1. HEPTI RRC “Kurchatov Institute” HYDROGEN SAFETY ASPECTS RELATED TO HIGH PRESSURE PEM WATER ELECTROLYSIS Fateev V.N. 1, Grigoriev S.A. 1, Millet P. 2, Korobtsev S.V. 1, Porembskiy V.I. 1, Pepic M. 3, Etievant C. 4, Puyenchet C. 4 1 Hydrogen Energy and Plasma Technology Institute of Russian Research Center “Kurchatov Institute”, Kurchatov sq. 1, Moscow, 123182, Russia, fat@hepti.kiae.rufat@hepti.kiae.ru 2 Institut de Chimie Moléculaire et des Matériaux, UMR CNRS n° 8182, Université Paris Sud, bât 420, 91405 Orsay Cedex France 3 DELTA PLUS Engineering & Consulting, Liege Science Park, Avenue Pré-Aily 1, B-4031, Angleur-Liège, Belgium 4 Compagnie Européenne des Technologies de l’Hydrogène, route de Nozay, Etablissements Alcatel, 91460 Marcoussis cedex France

2. РНЦ «Курчатовский Институт» HYDROGEN ENERGY & PLASMA TECHNOLOGY INSTITUTE HEPTI RRC “Kurchatov Institute”

3. ABSTRACT Polymer electrolyte membrane (PEM) water electrolysis has demonstrated its potentialities in terms of cell efficiency (energy consumption  4.0-4.2 kW/Nm 3 H 2 ) and gas purity (> 99.99% H 2 ). Current research activities are aimed at increasing operating pressure up to several hundred bars for direct storage of hydrogen in pressurized vessels. Compared to atmospheric pressure electrolysis, high-pressure operation yields additional problems, especially with regard to safety considerations. In particular the rate of gases (H 2 and O 2 ) cross-permeation across the membrane and their water solubility both increase with pressure. As a result, gas purity is affected in both anodic and cathodic circuits, and this can lead to the formation of explosive gas mixtures. To prevent such risks, two different solutions, reported in this communication, have been investigated. First, the chemical modification of the solid polymer electrolyte, in order to reduce cross-permeation phenomena. Second, the use of catalytic H 2 /O 2 recombiners to maintain H 2 levels in O 2 and O 2 levels in H 2 at values compatible with safety requirements.

4. Cell voltage (U), power (W) and energy efficiency dependence on current density for modern and developing electrolyzers 1 – industrial alkali electrolyzers and its modern modifications (70-95  С); 2 – electrolyzers for electrolysis in alkali melt (330-400  С; 0,1-1,0 MPa); 3 – solid polymer (PEM) electrolyzers (90-110  С; 0,1-3,0 MPa); 4 – high temperature electrolyzers (900  С; 0,1 MPa). Energy losses of power sources of electrolyzers and heat losses were not taken into account 1 – industrial alkali electrolyzers and its modern modifications (70-95  С); 2 – electrolyzers for electrolysis in alkali melt (330-400  С; 0,1-1,0 MPa); 3 – solid polymer (PEM) electrolyzers (90-110  С; 0,1-3,0 MPa); 4 – high temperature electrolyzers (900  С; 0,1 MPa). Energy losses of power sources of electrolyzers and heat losses were not taken into account 3

5. INTRODUCTION Polymer electrolyte membrane (PEM) water electrolysiss, is currently the subject of extensive studies. PEM technology provides an example of “zero-gap” configuration, in which electrodes (nano-sized electrocatalyst particles supported by a porous electronic conductor) are in direct contact with the surface of the ion exchange membrane. This cell concept offers some significant advantages compared to traditional electrolyzers with liquid electrolyte and not zero-gap design: (i) pure water being the only reactant provides high gas purity; (ii) low voltage losses in electrolyte and possibility to operate at high current density, (iii) low membrane gas permeability gives possibility for safe operation at high pressure. As a result, low energy consumption (4.0-4.2 kW/Nm 3 H 2 ), high specific productivity (current densities up to 2 A/cm 2 ), high hydrogen purity (>99.99%) and possibility to operate at 30-50 bars are realized. High-pressure (up to several hundred bars) electrolysers are currently needed for direct storage of hydrogen in pressurized vessels. Such electrolyzers would be of particular interest for small-scale (5-50 kW) energy systems powered by renewable energy sources, hydrogen filling stations and so on. But operation at high pressure results in increases the level of cross-contamination, decrease of current efficiency, gas purity and can lead to the formation of explosive H 2 /O 2 gas mixtures, either in the electrolyser itself or in the liquid-gas separators. Two different strategies are used in present research to avoid these problems (i) reduction of hydrogen cross-permeation; (ii) reduction of hydrogen contents in the oxygen-water output stream of anodic cells. Results reported in this communication were obtained mainly during the GenHyPEM STREP project, financially supported by the European Commission in the course of the 6th Framework Research Program.

6. Water electrolysis cell with PEM Overall reaction: H 2 O  1/2O 2 + H 2 Current collector Bipolar plate ANODE ZONE Proton exchange membrane Electrocatalytic layer O2O2 CATHODE ZONE H2H2 Н2ОН2О O 2 + Н 2 О H2H2 Н2ОН2О H 2 O  2H + + 1/2O 2 + 2e – 2H +  H 2 – 2e – Н+nH2OН+nH2O + - Н2ОН2О Н2ОН2О

7. Scheme of PEM electrolyser stack Membrane-electrode assembly Sealing elements Bipolar flow-field plates End plates

8. PEM electrolysers for high purity hydrogen production with productivity up to 2 m 3 /hour and operating pressure up to 30 bars Performances of PEM water electrolysers - Power consumption 4.0-4.2 kW*hour/m 3 of H 2 - Voltage on the cell 1.67-1.72 V at i=1 A/cm 2 and t=90  C - Operating pressure up to 30 bars and more - Hydrogen purity > 99.99% - Noble metal content in catalytic layer: anode 1.0-2.0 mg/cm 2 cathode 0.5-1.0 mg/cm 2 - Life time (average) > 20000 hours

9. Schematic diagram of the experimental setup including: (1) the PEM electrolysis cell; (1-a) PEM; (1-b) catalytic layers; (1-c) porous titanium current collectors; (1-d) gas collection compartments. Ancillary equipment: (2) liquid-gas separators; (3) pumps; (4) valves; (5) production valves; (6) thermocouples; (7) pressure transducers.

10. Lab-scale polarization curves measured during PEM water electrolysis at T = 90  C and different operating pressures: 1 – P = 1 bar; 2 – P = 50 bar. E=E 0 + RT/nF ln(P Н 2 Р О 2 1/2 )

11. spherical powder, porosity 35% SEM micrographs of a porous current collectors made of sintered titanium particles. irregular powder, porosity 57%

12. Lab-scale measurements of anodic and cathodic cell current efficiencies as a function of current density at 2 and 30 bar

13. Hydrogen content (vol.%) in the anodic oxygen-water vapour mixture, measured at 1, 6 and 30 bar in the liquid-gas separator as a function of operating current density. 50 cm 2 monocell. Pt as cathodic catalyst, Ir as anodic catalyst and Nafion®-117 as PEM.

14. Hydrogen detectorCatalytic hydrogen recombiner

15. The photograph of the experimental stand for measuring HPCM characteristics. Flow rate G, m 3 /min. Pressure drop dP, mm. water column Dependences of pressure drop for various HPCM samples (1 x 10 x 15 cm) of gas flow rate. Maximal productivity of recombiner 100 m 3 /h (for 4 vol. % of H 2 )

16. CONCLUSIONS PEM water electrolysers, operating at pressures up to 70 bar, can be used to produce hydrogen and oxygen of electrolytic grade suitable for PEM fuel cells, with high efficiencies. However, because of increasing rate of gas cross-permeation with pressure, the concentration of hydrogen in oxygen and the concentration of hydrogen in oxygen can reach critical levels. To avoid the formation of explosive gas mixtures, it is necessary to reduce gas cross-permeation. This can be done to a certain extend by surface modifying the solid electrolyte, for example by coating low-permeability protective layers and introduction inside the membrane inorganic proton conducting compounds. Contaminant concentration in the produced gases can also be reduced by adding catalytic gas recombiners, directly in the electrolysis cell or along the production line (gas separators). By using gas recombiners inside the electrolysis cell, it was possible to maintain hydrogen contents below 2 vol.% at large interval of current density at an operating pressure of 30 bar, with Nafion® 117 as solid electrolyte.