-energy vibration ball mill.

Preferably, in the step (1), the purities of the Ni, the Co, the Mn and the Al are all higher than 99.8%.

Preferably, in the step (2), the ball milling pot is placed in a glove box vacuum chamber for vacuumizing, nitrogen gas filling and vacuumizing again, and the nitrogen gas is repeatedly used for flushing for 2-3 times.

In the step (2), in order to prevent the loss of Mg element in the ball milling process, the excessive addition is carried out to calculate 10% of the Mg powder.

Preferably, in the step (2), the time for the mechanical ball milling is 6 h.

A direct borohydride fuel cell employing the above-described anode catalyst.

The invention has the advantages of

La-Mg-Ni system A₂B₇The type hydrogen storage alloy is a hydrogen storage alloy with a special superlattice structure, and the theoretical hydrogen storage capacity of the type hydrogen storage alloy is far higher than that of the commercialized AB₅Type alloy, but poor cycle stability due to the presence of Mg element in the alloy. A is described₂B₇Discharge capacity ratio AB of La-Mg-Ni type hydrogen storage alloy₅High alloy, high cyclic stability and AB₅The alloys are equivalent; and AB₅Compared with the alloy, because part of the rare earth elements are replaced by Mg elements and the rare earth elements adopt mixed rare earth, the cost for preparing the alloy is lower than AB₅And (3) alloying.

Electrochemical tests show that A₂B₇Hydrogen storage alloy (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21When x is 0.1-0.5, the electrochemical hydrogen storage performance of the alloy increases and then becomes smaller with the increase of the Mg content, and shows better electrochemical performance, especially when x is 0.2 and 0.3, the best electrochemical performance is shown.

The catalytic performance test as DBFC anode catalyst shows that the hydrogen storage alloy (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21When x is 0.1 to 0.5, the catalytic performance of the alloy increases and then becomes smaller with the increase of the Mg content, and the alloy shows better catalytic performance. Meanwhile, when x is 0.1-0.5, the number of electrons transferred by the alloy is obviously higher than AB₅Number of transferred electrons of the alloy.

Drawings

FIG. 1 is a drawing of BH in accordance with the present invention₄ ^-Oxidation processSchematic representation.

FIG. 2 shows the present invention (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21XRD diffraction pattern of hydrogen storage alloy.

FIG. 3 shows the present invention (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21The discharge capacity of the hydrogen occluding alloy varies with the number of cycles.

FIG. 4 shows the present invention (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21CV curve of hydrogen storage alloy.

FIG. 5 shows the present invention (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21CA profile of hydrogen storage alloy.

Detailed Description

Reaction mechanism of hydrogen storage alloy as DBFC anode catalyst

As shown in FIG. 1, BH₄ ^-In the course of oxidation, BH₄ ^-The medium hydrogen is oxidized from negative to positive valence in two processes: first, BH₄ ^-The B-H bond of (A) is broken, and the monovalent-minus hydrogen is oxidized to the zero valence to generate the surface-adsorbed hydrogen atom H_ads(Process 1), then hydrogen atoms H are adsorbed on the surface_adsContinued oxidation to a positive valence (Process 2), BH₄ ^-Direct oxidation process, corresponding to the equation of the reaction (Process 5)

BH₄ ^-+xOH^-→BO₂ ^-+(x-2)H₂O+(4-x)H₂+xe^-(Process 5)

In the formula, x represents each BH in the actual reaction process₄ ^-The number of electrons released.

Secondly, surface adsorbed hydrogen atoms H_adsAlso possible to combine into H₂Escape (Process 3), this is BH₄ ^-The side reaction of the oxidation process (hydrolysis reaction) corresponds to process 4. It can be seen that the anode catalyst with excellent performance needs to have high catalytic activity for the

processes

1 and 2 and also for the side productsThe reaction (process 3) has a strong inhibitory effect. Generally, noble metal catalysts have good catalytic performance for processes 1-3, so that side reactions are difficult to eliminate.

Hydrogen atoms H adsorbed on surface by hydrogen storage alloy with hydrogen storage function_adsHas strong binding force (process 4, hydrogen can be absorbed and stored in the crystal lattice of the hydrogen storage alloy, namely, the hydrogen storage function is realized). By competing process 4 with process 3, it is achieved that the occurrence of hydrolysis side reactions is suppressed. The hydrogen stored in the lattice of the hydrogen storage alloy is absorbed and oxidized again by the processes 5 and 6, so that the number of released electrons is increased, thereby improving the utilization rate of the fuel, corresponding to the reaction equation (6):

MH_x+xOH^-＝M+xH₂O+xe^- (6)

from the above analysis, it can be seen that a good anode catalyst needs to have both high catalytic activity for

processes

1 and 2 and strong inhibition effect on side reaction (process 3). The hydrogen storage alloy as an anode catalyst is required to suppress the side reaction (process 3) first, while enhancing the process 4. The occurrence of process 4 is closely related to the hydrogen storage capacity, etc. of the hydrogen storage alloy. Secondly, hydrogen stored in the crystal lattice of the hydrogen storage alloy is absorbed and oxidized again (process 5 and process 6) mainly determined by the hydrogen releasing ability, reversibility, discharge performance, etc. of the hydrogen storage alloy. It is understood that the inhibition of the side reaction and the hydrogen reoxidation is smoothly performed by the hydrogen storage characteristics (hydrogen storage capacity, reversibility of hydrogen absorption and desorption, kinetics of hydrogen absorption and desorption, etc.) of the hydrogen storage alloy.

Preferably, x is 0.2.

Preferably, x is 0.3.

Preferably, in the step (2), the time for the mechanical ball milling is 6 h.

A direct borohydride fuel cell employing the above-described anode catalyst.

Structural testing

The alloy sample was subjected to phase structure analysis using an XRD diffractometer for PW1830 type Cu target Ka rays manufactured by Philips, Netherlands. Scanning by adopting a step scanning method, wherein the measured angle range 2 theta is 20-80 degrees, the measured voltage is 30KV, the current is 30mA, and the step length is 0.200. The unit cell parameters were calculated using jade6.0 software, and the XRD patterns measured are shown in FIG. 2, which shows a hydrogen occluding alloy (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21When x is 0.1-0.5, all alloys are composed of LaNi₅Phase and La₂Ni₇The phase composition varies with the content of Mg, the proportion of two phases in the alloy varies, and when x is 0.2 and x is 0.3, the La contained in the alloy₂Ni₇The phase content is much greater. XRD diffraction pattern analysis shows that (Mm) prepared by the method_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21And x is 0.1-0.5 alloy A₂B₇A hydrogen storage alloy.

Preparation of electrode slice

A is to be₂B₇Hydrogen storage alloy (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21And x is 0.1-0.5, evenly mixing the mixture with hydroxyl nickel powder according to the mass ratio of 1:3, weighing 1g of the mixture, putting the mixture into a special steel die with the diameter of 17mm, pressing the mixture into a wafer-shaped sheet by a 769YP-24B powder pressing machine produced by Tianjin Corp Highesta technologies company under the pressure of 11t for 25min, then clamping the cylindrical sheet and one end of an electrode lead in the middle of foam, and fixing the cylindrical sheet and one end of the electrode lead by spot welding around the cylindrical sheet to be used as an electrode to be detected.

Electrochemical performance test

The electrochemical performance test was performed in an open glass three-electrode system. The electrode to be researched is a hydrogen storage alloy electrode, and the auxiliary electrode is high-capacity sintered Ni (OH)₂The electrode is NiOOH, the reference electrode is Hg/HgO, and the electrolyte is 6M KOH solution. The test is carried out on a LAND CT2001A battery tester by adopting a constant current charging and discharging mode. The charge-discharge system of the alloy is as follows: the electrode was charged at a current density of 100mA/g for 300min, allowed to stand for 5min, and then discharged at a current density of 60mA/g with a discharge cut-off voltage of-0.6V (relative to Hg/HgO reference electrode).

The electrochemical properties of the hydrogen storage alloy mainly include: the activation times, the maximum discharge capacity and the cycle stability are generally represented by a curve of the discharge capacity along with the cycle times. FIG. 3 is A₂B₇Hydrogen storage alloy (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21And x is 0.1-0.5 in 6M KOH solution with the change of the discharge capacity along with the cycle number. For ease of analysis, the main electrochemical performance data corresponding to FIG. 3 are shown in Table 1. It can be seen that the electrochemical performance (whether discharge capacity or cycling stability) of the 0.3 x alloy is significantly better than that of AB₅Electrochemical properties of the alloy.

TABLE 1A₂B₇Type (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21Hydrogen-absorbing alloy AB₅Number of activation times N of hydrogen occluding alloy_aMaximum discharge capacity C_maxAnd discharge capacity after 50 cycles C₅₀And capacity retention rate S₅₀

As DBFC anode catalyst pair BH₄ ^-Testing of catalytic performance of

All tests were performed in an open glass three-electrode system. The electrode to be researched is a hydrogen storage alloy electrode, the auxiliary electrode is a foamed nickel electrode with the thickness of 3cm multiplied by 3cm, the reference electrode is a Hg/HgO electrode, and the electrolyte is 2M KOH +0.1M KBH₄The mixed solution of (1). The catalytic performance test of the DBFC anode catalyst was performed using CHI660E B14637 electrochemical workstation of shanghai chenghua. Cyclic voltammetry characteristic test conditions: the scanning speed is 0.05V/s, the scanning range is-0.6V-0.6V, and the testing temperature is room temperature. Time counting current test conditions: the initial voltage was 0.2V. All tests were performed at room temperature.

The catalytic performance of the anode catalyst is generally characterized by a CV curve and a CA curve. In the CV curve, the lower the oxidation peak potential and the higher the oxidation peak current density of the catalyst, the more likely the direct electrooxidation reaction of the borohydride group occurs, indicating that the catalyst has better catalytic activity. FIG. 4 is A₂B₇Type (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21The hydrogen storage alloy is 2M KOH +0.1M KBH₄CV curve in mixed solution. Corresponding to the alloy sample in FIG. 3The oxidation peak values and the corresponding current densities of (a) are shown in Table 2.

TABLE 2A₂B₇Type (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21Hydrogen-absorbing alloy AB₅Oxidation peak potential and current density values of hydrogen storage alloys

Generally, the chronoamperometric method (CA curve) is a method that reflects the change of the oxidation current of borohydride with time, thereby studying the stability of the catalyst in the catalytic process. In general, the higher the current density, the better the catalytic stability. The number of electrons transferred during the electrocatalytic oxidation can also be calculated from the CA curve and the Cottrell equation. FIG. 5 shows the electrolyte at a constant voltage of-0.95V and 2M KOH +0.1M KBH₄Chronoamperometric profile of hydrogen storage alloy in solution. Table 3 shows the values of the stable current density and the number of electrons transferred by electrooxidation in the current curves corresponding to the samples of the alloy of FIG. 5.

TABLE 3A₂B₇Type (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21Hydrogen-absorbing alloy AB₅Current density and number of transferred electrons when hydrogen storage alloy reaches stability

Alloy sample	Currentdensity(mA/cm²)	Numberofelectrons(n)
			AB₅Alloy (I)	-0.0096	-----
A₂B₇，x＝0.1	4.5148	1.01
			A₂B₇，x＝0.2	4.8501	1.09
A₂B₇，x＝0.3	5.2939	1.19
			A₂B₇，x＝0.4	4.8264	1.09
A₂B₇，x＝0.5	2.4004	0.54

From the literature, BH is calculated according to the Kotler equation₄ ^-The number of electrons (n) transferred by electrooxidation over the 5 catalysts is shown in Table 3. As can be seen from Table 3, AB₅The number of transferred electrons of the type alloy alone as a catalyst is very small and cannot be calculated, and all A₂B₇Type (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21The current density and the number of transferred electrons of the hydrogen storage alloy are better than AB₅The alloy is prepared. A. the₂B₇Type (Mm)_1-xMg_x)Ni_2.485Co_0.525Mn_0.28Al_0.21Among hydrogen storage alloys, when x is 0.3, the catalyst has the best catalytic stability and the number of transferred electrons is the largest.

It is to be understood that the invention is not limited to the examples described above, but that modifications and variations are possible in light of the above teachings, as those skilled in the art will recognize, and all such modifications and variations are within the scope of the appended claims.

Claims

1. An anode catalyst of a direct borohydride fuel cell is characterized in that the anode catalyst is La-Mg-Ni A2B7 type hydrogen storage alloy, and the molecular formula of the hydrogen storage alloy is as follows: (Mm1-xMgx) Ni2.485Co0.525Mn0.28Al0.21, where x is 0.1-0.5, Mm is a misch metal consisting of 63.1 wt% La, 26.0 wt% Ce, 2.7 wt% Pr, and 8.2 wt% Nd.

2. Anode catalyst according to claim 1, characterized in that the Ni, Co, Mn, Al purity is preferably higher than 99.8%.

3. The anode catalyst according to claim 1, wherein x is 0.2.

4. The anode catalyst according to claim 1, wherein x is 0.3.

5. A method of preparing an anode catalyst for a direct borohydride fuel cell according to claim 1, wherein: (1) the metal Ni, Co, Mn, Al and mixed rare earth Mm are used as raw materials, wherein the Mm consists of 63.1 wt% of La, 26.0 wt% of Ce, 2.7 wt% of Pr and 8.2 wt% of Nd, the non-consumable vacuum electric arc furnace is adopted for smelting, the smelting temperature is higher than the melting point temperature of each metal, and the Mm1-xNi2.485Co0.525Mn0.28Al0.21 is obtained, and x is 0.1-0.5 alloy. (2) The Mm1-xNi2.485Co0.525Mn0.28Al0.21 alloy prepared by smelting is crushed and mixed with Mg powder according to the mixing ratio of (Mm1-xMgx) Ni2.485Co0.525Mn0.28Al0.21, wherein x is 0.1-0.5, and the mixture is mechanically ball-milled by adopting a high-energy vibration ball mill.

6. The method for preparing anode catalyst of direct borohydride fuel cell according to claim 5, wherein in step (1), the purity of Ni, Co, Mn, Al is preferably higher than 99.8%.

7. The method for preparing anode catalyst of direct borohydride fuel cell according to claim 5, wherein in the step (2), preferably, the ball mill pot is placed in a glove box vacuum chamber to be vacuumized, filled with nitrogen gas, vacuumized again, and then repeatedly flushed with nitrogen gas for 2-3 times.

8. The method for preparing anode catalyst of direct borohydride fuel cell according to claim 5, wherein in step (2), excess addition is calculated to be 10% of Mg powder, preferably to prevent Mg loss during ball milling.

9. The method for preparing anode catalyst of direct borohydride fuel cell according to claim 5, wherein in the step (2), the time of mechanical ball milling is preferably 6 h.

10. A direct borohydride fuel cell employing the anode catalyst of claim 1.