Boosting Electrochemical Denitrification and Resource Recovery in Low-Concentration Nitrate via Proton Transfer Adjustment.

On the Conversion Efficiency of CO2 Electroreduction on Gold

ABSTRACTAcidic CO2 electroreduction reaction (CO2RR) offers a carbon-negative pathway for synthesizing value-added chemicals with high carbon efficiency but is significantly hindered by the competing hydrogen evolution reaction (HER). While concentrated K+ cations have been extensively employed to suppress HER and improve CO2RR selectivity, they inevitably trigger catastrophic salt precipitation that degrades the durability of the electrolyzer. Here, we pioneer an anion engineering strategy that breaks the cation-concentration paradigm through manipulating proton transfer dynamics. Combining mass spectrometry, spectroscopic techniques and theoretical calculations, we reveal that hydrolyzable anions improve proton transfer via forming protonated species that simultaneously strengthen hydrogen-bond networks and lower the kinetic barrier for *H intermediate formation, thereby promoting HER. By leveraging this fundamental insight, we achieved highly selective CO2RR with 87.3% Faradaic efficiency in strong acidic conditions (pH 1) at a low K+ concentration (0.2 M) using non-hydrolyzable Cl−. Our work provides a paradigm shift from cation-centric to anion-dominated electrolyte design, and establishes anion hydrolysis tendency as a crucial descriptor of electrocatalytic performance in acidic CO2RR systems.

Suppressing the competing hydrogen evolution reaction in CO2 electroreduction: A review

Comprehensive SummaryTuning electrolyte properties is a widely recognized strategy to enhance activity and selectivity in electrocatalysis, drawing increasing attention in this domain. Despite extensive experimental and theoretical studies, debates persist about how various electrolyte components influence electrocatalytic reactions. We offer a concise review focusing on current discussions, especially the contentious roles of cations. This article further examines how different factors affect the interfacial solvent structure, particularly the hydrogen‐bonding network, and delves into the microscopic kinetics of electron and proton‐coupled electron transfer. We also discuss the overarching influence of solvents from a kinetic modeling perspective, aiming to develop a robust correlation between electrolyte structure and reactivity. Lastly, we summarize ongoing research challenges and suggest potential directions for future studies on electrolyte effects in electrocatalysis. Key ScientistsIn 1956, Marcus theory was developed to describe the mechanism of outer‐sphere electron transfer (OS‐ET). In 1992, Nocera et al. directly measured proton‐coupled electron transfer (PCET) kinetics for the first time, and their subsequent research in 1995 investigated the effects of proton motion on electron transfer (ET) kinetics. In 1999 and 2000, Hammes‐schiffer et al. developed the multistate continuum theory for multiple charge reactions and deduced the rate expressions for nonadiabatic PCET reactions in solution, laying the theoretical foundation for the analysis of PCET kinetics in electrochemical processes. In 2006, Saveant et al. verified the concerted proton and electron transfer (CPET) mechanism in the oxidation of phenols coupled with intramolecular amine‐driven proton transfer (PT). Their subsequent work in 2008 reported the pH‐dependent pathways of electrochemical oxidation of phenols.Electrolyte effects in electrocatalysis have gained emphasis in recent years. In 2009, Markovic's pioneering work proposed non‐covalent interactions between hydrated alkaline cations and adsorbed OH species in oxygen reduction reaction (ORR)/hydrogen oxidation reaction (HOR). In 2011, Markovic et al. significantly enhanced hydrogen evolution reaction (HER) activity in alkaline solution by improving water dissociation, which was assumed to dominate the sluggish HER kinetics in such media. In comparation, Yan et al. applied hydrogen binding energy (HBE) theory in 2015 to explain the pH‐dependent HER/HOR activity. Cations play a significant role in regulating the selectivity and activity of carbon dioxide reduction (CO2RR). In 2016 and 2017, Karen Chan et al. introduced the electric field generated by solvated cations to explain the cation effects on electrochemical CO2RR. Conversely, in 2021, Koper et al. suggested that short‐range electrostatic interactions between partially desolvated metal cations and CO2 stabilized CO2 and promoted CO2RR.Recent researches have combined the exploration of the electrical double layer (EDL) structure with theoretical analysis of PCET kinetics. In 2019, Huang et al. developed a microscopic Hamiltonian model to quantitatively understand the sluggish hydrogen electrocatalysis in alkaline media. In 2021, two meticulous studies from Shao‐Horn's group analyzed the effects of cations on reorganization energy and the impacts of hydrogen bonds between proton donors and acceptors on proton tunneling kinetics, respectively. Electrolyte effects on proton transport process were researched in recent years. In 2022, Hu et al. and Chen et al. proposed that the cation‐induced electric field distribution and pH‐dependent hydrogen bonding network connectivity played essential roles in proton transport, separately.

Proton-coupled electron transfer (PCET) is central to the reactivity of porphyrins. The coupling of the electron to the proton is central to a porphyrin’s ability to catalyze energy conversion reactions of which the hydrogen evolution reaction (HER) is exemplary. To understand the mechanistic details of the PCET chemistry of porphyrins and related macrocyclic congeners, we have designed hangman constructs that allow a proton, placed in the secondary coordination sphere (off of the hangman backbone), to be coupled to redox transformations at the macrocycle. For metals whose reduction potentials are positive of the porphyrin macrocycle, such as Co and Fe, HER catalysis is confined to PCET transformations of the metal center where the active catalyst for HER is a reduced metal hydride. Alternatively, the reduction potentials of Ni, Zn, and 2H (freebase) porphyrins allow for redox non-innocence of the macrocycle; here the active “hydridic” catalyst is a phlorin, which gives rise to elaborate HER reaction sequences. Beyond HER catalysis, redox non-innocence of Ni, Zn, and 2H porphyrins and related compounds has been informative for providing detailed mechanistic insight into the multi-site PCET hydrogenation of olefinic bonds of the macrocycle. This mini-review unravels the PCET dichotomy between the metal and macrocycle in promoting HER catalysis and novel chemical transformations that give rise to unusual macrocyclic structures.

To establish electrochemical CO2 reduction (CO2RR) as a viable industrial route for fuel and chemical production, it is crucial to sustain CO2RR over the competing hydrogen evolution reaction (HER) even at high current densities. However, the underlying mechanism of HER dominance at higher overpotentials remains poorly understood. Here, using operando Raman spectroscopy, we first probe the CO2-to-CO pathway on Ag catalysts modified with alkaline earth metals (AgMg, AgCa, AgSr, AgBa) in a Na+-containing electrolyte. These modified catalysts exhibit more pronounced Raman features than pure Ag, enabling the detection of key CO2RR intermediates. Notably, AgBa shows the clearest progression of intermediates with increasing cathodic potential: CO2 → *COO- → *COOH → *CO, providing direct spectroscopic evidence for the proposed CO formation mechanism. At potentials more negative than -0.3 V vs. RHE, CO2RR-related signals diminish, but this is accompanied by the emergence of a broad band at ∼532 cm-1, which is assigned to the libration of interfacial water. This feature strongly correlates with the visible occurrence of the HER current, suggesting its role in HER initiation. We propose that an increasingly negatively charged electrode drives the reorientation of interfacial water molecules into an "H-down" configuration, creating a favorable geometry to trigger HER. The accumulation of this ordered interfacial water structure may represent the molecular origin of HER dominance at high overpotentials. We hope that these insights provide a framework for designing strategies to suppress HER and promote CO2RR by controlling interfacial water reorientation.

Thermal chemical hydrogenation (TCH) is a key reaction for converting petroleum and biomass feedstocks to value-added fuels and chemicals. Electrochemical hydrogenation (ECH) is a mild alternative where applied potential drives the hydrogenation of hydrocarbons with protons sourced from the aqueous electrolyte near ambient conditions. Lignocellulose is an abundant renewable feedstock comprised of complex phenolic compounds that can be valorized via ECH. Phenol and benzaldehyde are simple model compounds for lignocellulose that have been the focus of recent ECH studies. When both phenol and benzaldehyde are present as reactants, phenol has been shown to act as a proton shuttle, enhancing benzaldehyde ECH rates but with negligible phenol turnover. [1] The study of the role of electrolyte on the ECH of these aromatic hydrocarbons is limited to a few buffer systems (typically acetate, phosphate, or sulfate) and in a pH range of 1-5. However, it well-known that the competing hydrogen evolution reaction (HER) kinetics are slower in base, and that molecules like phenol dissociate within a pH range of 1-14. Thus, a complete understanding of the role of electrolyte pH, especially in alkaline media where slowed HER rates may improve faradaic efficiency of ECH, is desirable.In this work, we show using cyclic voltammetry and chronoamperometry with product analysis that there are contrasting pH trends between platinum and rhodium for phenol ECH, with the highest ECH rates on platinum at pH 1 and the highest ECH rates on rhodium at pH 10. In-situ electrochemical FTIR and electrochemical impedance spectroscopy reveal that this is the result of differences in hydrogen adsorption kinetics, phenol/H coverage, and competition with HER between the two metals. For phenol ECH on rhodium, we show that pH-driven changes dictate the balance between a hydrogen atom transfer (LH) mechanism and a phenol-mediated proton coupled electron transfer mechanism. We also discuss phenol’s role as a proton shuttle towards benzaldehyde hydrogenation in alkaline pH, near the pka of phenol. With this insight, we show that the unique properties of rhodium and strategic tuning of the acid-base chemistry of phenol and the electrolyte can be utilized to yield elevated ECH rates with enhanced FE through slowed HER.[1] Sanyal, U., et al., Hydrogen Bonding Enhances the Electrochemical Hydrogenation of Benzaldehyde in the Aqueous Phase. Angew Chem Int Ed 2021, 60 (1), 290–296. https://doi.org/10.1002/anie.202008178.

The driving force dependence of the rate constants for nonadiabatic electron transfer (ET), proton transfer (PT), and proton-coupled electron transfer (PCET) reactions is examined. Inverted region behavior, where the rate constant decreases as the reaction becomes more exoergic (i.e., as DeltaG(0) becomes more negative), has been observed experimentally for ET and PT. This behavior was predicted theoretically for ET but is not well understood for PT and PCET. The objective of this Letter is to predict the experimental conditions that could lead to observation of inverted region behavior for PT and PCET. The driving force dependence of the rate constant is qualitatively different for PT and PCET than for ET because of the high proton vibrational frequency and substantial shift between the reactant and product proton vibrational wave functions. As a result, inverted region behavior is predicted to be experimentally inaccessible for PT and PCET if only the driving force is varied. This behavior may be observed for PT over a limited range of rates and driving forces if the solvent reorganization energy is low enough to cause observable oscillations. Moreover, this behavior may be observed for PT or PCET if the proton donor-acceptor distance increases as DeltaG(0) becomes more negative. Thus, a plausible explanation for experimentally observed inverted region behavior for PT or PCET is that varying the driving force also impacts other properties of the system, such as the proton donor-acceptor distance.

Refractory metals possess unique properties including high melting points, high hardness at room temperature, chemical resistance, and high density. Electrodeposition of refractory metals is of interest as a scalable approach to apply uniform coatings to enable a wide range of applications in aerospace, nuclear, catalysis, biomedical fields. This talk will focus on pulse electrochemical variables to enable methods to electrodeposit high quality metallic rhenium (Re).Re has traditionally been difficult to deposit from aqueous solutions due to the over potential for hydrogen evolution and complex electrochemical reactions. Electrodeposition of rhenium usually occurs from the perrhenate ion (ReO4-) with an oxidation state of +7. The exact mechanism to reduce the perrhenate ion from +7 to metallic rhenium is still unknown. It is unlikely that the 7 electrons transfer in a single step, so reduction likely occurs through intermediate oxide species including ReO2, ReO3 and Re2O5. Combined with the hydrogen evolution reaction, Re coatings are generally deposited at low faradaic efficiency, are brittle, and limited in thickness (sub-micron). In our previous work, water-in-salt electrolytes and complexing additives have enabled resolution of some of the issues associated with this complexity and a range of deposit qualities, though cracking issues remain and coating quality issues still needed optimization.Herein, we present on the effects of pulsed electrochemical approaches to eliminate cracking in the film by balancing nucleation and growth and the competing hydrogen evolution reaction. A custom high-throughput cell was developed to rapidly screen parameters and explore time-dependent evolution of the electrodeposition processes.

Direct electroreduction of nitric oxide offers a promising avenue to produce valuable chemicals, such as ammonia, which is an essential chemical to produce fertilizers. Direct ammonia synthesis from NO in a polymer electrolyte membrane (PEM) electrolyzer is advantageous for its continuous operation and excellent mass transport characteristics. However, at a high current density, the faradaic efficiency of NO electroreduction reaction is limited by the competing hydrogen evolution reaction (HER). Herein, we report a CO-mediated selective poisoning strategy to enhance the faradaic efficiency (FE) towards ammonia by suppressing the HER. In the presence of only NO at the cathode, Pt/C and Pd/C catalysts showed a lower FE towards NH3 than to H2 due to the dominating HER. Cu/C catalyst showed a 78 % FE towards NH3 at 2.0 V due to the stronger binding affinity to NO* compared to H*. By co-feeding CO, the FE of Cu/C catalyst towards NH3 was improved by 12 %. More strikingly, for Pd/C, the FE towards NH3 was enhanced by 95 % with CO co-feeding, by effectively suppressing HER. This is attributed to the change of the favorable surface coverage resulting from the selective and competitive binding of CO* to H* binding sites, thereby improving NH3 selectivity.

The interface engineering strategy has been an emerging field in terms of material improvisation that not only alters the electronic band structure of a material but also induces beneficial effects on electrochemical performances. Particularly, it is of immense importance for the environmentally benign electrochemical nitrogen reduction reaction (NRR), which is potentially impeded by the competing hydrogen evolution reaction (HER). The main problem lies in the attainment of the desired current density at a negotiable potential where the NRR would dominate over the HER, which in turn hampers the Faradaic efficiency for the NRR. To circumvent this issue, catalyst development becomes necessary, which would display a weak affinity for H-adsorption suppressing the HER at the catalyst surface. Herein, we have adopted the interfacial engineering strategy to synthesize our electrocatalyst NPG@SnS2, which not only suppressed the HER on the active site but yielded 49.3% F.E. for the NRR. Extensive experimental work and DFT calculations regarded that due to the charge redistribution, the Mott-Schottky effect, and the band bending of SnS2 across the contact layer at the interface of NPG, the d-band center for the surface Sn atoms in NPG@SnS2 lowered, which resulted in favored adsorption of N2 on the Sn active site. This phenomenon was driven even forward by the upshift of the Fermi level, and eventually, a decrease was seen in the work function of the heterostructure that increased the conductivity of the material as compared to pristine SnS2. This strategy thus provides a field to methodically suppress the HER in the realm of improving the Faradaic efficiency for the NRR.

Electrochemical CO2 reduction reaction (CO2RR) is a promising technique for converting the greenhouse gas CO2 into valuable fuels and chemicals in a clean energy society. To understand the underlying electrocatalytic reaction mechanism from the atomic level, researchers are investigating the role of electrolyte ions, particularly alkali metal cations, in various electrocatalytic reactions including CO2RR. Despite this attention, the impact of alkali metal cations is still a topic of debate, and it remains unclear how cations affect the CO2RR and the competing hydrogen evolution reaction (HER).In this study, explicit cations and water solvents were added to Au-water interfacial models to simulate the pathways of CO2RR and HER. Two cations (K+, Li+, and/or H+) were used in the model system to create similarly charged Au surfaces, and the local concentration of alkali metal cations (AM+) was adjusted by replacing AM+ (K+ and Li+) with H+. The first electron transfer step was considered critical in electrocatalytic reductions, so CO2 activation and water dissociation were evaluated for CO2RR and HER, respectively. Ab initio molecular dynamics (AIMD) simulations with the slow-growth sampling approach (SG-AIMD) were used to simulate the corresponding reaction mechanisms, and kinetic barriers were obtained through thermodynamic integrations. With these Au-water-cations interfacial models, a systematic study was conducted to investigate the mechanism of CO2 activation at Au-water-2AM+, Au-water-1AM+, and Au-water-0AM+ interfaces. These results show that a high concentration of metal cations with 2AM+ promotes CO2 activation through short-range electrostatic interactions between cations and key intermediates.Besides CO2 activation, this study also investigated water dissociation during the competing HER. Contrary to the promotion effect observed in CO2RR, local alkali metal cations were found to suppress water dissociation with a high reaction barrier (Figure 1). This can be attributed to the broken connectivity of the hydrogen bond network at Au-water-2AM+ interfaces. The reaction kinetics can be improved by reducing the metal cation concentration. Notably, K+ was found to have a more pronounced promotion effect than Li+ on CO2 activation, while the opposite suppression effect was observed on HER. By tuning the local alkali metal cation concentration, it is anticipated that the overall performance of CO2RR, including both activity and selectivity, can be engineered. Figure 1

Ammonia is produced commercially via the Haber-Bosch (HB) process, which produced 144 million tonnes of ammonia 2014, approximately 88% of which is used in the fertilizer industry [1,2]. However the HB process requires very high temperatures and pressures and as a result it is only be done in large plants which consume large amounts of power and generate large quantities of CO2 from the production of the hydrogen used in the HB process[1,2]. It is estimated that the HB process accounts for nearly 1% of annual global power consumption [3]. Due to the sheer scale of the HB process, distribution from the point of production to the point of use becomes an additional carbon and energy burden. Alternative methods of manufacturing ammonia that enable small scale, distributed generation at the point of use using renewable energy and sustainable feedstocks could have a great impact on global CO2 emissions. Electrochemical synthesis of ammonia at low temperatures and pressures is one promising solution that has recently begun to gain more attention as a viable method of sustainable, on-site generation of NH3. Several electrochemical approaches to NH3 synthesis have been reported in the literature with varying success, included high temperature proton-conducting ceramic electrolytes, molten hydroxides, PEM membrane and AEM membrane systems and more [4,5,6]. Most low temperature and low pressure production rates range between 10-12 and 10-8 mol NH3 cm-2 s-1 with Faradaic efficiencies that are typically very low on the order of 5% due to the competing hydrogen evolution reaction (HER) [4,5,6]. Cathode catalysts that enable increased NH3 production rates while suppressing the HER are critical to realizing the benefits of sustainable electrochemical synthesis of ammonia. As a result catalysts that adsorb nitrogen more strongly than hydrogen must be investigated for an efficient electrochemical synthesis. By utilizing alkaline media, non-platinum group metals (PGMs) may become feasible catalysts, thus lowering costs and use of very limited global supply of PGM. The electrochemical synthesis of ammonia was run under mild temperatures and pressures in alkaline media according to the following reactions: (1) (2) where reactions (1) and (2) take place at the cathode and anode of the electrochemical cell, respectively. The overall reaction leads to the synthesis of ammonia with a theoretical cell voltage of 0.059 V, according to: (3) Humidified nitrogen is flowed over a Pt/Ir electrode where it is reduced to ammonia (Eq. 1), and hydroxide ions are transported through a gel electrolyte to the anode where they are oxidized hydrogen to produce water (Eq. 2). The major challenge associated with electrochemical production of ammonia is the competing HER which can occur at the cathode. In order to mitigate this issue we hypothesize that using a polymer based gel electrolyte to control the amount of water present in the system could allow us to limit the HER, thus increasing the efficiency of the reaction. We believe that by using this process we will be able to examine different catalysts for nitrogen reduction in order to find a catalyst where the HER is less dominant [7].

Insight into the overpotentials of electrocatalytic hydrogen evolution on black phosphorus decorated with metal clusters

Electrocatalytic reduction of CO2 has the potential to produce fuels and chemicals from renewable feedstocks: It can be used as an energy carrier (if converted into energy-dense, transportation-ready forms) to store energy generated from intermittent, distributed sources such as wind and the sun. It can also be used as a renewable source of carbon for chemical production. The key to enabling these uses of CO2 is maximizing the selectivity and energy efficiency of electrocatatlytic CO2 reduction. Hori and co-workers have shown that various metals (Au, Ag and Cu, etc.) have the ability to catalyze the reaction and produce CO or hydrocarbons.(1) A significant challenge is posed by the competing hydrogen evolution reaction (HER), which tends to occur with a smaller overpotential and consumes a significant fraction of the total current. Nørskov and co-workers have shown that the catalytic activity for CO2 electro-reduction is limited by the strong scaling relation that exists between the key reaction intermediate, COOH, and the reaction product, CO.(2) Furthermore, the catalyst particles would have to be nano-sized in order to maximize the activity of metals such as Cu and Au.(3, 4) In present work, we take inspiration from biological enzymes that catalyze CO2 reduction (e.g. carbon monoxide dehydrogenase) and attempt to mimic their active sites by functionalizing Au electrodes with a series of organic thiol-based ligands, to seek to potentially break the constraints imposed by the scaling relations. This has resulted in significantly altered activity for CO formation and selectivity between CO and H2. As shown in the Figure (measured in 0.1 M KHCO3), the CO yield was increased dramatically and nearly doubled in the presence of 2-phenylethanethiol (2-PET) compared to the blank gold foil electrode. On the contrary, 2-merceptanpropanic acid (MPA) suppresses the yield of CO to negligible amounts. Meanwhile, the 2-PET-modified gold electrode reduced the faradaic efficiency for HER by half, whereas the MPA-modified Au electrode doubled it. Density function theory based modeling suggest that ligand-induced surface reconstruction of the Au surface is a major factor in the altered catalytic activity compared to blank Au, which creates sites that favor CO production over HER. Moreover, the surface-bound thiol ligands modify the adsorption energies of surface reduction intermediates through electronic interaction and further contribute to the altered activities for CO2 reduction and HER. Reference 1. Y. Hori, in Modern Aspects of Electrochemistry, C. Vayenas, R. White and M. Gamboa-Aldeco Editors, p. 89, Springer New York (2008). 2. C. Shi, H. A. Hansen, A. C. Lausche and J. K. Norskov, Physical Chemistry Chemical Physics, 16, 4720 (2014). 3. D. R. Kauffman, D. Alfonso, C. Matranga, H. Qian and R. Jin, Journal of the American Chemical Society, 134, 10237 (2012). 4. F. Studt, M. Behrens, E. L. Kunkes, N. Thomas, S. Zander, A. Tarasov, J. Schumann, E. Frei, J. B. Varley, F. Abild-Pedersen, J. K. Nørskov and R. Schlögl, ChemCatChem, 7, 1105 (2015). Figure 1