  In the growing field of capacitive deionization (CDI), a number of
performance metrics have emerged to describe the desalination process.
Unfortunately, the separation conditions under which these metrics are measured
are often not specified, resulting in optimal performance at minimal removal.
Here we outline a system of performance metrics and reporting conditions that
resolves this issue. Our proposed system is based on volumetric energy
consumption (Wh/m$^3$) and throughput productivity (L/h/m$^2$) reported for a
specific average concentration reduction, water recovery, and feed salinity. To
facilitate and rationalize comparisons between devices, materials, and
operation modes, we propose a nominal standard testing condition of removing 5
mM from a 20 mM NaCl feed solution at 50% water recovery for CDI research.
Using this separation, we compare the desalination performance of a
flow-through electrode (fte-CDI) cell and a flow between membrane (fb-MCDI)
device, showing how significantly different systems can be compared in terms of
generally desirable desalination characteristics. In general, we find that
performance analysis must be considered carefully so to not allow for ambiguous
separation conditions or the maximization of one metric at the expense of
another. Additionally, for context we discuss a number of important underlying
performance indicators and cell characteristics that are not performance
measures in and of themselves but can be examined to better understand
differences in performance.
