Electrochemical Infrared Spectroscopy Research Articles

Ti3+ redox dynamics enabling efficient nitrate reduction to ammonia on Ti2O3

The electrochemical nitrate reduction reaction (NO3−RR) demonstrates significant promise as a sustainable pathway for ammonia (NH3) synthesis and regulating the Earth's nitrogen cycle. However, its advancement is currently hindered by low catalytic activity, particularly limited by the sluggish rate-determining step of NO2− formation. Herein, we capitalize on the highly active and versatile redox properties of Ti3+ in Ti2O3, exploiting its spontaneous reduction of NO3− to NO2− to overcome the sluggish rate-determining step. Subsequently, the electrochemical reduction process continuously reduces the generated high valence titanium species back to Ti3+, creating a redox closed loop, and enabling the continuous NH3 synthesis. The catalyst achieves an outstanding NH3 production rate of 19.04 mg h−1 mgcat.–1 with a Faradaic efficiency of 97.8 %. Using in-situ electrochemical electron paramagnetic resonance technology, we explored dynamic changes of Ti3+ in Ti2O3 during the process, while in-situ electrochemical infrared spectroscopy provided insights into intermediates' evolution. Density functional theory calculations confirmed Ti2O3′s highly active NO3−RR.

Electroreduction of CO2 in a Non-aqueous Electrolyte-The Generic Role of Acetonitrile.

Transition metal carbides, especially Mo2C, are praised to be efficient electrocatalysts to reduce CO2 to valuable hydrocarbons. However, on Mo2C in an aqueous electrolyte, exclusively the competing hydrogen evolution reaction takes place, and this discrepancy to theory was traced back to the formation of a thin oxide layer at the electrode surface. Here, we study the CO2 reduction activity at Mo2C in a non-aqueous electrolyte to avoid such passivation and to determine products and the CO2 reduction reaction pathway. We find a tendency of CO2 to reduce to carbon monoxide. This process is inevitably coupled with the decomposition of acetonitrile to a 3-aminocrotonitrile anion. Furthermore, a unique behavior of the non-aqueous acetonitrile electrolyte is found, where the electrolyte, instead of the electrocatalyst, governs the catalytic selectivity of the CO2 reduction. This is evidenced by in situ electrochemical infrared spectroscopy on different electrocatalysts as well as by density functional theory calculations.

In Situ Electrocatalytic Infrared Spectroscopy for Dynamic Reactions

The modern society is confronted with severe problems of energy consumption, climatic variation, and environmental pollution. A series of electrocatalysis reactions, including oxygen evolution reaction (OER), hydrogen evolution reaction (HER), CO2 reduction reaction (CO2RR), N2 or nitrate reduction reaction (NRR or NOxRR), and methanol or ethanol oxidation reaction (MOR or EOR), have become promising ways to solve these serious issues and realize sustainable development. However, the limited understanding of the dynamic process of electrocatalysis still hinders the rational design of these electrocatalysts. Combining infrared spectroscopy with electrocatalysis, in situ electrochemical infrared spectroscopy (in situ EIRS) is a mighty tool to acquire deep insight of dynamic electrocatalytic reaction process at molecule level. Moreover, the surface sensitive in situ EIRS could acquire information about the adsorbates and the key intermediate, making it broad prospects in the study of electrocatalytic dynamic process. In this short review, we summarize recent progress of in situ EIRS with the focus of probing dynamic reactions. In particular, we first briefly introduce the fundamental and the methodological development of in situ EIRS. Then, the recent advances of in situ EIRS applications in electrocatalytic reaction is demonstrated, including OER, HER, CO2RR, NRR, and MOR. Finally, challenges and perspectives of future research in related fields are discussed.

Modification of the Coordination Environment of Active Sites on MoC for High‐Efficiency CH4 Production

AbstractModulating the coordination environment of active sites on catalyst surfaces is crucial to developing effective catalysts and controlling catalysis. However, this may be a highly challenging procedure. Guided by the first‐principles calculations, the modification of the coordination environment of active sites on MoC nanoparticle surfaces is experimentally accomplished by anchoring pyridinic N atom rings of holey graphene on Mo atoms. The rings produce electrostatic forces that enable the tuning of the Mo sites′ affinity to reaction intermediates, which passivates Mo hollow sites, activates Mo top sites, and reduces the overadsorption of OH on the Mo active sites, as predicted by calculations. The atomic‐level modification is well confirmed by atomic‐resolution imaging, high‐resolution electron tomography, synchrotron soft X‐ray spectroscopy, and operando electrochemical infrared spectroscopy. Consequently, the Faradaic efficiency for CO2 reduction to CH4 is enhanced from 16% to 89%, a record high efficiency so far, in aqueous electrolytes. It also exhibits a negligible activity loss over 50 h.

Highly Dispersed Mo Sites on Pd Nanosheets Enable Selective Ethanol-to-Acetate Conversion.

The fermentation of biomass allows for the generation of major renewable ethanol biofuel that has high energy density favorable for direct alcohol fuel cells in alkaline media. However, selective conversion of ethanol to either CO2 or acetate remains a great challenge. Especially, the ethanol-to-acetate route usually demonstrates decentoxidation current density relative to the ethanol-to-CO2 route that contains strongly adsorbed poisons. This makes the total oxidation of ethanol to CO2 unnecessary. Here, we present a highly active ethanol oxidation electrocatalyst that was prepared by in situ decorating highly dispersed Mo sites on Pd nanosheets (MoOx/Pd) via a surfactant-free and facile route. We found that ∼2 atom % of Mo on Pd nanosheets increases the current density to 3.8 A mgPd-1, around 2 times more active relative to the undecorated Pd nanosheets, achieving nearly 100% faradic efficiency for the ethanol-to-acetate conversion in an alkaline electrolyte without the generation of detectable CO2, evidenced by in situ electrochemical infrared spectroscopy, nuclear magnetic resonance, and ion chromatography. The selective and CO2-free conversion offers a promising strategy through alcohol fuel cells for contributing comparable current density to power electrical equipment while for selective oxidation of biofuels to useful acetate intermediate for the chemical industry.

Electrochemical CO2 reduction on Pd-modified Cu foil

Bimetallic catalysts can improve CO2 reduction efficiency via the combined properties of two metals. CuPd shows enhanced CO2 reduction activity compared to copper alone. Using differential electrochemical mass spectrometry (DEMS) and electrochemical infrared (IR) spectroscopy, volatile products and adsorbed intermediates were measured during CO2 and CO reduction on Cu and CuPd. The IR band corresponding to adsorbed CO appears 300 mV more positive on CuPd than that on Cu, indicating acceleration of CO2 reduction to CO. Electrochemical IR spectroscopy measurements in CO-saturated solutions reveal similar potentials for CO adsorption and CO32− desorption on CuPd and Cu, indicating that CO adsorption is controlled by desorption of CO32−. DEMS measurements carried out during CO reduction at both electrodes showed that the onset potential for reduction of CO to CH4 and CH3OH on CuPd is about 200 mV more positive than that on Cu. We attribute these improvements to interaction of Cu and Pd, which shifts the d-band center of the Cu sites.

Toward a quantitative theoretical method for infrared and Raman spectroscopic studies on single-crystal electrode/liquid interfaces.

In situ electrochemical infrared spectroscopy and Raman spectroscopy are powerful tools for probing potential-dependent adstructures at solid/liquid electrochemical interfaces. However, it is very difficult to quantitatively interpret the observed spectral features including potential-dependent vibrational frequency and spectral intensity, even from model systems such as single-crystal electrode/liquid interfaces. The clear understanding of electrochemical vibrational spectra has remained as a fundamental issue for four decades. Here, we have developed a method to combine computational vibrational spectroscopy tools with interfacial electrochemical models to accurately calculate the infrared and Raman spectra. We found that the solvation model and high precision level in the self-consistent-field convergence are critical elements to realize quantitative spectral predictions. This method's predictive power is verified by analysis of a classic spectroelectrochemical system, saturated CO molecules electro-adsorbed on a Pt(111) electrode. We expect that this method will pave the way to precisely reveal the physicochemical mechanism in some electrochemical processes such as electrocatalytic reactions.

Novel Electrochemical Pretreatment for Preferential Removal of Nonylphenol in Industrial Wastewater: Biodegradability Improvement and Toxicity Reduction.

Preferential pretreatment of nonylphenol (NP) before biological treatment is of great significance due to its horizontal gene transfer effect and endocrine disruption activity. A novel molecular imprinting high-index facet SnO2 (MI-SnO2,HIF) electrode is designed. NP was effectively removed from industrial wastewater at 1.8 V with totally suppressing human estrogen activity. The ratio of 5 day biological oxygen demand to chemical oxygen demand (BOD5/CODCr) was enhanced to 0.412 from 0.186 after preferential pretreatment. The effluent concentration of NP was 6.4 μg L-1 after further simulating anaerobic-anoxic-oxic treatment, which was about 1/10 of that without pretreatment. This preferential electrochemical pretreatment is interpreted as prior adsorption and enrichment of target pollutants on the MI-SnO2,HIF surface. The reactive oxygen species and subsequent oxidation products were investigated by in situ electron paramagnetic resonance and electrochemical infrared spectroscopy. The degradation pathway of NP was further analyzed by liquid chromatography-mass spectrometry. This unique pretreatment method for a complex tannery wastewater system has irreplaceable status because no methods with similar advantages have been reported, expecting to be widely used in preferential pretreatment of toxic contaminants blended with highly concentrated nontoxic organics.

Origin of Carbon Dioxide Evolved during Cycling of Nickel-Rich Layered NCM Cathodes.

Gas formation caused by parasitic side reactions is one of the fundamental concerns in state-of-the-art lithium-ion batteries because gas bubbles might block local parts of the electrode surface, hindering lithium transport and leading to inhomogeneous current distributions. Here, we elucidate on the origin of CO2, which is the dominant gaseous species associated with the layered lithium nickel cobalt manganese oxide (NCM) cathode, by implementing isotope labeling and electrolyte substitution in differential electrochemical mass spectrometry-differential electrochemical infrared spectroscopy measurements. Li2CO3 on the NCM surface was successfully labeled with 13C via a process that involves its removal followed by intentional growth. In situ gas analytics on such NCM samples with 13C-labeled Li2CO3 clearly indicate that Li2CO3 decomposition contributes to CO2 evolution, especially during the first charge. At the same time, the greater contribution of electrolyte decomposition was indicated by the large amount of 12CO2 observed. Employment of butyronitrile as the electrolyte solvent in further measurements helped determine that the majority of electrolyte decomposition occurs via a reaction that involves the lattice oxygen of NCM.

Investigation on Sulfide-Enhanced Oxygen Reduction Reaction Activity By in-Situ Electrochemical Infrared Spectroscopy

Oxygen reduction reaction (ORR) has been intensively investigated due to its relevance to fuel cell applications. It was discovered recently that sulfide (S2-), a potent poison of Pt, could be a promotor for both ORR activity and stability of Pt. However, the underlying chemistry of working mechanism is still unclear. Understanding the mechanistic effect of non-metallic promotor(s) should help broaden the prospect of developing better ORR catalysts. In this presentation we will discuss the insights we have gained in this respect by in-situ electrochemical infrared spectroscopy and theoretical simulations. Figure 1a and b show the δ(HOO) infrared vibration bands of ORR intermediates HOOad and HOOHad and v(O-O) vibration of O2− ad under the kinetic potential control regime (at ~0.9 V vs. RHE) on PtSad with different sulfur coverage. The IR bands of intermediates HOOad and HOOHad are larger at lower Sad coverages at which the enhanced ORR activity was observed. The observation of the O2− ad band at higher Sad coverage is consistent with the increase in H2O2 production during ORR. Interestingly, even a small fraction of Sad, say a coverage of 0.1, can play a significant role in modifying the ORR reaction pathway by preventing the cleavage of O-O bonding of O2 molecule. The theoretical simulations in Figure 1c~d confirmed that the turnover frequency (TOF) of H2O produced at 0.9 V by ORR on PtSlow is much bigger than that on PtShigh. As a conclusion, our finding suggests that the catalyst design by shifting the pathway of ORR from cleavage of O-O bonding on pure Pt to HOOad or HOOHad pathway would accelerate water production on the fuel cell cathode.

Evidence for diffusional coupling in electrochemical thin layers: implications for surface coverage calibration via electrochemical infrared spectroscopy

The time dependence of the concentration of CO2 in an electrochemical thin layer cavity is studied with Fourier transform infrared spectroscopy (FTIR) in order to evaluate the extent to which the thin layer cavity is diffusionally decoupled from the surrounding bulk electrolyte. For the model system of CO on Pt(111) in 0.1 M HClO4, it is found that the concentration of CO2, formed by electro-oxidation of CO, equilibrates rapidly with the surrounding bulk electrolyte. This rapid equilibration indicates that there is diffusion out of the thin layer, even on the short time scales of typical infrared experiments (1–3 min). However, since the measured CO2 absorbance intensity as a function of time is reproducible to within 10%, a new time-dependent method for surface coverage calibration using solution-phase species is proposed.

A Versatile Surface Modification Scheme for Attaching Metal Nanoparticles onto Gold: Characterization by Electrochemical Infrared Spectroscopy

A simple method for preparing metal nanoparticle films immobilized on gold substrates is described. A variety of nanoparticle films are characterized by electrochemical infrared reflection-absorption spectroscopy (EC-IRAS) in which the anomalous negative-absorbance properties commonly observed in metal particle arrays can be completely controlled. Such anti-absorbance v C O band components obfuscating the EC-IRAS data interpretation are associated with the complex dielectric behavior induced by metal nanoparticles aggregates. To achieve a well distributed metal nanoparticle array, the substrates were pretreated with 3-mercaptopropyltrimethoxysilane before anchoring gold, platinum or platinum-ruthenium alloy nanoparticles on the gold substrates. The prepared nanoparticle films displayed excellent electrochemical properties implying facile electronic communication through the organic glue matrix between the nanoparticle arrays and the gold substrates. Coating of the gold nanoparticle arrays with platinum via copper underpotential deposition (UPD) steps furthermore demonstrates optimal electronic response between the nanoparticle arrays and the underlying substrate. These findings will facilitate better nanoparticle analysis by electrochemical and optical spectroscopic means.

New developments in electrochemical infrared spectroscopy: adlayer structures of carbon monoxide on monocrystalline metal electrodes

Coverage- and electrode potential-dependent infrared spectra for carbon monoxide on low-index platinum- and rhodium-aqueous interfaces are examined alongside related data for the corresponding metal-uhv interfaces in order to compare the CO adlayer structures in these two types of surface environment. Broadly speaking, the electrochemical systems display a greater propensity for bridging versus terminal CO coordination, especially at the lowest electrode potentials and smallest coverages. These findings can be understood largely in terms of the differences in surface potential between the electrochemical and uhv systems together with the influence of water and/or hydrogen coadsorption in the former case. The formation of extensive CO domains (“islands”) during adiayer electrooxidation, which are largely absent during CO adsorption, is deduced from dipole coupling measurements using 12CO/ 13CO mixtures. The substantial shifts in CO site occupancy in the presence of coadsorbed Bi is also noted. Some implications of the adlayer structures to electrooxidation kinetics are pointed out. The charge-dependent properties of soluble high-nuclearity platinum carbonyl clusters are briefly noted in relation to the behavior of electrosorbed CO.