Advanced Manufacturing Office Research Articles

(Invited) Point Defects and Doping in Ga2O3 and Related Alloys

Ga2O3 and related alloys are a promising platform for realizing wide-band gap semiconductors for use in a number of next-generation electronic devices exploiting their large band gaps and controllable electrical conductivity. Ga2O3 exhibits a band gap (4.8 eV) that can be tuned by In and Al incorporation, with Al-containing alloys increasing the band gap in contrast to In incorporation. While control over defect concentrations and n-type doping of Ga2O3 is rapidly improving, the dopability of alloys has remained largely unexplored. Here we describe the behavior of point defects and n-type dopants in Ga2O3 and consider the prospects of doping in the larger-gap Al-containing alloys using first-principles modelling approaches based on hybrid functional calculations. We consider a number of conventional dopants such as Si, Ge and Sn, as well as newer dopants that have been identified as effective alternatives. These results provide guidance for doping in Ga2O3 and related alloys incorporated into heterostructure devices.This work was partially performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344 and partially supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. DOE, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office.

In Situ X-Ray Scattering Characterization of PEMFC Catalyst Ink Microstructure during Ink Processing

Catalyst layers for proton exchange membrane fuel cells (PEMFCs) are typically prepared by first dispersing a catalyst powder, comprised of platinum or platinum alloy nanoparticles supported on carbon blacks, with ionomer in solvent followed by extensive mixing using a variety of methods. The purpose of the ink mixing and processing procedure it to break up large agglomerates of the catalyzed carbon powder (>10 µm) and form a uniform dispersion, However, care must be taken to limit damage to the catalyst particles. Following the dispersing process, the catalyst ink is applied to the surface of the substrate (membrane, diffusion media, or transfer liner) through a variety of coating processes (painting, spraying, screen printing, etc.) [1]. In addition to creating coating defects, large agglomerates resulting from inadequate ink processing can limit catalyst utilization, inhibit mass transport in the catalyst layer, and damage the membrane and possibly also the gas diffusion layer. This can lower beginning-of-life performance by creating transport barriers for oxygen, hydrogen, and protons, and can increase membrane pinhole formation and electrical shorts, thus limiting the cell lifetime. This presentation will describe ink compositions and processing techniques for efficient coating of catalyst layers while optimizing cell performance and lifetime and minimizing ink processing time and cost [2, 3]. Ultra-small angle X-ray scattering (USAXS) was used to measure the agglomerate size distribution during ink mixing/processing. The experimental setup for the USAXS evaluation of inks during sonication is shown in Figure 1. A flask containing catalyst ink was immersed in an ice bath and sonicated using a water bath and/or a horn sonicator and pumped through a thin-walled capillary tube situated in the X-ray beam using a peristaltic pump. The effects of catalyst ink composition and processing variables on catalyst agglomerate size and ink properties were evaluated using different solvents, ionomer-carbon ratios, support types, and sonication time. The proper ink composition and sonication method that facilitate the break-up of carbon agglomerates with the minimum amount of time were determined. The electrochemically-active surface areas of the catalyst after the ink processing were determined using aqueous electrochemical methods to determine the extent of damage to the catalyst after processing (e.g., dislodging of catalyst particles from the support). The correlation of ink processing conditions and agglomerate structure with fuel cell performance will also be described. Acknowledgements This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency&Renewable Energy, Advanced Manufacturing Office, Roll-to-Roll Advanced Materials Manufacturing DOE Laboratory Consortium. This research used the resources of the Advanced Photon Source (APS), a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

Developments in Ultra-High Efficiency Fuel Cell/Turbine Hybrid Systems and Supporting Technologies

FuelCell Energy (FCE) is developing high temperature fuel cell/turbine-based hybrid systems. The development includes both commercial Direct FuelCell/Turbine (DFC/T®) systems as well as Solid Oxide Fuel Cell/Turbine (SOFC/T) systems. It consists of system concepts, design/engineering/demonstration of an alpha-unit sub-megawatt hybrid system, design/engineering of a commercial MW-scale hybrid system, and development of associated supporting technologies. Initial development originated under a Department of Energy (DOE) Vision 21 Program. Follow-on programs have supported further advancement of high temperature fuel cell/turbine hybrids through supporting development of advanced control modules and development of advanced high temperature recuperators (HTR’s), under, respectively, DOE SBIR and EERE Advanced Manufacturing Office (AMO) initiatives. Activities under the former included development of a novel paradigm for advanced control of power plant processes with specific adaptation to hybrid systems and smart grid dispatch. The paradigm involves intelligent control algorithms based on feedforward and feedback dynamic structures. It consists of a combined model-based robust controller and a neural network supervisor (NNS) that together manipulate the power plant actuators to suboptimally control the hybrid system during load ramping maneuvers. Under the latter ongoing effort, FCE is further developing system concepts and components based on microchannel HTR’s. Activities include candidate material screening and environmental testing as well as fabrication and testing of pilot-scale recuperators. The project will culminate in a MW-duty recuperator design that would support FCE’s development efforts toward a commercial MW-scale hybrid system.