LBL Researchers Study Batteries with Photothermal Deflection Spectroscopy

Spring 1990

By Jeffery Kahn, [email protected]

The secret to a battery's performance, whether stellar or abysmal, resides in a chain of unique and often baffling electrochemical reactions. Researchers know the chemical anatomy of a new, charged cell and after it has been discharged, they can dissemble it to identify the compounds and reactants that remain. But what goes on inside the battery while it is discharging or recharging -- the chain of critical interim reactions that dictates whether a rechargeable battery has nine lives or 900 -- is not well understood.

LBL researchers are studying the chemical changes within test cells as they operate using a technique called photothermal deflection spectroscopy, or PDS. Their effort to identify these perplexing intermediate reactions is providing scientists a better understanding of electrochemical processes which, ultimately, will allow researchers to build better batteries and fuel cells.

PDS research is conducted by a group led by Applied Science Division Director Elton Cairns and the division's Frank McLarnon. Relatively new, PDS is a spectroscopic technique finding use in many scientific fields. Cairns realized PDS possibly could provide unique insights into electrochemical systems, and began the LBL project. Scientists Rick Russo and research associate Jon Spear explored the concept, and devised a working analytical system. Currently, PDS techniques are being refined and studies performed by graduate students Jim Rudnicki and Jeff Weaver.

PDS allows scientists to study a battery or fuel cell as it operates with negligible interference. Alternative techniques which require the dissembling of the battery prior to analysis potentially can provide misleading information.

"Let's say you have a battery that performs poorly," says Rudnicki. "The intermediate chemicals which have formed on the battery electrodes can be the missing piece to the puzzle of what went wrong. You can remove an electrode from the cell for analysis by various techniques. But removing an electrode from its electrolyte bath and the electric field in the cell alters it. Only an in situ tool like PDS can identify these chemical intermediates with any certainty."

PDS is ideally suited to identify chemicals that form on electrodes. Rudnicki is using the technique to study such surface reactions.

Weaver and Rudnicki also are using the technique to probe chemical activity within the electrolyte solutions that are the internal current carriers in electrochemical systems. Gradients in concentration develop in these electrolyte solutions. PDS can be used to detect and study these concentration gradients, revealing critical corollary information about how an electrode is being utilized, and how its performance might be improved.

In the past, scientists have used light reflected off the surface of electrodes in an attempt to spectroscopically study their surface chemistry. But unless an electrode has a smooth, mirror finish, this technique doesn't work well. This is a major handicap in that shiny, smooth electrodes have much less surface area to accommodate reactions than irregular, folded electrode surfaces which, consequently, are higher performance.

Rather than using reflected light, PDS relies on absorbed light. Through PDS, the absorption spectrum of the surface can be determined. This spectrum then allows scientists to identify surface layers of chemicals as thin as a single molecule.

Experiments involving PDS are performed with electrochemical cells enclosed within transparent cases. Monochromatic light is shone on the electrode surface at a succession of wavelengths. Light at each wavelength is either absorbed or reflected by the surface, depending on the surface species' unique spectroscopic characteristics. When light is absorbed by the electrode surface, the absorbed energy is converted into heat. The more strongly the surface absorbs each particular wavelength of light, the more it heats up.

As the electrode surface absorbs light and is heated, heat is conducted into the liquid electrolyte. Temperatures in the electrolyte drop as the distance from the electrode increases, forming what is called a temperature gradient.

Like the temperature gradients, gradients also form in electrolyte concentrations. Differing temperatures and concentrations at varying layers create an optical effect, changing the degree to which a beam of light is refracted as it is passed through the electrolyte. These changes in the refractivity of the electrolyte solution are the basis of PDS.

For these phenomena to be meaningful, they must be measurable. A laser probe beam and a position detector make this possible.

The probe beam is aimed parallel to the electrode, skimming or grazing its surface, functioning as close as several microns. As it passes through the temperature and concentration gradients in the electrolyte, the laser beam is refracted. The position detector reads how far the beam has been deflected.

Both temperature and concentration gradients occur simultaneously in electrochemical cells. These two signals must be separated if meaningful data are to be obtained. Both deflect the beam. To separate the signals, the monochromatic light is turned on and off, causing a modulation in the temperature signal. The concentration signal, created by reactions on the electrode rather than the absorption of light, does not modulate, allowing both signals to be distinguished and quantified.

The sensitivity of the position detector may be the most remarkable aspect of PDS. To measure the degree of deflection of the laser, the position detector must have a sensitivity that is rated in nanoradians (an angle equal to about 1/5000 second).

"To visualize the minuteness of a nanoradian," says Rudnicki, "imagine a researcher at LBL is shining a laser beam on a white card held by an assistant standing in Los Angeles. If the researcher twisted the laser one nanoradian, the assistant would see the spot on the white card move half a millimeter. The deflections, when measured six inches from the test cell, can be on the order of nanometers (billionths of a meter)."

Even slight air currents or sounds can distort PDS measurements. Foam enclosures to dampen air currents, and a test platform insulated by air piston shock absorbers help. Nevertheless, many of Rudnicki and Weaver's experiments are performed at night.

PDS is a versatile technique. Weaver is probing electrolyte reactions within a simulated electrode pore where the uncertain succession of reactions that power electrochemical cells begins. Rudnicki is examining the platinum electrode in a promising fuel cell that, for one moment, successfully converts methanol directly into electricity. Then the reaction halts, fouled by a compound that coats the electrode, but which has yet to be definitively identified despite a decade of debate.

"PDS should help resolve this debate," says Rudnicki. "The development of many electrochemical systems hangs on understanding intermediate reactions like this. With PDS, we have a tool that can look at electrochemical puzzles and provide the missing pieces."