2.2 Common parameters for measuring PEC performance
2.2.1 Faradaic efficiency
Faradaic efficiency (FE) is crucial for evaluating selectivity towards
target products during the PEC reaction. It can be determined using the
following equation:
\begin{equation}
FE=\frac{\varepsilon\times n\times F\ }{Q}\times 100\%\nonumber \\
\end{equation}where ε represents the number of electrons transferred during the
formation of the target product molecules, n is the quantity of the
target products, F is Faraday’s constant, and Q is the total charge
passed. It is the proportion of the charge transferred to a specific
product relative to the total charge delivered during the PEC reaction.
2.2.2 Quantum efficiency
Quantum efficiency (QE) comprises both incident photon-to-current
efficiency (IPCE) and absorbed photon-to-current efficiency (APCE). They
can be determined using the following equations:
\begin{equation}
IPCE=\frac{\varepsilon\times\text{n\ }}{N}\times 100\%\nonumber \\
\end{equation}\begin{equation}
APCE=\frac{\varepsilon\times\text{n\ }}{N\times A}\times 100\%\nonumber \\
\end{equation}where N is the total number of incident photons, and A is the light
absorption of the photoelectrode. IPCE assesses the efficiency of
converting incident photons into electrons transferred to the product at
a specific wavelength. APCE, on the other hand, measures the efficiency
of converting absorbed photons into electrons transferred to the
product.[27]
2.2.3 Photocurrent density
Photocurrent density is a crucial parameter indicating the activity of
the PEC reaction. It is calculated by dividing the photocurrent, which
represents the flow of electric current through the photocathode, by the
photocathode’s geometric area during the PEC reaction. It can be
categorized into total photocurrent density, representing the current
density associated with all products generated during the PEC reaction,
and partial photocurrent density, which corresponds to the current
density for a specific target product during the PEC reaction.
2.2.4 Production rate (yield rate)
Production rate or yield rate is another critical parameter representing
the activity of photocathodes in PEC reactions. Gas products such as CO,
CH4, and C2H4 are
quantified using gas chromatography (GC), while liquid products like
HCOOH, CH3OH, and C2H5OH
are detected using nuclear magnetic resonance (NMR) to determine
production yield. The production rate is then normalized by dividing it
by the geometric area of the photocathode.[26]
2.2.5 Solar-to-fuel efficiency
Solar-to-fuel efficiency (STF) is the ratio of the chemical energy
stored in products to the overall energy input from sunlight
illumination. STF can be calculated using the following equation:
\begin{equation}
STF=\frac{G\times\text{n\ }}{P\times S\times t}\times 100\%\nonumber \\
\end{equation}where \(G\) is the standard Gibbs free energy of formation for the
product, P is the light illumination power density, S is the geometric
area of the photoelectrode, and t is the duration of the PEC reaction.
This parameter is crucial for evaluating the effectiveness of PEC
performance. Factors such as charge separation efficiency,
light-absorption intensity, surface area and morphology, and catalyst
activity can influence STF.[27]