5 Carbon
Neutral Technologies for Hydrogen Generation
When reading the lines that follow, please
start from the fundament that $/kW are not the only reference for measuring the
cost-benefit of an energy carrier. If a cheap, easy to get energy carrier –
call it petroleum derivates- cost us to lose the planet, then the dollar amount
per kW losses ground as point of reference and alternatives like a clean
hydrogen energy carrier become the low-cost, option of preference after all.
Contrary to most common
understanding, CO2 neutral hydrogen can be produced through several, many
methods. For the hydrogen economy to be carbon-neutral we are not stuck with
electrolysis that has been classified as a capital intensive, energy intensive
process. Although the several ways to produce hydrogen have
in common the need to separate it from other elements [1].
CO2
Neutral methods
Carbon free hydrogen can be produced
in several manners, but only a few methods have received attention because are
considered commercially viable. These processes can be classified as:
a.
Electrolytic processes.
b.
Biological processes that rely in microbial biomass conversion, or
photobiological conversion;
c.
Photoelectrochemical processes that rely on direct solar water splitting.
d.
Thermochemical processes that rely on high temperature dissociation of hydrogen
from other components and recycling of the original species. [2]
1. Photobiological: The use of microorganisms and
sometimes organic matter and sunlight is harnessed to produce hydrogen. This
technology may provide economical hydrogen production from sun light with low-
to net – zero carbon emissions. The attractiveness of this method relies in
those algae that grow in non-drinkable water or wastewater can potentially be
used to generate hydrogen [3]
o
In
photolytic biological systems, microorganisms such as green microalgae or
cyanobacteria use sunlight to split water into oxygen and hydrogen.
o
The
challenge of this process results from being able to capture the hydrogen ions
and combine them to be released as hydrogen gas. Simultaneously being able to
safely extract the oxygen without creating explosive mixtures.
o
Another
challenge is finding a way to increase the rates of hydrogen production to make
it commercially viable.
o
The
process also presents a low Solar-to-Hydrogen (STH) efficiency.
o
Researchers
are looking at ways of changing the normal biological pathways of bacteria
involved in the process to increase the rate of hydrogen production.
o
There
are 2 methods for photobiological production of hydrogen.
§ Direct
photobiological
where hydrogen is generated from the activity of hydrogenase without
intermediate molecules. Just picture a population of algae at the bottom of a
pond at night. There is respiration going on but not photosynthesis. There are
mostly anaerobic conditions. When light first hit, water splitting starts. A
few minutes follow before light activates the enzymes for the carbon fixing
reaction. The electrons produced by the water splitting would destroy the
organisms if the excess energy were not dissipated. A second reaction combines
the electrons and protons to form hydrogen molecules, thus getting rid of the
excess energy. This short period of time which only lasts for a few minutes is
what scientists need to tap, since once enough oxygen builds up, the algae
switches to carbon fixation to provide the food source.
§ Indirect
photobiological
where hydrogen is generated after the storage of carbohydrates or glycogen [4]
2. Photoelectrochemical: Semiconductors are used to convert
energy directly into chemical energy in the form of hydrogen. The
semiconductors used are like those used in photovoltaic electricity generation
but immersed in a water-based electrolyte. Sunlight energizes the water
splitting process [5]
o
The
process offers the potential for high conversion at low operating temperatures
o
Compared
to an electrolysis system (65% eff) operated from a 12% PV system, the
electrolysis results in a solar-to-hydrogen efficiency of 7.8%; while a direct photo
electrolysis system (operating at an equivalent 1.45 V) can have a
solar-to-hydrogen efficiency of 10.2% [6]
o
Furthermore,
this system eliminates the need for costly electrolytic cells.
3. Photovoltaic
electrolysis: A
study at the University of Illinois at Urbana [7] demonstrated a system where two
polymer electrolyte membrane electrolysers in series with one InGaP/GaAs/GalnNAsSb
triple junction solar cell produced a large enough voltage to drive electrolysis.
The Solar to Hydrogen (STH) efficiencies achieved reached between 28% and 32%.
The STH efficiency depends on Jsc, the short -circuit photocurrent
density, nf, the Faradaic efficiency for hydrogen evolution and P,
the incident illumination power density; all measured under standard solar
illumination power density [8]. A metallic coating then brings out the
conversion of water to hydrogen. On the semiconductor surface, water is
oxidized to oxygen by the hole (h+} A
solar cell is a semiconductor device which can be represented as a PN junction
diode which operates by the Photovoltaic Effect [9]. The production of
multi-layer solar cells presents challenges since lattice matching of layers is
essential for output efficiency. It is a case for ease of growth of layers. To
fabricate a cell with a larger number of junctions, lattice mismatching becomes
an issue for the free flow of electrons and “holes”. The use of 3J cells makes
fabrication simpler and leads to acceptable efficiencies.
4. Photocatalysis: A Photocatalysis process is the
result of a chemical reaction that is initiated by photons. Several
semiconductors have been reported to catalyze the decomposition of water. In
the process, a semiconductor absorbs the solar radiation (light) and excites
electrons from the valence band of the semiconductor moving them into the
conduction band [10]. Recently, Zn-dopped K2La2Ti3O10
has been found to have high photocatalytic activity for hydrogen production
under exposure to visible light. Another photocatalytic material using visible
light is Cr3+-dopped Bi4Ti3O12.
Hydrogen evolution was measured at 8 L/m2 using a more complex sulfur compound.
5. Thermochemical: Decomposition into hydrogen is
carried out through the enabling of cyclic chemical reactions. There is a
substantial number of redox reactions that can be used in a cyclic manner to
product hydrogen. Only a handful of them have been considered due to practical
reasons. I general and simplified terms, there is a two-step process for
decomposition that can be represented as:
XO à X + 1/2O2
X + H2O à XO + H2
Therefore,
in theory the species XO can be reused and recycled through the process over
and over. Incomplete reactions and side reactions bug the system and require
the continuous addition of fresh material, as well the elimination of
impurities through auxiliary systems resulting in overall efficiencies reduced
drastically. A thermochemical cycle using CuCl has been proposed and studied at
UOIT in Oshawa ON have been performed. The cycle requires temperatures in the
range of 550 oC. For this reason, it was considered a good candidate to tie
into nuclear plants for the efficient use of wasted heat. [11]. The
copper-chlorine cycle became increasingly attractive due to the temperature
requirements being in range with wasted heat from some industries and nuclear
energy operations. The same temperatures can be obtained from solar
concentrators using a field of mirror heliostats [12]. The cycle was initially
designed without electrolysis, but eventually evolved in the need for inclusion
of electrolysis due to increased complexity and side reactions. The overall
cell reaction is given by:
2CuCl(aq)+2HCl(aq)→2CuCl2(aq)+H2(g)
The
efficiency of the electrolytic process depends as expected on electrode
materials, membrane performance and electrolytic solution [13]
Conclusions:
Clean
hydrogen production is arguably expensive and perhaps inefficient as some debates
insist with powerful arguments based on the status of things. We have grown
used to fulfill our thirst for convenient energy from sources that are cheap,
practical, and for which we have learned and established the infrastructure for
the last 100 years or more. Petroleum derivates, gasolines, natural gas have
become so cheap that every other possibility to generate, store or transport
energy is quickly seen as expensive, unpractical. The hydrogen economy won’t
come cheap for our pocket, but highly profitable for the health of the planet.
It will force us to change our unhealthy, gobbling habits of energy
consumption.
Clearly if we continue to measure other
alternatives against the existing status quo, we will probably stay there. The
real cost and practicality of alternatives such as hydrogen as energy carrier
needs be measured in terms of the cost-benefit equation. If the alternative
energy carrier is generated without generating pollution, without destroying
our environment and jeopardizing life in the planet, then the dollar per kW
figure is meaningless and losses weight.
References
1.
Office of Energy Efficiency and Renewable
Energy; Hydrogen Production: Natural Gas Reforming; https://www.energy.gov/eere/fuelcells/hydrogen-production-natural-gas-reforming
; Aug 10, 2021
2.
Maximiliam B.G, Claudio C., John A. S. and John
W.Solar Thermochemical Hydrogen (STCH) processes, Electrochemical Society
Interface 27 53, 2018
3.
Bringing Thermochemical Water Splitting to the
Next Level, DLR Institute of Solar Research, HEST-HY
4.
Roshan Sharma et al, Hydrogen production using
photobiological methods; DOI: 10.1016/B978-1-78242-361-4.00010-8, July 2015
5.
Hydrogen Production: Photoelectrochemical Water
Splitting; Office of Energy Efficiency and Renewable Energy; Hydrogen and Fuel
Cell Technologies Office; Washington DC
6.
D Brent MacQueen, Photochemical Water Splitting
GCEP Hydrogen Workshop, April 2019, Stanford University, SRI international,
Menlo Park CA.
7.
Jieyang Jia et al, Solar Water Splitting by
Photovoltaic-Electrolysis with a solar-to-hydrogen efficiency over 30%, Nature
Communications, Oct 31 2016. DOI 10.1038/ncomms13237
8.
On the Solar to Hydrogen Conversion Efficiency
of Photoelectrodes for Water Splitting, The Journal of Physical Chemistry
Letters, pubs.acs.org/JPCL,
9.
Sanat Pandey, GE/GaAS/InGaP Triple Junction
Solar Cells fro Space Exploration, Department of electrical and Computer
Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA,
April 2019
10.
K. S. V Santhanan et al, Introduction to
Hydrogen Technology, 2018 John Wiley & Sons, www.wiley.com/go/santhanam/hydrogentech_2e
11.
Gabriel D Marin. Kinetics and Transport
Phenomena in the Chemical Decomposition of
Copper Oxychloride in the Thermochemical Cu-Cl Cycle, Faculty of Engineering
and Applied Sciences, UOIT, April 2012
12.
Thermochemical Water Splitting, Hydrogen and
Fuel Cells Technology Office, https://www.energy.gov/eere/fuelcells/hydrogen-production-thermochemical-water-splitting
, Sept 2, 2021
13.
Copper Chlorine Cycle, https://www.sciencedirect.com/topics/engineering/cu-cl-cycle,
Sept 2, 2021.
