1383When the solar-hydrogen (STH) ratio reaches 5-10%, photocatalytic water splitting to produce hydrogen is economically feasible [1]. At present, in the research of photocatalytic water splitting for hydrogen production, the most important activity evaluation parameters are the normalized photocatalytic hydrogen production rate (μmol·h-1·g-1 or mmol·h-1·g-1), Quantum yield (AQY) and STH energy conversion efficiency. However, the normalized photocatalytic water splitting rate of hydrogen production is affected by factors such as the wavelength range of incident light, the intensity of incident light, the type of reactor, and the reaction temperature, resulting in data results between different laboratories that cannot be compared under a unified standard [2]. Therefore, the two most important indicators to evaluate the photocatalytic water splitting of the catalyst for hydrogen production are AQY and STH energy conversion efficiency. About AQY, it has been elaborated in the article "Quantum Yield (AQY) Calculation Nanny Tutorial, You Deserve It!"
STH energy conversion efficiency is the efficiency of converting input solar energy into hydrogen energy, and it is the practical application standard for measuring the water splitting of photocatalysts [3].
1. The calculation formula of STH energy conversion efficiency in photocatalytic water splitting reaction is as follows [2]:
RH2: the rate of photocatalytic water splitting for hydrogen production (mmol s-1);
∆Gr: Molar Gibbs free energy of water splitting reaction (J mol-1);
Psun: Optical power density of AM 1.5G standard solar spectrum (100 mW cm-2);
S: light area (cm2).
Standard Molar Gibbs Free Energy of Water Splitting
, formula (1-1) can be simplified to [4]:
Theoretically, the STH energy conversion efficiency can also be calculated by integrating the quantum efficiencies of all wavelengths, the formula is as follows [5]:
λI: AM1.5G standard solar spectrum starting wavelength;
λF: AM1.5G standard solar spectrum termination wavelength;
QE: Quantum efficiency under standard conditions.
2. The calculation formula of STH energy conversion efficiency in photoelectric catalytic water splitting reaction is as follows [6]:
Jsc: short-circuit photocurrent density (mA cm-2);
E: thermodynamic decomposition potential of water (V);
ηF: Faraday efficiency;
Psun: The optical power density of the AM1.5G standard solar spectrum (100 mW·cm-2).
Likewise, the standard thermodynamic decomposition potential of water
=1.23 V, Equation (2-1) can be simplified to [6]:
When using the above formula, the following points should be paid attention to [2]:
1. The calculation of the energy conversion efficiency of STH is only for the total water splitting reaction with a molar ratio of H2:O2 of 2:1, and it does not hold for the half-reaction of hydrogen production in the presence of a hole sacrificial agent. Because the hole sacrificial agent will also participate in the reaction, the ΔGr and E values will change;
2. The ΔGr and E values under different temperatures and pressures are different, and should be corrected according to the actual reaction temperature and pressure;
3. The spectrum of the light source must conform to the AM1.5G standard solar spectrum, and the optical power density must be 100 mW·cm-2.
It can be seen from the above formula that the two necessary conditions to accurately measure the photocatalytic/photocatalytic STH energy conversion efficiency are:
① Accurate measurement of the hydrogen production rate of photocatalytic water splitting;
②AM1.5G standard solar spectrum.
Regarding the accurate measurement of the hydrogen production rate of photocatalytic water splitting, the reaction system should pay attention to the following interference factors:
1. Avoid measurement errors caused by offline manual injection;
2. Avoid uneven mixing of H2 and O2 in a short time.
Regarding the acquisition of the AM1.5G standard solar spectrum, there are two common ways: 1) Use the solar simulator directly; 2) The xenon light source cooperates with the AM1.5G filter, as shown in Figure 1.
Figure 1. (a) Physical image of solar simulator and xenon lamp with AM1.5G filter, (b) spectrum of AM1.5G standard solar spectrum and Microsolar 300 xenon lamp with AM1.5G filter
Professor Zhu Yongfa of Tsinghua University and researcher Zhang Tierui of the Institute of Physics and Chemistry of the Chinese Academy of Sciences used the Beijing Porphyran PLS-FX300HU high uniformity integrated xenon lamp as the light source for the photocatalytic total water splitting experiment. On-line analysis was performed, and the measurement of the energy conversion efficiency of STH in the photocatalytic water splitting reaction was performed. Related results have been published in Nano Energy [7].
The research group of Mr. Zhang Jiatao from Beijing Institute of Technology used the PLS-FX300HU high uniformity integrated xenon lamp as the light source for the photoelectric catalytic water splitting experiment through this scheme, and the H2 content was analyzed online through the Labsolar-6A all-glass automatic online trace gas analysis system. Measurement of STH energy conversion efficiency for photocatalytic water splitting reaction. Related results have been published in Advanced Energy Materials [8].
Figure 3. Classic case—measurement of photoelectric catalytic water splitting STH
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[8]Wang Hongzhi, Guo Yuying, Zhang Jiatao* et. al., Efficient plasmonic Au/CdSe nanodumbbell for photoelectrochemical hydrogen generation beyond visible region[J]. Advanced Energy Materials, 2019, 9, 1803889.
