Development of an ambient temperature alkaline electrolyser for dynamic operation with renewable energy sources

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 7 2 3 e7 3 9

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Development of an ambient temperature alkaline electrolyser for dynamic operation with renewable energy sources Tamunosaki Graham Douglas*, Andrew Cruden, David Infield Department of Electronics and Electrical Engineering, University of Strathclyde, 204 George Street, G1 1XW Glasgow, United Kingdom

article info

abstract

Article history:

A comparison is made between the ambient and conventional temperature alkaline elec-

Received 25 September 2012

trolysers in terms of operational system, voltage efficiency and corrosion rates. The capital,

Received in revised form

operational and maintenance costs are reduced by reducing auxiliary equipment as well as

19 October 2012

auxiliary utilities in the ambient temperature alkaline electrolyser. Also, since auxiliary

Accepted 20 October 2012

electricity consumption is reduced, the alkaline electrolyser is capable for dynamic,

Available online 19 November 2012

continuous and fast-response operation with renewable energy sources. The ambient temperature alkaline electrolyser is capable for wider operational range and faster

Keywords:

response time when powered by wind energy sources. Although the voltage efficiency for

Alkaline electrolyser

hydrogen production is increased by about 12% at the conventional operating temperature,

Renewable energy sources

corrosion rate of the electrode is increased by a factor of about 6.3. The voltage efficiency

Hydrogen energy systems

for hydrogen production, however, is increased by about 12% by employing electrocatalyst

Electrode

in the ambient temperature alkaline electrolyser, and there is benefit of enhancing lifetime durability of the electrode as well as cell components at relatively lower operating temperature. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

For about 85 years commercial manufacturers such as Norsk Hydro (Now NEL Hydrogen, Norway) has developed the alkaline electrolyser technology [1e3] to mainly produce hydrogen that can be used in the manufacture of ammonia-based fertiliser, methanol, metal processing and for hydrogenation of fats and oils. In some instances the oxygen by-product was captured to manufacture hydrogen peroxide [2]. In recent times however, alkaline electrolyser has become attractive for energy storage applications owing to the fact that the technology is relatively less expensive [4e6] compared to PEM or solid-oxide electrolysers for large-scale production of electrolytic hydrogen that can be utilised with oxygen/air in fuel cells to essentially generate back electricity. For this purpose, electrolytic hydrogen is considered as energy carrier in the hydrogen energy system

(HES). Although electrolyser and fuel cell technologies are yet to be fully commercialised in spite of decades of research, instability in the market price of crude oil [7] and natural gas still makes it imperative to look for alternative energy sources in order to not only diversify the energy economy but to minimise dependence on fossil fuels and to also ensure energy security. In the last 12 years, several pilot-scale renewable energy systems involving electrolysers and fuel cells have been demonstrated [4,8e14], in view of the so called future hydrogen economy [15e21]. However, the hydrogen energy systems mostly involve conventional temperature alkaline electrolysers and PEM fuel cells [4,8e14] which are limited in terms of cost, efficiency, durability, safety, storage and distribution infrastructure. Little wonder some people are sceptical [17,18] about the feasibility of hydrogen as future energy carrier in place of existing battery technology, fossil fuel or electricity

* Corresponding author. Tel.: þ44 (0) 1415482949; fax: þ44 (0) 1415484872. E-mail address: [email protected] (T.G. Douglas). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.071

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Nomenclature standard specific heat capacity of water, 4.18 J/ mol/g cell potential, V Ecell standard reversible cell potential, V Eocell thermoneutral voltage, V Eotherm F Faraday’s constant, 96,500 C/mol Gibb’s free energy, J/mol Go i current density, A/cm2 io exchange current density, A/cm2 Japp applied current density, A/cm2 Jcorr corrosion rate and current density, A/cm2 m mass of aqueous electrolyte, g n number of equivalent of electron transfer R gas constant, J/mol K Ohmic resistance, U RU polarisation resistance, U Rp electrical resistance, U R1 anode resistance, U Ranode Rcathode cathode resistance, U Rbubble H2 resistance of hydrogen bubbles, U Rbubble O2 resistance of oxygen bubbles, U Rmembrane membrane resistance, U transport resistance of ions, U Rions T absolute temperature, K ε electrode overpotential, V a charge transfer co-efficient of electrode Cp

infrastructure. This paper presents an innovative concept of an integrated renewable energy system that consists of the ambient temperature alkaline electrolyser and alkaline fuel cell. The ambient temperature alkaline electrolyser is an alkaline electrolyser that is operated without external heating of the electrolyte solution as opposed to the conventional temperature alkaline electrolyser. Therefore, a comprehensive analysis was carried-out on the ambient and conventional temperature alkaline electrolysers that are integrated with renewable energy sources, thereby identifying the scope for sustainable production of hydrogen and oxygen that can be directly utilised in the alkaline fuel cell to essentially generate back electricity. The efficiency and corrosion rates of electrodes were investigated at the ambient and conventional temperatures in order to identify conditions that can enhance efficiency as well as lifetime durability of the alkaline electrolyser. An electrocatalyst was synthesised and characterised in the ambient and conventional temperature alkaline electrolyser in order to compare the effects of temperature and catalyst to enhance efficiency as well as durability.

2. Comparison of hydrogen energy system (HES) and hydrogen and oxygen energy system (HOES) 2.1.

HES

The HES involves production, storage, transport and utilisation of hydrogen as energy carrier, by integrating

b ba bc hcell hact Uheater Yheater HH2 O ðTÞ

Tafel parameter, V/dec anode Tafel constant, V/dec cathode Tafel constant, V/dec cell efficiency activation overvoltage, V energy consumption in heater, kW h cost of energy consumption in heater, $/h enthalpy of water (J/mol) at a specific temperature in Kelvin standard enthalpy, J/mol Ho standard entropy, J/K/mol So water flow rate, kg/h XH2 O Yelectricity cost of electricity consumption in cell, $/h Yheat exchanger cost of energy consumption in heat exchanger, $/h Qheat exchanger energy consumption in heat exchanger, kW h XH2 O=KOH aqueous KOH electrolyte flow rate, kg/h Ydemin eralised water cost of demineralised water consumption, $/h Yelectricity for H2 production cost of electricity consumption in cell for hydrogen production, $/h Yelectricity for O2 production cost of electricity consumption in cell for oxygen production, $/h Ytotal cost for H2 production total cost for hydrogen production, $/h Ytotal cost for O2 production total cost for oxygen production, $/h total operational cost, $/h YTotal

electrolysers and fuel cells or hydrogen engines with renewable energy sources for stand-alone or electrical grid application. Table 1 presents the best known installed HESs, in order to identify the common types of electrolyser and fuel cell in this application. Notable amongst the HESs are the autonomous wind/ hydrogen system that was developed at Utsira in Norway and the HARI project that was developed at the West Beacon Farm in the UK. The two major HESs have similar concept that is illustrated in Fig. 1, whereby the conventional temperature alkaline electrolyser converts excess renewable electricity i.e. electricity generated from the wind turbine or electrical grid that is above demand capacity by consumer e.g. during night time when demand is relatively low; and water into hydrogen and oxygen. The oxygen gas is sometimes stored but most times it is vented to the atmosphere. The hydrogen gas is usually stored and subsequently utilised with air in a PEM fuel cell to generate back electricity when it is needed most by consumer e.g. during day time when demand is relatively high. In order words, in HES the role of conventional temperature alkaline electrolyser was to mainly produce hydrogen gas as part of a short/long-term energy storage medium. However, the limited operational range of conventional temperature alkaline electrolysers, the expensive requirement for storage and distribution of hydrogen gas, and lowround trip energy efficiency are three main barriers to the sustainability of HES. The conventional temperature alkaline electrolyser has limited operational range of 20%e100% of rated electrical power capacity [2,4,9]. The relatively high

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Table 1 e Overview of installed HESs worldwide [4,8e14]. Year of installation 2000 2001 2004

Location ENEA Research centre Cassicia, Italy [8] University of Quebec, Trios-Rivere’res Canada [10] Utsira Island, Norway [9]

2004

West Beacon Farm, Loughborough, UK [4]

2005

Unst, Shetland island, UK [11]

2006

NREL, Golden, Colorado, USA [12]

2007

PicoTruncado, Argentina [13]

2007

Keratea, Greece [14]

Name of project

Type of electrolyser

Type of fuel cell

Prototype wind/electrolyser testing system for stand-alone operation Renewable system based on hydrogen for remote applications Wind/electrolyser demonstration system for stand-alone operation by Statoil-Hydro Electrical-grid integrated renewable energy/ electrolyser system by the Hydrogen And Renewable Integration (HARI) project team Wind/electrolyser system for stand-alone operation by PURE Energy Electrical-grid integrated renewable energy/ electrolyser system Wind/electrolyser system for stand-alone operation by CNEA Electrical-grid integrated wind/electrolyser system by the Centre For Renewable Energy Sources (CRES)

10 kW alkaline

Not stated

5 kW alkaline

5 kW PEM

50 kW alkaline

10 kW PEM

36 kW alkaline

2 kW PEM

Alkaline

Not stated

6 kW PEM

Not stated

6 kW PEM

Not stated

25 kW alkaline

Not stated

minimum input power (20% of rated power) means that the alkaline electrolyser and its auxiliary units have to be properly sized [4,9] in order to be operated continuously under dynamic conditions of wind electricity. Since part of the electric load is for heating purposes especially in remote communities where heat sources are not readily available, the alkaline electrolyser was not operational almost all the time when there was power deficit from the wind turbine. For example in both the HARI and Utsira projects the alkaline electrolyser was turned on/off several times due to power shortage from the wind turbine that is below the minimum operational limit. Grid stabilising equipment such as batteries, flywheel and synchronous machines were used instead for energy storage, load-levelling or peak shaving applications. During the periods of sufficient

wind power, however, it took more than 30 min [9] to turn on the electrolyser from stand-by mode into full operational mode partly due to nitrogen purging that is normally done when the electrolyser is on stand-by mode and also the requirement to maintain optimum temperature of the stack by continued heating and cooling of electrolyte [4,9]. Another barrier to the sustainability of HES is the requirement for storage and distribution of hydrogen gas, which requires significant capital and operational cost investments. The project that was undertaken at CRES [14] has demonstrated the possibility to store hydrogen in metal hydride tanks thereby reducing storage capacity in accordance with the US DOE target [15]. However, the metal hydrides were made of rare earth metals such as lanthanum and cerium

Fig. 1 e Schematic illustration of currently installed HES involving conventional temperature alkaline electrolyser and PEM fuel cell.

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which are expensive and thus require major scientific and technological breakthrough for commercialisation. Also, the need to store hydrogen brings the issue of safety into consideration. The lower explosion limit (LEL) of hydrogen mixed with air or oxygen is about 4%, therefore care must be taken to avoid leakage of hydrogen gas from tanks or tube cylinders that are located in residential areas. Hydrogen can be stored in underground steel tanks as it is done at the hydrogen filling station in Porsgrunn [6,19]. However, storing hydrogen gas in vehicles poses high risk because accident might occur and if there is hydrogen leakage it can cause explosion. The HES is also limited due to relatively low-round trip energy efficiency [20]. The round-trip efficiency for an alkaline electrolyser and PEM fuel cell system is below 42% [21]. The round-trip efficiency is relatively low mainly due to unreliability of the PEM fuel cell. For example the Utsira PEM fuel cell [9] had severe problems of degradation which has made it impossible to reliably convert hydrogen and air into electricity. Apparently, oxygen that is produced from the alkaline electrolyser was not stored and air was used instead to operate the PEM fuel cell. Oxygen from air could likely be contaminated with oxides of carbon which accelerates deterioration of the synthetic polymer (Nafion) membrane in PEM fuel cells [21]. Pure oxygen is required as an oxidant in fuel cells. The oxygen is sometimes vented to the atmosphere in conventional temperature alkaline electrolysers because the operators are mainly concerned to produce hydrogen that can also be sold as chemical feedstock raw material. However, in the case of utilising hydrogen for energy conversion, the oxygen that is produced from the alkaline electrolyser should be stored and utilised as well. The production ratio (2:1) of hydrogen and oxygen respectively is the same as needed for the back conversion and fuel cell that is operated with pure oxygen (instead of air which contains only 21 percent oxygen) can perform at higher efficiency up to 70% due to faster electrode reactions [21].

2.2.

HOES

The HOES involves production, storage, transport and utilisation of hydrogen and oxygen as energy carrier by integrating the alkaline electrolysers and alkaline fuel cells with renewable energy sources either for stand-alone or electrical grid application. The alkaline electrolyser is capable to convert water and renewable electricity into hydrogen and oxygen

gases that can be directly utilised in the alkaline fuel cell to generate back water and electricity. For example Fig. 2 shows that the alkaline electrolyser produces hydrogen and oxygen gases that can be stored and directly utilised in the alkaline fuel cell to essentially generate back electricity. It should be noted that this figure suggests that the product gases might not be separated from the aqueous KOH electrolyte solution, since the liquidegas mixtures i.e. aqueous KOH þ hydrogen and aqueous KOH þ oxygen can also be directly utilised in the alkaline fuel cell. An alkaline fuel cell requires the input of hydrogen and oxygen gases as well as aqueous KOH electrolyte solution in order to generate electricity, heat and water by-product [21]. A regenerative alkaline fuel cell is a type of cell that integrates the alkaline electrolyser and alkaline fuel cell, and is suitable for distributed energy generation, portable and vehicular applications. There are relatively few publications on a regenerative alkaline fuel cell that directly utilises the hydrogen and oxygen product gases from the alkaline electrolyser; majority of the researchers like Markgraf et al. [22] have developed a regenerative alkaline fuel cell that is operated with air, thereby requiring a gas-diffusion layer that is made of carbon-based materials such as PTFE (Teflon) to selectively allow the reaction of oxygen gas, and to prevent CO2 ‘poisoning’ of the electrode and electrolyte. Verma et al. [23] however, has developed a unitised regenerative alkaline fuel cell that directly utilises the hydrogen and oxygen gases from the alkaline electrolyser; thus the product gases in the form of KOH þ water þ hydrogen and KOH þ water þ oxygen can be directly utilised in the alkaline fuel cell, and there is no need for a gas-diffusion layer in the alkaline fuel cell. The HOES is capable to increase the round-trip efficiency and reduce auxiliary utilities. The round-trip efficiency can be increased by least 4.5% by utilising pure oxygen rather than air in the alkaline fuel cell [24]. The alkaline electrolyser produces hydrogen and oxygen gases that are mixed with aqueous KOH electrolyte solution, which can be directly utilised in the alkaline fuel cell thereby eliminating separators, deoxidisers and driers, and consequently reducing capital, operational and maintenance cost. Further, by operating the alkaline electrolyser at the ambient temperature of say 296 K, heaters and heat exchangers can be eliminated thereby reducing auxiliary electricity consumption and consequently widening the operational range of the alkaline electrolyser. The operational range of conventional temperature alkaline electrolyser is limited at 20%e100% of rated electrical power [2,4,9]. Several reasons have been stated for this limited operational range such as: (a) improved gas purity [2,3] and (b)

Fig. 2 e Schematic illustration of proposed HOES involving the ambient temperature alkaline electrolyser and alkaline fuel cell.

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number of cells in the stack and balance of plants for optimum production [4,9]. However, the gas purity largely depends on: (a) purity of the KOH/water electrolyte; and it is customary to use deionised or distilled water in the alkaline electrolyser; (b) the nature of membrane; which should be chemically and structurally stable, has pore size less than 10 mm and porosity of at least 50% [5] in order to promote transport of electrolytic ions and to prevent mixing of gases; and (c) inter-electrode distance; which should be based on ‘zero-gap’ cell configuration [5] in order to facilitate gas-bubble removal and to prevent gas crossover between the electrodes. Thus the limited operational range is mainly due to the number of cells in the stack and associated balance of plants. The relatively high minimum operational limit (20% of rated electrical power) means that this much electricity has to be supplied in order to produce hydrogen and oxygen from the alkaline electrolyser, notwithstanding that part of the electrical load is in auxiliary equipment such as heaters, heat exchangers, gas driers and deoxidisers. Thus the auxiliary equipment that consumes additional electricity limits operational range of the conventional temperature alkaline electrolyser. The auxiliary equipment as well as auxiliary electricity consumption, however, is significantly reduced by operating the alkaline electrolyser at the ambient temperature and by integrating the alkaline electrolyser and alkaline fuel cell. The operational range of ambient temperature alkaline electrolyser can be extended within 5%e100% of rated electrical power thereby allowing continuous production of hydrogen and oxygen under dynamic and intermittent electricity input from the wind turbine.

auxiliary sub-units are required to operate the conventional temperature alkaline electrolyser. These auxiliary sub-units include: heaters, heat exchangers, separators, deoxidisers and driers, which are needed for heating and cooling of the electrolyte solution and for effective separation of hydrogen gas from the liquidegas mixtures. However, the auxiliary subunits incur additional investment cost. In particular, heaters, heat exchangers, deoxidisers and driers consume electricity and consequently increase operational cost of the HES. For example, Fig. 3 illustrates an operational system that optimises heating and cooling requirement of the conventional temperature alkaline electrolyser, in order to determine the operational and hydrogen production cost. The hydrogen production cost is the ratio of total operational cost to the hydrogen production rate. Table 2 presents commercial data for a conventional temperature alkaline electrolyser that has an efficiency of 85% (HHV) [25,26], and in Table 3 it is shown the feedstock and utility costs. By assuming 70% efficiency for the heater and heat exchanger [25], the hydrogen production cost is estimated based on Equations (1)e(9) and presented in Table 4. Yelectricity ¼ Cell energy requirement  Hydrogen production rate  Electricity price (1)
Uheater ¼ DHH2 O ðT353 K Þ  DHH2 O ðT316 K Þ  XH2 O

(2)

Yheater ¼ Uheater  Heat price  Efficiency

(3)

where Yheater

3. Comparison of operational system for the conventional and ambient temperature alkaline electrolysers

exchanger

¼ Qheat

exchanger 1

þ Qheat


exchanger 2

 Electricity price Qheat

exchanger 1


¼ DHH2 O ðT353 K Þ  DHH2 O ðT296 K Þ  XH2 O=KOH  Efficiency

3.1. Operational system for the conventional temperature alkaline electrolyser

Qheat

The conventional temperature alkaline electrolyser mainly produces hydrogen as energy carrier in the HES. Significant

Ydemineralised

exchanger 2

(4)

¼ DHH2 O ðT373

water

(5) KÞ


 DHH2 O ðT316 K Þ  XH2 O=KOH

 Efficiency

(6)

¼ XH2 O  Demineralised water price

(7)

Fig. 3 e Schematic illustration of an optimised system to produce hydrogen from the conventional temperature alkaline electrolyser.

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Table 2 e Commercial data for the conventional temperature alkaline electrolyser [25,26]. Manufacturer model Norsk Atmospheric bi-polar Type No. 5010 [26]

YTotal ¼ Yelectricity þ Yheater þ Yheat

Cell energy requirement (kW h/kg)

Hydrogen production rate (kg/h)

Outlet pressure (bar)

Operating temperature (K)

Power required for maximum hydrogen production (kW)

46.36

4.49

0.02

353

240

exchanger

þ Ydemineralised

water

YTotal H2 production cost ¼ H2 production rate

(8) (9)

As can be seen from Table 4, heating costs from heaters and heat exchangers are insignificant compared with electricity cost which contributes about 92% to the cost of electrolytic hydrogen. This means electricity cost contributes significantly to the cost of hydrogen. Therefore the industrial practice is to reduce electricity consumption by increasing heat consumption. This practice is economical as long as heat can be sourced from industrial plants or from geothermal and nuclear power plants as waste by-product. This practice, however, is only profitable for the people in the business of producing hydrogen that can also be sold as chemical feedstock raw material. In the case of producing hydrogen for energy storage applications like in remote communities for example, electrical heating of the electrolyte solution is the only option since alternative heat sources are not readily available. This means the electricity consumption in heaters and heat exchangers will be increased significantly for the production of hydrogen in remote areas. Apart from increasing the operational cost for hydrogen production, additional auxiliary utilities will reduce electricity that is available for electrolysis, thereby limiting dynamic and continuous operation of the alkaline electrolyser with renewable energy sources.

3.2. Operational system for the ambient temperature alkaline electrolyser The auxiliary utilities are reduced in the operation of ambient temperature alkaline electrolyser with renewable energy sources. For example in Fig. 4, since the alkaline electrolyser is operated at the ambient temperature, there is no need for auxiliary equipment such as heaters and heat exchangers. From Fig. 4, it can be seen that the electrolyte solution temperature is increased by about 30 K due to internal heat generation in the alkaline electrolyser. Nonetheless, the electrolyte solution can be recycled in order to enhance efficiency of the alkaline electrolyser. The electrolyte solution temperature rise is determined based on Equation (10):

DT ¼

nF Ecell  Eocell Cp m


(10)

Thus, assuming that total cell voltage of the alkaline electrolyser (Ecell) is 2.12 V at 200 mA/cm2 and at 296 K, the thermodynamic cell voltage (Eocell ) is 1.48 V, specific heat capacity of water (Cp) is 4.18 J/g K, number of moles of electrons (n) is 2 mol, and Faraday’s constant (F ) is 96,500 C/mol; it is estimated based on Equation (10) that the electrolyte solution temperature will rise by 30 K for electrolysis of 1000 g of aqueous electrolyte solution at the ambient temperature. There is no doubt about this prediction considering that in the conventional temperature alkaline electrolyser that is shown in Fig. 3, the electrolyte solution temperature rise by about 20 K, which is 10 K lower than in the ambient temperature alkaline electrolyser that is shown in Fig. 4 due to relatively higher energy consumption. In the conventional temperature alkaline electrolyser, the electrolyte solution temperature rise to about 373 K which is the boiling point of water that results to evaporation of water, leading to increase in the concentration of KOH electrolyte, and consequently increase in corrosion rate of the electrodes and cell components. Therefore heat removal from the electrolyte is necessary as shown in Fig. 3 in order to maintain the electrolyte solution temperature around 343 Ke353 K. In contrast, however, in the ambient temperature alkaline electrolyser that is shown in Fig. 4, the electrolyte solution temperature rise to about 326 K, which is below the boiling point of water, therefore there is no need for heat removal and the electrolyte solution can be recycled in order to enhance efficiency of the alkaline electrolyser. Moreover, corrosion rates of the electrode and cell components are reduced at relatively lower operating temperature of the electrolyte [5]. It should be emphasised that in Fig. 4, the hydrogen and oxygen that is produced are stored so that it can be subsequently utilised in the alkaline fuel cell. Hence, the production cost of hydrogen and oxygen will be accounted based on Fig. 4, which is in stark contrast to the HES consisting of conventional temperature alkaline electrolyser that is shown in Fig. 3. The hydrogen and oxygen production costs are estimated as the ratio of total operational cost to their respective production rates. For example assuming 80% (HHV) efficiency for the

Table 3 e Assumptions for feedstock and utility requirement for the conventional temperature alkaline electrolyser [27,28]. XH2 O=KOH (kg/h) 4.49

XH2 O (kg/h)

Uheater (kW h)

2.2

0.098

Qheat exchanger (kW h) 0.29

1

Qheat exchanger (kW h) 0.307

2

Industrial electricity price ($/kW h)

Industrial heating price ($/kW h)

Demineralised water price ($/kg)

0.21

0.09

1.61

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Table 4 e Operational and hydrogen production costs for the conventional temperature alkaline electrolyser. Yelectricity ($/h)

Yheater ($/h)

43.71

Yheat

exchanger

0.0062

($/h)

Ydemineralised water ($/h)

YTotal ($/h)

H2 production cost ($/kg)

3.54

47.88

10.66

0.621

ambient temperature alkaline electrolyser, Tables 5 and 6 present preliminary operational data for the electrolyser. The hydrogen production rate of the electrolyser is 107.76 kg/ day which is in the category of small forecourt size electrolysers [26], and based on Equations (11)e(17) the operational costs as well as hydrogen and oxygen production costs are estimated and presented in Tables 7 and 8. It is reasonable to assume 80% efficiency for the ambient temperature alkaline electrolyser, considering the fact that at the ambient temperature efficiency of the electrode metal is within 60e70% as evident in Section 4, and it is possible to increase the efficiency by a further 10% by electrocatalysis, and good electrochemical engineering of the cell in terms of ‘zero-gap’ cell configuration, forced or natural circulation of the electrolyte and product gases in order to enhance ionic conductivity and facilitate gas-bubble removal from the electrode surface [29]. Yelectricity

for H2 production

 Hydrogen production rate

Yelectricity

for O2 production

(11)

¼ Cell energy requirement  Oxygen production rate

Ydemineralised Ytotal

water

 Electricity price

(12)

¼ XH2 O  Demineralised water price

(13)

cost for H2 production

¼ Yelectricity

for H2 production

þ Ydemineralised Ytotal

cost for O2 production

¼ Yelectricity

(14)

water

for O2 production

þ Ydemineralised

(15)

water

Ycost for H2 production H2 production rate

(16)

O2 production cost ¼

Ycost for O2 production O2 production rate

(17)

The hydrogen production cost in Table 8 is reduced by $2.48/ kg compared to current hydrogen production cost of about $13.61/kg from small forecourt size conventional electrolysers [26,30]. Although, the US DOE has set a target of $3.70/kg for hydrogen selling price by the year 2015 [31], it is apparent that the focus has been on hydrogen production from the alkaline electrolyser, whereas the alkaline electrolyser produces hydrogen and oxygen that can both be utilised in an alkaline fuel cell to essentially generate back electricity. For this reason, the hydrogen and oxygen production costs should be accounted as shown in Table 8, which also indicates that there is an added value for producing and storing oxygen. There would be cost savings for utilising oxygen from the alkaline electrolyser as opposed to utilising air that requires additional cost of processing. The ambient temperature alkaline electrolyser is cost effective and energy efficient to produce hydrogen and oxygen that can be directly utilised in the alkaline fuel cell to essentially generate back electricity. For example, Fig. 5 shows an integrated alkaline electrolyser and alkaline fuel cell system that is otherwise referred to as a regenerative alkaline fuel cell [32e35]. The alkaline electrolyser produces hydrogen and oxygen as liquidegas mixtures i.e. in the form of KOH þ H2O þ O2 and KOH þ H2O þ H2 that can be directly utilised in the alkaline fuel cell. Therefore the round-trip efficiency can be increased by operating the cells reversibly either as a regenerative alkaline fuel cell or unitised regenerative alkaline fuel cell. The round-trip efficiency of an integrated system of alkaline electrolyser and alkaline fuel cell can be increased by

¼ Cell energy requirement  Electricity price

H2 production cost ¼

Electrolytic Hydrogen and Oxygen Production Wind Electricity Electrical grid network or stand-alone generation Electricity 273.23 kW H2O +KOH at 296K KOH +H O Storage

Alkaline Electrolyser

Drier/ Purifier

KOH +O +H O at ~326K

O

O Storage

KOH +H O at ~326K Separator

KOH +H +H O at ~326K Separator

H Drier/ H2 Storage Purifier KOH +H O at ~326K

KOH +H O Storage

KOH +H O at ~326K

Fig. 4 e Schematic illustration of operational system for hydrogen and oxygen production from the ambient temperature alkaline electrolyser.

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Table 5 e Assumed data for the ambient temperature alkaline electrolyser. Cell energy requirement (kW h/kg)

Hydrogen production rate (kg/h)

Oxygen production rate (kg/h)

Outlet pressure (bar)

Operating temperature (K)

4.49

2.24

1

296

49.26

at least 4.8% by utilising pure oxygen [21] rather than air which contains only about 21% oxygen. The energy that is stored in the hydrogen and oxygen product gases can be converted back into electricity, and based on 65% efficiency for each of the alkaline electrolyser and alkaline fuel cell, the round-trip efficiency can be up to 42%. This means at least 42% of electricity that is consumed in the ambient temperature alkaline electrolyser can be recovered. Electricity consumption contributes >80% to the cost of hydrogen production. In a regenerative or unitised regenerative alkaline fuel cell there is the potential to recover at least 42% of electricity that is consumed in the alkaline electrolyser; the remaining 58% is converted into heat that can be inherently utilised by recycling the electrolyte solution in order to enhance efficiency of the cell. The round-trip efficiency can be increased further by increasing rates of hydrogen and oxygen production from the ambient temperature alkaline electrolyser, which can be achieved by electrocatalysis and good electrochemical engineering of the cell in order to enhance ionic conductivity and facilitate mass transport of the produced gas-bubbles [29]. The auxiliary units are significantly reduced in the HOES that involve the ambient temperature alkaline electrolyser and alkaline fuel cell, thereby reducing operational, maintenance and capital cost investments. Since the alkaline electrolyser is operated at the ambient temperature of say 296 K, heaters and heat exchangers are no longer necessary. Also, since the alkaline electrolyser produces hydrogen and oxygen liquidegas mixtures that can be directly utilised in the alkaline fuel cell, separators, deoxidisers and gas driers may no longer be necessary thereby further reducing operational, maintenance and capital cost investments. Moreover, by reducing auxiliary electricity consumption, the ambient temperature alkaline electrolyser is capable for continuous and dynamic operation with wind energy sources. This implies that the ambient temperature alkaline electrolyser is capable for wider operational range (below 20% of rated power) and faster response time (less than a second) compared to the conventional temperature alkaline electrolyser. Gas losses can be reduced in the HOES by eliminating separation units such as gas driers. About 5% of gas losses from drier regeneration unit have been recorded [30] for the HES. The drier unit is no longer required in the HOES if the

liquidegas mixtures that are produced from the alkaline electrolyser can be directly utilised in the alkaline fuel cell. In order words, there might not be need to separate hydrogen and oxygen gases from their respective liquidegas mixtures as the product gases can be directly utilised for alkaline fuel cell operation (see Fig. 5). Therefore, hydrogen and oxygen production rates will be increased in the HOES that integrates the ambient temperature alkaline electrolyser and alkaline fuel cell. From the above sections, it is shown the benefit of the ambient temperature alkaline electrolyser which is to reduce auxiliary equipment. As the auxiliary equipment as well as auxiliary utilities is reduced, the ambient temperature alkaline electrolyser is capable for dynamic and continuous operation with renewable energy sources; the operational limit can be extended within 5%e100% of rated electrical power capacity and the response time can be less than a second. Also, by reducing auxiliary equipment, capital, operational and maintenance cost investments can be reduced. Although, the hydrogen production cost is relatively higher due to higher energy consumption, nonetheless, as evident in the subsequent sections, the energy consumption can be reduced by employing electrocatalysts in the ambient temperature alkaline electrolyser. Further, there is an added value for producing and storing oxygen that can be utilised alongside hydrogen in the alkaline fuel cell in order to generate back the electrical energy. The ambient temperature alkaline electrolyser and alkaline fuel cell system is up to 42% efficient, and is cost effective by reducing the cost of auxiliary utilities.

4. Efficiency and corrosion rate of the electrode in the alkaline electrolyser The voltage efficiency of the alkaline electrolyser can be determined based on the thermodynamic voltage and cell voltage as expressed in Equation (18). hCell ¼

1:48 Ecell

(18)

where Ecell ¼ Eocell þ jhact j þ iRU

(19)

The equilibrium cell voltage is related to the Gibbs free energy based on Equation (20): Table 6 e Assumptions for feedstock and utility for the ambient temperature alkaline electrolyser. XH2 O=KOH XH2 O Industrial electricity (kg/h) (kg/h) price ($/kW h) 4.49

2.2

0.21

Demineralised water price ($/kg) 1.61

DGo ¼ nFEocell o

(20) 1

(DG ¼ þ237.2 kJ mol ) [36]. The reversible or equilibrium cell voltage is the minimum electrical energy that is required for electrolysis to take place. For an isolated cell, the thermoneutral voltage (Eotherm ¼ þ1:48 V) [5,36] is used to describe the

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Table 7 e Operational costs for the ambient temperature alkaline electrolyser. Yelectricity for H2

Yelectricity for O2

production

production

($/h)

Ydemineralised

($/h)

46.45

water

23.17

DHo ¼ DGo þ TDSo

(21)

The thermoneutral voltage is almost independent of the operating temperature, but the reversible voltage is slightly reduced as the operating temperature is increased as shown in Table 9. The overvoltages reduce cell efficiency. The cell overvoltages are mainly due to activation and Ohmic overvoltages which increase with current density. The activation overvoltage is expressed as the Tafel equation (22): jij io

(22)

where the Tafel constant is jbj ¼ 2:3

RT aF

(23)

Considering Equations (22) and (23) it seems like activation overvoltage for hydrogen production increases as temperature increases, but in practise the activation overvoltage is reduced as temperature is increased due to increase in the kinetics of electrode reaction [5,36e39]. The activation overvoltage is significantly influenced by exchange current density of the electrode, which is also influenced by the catalyst material. The exchange current density is the equilibrium rate constant of electrode kinetics. Therefore activation overvoltage can be minimised and efficiency can be enhanced by employing electrocatalysts in the ambient temperature alkaline electrolyser in order to increase exchange current density and reduce the Tafel slope. The Ohmic overvoltage, however, depends on temperature, electrolyte conductivity, nature of membrane, cell configuration and inter-electrode distance. The Ohmic overvoltages can be minimised in the ambient temperature alkaline electrolyser by: (a) reducing the interelectrode distance such as ‘zero-gap’ cell configuration in order to enhance electrolyte conductivity and facilitate mass transport of gas-bubbles; (b) use of membrane separator that is chemically and structurally stable, possess pore size below 10 mm, porosity of at least 50% and surface-specific resistance not more than 0.01 U-cm2 [5,29]. The internal and electrical resistances cause inefficiency of the cell. The internal resistances are Ohmic/transport and

Table 8 e Production costs for the ambient temperature alkaline electrolyser.

11.13

for O2 production

($/h)

($/h)

49.99

26.71

($/h)

polarisation resistances. The transport resistance depends on the current path length of electroactive ions, electrolyte conductivity and nature of the membrane. The polarisation resistance depends on activation of the electrode and gasbubble coverage on the electrode surface. The electrical resistance depends on the electrical wiring and current collector that are externally connected to the electrical power source as illustrated in Fig. 6. The polarisation resistance is important for determining reaction rate of the electrode based on exchange current density. According to Equation (24) [40] reaction rate is increased as polarisation resistance is reduced. Thus the activation energy reduces as polarisation resistance reduces. The polarisation resistance depends on the nature of electrode in terms of its microstructure (i.e. specific density of active sites) and electrochemical surface area that is available for interfacial electron transfer.   RT (24) Rp ¼ nFio The polarisation resistance is also important for determining corrosion rate of the electrode. According to Equation (25) [41], corrosion rate of the electrode is increased as polarisation resistance is reduced. This is because as reaction rate is increased corrosion takes place due to reaction products such as hydrides or oxides that are formed on the electrode surface. The corrosion rate of electrode, however, depends on its composition, microstructure, morphology, physicochemical adhesion of catalyst on the electrode substrate, temperature, current density of operation, time of operation and electrolyte concentration .  Rp ¼

O2 production cost ($/kg) 11.92

 Dε b ε/0 ¼ DJapp Jcorr

(25)

where b¼

jbabcj 2:3ðjbaj þ jbcjÞ

(26)

4.1. Investigation of efficiency and corrosion rate of the electrode in the ambient and conventional temperature alkaline electrolysers 4.1.1.

H2 production cost ($/kg)

Ytotal cost

production

3.54

energy balance between electrical and thermal energy requirement based on Equation (21):

jhact j ¼ jbjlog

Ytotal cost for H2

Experimental measurements

DC polarisation and EIS measurements were carried out using Solartron 1287 (potentiostat/galvanostat) and 1255B frequency response analyser (FRA). The alkaline electrolyser cell is a monopolar tank-type that consists of working and counter electrodes that are separated by a poly-propylene based membrane at a distance of about 1 mm, and the electrolyte was 30% aqueous KOH solution. The working electrode was

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Hydrogen and Oxygen Energy System Wind Electricity Electrical grid network or stand-alone generation Electricity

H2O +KOH at 296 K KOH+ HO Storage

KOH +O +H O at ~326 K Alkaline Electrolyser

KOH +O +H O at ~326 K

Electricity

Storage

KOH +H +H O at ~326 K Storage

KOH +H +H O at ~326 K

Alkaline Fuel Cell

Fig. 5 e Hydrogen and oxygen energy system utilising the ambient temperature alkaline electrolyser and alkaline fuel cell.

nickel mesh that has a geometric active area of about 2.52 cm2. The counter electrode was a stainless steel sheet that has a geometric area of about 15 cm2. Polarisation of the working electrode was done for HER (cathodic) and for OER (anodic) in order to determine activity of the electrode for hydrogen and oxygen production respectively. The polarisation measurement was carried-out at a scan rate of 5 mA/s; the data were collected in triplicate measurements and the steady-state potentials were recorded as average of the triplicate measurements. The EIS measurements were carried out at DC potential of 2 V, AC amplitude of 10 mV and frequency range of 100 kHz to 0.1 Hz.

4.1.2.

coverage on the electrode as well as gas-bubble void fraction in the electrolyte solution [29]. The reversible cell voltage is only slightly reduced by about 4e5% for a 57 K increase in the electrolyte temperature (see also Table 9). The activation overvoltage is reduced at higher operating temperature possibly due to increase in the reaction rate based on exchange current density. The total cell voltage (Ecell) and voltage efficiency (hcell) are determined based on Equations (19) and (18) respectively, and are presented in Table 10 which indicates about 19% and 12% increase in efficiency for oxygen (anodic) and hydrogen (cathodic) production respectively at the conventional operating temperature. The exchange current density and corrosion rate of the electrode can be determined based on polarisation resistance of the electrode. The polarisation resistance can be determined from EIS measurement on the electrode. For example, the corresponding Nyquist impedance on nickel mesh electrode in the alkaline electrolyser is shown in Fig. 8 from which Ohmic and polarisation resistances, exchange current density and corrosion rate of the electrode were derived and presented in Table 11. It should be noted that the polarisation resistance was estimated as the diameter of the impedance arc, whereas the Ohmic resistance was estimated as distance of the impedance arc from the origin on the ‘real’ impedance axis. The impedance arc is revealed at the positive and negative axis regions of ‘real’ and ‘imaginary’ impedance respectively, which could be due to resistance as well as double-layer capacitance between the electrode and

Discussion of the results

As can be seen in Fig. 7, the cell overvoltages are reduced at the conventional operating temperature. In particular, for hydrogen and oxygen production on nickel mesh electrode at 200 mA/cm2, the half-cell overvoltages are reduced by 300 mV and 700 mV respectively at the conventional operating temperature of 80  C (353 K). The cell overvoltages are reduced at higher operating temperature due to increase in the electrolyte conductivity as well as electrode kinetics. The conductivity of 30% aqueous solution of KOH electrolyte is increased by about 3% per degree Celsius [41], thus ohmic overvoltage or iRU drop is significantly reduced at higher operating temperature. The Ohmic overvoltage for oxygen evolution (anodic) is significantly reduced at higher operating temperature, probably due to a reduction in gas-bubble

Table 9 e Comparison of thermodynamic voltages of the ambient and conventional temperature alkaline electrolysers [5]. Temperature dependence of the thermodynamic voltage at atmospheric pressure Eocell ¼ 1:5184  ð1:5421  103  TÞ þ 9:523  105  T  lnT

þ 9:84  108  T2

Voltage (V) of ambient temperature alkaline electrolyser at 296 K

Voltage (V) of conventional temperature alkaline electrolyser at 353 K

1.23

1.18

1.48

1.47







Eotherm ¼ 1:5187  ð9:763  105  TÞ
 9:50  108  T2

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Fig. 6 e Schematic illustration of electrical and internal resistances of the alkaline electrolyser [36].

electrolyte. The ‘imaginary’ impedance, however, extends to the positive axis region, which could be attributed to Joule heating that leads to internal heat generation in the alkaline electrolyser. The Ohmic and polarisation resistances are reduced for hydrogen and oxygen production at the conventional temperature. The Ohmic resistance is reduced mainly due to increase in electrolyte conductivity at higher temperature. The polarisation resistance is reduced because at higher temperature the gas-bubbles are relatively less dense and can easily migrate, thereby minimising gas-bubble coverage on the electrode surface and consequently enhancing the electrode kinetics. The gas-bubbles are non-conductive so the reaction rate is increased by minimising gas-bubble coverage on the electrode surface. The exchange current density is increased at the conventional temperature due to activation of the electrode. However, corrosion rate of the electrode is increased by a factor of about 6.3 and 2.6 respectively during hydrogen and oxygen production at the conventional temperature. This could be attributed to formation of reaction

products such as hydrides and oxides on the electrode surface during electrolysis. Thus hydrogen embritlement and passivation of the electrode take place under cathodic and anodic polarisation respectively, and are accelerated at the conventional operating temperature. At higher operating temperature, the electrolyte becomes more corrosive in accordance with the literature [5] that hydrogen embritlement is accelerated on the electrode at higher operating temperature under cathodic polarisation. It should be stated here that half-way between ambient and conventional temperature i.e. at 323 Ke336 K, the corrosion rate will be reduced compared with the conventional operating temperature of about 343 Ke353 K. From Table 11, the average corrosion current density is estimated; although not presented, which suggests that corrosion rate will be reduced by a factor of 2 at low to intermediate temperatures compared with the conventional operating temperatures for hydrogen and oxygen production. In order words, in the operation of ambient temperature alkaline electrolyser, although, as shown in Fig. 4, the

Fig. 7 e Polarisation of the electrode in the ambient and conventional temperature alkaline electrolysers that consist of 30% aqueous KOH electrolyte solution.

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Table 10 e Summary of cell voltages and efficiencies at the ambient and conventional temperature alkaline electrolyser. a

Electrode Ni Ni Ni Ni

mesh mesh mesh mesh

at at at at

23 23 80 80



C (296 C (296  C (353  C (353 

K) (anodic) K) (cathodic) K) (anodic) K) (cathodic)

Ecell200 (V)

hcell (%)

2.50 2.12 1.90 1.80

59.20 70.00 78.00 82.00

2

a Cell voltage at 200 mA/cm .

electrolyte temperature would rise to about 323 Ke336 K as a result of internal heat generation, nonetheless, corrosion rate of the electrode is reduced at low to intermediate temperatures compared with relatively higher operating temperatures of the conventional alkaline electrolysers. From the foregoing, it is clear that efficiency for hydrogen and oxygen production is increased in the conventional temperature alkaline electrolyser. The cell overvoltages are reduced thereby reducing the energy consumption. To be more precise, the efficiency is increased by about 12% and 19% for hydrogen and oxygen production respectively at the conventional operating temperature. However, corrosion rate of the electrode is increased by a factor of about 6.3 and 2.6 during hydrogen and oxygen production respectively at the conventional operating temperature. In general, corrosion rate of the electrode is reduced at low to intermediate temperatures compared with relatively higher operating temperatures of conventional alkaline electrolysers. This means the electrode is not durable at the conventional operating temperature. The efficiency for hydrogen and oxygen production, however, can be increased by employing electrocatalysts in the ambient temperature alkaline electrolyser, and there is benefit of enhancing durability of the electrode as

well as cell components by operating at relatively lower temperatures.

4.2. Investigation of efficiency and corrosion rate of the electrocatalyst in the ambient and conventional temperature alkaline electrolysers 4.2.1.

Literature review

Raney nickel, an alloy that consists mainly of nickel, is a preferred electrocatalyst that is utilised in conventional temperature alkaline electrolyser to enhance the efficiencies of hydrogen and oxygen production. This is partly due to high surface area and porosity of Raney nickel electrocatalyst that is obtained by leaching the electrocatalyst in alkaline solution. Also nickel has the ability to form complex compounds with the transition metals such as cobalt, lithium, chromium, molybdenum, etc, thereby increasing the active sites for electrochemical reaction to take place. For example Raney nickel that was made by electrodeposition from chloride electrolyte [42] has high electrochemical activity due to a high surface area structure that was obtained by leaching of the alloying metals in an alkaline solution. The BET specific surface area was greater than 25 m2/g for the leached anode at a deposition potential of 1.07 V/SCE [43]. The Raney nickel electrodes were further improved by doping with either lithium or cobalt. Best results were obtained by impregnating Raney nickel with cobalt oxides in order to reduce overpotentials by more than 130 mV after 24 h of electrolysis at 80  C and current density of 1000 mA/cm2 [44]. The work of Rosalbino et al. [45] has demonstrated the unique synergy on electro-activity of the transition metal alloys. On the basis of the BrewereEngel valence bond theory [46,47], the HER in particular, is enhanced over a wide range of

Fig. 8 e Nyquist impedance of nickel mesh electrode in the ambient and conventional temperature alkaline electrolysers that consist of 30% aqueous KOH electrolyte solution.

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Table 11 e Electrochemical and kinetic parameters of the electrode in the ambient and conventional temperature alkaline electrolysers. Electrode

Ni mesh at 23  C (296 K) Ni mesh at 80  C (353 K)

RU (U) Anodic

Rp (U) Anodic

io (A/cm2) Anodic

Jcorr (A/cm2) Anodic

RU (U) Cathodic

Rp (U) Cathodic

io (A/cm2) Cathodic

Jcorr (A/cm2) Cathodic

0.35 0.18

0.64 0.08

0.02 0.19

0.06 0.38

0.37 0.17

0.16 0.05

0.08 0.30

0.23 0.60

current densities by alloying appropriate combinations of the transition metals. The HER takes place on the electrode surface and free electrons facilitate the electron transfer mechanism for adsorption of reactant water (i.e. the Volmer step). Also, empty orbitals facilitate the electron transfer mechanism for hydrogen desorption (i.e. the Heyrovsky step). This synergy of electron transfer process for the HER can be achieved by alloying different d-series of the transition metals. Thus the HER can be enhanced by alloying metals of filled d-orbitals that are on the right-half of the transition metal series with metals of empty d-orbitals that are on the left-half of the transition metal series. For example alloying of Ni metal with metals such as Cr, V, Mo, La, Hf and Zr. By alloying of nickel that has electronic configuration of [Ar] 4s2 3d8 and molybdenum that has electronic configuration of [Kr] 5s1 4d5, the electro-activity of NieMo alloy is enhanced due to synergy. Nickel metal has free electrons to facilitate

adsorption of reactant water on the electrode surface and molybdenum has vacant orbitals to facilitate desorption of hydrogen from the electrode surface. For this reason, NieMo is considered as a superior electrocatalyst to enhance the HER activity of the alkaline electrolyser. The majority of researchers [48e54] have tried investigating NieMo electrocatalyst to enhance the HER activity of conventional temperature alkaline electrolyser. However, in this study NieMo electrocatalyst is investigated in the ambient and conventional temperature alkaline electrolysers in order to identify conditions that can enhance the efficiencies for hydrogen and oxygen production as well as durability of the electrocatalyst material.

4.2.2.

Experimental measurement

The experimental set-up is similar as described in Section 4.1.1, except that the working electrodes are SS (stainless steel) mesh

Pre-treatment of Stainless Steel (SS) electrode substrate 1. Mechanical polishing with sandpaper followed by immersion in water. 2. Chemical degreasing in isopropanol at 60oC followed by immersion in water. 3. Electrochemical degreasing by anodic treatment at 0.03 A/cm2 in 30%KOH at 70oC for 5 min followed by immersion in water. 4. Etching or pickling at 0.03 A/cm2 in 70 % H2SO4 at 70oC for 5 min followed by immersion in water.

Pre-deposition of Nickel Cathodic treatment at 0.05 A/cm2 in Wood’s nickel solution (240 g/L NiCl2 and 120 mL HCl) for 5 min followed by immersion in water.

Electrodeposition of Nickel(Ni) and Molybdenum(Mo) Cathodic treatment at 50 mA/cm2 at 333 K for up to 2 hours in 150 g/L NiSO4.6H2O (nickel sulphate hexahydrate), 20 g/L Na2MoO4.2H2O (sodium molybdate dihydrate), 30 g/L HO(COONa)(CH2COONa)2.2H2O (sodium citrate dihydrate), and citric acid for control of solution pH of 3 with continued stirring of solution. Auxiliary anodes were positioned at approximately 1mm at opposite sides of the cathode.

Characterisation of SS-Ni-Mo SEM imaging Anodic (OER) and Cathodic (HER) polarisation at scan rate of 5 mA/sec, in 30 % aqueous KOH solution at 296 K (23oC) and 353 K (80oC). EIS at anodic and cathodic DC potentials of 1.8 V superimposed on 10 mV AC potential at the frequency range of 0.1 Hz to 100 kHz, in 30 % aqueous KOH solution at 296 K(23oC) and 353 K (80oC).

Fig. 9 e Schematic for fabrication and characterisation of SSeNieMo electrocatalyst.

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Table 12 e Kinetic parameters for SS mesh and SSeNieMo electrocatalyst. Electrode SS mesh (anodic) SS mesh (cathodic) SSeNieMo (anodic) SSeNieMo (cathodic)

Fig. 10 e SEM image of SSeNieMo electrocatalyst prepared by electrodeposition.

and SSeNieMo (stainless steelenickelemolybdenum) electrocatalyst. The electrocatalyst was fabricated and characterised as illustrated in Fig. 9:

4.2.3.

Discussion of the results

The SEM image of SSeNieMo electrocatalyt is shown in Fig. 10, which indicates a multi-layered rough and cracked surface structure that could be evident of high surface area of the electrocatalyst. The cathode efficiency is 26% indicating relatively high rate of electrodeposition of NieMo on SS mesh; the catalyst loading is 0.016 g/cm2 and the thickness of deposit is 27.40 mm. The polarisation of SS mesh and SSeNieMo electrocatalyst in the ambient temperature alkaline electrolyser is shown in

jbj (mV/dec)

Rp (U)

io (mA/cm2)

120.33 162.80 115.30 131.45

1.43 2.90 0.96 0.15

18.0 8.96 27.1 173.3

Fig. 11, which indicates that overvoltages are reduced by electrocatalysis. In particular, for hydrogen and oxygen production on SSeNieMo electrocatalyst at 200 mA/cm2, the half-cell overvoltages are reduced by about 400 mV and 200 mV respectively; the voltage efficiency is increased by about 12.4% and 4.8% for hydrogen and oxygen production respectively. It is apparent that the SSeNieMo electrocatalyst significantly enhance the efficiency for hydrogen production compared to oxygen production. This is expected as NieMo alloy catalyst is particularly effective to enhance the kinetics of HER; the NieMo alloy catalyst has superior HER activity due to high electronic conductivity, high surface roughness, high apparent and real surface area [51e55]. The Tafel slopes for the electrode reactions were determined and presented in Table 12. It should be noted that in Table 12 it is also presented the polarisation resistance and exchange current density of the electrodes. The polarisation resistance was determined as the diameter of the impedance arc that is shown in Fig. 12; and the exchange current density was determined based on Equation (24). Table 12 indicates that SSeNieMo electrocatalyst reduces the Tafel constant by 5.03 mV/dec and 31.35 mV/dec, and increases the exchange current density by 9.1 mA/cm2 and 164.34 mA/cm2 for OER and

Fig. 11 e Polarisation of SS mesh and SSeNieMo electrocatalyst in the ambient temperature alkaline electrolyser that consists of 30% aqueous KOH electrolyte solution.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 7 2 3 e7 3 9

737

Fig. 12 e Nyquist impedance of SS mesh and SSeNieMo electrocatalyst in the ambient temperature alkaline electrolyser that consists of 30% aqueous KOH electrolyte solution.

HER respectively; thus the electrode kinetics is enhanced by employing electrocatalyst in the ambient temperature alkaline electrolyser. From Fig. 12 it can be seen that the impedance arc is more depressed on SSeNieMo, which could be due to several factors including: surface roughness as evident in Fig. 10 and charge accumulation/distribution on the electrode surface [56]. The polarisation of SSeNieMo electrocatalyst in the ambient and conventional temperatures is shown in Fig. 13,

which indicates that voltage efficiency is significantly increased at the conventional operating temperature. For example at 200 mA/cm2 the half-cell overvoltages are reduced by about 323 mV and 241 mV for oxygen and hydrogen production respectively at the conventional operating temperature; the electrocatalyst increases the voltage efficiency by about 12% and 9% for oxygen and hydrogen production respectively at the conventional operating

Fig. 13 e Polarisation of SSeNieMo electrocatalyst in 30% aqueous KOH electrolyte solution at the ambient and conventional temperatures.

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temperature. However, the corrosion rate was determined based on Equation (25), which indicated increase in the corrosion rate of SSeNieMo electrocatalyst by a factor of about 2 and 5 during oxygen and hydrogen production respectively at the conventional operating temperature. As the conductivity of KOH electrolyte increases with temperature, the corrosion rate also increases with temperature. Therefore efficiency improvement should be balanced against corrosion rate based on the electrolyte temperature, which can be achieved by operating at relatively low to intermediate temperatures compared with the conventional operating temperature of alkaline electrolysers. It is clear from this section that efficiency can be enhanced by employing electrocatalyst in the alkaline electrolyser. Although at higher operating temperature the electrocatalyst increases the voltage efficiency for hydrogen production up to 12%, nonetheless, by employing electrocatalyst in the ambient temperature alkaline electrolyser the voltage efficiency for hydrogen production can be up by about 12.4%. The corrosion rate of electrocatalyst is significantly reduced at the ambient temperature. Therefore efficiency and durability of the electrocatalyst can be enhanced in the ambient temperature alkaline electrolyser. The SSeNieMo electrocatalyst is particularly effective to enhance the electrode kinetics for hydrogen production. In the literature [25,37,39,52,54,57], several different types of electrocatalysts are suggested for the anode and cathode electrodes, and activity of the electrocatalyst largely depends on the preparation methods, its composition, microstructure, morphology and surface area. Nevertheless, the aim is achieved which is to demonstrate the possibility to enhance efficiency and durability of the alkaline electrolyser by employing electrocatalyst and operating at relatively lower temperatures compared with the conventional operating temperature. The ambient temperature alkaline electrolyser is particularly considered for efficient, reliable, dynamic and continuous operation with renewable energy sources.

5.

Conclusions

The ambient and conventional temperature alkaline electrolysers have been compared for integrating with renewable energy sources. The ambient temperature alkaline electrolyser is identified to be better suited for integrating with renewable energy sources and for energy storage applications. This is because the auxiliary equipment as well as auxiliary utilities are reduced, thereby allowing for dynamic, continuous and fast-response operation of the alkaline electrolyser with renewable energy sources. Also, by reducing auxiliary utilities, capital, operational, and maintenance cost investments can be reduced. The hydrogen production cost from ambient temperature alkaline electrolyser is less than current hydrogen selling price from small forecourt size conventional electrolysers, and there is additional value for producing and storing oxygen that can be utilised alongside hydrogen in the alkaline fuel cell to essentially generate back the electricity. Although efficiency of the electrode metal is increased by about 12% for hydrogen production at the conventional operating temperature, corrosion rate of the electrode metal is

increased by a factor of about 6.3, thereby reducing lifetime durability of the electrode metal. In general, durability of the electrode is reduced at relatively higher operating temperatures of the conventional alkaline electrolysers. The efficiency for hydrogen production, however, can be increased by about 12% by employing electrocatalyst in the ambient temperature alkaline electrolyser, and there is benefit of enhancing lifetime durability of the electrode, electrocatalyst as well as cell components by operating at relatively lower temperatures.

Acknowledgement The UK EPSRC Consortium on Delivery of Sustainable Hydrogen (DOSH2) for supporting this research work.

references

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