Cathode loadings

An advanced cell configuration for underwater application has been developed using high-surface-area Raney nickel anodes loaded at 120 mg/cm (1-2% Ti) and Raney silver cathodes loaded at 60 mg/cm containing small amounts of Ni, Bi, and Ti (6). 

Surface characterization includes also the study of the modification of a surface under cathodic load or after some pretreatments. The presence of residual surface oxides can explain some observations otherwise inexplicable. Activation in situ usually results in composite structures which are difficult to identify by X-ray, and may contain metallic and non-metallic components. Particularly crucial is the case of the surface structure of glassy metals or amorphous alloys. 

The most outstanding source of instability, which from an academic point of view may be difficult to realize, is the corrosion that can arise at open circuit when a cell is shut down for maintenance or other reasons [108]. The cathode can simply corrode, or can be oxidized. In the latter case, the residual oxide can be deleterious for the catalytic activity since it may remain unreduced even under cathodic load. Therefore, cathode materials usually contain additives whose function is to reduce the consequences of shut-downs [109]. Laboratory experiments should thus include also this kind of test if a complete analysis of the material is to be done [105, 108, 110]. If the cathode corrodes, some cathodic protection may need to be maintained during shut-downs [7], which is of course to be accounted for in the evaluation of the economic efficiency. 

Modifications of the chemical nature of the catalyst under cathodic load are also possible. Sulphides can be reductively dissolved with liberation of H2S [139]. Oxides can be progressively reduced with loss of the specific activity [140]. In the latter case, an additive can be used to diminish the rate of reduction. Intermetallic compounds or alloys may exhibit preferential dissolution of one of the components during cathodic performances in concentrated alkali [141],... 

In other cases, thermal decomposition is used to prepare the active layer which is then activated either by a high temperature treatment in H2 atmosphere, or by in situ reduction under cathodic load. It has been reported that for molybdate-activated cathodes the latter procedure is less satisfactory than the former [153]. Thus, temperature, procedure of preparation and activation are all crucial parameters which can dramatically influence the final activity. 

Studies on single crystal faces have indicated that, on the whole, the crystallographic orientation has a minor influence on the electrocatalytic activity. Of the possible explanations which can be offered [292], a reasonable one is that the high coverage with adsorbed hydrogen far from equilibrium smoothes down the differences in A//ad from face to face. Actually, quantum chemical calculations reproduce more closely the situation at 0H-O, which does not usually correspond to the real situation under cathodic load. Another possibility is that the hydridic... 

The mechanism may change from acids to alkalis in some cases [365], This may be related to the higher sensivity of the Fe surface to oxidation in alkaline solutions [365, 367], Actually, the corrosion of Fe proceeds also under moderate cathodic load [368]. Impedance measurements have suggested that the classical mechanisms of hydrogen evolution is probably inadequate to describe the situation on Fe [377], A surface diffusion step with spillover of hydrogen to sites with lower M-H energy has been suggested. Adsorption of CN- interferes with such a diffusion. 

The activity of Ni cathodes decreases under prolonged cathodic load [380, 381]. This is primarily due to absorption of hydrogen which can reach quite considerable concentrations [382]. The diffusivity of H in Ni has been measured [383]. However, the mechanism does not appear to change on deactivated electrodes [380]. An increase in temperature appears to conform to the kinetic predictions in some cases [378], but at temperatures above 80°- 100°C, a dramatic activation is observed [380, 384] which must be related to some modifications occurring on the electrode surface. 

Raney Ni with additives is also used [77, 276]. In particular, valve metals are added to stabilize the catalyst structure [102,410, 411], thus decreasing the recrystallization and sintering which always takes place as the solution temperature is raised [412] (which points to the high energy state of such an electrode structure). In this respect, potential cycling has also been observed to be detrimental since it can induce recrystallization [407]. This is probably the reason why surface oxidation may be deleterious with Raney structures [390] while it normally results in improved electrocatalytic properties with bulk Ni electrodes [386]. However, after prolonged cathodic load resulting in deactivation, Raney Ni electrodes can be reactivated (temporarily) by means of anodic sweeps [405]. 

The specific role and the fate of Mo in the alloy has been investigated [141]. It has been found that Mo is not at all stable but tends to be leached out, which would be the origin of the deactivation observed on cathodic load. The deactivation results in a progressive increase in the Tafel slope, which cannot be reactivated in situ by addition of molybdenum salts. On the other hand, that Mo is leachable can be inferred also from the observation that in situ deposited Co-Mo alloys are quickly dissolved as the current is interrupted [528, 529]. This seems to indicate a provisory activation of the cathode by Mo, which cannot be recovered in a simpley way once decayed [141]. However, this contrasts somewhat with the claim of long term stability and resistance to cell shut-downs for the thermally prepared Ni-Mo coating [5]. The structure of the layer may differ depending on the details of the preparation procedure. 

Similarly, a number of amorphous alloys based on Fe-Zr, Ni-Zr, Co-Zr, Ni-Nb, have not shown any increase in activity over that expected for the mechanical mixture of the crystalline components [571]. For Ni-Nb the overpotential has even increased. Only Cu-Ti alloys have shown apparent synergetic effects, but the results of Machida et al. [89] (cf. Fig. 32) should also be taken into account. Jorge et al. [152] have observed higher activity for the amorphous form of Cu-Ti alloys, but they have attributed it to the preferential dissolution of Ti in the amorphous sample under cathodic load, with formation of a relatively porous Cu layer. The same effect was obtained more rapidly by means of HF etching [89,152]. 

Among the various classes of materials, some have not yet kept their early promise. This is the case of amorphous compounds, whose use is also hampered by the severe conditions often employed in electrolysis cells. In the case of sulphides it is not yet clear how much of their activity is due to the chemical composition of the surface and how much to the structure resulting from the modification of the surface under cathodic load. In the case of composite materials, it is necessary to take into account that the surface area is normally higher for multicomponent phases, depending in particular on the method of preparation. 

The results of reductions in catalyst loading to effect a cost improvement with minimum impact on cell performance have also been encouraging. Techniques for reducing both anode and cathode loadings up to 93% have been identified and shown feasible. 

Figure 2 shows the material cost ( /kilowatt electrical [kWe]) versus cathode platinum loading for stacks operating at 3 atmospheres, 160°C, and 0.8 volts with direct hydrogen and reformate. Assumptions in this analysis include use of an alloy catalyst having a kinetic activity two times that of platinum, a unit cell resistance of 0.1 ohm centimeter squared (cm ), and an anode catalyst loading equal to one half the cathode loading. 

Figure 13.1 Histogram illustrating the number of early experiments at SRI and ENEA, showing measurable excess power as a function of maximum cathode loading. Also illustrated are points for the MIT calorimetric experimental result, with a stated loading of 0.75 0.05 (Ref [25]) and for the Caltech null experimental result, with loading measurements quoted at 0.77, 0.78, and 0.80.

Figure 20.5. Comparison of hydrogen/air polarization curves for spray-based and conventional electrocatalysts at 0. 2 mgPt/cm cathode loadings anode catalyst is identical at 0.05 mgPt/cm. Single 50 cm cell PEM MEA performance measured at 50 °C and atmospheric pressure at constant flow, corresponding to stoichiometry of 1.2 for hydrogen and 2.2 for air at 1 A/cm, and with 100% humidification of the gases, Nafion 112 membrane. (Reproduced with permission of Cabot Corporation.)...

Figure 2.4 indicates the polarisation losses for PAFC with anode and cathode loading of 0.5 mg Pt/cm, pressure 1 atm, 100% phosphoric acid and operating temperature of 180°C. The anode losses are very small on pure Ha, whereas the cathode polarisation is greater with air and comparatively small for pure oxygen. As discussed, it is known that an increase in cell pressure enhances the performance of the cell (Eq. 2.72). 

C, hydrogen/oxygen, 25% humidity. Herring et al. anode and cathode loading 0.5 mg (2004)... 

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