At fixed discharge current, delivered voltage can be represented aswhere is discharged charge and the terms denote distinct physical losses. A typical trace has an initial drop from open circuit, an approximately flat useful region and a final decline toward cutoff. Its exact shape depends on the cell chemistry and current; not every battery has an equally flat plateau.
Four distinguishable contributions are:
- Ohmic polarization: electronic, ionic and contact resistance cause a rapid drop on applying current. This need not wait for a concentration gradient to develop.
- Activation overpotential: finite electrode-reaction kinetics require a departure from equilibrium to sustain charge transfer. It is important when the exchange current is small.
- Concentration overpotential: electrolyte transport and diffusion within active particles can deplete reactants or accumulate products near reaction sites. These losses often grow near the end of discharge or at high current.
- Phase-transformation polarization: nucleation and growth of a new phase can require extra driving force, produce an initial voltage dip or cause hysteresis. It is especially relevant to conversion and two-phase electrodes, rather than a universal independent loss in every chemistry.
Terminology sometimes groups solid-state transport or phase-change effects differently. The important distinction is the mechanism, not counting the same diffusion loss under two names. A falling equilibrium voltage as composition changes is also not, by itself, polarization.
An illustrative discharge voltage combines equilibrium changes and distinct losses
. Original stipulated model, not battery measurements. The loss components show an instantaneous resistive contribution, activation loss, a transient nucleation-related contribution and increasing late-discharge transport loss. The equilibrium curve is plotted separately. Details.
The hydrogen fuel cell is the more likely high-power choice at low temperature, for otherwise comparable electrodes, oxidant supply and device design. Similar equilibrium oxidation potentials imply similar thermodynamic voltage opportunities; they do not imply similar current densities.
Hydrogen oxidation on an appropriate catalyst is relatively facile. Ammonia oxidation requires multiple bond-breaking and nitrogen-coupling steps; adsorbed intermediates can occupy catalytic sites and inhibit continued reaction. A larger anodic activation overpotential and slower turnover reduce the useful current and operating voltage of a low-temperature ammonia cell.
Since areal power is , a cell retaining substantial voltage at a larger current density can deliver more power even when its equilibrium voltage is similar. Ammonia's favorable storage characteristics are a separate system-level consideration; a comparison of power cannot be made by counting the electrons per fuel molecule alone. This is a kinetic expectation, not a universal numerical ranking independent of catalysts, temperature and transport.
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