Research on fully automatic analysis methods for hydrogen energy

An ion chromatograph equipped with an anion exchange column was used to study the analytical methods for total halides and formic acid in hydrogen fuel. An automatic sampling method was established and compared with the manual injection method. A good linear relationship was observed between the peak area and concentration (r2 = 0.999), with a relative standard deviation (RSD) of less than 3% and a detection limit of the instrument at 0.01 mg/L. The experimental results indicate that this method has excellent repeatability and high sensitivity, enabling fully automatic analysis of total halides and formic acid content in hydrogen energy. This provides a fully automatic online analysis solution for the quality of hydrogen energy in the upstream, midstream, and downstream sectors.


Introduction
Hydrogen energy is the energy released by hydrogen during its physical and chemical changes, which can be used for power generation, or as vehicle and aircraft fuel, household fuel, etc.As an energy source, hydrogen has the following characteristics: high calorific value per unit, with the lower heating value of hydrogen gas being 120 kJ/kg, which is 2.6 to 4 times the calorific value of fossil fuels such as natural gas, oil, and coal of the same weight; abundant reserves, hydrogen is mainly stored in the form of compounds in water, therefore the abundant water resources on Earth contain a large amount of hydrogen energy available for development; clean and environmentally friendly, hydrogen itself is tasteless and non-toxic, and the by-products of hydrogen combustion or power generation are water, achieving zero emissions [1] .Therefore, hydrogen energy is hailed as the cleanest energy of the 21st century.Utilizing new energy for hydrogen production can enhance the consumption level of new energy, promote the large-scale development of new energy, and at the same time, hydrogen energy storage can achieve long-term cross-seasonal power volume regulation, meeting the multi-time scale adjustment needs of the power system, and supporting the safe and reliable operation of the power system [2] .
The supply side of the hydrogen fuel industry chain includes three major links: upstream hydrogen production, midstream hydrogen storage and transportation, and hydrogen refueling station construction.The demand side of the hydrogen fuel industry chain is the comprehensive application of downstream hydrogen fuel.Each link has strict quality requirements for hydrogen fuel.For example, the national standard "GB/T 37244-2018 Proton Exchange Membrane Fuel Cell Vehicle Fuel -Hydrogen" [3] limits the content of various impurities for hydrogen fuel cells, where the total halide content must be less than 0.05 μmol/mol, and the formic acid content must be less than 0.2 μmol/mol.Currently, the detection methods for these two impurities generally use ion chromatography [4][5][6] .Ion chromatography technology is a unique and effective trace ion analysis method developed in the late 1970s [7] .Its analytical principle is to rapidly separate various ions in an ion exchange column using the ion exchange chromatography principle.The suppressor connected in series after the separation column effectively reduces the acidity or alkalinity of the strong electrolyte eluent using the acid-base neutralization principle, greatly enhancing the sensitivity of the sample to be tested.The conductivity detector then converts the conductivity value of the effluent into the corresponding voltage value, which is input to the microcomputer through A/D conversion to obtain the chromatographic peak of the ion to be tested, achieving the purpose of separation, qualitative, and quantitative analysis [8] .However, most ion chromatography methods for testing total halides and formic acid have low sensitivity, require manual operation, have a cumbersome sample preparation process, and are limited to laboratory conditions, making them inconvenient for most industrial analysis fields.In response to existing problems and the uniqueness of the sample, this paper has developed a fully automatic sample pretreatment system, which, in conjunction with ion chromatography, can achieve ultra-sensitive detection of total halides and formic acid in hydrogen.The instrument's limit of detection limit can reach 0.01 mg/L, with a relative standard deviation (RSD) of less than 3%, and it has good repeatability.
Automatic sampling device: the absorbent is deionized water (resistivity not less than 18 MΩ•cm), manufactured by Shanghai Huaai Chromatography Analytical Technology Co., Ltd.

Preparation of standard solutions
We separately weigh 2.2 g of NaF, 1.6 g of NaCl, 1.48 g of KBr, and 1.7 g of NaCOOH and dissolve in deionized water, then transfer them to a 1 L volumetric flask and dilute to the mark to mix well, serving as the anion standard stock solution.
Using a pipette, we draw several milliliters of the above standard stock solution into a 100 mL volumetric flask, dilute to the corresponding mark with deionized water, and shake well to obtain mixed solutions of various concentrations.The ion concentrations in the prepared standard solutions are shown in Table 1.

Manual injection device operating procedure
The manual injection device consists of a pressure-reducing valve, a washing bottle, a flow meter, and a soluble polytetrafluoroethylene (PFA) tube, as shown in Figure 1.where 1-hydrogen gas cylinder; 2-pressure reducing valve; 3, 4-washing bottles; 5-flow meter.50 mL of deionized water is added to each washing bottle, using a two-stage absorption method.The sample hydrogen gas is depressurized and its flow rate is controlled at a stable 200 mL/min.The sample collection time is 50 minutes, with an absorption volume of 10 L. After sampling is completed, it is brought up to a volume of 250 mL with deionized water and then analyzed by using the ion chromatograph.
The specific operating procedure of the fully automatic sampling device is as follows: first, we use the peristaltic pump (1) to inject deionized water into four 100 mL absorption bottles, control the start and stop of the pump and the opening and closing of the valve through the liquid level sensor, and draw the liquid from the 100 mL absorption bottles to the 50 mL absorption bottles.The sample hydrogen gas is depressurized at a stable flow rate of 200-500 mL/min and sequentially passed through four washing bottles, allowing for the complete absorption of formic acid in the hydrogen gas.Meanwhile, the time is 20 min-50 min, with an absorption volume of 10 L and discharge of the hydrogen gas.Then, we collect the absorption liquid from the four washing bottles into a 300 mL absorption bottle and wash the four absorption bottles.The wash liquid is also collected into the 300 mL absorption bottle.Next, we use the peristaltic pump (2) to inject deionized water into the 300 mL absorption bottle, bringing the absorption liquid up to a volume of 250 mL.We allow it to stand for 5 minutes for thorough mixing.Finally, the sample is sent to the ion chromatograph for analysis through the peristaltic pump (3).The absorption time can be adjusted based on the content of total halides and formic acid in the hydrogen gas.If the content exceeds the detection range, the absorption time can be shortened; if the content is low, the absorption time can be appropriately increased.

Establishment of the standard curve
By using ion chromatography, five standard solutions of different concentrations were tested.The peak area response values for each concentration were obtained, as shown in Table 2.The corresponding spectra can be found in Figures 3-7.

Comparison of data measured by fully automatic sampling device and manual injection device
The ion impurity content in hydrogen gas samples was tested by using both the fully automatic sampling device and the manual injection device.The results from three parallel experiments are presented in Table 3.The experimental results demonstrate that the fully automatic sampling device exhibits good repeatability.There is no cross-contamination issue with the entire apparatus, allowing for the sensitive measurement of total halides and formic acid in hydrogen.
Table 3.Comparison table of the concentration of various impurity ions in the hydrogen sample tested by the fully automatic sampling device and the manual injection device.

Sampling method
The concentration of impurity ions in hydrogen gas samples (mg/L)

Determination of the limit of detection (LOD)
The Limit of Detection (LOD) refers to the smallest peak value that the instrument itself can detect.Its magnitude depends on the intrinsic noise value of the instrument and the detection peak value of the minimum concentration of impurities.The calculation methods are divided into theoretical values and actual detection values [9,10] .In this study, by calculating the actual detection values of total halides and formic acid, when the peak height is just equal to or greater than twice the noise peak height, that concentration is the LOD for that component.Instrument noise: 100 (nS/cm), instrument drift: 1000 (nS/cm).The equation for the LOD is shown in Equation (1).The LODs for each component are presented in Table 4.
where -D represents the LOD, in mg/L; -N stands for the noise, in nS/cm; -W is the concentration of the component, in mg/L; -H signifies the peak height, in nS/cm.
where -X represents the volume fraction of each substance in hydrogen gas; -C stands for the concentration of each substance in the absorbent solution, in mg/L; -V1 is the volume of the absorbent solution, in mL; -V0 signifies the sampling volume of the hydrogen gas to be tested under standard conditions, in L. When the volume of the absorbent solution is 100 mL and the sampling volume of the hydrogen gas to be tested is 100 L, the calculation results are as follows Table 5: Table 5.The actual detection limit of F -, COOH -, Cl -, and Br -.

Conclusion
This study established a fully automatic sampling device for sample pretreatment in conjunction with the ion chromatograph for sensitive measurement of the total halide compounds and formic acid content in hydrogen gas.Through actual detection values, the limit of detection for total halide compounds in hydrogen was determined to be 0.02 μmol/mol, and for formic acid, it was 0.006 μmol/mol.Moreover, the standard curve linear correlation coefficients were all greater than 0.999.Repeatability experiment results demonstrated that the relative standard deviation (RSD) is less than 3%, indicating that the method has high sensitivity, good repeatability, and stability.It can meet the analytical requirements for total halide compounds and formic acid in hydrogen energy.Additionally, the developed fully automatic sampling device can replace manual analysis, offering a fully automated online solution for assessing the quality of hydrogen energy for upstream, midstream, and downstream sectors.

Figure 8 .
Figure 8. Linear diagram of different concentrations of F -.

Table 1 .
The concentration of each ion in the standard solution.

Table 2 .
Response values of ion peak area at different concentrations.