Abstract:In addition to the XANES region, at higher energies the extended X-ray absorption fine structure (EXAFS) region is found. The spectral shape in the near-edge region is determined by electronic density of states effects and gives mainly information about the electronic properties and the local geometry of the absorbing atom. The EXAFS region is dominated by single scattering events of the outgoing electron on the neighboring atoms, providing mainly information about the local geometric structure around the abso… Show more
“…The measurements of the LiMn 2 O 4 electrodes had to be made in fluorescence yield (FY) mode due to the comparably low loading required by the electrocatalytic measurements. The FY mode only approximates an X-ray absorption spectrum [23,45] and depends on the measurement geometry. Nonetheless, the spectrum of a sample held at 1.55 V vs. RHE for 1 h at pH 14 is congruent with the pristine sample and only deviates at the maximum near 6560 eV, where lower amplitudes are a common artefact of FY measurements.…”
Chemical and structural changes preceding electrocatalysis obfuscate the nature of the active state of electrocatalysts for the oxygen evolution reaction (OER), which calls for model systems to gain systematic insight. We investigated the effect of bulk oxidation on the overpotential of ink‐casted LiMn2O4 electrodes by a rotating ring‐disk electrode (RRDE) setup and X‐ray absorption spectroscopy (XAS) at the K shell core level of manganese ions (Mn−K edge). The cyclic voltammogram of the RRDE disk shows pronounced redox peaks in lithium hydroxide electrolytes with pH between 12 and 13.5, which we assign to bulk manganese redox based on XAS. The onset of the OER is pH‐dependent on the scale of the reversible hydrogen electrode (RHE) with a Nernst slope of −40(4) mV/pH at −5 μA monitored at the RRDE ring. To connect this trend to catalyst changes, we develop a simple model for delithiation of LiMn2O4 in LiOH electrolytes, which gives the same Nernst slope of delithiation as our experimental data, i. e., 116(25) mV/pH. From this data, we construct an ERHE‐pH diagram that illustrates robustness of LiMn2O4 against oxidation above pH 13.5 as also verified by XAS. We conclude that manganese oxidation is the origin of the increase of the OER overpotential at pH lower than 14 and also of the pH dependence on the RHE scale. Our work highlights that vulnerability to transition metal redox may lead to increased overpotentials, which is important for the design of stable electrocatalysts.
“…The measurements of the LiMn 2 O 4 electrodes had to be made in fluorescence yield (FY) mode due to the comparably low loading required by the electrocatalytic measurements. The FY mode only approximates an X-ray absorption spectrum [23,45] and depends on the measurement geometry. Nonetheless, the spectrum of a sample held at 1.55 V vs. RHE for 1 h at pH 14 is congruent with the pristine sample and only deviates at the maximum near 6560 eV, where lower amplitudes are a common artefact of FY measurements.…”
Chemical and structural changes preceding electrocatalysis obfuscate the nature of the active state of electrocatalysts for the oxygen evolution reaction (OER), which calls for model systems to gain systematic insight. We investigated the effect of bulk oxidation on the overpotential of ink‐casted LiMn2O4 electrodes by a rotating ring‐disk electrode (RRDE) setup and X‐ray absorption spectroscopy (XAS) at the K shell core level of manganese ions (Mn−K edge). The cyclic voltammogram of the RRDE disk shows pronounced redox peaks in lithium hydroxide electrolytes with pH between 12 and 13.5, which we assign to bulk manganese redox based on XAS. The onset of the OER is pH‐dependent on the scale of the reversible hydrogen electrode (RHE) with a Nernst slope of −40(4) mV/pH at −5 μA monitored at the RRDE ring. To connect this trend to catalyst changes, we develop a simple model for delithiation of LiMn2O4 in LiOH electrolytes, which gives the same Nernst slope of delithiation as our experimental data, i. e., 116(25) mV/pH. From this data, we construct an ERHE‐pH diagram that illustrates robustness of LiMn2O4 against oxidation above pH 13.5 as also verified by XAS. We conclude that manganese oxidation is the origin of the increase of the OER overpotential at pH lower than 14 and also of the pH dependence on the RHE scale. Our work highlights that vulnerability to transition metal redox may lead to increased overpotentials, which is important for the design of stable electrocatalysts.
“…Detailed explanations of the XAS and XES theories fall outside the scope of this paper and can be found in numerous recent reviews, on core‐level spectroscopy generality (de Groot and Kotani, 2008), XANES (Henderson et al, 2014; Joly and Grenier, 2016), EXAFS (Newville, 2014), XAS (Rehr and Albers, 2000; Ortega et al, 2012; Milne et al, 2014), XES (DeBeer and Bergmann, 2016), and HERFD (Bauer, 2014; Kowalska et al, 2016) applied to bioionorganic chemistry, and applications in environmental sciences (Gräfe et al, 2014). We will briefly recall the main principles of the core‐level spectroscopy techniques with special attention to effects that can be better analyzed with CAS.…”
The study of the speciation of highly diluted elements by X-ray absorption spectroscopy (XAS) is extremely challenging, especially in environmental biogeochemistry sciences. Here we present an innovative synchrotron spectroscopy technique: high-energy resolution fluorescence detected XAS (HERFD-XAS). With this approach, measurement of the XAS signal in fluorescence mode using a crystal analyzer spectrometer with a ∼1-eV energy resolution helps to overcome restrictions on sample concentrations that can be typically measured with a solid-state detector. We briefly describe the method, from both an instrumental and spectroscopic point of view, and emphasize the effects of energy resolution on the XAS measurements. We then illustrate the positive impact of this technique in terms of detection limit with two examples dealing with Ce in ecologically relevant organisms and with Hg species in natural environments. The sharp and well-marked features of the HERFD-X-ray absorption near-edge structure spectra obtained enable us to determine unambiguously and with greater precision the speciation of the probed elements. This is a major technological advance, with strong benefits for the study of highly diluted elements using XAS. It also opens new possibilities to explore the speciation of a target chemical element at natural concentration levels, which is critical in the fields of environmental and biogeochemistry sciences.
“…Optical and X‐ray absorption spectroscopies enable to probe TM speciation: XANES is an element‐specific technique while optical absorption spectroscopy probes only coloring species. XANES is widely used to determine the speciation and the redox state of TM in glass including ancient glasses . Optical absorption spectroscopy is a key complementary technique that can relate the glass color to glass composition and fabrication conditions …”
Potash‐ and soda‐lime‐stained glasses from the 12th–13th centuries, blue‐colored by cobalt, have been investigated by Mn, Fe, and Cu K‐edge X‐ray and optical absorption spectroscopies in order to determine the oxidation state of these elements and their impact on the blue color. Remelting these historical glasses in air at 1200°C, the estimated temperature of medieval furnaces, revealed that these four glasses are more reduced before remelting. This favors Mn as weakly absorbing Mn2+, Fe as Fe2+ and Cu as colorless Cu+. Therefore Fe2+ is the second blue chromophore and copper was not intentionally used by glassmakers to obtain a blue color. A colorimetric analysis indicates that these specific melting conditions have a limited effect on the blue color of these glasses. Based on the spectroscopic determination of the redox state of Fe, Mn, and Cu, we estimate the oxygen partial pressure in medieval furnaces to be 10−7–10−9 and 10−5 bar for the potash‐ and soda‐lime samples, respectively. The comparison with previous results enables to prove the evolution of furnace technology over centuries.
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