Highlights

Anodized Aluminum Oxide (AAO) Based Nanofabrication for Hydrogen Sensing

Scientific Achievement

The great surface activity, very small size, superior sensitivity and low-power consumption of nanostructures have recently attracted the attention of scientists to overcome the limitations of current hydrogen sensing devices.

We fabricated and investigated hydrogen sensing capabilities of a nanostructured sensing device based on anodic aluminum oxide (AAO) nanowells.  We focused on the AAO shallow region with an aspect ratio of less than 10 to produce an array of nanowells used as a substrate for thin films of palladium nanoparticles.  The work at the EMC allowed us to characterize the AAO nanowell – Pd nanostructure and correlate its structure with its hydrogen sensing performance. Hydrogen concentrations as low as 0.05 vol% (500 ppm) can be detected at room temperature. Response times of ~ 1 second were obtained for the AAO nanowell-Pd nanostructure detector which, when compared to current devices and nanostructures in development, is found to be very fast without compromising sensitivity and selectivity.

Significance

The prior work at the EMC allowed us to characterize an AAO nanowell – Pd based nanostructure and correlate its structure with its hydrogen sensing performance [Sensors and Actuators B 134, 869–877 (2008)]. Recently, our group developed a single-walled carbon nanotube (SWNT) based nanostructure decorated with Pd nanoparticles and tested it for hydrogen sensing [Appl. Phys. Lett. 90, 213107 (2007)].  Results showed that SWNT based sensors have superior hydrogen sensing capabilities.  We are ontinuing with our investigation by working on a nanostructure with double-walled nanotubes (DWNT) decorated with Pd nanoparticles.  It has been proposed in the literature that DWNTs exhibit superior hydrogen absorption compared to SWNTs.  Our preliminary results comparing SWNT to DWNT showed that the nanostructured based on the latter exhibit superior sensitivity for hydrogen sensing.  An electron microscopy characterization will allow us to evaluate the structure of our DWNT nanostructure and correlate it with its hydrogen sensing capabilities.  In our knowledge, there is no work reported in this specific subject.  In addition, the study of the structure of our DWNT device will contribute to understand the electronic characteristics and mechanisms of hydrogen interaction with DWNTs.  Our work on the AAO template based nanofabrication will continue.  We will also characterize nanowires and nanotubes with use of the FE-SEM instruments.

Performers

Utilizing a dual-beam focused ion beam scanning electron microscope (FIB-SEM), we have developed a 3D reconstruction method for solid oxide fuel cell (SOFC) electrodes.  This technique has enabled us to probe many microstructural properties such as Triple Phase Boundary length (TPB), phase connectivity, and tortuosity, and to establish that these have a significant impact on electrochemical performance.

We have analyzed the effects of compositional variations in Ni-YSZ anodes (YSZ = Y-stabilized Zirconia).  It was found that substantial isolated Ni and porosity existed in the sample with 40 wt% NiO, such that the electrochemically active TPB density was much lower than the total TPB density, and hence, the polarization resistance was greatly increased.  Additionally, high YSZ tortuosity in samples with low amounts of YSZ also contributed to reduced electrode performance.  

We have also quantitatively observed the evolution of Ni-YSZ electrodes due to Ni-coarsening at high temperatures.  Experimentally, an 11% decrease in Ni specific surface area was observed after 400 hours as well as a 9% decrease in TPB length.  The original structure was also input for a phase-field simulation of the microstructure evolution based on Ni surface diffusion.  Comparing experimental coarsening structures with simulated ones has allowed us to better understand this degradation mechanism that over time contributes to the unstable operation of SOFCs.

Significance

State-of-the-art fuel cell electrodes typically have a complex micro/nano-structure involving interconnected electronically and ionically conducting phases, gas-phase porosity, and catalytic surfaces. Understanding this microstructure is a critical and typically missing link to understanding the electrode performance, given a set of processing conditions.  This work was the first to three-dimensionally reconstruct a solid oxide fuel cell (SOFC) electrode and continues to lead the field in correlating performance to processing.  Additionally, measurements of electrochemically-active TPB density and phase tortuosity in anodes were the first published results of this kind, and were used to better understand microstructure – performance relationships between anodes of different compositions.  These methods also allow, for the first time, a way to study the underlying fundamental properties of Ni coarsening by comparing experimental coarsened structures to those achieved through computer simulations.

Performers