Sodium Silicide

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The final breakthrough for hydrogen fuel cells?

March 31, 2011
Elizabeth Engler Modic

Hydrogen fuel cells have promised for years to be the solution that moves the United States from an economy that relies on petroleum and toxic batteries to a cleaner, hydrogen economy. But, lingering technical hurdles have prevented fuel cells from being commercially accepted; market viability always seems to be just a few years or one more technical breakthrough away.

However, a new technology is now on the market that is proving that hydrogen fuel cells are not only commercially viable, but even more cost effective than batteries or low-power internal combustion engines (less than 1kW). This material – sodium silicide – produces pure hydrogen gas in real time, as needed by the fuel cell, and at pressures less than those found in a common soda can. By producing hydrogen on demand, sodium silicide eliminates the two most significant challenges faced by low-temperature proton exchange membrane (PEM) fuel cell manufacturers; namely the need to store high-pressure hydrogen gas or to build a costly hydrogen refilling infrastructure.

Sodium silicide hydrogen-generation systems are inherently simple and enable low-cost, reliable fuel cell systems compared to other high-density hydrogen storage alternatives. Is this the breakthrough that PEM fuel cell makers have been waiting for?

Sodium silicide canister (left); General purpose hydrogen generator.
Clean, On Demand
Sodium silicide (NaSi) is a benign powder that is formed by combining sodium metal with silicon powder. Yet, when it comes in contact with water, sodium silicide reacts powerfully and creates pure, uncontaminated hydrogen gas. The reaction is instantaneous, safe, and can be easily controlled – allowing for fuel cells with a fast start/stop capability. Through a new breakthrough stabilization process, the sodium silicon powder reacts controllably with water and can be handled safely in air. The hydrogen yield from this material is impressive, producing greater than 9.8wt% hydrogen. For ultra-high-performance energy-dense applications, sodium silicide hybrids have been developed that demonstrate 15wt% hydrogen characteristics. 

The sodium silicide manufacturing process is straightforward and inexpensive. The primary materials are sodium and silicon, both abundant and renewable materials whose prices are not influenced by the cost of oil. For every 1kg of sodium silicide produced, the raw sodium and silicon materials cost less than $4/kg. The manufacturing process is also clean and creates no waste – every ingredient that goes into the reactor ends up in the final product. So, unlike other hydrogen-generation technologies, sodium silicide does not demand much energy to produce, require any purification steps, or create harmful emissions or by-products.

In fact, fuel cells powered by sodium silicide output only pure hydrogen and water vapor – there are zero greenhouse gases created. Once the sodium silicide in a canister is spent, the fuel cell is left with a non-toxic by-product (sodium silicate, Na2Si2O5), which is fully contained within the canister. Sodium silicate, which is often called water-glass, is a common material that is used as an industrial feedstock for many products, including glass and toothpaste. Disposal is not a problem; sodium silicate is categorized by the U.S. Environmental Protection Agency (EPA) as a non-hazardous solid waste material and can be readily disposed of in municipal waste streams or can be recycled by the enduser.

Low-Cost, Low Complexity
For fuel cell makers, sodium silicide eliminates the need to store and refill high-pressure hydrogen gas. The powder is stored in disposable or recyclable canisters (shown above), which are designed to work with any common low-temperature PEM fuel cell. The canisters allow for ease-of-use, low-cost, and low-complexity fuel cell systems compared to other high-density hydrogen-generation materials, like sodium borohydride or ammonia borane. Other advantages that sodium silicide offer include high/low temperature stability, fast start/stop capability, low temperature/pressure operation, and lightweight packaging.

The canisters are easily removable and hot swapped out, allowing endusers to replace empty canisters with no loss of power to the fuel cell. Since sodium silicide is already governed under existing regulatory guidelines, the fuel canisters can be easily distributed to endusers as a consumer commodity.

Potential Markets
Fuel cells powered by sodium silicide are compact, lightweight, safe for indoor or outdoor use, and can power any standalone application that requires from 1W to 3kW of power. While these features make this material a good fit for any application needing a portable or back up power source, three entry markets are anticipated for sodium silicide fuel cells:

  • Consumer electronics (laptops, cell phones, and cameras);
  • Emergency response and military (generators, water pumps, military, and emergency response equipment);
  • Portable power (lawn mowers and electric bicycles).

Consumer Electronics – Sodium silicide fuel cells have great potential in the consumer electronics market. Faster data networks and more power-hungry applications, like live-video streaming, are quickly increasing the need for higher-density power supplies. While lithium based batteries have advanced in recent years, internal heating issues limit further improvements. Sodium silicide fuel cells, however, have a much higher energy density than batteries and provide a significant increase in power availability and lifetime. Several modular canister formats have been developed to provide power to a variety of consumer electronics, including a flat design for small fuel stacks or planar fuel cells.

Emergency Response and Military Markets – Sodium silicide technology has a few surprising attributes that make it a fit for fuel cells that operate in harsh and unreliable environments. Sodium silicide is stable over all practical temperature ranges (-55°C to 300°C), is lightweight, and has an unlimited shelf life. For emergency responders and the military, this means that generators, radios, phones, computers, telecommunications stations, and temporary medical equipment will operate continuously, reliably, and safely with sodium silicide fuel cells. Given its indefinite shelf life, this material can be easily stockpiled and transported. 

Another interesting benefit, clean water is not required to generate hydrogen. Sodium silicide fuel cells can generate power with any type of water solution – including potable water, polluted water, sea water, or even urine. This is significant in battlefield or natural disaster response settings where clean water supplies may be disrupted. In those instances, even sea or puddle water could be used to provide a quiet supply of power to generators or other applications, ranging from battery recharging to medical evacuation stretchers.  

Portable Power – This novel hydrogen-generation technology has already been demonstrated for personal mobility applications. One recently demonstrated application is Pedego’s electric bicycle, which uses a sodium silicide fuel cell range extender. This sodium silicide fuel cell creates up to 200W of continuous power in a fuel cell/battery hybrid that stores excess power in existing lithium polymer batteries for acceleration and hill climbing. This application demonstrates the higher energy density that sodium silicide offers compared to lithium batteries. For example, assuming a 50% fuel cell conversion efficiency, the energy density for a sodium silicide fuel cell exceeds a net 1,000 W-hrs/kg as compared to approximately 75/W/hours/kg for the lithium polymer batteries used to power an electric bicycle.

For the electric bicycle rider, the fuel cell extender triples the range of the e-bike with only minimal additional weight. Existing e-bikes have a range of up to 20 miles without pedaling; the fuel cell extender model has up to 60 miles in range (for each carried fuel cartridge) without pedaling. 

Cost Comparison
It is often costly for markets to adopt new technologies due to the fixed costs associated with material and product manufacturing. Sodium silicide, however, uses only sodium and silicon, both of which are low-cost starting materials. The unique cost attributes associated with making sodium silicide, and the simple hydrogen control and low-temperature PEM fuel cell systems that it enables, allow for power solutions that are significantly cheaper than existing portable power sources.

For example, a sodium silicide fuel cell in reasonable production volumes will be 10 times less expensive than alkaline batteries and six times less expensive than disposable lithium batteries, including the cost of the fuel cell. Put another way, in order to buy enough alkaline batteries to charge a cell phone 100 times (800 W-hrs ), a consumer will pay about $967. Lithium batteries are a bit cheaper and will cost about $512 for 100 charges. For a sodium silicide fuel cell, consumers will pay only $89 for 100 uses. Including the cost of the fuel cell, consumers will save nearly $900 by charging their electronics with a sodium silicide fuel cell. 

Sodium silicide is proving itself as a high-performance, scalable hydrogen-generation technology with a low carbon footprint and a beneficial life cycle compared to other fuel sources used for fuel cells. With the success that sodium silicide has achieved to date, consumers can expect to see these fuel cell systems on store shelves within the next 12 months.

SiGNa Chemistry Inc.
New York, NY