60th Annual Report on Research 2015

One of the largest cost and efficiency limitations of electrochemical energy devices that convert organic molecules to electricity (e.g. methanol, ethanol) lies in the slow kinetics of the small molecule electro-oxidation at the anode. The ability to unravel the origin of this sluggish kinetics can assist with the design and the discovery of new electrocatalysts that can more efficiently facilitate the electro-oxidation reaction; in addition, such knowledge can pave a way toward the design of a new electrocatalyst that can utilize more difficult higher hydrocarbons for future electrochemical energy conversions. 

In an effort to address this challenge, our work has been to create, develop, and apply an in situ characterization method that can monitor and resolve the surface intermediate species during the electro-oxidation process. We utilize in situ stimulated Raman spectroscopy (SRS), which uses energy-matching photon to simulate the Raman scattering (Fig. 1). SRS has many advantages over traditional in situ vibrational spectroscopy. Notably, the challenge associated with infrared lies in the highly absorptive electrolyte. SRS circumvents this challenge by using a visible/near-infrared probe, which is electrolyte-transparent. In comparison to traditional Raman, SRS offers several higher signal and better time resolution.

Generously supported by the ACS PRF, we have been studying SRS for monitoring the electro-oxidation reaction in the past year. We organize our effort into two objectives. The first focuses on creating an in situ electrochemical cell. The second focuses on outfitting our Ti:sapphire laser for SRS. In the first objective, we adopted the geometry of an electrochemical cell that has been previously used for in situ (spontaneous) Raman spectroscopy. This geometry uses an upward-facing electrode to minimize the electrolyte signal while still retaining the benefit of a three-electrode cell. We have verified the efficacy of our electrochemical cell by reproducing the in situ Raman features (e.g. Au-O and O-O) that are formed during the electrochemical oxidation of Au (Fig. 2). These results confirmed that we had successfully built a functional, in situ electrochemical cell that is Raman-compatible.

After finishing with the in situ cell creation, we turned our attention to outfitting our laser system for SRS. Our first task focuses on narrowing the spectral width of our laser, which has a bandwidth that is too large for a Raman pump. To narrow down the bandwidth, we built a spectral filter based on a stretcher-filter-compressor scheme, which allows us to reduce the bandwidth to ~2.5 nm (Fig. 3). Our next step is to use an even narrower slit to bring the bandwidth down to ~1 nm, which corresponds to ~17 cm^-1 resolution. Our second task focuses on creating stable continuum generation as a Raman probe. To accomplish this, we picked off and focused the Ti:sapphire beam into a sapphire plate. This produces a white-yellow beam (Fig. 4a). We control the quality of this generation by optimizing the power, the aperture, and the focusing until the output looks visually stable and has the bandwidth needed to serve as a Raman probe. Our third task focuses on creating a delay line to ensure that the pump and probe arrived on the sample at the same time. We use a beta-barium-borate (BBO) crystal to determine the delay length that can synchronize this crossing (so-called ‘time-zero’). To find the delay, we carefully adjusted the delay optics until we can observe the sum frequency generation (SFG) between the two beams. At time-zero, the overlap between the two pulses is maximized, thereby producing the highest SFG intensity (Fig. 4.) Our next task is to integrate these individual components – an in situ electrochemical cell, the pump and the probe pulse generation, and the delay optics to create a functional, in situ SRS experiment to study the CO and methanol electro-oxidation reaction.

Throughout this past 1-year period, this ACS-PRF project has had incredible impact on my group’s research and education mission. This impact extends beyond the scientific capability; both my career and my student’s personal development had significantly benefited from this ACS-PRF program. On the capability front, in addition to the progress abovementioned, this program had allowed us to vastly improve our facilities. Through it, we had optimized and built a functioning Raman system with the Ti:sapphire laser and identified the temperature-humidity fluctuation in our laboratory. This latter finding had allowed us to recently negotiate a new space with better temperature-humidity control.

The impact the ACS PRF has on my career is significant. It enables me to pursue a high-risk, high-reward direction that integrates two normally distinct disciplines: non-linear spectroscopy and electrochemistry. This pursuit allows me to test the hypothesis of whether we can combine advances in spectroscopy and optics to drive discovery in catalysis. The impact on the education of the students in my group is immense. This ACS-PRF program is directly supporting a young, promising Cornell graduate student to learn cutting-edged concepts and state-of-the-art skills in in situ spectroscopy, surface science, and electrochemistry. The graduate student supported by the ACS PRF has also taken on a mentorship role of a younger undergraduate, who is sponsored by the research fund from Cornell. Together, they have built a highly collaborative team, where they are learning how to work together and be analytical, tenacious, and rigorous.

Figure 1. (a) Traditional Raman scattering occurs via the spontaneous emission of the Stoke transition. (b) Stimulated Raman Spectroscopy uses a separate, energy-matching pulse to stimulate the emission.   Figure 2. In situ Raman spectroscopy on Au electrode in 1M HClO₄ (left). The differential Raman spectra (right) shows the growth of the Au-O and O-O peaks at higher electrochemical oxidation as reported previously.  

Figure 3. (a) A spectral filter for reducing the bandwidth of the Ti:sapphire laser. (b) Our filter narrows the bandwidth from ~8 nm to ~2.5 nm.

  Figure 4. (a) Continuum generation using the Ti:sapphire laser. (b) At time-zero, both the pump and probe pulses arrive simultaneously, allowing the sum frequency generation (SFG) to occur. The middle blue spot is the result of the SFG (the right blue spot is the second harmonic generation of the pump). BBO: Barium-borate crystal.