Electrolyser performance

How to increase power

increase cell area (increases current), stack more cells (increases voltage)

What is a potentiostat

maintains specific voltage while measuring current using a 3 electrode system

What is a polarisation curve and what is ideal

Measures current density vs cell potential; indicates cell performance; ideal = flat line/same voltage for any current density

activation losses

overpotential required due to sluggish reaction kinetics associated with electrodes; largest effect at low current densities; catalysts can reduce these losses

Volcano plots

plot of exchange current density or overpotential vs enthalpy of ions binding to catalyst; indicates best performing catalysts; too strong and all the ions stuck on the catalyst, too weak and the catalyst doesn’t do its job; don’t tell us about stability or durability

Ohmic losses

losses from internal resistance in the cell; linear relationship

Where can resistances occur in a cell

electronic resistance in electrode, electrode contact with flow plates, ionic resistance across membrane (generally largest effect)

Resistance in a SOEC

lots of resistances due to contacts between many layers

Mass transport (concentration) losses

due to limited diffusion of reactant species (water or gases) to the electrodes; occurs at high current densities; tend to be more significant in fuel cells

Cut off voltage

set during testing to prevent materials from degrading in the electrodes, V_lim ~ 2.1V

What is a good polarisation curve

lower, flatter (close to thermoneutral voltage as possible), good electrolyser will reach 2A/cm before cut-off

How does temperature affect performance

high T is better: reduces activation losses, reduced ohmic losses as species are more mobile

How does anode water flow rate affect performance

at low T: high flow rate reduces performance as it has a cooling effect on the cell; at high T: high flow rate improves performance in ohmic and mass transport regions due to better conductivity and delivery to catalyst

Electrochemical impedance spectroscopy EIS

measures impedance in a system, allows us to decouple resistance processes in the cell to model them, measured on Nyquist curve

How does the Nyquist curve change with increasing: voltage, KOH conc, temperature

Voltage: higher V decreases resistances, easier for charges to get in and out; KOH conc: better ion mobility decreases resistance; Temp: decreases resistance due to favourable kinetics and better e mobility

Tomography

3D image from X rays in multiple directions, good for mesoscale PTLs

Neutron radiography/imaging

uses neutrons instead of xrays, good contrast, good for macroscale flow channels