Production of Pure Hydrogen from a Source of Waste and Steam sing Solid O ide Membrane Electrol erand Steam using Solid Oxide Membrane Electrolyzer

Soobhankar PatiKyung Joong YoonSrikanth Gopalan

Uday B. Pal

Materials Science & EngineeringMaterials Science & Engineering

ECS ECS 215215thth Conference, San FranciscoConference, San FranciscoMay 24 May 24 ‐‐ 3030thth, 2009, 2009

OUTLINE OF THE PRESENTATION

Hydrogen as an energy carrier ?

Conventional Solid Oxide Steam Electrolyzers (SOSE)

Novel Solid Oxide Membrane (SOM) ElectrolyzerNovel Solid Oxide Membrane (SOM) Electrolyzer

Experiment

Process model for the SOM ElectrolyzerProcess model for the SOM Electrolyzer

Evaluation of experimentally obtained data using the process d lmodel

Some ongoing experiments to improve the efficiency

ELECTROLYSIS: CONVENTIONAL SOLID OXIDE STEAM ELECTROLYZER (SOSE)O2 (g)

2e-Air (O2 , N2)

Porous anode

O2‐ O + 2e‐

O + O O2(g)

V

Porous cathode

H2O(g) + 2e‐ H2(g) + O2‐

Electrolyte V

Porous cathode

H2O (g) H2 (g) 2e-

60 – 70% of the energy REDUCTANT

Natural gas*

Partial pressure of oxygen in air (pO2)air > Partial pressure of oxygen in H2-H2O (pO2)H2O/H2

= THERMODYNAMIC

BARRIERTHERMODYNAMIC

BARRIER

REDUCTANT AT THE ANODE Coal

Hydrocarbont

PROBLEM: Conventional SOSE is not equipped to use reductants (waste, coal, etc.)waste

*J. Martinez‐Frias, Ai‐Quoc Pham and S. M. Aceves: Int. J. of Hydrogen Energy, 2003, vol.28, pp 483‐90

Reductant (R)(Coal, Hydrocarbon waste)

RO(g)Syn gas

CONFIGURATION: SOLID OXIDE MEMBRANE (SOM)ELECTROLYZER

Liquid metal anode

2e-

[O] R RO( )

(Coal, Hydrocarbon waste) Syn‐gas

Liquid metal anode(Sn1, Ag2, Cu3)

ElectrolyteO2‐ [O]LMA + 2e‐

[O]LMA + R RO(g)

O2‐ V

H2O(g) + 2e‐ H2(g) + O2‐

Porous cathode(Ni‐YSZ, Cu‐YSZ)

(YSZ)

H2O(g)

2e-H2 (g)

LIQUID METAL ANODE

1 , 2 Charge transfer at the YSZ/Anode interface

3 Reaction interface for oxidation of reductant by the [O]LMAy [ ]LMA

1 T. Ramanarayanan and R.A. Rapp: Metall. Trans. B,3, 3239 (1972)2 T. H. Etsell and S. N. Flengas, Metall. Trans. B, 2, 2829 (1971)3 A. Krishnan, U. Pal and X. Lu: Metall. Trans. B, 36,463 (2005)

1.2

ADVANTAGES : SOLID OXIDE MEMBRANE (SOM) ELECTROLYZER

200

H O ( ) H ( ) 1/2 O ( )

SOSE

0 6

0.9

100

Go (k

J / m

ol) H2O (g) = H2(g) + 1/2 O2(g)

Operating 0.3

0.6Δ

Eo (V

olts

0

H2O (g) + C = H2(g) + CO (g)

ΔG Temperature

SOM

-0.3

0.0

s)

400 600 800 1000 1200-100

2 2

Temperature ( oC )

If h d b (HC) i d If hydrocarbon waste (HC) is used ,

H2O(g) + (HC) H2(g) + H2O(g) + CO(g)

‐ Electrochemical conversion of H2O(g) High purity H2

‐ Efficient way of converting energy value in waste

EXPERIMENTAL SET UP: ELECTROCHEMICAL PERFORMANCE

Ni-YSZ cathode S~ 90 m~ 10% porous

YSZ electrolyteYSZ electrolyte~ 2 mm

O ti T t 1000oCOperating Temperature : 1000oC

ELECTROCHEMICAL CHARACTERIZATION: OPEN CIRCUIT POTENTIAL

ELECTROCHEMICAL CHARACTERIZATION AND PERFORMANCE

Open circuit potential (Eeq)*,** :

ELECTROCHEMICAL CHARACTERIZATION: OPEN CIRCUIT POTENTIAL

Cathodic gas compositionpo

O2(a)

[C/CO(g) equilibrium]Measured Values

(V)

3 % H2O - H2

3.999 × 10-19

-0.050

10 % H2O - H2 - 0.120

20 % H2O - H2 -0.154

40 % H2O - H2 - 0.207

A negative open circuit potential (OCP) indicates the process is spontaneous

OCP of SOSE (0.89 V) >> OCP SOM electrolyzer (-0.207 V) : 40%H2O – H2

* J. Martinez‐Frias, Ai‐Quoc Pham and S. M. Aceves: Int. J. of Hydrogen Energy, 2003, vol.28, pp 483‐90.** P. Soral, U. Pal, H. Larson and B. Schroeder: Metall. Trans. B, 1999, vol.30, pp 307‐21.

ELECTROCHEMICAL CHARCTERIZATION: POTENTIODYNAMIC SCAN5

40% H2O 0 4

3

4

(A/c

m2 )

)

2 20% H2O 10% H2O 3% H2O 0.3

0.4

2

3

nt D

ensi

ty

Cur

rent

(A)

0.2

0

1

Cur

renC

0.1Mass transfer limited

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50

Potential (V)

Current density increased with increase in steam content in the cathodic gas feed

Mass transfer limitation was observed only at 3% H2O content

~ Unavailability of H2O(g) at the TPBs

ELECTROCHEMICAL PERFORMANCE: POLARIZATION MODEL DEVELOPMENT

Open circuit potential (Eeq) :

Ohmic Polarization ( ohm) :ohm = i Rohm Rohm : YSZ , Electrodes, Contact, Lead wire

Activation Polarization ( act) : (ne=2, α=1/2)

(Anode + Cathode)(Anode + Cathode)

Concentration Polarization ( conc) :

Anodic concentration polarizationCathodic concentration polarization

ELECTROCHEMICAL PERFORMANCE: CURVE FITTING

Ohmic loss

Cathodic conc. polarization Anodic conc. polarizationActivation polarizationOpen CircuitPotential

Measured

Impedance

i0 : Fitting parameter

(Eff i bi diff i i )

kc (Mass transfer coeff.of dissolved oxygen

: Literature value*

Impedance spectroscopy (Effective binary diffusivity)

: Fitting parameter

ygin the liquid tin anode): Fitting parameter

Maximum of 2 fitting parameters used for the curve fitting Appropriate assumption

* T. Ramanarayanan and R.A. Rapp: Metall. Trans. B, 1972, vol.3, pp 3239‐46

ELECTROCHEMICAL PERFORMANCE: IMPEDANCE SPECTROSCOPYZreal(Ω-cm2)

5 10 15 20 25 30 35 40

1.2

1.64 X 10-2 Hz

3% H2O 10% H2O 20% H2O

5 10 15 20 25 30 35 40

16

0.8

1.2

1 Hz

Z imag

(Ω-c

m2 ) 40% H2O

- Zim

ag(Ω

)

8

12

0.0

0.4

4000 Hz

- Z

0

4

High frequency intercept : Ohmic resistance* : Independent of steam content**

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0Zreal(Ω)

g q y p p

Overlap of charge transfer resistance and diffusional (Warburg) impedance at higherfrequencies ***

* J.R. Macdonald : Impedance Spectroscopy, John Wiley, New York,NY,1987** M. A. Laguna‐Bercero, S. J. Skinner and J. A. Kilner, J. of Power Sources, doi:10.1016/j.jpowsour.2008.12.139,(2009)*** S. Britten and U. Pal, Metall. and Mat. Transactions B, 31, 733 (2000)

ELECTROCHEMICAL PERFORMANCE: CURVE FITTING (40% H2O in cathodic gas)

h d lOh i l A di l i iA i i l i iO Cathodic conc. polarizationOhmic loss Anodic conc. polarizationActivation polarizationOpen

CircuitPotential

Measuredi0 : Fitting parameter

Insignificant kc: Fitting parameterMeasured

Measured (Imp. Spectra)

Insignificant kc: Fitting parameter

ASSUMPTION: Cathodic concentration polarization ( ) is negligible*3 0

3.5

ASSUMPTION: Cathodic concentration polarization ( conc, c ) is negligible

‐Mass transfer limitation not observed ‐ SOM electrolyzer is electrolyte supported with thin Ni‐ YSZ cathode 1.5

2.0

2.5

3.0

ial (

V)

Fitting parameters Curve fitting results

Exchange current (i0 ) 1.00A

f ff ( k )0.0

0.5

1.0

Pote

nti

* M. Ni, M.K.H. Leung and D.Y.C. Leung: Int. J of Hyd. Energy, 2007, vol. 32, pp 2305‐13

Mass transfer coefficient ( kc) 0.00056 cm/sec0 1 2 3 4 5

-0.5

Current (A)

ELECTROCHEMICAL PERFORMANCE: CURVE FITTING (3% H2O in cathodic gas)

Activation polarizationOhmic lossOpen Cathodic conc polarization Anodic conc polarizationActivation polarizationCircuit

Potential

Cathodic conc. polarization Anodic conc. polarization

Measured i0 : Fitting parameterkc : Independent 0f cathodic gas comp.*

Measured (Imp. Spectra) : Fitting parameter

kc : 40 % curve fitting (0.00056 cm/sec

2.0 3% H2O

Curve Fitted

1.0

1.5

entia

l (V)

Fitting parameters Curve fitting results

Exchange current (i0 ) 0.12 A

Effective Binary Diffusivity ( DeffH2‐H2O ) 0.025 cm2/sec0.0

0.5Pote

*S. Yuan, K.C. Chou, U. Pal: J. Elec. Soc.,1994, vol. 141, pp. 467‐74

0.0 0.3 0.6 0.9 1.2 1.5 1.8

Current (A)

ELECTROCHEMICAL PERFORMANCE: CURVE FITTING ( 10 % and 20 % H2O)

Activation polarizationOhmic lossOpen

Circuit Cathodic conc. polarization Anodic conc. polarizationpCircuitPotential

p p

Measured i0 : Fitting parameterkc : 40 % curve fitting

(0.00056 cm/sec

3

10 % H2O

20 % H2O

Measured (Imp. Spectra)

:Independent of cathodic gas comp.*

:3% curve fitting (0.025 cm2/sec)

1

2 Curve Fitted

entia

l (V)

Cathode gas composition Exchange current (i )

0

Pote Cathode gas composition Exchange current (i0)

10 % H2O – H2 0.26 A

20 % H2O – H2 0.68 A

0 1 2 3 4

Current (A)R. B. Bird, W. E. Stewart and E. N. Lightfoot: Transport Phenomena, 2nd ed., John Wiley & Sons, New York, 2002

Cathode gas

ELECTROCHEMICAL PERFORMANCE : ACTIVATION POLARIZATION

g

compositionExchange current (i0)

3 % H2O – H2 0.12 A

10 % H2O – H2 0.26 A

20 % H2O – H2 0.68 A

40 % H2O – H2 1.00 A

0.4 3% H2O

H2O content

S f f H O ( ) TPB’ 0 2

0.3

0.4 10% H2O 20% H2O 40% H2O

ariz

atio

n (V

)

=> Surface coverage of H2O (g) at TPB’s

0.1

0.2

ctiv

atio

n po

la

Provides additional sites for the charge transfer reaction0 1 2 3 4 5

0.0Ac

Current (A)

ELECTRO. PERFORM.: CATHODIC CONC. POLARIZATION (40% H2O)

is composition independent

2 5

3.0

3.5 Total polarization Cathodic conentration polarization

Cathodic concentration polarization 1.5

2.0

2.5

rizat

ion

(V)

1% of the total overpotential0.0

0.5

1.0

Pola

r

0 1 2 3 4 5-0.5

Current (A)

Cathodic concentration polarization is insignificant at 40% H2O

: Consistent with our initial assumption

ELECTRO. PERFORM.: IMPEDANCE SPECTROSCOPY IN SUPPORT OF POLARIZATION MODELING

Zreal (Ω-cm2)

Cathodic gas composition 40 % H O-H

0 5 10 15 20 25 3020

Equivalent circuit describing the SOM electrolyzer Cathodic gas composition :40 % H2O

1.0

1.5

cm2 )

4.0 X 10-2 Hz

Ω)

Cathodic gas composition 40 % H2O H2

Curve fitted

CNLS Fitting Results: Rohm = 0.585 Ω

12

16

0.5

0

-Zim

ag(Ω

-c

4000 Hz10 Hz

-Zim

ag (Ω

1 Hz

ohm Rct = 0.061 Ω

4

8

0.5 1.0 1.5 2.0 2.5

0.0

Zreal (Ω)

0

At small activation polarization real ( )

At small activation polarization,

Polarization modeling CNLS Fitting

Oh i L (R ) 0 58Ω 0 585ΩOhmic Loss (Rohm) 0.58Ω 0.585Ω

Charge transfer resistance (Rct,c) 0.054Ω 0.061 Ω

40% Steam content 3% Steam content

ELECTROCHEMICAL PERFORMANCE: VARIOUS POLARIZATION LOSSES

2.5

3.0

3.5

on (V

)

Conc. polarization( Anode)Activation pol.

1.5

2.0

Conc. polarization(Anode)

n (V

)

1.0

1.5

2.0 Ohmic loss ( Electrode, Current collector)

Ohmic lossTota

l Pol

ariz

atio

0 5

1.0

Conc. polarization(Cathode)

tal P

olar

izat

ion

Ohmic loss (Electrodes,Contact, Current collector)

Activation polarization

0 1 2 3 4 50.0

0.5 Ohmic loss

( YSZ electrolyte)

T

Current (A)0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

0.0

0.5

Ohmic (YSZ electrolyte)

Tot

Current (A)

)

Current (A)

At 40% H2O ohmic resistance is 80.5% of the total polarization

At 3% H2O the ohmic part ≈ Overpotential due to electrode processes

Current (A)

2 pElectrodes, Contacts, Current collector 52 % of the ohmic lossYSZ electrolyte resistance 48 % of the ohmic loss

Ohmic loss due to the dominates the performance loss:Ohmic loss due to the dominates the performance loss:

‐Molybdenum current collectors on the anodic side‐ SOM electrolyzer design ( Electrode supported)

SUMMARY

The potential of hydrogen production from steam using a solid oxide membrane l t l ith li id d d t t delectrolyzer with a liquid anode was demonstrated.

Thermodynamic barrier was lowered using a reductant in the liquid metal anode.

U i l t l t t d d i d l i l 2 0 V t d it f Using an electrolyte supported design and applying only 2.0 V , a current density of 0.5 A/cm2 was achieved.

Polarization modeling results thus showed that the performance is rate‐controlled byh h i l the ohmic loss.

Experimental study and modeling will form the basis for redesigning the SOMelectrolyzer to improve its efficiency and for investigating various types of waste feed.