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Oct. 24, 2018

Volvo Cars has acquired a stake in electric car charging company FreeWire Technologies via the Volvo Cars Tech Fund, deepening the company’s commitment to a fully electric future. (See Industry Announcement Below)

While Volvo Cars’s electrification strategy does not envision direct ownership of charging or service stations, the investment in FreeWire reinforces its overall commitment to supporting a widespread transition to electric mobility together with other partners.

FWire mobisLeafsFreeWire is a San Francisco-based company that has been a pioneer in flexible fast-charging technology for electric cars. It specialises in both stationary and mobile fast charging technology, allowing electric car charging to be deployed quickly and widely. (Check Out FWT’s website – Featuring ‘MOBI’)  FreeWire Technologies – Electrification Beyond the Grid

Installing traditional fixed fast-charging stations is usually a cost- and labour intensive process that requires a lot of electrical upgrades to support the connection between charging stations and the main electrical grid. FreeWire’s charging stations remove that complication and use low-voltage power, allowing operators to simply use existing power outlets. This means drivers can enjoy all the benefits of fast charging without operators needing to go through the hassle of establishing a high-voltage connection to the grid.

Volvo Cars has one of the auto industry’s most ambitious electrification strategies. Every new Volvo car launched from 2019 will be electrified, and by 2025 the company aims for fully electric cars to make up 50 per cent of its overall global sales.

“Volvo Cars’ future is electric, as reflected by our industry-leading commitment to electrify our entire product range,” said Zaki Fasihuddin, CEO of the Volvo Cars Tech Fund. “To support wider consumer adoption of electric cars, society needs to make charging an electric car as simple as filling up your tank. Our investment in FreeWire is a firm endorsement of the company’s ambitions in this area.”

“FreeWire’s fast charging technology holds great promise to simplify the experience for customers of electrified Volvos,” said Atif Rafiq, chief digital officer at Volvo Cars. “With this move, we aim to make the future of sustainable, electric cars more practical and convenient.”

“We’re thrilled to partner with Volvo Cars to develop new markets and business models around our EV fast charging and ultra-fast charging technology,” said Arcady Sosinov, CEO of FreeWire. “Having a car maker with both the legacy and future vision of Volvo is going to give us access to technology, testing, and new strategies that will really accelerate the growth of the company.”

The Volvo Cars Tech Fund was launched earlier this year and aims to invest in high-potential technology start-ups around the globe. It focuses its investments on strategic technology trends transforming the auto industry, such as artificial intelligence, electrification, autonomous drive and digital mobility services.

Earlier this year, the Volvo Cars Tech Fund announced its first investment in Luminar Technologies, a leading start-up in the development of advanced sensor technology for use in autonomous vehicles, with whom Volvo Cars collaborates on the development and testing of its LiDAR sensing technology on Volvo cars.

Companies benefit from participation by the Volvo Cars Tech Fund as they may gain the ability to validate technologies, accelerate market introduction, as well as potentially access Volvo Cars’ global network and unique position in the Chinese car market.

 

 Volvo Car Group in 2017

For the 2017 financial year, Volvo Car Group recorded an operating profit of 14,061 MSEK (11,014 MSEK in 2016). Revenue over the period amounted to 210,912 MSEK (180,902 MSEK). For the full year 2017, global sales reached a record 571,577 cars, an increase of 7.0 per cent versus 2016. The results underline the comprehensive transformation of Volvo Cars’ finances and operations in recent years, positioning the company for its next growth phase.

About Volvo Car Group

Volvo has been in operation since 1927. Today, Volvo Cars is one of the most well-known and respected car brands in the world with sales of 571,577 cars in 2017 in about 100 countries. Volvo Cars has been under the ownership of the Zhejiang Geely Holding (Geely Holding) of China since 2010. It formed part of the Swedish Volvo Group until 1999, when the company was bought by Ford Motor Company of the US. In 2010, Volvo Cars was acquired by Geely Holding.

In 2017, Volvo Cars employed on average approximately 38,000 (30,400) full-time employees. Volvo Cars head office, product development, marketing and administration functions are mainly located in Gothenburg, Sweden. Volvo Cars head office for China is located in Shanghai. The company’s main car production plants are located in Gothenburg (Sweden), Ghent (Belgium), Chengdu, Daqing (China) and Charleston (USA), while engines are manufactured in Skövde (Sweden) and Zhangjiakou (China) and body components in Olofström (Sweden).

About Volvo Cars Tech Fund Volvo download

Volvo Cars Tech Fund is a new venture fund under the Volvo Cars umbrella, and is dedicated to advancing Volvo’s mission of ecology, safety, and technology across its vehicles. The fund was established in 2018, and is based out of Volvo Cars R&D Tech Center in Mountain View, California. Read more here.

 

 

Industry Announcement

Volvo is the latest business to take an interest in FreeWire.  Swedish luxury vehicles company Volvo Cars has bought a stake in FreeWire Technologies, a California-based electric car charging business. 

The acquisition has been made through the Volvo Cars Tech Fund, which was launched earlier this year. In an announcement Wednesday, Volvo described FreeWire as a “pioneer in flexible fast charging technology for electric cars.”Volvo becomes the latest major business to take an interest in FreeWire. In January 2018, BP Ventures announced it was investing $5 million in the business. 

From 2019, every new car that Volvo launches is set to be electrified. The business wants fully-electric cars to account for 50 percent of overall global sales by the year 2025.

“To support wider consumer adoption of electric cars, society needs to make charging an electric car as simple as filling up your tank,” Zaki Fasihuddin, the Volvo Cars Tech Fund CEO, said in a statement. “Our investment in FreeWire is a firm endorsement of the company’s ambitions in this area.”

In 2017, there were more than 3 million electric and plug-in hybrid cars on the planet’s roads, according to the International Energy Agency’s (IEA) Global Electric Vehicles Outlook. This represents an increase of 54 percent compared to 2016.

Almost 580,000 electric cars were sold in China last year, according to the IEA, while around 280,000 were sold in the U.S.

In terms of charging infrastructure, the IEA says that, globally, there were an estimated 3 million private chargers at homes and workplaces in 2017. The number of “publicly accessible” chargers amounted to roughly 430,000.

Scientists are exploring graphene’s ability to ‘ripple’ into the third dimension.

Image: REUTERS/Nick Carey

Graphene is a modern marvel. It is comprised of a single, two-dimensional layer of carbon, yet is 200 times stronger than steel and more conductive than any other material, according to the University of Manchester, where it was first isolated in 2004.

Graphene also has multiple potential uses, including in biomedical applications such as targeted drug delivery, and for improving the lifespan of smartphone batteries.

Now, a team of researchers at the University of Arkansas has found evidence to suggest graphene could also be used to provide an unlimited supply of clean energy.

The team says its research is based on graphene’s ability to “ripple” into the third dimension, similar to waves moving across the surface of the ocean. This motion, the researchers say, can be harvested into energy.

To study the movement of graphene, lead researcher Paul Thibado and his team laid sheets of the material across a copper grid that acted as a scaffold, which allowed the graphene to move freely.

Thibado says graphene could power biomedical devices such as pacemakers.

Image: Russell Cothren

The researchers used a scanning tunnelling microscope (STM) to observe the movements, finding that narrowing the focus to study individual ripples drew clearer results.

In analysing the data, Thibado observed both small, random fluctuations, known as Brownian motion, and larger, coordinated movements.

A scanning tunnelling microscope.

Image: University of Arkansas

As the atoms on a sheet of graphene vibrate in response to the ambient temperature, these movements invert their curvature, which creates energy, the researchers say.

Harvesting energy

“This is the key to using the motion of 2D materials as a source of harvestable energy,” Thibado says.

“Unlike atoms in a liquid, which move in random directions, atoms connected in a sheet of graphene move together. This means their energy can be collected using existing nanotechnology.”

The pieces of graphene in Thibado’s laboratory measure about 10 microns across (more than 20,000 could fit on the head of a pin). Each fluctuation exhibited by an individual ripple measures only 10 nanometres by 10 nanometres, and could produce 10 picowatts of power, the researchers say.

As a result, each micro-sized membrane has the potential to produce enough energy to power a wristwatch, and would never wear out or need charging.

Sheet of graphene as seen through Thibado’s STM

Image: University of Arkansas

Thibado has created a device, called the Vibration Energy Harvester, that he claims is capable of turning this harvested energy into electricity, as the below video illustrates.

This self-charging power source also has the potential to convert everyday objects into smart devices, as well as powering more sophisticated biomedical devices such as pacemakers, hearing aids and wearable sensors.

Thibado says: “Self-powering enables smart bio-implants, which would profoundly impact society.”

Have you read?

Graphene could soon make your computer 1000 times faster

Can graphene make the world’s water clean?

Energy Storage is fast finding favor among savvy investors as it looks likely to revolutionize the ‘Renewable Energy Landscape.’

University of Texas at Austin. This is the world’s thinnest wearable Health Monitor, designed and developed by the researchers at the University of Texas at Austin, in the form of a “Graphene-Ink Tattoo”.

Most health monitors in use today are bulky and tend to restrict patients movements. This graphene tattoo will eliminate these restrictions. It picks up electric signal given off by the body and transmits it to a smartphone app.

Read More: An Ultra-Thin – Wearable Health Monitor made possible by a ‘Graphene Ink Tattoo’

Rice University Expands LIG (laser induced graphene) Research and Applications: Supercapacitor, an Electrocatalyst for Fuel Cells, RFID’s and Biological Sensors

Rice University scientists who introduced laser-induced graphene (LIG) have enhanced their technique to produce what may become a new class of edible electronics.

The Rice lab of chemist James Tour, which once turned Girl Scout cookies into graphene, is investigating ways to write graphene patterns onto food and other materials to quickly embed conductive identification tags and sensors into the products themselves.

“This is not ink,” Tour said. “This is taking the material itself and converting it into graphene.” Read More: Rice University Expands LIG (laser induced graphene) Research

 

Small Drones Could Be Better for Climate Than Delivery Trucks, Says Study

Automated, unmanned drones are poised to revolutionize the package delivery industry, with a number of companies already testing drone-based delivery methods.

A new study in Nature Communications looks at the climate impact of a shift from truck-based to drone-based package delivery. It finds that while small drones carrying packages weighing less than 0.5 kg would reduce greenhouse gas emissions compared to diesel or electric trucks anywhere in the U.S., the same is not true for larger drones carrying heavier packages.

Read More: Small Drones Could Be Better for Climate Than Delivery Trucks, Says Study

Watch Our New Tenka Energy Video:

Tenka Energy, Inc. Building Ultra-Thin Energy Dense SuperCaps and NexGen Nano-Enabled Pouch & Cylindrical Batteries – Energy Storage Made Small and POWERFUL! YouTube Video


 

 

 

 

 

 

 

 

 

 

 

 

Promising New Research for High Performance Lithium Batteries – Engineering 2D Nanofluidic Channels 

Despite being a promising electrode material, bulk cobalt oxide (Co3O4) exhibits poor lithium ion storage properties. Nanostructuring, e.g. making Co3O4 into ultrathin nanosheets, shows improved performance, however, Co3O4-based nanomaterials still lack long-term stability and high rate capability due to sluggish ion transport and structure degradation. Read More …

 

MIT: Device makes power conversion more efficient New design could dramatically cut energy waste in electric vehicles, data centers, and the power grid

Power electronics, which do things like modify voltages or convert between direct and alternating current, are everywhere. They’re in the power bricks we use to charge our portable devices; they’re in the battery packs of electric cars; and they’re in the power grid itself, where they mediate between high-voltage transmission lines and the lower voltages of household electrical sockets. Read More …

 

Tesla Semi and Roadster could be relying on a “battery breakthrough”

Elon Musk and Tesla have made some bold claims for the new Tesla Semi and Roadster. Those who understand batteries have been scratching their heads trying to figure out how the company can deliver the specs it’s promising – and concluding that the only possible way is some as-yet-unannounced advancement in battery technology. Read More …

Watch Our Video on New Energy Storage Technology: Supercapacitors and Batteries

 

 

 

 

Battery stores energy in nontoxic, noncorrosive aqueous solutions

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a new flow battery that stores energy in organic molecules dissolved in neutral pH water.

This new chemistry allows for a non-toxic, non-corrosive battery with an exceptionally long lifetime and offers the potential to significantly decrease the costs of production.

The research, published in ACS Energy Letters, was led by Michael Aziz, the Gene and Tracy Sykes Professor of Materials and Energy Technologies and Roy Gordon, the Thomas Dudley Cabot Professor of Chemistry and Professor of Materials Science.

Flow batteries store energy in liquid solutions in external tanks — the bigger the tanks, the more energy they store.

 

Read The Full Article Long-lasting flow battery could run for more than a decade with minimum upkeep – Harvard Paulson School of Engineering

A new company Tenka Energy, LLC ™ has been formed to exploit and commercialize the Next Generation Super-Capacitors and Batteries. The opportunity is based on Nanoporous-Nickel Flexible Thin-Form, Scalable Super Capacitors and Si-Nanowire Battery Technologies with Exclusive IP Licensing Rights from Rice University.

… Problem 1: Current capacitors and batteries being supplied to the relevant markets lack the sustainable power density, discharge and recharge cycle and warranty life. Combined with a weight/ size challenge and the lack of a ‘flexible form factor’, existing solutions lack the ability to scale and manufacture at Low Cost, to satisfy the identified industries’ need for solutions that provide commercial viability & performance.

Solution: For Marine & Drone Batteries – Medical Devices

  • High Energy Density = 2X More Time on the Water; 2X Flight Time for Drones
  • Simplified Manufacturing = Lower Costs
  • Simple Electrode Architecture = Flex Form Factor (10X Energy Density Factor)
  • Flexible Form = Dramatically Less Weight and Better Weight Distribution
  • Easy to Scale Technology

To Read the Full Article Click on the Link Below:

  

 

Energy Storage: New Si-Nanowire Battery for Applications in Marine and Drone Battery Markets: w/Video

            

 

Image: UCF

Technology I: University of Central Florida

Leaving your phone plugged in for hours could become a thing of the past, thanks to a new type of battery technology that charges in seconds and lasts for over a week.

Watch the Video

While it probably won’t be commercially available for a years, the researchers said it has the potential to be used in phones, wearables and electric vehicles.

“If they were to replace the batteries with these supercapacitors, you could charge your mobile phone in a few seconds and you wouldn’t need to charge it again for over a week,” said Nitin Choudhary, a UCF postdoctoral associate, who conducted much of the research, published in the academic journal ACS Nano.

How does it work?

Unlike conventional batteries, supercapacitors store electricity statically on their surface which means they can charge and deliver energy rapidly. But supercapacitors have a major shortcoming: they need large surface areas in order to hold lots of energy.

To overcome the problem, the researchers developed supercapacitors built with millions of nano-wires and shells made from two-dimensional materials only a few atoms thick, which allows for super-fast charging. Their prototype is only about the size of a fingernail.

“For small electronic devices, our materials are surpassing the conventional ones worldwide in terms of energy density, power density and cyclic stability,” Choudhary said.

Cyclic stability refers to how many times a battery can be charged, drained and recharged before it starts to degrade. For lithium-ion batteries, this is typically fewer than 1,500 times.

Supercapacitors with two-dimensional materials can be recharged a few thousand times. But the researchers say their prototype still works like new even after being recharged 30,000 times.

 

wearable-textiles-100616-0414_powdes_ti_f1Those that use the new materials could be used in phones, tablets and other electronic devices, as well as electric vehicles. And because they’re flexible, it could mean a significant development for wearables.

 

 

 

 

 

 

 

 

Technology II: Rice University

391f84fd-6427-4c06-9fb4-3d3c8a433f41A new company has been formed (with exclusive licensing rights) to exploit and commercialize the Next Generation Super-Capacitors and Batteries. The opportunity is based on Nanoporous-Nickel Flexible Thin-form, Scalable Super Capacitors and Si-Nanowire Battery Technologies, developed by Rice University and Dr. James M. Tour, PhD – named “One of the Fifty (50) most influential scientists in the World today” is the inventor, patent holder and early stage developer. tourportrait2015-300

tenka-flex-med-082616-picture1Identified Key Markets and Commercial Applications 

  • Medical Devices and Wearable Electronics
  • Drone/Marine Batteries and Power Banks
  • Powered Smart Cards and Motor Cycle/ EV Batteries
  • Sensors & Power Units for the iOT (Internet of Things) [Flexible Form, Energy Dense]  

 

The Coming Power Needs of the iOTiot-picture1

  • The IoT is populated with billions of tiny devices.
  • They’re smart.
  • They’re cheap.
  • They’re mobile.
  • They need to communicate.
  • Their numbers growing at 20%-30%/Year.

The iOT is Hungry for POWER! All this demands supercapacitors that can pack a lot of affordable power in very small volumes …Ten times more than today’s best supercapacitors can provide.

 

iot-img_0008

 

Highly Scalable – Energy Dense – Flexible Form – Rapid Charge

 Problem 1: Current capacitors and batteries being supplied to the relevant markets lack the sustainable power density, discharge and recharge cycle, warranty life combined with a ‘flexible form factor’ to scale and satisfy the identified industry need for commercial viability & performance.

tenka-smartcard-picture1Solution I: (Minimal Value Product) Tenka is currently providing full, functional Super Capacitor prototypes to an initial customer in the Digital Powered Smart Card industry and has received two (2) phased Contingent Purchase Orders during the First Year Operating Cycle for 120,000 Units and 1,200,000 Units respectively.

Solution II: For Drone/ Marine Batteries – Power Banks & Medical Devices

  • Double the current ‘Time Aloft’ (1 hour+)drone1
  • Reduces operating costs
  • Marine batteries – Less weight, longer life, flex form
  • Provides Fast Recharging,  Extended Life Warranty.
  • Full -battery prototypes being developed

Small batteries will be produced first for Powered Digital Smart Cards (In addition to the MVP Super Caps) solving packaging before scaling up drone battery operations. Technical risks are mainly associated with packaging and scaling.

The Operational Plan is to take full advantage of the gained ‘know how’ (Trade Secrets and Processes) of scaling and packaging solutions developed for the Powered Digital Smart Card and the iOT, to facilitate the roll-out of these additional Application Opportunities. Leveraging gained knowledge from operations is projected to significantly increase margins and profitability. We will begin where the Economies of Scale and Entry Point make sense (cents)!

tenka-mission-082516-picture1

“We are building and Energy Storage Company starting Small & Growing Big!”

Watch the YouTube Video

Fuel cell electric vehicles have a long way to go before they can compete with their battery EV cousins, and energy storage is a key sticking point when the fuel is hydrogen. Hydrogen is light, plentiful, and fabulously energy dense, but energy storage in a personal mobility unit racing down a crowded highway is a different kind of chicken. Safety, cost, and performance are critical sticking points, and a research team at Lawrence Berkeley Laboratory is on to a solution for at least one of those.

hydrogen energy storage with graphene

Energy Storage Challenges For Hydrogen Fuel Cell EVs

The US Energy Department’s 2015 annual report provides a birds-eye view of the array of energy storage solutions that are emerging for hydrogen fuel cells, including advancements in hydrogen tank technology as well as solids-based storage.

Despite the progress, according to the Energy Department, challenges still remain for stationary and portable fuel cells in terms of raising the energy storage density, and there are “significant challenges” for fuel cell EVs. The problem is this:

Hydrogen has the highest energy per mass of any fuel; however, its low ambient temperature density results in a low energy per unit volume, therefore requiring the development of advanced storage methods that have potential for higher energy density.

The Energy Department has set a goal of 2020 for achieving verifiable hydrogen storage systems for light duty fuel cell EVs that meet the driving public’s thirst for range, comfort, refueling convenience, and performance. Here are the targets:

1.8 kWh/kg system (5.5 wt.% hydrogen)

1.3 kWh/L system (0.040 kg hydrogen/L)

$10/kWh ($333/kg stored hydrogen capacity)

Fuel cell EVs are already leaking into the transportation scene, particularly in California, Japan, and the European Union, notably including Wales.

However, the Energy Department is already looking beyond the current state of on-road technology to meet its 2020 goal. According to the agency, the 300-mile range is being met by using compressed gas, high pressure energy storage technology, and the problem is that competing technology on the market today — primarily gasmobiles and hybrids — already exceeds that range.

To compete for consumers on the open market, the agency is pursuing a near-term goal of improving compressed gas storage, primarily by deploying fiber reinforced composites that enable 700 bar pressure.

The long term goal consists of two pathways. One is to improve “cold” compressed gas energy storage technology, and the other is to go a different route altogether and store hydrogen within materials such as sorbents, chemical hydrogen storage materials, and metal hydrides.

The Berkeley Lab Energy Storage Solution

Where were we? Oh right, Berkeley Lab. Berkeley Lab has been tackling the metal hydride pathway.

Metal hydrides are compounds that consist of a transition metal bonded to hydrogen. They are believed to be the most “technologically relevant” class of materials for storing hydrogen, partly due to the broad range of applications.

That’s the theory. The problem is that when it comes to real world performance, metal hydrides are highly sensitive to contamination and they degrade somewhat rapidly unless properly shielded.

The Berkeley Lab energy storage solution consists of a graphene “filter” encasing nanocrystals of magnesium. With the addition of the graphene layer, the magnesium crystals act as a sort of sponge for absorbing hydrogen, providing both safety and compactness without causing performance issues:

The graphene shields the nanocrystals from oxygen and moisture and contaminants, while tiny, natural holes allow the smaller hydrogen molecules to pass through. This filtering process overcomes common problems degrading the performance of metal hydrides for hydrogen storage.

Berkeley Lab has provided this photo to show off how stable the crystals are when exposed to air (for scale, the bottle cap is about the size of a thumbnail):

graphene hydrogen energy storage

At one atom thick (yes, one atom), graphene is known to be an incredibly finicky material to work with. It is extremely difficult to synthesize it without defects, but that’s not a problem for this energy storage solution. The defects are actually desirable in this case. The tiny gaps enable molecules of hydrogen gas to wriggle through, but oxygen, water, and other contaminants are too large to penetrate the shield.

The new energy formula also solves another key challenge for metal hydrides. They tend to take in and dispense hydrogen at a relatively slow pace, but the Berkeley Lab solution has sped up the intake-outflow cycle significantly. That effect is attributed to the nanoscale size of the graphene-shielded crystals, which provide a greater surface area.

Energy Department Gets The Last Word?

We’ve been having a lively debate about fuel cell electric EVs over here at CleanTechnica, so let’s hear from the Berkeley Lab team:

A potential advantage for hydrogen-fuel-cell vehicles, in addition to their reduced environmental impact over standard-fuel vehicles, is the high specific energy of hydrogen, which means that hydrogen fuel cells can potentially take up less weight than other battery systems and fuel sources while yielding more electrical energy.

However, the team also makes it clear that:

More R&D is needed to realize higher-capacity hydrogen storage for long-range vehicle applications that exceed the performance of existing electric-vehicle batteries…

Among other issues, the next step for a sustainable fuel cell EV future is to develop sustainable and renewable sources for hydrogen fuel. Currently the main source of hydrogen is natural gas, which puts fuel cell EVs in the same boat as battery EVs that draw electricity from a coal or natural gas-fired grid.

Nanowires 020316 bf8802f7297fd2bfea985c26d0b9a636_w1440

California is committed to 33 percent energy from renewable resources by 2020. With that deadline fast approaching, researchers across the state are busy exploring options.

Solar energy is attractive but for widespread adoption, it requires transformation into a storable form. This week in ACS Central Science, researchers report that nanowires made from multiple metal oxides could put solar ahead in this race.

One way to harness solar power for broader use is through photoelectrochemical (PEC) water splitting that provides hydrogen for fuel cells. Many materials that can perform the reaction exist, but most of these candidates suffer from issues, ranging from efficiency to stability and cost.

Peidong Yang and colleagues designed a system where nanowires from one of the most commonly used materials (TiO2) acts as a “host” for “guest” nanoparticles from another oxide called BiVO4. BiVO4 is a newly introduced material that is among the best ones for absorbing light and performing the water splitting reaction, but does not carry charge well while TiO2 is stable, cheap and an efficient charge carrier but does not absorb light well.

Together with a unique studded nanowire architecture, the new system works better than either material alone.

The authors state their approach can be used to improve the efficiencies of other photoconversion materials.

Synopsis

We report the use of Ta:TiO2|BiVO4 as a photoanode for use in solar water splitting cells. This host−guest system makes use of the favorable band alignment between the two semiconductors. The nanowire architecture allows for simultaneously high light absorption and carrier collection for efficient solar water oxidation.

nanowires II 020316 oc-2015-004025_0008.gif

Metal oxides that absorb visible light are attractive for use as photoanodes in photoelectrosynthetic cells. However, their performance is often limited by poor charge carrier transport. We show that this problem can be addressed by using separate materials for light absorption and carrier transport. Here, we report a Ta:TiO2|BiVO4 nanowire photoanode, in which BiVO4 acts as a visible light-absorber and Ta:TiO2 acts as a high surface area electron conductor. Electrochemical and spectroscopic measurements provide experimental evidence for the type II band alignment necessary for favorable electron transfer from BiVO4 to TiO2. The host–guest nanowire architecture presented here allows for simultaneously high light absorption and carrier collection efficiency, with an onset of anodic photocurrent near 0.2 V vs RHE, and a photocurrent density of 2.1 mA/cm2 at 1.23 V vs RHE.

Introduction


Harnessing energy from sunlight is a means of meeting the large global energy demand in a cost-effective and environmentally benign manner. However, to provide constant and stable power on demand, it is necessary to convert sunlight into an energy storage medium.(1) An example of such a method is the production of hydrogen by photoelectrochemical (PEC) water splitting. The direct splitting of water can be achieved using a single semiconductor; however, due to the voltage requirement of the water splitting reaction and the associated kinetic overpotentials, only wide-band-gap materials can perform overall water splitting, limiting the efficiency due to insufficient light absorption.(2) To address this issue, a dual-band-gap z-scheme system can be utilized, with a semiconductor photoanode and photocathode to perform the respective oxidation and reduction reactions.(3) This approach allows for the use of lower-band-gap materials that can absorb complementary portions of the solar spectrum and yield higher solar-to-fuel efficiencies.(4, 5) In this integrated system, the charge flux is matched in both light absorbers of the photoelectrochemical cell. Therefore, the overall performance is determined by the limiting component. In most photoelectrosynthetic cells, this limiting component is the semiconductor photoanode.(6)
Metal oxides have been heavily researched as photoanode materials since few conventional light absorber materials are stable at the highly oxidizing conditions required for water oxidation.(7) However, the most commonly studied binary oxide, TiO2, has a band gap that is too large to absorb sunlight efficiently (∼3.0 eV), consequently limiting its achievable photocurrent.(8) While promising work has recently been done on stabilizing conventional light absorbers such as Si,(9) GaAs,(10) and InP,(11) the photovoltage obtained by these materials thus far has been insufficient to match with smaller-band-gap photocathode materials such as Si and InP in a dual absorber photoelectrosynthetic cell.(12, 13) Additionally, these materials have high production and processing costs. Small-band-gap metal oxides that absorb visible light and can be inexpensively synthesized, such as WO3, Fe2O3, and BiVO4, are alternative materials that hold promise to overcome these limitations.(14-16) Among these metal oxides, BiVO4 has emerged as one of the most promising materials due to its relatively small optical band gap of ∼2.5 eV and its negative conduction band edge (∼0 V versus RHE).(17, 18) Under air mass 1.5 global (AM1.5G) solar illumination, the maximum achievable photocurrent for water oxidation using BiVO4 is ∼7 mA/cm2.(16) However, the water oxidation photocurrent obtained in practice for BiVO4 is substantially lower than this value, mainly due to poor carrier transport properties, with electron diffusion lengths shorter than the film thickness necessary to absorb a substantial fraction of light.(17)
One approach for addressing this problem is to use two separate materials for the tasks of light absorption and carrier transport. To maximize performance, a conductive and high surface area support material (“host”) is used, which is coated with a highly dispersed visible light absorber (“guest”). This architecture allows for efficient use of absorbed photons due to the proximity of the semiconductor liquid junction (SCLJ). This strategy has been employed in dye sensitized (DSSC) and quantum dot sensitized solar cells (QDSSC).(19, 20) Using a host–guest scheme can improve the performance of photoabsorbing materials with poor carrier transport but relies upon appropriate band alignment between the host and guest. Namely, the electron affinity of the host should be larger, to favor electron transfer from guest to host without causing a significant loss in open-circuit voltage.(21) Nanowire arrays provide several advantages for use as the host material as they allow high surface area loading of the guest material, enhanced light scattering for improved absorption, and one-dimensional electron transport to the back electrode.(22) Therefore, nanowire arrays have been used as host materials in DSSCs, QDSSCs, and hybrid perovskite solar cells.(23-25) In photoelectrosynthetic cells, host–guest architectures have been utilized for oxide photoanodes such as Fe2O3|TiSi2,(26) Fe2O3|WO3,(27) Fe2O3|SnO2,(28) and Fe2TiO5|TiO2.(29) For BiVO4, it has been studied primarily with WO3|BiVO4,(30-32) ZnO|BiVO4,(33) and anatase TiO2|BiVO4.(34) While attractive for its electronic transport properties, ZnO is unstable in aqueous environments, and WO3 has the disadvantage of having a relatively positive flatband potential (∼0.4 V vs RHE)(14) resulting in potential energy losses for electrons as they are transferred from BiVO4 to WO3, thereby limiting the photovoltage of the combined system. Performance in the low potential region is critical for obtaining high efficiency in photoelectrosynthetic cells when coupled to typical p-type photocathode materials such as Si or InP.(12, 13) TiO2 is stable in a wide range of pH and has a relatively negative flat band potential (∼0.2 V vs RHE)(7) which does not significantly limit the photovoltage obtainable from BiVO4, while still providing a driving force for electron transfer. While TiO2 has intrinsically low mobility, doping TiO2 with donor type defects could increase the carrier concentration and thus the conductivity. Indeed, niobium and tantalum doped TiO2 have recently been investigated as potential transparent conductive oxide (TCO) materials.(35, 36) A host material with high carrier concentration could also ensure low contact resistance with the guest material.(37)
Using a solid state diffusion approach based on atomic layer deposition (ALD), we have previously demonstrated the ability to controllably and uniformly dope TiO2.(38) In this study we demonstrate a host–guest approach using Ta-doped TiO2 (Ta:TiO2) nanowires as a host and BiVO4 as a guest material. This host–guest nanowire architecture allows for simultaneously high light absorption and carrier collection efficiency, with an onset of anodic photocurrent near 0.2 V vs RHE, and a photocurrent of 2.1 mA/cm2 at 1.23 V vs RHE. We show that the synergistic effect of the host–guest structure results in higher performance than either pure TiO2 or BiVO4. We also experimentally demonstrate thermodynamically favorable band alignment between TiO2 and BiVO4 using spectroscopic and electrochemical methods, and study the band edge electronic structure of the TiO2 and BiVO4 using X-ray absorption and emission spectroscopies.

Article adapted from a American Chemical Society news release. To Read the FULL release, please click on the link provided below.

Publication: TiO2/BiVO4 Nanowire Heterostructure Photoanodes Based on Type II Band Alignment. Resasco, J et al. ACS Central Science (3 February, 2016): Click here to view.

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