Techno-Economic Study of Generating/Compressing

Hydrogen Electrochemically

A.B. LaConti, T. Norman, K.D. Patch. W. Schmitt, & L.J. Gestaut

Giner Electrochemical Systems L.L.C. (GES)

89 Rumford Avenue

Newton, MA 02466 USA

Tel: 781-529-0533 ··E-mail: lgestaut@ginerinc.com

A. Rodrigues

General Motors, Fuel Cell Activities (GM)

10 Carriage Street

Honeoye Falls, NY 14472 USA

Abstract

The impending hydrogen economy will utilize hydrogen from a number of sources; most notably reformers

and water electrolyzers. In the later case, the primary energy source can be conventional (fossil fuels,

hydroelectric, nuclear) or renewable (solar, wind, biomass).

However, regardless of the source of the primary energy or the method of hydrogen production, there is a

common requirement that the hydrogen be sufficiently compressed to achieve adequate energy density

storage and to allow the rapid transfer of gas from central to local or mobile storage systems. In the case of

water electrolyzers, the hydrogen can be directly produced at elevated pressures. Independent of the source

of the hydrogen, pressurization can also be accomplished subsequent to its production by the use of

mechanical or electrochemical compressors.

While current electrolyzer developments have targeted hydrogen production at pressures of 340 bar and

higher, careful attention must be paid to trade-offs between the electrolyzer system capital costs, operating

costs, and system reliability. The technical and economic impact of varying scenarios has a profound effect

on the overall economics of the hydrogen production and, ultimately, on the economics of the hydrogen

economy.

Keywords: electrolysis, economics, pressure, membrane, PEM

Introduction The movement to a hydrogen economy requires the development of technologies that make that transition

economically viable. This implies that competitiveness with energy derived from carbon-based, non-renewable

sources such as coal or oil must be achieved for the new fuels to be adopted. While tax policies can be used to alter

the competitive landscape, a far better solution is for hydrogen fuel to be inherently competitive.

In the United States the price of a gallon of “unleaded regular” gasoline is approximately $ 0.37/l

($1.40/gallon), including approximately $ 0.40 /gallon ($ 0.11/l) in various federal, state, and local taxes. This price

results, in part, from the from a crude oil price of $ 20 – 25/barrel. The average light duty vehicle fuel consumption

was 19.6 miles/gallon (8.3 km/l) in 1999.[1]

A liter of gasoline contains approximately 32.8 MJ of energy (LHV) [2]. Hydrogen fuel cell vehicles have

demonstrated an efficiency improvement 114% compared to equivalent gasoline-powered internal combustion

engine vehicles[3]. Thus, an equivalent hydrogen fuel cell vehicle would require only 15.3 MJ of energy (LHV) to

move an identical distance. With an energy content of 120 MJ/kg (LHV), this is the equivalent of 0.47 kg of

hydrogen. Thus, economic equivalency to gasoline would require a pretax cost of $ 2.20/kg H2. The results of this

study indicate that such a cost isI within reach when the hydrogen is produced by water electrolysis.

Description Of Electrolysis Process High-volume water electrolyzers introduce water to the anode side of an electrolytic cell, which utilizes

direct current to electrochemically produce pure, high-pressure hydrogen gas on the opposite (cathode) side of the

cell (Figure 1). High-purity water is provided to the oxygen-evolution electrode (anode), where it is

electrochemically decomposed to provide oxygen gas, hydrogen ions and electrons. The hydrogen ions move

through the proton exchange membrane (PEM) to the hydrogen-evolving electrode (cathode), while the electrons

move through the external circuit. An excess of water is supplied to the oxygen side of the cell and recirculated to

remove any waste heat. A portion of the excess water is electro-osmotically transported across the PEM with the

hydrogen ions. Provisions are required to separate and recover the electrochemically transported water. A PEM

electrolysis cell can withstand large differential pressures (>172 bar) when properly supported, as well as high

absolute pressures.

A PEM electrolyzer stack consists of multiple cells in a series electrical configuration. A complete

electrolyzer is supported by a fluid handling system, power conditioning equipment, and a control package. The

simplified major subsystems (Figure 2) of the high-pressure electrolyzer system include the electrolyzer, the

electrical feed subsystem, the deionized (DI) water feed and deionized water handling system, low-pressure

deionized water pump, oxygen gas/liquid separator and oxygen vent subsystem, and the hydrogen gas train

(hydrogen separator, dryer and high-pressure hydrogen outlet.) These subsystems have been packaged and

demonstrated in a compact 25 kW (0.37 kg H2/hr) hydrogen generator (Figure 3).

Figure 1. Water Electrolysis Cell Cross Section

Figure 2. Process Flow Diagram

DI W ater

Han d lin g

P u m p

H 2 Sep ara tor

H 2 Dr yer

E lec tr olyzer

O 2

V en t

W ater

Feed

5 ,000 p s ig

H2

O u t le tE lec t ric i ty

Feed

O 2 S ep ara to r

DI W ater

Han d lin g

P u m p

H 2 Sep ara tor

H 2 Dr yer

E lec tr olyzer

O 2

V en t

O 2

V en t

O 2

V en t

W ater

Feed

W ater

Feed

W ater

Feed

5 ,000 p s ig

H2

O u t le t

5 ,000 p s ig

H2

O u t le t

5 ,000 p s ig

H2

O u t le tE lec t ric i ty

Feed

E lec t ric i ty

Feed

E lec t ric i ty

Feed

O 2 S ep ara to r

Figure 3. Compact 25 kW water electrolyzer

Current Status Of Development In 1968, PEM electrolyzers were only capable of 3.4 bar differential pressure, and when run at an 85%

electrical efficiency were only capable of operating at a current density of 0.7 kA/m2. By 2002, a differential pressure

capability of 172 bar has been demonstrated in single cells as well as current densities as high as 34 kA/m2 at 1.74V

(an 85% electrical efficiency based on an HHV of 1.48V). Higher current densities allow fewer cells (and lower costs)

for equal hydrogen production. In the period of 1968 to the present, a ratio of 50 to 1 reduction number of cells required

to produce a given amount of hydrogen was achieved. During the same period, total noble metal catalyst loadings

have been decreased from 10 mg/cm2 in 1968 to 1 mg/cm2 in 2002. The electrolyzer performance reported in Figure

4 is for a PEM electrolyzer with these low catalyst loadings.

241 bar

0 bar

Improvements in electrolyzer technology have been accompanied by substantial improvement in the

supporting subsystems which in addition to the electrolyzer include the electrical feed subsystem, the deionized (DI)

water feed and handling system, low-pressure deionized water pump, oxygen gas/liquid separator and oxygen vent

subsystem, and the hydrogen gas train (hydrogen separator, dryer and high-pressure hydrogen outlet.)

0

500

1000

1500

2000

2500

3000

3500

1967 1968 1980 1998 2002

Year

Dif

fere

nti

al P

res

su

re (

ps

id)

or

Cu

rren

t D

en

sit

y a

t 85%

Eff

icie

nc

y

(1.7

4 V

)

Differential Pressure

Current Density

Figure 4. Advancements in Electrolyzers

Future Electrolyzer Developments Electrolyzer improvements are underway to further reduce the energy required for hydrogen production. The

use of elevated temperatures, thinner membranes, alternative membranes, and novel catalysts promise to reduce the

energy required by the electroyzer. Each of these improvements is an active area of research, the potential result of

which is indicated in the accompanying table.

Table. Electrolyzer Improvements

Nafion 117

50 ºC

Nafion

117

80 ºC

Membrane A

80 ºC

Membrane B

80 ºC

Membrane B

80 ºC

Adv. Cat.

Calculated Open-Circuit

Voltage (V)

1.210 1.180 1.180 1.180 1.180

Anode Overpotential (V) 0.300 0.275 0.275 0.275 0.235

Cathode Overpotential (V) 0.015 0.010 0.010 0.010 0.010

Membrane Resistance

(iR voltage loss)

0.450 0.250 0.140 0.080 0.080

Contact resistance

(iR voltage loss)

0.025 0.025 0.025 0.025 0.025

Cell Voltage (V) 2.00 1.74 1.63 1.57 1.53

Energy Consumption

(kW-hr/kg H2)

54 47 44 42 41

High Pressure Hydrogen Production Hydrogen gas occurs only in trace amounts in the atmosphere and, as such, does not itself represent a source

of energy, but rather a material for the storage and transportation of energy. Steam reforming of fossil fuels and

thermal decomposition of water are potential sources of hydrogen [4]. Another is the electrolysis of water to produce

hydrogen and oxygen gas. The prime sources of the electrical energy required for electrolysis can come from

traditional sources such as coal, hydroelectric, nuclear or oil, or it can come from renewable sources such as wind,

solar energy, or biomass.

Hydrogen possesses a relatively low energy content on a volume basis compared to liquid hydrocarbon fuels.

Unlike liquid or solid fuels, these factors make it difficult to store sufficient hydrogen at low pressures to meet

anticipated requirements. Though cryogenic storage, metal hydride storage, and chemical hydride storage have been

identified as potential means of hydrogen containment, currently high pressure storage seems to present the best option

for storing sufficient energy to accomplish transportation objectives with existing technologies.

The efficiency of hydrogen production must consider both the electrolytic and auxiliary energy costs.

Electrolyzer efficiency may be expressed as the electric energy required to generate a specific amount of gas (such as

kW-hr/kg), we can refer to this measure as the energy cost of production. That cost of production is dependent on

factors such as temperature, contact resistance, electrode polarization, and electrolyzer operating pressure. The

operating pressure effects the cross-membrane diffusion of the hydrogen and oxygen. When hydrogen crosses to the

oxygen side (and the reverse) the two reaction products recombine to form water and the product gases are lost.

Membrane permeability is a function of the PEM material, its water content and thickness, cell operating temperature,

and partial pressures. Crossover losses are proportional to membrane permeability and cross-sectional area, which is

dependent on current density for a given production rate. For Membrane A cells at 28 bar, 80C, and 21.5 kA/m² ,

the diffusional losses are equivalent to approximately 1.5 kW-hr/kg. For the Membrane B cells under similar

conditions the back-diffusion losses are 2.0 kW-hr/kg.

The cell must also provide the thermodynamic work required to compress the hydrogen gas. For the 26 bar

differential pressure case, the isothermal compression of hydrogen is equivalent to 46 mV or 1.3 kW-hr/kg, and is a

function of pressure and temperature only. The energy cost of mechanical compressors is higher still. Compressor

manufacturers typically measure their system efficiency against an isentropic compression ideal. Isentropic

compression of an ideal gas(assuming a constant specific heat ratio of 1.4) to a pressure ratio of 26 requires 67% more

energy than isothermal compression. Diaphragm compressors typically operate at efficiencies of 50% (or 4.4 kW-

hr/kg) and considering the cost penalty of the compressors and their associated cooling and drying equipment the

high-pressure electrolyzer is an intriguing choice.

Some strategies for the production of hydrogen anticipate producing the hydrogen at low pressures (<35 bar)

and subsequently mechanically compressing the hydrogen to the storage pressure (~345 bar) while other strategies

seek to produce the hydrogen in the electrolytic cell directly at the pressure required. In this study we have chosen to

model these two strategies as well as an intermediate strategy of producing the hydrogen at moderate pressures

(35 – 345 bar) and utilizing mechanical compression to reach the final storage pressure.

Methods

The first consideration is the electrochemical process. Factors considered include the reversible cell potential,

the target current density, electrode Tafel corrections, membrane types and resistances, Nernstian voltages associated

with the compression of the hydrogen produced, and contact resistances. Next considered is the size of the individual

cells, the number of cells per electrolyzer, and the number of electrolyzers required for a particular production target.

Electric power losses are considered in the power supplies, parasitic losses (pumps, fans, heaters, etc.), and in

supplemental compression requirements and efficiencies. Capital costs for repeating and non-repeating elements of

the electrolyzer stack as well as the labor requirements and rates for assembly are included.

Estimates are included for the costs of balance of plant items such as power supplies, vessels, pumps, controls,

hardware, and supplemental compression equipment. Maintenance and installation labor are included. Economic

factors such as amortization, interest, term, profit, load factors and tax rates are likewise included.

Results

Our proprietary integrated model can be used to calculate the costs of producing electrolytic hydrogen

utilizing differing production strategies. In the specific case of supplying 330 kg H2/day at 345 bar operating at a

typical 70% load factor the comparison of manufacturing hydrogen at low pressures (23 bar) with two-stage

supplemental compression, moderate pressure (138 bar) with one stage of compression, and directly at 345 bar with

no supplemental compression. Within each of these strategies we examined the effects of current density and power

costs to determine the relative economics of hydrogen production, using material and labor estimates consistent with

low volume production of the electrolyzer systems. Those results are illustrated in Figure 5 below.

Not surprisingly, the results indicate that lower power costs uniformly result in lower production costs. Not as

intuitive are other trends:

1. For 330 kg/day H2 production rates, moderate current densities are generally more economic than either low

or high current densities. At low current densities the cost of the additional cells required to produce the

targeted amount of hydrogen becomes excessive. At high current densities the additional electrolyzer

polarizations adversely affect the operating economics, particularly at high power costs. Further, higher

differential pressure results in increased hydrogen crossover to the anodic side of the PEM electrolyzer.

2. For 330 kg/day H2 production rates, low-pressure (23 bar) hydrogen production from the electrolyzer

followed by supplemental compression is more favorable than in-cell compression. Several factors account

for this result. In order to support the in-electrolyzer high differential electrolyzer pressures (345 bar) it is

probable that thicker membrane materials will be required. While this has a direct economic effect, the

increased resistance of the thicker membranes further increases the electrolyzer stack operating voltage, but

decreases crossover losses.

Care must be taken in interpreting these results. The study outlined above describes a relatively large

electrolyzer as might be used in a commercial fueling station. Other scenarios such as remote locations or home

fueling will likely have smaller electrolysis units and may or may not require pressures as high as 345 bar. Such

scenarios require further study to reach reasonable conclusions.

Certain considerations are difficult to model such as availability of maintenance, the value of eliminating

compressor noise, and size constraints. Such considerations may ultimately result in operating strategies that would

not be predicted from direct economic considerations.

[1] Effectiveness and Impact of Corporate Average Fuel Economy (CAFE) Standards, Federal Highway

Administration Research Board, 2002 Report to Congress, page 16.

23 b

ar;

& 1

0.7

5kA

/m²

23 b

ar;

& 2

1.5

kA

/m²

23 b

ar;

& 3

2.2

5kA

/m²

23 b

ar;

& 4

3kA

/m²

23 b

ar;

& 5

3.7

5kA

/m²

23 b

ar;

& 6

4.5

kA

/m²

23 b

ar;

& 7

5.2

5kA

/m²

23 b

ar;

& 8

6kA

/m²

138 b

ar;

& 1

0.7

5kA

/m²

138 b

ar;

& 2

1.5

kA

/m²

138 b

ar;

& 3

2.2

5kA

/m²

138 b

ar;

& 4

3kA

/m²

138 b

ar;

& 5

3.7

5kA

/m²

345 b

ar;

& 1

0.7

5kA

/m²

345 b

ar;

& 2

1.5

kA

/m²

345 b

ar;

& 3

2.2

5kA

/m²

345 b

ar;

& 4

3kA

/m²

$0.02

$0.03

$0.04

$0.05

$0.06

$0.07

$3.00

$4.00

$5.00

$6.00

$7.00

$8.00

$9.00

$10.00

$11.00

co

st

of

hyd

rog

en

($/k

g)

electrolyzer outlet pressure & current density

$/kWh

Figure 5. Cost of Hydrogen Vs Current Density, Outlet Pressure, and Power Cost

$3.00 -$4.00 $4.00 -$5.00 $5.00 -$6.00 $6.00 -$7.00 $7.00 -$8.00 $8.00 -$9.00 $9.00 -$10.00 $10.00 -$11.00

[2] Internal Combustion Engine Fundamentals, John B. Heywood, McGraw-Hill Books, New York,1998, p.915

[3] Wheel-to-Wheel Energy Use & Greenhouse Gas Emissions of Advanced Fuel/Vehicle Systems, General

Motors, Argonne National Laboratory, BP, ExxonMobile, Shell, June 2001, Volume 2, p2-10.

[4] “Large-Scale Production of Hydrogen by Nuclear Energy for the Hydrogen Economy”, Lloyd C. Brown, et. al.,

AIChE 2002 Spring National Meeting, New Orleans, LA March 2002.