Mixed Ni–Fe oxides are attractive anode catalysts for efficient water splitting in solar fuels reactors. Because of conflicting past reports, the catalytically active metal redox state of the ...catalyst has remained under debate. Here, we report an in operando quantitative deconvolution of the charge injected into the nanostructured Ni–Fe oxyhydroxide OER catalysts or into reaction product molecules. To achieve this, we explore the oxygen evolution reaction dynamics and the individual faradaic charge efficiencies using operando differential electrochemical mass spectrometry (DEMS). We further use X-ray absorption spectroscopy (XAS) under OER conditions at the Ni and Fe K-edges of the electrocatalysts to evaluate oxidation states and local atomic structure motifs. DEMS and XAS data consistently reveal that up to 75% of the Ni centers increase their oxidation state from +2 to +3, while up to 25% arrive in the +4 state for the NiOOH catalyst under OER catalysis. The Fe centers consistently remain in the +3 state, regardless of potential and composition. For mixed Ni100–x Fe x catalysts, where x exceeds 9 atomic %, the faradaic efficiency of O2 sharply increases from ∼30% to 90%, suggesting that Ni atoms largely remain in the oxidation state +2 under catalytic conditions. To reconcile the apparent low level of oxidized Ni in mixed Ni–Fe catalysts, we hypothesize that a kinetic competition between the (i) metal oxidation process and the (ii) metal reduction step during O2 release may account for an insignificant accumulation of detectable high-valent metal states if the reaction rate of process (ii) outweighs that of (i). We conclude that a discussion of the superior catalytic OER activity of Ni–FeOOH electrocatalysts in terms of surface catalysis and redox-inactive metal sites likely represents an oversimplification that fails to capture essential aspects of the synergisms at highly active Ni–Fe sites.
In-situ X-ray absorption spectroscopy (XAS) at the oxygen K-edge was used to investigate the role of oxygen during the oxygen evolution reaction (OER) in an electrodeposited Ni-Fe(O
H
) ...electrocatalyst in alkaline pH. We show the rise of a pre-peak feature at 529 eV in the O K-edge spectra, correlated to the appearance of a shoulder at the Ni L
-edge and formation of oxidized Ni
-O. Then, for the first time, we track the spectral changes in a dynamic fashion in both the soft and hard X-ray regimes during cyclic voltammetry (in situ CV-XAS) to obtain a fine-tuned resolution of the potential-related changes. The pre-peak feature at the O K-edge likely signifies formation of an electron deficient oxygen site. The electrophilic oxygen species appears and disappears reversibly in correlation with the Ni
↔ Ni
process, and persists during OER catalysis as long the metal is oxidized. Our study provides new insight into OER electrocatalysis: Before onset of the O-O bond formation step, the catalytic oxyhydroxide has accumulated electron deficiencies by both, oxidation of transition metal ions and formation of partially oxidized oxygen sites.
Increased attention has been directed toward generating nonequilibrium hot carriers resulting from the decay of collective electronic oscillations on metal known as surface plasmons. Despite numerous ...experimental endeavors, demonstrating hot carrier-mediated photocatalysis without a heating contribution has proven challenging, particularly for single electron transfer reactions where the thermal contribution is generally detrimental. An innovative engineering solution is proposed to enable single electron transfer reactions with plasmonics. It consists of a photoelectrode designed as an energy filter and photocatalysis performed with light function modulation instead of continuously. The photoelectrode, consisting of FTO/TiO2 amorphous (10 nm)/Au nanoparticles, with TiO2 acting as a step-shape energy filter to enhance hot electron extraction and charge-separated state lifetime. The extracted hot electrons were directed toward the counter electrode, while the hot holes performed a single electron transfer oxidation reaction. Light modulation prevented local heat accumulation, effectively decoupling hot carrier catalysis from the thermal contribution.
The zinc/copper hexacyanoferrate (Zn/CuHCF) cell has gained attention as an aqueous rechargeable zinc-ion battery (ZIB) owing to its open framework, excellent rate capability, and high safety. ...However, both the Zn anode and the CuHCF cathode show unavoidable signs of aging during cycling, though the underlying mechanisms have remained somewhat ambiguous. Here, we present an in-depth study of the CuHCF cathode by employing various X-ray spectroscopic techniques. This allows us to distinguish between structure-related aging effects and charge compensation processes associated with electroactive metal centers upon Zn2+ ion insertion/deinsertion. By combining high-angle annular dark-field-scanning electron transmission microscopy, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy, and elemental analysis, we reconstruct the picture of both the bulk and the surface. First, we identify a set of previously debated X-ray diffraction peaks appearing at early stages of cycling (below 200 cycles) in CuHCF. Our data suggest that these peaks are unrelated to hypothetical Zn x Cu1–x HCF phases or to oxidic phases, but are caused by partial intercalation of ZnSO4 into graphitic carbon. We further conclude that Cu is the unstable species during aging, whose dissolution is significant at the surface of the CuHCF particles. This triggers Zn2+ ions to enter newly formed Cu vacancies, in addition to native Fe vacancies already present in the bulk, which causes a reduction of nearby metal sites. This is distinct from the charge compensation process where both the Cu2+/Cu+ and Fe3+/Fe2+ redox couples participate throughout the bulk. By tracking the K-edge fluorescence using operando XAS coupled with cyclic voltammetry, we successfully link the aging effect to the activation of the Fe3+/Fe2+ redox couple as a consequence of Cu dissolution. This explains the progressive increase in the voltage of the charge/discharge plateaus upon repeated cycling. We also find that SO4 2– anions reversibly insert into CuHCF during charge. Our work clarifies several intriguing structural and redox-mediated aging mechanisms in the CuHCF cathode and pinpoints parameters that correlate with the performance, which will hold importance for the development of future Prussian blue analogue-type cathodes for aqueous rechargeable ZIBs.
Graphite electrodes offer remarkable electrochemical properties, emerging as a viable alternative to glassy carbon (GCE) and other carbon-based electrodes for fundamental electrochemistry research. ...We report the fabrication and characterization of high-purity graphite disk electrodes (GDEs), made from cost-effective materials and a solvent-free methodology employing readily available laboratory equipment. Analysis of their physical properties via SEM, EDX and XPS reveals no metallic interferences and a notably high porosity, emphasizing their potential. The electrochemical performances of GDEs were found to be comparable to those of GCE. Immobilization of peptides and enzymes, both via covalent coupling and surface adsorption, was used to explore potential applications of GDEs in bioelectrochemistry. Enzyme activity could be addressed both via direct electron transfer and mediated electron transfer mechanism. These results highlight the interesting properties of our GDEs and make them a low-cost alternative to other carbon-based electrodes, with potential for future real-world applications.
Metal–organic frameworks (MOFs) are periodic organic–inorganic materials that have garnered considerable attention for electrocatalytic applications due to their wide tunability. Metal-hydroxide ...organic frameworks (MHOFs), a subset of MOFs that combine layered metal hydroxides with organic ligands of various π–π stacking energy, have shown promising catalytic functions, such as for the oxygen evolution reaction (OER). The long-term electrochemical stability of these materials for the OER is unfortunately not well understood, which is critical to design practical devices. In this study, we investigated how Ni-based MHOFs composed of two linkers with different π–π interaction strength (terephthalate; L1 and azobenzene-4,4′-dicarboxylate; L4) change as a function of cycle number and potential for the OER. All MHOFs tested showed significant increases in the number of electrochemically active Ni sites and OER activity when cycled. MHOFs constructed using the linkers with stronger π–π stacking energy (L4) were observed to remain intact in bulk with only near-surface transformations to NiOOH2–x -like phases, whereas MHOFs with linkers of weaker π–π stacking energy (L1) showed complete reconstruction to NiOOH2–x -like phases. This was confirmed using X-ray diffraction, X-ray absorption spectroscopy, and electron microscopy. Further, in situ characterization using Raman and UV–vis revealed that the presence of stable linkers within the MHOF structure suppresses the Ni2+/Ni(3+δ)+ redox process. We further identify NiOOH2–x as the OER active phase, while the MHOF phase serves as a precatalyst. We further propose a detailed mechanism for the phase transformation, which provides valuable insights into the future challenges for the design of both stable and catalytically active MOF-based materials for water oxidation.
Nickel–vanadium layered double hydroxide has recently been considered as a highly active, low-cost electrocatalyst and as a benchmark non-noble metal-based electrocatalyst for water oxidation. The ...material showed a current density of 27 mA/cm2 at an overpotential of 350 mV, which is comparable to the best-performing nickel–iron-layered double hydroxides for water oxidation in alkaline media. The enhanced conductivity and facile electron transfer were suggested among important factors for the high activity of nickel–vanadium layered double hydroxide. In the present study, the stability of an Ni–V catalyst was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and electrochemical characterization methods. These methods show that the initial Ni–V catalyst during water oxidation in alkaline conditions is converted from an initial α-Ni(OH)2 phase to a partially oxidized α-Ni(OH)2/NiOOH phase and VO4 3– ions. We carefully evaluate the stability of the catalysts and analyze the compositional changes during prolonged water-oxidation conditions using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The experiments using both Fe-free electrolyte and Fe-free nickel–vanadium layered double hydroxide reveal that vanadium do not affect the water-oxidizing activity of α-Ni(OH)2.
•Deactivation of the OER activity is observed in the NiFeOx catalyst in pH 7.•Oxygen evolution activity is partly recovered when transferred back to pH 13.•Correlations between crystallinity, ...composition, and activity are established.•Limited correlation is found between the Ni redox charge and the OER activity.•In situ DEMS analysis shows formation of degradation products at lower pH.
Mixed Ni-Fe oxide electrocatalysts have shown high catalytic activity for the oxygen evolution reaction (OER) in alkaline electrolyte. Fundamental research on mixed Ni-Fe OER catalysts has largely focused on high pH, while the OER activity near neutral pH has remained poorly studied.
Here we review the activity of an amorphous mixed Ni-Fe oxyhydroxide catalyst supported on carbon (NiFeOx/C) in 0.1M KOH pH 13, in 0.1M borate buffer (Bi) pH 9.2, and in 0.1M phosphate buffer (Pi) pH 7.0. The OER catalytic performance was found to decrease in the order of pH 13>pH 9.2>pH 7. In contrast to pH 13 and 9.2, the catalyst cycled in pH 7 showed an instantaneous decrease in OER activity and a simultaneous loss of the Ni(OH)2/NiOOH redox peak. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) showed the formation of crystalline areas upon CV cycling, which appeared more Ni enriched after cycling in pH 7. Deactivated electrodes cycled in pH 13 recovered the OER activity along with a partial reappearance of the Ni redox peak when subsequently cycled in pH 13. SEM-EDX spectroscopy confirmed compositional changes in the bulk during cycling at different pH, with an extensive leaching of Ni in pH 7. Our study provides new insight into the OER activity upon exposure to different electrolyte conditions, which unveils a highly dynamic Ni-Fe oxide framework.
We present an unusual, yet facile, strategy towards formation of physically mixed Ni-Fe(OxHy) oxygen evolution electrocatalysts. We use in situ X-ray absorption and UV-vis spectroscopy, and ...high-resolution imaging to demonstrate that physical contact between two inferior Ni(OH)2 and Fe(OOH) catalysts self-assemble into atomically intermixed Ni-Fe catalysts with unexpectedly high activity.
Ni–Fe oxyhydroxides are the most active known electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes and are therefore of great scientific and technological importance in ...the context of electrochemical energy conversion. Here we uncover, investigate, and discuss previously unaddressed effects of conductive supports and the electrolyte pH on the Ni–Fe(OOH) catalyst redox behavior and catalytic OER activity, combining in situ UV–vis spectro-electrochemistry, operando electrochemical mass spectrometry (DEMS), and in situ cryo X-ray absorption spectroscopy (XAS). Supports and pH > 13 strongly enhanced the precatalytic voltammetric charge of the Ni–Fe oxyhydroxide redox peak couple, shifted them more cathodically, and caused a 2–3-fold increase in the catalytic OER activity. Analysis of DEMS-based faradaic oxygen efficiency and electrochemical UV–vis traces consistently confirmed our voltammetric observations, evidencing both a more cathodic O2 release and a more cathodic onset of Ni oxidation at higher pH. Using UV–vis, which can monitor the amount of oxidized Ni+3/+4 in situ, confirmed an earlier onset of the redox process at high electrolyte pH and further provided evidence of a smaller fraction of Ni+3/+4 in mixed Ni–Fe centers, confirming the unresolved paradox of a reduced metal redox activity with increasing Fe content. A nonmonotonic super-Nernstian pH dependence of the redox peaks with increasing Fe contentdisplaying Pourbaix slopes as steep as −120 mV/pHsuggested a two proton–one electron transfer. We explain and discuss the experimental pH effects using refined coupled (PCET) and decoupled proton transfer–electron transfer (PT/ET) schemes involving negatively charged oxygenate ligands generated at Fe centers. Together, we offer new insight into the catalytic reaction dynamics and associated catalyst redox chemistry of the most important class of alkaline OER catalysts.
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