Direct observation of bicarbonate and water reduction on gold: understanding the potential dependent proton source during hydrogen evolution

Investigating Surface pKa and pH Using Surface-Enhanced Raman Scattering Spectroscopy with 4-Mercaptobenzoic Acid in Deionized Water and Sodium Bicarbonate Electrolytes

Our research presents spectroscopic measurements of the surface pKa at electrode/electrolyte interfaces using surface-enhanced Raman scattering spectroscopy of 4-mercaptobenzoic acid (4-MBA). As the electrochemical potential is varied from negative to positive, the Raman intensity of the -COOH functional group (at 1700 cm-1) decreases while the -COO- Raman intensity (at 1410 cm-1) increases. The protonation-deprotonation of this surface-bound molecule reflects an electrochemically induced shift to more acidic conditions at negative potentials and more basic conditions at positive potentials. By fitting the data to a normalized sigmoid function, we obtain the percentage of surface protonation/deprotonation, which can be related to the surface pKa of the system. The percentage of surface protonation, which gives a proxy of the two-dimensional surface pKa, follows the Fermi-Dirac distribution as a function of the applied potential. The electrolyte-electrode pH-neutral conditions at the interface are extracted by the linear fitted intercepts of -log(COO-/COOH) as a function of the applied potential based on the Nernst equation, which are 0.25, 0.07, 0.08, and -0.46 V for DI water and 0.5 M sodium bicarbonate solutions with and without CO2 purging, respectively. The shift of surface neutral conditions toward more positive voltages in the electrolytes with CO2 purging indicates that the bulk solutions dissolved in the CO2-dissolved form become more acidic. The 25% reduction of protonation at negative applied potentials in CO2-purged DI water suggests that the direct reduction of hydronium ions and/or carbonic acid increases the surface pKa in the microenvironment. Adding alkali cations (Na+) attracts more proton donors toward the working electrode, resulting in the protonation capacity near the electrode surface, approximately -1.9 V-1, being double that of DI water, which is around -1 V-1. Hydrogen evolution reaction pathways are not detected in neutral or basic conditions due to the low concentration of hydronium ions (<10-6 M). The independence of the carbonic acid concentration with applied negative potentials, as measured by the surface pKa in the Helmholtz plane, indicates that changes in the local pH/surface pKa under neutral or basic bulk conditions are governed by the acid-base equilibrium of water, carbonic acid, bicarbonate, and carbonate ions.

Insights into Electrochemical Nitrate Reduction to Nitrogen on Metal Catalysts for Wastewater Treatment

Electrocatalytic nitrate reduction reaction (NO3RR) to harmless nitrogen (N2) presents a viable approach for purifying NO3--contaminated wastewater, yet most current electrocatalysts predominantly produce ammonium/ammonia (NH4+/NH3) due to challenges in facilitating N-N coupling. This study focuses on identifying metal catalysts that preferentially generate N2 and elucidating the mechanistic origins of their high selectivity. Our evaluation of 16 commercially available metals reveals that only Pb, Sn, and In demonstrated substantial N2 selectivity (79.3, 70.0, and 57.0%, respectively, under conditions of 6 h electrolysis, a current density of 10 mA/cm2, and an initial NO3--N concentration of 100 mg/L), while others largely favored NH4+ production. Comprehensive experimental and theoretical analyses indicate that NH4+-selective catalysts (e.g., Co) exhibited high water activity that enhances •H coverage, thereby promoting the hydrogenation of NO3- to NH4+ through the hydrogen atom transfer mechanism. In contrast, N2-selective catalysts, with their lower water activity, promoted the formation of N-containing intermediates, which likely undergo dimerization to form N2 via the proton-coupled electron transfer mechanism. Enhancing NO3- adsorption was beneficial to improve N2 selectivity by competitively reducing •H coverage. Our findings highlight the crucial role of water activity in NO3RR performance and offer a rational design of electrocatalysts with enhanced N2 selectivity.

How to Assess and Predict Electrical Double Layer Properties. Implications for Electrocatalysis

The electrical double layer (EDL) plays a central role in electrochemical energy systems, impacting charge transfer mechanisms and reaction rates. The fundamental importance of the EDL in interfacial electrochemistry has motivated researchers to develop theoretical and experimental approaches to assess EDL properties. In this contribution, we review recent progress in evaluating EDL characteristics such as the double-layer capacitance, highlighting some discrepancies between theory and experiment and discussing strategies for their reconciliation. We further discuss the merits and challenges of various experimental techniques and theoretical approaches having important implications for aqueous electrocatalysis. A strong emphasis is placed on the substantial impact of the electrode composition and structure and the electrolyte chemistry on the double-layer properties. In addition, we review the effects of temperature and pressure and compare solid-liquid interfaces to solid-solid interfaces.

Integrated Carbon Dioxide Capture by Amines and Conversion to Methane on Single-Atom Nickel Catalysts

Direct electrochemical reduction of carbon dioxide (CO2) capture species, i.e., carbamate and (bi)carbonate, can be promising for CO2 capture and conversion from point-source, where the energetically demanding stripping step is bypassed. Here, we describe a class of atomically dispersed nickel (Ni) catalysts electrodeposited on various electrode surfaces that are shown to directly convert captured CO2 to methane (CH4). A detailed study employing X-ray photoelectron spectroscopy (XPS) and electron microscopy (EM) indicate that highly dispersed Ni atoms are uniquely active for converting capture species to CH4, and the activity of single-atom Ni is confirmed using control experiments with a molecularly defined Ni phthalocyanine catalyst supported on carbon nanotubes. Comparing the kinetics of various capture solutions obtained from hydroxide, ammonia, primary, secondary, and tertiary amines provide evidence that carbamate, rather than (bi)carbonate and/or dissolved CO2, is primarily responsible for CH4 production. This conclusion is supported by 13C nuclear magnetic resonance (NMR) spectroscopy of capture solutions as well as control experiments comparing reaction selectivity with and without CO2 purging. These findings are understood with the help of density functional theory (DFT) calculations showing that single-atom nickel (Ni) dispersed on gold (Au) is active for the direct reduction of carbamate, producing CH4 as the primary product. This is the first example of direct electrochemical conversion of carbamate to CH4, and the mechanism of this process provides new insight on the potential for integrated capture and conversion of CO2 directly to hydrocarbons.

Efficient ammonia synthesis from the air using tandem non-thermal plasma and electrocatalysis at ambient conditions

While electrochemical N2 reduction presents a sustainable approach to NH3 synthesis, addressing the emission- and energy-intensive limitations of the Haber-Bosch process, it grapples with challenges in N2 activation and competing with pronounced hydrogen evolution reaction. Here we present a tandem air-NOx-NOx--NH3 system that combines non-thermal plasma-enabled N2 oxidation with Ni(OH)x/Cu-catalyzed electrochemical NOx- reduction. It delivers a high NH3 yield rate of 3 mmol h-1 cm-2 and a corresponding Faradaic efficiency of 92% at -0.25 V versus reversible hydrogen electrode in batch experiments, outperforming previously reported ones. Furthermore, in a flow mode concurrently operating the non-thermal plasma and the NOx- electrolyzer, a stable NH3 yield rate of approximately 1.25 mmol h-1 cm-2 is sustained over 100 h using pure air as the intake. Mechanistic studies indicate that amorphous Ni(OH)x on Cu interacts with hydrated K+ in the double layer through noncovalent interactions and accelerates the activation of water, enriching adsorbed hydrogen species that can readily react with N-containing intermediates. In situ spectroscopies and density functional theory (DFT) results reveal that NOx- adsorption and their hydrogenation process are optimized over the Ni(OH)x/Cu surface. This work provides new insights into electricity-driven distributed NH3 production using natural air at ambient conditions.

Tuning the Interfacial Reaction Environment for CO2 Electroreduction to CO in Mildly Acidic Media

A considerable carbon loss of CO2 electroreduction in neutral and alkaline media severely limits its industrial viability as a result of the homogeneous reaction of CO2 and OH- under interfacial alkalinity. Here, to mitigate homogeneous reactions, we conducted CO2 electroreduction in mildly acidic media. By modulating the interfacial reaction environment via multiple electrolyte effects, the parasitic hydrogen evolution reaction is suppressed, leading to a faradaic efficiency of over 80% for CO on the planar Au electrode. Using the rotating ring-disk electrode technique, the Au ring constitutes an in situ CO collector and pH sensor, enabling the recording of the Faradaic efficiency and monitoring of interfacial reaction environment while CO2 reduction takes place on the Au disk. The dominant branch of hydrogen evolution reaction switches from the proton reduction to the water reduction as the interfacial environment changes from acidic to alkaline. By comparison, CO2 reduction starts within the proton reduction region as the interfacial environment approaches near-neutral conditions. Thereafter, proton reduction decays, while CO2 reduction takes place, as the protons are increasingly consumed by the OH- electrogenerated from CO2 reduction. CO2 reduction reaches its maximum Faradaic efficiency just before water reduction initiates. Slowing the mass transport lowers the proton reduction current, while CO2 reduction is hardly influenced. In contrast, appropriate protic anion, e.g., HSO4- in our case, and weakly hydrated cations, e.g., K+, accelerate CO2 reduction, with the former providing extra proton flux but higher local pH, and the latter stabilizing the *CO2- intermediate.