CN1039506A - Water deficiency type gas diffusion porous electrode - Google Patents

Abstract

The present invention relates to a kind of electrode.It provides thickness of liquid film on a kind of minimizing Catalytic Layer three phase boundary according to the out-of-flatness liquid film model theory of gas diffusion porous electrode, diffuses through the water deficiency type gas diffusion porous electrode that the liquid film process improves electrode performance thereby improve gas reactant.It also is provided with the electrichydraulic control layer except that the watertight composition of common usefulness, Catalytic Layer, conductive mesh.The present invention few with even not using under the condition of precious metal catalyst, can significantly improve in, electrode performance under the high current density load.Be suitable for using the various series fuel battery of the gas-diffusion electrode of variety of catalyst types and medium.

Description

Water deficiency type gas diffusion porous electrode

The present invention relates to a kind of electrode.

Gas diffusion porous electrode the electro-chemical conversion of the energy and energy-conservation aspect have great importance.Conventional gas-diffusion electrode is made up of conductive mesh, watertight composition and Catalytic Layer.Though adopt the catalyst of precious metal or novel high-performance can improve the activity of electrode, the medium current density (50-500mA/cm of Practical significance arranged most ²) in the scope, the structural factor of electrode has decisive influence to the performance of electrode.

The inventor once proposed a kind of out-of-flatness liquid film type theory (Tian Zhaowu, Lin Zugeng, Eugene stride, Chinese science, 5(1981) 581-587) of gas diffusion porous electrode.

The objective of the invention is theory according to above-mentioned gas diffusion porous electrode out-of-flatness liquid film model, adopt novel electrode structure and battery design, reduce the thickness of liquid film on the Catalytic Layer three phase boundary, thereby provide a kind of obvious gas reactant that improves to diffuse through the water deficiency type gas diffusion porous electrode that the liquid film process improves the electrode performance of gas-diffusion electrode.

Electrode of the present invention and battery design version are to be provided with the electrichydraulic control layer in addition on the Catalytic Layer surface of conventional type gas-diffusion electrode, and this layer is close to the Catalytic Layer surface.The effect of electrichydraulic control layer is: 1, utilize its capillarity that the inner superfluous electric liquid of Catalytic Layer is extracted out; 2, scattering and disappearing and the difference of supply or the synergy of the two by key-course moisture content, make topped out-of-flatness thickness of liquid film become book on wet surface of catalyst, cause the inner lack of water condition of Catalytic Layer, diffuse through the liquid film process thereby obviously improve gas reactant, reach the purpose that improves the gas-diffusion electrode performance.

According to the out-of-flatness liquid film model of porous electrode polar theory one gas diffusion porous electrode, polarization curve straightway slope satisfies following relational expression:

$\frac{\mathrm{△\; \phi }}{\mathrm{△\; I}}=\frac{\mathrm{2\; b\; e}}{\sqrt{I^{*}{}_F{\mathrm{·\; I}}^{*}{}_{\Omega }}}\mathrm{·\; C\; o\; t\; h}\sqrt{\frac{I^{*}{}_F}{I^{*}{}_{\Omega }}}\mathrm{(\; 1\; )}$

Wherein reactant diffuses through the characteristic current I of liquid film process ^* _FCharacteristic current I with porous electrode internal solution conducting process ^* _ΩBe defined as respectively:

I ^＊ _F≡nFSX _mD _LσC ^° _G/hm

I ^＊ _Ω≡2be/ρ ^＊Xm

With I ^* _F, I ^* _ΩSubstitution (1) Shi Kede:

(△ψ)/(△I) ＝ρ ^＊XgCoth（ (Xm)/(Xg) ）（2）

Xg≡（2behm/nFSD _LσC ^° _Gρ ^＊） ^1/2

Xm is a Catalytic Layer thickness in the formula; ρ ^*Effective resistivity for the Catalytic Layer internal electrolyte solution; Xg is best Catalytic Layer thickness; Be is Tafel slope/2.303; Hm is the maximum of out-of-flatness thickness of liquid film; N is the electrochemical reaction electron number; F is a Faraday constant; S is the wet thin layer total outer surface area in the unit volume Catalytic Layer; D _LBe the diffusion coefficient of reactive material in liquid film; σ is the dissolution equilibrium constant of gas reactant in electrolyte solution; C ^° _GConcentration for gas reactant.

Following formula shows that electrode structure has direct influence to the slope of straightway, reduce the slope of straightway, then requires Catalytic Layer to satisfy: solution resistance rate (ρ in the Catalytic Layer ^*) and buckling factor (T) low, (hm) is little for the liquid film maximum ga(u)ge, the gross area (S) of the solution-air interface in the Catalytic Layer is big.Has corresponding numerical value at the above-mentioned electrode parameter of certain technology for preparing electrode.The present invention is based on the result that above-mentioned theory is analyzed, under the preparation process condition that does not change any electrode, utilize the electric liquid of the above-mentioned various effects of electrichydraulic control layer or/and control electrode to dissipate and supply poor, liquid film sucking-off with Catalytic Layer inside, cause the lack of water condition, make topped out-of-flatness thickness of liquid film (hm) attenuation on wet surface of catalyst, thereby significantly improve gas reactant to diffuse through the liquid film process and improve the performance of gas-diffusion electrode, special task is in having the middle and high current density range of Practical significance.

Current, more urgent in the practicality for the gas-diffusion electrode requirement of under medium and high current density load, working, and the slope of polarization curve straightway is for than the electrode performance under the high current density load decisive influence being arranged.We compare (as Fig. 1) with the electrode of different electrode processes and catalyst content, and as can be seen, conventional type oxygen electrode (seeing curve 2) is owing to the catalyst (20mg/cm that has adopted than the higher carrying capacity of water deficiency type oxygen electrode (seeing curve 1) ²The Ag+0.5%Ni catalyst, full jet printing type technology; Curve 1 corresponding catalyst is 1mg/cm ²Ag, compaction type technology), therefore, under the low current density load, lower overpotential is arranged.But under the high current density load, because the water deficiency type oxygen electrode structure is reasonable, helps improving the transmission performance of various reactants and product, thereby make polarization curve straightway slope low, electrode performance is better than the former as a result.Therefore, if an amount of catalyst loading that improves adopts the water deficiency type electrode, it is lower to obtain overpotential, and the straightway slope is little, well behaved gas-diffusion electrode.

Therefore, adopt water deficiency type electrode of the present invention that following outstanding effect is arranged:

(1) can improve the electrode performance of electrode under medium and high current density load, as shown in Figure 2.Curve 1 is the water deficiency type electrode among the figure, and (electrode of the same terms preparation: catalyst is 0.03mg Pt/cm to curve 2 for the conventional type electrode ², 5NH ₂SO ₄, V=1mv/s, room temperature.), as seen, the water deficiency type electrode has tangible advantage.Listed according to (1) formula by the result such as the table 1 of computer curve fitting.

Table 1, water deficiency type and conventional type oxygen electrode cathodic polarization curve straightway curve fitting parameter

（mA/cm ²）

Fitting parameter water deficiency type conventional type

I ^＊ _F2000-4000 400-500

I ^＊ _Ω170 120

I ^＊ _L140 50

The improvement of water deficiency type electrode performance is mainly due to I as can be seen from Table 1 ^* _FVariation, water deficiency type electrode parameter I ^* _FIncrease by 500 to 10 times than conventional type electrode, and I ^* _FVariation be relevant with out-of-flatness liquid film maximum ga(u)ge (hm), hm is more little to be that liquid film is thin more, then I ^* _FBig more.As seen the water deficiency type electrode mainly is that the result of analysis foretells consistent with the out-of-flatness liquid film model owing to the cause of the inner thinning of liquid film of Catalytic Layer.

(2) under the identical electrodes polarization condition, can then can save catalyst consumption less with even not using precious metal catalyst, reduce cost (catalyst is the higher important component part of gas-diffusion electrode cost).

(3) the many physical parameters of neutral medium can not show a candle to acidity, alkaline medium, and many catalyst activity in neutral medium is not high simultaneously, especially poor-performing under medium current density.But adopt the water deficiency type electrode can overcome above weak point.Fig. 3 is oxygen electrode (Mn catalyst) cathodic polarization curve of (25%w/o Nacl) in neutral medium.Adopt the water deficiency type electrode under the equipolarization current potential, its polarization current is bigger 10 times to 20 times than conventional type polarization of electrode electric current.

This neutral medium water deficiency type oxygen electrode is worth for the various series metal-fuel cells of exploitation neutral medium have important practical.

Embodiment:

Fig. 4 is the water deficiency type gas diffusion porous electrode structural representation.It is made up of watertight composition (1), conductive mesh (2), Catalytic Layer (3) and electrichydraulic control layer (4).Utilize conventional compaction type prepared Catalytic Layer and watertight composition, be cut into identical size with the watertight composition thin slice, sandwich conductive mesh, extrusion forming being rolled into certain thickness Catalytic Layer.The electrichydraulic control layer is selected the micropore lyophilic material for use, as microcrystalline cellulose, modified polyvinyl chloride, polypropylene etc. or macromolecule modified layer.The electrichydraulic control layer preferably is close to the Catalytic Layer surface.

For power on dissipation and the supply of liquid of control electrode forms necessarily poorly, electrode preferably is provided with the feeding controller (7) of dissipative control transducer (5) and electrolyte (6), and (8) are air inlet among the figure, and (9) are the gas outlet.

Claims (4)

1, a kind of water deficiency type gas diffusion porous electrode comprises conductive mesh, watertight composition, Catalytic Layer, it is characterized in that the Catalytic Layer surface is provided with the electrichydraulic control layer.

2, electrode as claimed in claim 1 is characterized in that the electrichydraulic control layer preferably is close to the Catalytic Layer surface.

3,, it is characterized in that the electrichydraulic control layer adopts the micropore lyophilic material, as microcrystalline cellulose, modified polyvinyl chloride, polypropylene etc. or macromolecule modified layer as claim 1 and 2 described electrodes.

4, electrode as claimed in claim 1 is characterized in that preferably being provided with dissipative control transducer and electrolyte feeding controller.

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