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]