X-ray absorption spectroscopy (XAS) plays a critical role in the characterization of energy materials, including thin-film electrocatalysts and battery materials. XAS is well-suited for this purpose ...because it is element-specific and can target distinct chemical environments within a material, even in a mixed or complicated matrix. Even so, some key energy materials are far from “ideal” XAS samples. This means that both sample preparation and experimental conditions need to be considered when collecting and interpreting data to ensure that conclusions are correct. This review outlines some of the key questions that an XAS experiment is well-suited to answering, including speciation of amorphous materials, understanding how multi-metal systems interact, and the different ways that we may observe single atoms. In addition, we show how XAS can be highly complementary to other analytical techniques in developing a full picture of a material over different scale bars. Importantly, we also examine instances where the sample matrix can distort XAS data, show an example where bond-length disorder can be confused with a change in the coordination number, and discuss some of the advantages and challenges of in situ electrocatalysis. Finally, we examine the future role that XAS will play in innovations in energy materials.
Development of catalysts for the oxygen evolution reaction (OER) that are capable of robust operation at low pH and elevated temperatures, but do not contain scarce ruthenium and iridium, presents a ...challenging yet very attractive strategy in decreasing the high cost of efficient water electrolyzers paired with proton-exchange electrolytes. Toward this aim, combinations of both catalytically active and acid-stable components offer an appealing approach to cost-effective anode catalysis for low-pH water electrolysis. The current work presents an oxygen-evolving Ag + BiO x catalyst based on intermixed silver and bismuth oxides, prepared by a simple anodic electrodeposition. We demonstrate that numerous electrode substrates can be functionalized and operate stably with the Ag + BiO x catalyst in nominally pure aqueous H2SO4 solutions. Moreover, this catalyst maintains robust operation at pH 0.3 and temperatures as high as 80 °C. Under these conditions, the Ag + BiO x catalyst can deliver an OER rate of 100 mA cm–2 at an overpotential of 0.70 ± 0.02 V vs reversible hydrogen electrode (RHE). In situ X-ray absorption spectroscopic and Fourier transformed alternating current cyclic voltammetric studies of the Ag + BiO x system demonstrate the stabilizing role of the bismuth oxide matrix that facilitates the transformation of silver into a highly oxidized state catalyzing the acidic water electrooxidation.
The hydrogen evolution reaction (HER) under alkaline conditions is subject to significant kinetic limitations even with the most active platinum-based catalysts, while more affordable ...non-noble-metal-based catalytic materials present further challenges in terms of activity and durability in operation. To improve on these aspects, we present a new microwave-assisted synthetic route to fabricate sulfides of nickel and cobalt integrated into a layer expanded molybdenum sulfide (NiS x /MoS2 LE and CoS x /MoS2 LE), which efficiently catalyze H2 evolution in 1 M KOH. The use of the microwave-synthesis conditions enables the formation of nanoscale Ni and Co sulfides distributed homogeneously within the highly disordered layered molybdenum sulfide, as established using a comprehensive suite of physical methods. Synthesis of FeS x /MoS2 LE is also presented, but the resulting material did not exhibit promising properties. Electrocatalytic tests reveal higher activity of the Ni-based catalyst as compared to CoS x /MoS2 LE and especially unmodified MoS2 LE. The performance of NiS x /MoS2 LE at a HER overpotential of 0.15 V at ambient temperature and 60 °C corresponds to specific H2 evolution rates of 28 ± 4 and 58 ± 10 A g–1, respectively. Analysis of the electrokinetic data indicates that the exchange current density of the HER per an electrochemically active surface area of the sulfide-based materials is not high (∼0.001 mA cm–2 at ambient temperature), and that the high performance per unit mass observed here is supported by the well-developed surface area of the material (corresponding to a specific capacitance of ∼71 F g–1). A similar conclusion likely applies to many nickel and cobalt sulfide-based alkaline hydrogen evolution catalysts reported previously. Durability in operation of NiS x /MoS2 LE and CoS x /MoS2 LE is also demonstrated, in particular through a 2-week-long two-electrode water splitting test.
Herein, we demonstrate that introduction of secondary metals into the promising manganese-antimony oxide acidic water oxidation catalysts endows the resulting materials with superior activity and ...stability in operation. Using a simple solution-based method, we synthesised multi-metallic manganese antimonates MnM + SbO
x
with M = Ru, Co, Pb and Cr. All of the secondary metals examined notably increase the initial activity of the mixed oxides towards the oxygen evolution reaction (OER) in 0.5 M H
2
SO
4
at ambient temperature, though MnCr + SbO
x
and MnRu + SbO
x
were found to be less stable. In contrast, MnCo + SbO
x
, MnPb + SbO
x
and MnCoPb + SbO
x
maintained high stability and improved the overpotential required to achieve the water oxidation rate of 10 mA cm
−2
by 0.040 ± 0.014, 0.08 ± 0.03 and 0.050 ± 0.014 V with respect to the parent Mn + SbO
x
catalyst, respectively. Similar improvements in performance were found after tests at elevated temperature of 60 ± 1 °C. The MnPb + SbO
x
system exhibits one of the highest activities for the OER at low pH reported for the noble-metal free catalysts so far,
viz
. 10 mA cm
−2
OER at 0.60 ± 0.03 V overpotential at 24 ± 2 °C with a flat electrode. We ascribe this improved performance to the stabilising effect of lead oxides on the catalytically active manganese sites, which is demonstrated herein by
in situ
spectroelectrochemical X-ray absorption analysis.
Modification of the manganese-antimony oxide nanocomposites with lead produces an active and highly robust electrocatalyst for anodes of hydrogen-generating water electrolysers with proton conducting electrolytes.
Abstract
Aiming to design a catalyst for stable electrooxidation of water at low pH, the present work explores the properties and structural features of electrodeposited composite oxides based on Bi ...and Co, which were anticipated to provide stability and catalytical activity, respectively. Materials deposited as very thin (
ca
50 nm) films on F‐doped SnO
2
(FTO) substrate do not initially exhibit high activity in 0.1 M H
2
SO
4
, but are activated during operation through the electrooxidatively‐induced enrichment of the catalytic surface with Co and Sn oxides. The latter originate from the FTO support and are identified as an important component of the catalyst through control experiments with a Sn‐free substrate and with Sn
2+
intentionally added at the electrodeposition stage. A distinctive feature of the Co−Bi−Sn‐based electrocatalyst is the slow but persistent improvement in the activity during operation in 0.1 M H
2
SO
4
at both ambient and elevated (60 °C) temperatures, which contrasts with the continuously degrading behaviour of state‐of‐the‐art oxygen evolution catalysts at low pH. This is demonstrated by 9‐day‐long galvanostatic tests at 10 mA cm
−2
, during which the Co−Bi−Sn‐based thin film catalyst shows no degradation and sustains stable water oxidation at
ca
1.9 V
vs
. reversible hydrogen electrode. The effects of tin leaching from the support detected herein might have implications to other acidic water oxidation catalysts supported on high‐surface area doped SnO
2
materials.
Aiming to design a catalyst for stable electrooxidation of water at low pH, the present work explores the properties and structural features of electrodeposited composite oxides based on Bi and Co, ...which were anticipated to provide stability and catalytical activity, respectively. Materials deposited as very thin (ca 50 nm) films on F‐doped SnO2 (FTO) substrate do not initially exhibit high activity in 0.1 M H2SO4, but are activated during operation through the electrooxidatively‐induced enrichment of the catalytic surface with Co and Sn oxides. The latter originate from the FTO support and are identified as an important component of the catalyst through control experiments with a Sn‐free substrate and with Sn2+ intentionally added at the electrodeposition stage. A distinctive feature of the Co−Bi−Sn‐based electrocatalyst is the slow but persistent improvement in the activity during operation in 0.1 M H2SO4 at both ambient and elevated (60 °C) temperatures, which contrasts with the continuously degrading behaviour of state‐of‐the‐art oxygen evolution catalysts at low pH. This is demonstrated by 9‐day‐long galvanostatic tests at 10 mA cm−2, during which the Co−Bi−Sn‐based thin film catalyst shows no degradation and sustains stable water oxidation at ca 1.9 V vs. reversible hydrogen electrode. The effects of tin leaching from the support detected herein might have implications to other acidic water oxidation catalysts supported on high‐surface area doped SnO2 materials.
SYNOPSIS. Stable oxygen evolution reaction at low pH catalysed by cobalt‐bismuth oxides is promoted by small amounts of tin leaching from the F‐doped SnO2 support.
Studies have shown that increased student engagement is correlated with improved learning outcomes and overall positive results for students. While engagement can be viewed as a precursor to other ...outcomes, it should also be examined as an outcome itself. To increase student engagement and improve the learning experience for students, we must understand which factors can facilitate engagement and how educators can positively affect these factors. This research explored the influence of three proposed facilitators of engagement: autonomy, competence, and relatedness. Self-determination theory, a theory of motivation, posits that these are three innate psychological needs that must be fulfilled to experience the highest level of motivation, for which engagement has been used as a proxy. In the format of a multiple-article dissertation, I present three articles. The first article reviewed the literature concerning self-determination theory and student engagement in both K-12 and higher education settings. It answered the following research question: "What has been found regarding the influence of autonomy, competence, and relatedness on student engagement?" The second article built upon findings from the first article and outlined the process to create and validate an instrument to measure autonomy, competence, relatedness with peers, relatedness with professors, and emotional engagement. Data were collected from university students through an online survey (n = 340). Confirmatory factor analysis results showed that survey items performed well and measured the intended constructs. Structural equation modeling was then used to identify the best fitting model for the data collected. Results showed that sense of competence had the largest predicted effect on emotional engagement. The third article employed the validated survey discussed in the second article. It was administered to students in an online higher education program (n = 3092). Confirmatory factor analysis and structural equation modeling were conducted on the sample. Students' sense of autonomy was found to have the greatest effect on emotional engagement. The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.
Studies have shown that increased student engagement is correlated with improved learning outcomes and overall positive results for students. While engagement can be viewed as a precursor to other ...outcomes, it should also be examined as an outcome itself. To increase student engagement and improve the learning experience for students, we must understand which factors can facilitate engagement and how educators can positively affect these factors. This research explored the influence of three proposed facilitators of engagement: autonomy, competence, and relatedness. Self-determination theory, a theory of motivation, posits that these are three innate psychological needs that must be fulfilled to experience the highest level of motivation, for which engagement has been used as a proxy.In the format of a multiple-article dissertation, I present three articles. The first article reviewed the literature concerning self-determination theory and student engagement in both K-12 and higher education settings. It answered the following research question: What has been found regarding the influence of autonomy, competence, and relatedness on student engagement? The second article built upon findings from the first article and outlined the process to create and validate an instrument to measure autonomy, competence, relatedness with peers, relatedness with professors, and emotional engagement. Data were collected from university students through an online survey (n = 340). Confirmatory factor analysis results showed that survey items performed well and measured the intended constructs. Structural equation modeling was then used to identify the best fitting model for the data collected. Results showed that sense of competence had the largest predicted effect on emotional engagement. The third article employed the validated survey discussed in the second article. It was administered to students in an online higher education program (n = 3092). Confirmatory factor analysis and structural equation modeling were conducted on the sample. Students' sense of autonomy was found to have the greatest effect on emotional engagement.
