Cheap Hydrogen Gets Closer to Reality
Mark Atwater posted on February 15, 2015 |

The cost of products is tied to two basic factors: the cost of the raw materials and the cost of processing those materials into the final product. In the “ideal” case, hydrogen could be generated from water. Unless you’re in the habit of buying water harvested directly from a glacier or hundreds of feet under a volcano, then water is about as cheap as it gets. The process is where the problems arise, but researchers are making inroads on bringing down the cost. 

The process of creating hydrogen from water involves splitting the molecules to separate oxygen and hydrogen (i.e., electrolysis). While this can be simple enough for a middle school science experiment, to make it efficient enough for commercial hydrogen production is decidedly more complex. 

A catalyst is required to drive the process efficiently, and platinum has long been the standard. It is an excellent catalyst, but it is exceptionally expensive. An alternative, molybdenum disulfide (MoS2), which is often used a dry lubricant, has intrigued researchers for years, but it has never been able to match, or even reasonably compete with platinum for efficiency.

Researchers at North Carolina State University have been working on understanding how MoS works so they can improve its activity. Unlike platinum, MoS2, can vary in composition (between 2 and 3 oxygen atoms per molecule). Also of interest are the structural factors determining the behavior of MoS2.

There are some basic parameters you want in this catalyst. Those factors are a low Tafel slope, which determines the voltage required to drive the reaction at a given rate, the exchange current density, which measures the intrinsic activity of the catalyst, and the stability, which indicates the longevity of the catalyst under operating conditions. These characteristics should be low, high and high, respectively.

The NCSU research group, headed by Linyou Cao, assistant professor of materials science and engineering, discovered that MoS2 behaves differently when amorphous than when crystalline. When amorphous the Tafel slope is lower, but the activity and stability are also lower. When crystalline, the Tafel slope is higher, but so are the activity and stability.

Understanding the structure-performance relationship is vital. Neither form has the ideal set of properties, but they may be combined to reach a workable balance. Cao describes it this way, "Now that we understand this, we need to carefully engineer MoS2 with a balanced structure to control the effects on all of the aspects of catalysis."

As it turns out, the exact composition is not critical, so the structural factors are of primary importance. Currently their research is focused on nanoclusters of MoS2 only 5-30 nm in diameter, which are described in the related publication in ACS Catalysis. The refinement and improvement of this low-cost catalyst could “catalyze” more rapid development and commercialization of hydrogen technology.

 

Image: Water Cooler World (UK)

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