PEM water electrolysis fundamentals. Dimitrios Tsiplakides

Transcription

1 PEM water electrolysis fundamentals Dimitrios Tsiplakides

2 Hydrogen production Production and consumption of hydrogen H Y D R O G E N Only 0.25% of hydrogen produced worldwide was produced by electrolysis Electrolytic process is only competitive when the cost of the electricity is low The electrolysis of water has received considerable attention in the context of the socalled hydrogen economy Encyclopedia of Electrochemistry, Volume 5: Electrochemical Engineering, Wiley (2007)

3 Hydrogen production Hydrogen is fastest growing industrial gas Central production Distributed production Commercial plants using alkaline electrolysis have operated at capacities over 60,000 kg /day for industrial chemical processing Produced at station to enable low-cost delivery Renewable plants using renewable feedstocks (wind, solar, etc.) are envisioned in longer term Currently available using grid electricity 2015 Targets * cost of hydrogen delivered at a refuelling station of less than 5 /kg (0.15 /kwh) capital cost of 1,500-2,500 /kw for industrial units and 4,000-5,000 /kw for micro-chp Renewable demonstration systems using PEM electrolysis already in place at select locations * (FCH-JU)

4 What is water electrolysis? Electrolysis of water is the decomposition of water (H 2 O) into oxygen (O 2 ) and hydrogen gas (H 2 ) due to an electric current being passed through the water. H O H + 1/2O 2 2 2

5 Thermodynamics H O H 1 / 2 O H 286 kj/ mol o * r (endothermic: energy has to be applied) Ideal gas enthalpy of formation, kj/mol Ideal gas entropy, kj/mol K O H H 2 O (liquid) Perry's Chemical Engineers' Handbook, Section 2.Physical and Chemical Data o o o G H S ( T) kj/mol r S o S o 1/ 2S o S o kJ/mol K H O H O H o H o 1/ 2H o H o kJ/ mol H O H O ΔG o corresponds to the minimum share of ΔH o which has to be applied as work, e.g. electricity TΔS o corresponds to the maximum share of ΔH o which can be applied as thermal energy to the process. * water is liquid

6 Thermodynamics The minimum necessary cell voltage to start water electrolysis is the reversible (no losses in the process) potential: o G V rev nf n=number of electrons transferred (n=2) F=Faraday s constant (F=96487 C/mol) Example: at 25 o C V rev=1.23 V Precondition: heat corresponding to TΔS CAN be integrated into the process The potential at which the cell operates adiabatically (heat is not lost or required) is the thermoneutral potential: o H V th nf Example: at 25 o C V th=1.48 V V th accounts for the missing energy when thermal energy cannot be added from the surroundings (low temperature process)

7 Thermodynamics The importance of thermoneutral potential for water electrolysis V th (1.48V) V rev (1.23V) The cell is heated by the excess energy generated by the joule heat caused by the overpotentials The cell will cool as the cell dissipates the heat associated with the entropy change, irreversibly V th is higher than V rev as it contains the heat associated with the entropy change for the reaction. cell potential

8 Thermodynamics

9 Thermodynamics The electrolysis efficiency is defined as the thermoneutral potential over the operating voltage (the higher heating value (HHV) of one mole of the product divided by the energy consumption): H o G V th V Isothermal operation, i.e. by applying the thermoneutral potential, corresponds to 100% efficiency in electricity-to-chemical energy conversion. Theoretical efficiencies greater than 100% are possible when the cell operates at a lower potential than the thermoneutral potential (and greater than reversible potential, V rev ). In this case, external heat is required to supply the remaining energy required (-Τ ΔS). Example: electrolysis at 25 o C: ε max = (1.47V / 1.23V) = 120%! The operational potential is defined by the optimization of production economy. The higher the cell voltage is increased above V rev the higher is the current density and in turn the production rate.

10 Thermodynamics Effect of temperature At elevated temperatures a significant part of the total energy demand can be provided as heat This provides an opportunity to utilize the Joule heat that is inevitably produced due to the passage of electrical current through the cell In this way, the overall electricity consumption and, thereby, the H 2 production price can be reduced

11 Thermodynamics Effect of pressure The equilibrium potential, E, is described by the Nernst equation: E o Nernst potential at standard pressure (=ΔG o /nf) p RT 2 V V ln nf p p H HO 1/ 2 O 2 2 If we assume that the overall pressure is equal at both electrodes, the increase of E with pressure is given by: RT 1 V (V V ) ln 12 / nf p Example: an increase in the overall pressure from 1 atm to 200 atm corresponds to an increase in E by only 34mV (@ 25 o C) and 48mV (@150 o C)! Pressurization is advantageous due to: a) Reduction of internal cell resistance b) Production of compressed hydrogen facilitates storage c) Reduction of volume of gas bubbles facilitates water transport

12 Thermodynamics Current density-voltage diagrams In a working electrolysis cell, irreversibility of various processes is unavoidable, which leads to a cell voltage V > V rev : η cat overpotential at the cathode η an overpotential at the anode i R A ohmic losses within the cell and its components R A area-specific ohmic resistance concentration overpotential (mass transport limitations) η an V V ir rev cat an A conc Due to the more complex oxidation at the anode (the OER contains an overall transfer of 4 electrons), the overpotential at this electrode is dominant (η cat > η an ).

13 Types of electrolyzers Type Alkaline Acid Polymer electrolyte Solid oxide Charge carrier OH - H + H + O 2- Reactant water water water water, CO 2 Electrolyte Sodium or Potassium hydroxide Sulphuric or Phosphoric acid polymer ceramic Electrode Raney Ni Pt Pt, Ir Ni-cermet Temperature o C 150 o C o C o C

14 Types of electrolyzers Overall reaction: Half-cell reactions H O H + 1/2O Type Alkaline Acid Polymer electrolyte Solid oxide Charge carrier OH - H + H + O 2- Cathode reaction (HER) 2H O 2e H 2 2 2OH 2H 2e H 2 2H 2e H 2 H O2 e H +O Anode reaction (OER) 2OH H O 2 2 1/ 2O 2e H O 1/ 2O H 2 2 2e 2 H O 1/ 2O H 2 2 2e 2 2 O 1/ 2O 2e 2

15 Classical water electrolysis Alkaline electrolysis Alkaline electrolyte electrolyzers represent a very mature technology that is the current standard for large-scale electrolysis. Common electrolyte: aqueous potassium hydroxide (KOH) at 30% concentration Operation Conditions: o C and 1-30bar Operational voltage: V (@ Acm -2 ) Can utilize cost effective electrode materials (iron, nickel, nickel compounds) Diaphragm often asbestos Efficiency: 70-80% (based on hydrogen HHV) 2OH H O 1/ 2O 2e 2 2 2H O 2e H 2OH 2 2

16 Advantages Classical water electrolysis Alkaline electrolysis Require cheap electrodes Cheap construction material Low specific production rate Low efficiency Large system size Drawbacks

17 Operational principle PEM electrolysis is a process just reverse of a PEM fuel cell process; however the materials are typically different from PEM-FC The heart of a PEM or SPE electrolyser is a proton exchange membrane (or solid polymer) electrolyte H O 1/ 2O H 2e H e H 2 H O H + 1/2O 2 2 2

18 Historical facts The first solid polymer electrolysers (SPE) were developed by the General Electric Company as fuel cells for the NASA space programme (project Gemini). Subsequently, small-scale SPE water electrolysers were used for military and space applications in the early 1970s (active cathode areas per cell: 0.02 m 2 ) In 1975, a development programme was established to scale-up the technology targeted towards large-scale, commercial applications such as energy storage (active area: m 2 and then 0.23m 2 per cell involving a 200 kw module capable of producing 55 m 3 h -1 H 2 )

19 Performance optimization Components resistivity Membrane thickness, ion-exchange capacity Oxygen kinetics i E ln i R cell i o R R R R R dif bipolar electrolyte contact * Current efficiency (or Faradaic efficiency): molar H 2 rate production divided by (i/2f)

20 Components Membrane Electrode Assembly (MEA) Membrane Anode and Cathode electrode Gas diffusion Layer (Current collector) Bipolar plates Schematic of PEM electrolyser showing the location of the components Components cost

21 Proton exchange membrane Requirements high protonic conductivity for the transport of ions high oxidative stability sufficient mechanical and thermal stability low permeability for gases to prevent mixing of the gases produced electric insulator to prevent short-circuits within the cell for electrode to electrode Ionomer must have high chemical and mechanical stability to withstand the harsh conditions in a PEM electrolysis cell

22 PFSA PEM electrolysis Proton exchange membrane Supported Membranes (Dimensionally Stable Membranes, DSM TM ) Perfluorosulfonic acid (PFSA) ionomer incorporated in an engineering plastic support (Nafion, Fumapem, Flemion, Aciplex ) High strength High-effisciency No x-y dimensional changes upon wet/dry or freeze-thaw cycling Alternative Membranes Bi-Phenyl Sulfone Membrane (BPSH) Hydrocarbon/Phosphonate Membrane Inexpensive starting materials Trade-off between conductivity and mechanical properties PFSA (700 EW & 850 EW) membranes Optimization of ionomer EW and thickness, scale-up fabrication methods and techniques, and improve costs PA doped poly [m-phenylene-bis(5,50-benzimidazole)] (PBI) Aromatic polyethers containing pyridine groups (H 3 PO 4 doped) BPSH N O N O X x O Y O X O y N O X z ADVENT TPS

23 Proton exchange membrane: Performance

24 Proton exchange membrane: Durability Indication of membrane degradation Membrane degradation F - release rate: 3.7 μg/hr (<10 ppb) Estimate lifetime: ~55,000 hrs

25 Catalysts Mixture of catalyst and ionomer in order to enlarge the area of threephase-boundary where half-cell reactions take place. Hydrogen side: Supported or unsupported Platinum (loading: 1-6 mg cm -2 ). Oxygen side: Unsupported iridium, ruthenium and their oxides and mixtures (loading: 1-2 mg cm -2 ): Ruthenium oxide (RuO 2 ) is the most active material for OHR; yet it is highly unstable (the oxidation of RuO 2 to RuO 4 occurs at potentials more positive than 1.387V) Iridium oxide (IrO 2 ) is the standard material compromising activity and stability. Used pure or mixed with other precious (e.g. Pt) or non-precious metals. Binary and ternary unsupported catalysts (SnO 2, TaO 2, SbO 2, TiO 2, Ti 4 O 7 ) Cross-section of a membrane electrode assembly consisting of Nafion 117 and electrodes made of pure Pt (cathode) and pure Ir (anode). Courtesy: Fraunhofer ISE

26 Catalysts Ir x Ru y Ta z O 2 V-I characteristics comparing the performance of different binary and ternary unsupported electrocatalysts for the OER. T. Smolinka et al., Polymer Electrolyte Membrane (PEM) electrolysis in Hydrogen and Fuel Cells: Fundamentals, Technologies and Applications (Ed. D. Stolten), Wiley-VCH (2010) Cell potential of PEM water electrolysis cell at high current density (1 A cm -2 ) and 80 o C. ( ) 0% Ta; () 10% Ta; () 20% Ta; () 30% Ta. A.T. Marshall et al., Int. J. Hydrogen Energy 32 (2007)

27 Catalysts: Research trends Improvement of the electrocatalytic activity for OER using different binary and ternary supported catalyst systems (metal oxides) Use of stabilizing agents, e.g. conductive titanium sub-oxides Reduction of metal loading of Pt-based catalysts for HER to values smaller than 1 mg cm -2 Substitution of Pt as a catalyst for HER: e.g. use of nickel or cobalt glyoximes as low-cost compounds which are stable in strong acidic media Use of alternative catalysts synthesis methodologies (Adams fusion, sputtering, sol-gel, sonochemistry ) Application of nanostructured thin film electrodes

28 Current collectors Hydrogen side: electrode potentials are close to zero; it is possible to use carbonbased materials such as carbon papers or felts as known in PEM FCs. Oxygen side: graphite or carbon-based materials are not stable at the anode s working potential. Carbon undergoes electrochemical oxidation at voltages > 0.9V according to: C 2H O CO 4H 4e 2 2 Instead of carbon, porous titanium structures made from titanium are mainly used (main drawbacks: formation of oxide layers, high cost). sintered Ti powder sintered Ti felt expanded Ti mesh C-based paper

29 Bipolar plates (cell-separators) Requirements High bulk electrical conductivity Gas-impermeable (separates H 2 and O 2 compartments) Facilitate water flow and gas evacuation High electrical conductivity and high surface conductivity Resistant to hydrogen embrittlement Stable in oxidizing environment Low-Cost Approach Replacement of carbon (C) with titanium (Ti) Development of low-cost dual-layer structure (e.g. C/Ti) Application of alternative coating strategies Unitized parts and laminate designs

30 Bipolar plates (cell-separators)

31 Stack design: efficiency/power consumption Operating current density: A cm -2 ( V) Pressure: Mpa Current efficiency: up to 98% Stack power consumption: kwh Nm -3

32 Stack design: lifetime Courtesy: Hydrogenics Thermal management is facilitated due to endothermic reaction (water recirculation) High mechanical stresses (pressurized operation, pressure fluctuations at start/stop) Degradation mechanisms: membrane thinning titanium embrittlement by hydrogen corrosion/dissolution/agglomeration of catalyst and support deterioration of seals

33 System design Small systems (< 1 Nm 3 h -1 ): 6-8 kwh Nm -3 Large systems (> 10 Nm 3 h -1 ): < 6 kwh Nm -3 Power conditioning unit Feed water pump (Low pressure) circulation loop for oxygen side with heat exchanger, water purification stage and gaswater separator (High pressure) circulation loop for hydrogen side with gas-water separator Hydrogen conditioning unit with a demister (to remove droplets and aerosols), heat exchanger and condensate trap (to reduce dew point to room temperature) Water drain to recapture the water transported via the membrane to the cathode (electroosmotic drag) Control and safety installation

34 Advantages over classical (alkaline) technology no corrosive electrolytes good chemical and mechanical stability high protonic conductivity high gas impermeability excellent gas separator high current density at higher efficiency reduced number of moving parts easy maintenance excellent partial-load range and rapid response to fluctuating power inputs compact stack design allowing high pressure operation

35 Key benefits Produces virtually no pollution when combined renewable energy sources Uses existing infrastructure Uses fuel cell advances Critical challenges System efficiency and capital/feedstock costs Integration with renewable energy sources Design for manufacturing

36 Major R&D needs Develop of more durable and less expensive membranes Develop of long-lasting membranes and corrosion resistance interconnects (bipolar plates) Develop of durable, low-cost and active anode electrocatalysts Design of novel architectures for large-scale production Balance storage and production rate capacity for variable demand Develop of flexible, scalable systems using low-cost materials Increase reliability for high-temperature units

37 Thank you!