Review
Electrochemical hydrogen compression and purification versus competing technologies: Part I. Pros and cons

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Abstract

It is undisputed that hydrogen will play a great role in our future energetic mix, because it enables the storage of renewable electricity (power-to-H2) and the reversible conversion into electricity in fuel cell, not to speak of its wide use in the (petro)chemical industry. Whereas in these applications, pure hydrogen is required, today's hydrogen production is still largely based on fossil fuels and can therefore not be considered pure. Therefore, purification of hydrogen is mandatory, at a large scale. In addition, hydrogen being the lightest gas, its volumetric energy content is well-below its competing fuels, unless it is compressed at high pressures (typically 70 MPa), making compression unavoidable as well. This contribution will detail the means available today for both purification and for compression of hydrogen. It will show that among the available technologies, the electrochemical hydrogen compressor (EHC), which also enables hydrogen purification, has numerous advantages compared to the classical technologies currently used at the industrial scale. EHC has their thermodynamic and operational advantages, but also their ease of use. However, the deployment of EHCs will be viable only if they reach sufficient performances, which implies some specifications that their base materials should stick to. The present contribution will detail these specifications.

Graphical Abstract

This contribution details the means available today for hydrogen purification and compression. It shows that among the available technologies, the electrochemical hydrogen compressor has numerous advantages compared to the classical technologies presently used in the industry.

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Introduction

The recent COP21 in Paris demonstrated that most countries are willing to reduce their carbon footprint. One convenient manner to do so is to increase renewable energies (e.g. solar panel and windmills) and to store their excess production into chemicals via a power-to-gas (and in particular power-to-hydrogen) strategy, so that “green” electricity can be produced on demand peaks by converting the gas (hydrogen) into electricity, for instance in fuel cells [1]. Electrolyser systems have a maximum delivery pressure equal to 44.8 MPa with an untreated hydrogen gas purity equal to 99.5%, excluding water vapor. The Impurity is mainly O2 and after deoxidizer, the treated gas purity reaches 99.999%, excluding water vapor [2]. Therefore, purification is not required for water electrolysis except for O2 and water vapor. This is the ideal scenario, and unfortunately, the present hydrogen is still not widely produced using this strategy. Today, more than 95% of hydrogen is produced from fossils (and in a minor amount from bio-processes) and is therefore not pure [3]. Nevertheless, supplementary hydrogen sources from biomass can contribute to the penetration of renewable energies ([4, 5]), e.g. photobiohydrogen exhibits a positive global warming potential, low acidification potential, relevant social cost of carbon and a low potential production cost [6]. Hence, the biomass processes appear as promising technologies to produce renewable hydrogen [7], but with a large amount of impurities.

This means that widespread usage of hydrogen, e.g. in fuel cell vehicles, stationary power production or for specialty chemistry, requires its efficient purification. If one takes the case of proton exchange membrane fuel cells (PEMFC) for automotive applications as a benchmark, the level of purity required is as follows: H2 > 99.97 mol%, hydrocarbons CO < 0.2 ppm (mol), H2S < 0.004 ppm (mol), NH3 < 0.1 ppm (mol), O2 < 5 ppm (mol), N2 and Ar < 300 ppm (mol) ([8, 9]). Besides, the volumetric energetic content of hydrogen being small at room pressure, this gas needs to be compressed to fairly high pressures to compete with the usual fuels (e.g. gasoline, natural gas, etc.) [10].

At present, there are many ways to purify and to compress hydrogen. The present contribution will review the most relevant (and widespread) ones, and will particularly focus on their interest for the application of hydrogen-energy (e.g. for fuel cell vehicles). In this application, in order to minimize the hydrogen transportation cost, one option would be to widespread hydrogen production/purification/compression into small units. The present work will particularly address these items and check the pros and cons of each technology for these aspects. It will notably be shown that electrochemical hydrogen compression (EHC) systems enable both purification and compression at reasonable efficiency, and could therefore be the technological solution for such application.

Section snippets

Purification methods

In order for hydrogen to become a widespread renewable-energy carrier, its purification and compression are unavoidable industrial processes [10]. Currently, these two processes are usually physically separated in, firstly, a purification step to convert impure hydrogen into ultra-pure hydrogen, and then a compression step to make the purified (and low-pressure) hydrogen gas storage sufficiently dense (in terms of gravimetric and volumetric density) to compete with the usual energy vectors

Thermodynamic comparison of gas compression

As stated in preamble, most industrial techniques of hydrogen post-treatment upon production use different steps for the purification and the compression. Hereafter are detailed the classical means that are used to compress hydrogen at an industrial scale. Most of them are based on a mechanical compression (MC).

According to Petitpas et al. [32], the ideal gas law is valid until 100 bar for hydrogen (23 °C). At 23 °C between 100 to 800 bar, the isentropic coefficient γ increases. The adiabatic

General layout

Currently, all the electrochemical compression systems involved in the hydrogen industry are described as non-thermal devices. Therefore, the main purpose of most research works developed in this domain is to increase their efficiency. Some researchers have focused on the electrochemical principle of the compression [62] and others have chosen to work on the purification aspects of this process [63, 64]. Casati et al. [55] have investigated some fundamental aspects in the EHC using a PEMFC;

Conclusion: why EHC is relevant for hydrogen purification/compression and why they need adapted electrocatalysts

The present contribution highlights that, among the different means to purify and compress hydrogen, the EHC exhibits a wealth of assets. This system appears to be the best compromise if one needs to simplify the purification/compression steps of hydrogen. Compared to classical means of hydrogen purification, the EHC combines a low energetic cost, high H2 recovery and purity, little maintenance, low cost and low temperature of operation, which neither the pressure swing adsorption, the

Nomenclature

    cf

    Fixed charge site concentration (mol cm−3)

    CH2O

    Water concentration (mol m3)

    D

    Diffusion coefficient (m2s−1)

    EHC

    Electrochemical hydrogen compressor

    F

    Faraday's constant (C mol−1)

    HHV

    Higher Heating Value

    Hi

    Solubility coefficient, (mol m−3 Pa−1)

    I

    Current (A)

    j

    Current density (A cm−2)

    ki

    Gas permeability coefficient (mol m−1 s−1 Pa−1)

    m˙

    Hydrogen mass flow (kg s−1)

    M

    Molar mass (kg mol−1)

    MEA

    Membrane Electrode Assembly

    MHC

    Mechanical hydrogen compressor

    Ni

    Gas permeation rate (mol s−1 m−2)

    n

    Mole number (mol)

    pi

    Pressure

Superscripts & subscript

    eq

    Equivalent

    f

    Faradaic

    i, n

    Species

    h

    High pressure

    l

    Low pressure

    m

    Membrane

    w

    Work

Acknowledgments

The authors thank the Auvergne Rhône-Alpes region for the funding of the PhD thesis of Marine Trégaro. Part of the work has been performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials “CEMAM” no. ANR-10-LABX-44-01. Both MR and MT make their PhD in the frame of the Eco-Sesa project, funded by IDEX Université Grenoble Alpes.

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