EES Blog

Chemists and chemical engineers from Cal-Tech, Stanford, and the US Department of Energy collaborated to investigate four methods of industrial-scale hydrogen gas generation directly from sunlight, via photocatalysis.

The lowest-tech solution, giant ‘baggies’ containing water and a photocatalytic slurry that co-generate hydrogen and oxygen, is the least expensive option, but in terms of land use is surprisingly as efficient as higher-tech panel-based options. Though this is a technical report, anyone interested in renewable energy and the hydrogen economy on a larger scale would find this interesting.

Hydrogen is the single most promising fuel for the future economy. Although >95% of it is presently made by steam reforming, it can be generated independently from fossil fuels. Thermal reforming of biofuels, renewably powered electrolysis, and photolytic generation together represent the possibilities for renewable H₂ generation, and this paper investigates the viability of the last, modelling the scale-up of present technologies for H₂ generation to ten tonnes per day. The US Department of Energy (DoE) guidelines set $2-4/kg H₂ as a reasonable threshold for viability; based on an ideal location for gathering solar energy, the authors found that two of the four designs viably meet this goal.

Photocatalysis couples the hydrogen evolution reaction (HER) to the oxygen evolutio
lving the proposed overall efficiency. The first design predominates, and is proposed to produce H₂ at $1.60/kg.n reaction (ORR). Two designs are based on slurries of photocatalytically active particles in water-filled HDPE bags. The single-bed system evolves H₂ and O₂ simultaneously, posing an explosive risk and requiring separation, but is simple and cost-effective. The dual-bed system evolves the two gases separately, but requires both the redox reaction of an efficiently redox-cycling ‘mediator’ (e.g. Fe²⁺/Fe³⁺) to be coupled to the OER and HER reactions and porous bridges between half-cells to transport the mediator between the cells, significantly complicating the design and halving the efficiency.

The other two designs are based on panels composed of two photocatalytically active layers between a transparent anode exposed to the sun where the HER occurs and a metal cathode where the OER occurs. The third design involves flat panels. The fourth design uses cylindrical parabolas for a 10:1 light concentration to increase efficiency. In both designs, the two reactions are inherently separated, and although panels are more efficient for the area collecting sunlight, panel spacing makes their footprint larger. Of these, the parabolic design predominates, producing at $3.20/kg.

Thus, both the giant baggie and concentrator panel options are economically viable – the baggies are cheaper, but the panels have fewer unknowns in terms of their safety, lifetime, and mass production. Compare these efficiencies with those of electrolyzer apparatuses paired to solar electrical generation, a further study the authors recommend, and we’ll have a good picture of the future of renewable fuel generation.

Read the article in EES:

Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry
Blaise A. Pinaud, Jesse D. Benck, Linsey C. Seitz, Arnold J. Forman, Zhebo Chen, Todd G. Deutsch, Brian D. James, Kevin N. Baum, George N. Baum, Shane Ardo, Heli Wang, Eric Miller and Thomas F. Jaramillo
DOI: 10.1039/C3EE40831K, Analysis

By Benjamin Britton

While the idea of micro/ nanostructured electrode materials has been very effective for developing high-performance lithium-ion batteries, integrated 3D electrodes with hierarchical arrays are the boutique avenues going forward. A recent Energy and Environmental Science paper by Yu et al presents morphology-controlled tunable CoxMn3-xO4 structures that exhibit interesting electrochemical performance.

The strategy of growing electroactive nanostructures on conductive substrates to form integrated 3D electrodes is interesting not only from a device-design standpoint but also for efficient electron transport, as noted by the authors. The synthesis strategy used is robust and well controlled. I find their approach and implementation of direct growth of free-standing and tailored CoxMn3-xO4 structures on current-collecting substrates to be an important and logical step towards integrated electrodes for high performance energy storage devices.

Electrode materials for lithium-ion batteries are the poster child for structure-performance relationships. The interplay between electrochemical performance and morphology of the assembled structure is intriguing and for me makes a strong case for studying these materials in-operation. How does each of the components alter during charging-recharging, what is their role and could the entire framework look like a symphony?

Intrigued by this symphony? Read the paper!

Controlled synthesis of hierarchical CoxMn3−xO4 array micro-/nanostructures with tunable morphology and composition as integrated electrodes for lithium-ion batteries
Le Yu, Lei Zhang, Hao Bin Wu, Genqiang Zhang and Xiong Wen (David) Lou
DOI: 10.1039/C3EE41181H

By Prineha Narang

In a recent paper in Energy & Environmental Science, UCLA Parsons Foundation Professor James C. Liao and his lab report the metabolic engineering of cyanobacteria to efficiently produce photosynthetic n-butanol.

The production of sustainable energy and fuels is one of the great challenges facing chemistry today.  There are many approaches to this immense task, including the engineering of photosynthetic organisms to directly produce chemical fuels.

Ethanol is great for many reasons, but n-butanol is a better target for renewable fuel systems, as it can be blended seamlessly into the current fossil fuel infrastructure.  Unfortunately for synthetic biologists, the microbe that best produces n-butanol is a strict anaerobe, while the cyanobacterial photosynthetic machinery is responsible for the better part of the molecular oxygen in Earth’s atmosphere (For the record, this blogger is in favor of cyanobacterial oxygen evolution).

The authors of this paper find a clever way around this fundamental incompatability by using a different, air-tolerant enzyme for a key step in the biosynthetic pathway.  A propionaldehyde oxidation catalyst (PduP) demonstrated catalytic reduction of butyryl-CoA to n-butyraldehyde, which could subsequently be converted to n-butanol.  This is just one of 7 enzymes from 7 different organisms functioning in concert to produce the photosynthetic butanol.  Much of this process has been previously been reported by the Liao group.

The challenges in stitching together these genomes are enormous.  After all, no enzyme works in a vacuum (no offense, computationalists), and getting the right cocktail of expressed enzymes and complementary cofactors requires well-engineered organisms.   Even the best systems have yields discussed in milligrams per liter per day.  And even then, the butanol produced is toxic to the cells that make it.

Using synthetic biology to properly engineer these organisms will render them hardly recognizable.  The challenges are daunting, but the possibilities seem limitless.  By combining enzymes from many different organisms, each evolved over billions of years for efficient catalysis, or even human-designed enzymes tailored to exotic new schemes, metabolic engineers could produce stripped-down microbial factories to fuel our transportation and synthesize fine chemicals.  Perhaps, given the diversity of life and biochemistry on Earth, microbial engineering could one day be used to bioterraform the solar system for human colonization!  Well, I can dream, right?

Read Liao and co-workers’ article in EES:

Oxygen-tolerant coenzyme A-acylating aldehyde dehydrogenase facilitates efficient photosynthetic n-butanol biosynthesis in cyanobacteria
Ethan I. Lan, Soo Y. Ro and James C. Liao
DOI: 10.1039/C3EE41405A

By Michael Doud

Alexandre Ponrouch and coworkers made significant progress in the optimization of the electrolyte system for sodium-ion batteries. They demonstrate a working sodium-ion battery with an energy density comparable to that of state-of-the-art lithium-ion batteries.

Sodium is both cheaper and more abundant than lithium, and thus sodium-ion batteries are an appealing alternative to lithium-ion batteries for a broad range of energy storage applications. Previous work on Na-ion batteries has demonstrated that hard carbon (HC) electrodes are well suited for the anode of a battery that uses an electrolyte based on a mixture of ethylene carbonate (EC) and propylene carbonate (PC).  However, the addition of a third co-solvent can decrease electrolyte viscosity and thus increase its ionic conductivity.  Dimethyl carbonate (DMC) was found to be a viable third co-solvent, combining decreased viscosity, increased ionic conductivity and a stable solid electrolyte interphase (SEI) layer (as demonstrated by XPS).

A full battery, utilizing a hard carbon anode, a Na₃V₂(PO₄)₂F₃ (NVPF) cathode, and an EC/PC/DMC electrolyte was assembled. The battery had an operation voltage of 3.65 V, 110 mA h g^-1 specific capacity for the cathode and 300 mA h g^-1 specific capacity for the anode.  This yields an overall specific energy of 78 Wh kg^-1, a figure that compares favorably with that of current Li-ion batteries.

For more information on this exciting advancement in Na-ion batteries, read the full article here:

Towards high energy density sodium ion batteries through electrolyte optimization
Alexandre Ponrouch, Remi Dedryvere, Damien Monti, Atif E. Demet, Jean Marcel Ateba Mba, Laurence Croguennec, Christian Masquelier, Patrik Johansson and M. Rosa Palacin
DOI: 10.1039/c3ee41379a

by Heather Audesirk

Researchers in Atlanta, GA have developed a device that produces hydrogen gas from waste heat, solar, and/or mechanical energies.

Recent research conducted at Georgia Institute of Technology by Yang and coworkers have developed a new hybrid energy harvesting device to aid in the effort of hydrogen fuel production. Currently, hydrogen gas for use in industry is predominantly generated by steam reformation of methane or natural gas. This method produces greenhouse gas, requires high temperatures, and utilizes non-renewable resources.

Electrolytic production of hydrogen from water is one popular contender, due to its simplicity and abundance of reactants, however it is currently a more expensive process than through steam reformation. By developing a hybrid device that can take advantage of multiple modes of otherwise wasted energy, an “ambient” fuel generator can be realized.

The novelty in the present work is the combination of solar, thermal, and mechanical energies in the electrolysis of water. By matching a Si-pyramid solar cell with a Bi₂Te₃ based thermoelectric cell, along with a polyamide-perfluoroalkoxy based triboelectric nanogenerator, simultaneous energy harvesting can occur under a variety of environmental conditions. The group has reported H₂ gas production at a rate of 4 x10^-4 mL/s under hybrid generation mode. The technology can also be made to charge a Li-ion battery for on-demand hydrogen production, to compensate for environmental fluctuations.

With optimization, technology such as this can one day exist as a remote platform on the open ocean, passively generating hydrogen gas to be either pipelined to land, or used immediately for fuel cell powered ships.

by David Novitski

Interested? Read the full communication in Energy and Environmental Science here:

A hybrid energy cell for self-powered water splitting
Ya Yang, Hulin Zhang, Zong-Hong Lin, Yan Liu, Jun Chen, Ziyin Lin, Yu Sheng Zhou, Ching Ping Wonga and Zhong Lin Wang
Energy Environ. Sci., 2013, Advance Article
DOI: 10.1039/C3EE41485J

In their recent perspective article in Energy and Environmental Science, Professor Doron Aurbach and co-workers highlight key developments in magnesium-ion battery technology.

Rechargeable batteries are ubiquitous in portable electronics, and are expected to play an important role in electric vehicles and grid storage in the future. While lithium-ion technology is the current state of the art, concerns remain about the supplies of lithium on Earth. As such, alternative systems such as magnesium-ion batteries are being developed. Magnesium is the fifth most abundant element in the Earth’s crust and its bivalency means that it can in principle store more energy per unit volume than lithium metal.

A key requirement in magnesium-ion battery technology is reversible deposition of magnesium on the magnesium metal anode. However, most organic solvents and simple magnesium salts react with magnesium to form a so-called passivation layer on the anode, precluding reversible magnesium deposition and further charge-discharge cycles. Vital progress was made with the discovery of electrolytes that were stable in the presence of the magnesium anode. In particular, reaction of AlCl_3-nR_n lewis acids with R₂Mg lewis bases in ether solvents yielded electrolyte solutions which were stable up to voltages of 2.1 V vs. magnesium metal. The combination of these so-called dicholoro-complex (DCC) electrolytes with Chevrel phase intercalation cathodes (e.g. Mo₆S₈) and magnesium anodes yielded the first working prototypes for reversible magnesium-ion batteries.

While these first prototypes constituted a great breakthrough, relatively low energy densities and lacking performance at high rates precluded their commercialisation. Ongoing work focuses on finding new electrolytes with wider electrochemical stability windows, as well as new cathode and anode materials. One interesting possibility is to replace the magnesium anode with metal alloys of bismuth and antinomy, sidestepping the problems associated with the reactivity of magnesium and allowing more conventional electrolytes to be used.

Read more in the perspective article in Energy and Environmental Science:

Mg rechargeable batteries: an on-going challenge
Hyun Deog Yoo, Ivgeni Shterenberg, Yosef Gofer, Gregory Gershinsky, Nir Pour and Doron Aurbach
Energy Environ. Sci., 2013, Advance Article
DOI: 10.1039/C3EE40871J

By Alexander Forse

Take a look at our selection of exciting articles that have been recently published online:

Post-combustion carbon dioxide capture using electrochemically mediated amine regeneration
Michael C. Stern, Fritz Simeon, Howard Herzog and T. Alan Hatton
DOI: 10.1039/C3EE41165F

Improving the long-term stability of PBDTTPD polymer solar cells through material purification aimed at removing organic impurities
William R. Mateker, Jessica D. Douglas, Clément Cabanetos, I. T. Sachs-Quintana, Jonathan A. Bartelt, Eric T. Hoke, Abdulrahman El Labban, Pierre M. Beaujuge, Jean M. J. Fréchet and Michael D. McGehee
DOI: 10.1039/C3EE41328D

New materials based on a layered sodium titanate for dual electrochemical Na and Li intercalation systems
Mona Shirpour, Jordi Cabana and Marca Doeff
DOI: 10.1039/C3EE41037D

Towards high energy density sodium ion batteries through electrolyte optimization
Alexandre Ponrouch, Rémi Dedryvère, Damien Monti, Atif E. Demet, Jean Marcel Ateba Mba, Laurence Croguennec, Christian Masquelier, Patrik Johansson and M. Rosa Palacín
DOI: 10.1039/C3EE41379A

Charge transport in lithium peroxide: relevance for rechargeable metal–air batteries
Maxwell D. Radin and Donald J. Siegel
DOI: 10.1039/C3EE41632A