Operation potential

When current is flowing, the actual cell operating potential is given by ... 

Graphs of operating potential versus current density are called polarization curves, which reflect the degree of perfection that any particular fuel cell technology has attained. High cell operating potentials are the result of many years of materials optimization. Actual polarization curves will be shown below for several types of fuel cell. 

Further, as the current density of the fuel cell increases, a point is inevitably reached where the transport of reactants to or products from the surface of the electrode becomes limited by diffusion. A concentration polarization is estabhshed at the elec trode, which diminishes the cell operating potential. The magnitude of this effect depends on many design and operating variables, and its value must be obtained empirically. 

The operating temperature also affects the fuel cell operating potential, A high operating temperature accelerates reaction rates but... 

Contact with steel, though less harmful, may accelerate attack on aluminium, but in some natural waters and other special cases aluminium can be protected at the expense of ferrous materials. Stainless steels may increase attack on aluminium, notably in sea-water or marine atmospheres, but the high electrical resistance of the two surface oxide films minimises bimetallic effects in less aggressive environments. Titanium appears to behave in a similar manner to steel. Aluminium-zinc alloys are used as sacrificial anodes for steel structures, usually with trace additions of tin, indium or mercury to enhance dissolution characteristics and render the operating potential more electronegative. 

Platinised Ti, Ta or Nb < 1000 Am/m (consumption) Discontinuities in Pt coat protected by oxide film on subtrate sensitive (< lOOHz) a.c. ripple in d.c. or negative current spikes causing electrode consumption maximum operating potential with Ti substrate 9 V... 

The driving voltage to bare steel, i.e. protection potential of steel - anode operating potential. 

Anode Operating Potential, Protection Potential and Driving Voltage... 

The operating potential of an anode material is its potential when coupled to a structure (i.e. the closed-circuit potential). Since all commercial anode materials are formulated to suffer only slight polarisation under most conditions of exposure, the operating potential approximates to the open-circuit potential. Indeed, any substantial difference (>50mV) between these two potentials will call into question the suitability of the anode in the particular environment. 

The driving voltage is the difference between the anode operating potential and the potential of the polarised structure to which it is connected. For design purposes, the driving voltage is taken as the difference between the anode operating potential and the required protection potential of the structure. 

The anode material must provide a driving voltage sufficiently large to drive adequate current to enable effective cathodic polarisation of the structure. This requirement implies that the anode must have an operating potential that is more negative than the structure material to be protected. 

The anode material must have a more or less constant operating potential over a range of current outputs. Consequently the anode must resist polarisation when current flows the polarisation characteristics must also be predictable. 

Although aluminium is a base metal, it spontaneously forms a highly protective oxide film in most aqueous environments, i.e. it passivates. In consequence, it has a relatively noble corrosion potential and is then unable to act as an anode to steel. Low level mercury, indium or tin additions have been shown to be effective in lowering (i.e. making more negative) the potential of the aluminium they act as activators (depassivators). Each element has been shown to be more effective with the simultaneous addition of zinc . Zinc additions of up to 5% lower the anode operating potential, but above this level no benefit is gained . Below 0 9 7o zinc there is little influence on the performance of aluminium anodes . Table 10.10 lists a number of the more common commercial alloys. 

The presence of HjS (from bacterial activity in anaerobic saline mud, for example) can result in a significant decrease (16%) in capacity and loss of operating potential for Al-Zn-ln anodes... 

Fig. 10.15 Anode operating potential in semi-saline water exposed for 30-38 days at 15-20°C. Current densities (mA/fp) 400 (A) 200 (O) 80 ( ) (after Schreiber and Murray" )...

The choice between Al-Zn-In and Al-Zn-Hg may well be influenced by their respective operating potentials and capacities. Where an additional driving voltage is required (such as in seabed mud), Al-Zn-In anodes may be preferred to ensure adequate structure polarisation. Alternatively, a lower driving potential may be acceptable where the additional capacity (and hence weight saving) is the predominant factor this favours Al-Zn-Hg anodes. 

Aluminium anodes are less constant in their electrochemical characteristics than zinc. This presents no major problem provided the designer is aware of their properties. They suffer from reduced capacity and increased operating potential (and hence risk of passivation) with increasing temperatures above approximately 50°C (Fig. 10.14), decreasing salinity (Figs. 10.15 and 10.16) and decreasing operating current density (Fig. 10.17). 

The application of sacrificial anodes for the protection of structures requires the development of suitable anode materials for the exposure environment. Screening tests enable the rapid selection of materials which show potential as candidates for the given application. These tests may typically use a single parameter (e.g. operating potential at a defined constant current density) as a pass/fail criterion and are normally of short duration (usually hours) with test specimen weights of the order of hundreds of grams. The tests are not intended to simulate field conditions precisely. 

The critical information required from testing may include one or all of the following tendency to passivation, anode operating potential and capacity. The tests, whilst all capable of producing information on the above, tend to be particularly suited to certain applications. For example potentiostatic testing is useful for evaluating passivation tendencies but not generally appropriate to anode capacity determination. 

Anode output is the current available from the anode under the design conditions. It will depend on the shape of the anode, the resistivity of the environment, the protection potential of the structure and the anode operating potential. It is defined as ... 

Besides its widespread use for investigating the mechanism of redox processes, spectroelectrochemistry can be usefiil for analytical purposes. In particular, the simultaneous profiling of optical and electrochemical properties can enhance the overall selectivity of different sensing (30) and detection (31) applications. Such coupling of two modes of selectivity is facilitated by the judicious choice of the operating potential and wavelength. 

The presence of an electrical potential drop, i.e., interfacial potential, across the boundary between two dissimilar phases, as well as at their surfaces exposed to a neutral gas phase, is the most characteristic feature of every interface and surface electrified due to the ion separation and dipole orientation. This charge separation is usually described as the formation of the ionic and dipolar double layers. The main interfacial potential is the Galvani potential (termed also by Trasatti the operative potential), which is the difference of inner potentials (p and of both phases. It is a function only of the chemical... 

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