ANALYSIS OF TRACE-LEVEL IMPURITIES IN HYDROGEN


To achieve sustainable development goals related to climate change and to improve air quality, the reduction of carbon emissions due to transport and mobility are fundamental [1]. Transport is currently responsible for over a quarter of greenhouse gas emissions in developed countries and is worldwide the primary source of urban air pollution [2]. The deployment of hydrogen as a sustainable fuel has the potential to substantially reduce emissions of greenhouse gases and harmful air pollutants. In 2050, hydrogen may account for 32% of the fuel demand in Europe [3]. The fuel cell system in a hydrogen vehicle requires very high-quality hydrogen because trace levels of impurities can adversely affect fuel cell performance and durability [1]. For example, formaldehyde and formic acid at concentrations higher than 200 nmol/mol can cause signifi cant fuel cell performance degradation [4]. To ensure the hydrogen quality, a specifi cation has been developed (ISO 14687), setting upper concentrations of a series of impurities (Table 1). To demonstrate the conformity with this standard it is required to validate by measurement that the levels of the impurities are below the required thresholds.


Existing analytical methods suitable for measuring ISO 14687 impurities in fuel cell graded hydrogen mainly involve techniques based on gas chromatography. However, a combination of several analytical techniques and methodologies are necessary to perform the full scope of analysis required. In Table 1 are summarized the analytical methods that we have chosen for the characterization of hydrogen purity. In this article, we will focus on the GC-FID and HPLC systems.


2 Materials and methods


In this section, we will develop the analytical systems and gas generators used for the characterization of trace-level impurities in hydrogen.


2.1 Gas supply


Air generators (airmopure, Chromatotec® below -15°C (Hydroxychrom, Chromatotec®


, France) and Hydrogen generators 99.9999% with dew point , France) were used for the fl ames of FIDs, valve actuations


of the GC-FIDs. The VOCs content of gas generated by both generators was verifi ed experimentally using auto-TDGC-FID and Non-Methanic Hydrocarbon Concentration (NMTHC) for both analyzers was below 0.1 µg.m-3


2.2 auto-GC-FID system for the measurement of Total hydrocarbons For the monitoring of total hydrocarbon compounds (THC), an automatic gas chromatograph (auto- GC-FID) equipped with a fl ame ionization detector (FID) has been used (ChromaTHC, Chromatotec®


France). For each analysis, sample was drawn into the system with a fl ow rate of 25 ml.min-1 . The


sample was injected in a three-dimensional columns system (one polar capillary column and tow packed columns) located inside the heated oven of the GC using hydrogen as carrier gas. The system allows the separation and quantifi cation of methane and non-methane hydrocarbons in two minutes. Total hydrocarbon content is calculated with the sum of methane and non-methane (NMTHC).


2.3 auto-GC-FID system with methanizer for CO and CO2 measurement


For the monitoring of carbon monoxide and carbon dioxide, an auto-GC-FID (ChromaCO, Chromatotec®


the system with a fl ow rate of 25 ml.min-1 and quantifi cation of CO and CO2 to CH4


, France) equipped with FID has been used. For each analysis, sample was drawn into . The sample was injected in a packed column system


located inside the heated oven of the GC using hydrogen as carrier gas. Before detection, the sample goes through a catalytic system which reduces CO and CO2


. The system allows the separation with a cycle time of 10 minutes. 2.4 auto-HPLC system for formaldehyde and formic acid


The reference ISO 16000-3 method for aldehydes detection is based on active sampling using 2,4-Dinitrophenylhydrazine (DNPH) tube followed by hydrazones quantifi cation by HPLC-UV (HPLC system, Chromatotec®


) [5]. This method allows the quantifi cation of all aldehydes present in ambient air


but can be applied for hydrogen impurities characterization [6]. Once sampling is achieved, DNPH tubes are eluted with 2–3 mL of acetonitrile (99.8%). An amount of 20 µL of the resulting hydrazones solution is then injected and quantifi ed by HPLC/UV using an external calibration. Hydrazones are separated through a nonpolar C18 column using acetonitrile / water (75:25) and detected at 360 nm. For formic acid analysis, similar method can be applied following the protocol described by Uchiyama et al. [7]. To ensure complete reaction with DNPH, half of the eluted solvent must be heated up to 80°C for 5h.


PIN April / May 2022 . Using these generators, only power supply and water are needed to run the analytical systems. Water


Total hydrocarbons Oxygen Helium


Nitrogen Argon


, Carbon dioxide Carbon monoxide


Total sulfur compounds Formaldehyde Formic acid Ammonia


Total halogenated


5 2 5


300 100 100 2


0.2


0.004 0.01 0.2 0.1


0.05 2.5 Other systems for hydrogen impurities measurement


For the other impurities, several analytical systems are required. Electrolytic water sensor will be used for the monitoring of water at ppm level in H2


gas (DETH2S, Chromatotec® in H2 , specifi c metal coating is required to ensure accurate measurements.


Gas chromatograph analyzer equipped with pulse ion discharge detector (DID) will be used for monitoring of permanent gases (Chroma DID, Chromatotec®


for Ar/O2 separation are used for the quantifi cation of N2 , France). , O2


, France). Helium carrier gas and specifi c column and Ar with a single analytical system [8].


The total sulfur compounds will be analyzed by auto-GC equipped with specifi c electrochemical detector (H2S TS MEDOR, Chromatotec®


Total halogenated compounds can be characterized by auto-GC-ELDC (Chroma ELCD, Chromatotec® , France).


Finally, ammonia will be characterized using Fourier Transform Spectrometry for UV-Visible with specifi c preconcentration system (DET NH3, Chromatotec®


, France). The techniques presented in this section will not be developed in the results section (please contact the author for more information). , France). For measurement


Table 1: Fuel quality requirements specifi ed by ISO/DIS 14687-2 for Types I & II Grade D and summary of analysis methods


Components


Maximum impurities concentration (µmol/mol)


Electrolytic water sensor GC-DID C-FID GC-ED HPLC-VU FT-UV


GC-ELCD


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