Hydrogen purity

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Hydrogen purity or hydrogen quality describes the presence of impurities in hydrogen when used as a fuel gas. Impurities in hydrogen can interfere with the proper functioning of equipment that stores, distributes, or uses hydrogen fuel.


Hydrogen Purity Requirements

The impact of impurities varies with the specific equipment used and on the physio-chemical nature of the impurity. For example, hydrogen boilers that combust hydrogen will generally tolerate higher concentrations of impurities than a vehicle using a polymer electrolyte membrane fuel cell (PEMFC) [1] and inert impurities such as nitrogen are usually less harmful than reactive species such as hydrogen sulphide. [2]

As the specific impurity matters it is not sufficient to rely on normal metrics of gas purity, often reported using nines (e.g. >99.9990% or 5.0N), [3] as this does not provide adequate information about which impurities may be present at trace levels. Instead, standards have been developed that provide more detailed requirements on fuel purity for specific applications. The international standard ISO 14687:2019 [2] specifies maximum permissible concentrations for many key impurities depending on use. This standard is being adopted into legislation in many jurisdictions. For example, in Europe the Directive 2014/94/EU [4] on the deployment of alternative fuels infrastructure states that the hydrogen purity dispensed by hydrogen refuelling points shall comply with the technical specifications included in ISO 14687-2.

Fuel Cell Electric Vehicles

Fuel cell electric vehicles commonly use polymer electrolyte membrane fuel cells (PEMFC) which are susceptible to a range of impurities. Impurities impact PEMFC using a range of mechanisms, these may include poisoning the anode hydrogen oxidation reaction catalysts, reducing the ionic conductivity of the ionomer and membrane, altering wetting behaviour of components or blocking porosity in diffusion media. The impact of some impurities like carbon monoxide, formic acid, or formaldehyde is reversible with PEMFC performance recovering once the supply of impurity is removed. Other impurities, for example sulphurous compounds, may cause irreversible degradation. [5] The permissible limits of hydrogen impurities are shown below.

Fuel Quality Specification For Gasseous Hydrogen Supplied to PEMFC Road Vehicles [6]
Maximum Permissible Concentration / μmol mol−1
Total non-hydrogen gasses300
Total Hydrocarbons Except Methane [Carbon atom basis]2
Carbon Dioxide2
Carbon Monoxide0.2
Total Sulphur Compounds [Sulphur atom basis]0.004
Formic Acid0.2
Halogenated Compounds [Halogen ion basis]0.05
Maximum Particulate Concentration1 mg kg−1

Efforts to assess the compliance of hydrogen supplied by hydrogen refuelling stations against the ISO-14687 standard have been performed. [7] [8] [9] While the hydrogen was generally found to be 'good' [7] violations of the standard have been reported, most frequently for nitrogen, water and oxygen.

Combustion Engines and Appliances

Combustion applications are generally more tolerant of hydrogen impurities than PEFMC, as such the ISO-14687 standard for permissible impurities is less strict. [10] This standard has itself been criticised with revisions proposed to make it more lenient and therefore suitable for hydrogen distributed through a repurposed gas network. [1]

Fuel Quality Specification For Gaseous Hydrogen Supplied to Combustion Engines and Appliances [11]
ImpurityMaximum Permissible Concentration / μmol mol−1
Total non-hydrogen gasses20 000
Total Hydrocarbons [Carbon atom basis]100
Carbon Monoxide1
Sulphur [Sulphur atom basis]2
Combined water, oxygen, nitrogen, argon19 000
Permanent ParticulatesShall not contain an amount sufficient to cause damage.

Sources of Hydrogen Impurities

The presence of impurities in hydrogen depends on the feedstock and the production process. Hydrogen produced by electrolysis of water may routinely include trace oxygen and water, which must be usually be removed prior to use. Hydrogen produced by reforming of hydrocarbons is produced as a mixture with a stoichiometric mixture with carbon dioxide and carbon monoxide which must be separated, additionally trace impurities from the feedstock such as sulphur compounds may be present in the final hydrogen supply. Impurities may also be introduced during storage, distribution, dispensing or as a result of equipment malfunction. Examples of this include distribution of hydrogen through repurposed gas networks which may be contaminated with a range of impurities or malfunctioning of equipment at refuelling stations. [1] Some impurities may be added deliberately, for example odorants to aid detection of gas leaks. [12]

Methods for Hydrogen Purity Analysis

As the permissible concentrations for many impurities are very low this sets stringent demands on the sensitivity of the analytical methods. Moreover, the high reactivity of some impurities requires use of a properly passivated sampling and analytical systems. [13] Sampling of hydrogen of is challenging and care must be taken to ensure that impurities are not introduced to the sample and that impurities do not absorb on or react within the sampling equipment, there are currently different methods for sampling but rely on filling a gas cylinder from the refuelling nozzle of a refuelling station. [14] Efforts are underway to standardise and compare sampling strategies. [15] [16] A combination of different instruments is needed to assess hydrogen samples for all of the components listed in ISO 14687-2. [17] Techniques suitable for individual impurities are indicated in the table below.

Example Analytical Methods for Asessing The Concentration of Impurities in Hydrogen [18] [19]
ImpurityPossible Analytical MethodsDetection Limits
Total non-hydrogen gasses
WaterQuartz crystal microbalance


1.3 or 0.030
Total Hydrocarbons Except Methane [Carbon atom basis]GC-Methaniser-FID0.1
MethaneGC-Methaniser-FID, GC-EPD0.1
Carbon DioxideGC-Methaniser-FID, GC-EPD0.02
Carbon MonoxideGC-Methaniser-FID, GC-EPD0.02
Total Sulphur Compounds [Sulphur atom basis]GC-SCD, GC-EPD0.001
Formic AcidFTIR0.2
AmmoniaGC-MS or UV-visible spectroscopy or FTIR1 or 0.03 or 0.1
Halogenated Compounds (Halogen Ion Equivalent)TD-GC-MS0.016

In addition to rigorous laboratory analysis analytical methods that can be operated in the field continuously assessing hydrogen for impurities are being developed. These include techniques such as electrochemical sensors [20] [21] and mass spectrometry. [22]

Methods for Purifying Hydrogen

See also: Hydrogen purifier

Purification of hydrogen is an important aspect of hydrogen distribution and there are a range of technologies available depending on the impurities present and process conditions. [1]

See also

Related Research Articles

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A fuel cell is an electrochemical cell that converts the chemical energy of a fuel and an oxidizing agent into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requiring a continuous source of fuel and oxygen to sustain the chemical reaction, whereas in a battery the chemical energy usually comes from substances that are already present in the battery. Fuel cells can produce electricity continuously for as long as fuel and oxygen are supplied.

A regenerative fuel cell or reverse fuel cell (RFC) is a fuel cell run in reverse mode, which consumes electricity and chemical B to produce chemical A. By definition, the process of any fuel cell could be reversed. However, a given device is usually optimized for operating in one mode and may not be built in such a way that it can be operated backwards. Standard fuel cells operated backwards generally do not make very efficient systems unless they are purpose-built to do so as with high-pressure electrolysers, regenerative fuel cells, solid-oxide electrolyser cells and unitized regenerative fuel cells.

<span class="mw-page-title-main">Proton-exchange membrane fuel cell</span> Power generation technology

Proton-exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable fuel-cell applications. Their distinguishing features include lower temperature/pressure ranges and a special proton-conducting polymer electrolyte membrane. PEMFCs generate electricity and operate on the opposite principle to PEM electrolysis, which consumes electricity. They are a leading candidate to replace the aging alkaline fuel-cell technology, which was used in the Space Shuttle.

<span class="mw-page-title-main">Steam reforming</span> Method for producing hydrogen and carbon monoxide from hydrocarbon fuels

Steam reforming or steam methane reforming (SMR) is a method for producing syngas (hydrogen and carbon monoxide) by reaction of hydrocarbons with water. Commonly natural gas is the feedstock. The main purpose of this technology is hydrogen production. The reaction is represented by this equilibrium:

<span class="mw-page-title-main">Water splitting</span> Chemical reaction

Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen:

<span class="mw-page-title-main">Methanol economy</span>

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A proton-exchange membrane, or polymer-electrolyte membrane (PEM), is a semipermeable membrane generally made from ionomers and designed to conduct protons while acting as an electronic insulator and reactant barrier, e.g. to oxygen and hydrogen gas. This is their essential function when incorporated into a membrane electrode assembly (MEA) of a proton-exchange membrane fuel cell or of a proton-exchange membrane electrolyser: separation of reactants and transport of protons while blocking a direct electronic pathway through the membrane.

Formic acid fuel cells (direct formic acid fuel cells or DFAFCs) are a subcategory of direct liquid-feed fuel cells (DLFCs), in which the liquid fuel is directly oxidized (electrochemically) at the anode instead of reforming to produce hydrogen. Formic acid-based fuel cells represent a promising energy supply system in terms of high volumetric energy density, theoretical energy efficiency, and theoretical open-circuit voltage. They are also able to overcome certain problems inherent to traditional hydrogen (H2) feed fuel cells such as safe handling, storage, and H2 transportation.

<span class="mw-page-title-main">Protonic ceramic fuel cell</span>

A protonic ceramic fuel cell or PCFC is a fuel cell based around a ceramic, solid, electrolyte material as the proton conductor from anode to cathode. These fuel cells produce electricity by removing an electron from a hydrogen atom, pushing the charged hydrogen atom through the ceramic membrane, and returning the electron to the hydrogen on the other side of the ceramic membrane during a reaction with oxygen. The reaction of many proposed fuels in PCFCs produce electricity and heat, the latter keeping the device at a suitable temperature. Efficient proton conductivity through most discovered ceramic electrolyte materials require elevated operational temperatures around 600-700 degrees Celsius, however intermediate temperature ceramic fuel cells and lower temperature alternative are an active area of research. In addition to hydrogen gas, the ability to operate at intermediate and high temperatures enables the use of a variety of liquid hydrogen carrier fuels, including: ammonia, and methane. The technology shares the thermal and kinetic advantages of high temperature molten carbonate and solid oxide fuel cells, while exhibiting all of the intrinsic benefits of proton conduction in proton exchange membrane fuel cells (PEMFC) and phosphoric acid fuel cells (PAFC). PCFCs exhaust water at the cathode and unused fuel, fuel reactant products and fuel impurities at the anode. Common chemical compositions of the ceramic membranes are barium zirconate (BaZrO3), cesium dihydrogen phosphate (CsH2PO4), and complex solid solutions of those materials with other ceramic oxides. The acidic oxide ceramics are sometimes broken into their own class of protonic ceramic fuel cells termed "solid acid fuel cells".

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<span class="mw-page-title-main">Hydrogen storage</span> Methods of storing hydrogen for later use

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<span class="mw-page-title-main">Metal hydride fuel cell</span>

Metal hydride fuel cells are a subclass of alkaline fuel cells that have been under research and development, as well as scaled up successfully in operating systems. A notable feature is their ability to chemically bond and store hydrogen within the fuel cell itself.

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<span class="mw-page-title-main">Reformed methanol fuel cell</span> Fuel Cell Type

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<span class="mw-page-title-main">Hydrail</span>

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<span class="mw-page-title-main">Polymer electrolyte membrane electrolysis</span>

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<span class="mw-page-title-main">Liquid organic hydrogen carriers</span> Organic compounds that can absorb and release hydrogen through chemical reactions

Liquid organic hydrogen carriers (LOHC) are organic compounds that can absorb and release hydrogen through chemical reactions. LOHCs can therefore be used as storage media for hydrogen. In principle, every unsaturated compound can take up hydrogen during hydrogenation. The sequence of endothermal dehydrogenation followed by hydrogen purification is considered as the main drawback which limits the overall efficiency of the storage cycle.

High Temperature Proton Exchange Membrane fuel cells (HT-PEMFC), also known as High Temperature Polymer Electrolyte Membrane fuel cells, are a type of PEM fuel cells which can be operated at temperatures between 120 and 200°C. HT-PEM fuel cells are used for both stationary and portable applications. The HT-PEM fuel cell is usually supplied with hydrogen-rich gas like reformate gas formed by reforming of methanol, ethanol, natural gas or LPG.


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