A Short Review on Hydrogen, Biofuel, and Electricity Production Using

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A short review on hydrogen, bio-fuel and electricity production using seawater as a medium Kumaravel Vignesh, and Ahmed Abdel-Wahab Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00995 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Energy & Fuels

Contents: 1. Introduction 2. Technologies available for energy production/conversion using seawater 2.1. Hydrogen generation 2.1.1. Photo-catalysis 2.1.2. Photo-electrochemical 2.1.3. Bio-chemical 2.2. Biofuel production 2.3. Seawater batteries 2.4. Electricity generation 2.4.1. Microbial desalination cells (MDC) 2.4.2. Salinity Gradient Energy Pressure-retarded osmosis (PRO) Reverse electro-dialysis (RED) Capacitive mixing (CAPMIX) Mixing entropy battery (MEB) 3. Summary Acknowledgement References

1. Introduction: In the 21st century, energy security and control of global warming are the two important challenges worldwide. Nearly 80-85 % of the world’s energy demand is obtained from the nonrenewable fossil fuels such as oil, coal and natural gas 1. The consumption of fossil fuels has been doubled every two decades

2, 3

. Moreover, the emission of greenhouse gases from fossil

fuels is the major cause for global warming. Therefore, harvesting energy from renewable sources is a mandate to devoid the effects of greenhouse gas pollution. At present, only 15-20 % of energy is produced from the renewable sources such as solar, wind, tidal, biomass, geothermal and hydropower 1. Governments and funding agencies allot a major portion of resources towards energy production via cost effective and environmental benign technologies to secure the energy supply for the future generation. Most of the renewable energy technologies have the following 2 ACS Paragon Plus Environment

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constraints: sources are located in remote area, not always available, limited commercialization potential and more expensive to produce compared to conventional energy sources. Seawater is the most earth-abundant natural resource (more than 90 % of earth’s water source). From the practical point of view, conversion/production of energy using seawater would be highly desirable. NaCl, MgCl2, MgSO4, CaSO4, K2SO4, K2CO3 and MgBr2 are the important contents of natural seawater. Among them NaCl (~3.5 %) is the major constituent for the salinity of seawater (salinity percentage: Chloride 55 % and Sodium 30 %). Several technologies have been investigated for extracting energy from it (Fig. 1). The percentages of articles that have been published to date on each technology are shown in Fig. 2 (the percentages have been estimated on the basis of the number of research articles reviewed in in each section). It can be observed that bio-chemical processes for the production of hydrogen or electricity have attracted considerable attention. The following sections discuss the technologies that have been evaluated for energy production using seawater.

Fig. 1. Various technologies used to produce energy using seawater

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Energy & Fuels

Fig. 2. The contribution of research publications on energy production using seawater 2. Technologies available for energy production from seawater 2.1. Hydrogen Production: 2.1.1.Photocatalysis: Photocatalytic water splitting has been identified as a promising technology for H2 production 4. When a semiconductor is irradiated with light energy equivalent to or higher than the band gap energy, electrons are excited from the valence band (VB) to the conduction band (CB), and therefore electron-hole pairs are formed (Fig. 3). Holes (h+) in the VB can oxidize the water into oxygen (O2). Electrons (e-) in the CB can reduce the hydrogen (H+) ions into molecular hydrogen (H2). To attain this oxidation–reduction reaction, the CB minimum potential of a photo-catalyst must be lower than the reduction potential of H+/H2 (0 V vs. NHE) and the VB maximum potential must be higher than the oxidation potential of H2O/O2 (+1.23 V vs. NHE), respectively. The photocatalytic water splitting is an uphill reaction and hence the experiments are generally performed in the presence of hole scavenging agents such as ethanol, methanol, triethanolamine and sodium sulfide/sodium sulfite mixture 4-7. Titanium dioxide (TiO2) and zinc oxide (ZnO) are the most commonly preferred photocatalysts

4, 8-13

. However, their wide band gap energy allows only a small portion of solar

spectrum (only UV light) to be utilized. Therefore, there have been appreciable amount of research work on developing visible light responsive photo-catalysts via doping with metals 14, 15,


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decorating with dyes

16, 17

, coupling with narrow band gap semiconductors

Page 4 of 38

18, 19

, Z-Scheme

systems 20, and constructing hetero-structures 21, 22. Moreover, H2 generation is also promoted by the existence of a solid co-catalyst (like noble metals (Ag, Pt, Pd, Rh.), metal oxides (NiO, RuO2)) via the formation of more active sites and minimizing the activation energy 19.

Fig. 3. Schematic representation of H2 production via photo-catalytic water splitting Most of the studies in this area are focused on using precious pure water. The studies regarding the utilization of natural or synthetic seawater for H2 production are very limited. The results are summarized in Table 1. In many cases, the H2 production efficiency of photocatalyst in natural seawater medium is higher than the synthetic seawater. The surface characteristics (surface area, co-catalyst, defects, light absorption capacity, and morphology) of the photocatalyst, NaCl concentration of seawater and concentration of sacrificial agent are the important parameters that influence the H2 production efficiency. The photo-generated electrons and holes have very short life time. If the hole scavenging action of sacrificial agent is poor, the electron and hole will recombine again, it will influence the H2 production efficiency of the photocatalyst. To attain the maximum electron-hole separation process, the sacrificial agent should be effectively adsorbed on the photocatalyst surface for its reaction with photo-generated holes. The NaCl content of seawater can support the adsorption of sacrificial agents on the photo-catalyst surface. Na+ ions tend to attach the sacrificial agents via ionic bond or electrostatic interaction. 5 ACS Paragon Plus Environment

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Energy & Fuels

Therefore, the seawater medium promotes the hole scavenging action of sacrificial agents and improves the electron-hole separation on the photocatalyst surface NaCl content to achieve maximum efficiency

24

23

. 3.5 % was the optimum

. The photocatalytic activity was decreased at

high salinity. In addition to that, most of the photocatalysts are chemically stable in the seawater medium under prolonged light irradiation. Ji et al. 24 evaluated the effect of different metal salts on the H2 production efficiency. They found that the efficiency was dropped significantly in the presence of MgCl2 and it was high in the presence of K2SO4 when compared to other salts. Therefore, the removal of Mg ions from seawater could further improve the photocatalytic efficiency. Gao et al. 25 investigated the formation of oxidized chloride compounds (e.g., Cl2 gas, hypochlorite, alkyl halides) during the photo-catalytic seawater splitting reaction. It was observed that the chloride content was not changed throughout the photochemical reaction. This is ascribed to chloride radicals are reduced back to chloride ions by photo-electrons. Table.1. Summary of the efficiency of different photocatalysts for H2 production using seawater

Photocatalyst

Water source

Activation

Sacrificial

method

agent

Comments

ZnS1-x-

Synthetic

Visible light

0.10 mol L-1

Catalyst synthesized

0.5yOx(OH)y-

seawater

(400 W Hg

Na2S and

by hydrothermal

with UV

0.040 mol

method showed very

cutoff filter)

L-1 Na2SO3

good performance.

ZnO/NiS (1 %)

Reference

22

The efficiency of catalyst in seawater is higher than in pure water. TiO2

Natural seawater

Sun light

Cyclohexen

H2 production rate

e (5 %)

for seawater is

26

higher than that of pure water Pt/Cd0.5Zn0.5S

Synthetic

Visible light

seawater

(400 W Hg

Glucose

H2 production rate increases with


23

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with UV

increase of NaCl

cutoff filter)

concentration upto 3 mol/L. The efficiency for seawater is higher than that of pure water.

Rutile TiO2

Pure water and

UV light (500

-

H2, H2O2 and O2 are

synthetic

W Xe) +

identified as

seawater

Sonication

products.

(4 %, 10 %, 20

(200 KHz)

Sono-photocatalytic

% and 25 %

efficiency decreases

NaCl)

with increase of

27

NaCl concentration. The efficiency of pure water system is higher than that of seawater. La2Ti2O7

Pure water (PW), UV light (400

Na2S and

Efficiency of

(for UV light)

natural seawater

Na2SO3

La2Ti2O7 :

CdS/TiO2

(NSW), and

(for visible

synthetic

light)

seawater (SSW)

W Hg)

24

PW > NSW > SSW

Efficiency of CdS/TiO2: SSW > NSW > PW

TiO2/CuO (2.5

Synthetic

UV-Vis light

%)

seawater (3.5 %

(300 W Xe)

-

H2 production efficiency of pure

NaCl)

water is higher than that of seawater. The activity of TiO2/CuO (2.5 %) is 7 ACS Paragon Plus Environment

28

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Energy & Fuels

7.8 times higher than TiO2 in seawater splitting. CuO/TNT

Synthetic

UV light (300

0.05 Oxalic

The efficiency of

seawater (3.5 %

W Xe)

acid

CuO/TNT is much

NaCl)

29

higher than that of TNT.

a)TiO2/CuO

Synthetic

UV-Vis light

0.05 M

H2 production

b)TiO2/NiO

seawater (3.5 %

(300 W Xe)

oxalic acid

efficiency of pure

30

water is higher than

NaCl)

seawater. CuO/nanoTiO2 showed better efficiency when compared to NiO/nano-TiO2. Ladder type

Synthetic

Simulated

WO2-NaxWO3

seawater (pH

solar light

of 37.5% WO2 and

6.5)

(1000 W Xe)

62.5% NaxWO3

+ IR light

showed maximum

(980 nm laser)

efficiency.

-

Catalyst composed

31

The photocatalytic activity is maintained well for 90 h. SiO2/Ag@TiO2 Synthetic

Visible light

20 % v/v

The nano-composite

core-shell

seawater (3. 5 %

(300 W Xe

glycerol

has photocatalytic,

(solar thermal

NaCl)

with UV

plasmonic and

cutoff filter)

photothermic

collector nanocomposite)

attributes. H2 generation efficiency: 8 ACS Paragon Plus Environment

25

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SiO2/Ag@TiO2 > SiO2@TiO2 > TiO2 . H2 production rate for seawater is lower than that of pure water. ZnS1-x-

Synthetic

Visible light

0.10 mol/L

The activity is

0.5yOx(OH)y-

seawater

(400 W Hg

Na2S and

strongly influenced

with UV cut-

0.040 mol/L

by the NaCl content.

off filter)

Na2SO3

When the NaCl

ZnO

32

concentration is high (3 mol/L), the efficiency of the catalyst in seawater is higher than in pure water Ni-ZnO@C

Synthetic

UV-Vis light

CH3OH

Catalyst with Zn/Ni

nanoreactor

seawater

(300 W Xe )

(2 %)

mole ratio of 1

(3.5 % NaCl)

33

showed maximum efficiency.

2.1.2.Photo-electrochemical (PEC): PEC is one of the most promising technologies for H2 production via electrolysis of water under light irradiation. The photocatalyst is deposited as a thin film on a suitable substrate to form a photo-anode or photo-cathode. During light irradiation electron-hole pairs are generated in the photo-electrode, which can participate in the redox reactions at the electrode/water (electrolyte) interface. Photon with energy sufficient to generate above 1.23 V is required for H2 production. An external circuit is also required to direct the photo-generated electrons. The schematic representation of a PEC cell is shown in Fig. 4 34. 9 ACS Paragon Plus Environment

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Fig. 4. Schematic representation of a PEC cell

34

The reactions occur in a PEC system is given as follows 34: Photo-electrode + hυ

e- + h+

(1)

H2O + ½ O2 (g)

(2)

2OH- + H2 (g)

(3)

H2 (g) + ½ O2 (g)

(4)

Oxygen evolution reaction (OER): 2OH- + 2h+ Hydrogen evolution reaction (HER): 2H2O + 2eOverall reaction: H2O + hυ

Photo-anodes with n-type semiconductors favor OER; photo-cathodes with p-type semiconductors promote HER. PEC experiments are usually carried out in a three-electrode system using platinum wire, Ag/AgCl and photo-catalyst as the counter, reference, and working electrodes, respectively. PEC performance was evaluated in terms of current density by measuring the linear sweep voltammetry (LSV) curves. Most of the highly efficient photo-electrodes are tested in an acidic electrolyte using pure water. Seawater is a naturally available electrolyte and it has high quantities of dissolved ions when compared to pure water. In 1997, Ichikawa

35

successfully utilized natural seawater as an


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electrolyte solution for the first time to produce H2 under sunlight irradiation. The results of this work encouraged other researchers to study the PEC performance using natural seawater electrolyte. Recently Li et al.

36-38

evaluated the PEC performance of WO3/g-C3N4, TiO2@g-

C3N4/Co-Pi and α-Fe2O3/WO3 nano-rod arrays using natural seawater. The main findings of their work are summarized in Table 2. The efficiency and stability of the nano-composites were found to be higher than that of bare metal oxides. This was attributed to maximum visible absorption and effective charge separation. These results also proved that the photoelectrodes are stable under prolonged light irradiation for PEC seawater splitting. The efficiency of natural seawater electrolyte is almost the same to pure water with Na2SO4 or K2SO4. Table 2. PEC performance of nano-rod arrays in seawater electrolyte Photo-anode

2D WO3/g-C3N4

LSV

Scan Current

rate

density

(mV s-1 )

(mA/cm2)

5

0.73

Comments

Reference

The efficiency of nano-

36

composite is higher than that of WO3. The photoanode is stable after 1 h of continuous light irradiation TiO2@g-C3N4 /Co-Pi

10

1.6

The performance of photo-

37

anode is not changed after 10 h of continuous light irradiation α-Fe2O3/WO3

10

1

The efficiency of nanocomposite

is

50

times

higher than that of Fe2O3. However, 35 % of current density is dropped after 5 h of

continuous

irradiation


light

38

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Nam et al. 39-42 evaluated PEC performance of TiO2 photo-anodes using natural seawater. H2 evolution rate of TiO2 photo-anode was ca. 215 µmol/cm2 h at an optimum external bias of 3.0 V. The efficiency was further increased by the deposition of Pt (0.2 wt %) co-catalyst

40

or

42

enzyme . Nam et al. suggested that nano-composite showed high performance when compared to single-phase electrode. The performance was further improved in concentrated seawater electrolyte

41

because, seawater with high total dissolved salts (TDS) favor the rapid ionic

transport across the electrode/electrolyte interface. This also offers the potential of using seawater brine from desalination plant as an electrolyte solution for PEC.

2.1.5. Bio-chemical: Seawater contains a lot of natural organic matter. Micro-organisms such as cyanobacteria, phototrophic bacteria and dark-acidogenic bacteria have the ability to produce H2 via fermentation of organic matter in seawater. These micro-organisms can break down the organic matter into H2 and volatile fatty acids (VFA)

43-46

. Among them, phototropic bacteria

have the only ability to further degrade the VFA into H2 and CO2 47. The fermentation reactions are carried out in the presence of essential nutrients related to the specific micro-organism. Microbial growth and H2 production require slight alkaline pH

47

. Hence, seawater is a more

attractive medium when compared to pure water. Table 3 summarizes the H2 production efficiency of different marine micro-organisms in seawater.

Table 3. Summary of H2 production efficiency of marine micro-organism in seawater Micro-

Water

organism

source

Phototrophic

Natural

bacterial consort seawater, China

Condition

Nutrient

Phototrophic Butyrate

Comments

Reference

Maximum

H2

and

production is achieved

sodium

at 30 °C, light intensity

glutamate

of

80

mmol

photons/m2s, and pH 8. The

yield

of

H2

production is decreased with 12 ACS Paragon Plus Environment

increase

of

47

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butyrate concentration. Cyanobacterium Natural

Dark

(Aphanothece

seawater, anaerobic

halophytica)

Thailand

NaNO3,

H2 production rate is

glucose,

higher

fermentation Ni2+

for

the

48

cells

and incubated in seawater

Fe3+ salts

without

NaNO3.

The

efficiency is influenced by NaCl, glucose, Ni2+ and Fe3+ concentration. Macroalgae

Natural

Dark

DSMZ

H2 production rate is

Sargassum sp

seawater, fermentation medium

influenced by inoculum

Portugal

concentration.

and

and

anaerobic

Cellobiose with 14-20% of H2 was

digestion

49

Bio-gas

produced.

Operating conditions such as pH, temperature and light intensity, availability of nutrients and nature of marine bacterial strains are important factors that determine the H2 production efficiency. Among them, pH and incubation temperature are the main influencing factors

47

.

Addition of some extra nutrients into seawater could further enhance the H2 production efficiency.

2.4. Bio-fuel production using seawater: Methane: Methane (CH4) gas can be produced from the anaerobic microbial conversion of marine microalgae

50

in seawater. However, the high concentration of sodium ions (Na+) is toxic to the

microbial communities (especially methanogens). Hence, marine sediments are used as salt tolerant microbial source to produce CH4 using seawater and microalgae. Three major steps are involved in the biomass conversion: (1) Formation of volatile fatty acids (VFA) by various bacteria; (2) Transformation of VFA into acetate and hydrogen by propionate and butyrate oxidizing bacteria, respectively; (3) Conversion of acetate into CH4 and CO2 by acetoclastic methanogens and conversion of H2 and CO2 into CH4 by hydrogenotrophic methanogens. The


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yield of CH4 highly depends on the activity of these three steps. Table 4 summarizes the yield of CH4 production using various microalgae and marine sediments in seawater. Table 4. Summary of CH4 production using marine sediments and seawater Algae

Seawater

Marine sediment Comments

Reference

(MS) Saccharina

Synthetic seawater Tokoyo

bay, MS showed high

japonica

(30 g/L NaCl)

Hiroshima

bay, butyrate

and

sea, acetate

Ariake

Japan

and

conversion when to

51

rates

compared mesophilic

methanogenic granules. Sea

wrack Natural seawater

biomass

Mangrove and sea grass

High CH4 yield

52

mudflat, was achieved by

Philippines

MS

when

compared to cow manure and sea wrack associated microflora Macrocystis pyrifera

Natural seawater

Jiaozhou China

Bay,

High CH4 yield

(Littoral) was achieved by

and Arabian Sea the (Sub-littoral)

when to

littoral

MS

compared sub-littoral

MS.

CH4

formation

was

achieved

via


53

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syntrophic acetate oxidation coupled with

hydroge-

notrophic methanogenesis.

The results reveal that microbial process and CH4 yield are strongly influenced by the nature of marine sediments. The efficiency was also affected by the change in physico-chemical parameters such as pH, temperature and salinity. CH4 was not formed in the initial days of microbial process, instead H2S and CO2 was detected. This was attributed to the existence of sulfate and sulfate-reducing bacteria (SRB) in seawater

53

. This could be rectified by the

application of specific SRB inhibitors. Fan et al. found that CH4 production was improved in the presence of 0.8 mM sodium molybdate as SRB inhibitor

53

. They also found that littoral

sediments showed high methanogenetic activity when compared to sub-littoral sediments. The difference in utilizing acetate by the marine sediments might be the reason for the variation in CH4 yield because acetate is considered as the general precursor in the anaerobic microbial production of CH4.

Biodiesel: The most significant steps of bio-diesel production from microalgae are lipid synthesis (e.g., triglycerides (TAG)), lipid extraction and its conversion to biodiesel

54, 55

. Nutrients are

very essential for the bio-synthesis of chlorophyll, DNA, RNA and protein. Significant amount of water and large area of land are required for the production of bio-diesel via conventional plant cultivation method

56

. Therefore, the utilization of seawater as a microalgal cultivation

media is greatly desirable to conserve the land and fresh water resources. Moreover, the use of seawater reduces the amount of mineral supplementation. The microbial process is mainly governed by the composition of nutrients and salinity of the cultivation medium. Depending on the nature of microalgae, the cultivation is carried out in two ways: phototrophic (requires light and organic carbon source) and heterotrophic (requires only organic carbon source). The following steps are involved in the biodiesel production: 1) algal strain isolation; 2) cultivation in the presence of carbon (C), nitrogen (N), and phosphorus (P) nutrients; 3) harvesting; 4) drying; 15 ACS Paragon Plus Environment

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Energy & Fuels

5) lipid extraction and 6) trans-esterification. Among these, strain selection is very important for the cost effective bio-diesel production. Strains that have the ability to grow under local climatic conditions are beneficial for economic success and environmental sustainability. Table 5 summarizes the yield of bio-diesel production using different micro-algae in seawater.

Table 5. Summary of bio-diesel production using marine microalgae and seawater Algae

Seawater

Growth

Fatty

acid Lipid

condition

carbon chain

Reference

yield (mg

L−1

day−1)

Nannochloropsis Natural

Phototrophic

-

13.70

57

58

gaditana Q6

seawater

Chlorococcum

Natural

Phototrophic

polyunsaturated

3.31

sp. RAP13

seawater

Heterotrophic

medium

20.33

(glucose)

saturated

chain and

monounsaturated Heterotrophic,

Medium

(glycerol waste)

saturated

chain and 22

monounsaturated Neochloris

Natural

Phototrophic

oleoabundans

seawater

(N-stress)

Tetraselmis sp.

Natural

Phototrophic

seawater Nannochloropsis Synthetic gaditana

56

59

Unsaturated and 16

60

monounsaturated

monounsaturated Phototrophic

Unsaturated and 13

seawater

monounsaturated

CCMP527 Nannochloropsis

18

salina 16 ACS Paragon Plus Environment

61

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Page 16 of 38

CCMP537

The results clearly show that the efficiency is strongly influenced by the nature of alga, growth condition and utilization of nutrient. Phosphorus (P) was identified as a key nutrient to determine the biomass productivity when compared to N and C

60

. The difference in total fatty

acid composition was ascribed to the significant difference in the metabolism of lipids within the genus

61

. The changes in fatty acid profile depend on the substrate and culture conditions. Fuel

properties such as oxidative stability, Cetane number, and melting point are directly linked to the length and degree of unsaturation of the fatty acid carbon chain 61. Beevi and Sukumaran 58 successfully utilized glycerol waste as nutrient source, indicating the potential recycling of waste to fuel. It was also found that the biomass productivity was higher in seawater medium when compared to fresh water. Recently, Taleb et al. 62 reported the results of screening reshwater and seawater microalgae strains in a fully controlled photobioreactor. Algal strains were screened based on their culture behavior and bio-mass productivity. The screening process was performed via two steps (Fig. 5): (1) a rapid prescreening step and (2) a final validation step.


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Fig.5. Schematic representation of microalgae screening procedure developed by Taleb et al. 62 Parachlorella kessleri UTEX2229 and Nannochloropsis gaditana CCMP527 were identified as promising freshwater (2.7 x 10-3 kg m-3 d-1 of TAG productivity) and seawater strains (2.3 x 10-3 kg m-3 d-1 of TAG productivity), respectively. The strains of Nannochloropsis genus were mainly composed of nine major fatty acids with varying carbon numbers and unsaturation: myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, eicosanoic acid, arachidonic acid and eicosapentae-noic acid 61. 2.3. Seawater batteries: 18 ACS Paragon Plus Environment

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High cost and limited energy storage of lithium ion batteries are the major reasons for the development of seawater batteries (SB) 63. SB is a kind of power source which employs seawater as an electrolyte

64

. Therefore, SB can be used for undersea vehicles. It has a wide range of

applications in lifebuoys, sonobuoys, torpedoes, life raft, and detection devices. The energy density of SB is higher than the normal batteries. The efficiency of SB highly relies on the corrosion of reactive metal anode in seawater and reduction of oxygen or water in an inert metal cathode (Silver chloride (AgCl) or copper chloride (CuCl)). Magnesium (Mg) based alloys are commonly preferred as anodes due to its high faradic capacity (2.2 Ah.g-1), negative electrode potential (-2.73 V (vs. NHE)) and discharge potential. The electrochemical reactions of Mg alloy can be described as follows: Anode:

Mg

Cathode: 2H2O + 2e-

Mg2+ + 2e-

(5)

2OH- + H2

(6)

The corrosion of Mg anode occurs via H2 evolution and magnesium hydroxide (Mg(OH)2) formation Mg + 2H2O

Mg(OH)2 + H2

(7)

The corrosion characteristic of Mg alloy is greatly influenced by morphology, size and distribution of secondary phases. Besides, the discharge reaction is affected by magnesium hydroxide (Mg(OH)2) formation on the electrode. The corrosion potential, specific energy and discharge performance of Mg alloys have been improved by its doping with other metals (ex. Sn, Pb, Hg, Zn, etc) 65. Table 6 summarizes the H2 production rate of different Mg based alloys. It is observed that most of the research works have been carried out using synthetic seawater.

Table 6. Summary of H2 production efficiency of Mg based alloys using synthetic seawater Alloy

Secondary

Activation Comments

Specific Reference

phase

time (s)

energy (W h/kg)

Mg−9%Al−2.5%Pb α-Mg and β- -

Mg−9%Al−2.5%Pb -

Mg17Al12

anode showed high efficiency

when

compared

to


66

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Energy & Fuels

Mg−9%Al−5%Pb Mg-6%Al-1%Sn

Mg17(Al,Sn)12 7.7

Mg-6%Al-1%Sn

(for Mg-6%Al-

anode showed high

1%Sn)

and

efficiency

Mg2Sn

(for

compared to Mg-

67

147

68 69

when

Mg-6%Al-

6%Al-5%Sn,

5%Sn)

Mg−6%Al−5%Pb and

94.2

Mg-3%Al-

1%Zn Mg–4%Ga–2%Hg

Mg3Hg

and 5.7

Mg–4%Ga–2%Hg

Mg21Ga5Hg3

showed

high

efficiency

when

compared

to

Mg−6%Al−5%Pb and

Mg-3%Al-

1%Zn

The efficiencies of metal doped Mg alloys are higher than commercial Mg alloys (AP 65, AZ 31). This was attributed to the improved corrosion via the formation of intermetallic phases. The currently available Mg based alloys have low concentration of heavy metals impurities, which is harmful to the environment. Hence, it is important to develop non-toxic Mg alloys with high efficiency. There are no studies have been investigated using natural seawater.

2.2.2. Microbial Cells: (a) Microbial Desalination Cells (MDC): Recently, Saeed et al.

70

and Sophia et al. 71 published comprehensive reviews on MDC.

Microbial desalination cells (MDCs) is an energy efficient and economic technology to achieve desalination, energy production and wastewater treatment in a single reactor. MDC can produce more energy than the energy required for its functioning. It is based on the concept of microbial fuel cell (MFC). It utilizes the microbes present in wastewater to convert bio-chemical energy into electricity

72, 73


. The schematic diagram of a

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Page 20 of 38

MDC is shown in Fig.6. Wastewater and salt water are fed in the anode and desalination chambers, respectively. A bio-film (accumulation of microbial colonies) is grown on the anode, which acts as an anodic catalyst

74

. At the anode chamber, the bacterium oxidizes the organic

matter and producing protons and electrons. The electrons are captured by the cathode through an external circuit 71. Different types of MDCs such as air cathode, bio-cathode, recirculation, capacitive, upflow, osmotic, bipolar membrane, de-coupled, separator coupled stacked circulation and ionexchange resin coupled have been studied 75-81. Synthetic seawater was used in most of the MDC studies 82-85. Only few researchers used natural seawater and/or actual wastewater

86 87 70, 88

.

Fig.6. The schematic diagram of a MDC 86 Recently, Sevda et al. 86 reported bio-electricity generation using petroleum refinery wastewater effluent and natural seawater (Collected from Doha, Qatar). The power generation and salt removal rate were enhanced in acidic catholyte when compared to neutral catholyte. Zhang and He

87

examined energy production using an up-flow MDC (UMDC) connected to an electro-

dialysis (ED) unit. They suggested that rechargeable batteries are more appropriate to extract energy when compared to external circuit. The required amount of wastewater depends on the initial salinity of water

70, 88

. The electricity generation can be affected by pH, temperature,

volume of water, membrane fouling and electrode stability. The electricity generation was increased by electrolyte recirculation process in MDC 73. 2.2.Electricity generation using seawater: 21 ACS Paragon Plus Environment

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2.2.1. Salinity Gradient Energy (SGE): Salinity gradient energy (SGE) is produced by mixing two solutions of different salinities (e.g., seawater and river water) using pressure-retarded osmosis (PRO), reverse electro-dialysis (RED), capacitive mixing (CAPMIX), and mixing entropy battery (MEB). A comprehensive review on salinity gradient energy published by Jia et al.

89

and Yip et al.

90

. However, a brief

description on the principle and the important results using natural seawater/seawater brine are provided in the following section.

Pressure Retarded Osmosis (PRO): The schematic diagram of a PRO process is shown in Fig. 7

91

. The osmotic pressure

difference between seawater and river water is used to convert SGE to mechanical or electrical energy 92. Water flows from river water side (low concentration (LC) and pressure) to seawater side (high concentration (HC) and pressure) via a semi-permeable membrane in order to make up for the chemical potential equilibrium on both sides. Mechanical energy is generated due to the expansion of volume in the HC compartment and this could be utilized to run a turbine to generate electricity 93, 94. HC and LC solutions are otherwise called as draw and feed solutions, respectively. Table 7 summarizes the results of PRO energy production using natural and synthetic seawater brine as a draw solution.

Fig.7. The schematic diagram of PRO process

91

Table 7. Summary of results for PRO power production using seawater 22 ACS Paragon Plus Environment

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Membrane

HC

LC

Page 22 of 38

Pressure

Power

(bar)

density

Reference

(W m-2) SiO2/poly

Seawater

acrylonitrile

brine

80 mM NaCl

15.2

15.2

95

10 mM NaCl

20

7.0

96

Wastewater

20

4.6

97

20

21.1

97

(1060

mM NaCl) Hollow

fiber Seawater

membrane

brine

(1000

mM NaCl) Thin

film Seawater

composite

– brine*

retentate

polyethersulfone Thin

film Seawater

composite

– brine*

Deionized water

polyethersulfone

*Collected from TuaSpring desalination plant, Singapore.

The results showed that energy production is strongly influenced by the membrane properties and concentration of LC/HC solutions. The performance of membrane is mainly affected by reverse solute permeation, internal concentration polarization and external concentration polarization

95 96

. Using of seawater brine as HC solution has more advantages

such as: retenate is already under high pressure and salinity, energy consumed during RO may be recovered in PRO, and the negative impact of concentrated seawater brine to the environment will be mitigated

97

. Utilizing of treated wastewater as the feed solution is also an attractive

choice for energy production using PRO (Fig. 8). However, pretreatment of wastewater by ultrafiltration and nano-filtration is beneficial attain higher efficiency 97.


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Energy & Fuels

Fig. 8. The schematic representation of PRO process utilizing wastewater as feed solution 97

More research on PRO technology is underway to minimize concentration polarization and membrane fouling and to enhance the mechanical properties and water permeability in order to make the PRO economically viable.

Reverse electro-dialysis (RED): In RED, HC and LC solutions are pumped into an array of alternating anion exchange membrane (AEM) and cation exchange membrane (CEM). Electrochemical potential is generated by the salinity difference and ionic selectivity of the membrane. The schematic representation of a RED system is shown in Fig. 9. A redox couple (Fe2+/Fe3+) or ion adsorption phenomena in the electrode compartment is used to carry out the current to the external circuit.


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Fig. 9. The schematic representation of RED 98

The design of a RED is significantly distinct from that of electro-dialysis. High flow velocities are required in electro-dialysis to reduce salt depletion at the boundary layers adjacent to the membranes. However, the salt depletion is not an issue in RED, because ions move in the direction of the concentration gradient. The efficiency of RED can be improved by tuning the design of membrane stack. Because the efficiency is influenced by the membrane selectivity (coion transport, water osmosis, and electro-osmosis), intermembrane distance, flow direction of HC/LC solutions, and number of electrode segments 99-101. HC and LC solutions are effectively utilized by the stacks with small inter-membrane distance when compared to the stacks with large inter-membrane distance

99

. However, the

disadvantages of using small inter-membrane distance are: large hydraulic friction of HC compartment and extra pretreatment to avoid fouling. These limitations are rectified with the help of profiled membranes (membrane with ridges). The electromotive force (related to electrical potential), ohmic loss (related to current density) and energy utilization are dependent on the flow direction of HC and LC solutions Vermaas et al.

100

100

.

studied the RED efficiency under different flow orientations such as co-flow

(flow of HC and LC in the same direction), cross-flow (flow in the perpendicular direction), and counter-flow (flow in the opposite direction) (Fig. 10). RED efficiency has the following order: counter flow > cross flow > co-flow. The higher efficiency of counter-flow direction is attributed 25 ACS Paragon Plus Environment

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Energy & Fuels

to: the electromotive force is non-zero at all positions, the concentration difference across the membrane is equally distributed. It was also found that the use of multiple electrode segments will further improve the current density.

Fig. 10. The schematic diagram of different flow directions of seawater and river water in RED 100

Selective membranes with ion conductive spacers and innovative channel design are essential to overcome the resistance and hydrodynamic constraints. Capacitive mixing (CAPMIX): CAPMIX is based on the electrochemical double layer (EDL) capacitor technology. Here, HC and LC solutions do not flow simultaneously to anode and cathode compartments, 26 ACS Paragon Plus Environment

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instead these solutions flow to the same compartment at different times

Page 26 of 38

102, 103

. Energy is

extracted by charging and discharging the porous electrodes using HC and LC solutions. The cyclic process involved in a CAPMIX is shown in Fig. 11 90. The process contains four steps: (i) The electrodes are immersed in HC and they are charged using an external electrical current. Cations and anions are accumulated on the oppositely charged electrode. Hence, EDL capacitance is increased at the electrode surface (ii) The circuit is opened and HC is replaced by LC and therefore, EDL capacitance is decreased (iii) The circuit is closed and the ions are discharged from the electrodes via an external load resistor. The energy produced in step (iii) is higher than that was consumed in step (i). (iv) The circuit is opened and LC is replaced by HC to complete the cyclic process.

Fig.11. The schematic diagram of cyclic process involved in a CAPMIX cell 90

To achieve maximum efficiency, the cyclic process is repeated by reusing the HC and LC solutions until the equilibrium concentration is reached. Ion exchange membranes are also used between the porous electrodes to increase the efficiency. This technology is also known as capacitive donnan potential (CDP) approach. This kind of CAPMIX cells does not need an external electric current. Recently, Hatzell et al.

104

reported energy production using a CDP

approach coupled with a bio-electrochemical system (BES) (Fig. 12). The combination of CDP with BES will increase the energy efficiency. Moreover, energy production and wastewater treatment can be attained at the same time. 27 ACS Paragon Plus Environment

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Energy & Fuels

Fig. 12. (a) The schematic representation of CDP-BES (b) theoretical energy extraction using CDP and CDP-BES 104 Here, the capacitive electrodes are immersed into BES. An ionic filed is generated via the microbial oxidation of organic matter by the exo-electrogenic bacteria. This approach can also be applied for thermolytic solutions. The only drawback is the requirement of expensive ionexchange membrane. CAPMIX is more beneficial in terms of using inexpensive electrodes when compared to RED and PRO. However, the charge leakage, low power density, and intermittent power production are the limitations of this method

105

. To address these limitations, Kim et al

105

developed an efficient and cost effective CAPMIX–concentration flow cell (Fig.13). The advantageous of CAPMIX-concentration flow cell are: using of battery electrodes, continuous power density during cyclic use, expensive ion exchange membranes are not required and achieving high efficiency when compared to normal CAPMIX cell.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig.13. The schematic representation of a CAPMIX –concentration flow cell 105

Mixing Entropy Battery (MEB): MEB is similar to CAPMIX but instead of using inert porous electrodes, it employs faradic electrodes with high specific capacity and low self-discharge. This technology does not need any expensive ion-exchange membranes. Here, charging and discharging are carried out in LC and HC solutions, respectively. The term “mixing” refers the mixing of two solutions with different salinity (LC and HC solution or river water and seawater). The mixing of these two solutions increases the entropy of the system. The entropic energy is extracted and stored in the form of electrochemical energy. Therefore, it is called as mixing entropy battery. The four steps of cyclic process involved in MEB are (Fig. 14 (a)): (i) the electrodes are immersed in LC and the battery is charged by extracting Na+ and Cl- ions from the respective electrodes, (ii) LC is exchanged with HC, which causes an increase in potential difference between the electrodes, (iii) the battery is discharged when the Na+ and Cl- ions are reincorporated into the respective electrodes, and (iv) the HC is then exchanged with LC, which results in a decrease of potential difference 106. The relationship between the battery cell voltage (∆E) and charge (q) is displayed in Fig. 14 (b). There is no consumption or production of energy during step 2 and 4. However, energy is required to drive the ions for step 1 and energy is produced in the battery at step 3 via the incorporation of ions. 29 ACS Paragon Plus Environment

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Energy & Fuels

Fig. 14 (a) The schematic diagram of cyclic process involved in a MEB (b) relationship between battery cell voltage (∆E) and charge (q) during the cyclic process 106

Ye et al.

107

examined the performance of a plate shaped MEB using waste water

effluent (LC; collected from Palo Alto Regional Water Quality Control Plant, USA) and seawater (HC; collected from Half Moon Bay, CA). The schematic representation of the cyclic process involved in a MEB is shown in Fig.15.

Fig.15. The schematic diagram of cyclic process involved in a MEB using waste water effluent and seawater 107

The charging reactions are given as follows: 30 ACS Paragon Plus Environment

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Cathode: 10Na4Mn9O18 Anode:

4AgCl + 4e-

Page 30 of 38

18Na2Mn5O10 + 4Na+ + 4e-

(8)

4Ag + 4Cl-

(9)

The discharging reactions are given as follows: Cathode: 18Na2Mn5O10 + 4Na+ +4eAnode:

4Ag + 4Cl-

10Na4Mn9O18

(10)

4AgCl + 4e-

(11)

It was found that more cells with high specific capacity and stability are required to achieve maximum efficiency.

3. Summary: This review article provides a detailed description on energy production/conversion technologies using seawater. We can generate energy in the form of H2 (photo-catalysis, PEC, bio-chemical, seawater activated battery), natural-gas (CH4), bio-diesel and electricity (PRO, RED, CAPMIX, MEB, MDC,) using seawater as a medium. Microbial process is the easiest methods to achieve high efficiency when compared to other technologies. Natural seawater was mainly collected from China, Korea, Taiwan, Turkey, Arabia and Spain. Pretreatment (such as microfiltration, nano-filtration and ultra-filtration) of natural seawater avoids the negative effect of impurities. Energy production efficiency is strongly influenced by the physico-chemical properties of materials (morphology, size, crystallinity, porosity, stability, durability, water permeability, surface modifiers/additives, surface area, defects, etc.), reaction conditions (volume of water, pH, salinity, temperature, etc.), reactor design and the formation of byproducts. Numerous studies should be carried out in future to validate the economic and environmental feasibility of each technology, reactor design, optimization reaction conditions, usage of natural seawater and the influence of real environmental conditions.

Acknowledgement: The authors are grateful to Texas A&M University at Qatar and Qatar Foundation for the financial support.

References: 31 ACS Paragon Plus Environment

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