The production of value-added chemicals and fuels from CO
2
and water by highly efficient, selective, robust electrocatalytic materials is of vital interest to address the energy challenges. In this ...work, we report a hierarchical CuO-derived inverse opal (CuO-HIO) electrocatalyst which demonstrated impressive CO selectivity and negligible H
2
evolution at high current densities (achieved ~35 mA cm
-2
at -1.2 V
vs.
RHE). Such a three-dimensional interconnected porous network with voids about 200 nm surprisingly exhibited a Faradaic efficiency (FE) of 72.5% for CO production at -0.6 V
vs.
RHE and was very stable during 24-hour CO
2
electrolysis. The
in situ
Cu K-edge XANES and Fourier transformed EXAFS spectra dynamically revealed the direct transformation of Cu
2+
to Cu
0
species within few minutes at -0.6 V
vs.
RHE and CuO was entirely reduced to metallic copper after 60 min. Such an observation was well consistent with the
in situ
time-resolved XRD patterns which illustrated the gradual emergence of (111), (200), (220), (311), and (222) diffractions of cubic copper under the working condition. CO
2
-to-CO outperformance of CuO-HIO catalyst over typical oxidized copper catalysts could be ascribed to (
i
) the mass transport limitation and local pH effect induced by unique hierarchical morphology, (
ii
) highly roughened surface, and (
iii
)
in situ
reduction of Cu
2+
to stabilized metallic Cu active sites during the CO
2
electrolysis. Our findings may open an opportunity for rational design of promising, high-performance, less expensive electrocatalysts for renewable fuel production from fossil fuel-generated CO
2
emission.
Strontium-doped lanthanum manganite (LSM) is one of the most widely used cathode material for yttria-stabilized zirconia (YSZ)-based solid oxide fuel cells (SOFCs), due to its superior chemical ...stability and small coefficient of thermal expansion (TEC) mismatch with YSZ. The activation process of the cell, which can be attributed to the cathode and cathode electrode interface, has a significant effect on the long-term electrochemical performance and stability of SOFCs. Therefore, it is essential to understand the origin of this underlying activation process. In this study, thin film LSM on YSZ half cells were fabricated with an LSCF counterelectrode and the effects of cathodic current density and overpotential on the polarization behavior of the LSM thin film electrode were investigated. The surface morphology as well as surface chemistry and bonding environment of LSM before and after polarization was investigated using scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The formation of SrO was found on the LSM surface after polarization. The cross-section microstructure of LSM, as well as interfacial chemistry of LSM/YSZ, were investigated using transmission electron microscope (TEM) and TEM-based energy dispersive X-ray spectroscopy (EDS). The presence of Mn-rich phases were observed before and after polarization. The pre- and post-activation electrode performances were investigated at various temperatures and oxygen partial pressures, which helps reveal the nature of the activation mechanism in combination with the microstructural and chemical analysis.
The activation behavior of a LSM based-cathode in the SOFC is characterized by a rapid reduction in the electrode polarization resistance after cathodic current passage treatment. This activation ...behavior has been attributed to mechanisms that either enhance the kinetics of oxygen reduction or remove impediments from the oxygen reduction mechanism. Due to the sensitivity of the activation process to the operation history, a comprehensive study was carried out to reveal the relationship between the activation conditions, the steady state cathode performance and the long term stability. This work demonstrates the application of a two electrode configuration half-cell operated for the impedance analysis of LSM-based electrodes. The effects of current density and overpotentials on the activation behavior of LSM based electrode were investigated. The impacts of temperature and oxygen partial pressure to the activation process and final performance are also discussed. Long term degradation test of LSM electrodes were carried out on both half-cells and full cells with various activation processes. Numerical simulation of the LSM-based cathode was conducted with a multi-step oxygen reduction reaction (ORR) mechanism for a better understanding of the nature of the activation process. The results are of great relevance as the electrode performance depends on not only the fabrication but also on the initial break-in procedure.
Solid oxide fuel cells (SOFCs) are a promising technology for clean, efficient electrochemical energy conversion, particularly for stationary applications. Performance degradation over their ...lifetime, however, still presents a hindrance to widespread SOFC commercialization. Understanding this degradation, which is tied in part to reaction rates and overpotentials associated with the electrodes, is therefore a crucial research direction. Many studies have used various tomographic techniques on research-grade SOFC electrodes to examine the porous, composite electrodes’ microstructures, which bear a strong relationship to their performance. This approach has yielded much insight. However, because commercially available, mass-produced SOFC electrodes must meet cost targets, they tend to exhibit more heterogeneous microstructures than research-grade cells (1). The required magnitude of measured volume in these more heterogeneous cells must be quantified.
One particular work by Harris & Chiu (2) provided a good theoretical framework for estimating the representative volume element (RVE) for such materials using the characteristic particle size as an input for the estimation. The results of that work appear to hold true for a wide range of tomographic SOFC studies in the literature, including those authors’ own application to their experimental data (3). However, we have found that for mass-produced cells that exhibit significant heterogeneity over 5-10 µm, larger RVEs are required, in contrast to the more homogeneous academically fabricated cells found in most of the literature. Specifically, the mass-produced cells’ characteristic particle sizes are in the range of 0.5-1 µm (similar to academically fabricated cells), yet we find that the RVEs for characterizing mass-produced cells are significantly larger than expected, reaching into the 20
3
- 30
3
µm
3
range – see, for example, an excerpt of our results in Figure 1, which illustrates the change in triple phase boundary density variability as the size of the studied region increases.
In this presentation, we will discuss our experimental results and analysis of the heterogeneity of commercial, mass-produced SOFC electrodes using microscale and nanoscale X-ray computed tomography. We will then discuss an updated theoretical framework for determining the RVE using limited knowledge of the electrode, similar to that of Harris & Chiu (2), but modified to account for the poorer mixing and greater heterogeneity that we observe in mass-produced cells.
Figure 1: Variability (in terms of 90% confidence interval according to Student’s t-test, expressed as a fraction of the overall mean) in triple phase boundary density shown as a function of the side-length of cubic regions of interest, for the cathode and anode active layers of a commercial SOFC, as determined using analysis of 3D nanoscale X-ray CT imagery (inset).
References
1. S. J. Dillon, L. Helmick, H. M. Miller, L. Wilson, R. Gemman, R. V. Petrova, K. Barmak, G. S. Rohrer, and P. a. Salvador,
J. Am. Ceram. Soc.
,
94
(11), 4045 (2011).
2. W. M. Harris and W. K. S. Chiu,
J. Power Sources
,
282
, 552 (2015).
3. W. M. Harris and W. K. S. Chiu,
J. Power Sources
,
282
, 622 (2015).
Figure 1
Surface-enhanced infrared absorption is a spectroscopic technique but unsuitable for gas spectroscopy due to the need for long absorption path lengths. We demonstrate a device using metal-organic ...framework integrated with plasmonic nanoantennas for CO 2 sensing.
We demonstrated an ultra-sensitive near-infrared (NIR) absorption CO 2 fiber-optic sensor at 1.57μm wavelength enhanced by metal-organic framework. We achieved 20 ppm detection limit with only 5cm ...length with 500× NIR enhancement from the MOF film.
Charged active sites are hypothesized to participate in heterogeneously-catalyzed reactions. For example, Au
δ
+
species at the catalyst surface or catalyst-support interface are thought to promote ...the thermally-driven CO oxidation reaction. However, the concept of charged active sites is rarely extended to electrochemical systems. We used atomically precise Au
25
q
nanoclusters with different ground state charges (
q
= −1, 0, +1) to study the role of charged active sites in Au-catalyzed electrochemical reactions. Au
25
q
clusters showed charge state-dependent electrocatalytic activity for CO
2
reduction, CO oxidation and O
2
reduction reactions in aqueous media. Experimental studies and density functional theory identified a relationship between the Au
25
q
charge state, the stability of adsorbed reactants or products, and the catalytic reaction rate. Anionic Au
25
−
promoted CO
2
reduction by stabilizing coadsorbed CO
2
and H
+
reactants. Cationic Au
25
+
promoted CO oxidation by stabilizing coadsorbed CO and OH
−
reactants. Finally, stronger product adsorption at Au
25
+
inhibited O
2
reduction rates. The participation of H
+
and OH
−
in numerous aqueous electrocatalytic reactions likely extends the concept of charge state-mediated reactivity to a wide range of applications, including fuel cells, water splitting, batteries, and sensors. Au
25
q
clusters have also shown photocatalytic and more traditional thermocatalytic activity, and the concept of charge state-mediated reactivity may create new opportunities for tuning reactant, intermediate and product interactions in catalytic systems extending beyond the field of electrochemistry.
Differently charged, atomically-precise Au
25
q
nanoclusters were used to establish trends between active site charge state and electrocatalytic activity.
Full text
Available for:
IJS, KILJ, NUK, UL, UM, UPUK
Charged Au
n+/−
sites are hypothesized as key reaction centers in gold catalysis, but their charge state and mechanistic roles remain controversial. Two examples include CO
2
reduction and CO ...oxidation. Converting CO
2
into value-added products is critical for green-house gas mitigation and renewable fuels discovery, and oxidizing CO in the presence of water is central to the industrially important water gas shift reaction (WGS: CO + H
2
O → CO
2
+ H
2
). Debate surrounds the charge state of Au active sites, and variously charged Au
n+/0/−
species and/or the catalyst-support have all been proposed as reaction centers for CO oxidation and CO
2
reduction. We used differently charged Au
25
q
clusters (
q
= −1, 0, +1) to precisely identify the role of active site charges in heterogeneous gold catalysis. Au
25
q
are unique because they have three stable charge states, their crystal structure has been solved, and their small size (~1nm) allows computational modeling of realistic cluster-adsorbate systems. In this regard, Au
25
q
can serve as well-defined active sites for probing the chemistry of charged catalyst species. Experimental studies and density functional theory identified a relationship between the active site charge, the stability of adsorbed reactants or products and the reaction rate. We found charge-dependent electrocatalytic activity for CO
2
reduction, CO oxidation and O
2
reduction reactions in aqueous media. Anionic Au
25‾
promoted CO
2
reduction by stabilizing CO
2
+ H
+
coadsorption. Cationic Au
25
+
promoted CO oxidation by stabilizing CO + OH
−
coadsorption. Finally, stronger product adsorption at Au
25
+
inhibited O
2
reduction. These results provide insight into the role of charged active sites and should help guide future catalyst design.
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