Theory-Guided Design of Unconventional Phase Metal Heteronanostructures for Higher-Rate Stable Li-CO2 and Li-Air Batteries.

Lithium-carbon dioxide (Li-CO2) and Li-air batteries hold great potential in achieving carbon neutral given their ultrahigh theoretical energy density and eco-friendly features. However, these Li-gas batteries still suffer from low discharging-charging rate and poor cycling life due to sluggish decomposition kinetics of discharge products especially Li2CO3. Here we report the theory-guided design and preparation of unconventional phase metal heteronanostructures as cathode catalysts for high-performance Li-CO2/air batteries. The assembled Li-CO2 cells with unconventional phase 4H/face-centered cubic (fcc) ruthenium-nickel heteronanostructures deliver a narrow discharge-charge gap of 0.65 V, excellent rate capability and long-term cycling stability over 200 cycles at 250 mA g-1. The constructed Li-air batteries can steadily run for above 150 cycles in ambient air. Electrochemical mechanism studies reveal that 4H/fcc Ru-Ni with high-electroactivity facets can boost redox reaction kinetics and tune discharge reactions towards Li2C2O4 path, alleviating electrolyte/catalyst failures induced by the aggressive singlet oxygen from solo decomposition of Li2CO3.

Light-Induced Dynamic Activation of Copper/Silicon Interface for Highly Selective Carbon Dioxide Reduction.

Numerous studies have shown a fact that phase transformation and/or reconstruction are likely to occur and play crucial roles in electrochemical scenarios. Nevertheless, a decisive factor behind the diverse photoelectrochemical activity and selectivity of various copper/silicon photoelectrodes is still largely debated and missing in the community, especially the possibly dynamic behaviors of metal catalyst/semiconductor interface. Herein, through in situ X-ray absorption spectroscopy and transmission electron microscope, a model system of Cu nanocrystals with well-defined facets on black p-type silicon (BSi) is unprecedentedly demonstrated to reveal the dynamic phase transformation of forming irreversible silicide at Cu nanocrystal-BSi interface during photoelectrocatalysis, which is validated to originate from the atomic interdiffusion between Cu and Si driven by light-induced dynamic activation process. Significantly, the adaptive junction at Cu-Si interface is activated by an expansion of interatomic Cu-Cu distance for CO2 electroreduction, which efficiently restricts the C-C coupling pathway but strengthens the bonding with key intermediate of *CHO for CH4 yield, resulting in a remarkable 16-fold improvement in the product ratio of CH4/C2 products and an intriguing selectivity switch. This work offers new insights into dynamic structural transformations of metal/semiconductor junction and design of highly efficient catalysts toward photosynthesis.

Tailoring Oxygen Reduction Reaction Kinetics of Fe-N-C Catalyst via Spin Manipulation for Efficient Zinc-Air Batteries.

The interaction between oxygen species and metal sites of various orbitals exhibits intimate correlation with the oxygen reduction reaction (ORR) kinetics. Herein, a new approach for boosting the inherent ORR activity of atomically dispersed Fe-N-C matrix is represented by implanting Fe atomic clusters nearby. The as-prepared catalyst delivers excellent ORR activity with half-wave potentials of 0.78 and 0.90 V in acidic and alkaline solutions, respectively. The decent ORR activity can also be validated from the high-performance rechargeable Zn-air battery. The experiments and density functional theory calculations reveal that the electron spin-state of monodispersed Fe active sites is transferred from the low spin (LS, t2g 6 eg 0) to the medium spin (MS, t2g 5 eg 1) due to the involvement of Fe atomic clusters, leading to the spin electron filling in σ∗ orbit, by which it favors OH- desorption and in turn boosts the reaction kinetics of the rate-determining step. This work paves a solid way for rational design of high-performance Fe-based single atom catalysts through spin manipulation.

Dynamic (Sub)surface-Oxygen Enables Highly Efficient Carbonyl-Coupling for Electrochemical Carbon Dioxide Reduction.

Nowadays, high-valent Cu species (i.e., Cuδ +) are clarified to enhance multi-carbon production in electrochemical CO2 reduction reaction (CO2RR). Nonetheless, the inconsistent average Cu valence states are reported to significantly govern the product profile of CO2RR, which may lead to misunderstanding of the enhanced mechanism for multi-carbon production and results in ambiguous roles of high-valent Cu species. Dynamic Cuδ + during CO2RR leads to erratic valence states and challenges of high-valent species determination. Herein, an alternative descriptor of (sub)surface oxygen, the (sub)surface-oxygenated degree (κ), is proposed to quantify the active high-valent Cu species on the (sub)surface, which regulates the multi-carbon production of CO2RR. The κ validates a strong correlation to the carbonyl (*CO) coupling efficiency and is the critical factor for the multi-carbon enhancement, in which an optimized Cu2O@Pd2.31 achieves the multi-carbon partial current density of ≈330 mA cm-2 with a faradaic efficiency of 83.5%. This work shows a promising way to unveil the role of high-valent species and further achieve carbon neutralization.

Inter-site structural heterogeneity induction of single atom Fe catalysts for robust oxygen reduction.

Metal-nitrogen-carbon catalysts with hierarchically dispersed porosity are deemed as efficient geometry for oxygen reduction reaction (ORR). However, catalytic performance determined by individual and interacting sites originating from structural heterogeneity is particularly elusive and yet remains to be understood. Here, an efficient hierarchically porous Fe single atom catalyst (Fe SAs-HP) is prepared with Fe atoms densely resided at micropores and mesopores. Fe SAs-HP exhibits robust ORR performance with half-wave potential of 0.94 V and turnover frequency of 5.99 e-1s-1site-1 at 0.80 V. Theoretical simulations unravel a structural heterogeneity induced optimization, where mesoporous Fe-N4 acts as real active centers as a result of long-range electron regulation by adjacent microporous sites, facilitating O2 activation and desorption of key intermediate *OH. Multilevel operando characterization results identify active Fe sites undergo a dynamic evolution from basic Fe-N4 to active Fe-N3 under working conditions. Our findings reveal the structural origin of enhanced intrinsic activity for hierarchically porous Fe-N4 sites.

Crystal Phase Engineering of Ultrathin Alloy Nanostructures for Highly Efficient Electroreduction of Nitrate to Ammonia.

Electrocatalytic nitrate reduction reaction (NO3RR) toward ammonia synthesis is recognized as a sustainable strategy to balance the global nitrogen cycle. However, it still remains a great challenge to achieve highly efficient ammonia production due to the complex proton-coupled electron transfer process in NO3RR. Here, the controlled synthesis of RuMo alloy nanoflowers (NFs) with unconventional face-centered cubic (fcc) phase and hexagonal close-packed/fcc heterophase for highly efficient NO3RR is reported. Significantly, fcc RuMo NFs demonstrate high Faradaic efficiency of 95.2% and a large yield rate of 32.7 mg h-1 mgcat -1 toward ammonia production at 0 and -0.1 V (vs reversible hydrogen electrode), respectively. In situ characterizations and theoretical calculations have unraveled that fcc RuMo NFs possess the highest d-band center with superior electroactivity, which originates from the strong Ru─Mo interactions and the high intrinsic activity of the unconventional fcc phase. The optimal electronic structures of fcc RuMo NFs supply strong adsorption of key intermediates with suppression of the competitive hydrogen evolution, which further determines the remarkable NO3RR performance. The successful demonstration of high-performance zinc-nitrate batteries with fcc RuMo NFs suggests their substantial application potential in electrochemical energy supply systems.

Vacancy Defects Inductive Effect of Asymmetrically Coordinated Single-Atom Fe─N3 S1 Active Sites for Robust Electrocatalytic Oxygen Reduction with High Turnover Frequency and Mass Activity.

The development of facile, efficient synthesis method to construct low-cost and high-performance single-atom catalysts (SACs) for oxygen reduction reaction (ORR) is extremely important, yet still challenging. Herein, an atomically dispersed N, S co-doped carbon with abundant vacancy defects (NSC-vd) anchored Fe single atoms (SAs) is reported and a vacancy defects inductive effect is proposed for promoting electrocatalytic ORR. The optimized catalyst featured of stable Fe─N3 S1 active sites exhibits excellent ORR activity with high turnover frequency and mass activity. In situ Raman, attenuated total reflectance surface enhanced infrared absorption spectroscopy reveal the Fe─N3 S1 active sites exhibit different kinetic mechanisms in acidic and alkaline solutions. Operando X-ray absorption spectra reveal the ORR activity of Fe SAs/NSC-vd catalyst in different electrolyte is closely related to the coordination structure. Theoretical calculation reveals the upshifted d band center of Fe─N3 S1 active sites facilitates the adsorption of O2 and accelerates the kinetics process of *OH reduction. The abundant vacancy defects around the Fe─N3 S1 active sites balance the OOH* formation and *OH reduction, thus synergetically promoting the electrocatalytic ORR process.

Photosynthesis of Benzonitriles on BiOBr Nanosheets Promoted by Vacancy Associates.

Photocatalytic organic functionalization reactions represent a green, cost-effective, and sustainable synthesis route for value-added chemicals. However, heterogeneous photocatalysis is inefficient in directly activating ammonia molecules for the production of high-value-added nitrogenous organic products when compared with oxygen activation in the formation of related oxygenated compounds. In this study, we report the heterogeneous photosynthesis of benzonitriles by the ammoxidation of benzyl alcohols (99 % conversion, 93 % selectivity) promoted using BiOBr nanosheets with surface vacancy associates. In contrast, the main reaction of catalysts with other types of vacancy sites is the oxidation of benzyl alcohol to benzaldehyde or benzoic acid. Experimental measurements and theoretical calculations have demonstrated a specificity of vacancy type with respect to product selectivity, which arises from the adsorption and activation of NH3 and O2 that is required to promote subsequent C-N coupling and oxidation to nitrile. This study provides a better understanding of the role of vacancies as catalytic sites in heterogeneous photocatalysis.

Regulating Intramolecular Electron Transfer of Nickel-Based Coordinations through Ligand Engineering for Aqueous Batteries.

The integration of electronic effects into complexes for the construction of novel materials has not yet attracted significant attention in the field of energy storage. In the current study, eight one-dimensional (1D) nickel-based salicylic acid  complexes (Ni-XSAs, X = pH, pMe, pMeO, mMe, pBr, pCl, pF, and pCF3 ), are prepared by ligand engineering. The coordination environments in the Ni-XSAs are explored using X-ray absorption fine structure spectroscopy. The charge transfer of the complexes is modulated according to the difference in the electron-donating ability of the substituents, in combination with frontier orbital theory. Furthermore, density functional theory is used to investigate the effect of the substituent position on the electronic properties of the complexes. Ni-mMeSA exhibits better electrical conductivity than Ni-pMeSA. The electrochemical performance of Ni-mMeSA as an aqueous battery cathode is remarkably improved with a maximum energy density of 0.30 mWh cm-2 (125 Wh kg-1 ) and a peak power density of 33.72 mW cm-2 (14.03 kW kg-1 ). This study provides ideas for the application of new coordination chemistry in the field of energy materials science.

Activating dynamic atomic-configuration for single-site electrocatalyst in electrochemical CO2 reduction.

One challenge for realizing high-efficiency electrocatalysts for CO2 electroreduction is lacking in comprehensive understanding of potential-driven chemical state and dynamic atomic-configuration evolutions. Herein, by using a complementary combination of in situ/operando methods and employing copper single-atom electrocatalyst as a model system, we provide evidence on how the complex interplay among dynamic atomic-configuration, chemical state change and surface coulombic charging determines the resulting product profiles. We further demonstrate an informative indicator of atomic surface charge (φe) for evaluating the CO2RR performance, and validate potential-driven dynamic low-coordinated Cu centers for performing significantly high selectivity and activity toward CO product over the well-known four N-coordinated counterparts. It indicates that the structural reconstruction only involved the dynamic breaking of Cu-N bond is partially reversible, whereas Cu-Cu bond formation is clearly irreversible. For all single-atom electrocatalysts (Cu, Fe and Co), the φe value for efficient CO production has been revealed closely correlated with the configuration transformation to generate dynamic low-coordinated configuration. A universal explication can be concluded that the dynamic low-coordinated configuration is the active form to efficiently catalyze CO2-to-CO conversion.

2D MOF-assisted Pyrolysis-displacement-alloying Synthesis of High-entropy Alloy Nanoparticles Library for Efficient Electrocatalytic Hydrogen Oxidation.

Multimetallic alloy nanoparticles (NPs) have received considerable attention in various applications due to their compositional variability and exceptional properties. However, the complexity of both the general synthesis and structure-activity relationships remain the long-standing challenges in this field. Herein, we report a versatile 2D MOF-assisted pyrolysis-displacement-alloying route to successfully synthesize a series of binary, ternary and even high-entropy NPs that are uniformly dispersed on porous nitrogen-doped carbon nanosheets (PNC NSs). As a proof of utility, the obtained Co0.2 Ru0.7 Pt0.1 /PNC NSs exhibits apparent hydrogen oxidation activity and durability with a record-high mass specific kinetic current of 1.84 A mg-1 at the overpotential of 50 mV, which is approximately 11.5 times higher than that of the Pt benchmark. Both experimental and theoretical studies reveal that the addition of Pt engenders a phase transition in CoRu alloys from hexagonal close-packed (hcp) to face-centered cubic (fcc) structure. The elevated reactivity of the resulted ternary alloy can be attributed to the optimized adsorption of hydrogen intermediate and the decreased reaction barrier for water formation. This study opens a new avenue for the development of highly efficient alloy NPs with various compositions and functions.

Leveraging Metal Nodes in Metal-Organic Frameworks for Advanced Anodic Hydrazine Oxidation Assisted Seawater Splitting.

Metal-organic frameworks (MOFs) show great promise for electrocatalysis owing to their tunable ligand structures. However, the poor stability of MOFs impedes their practical applications. Unlike the general pathway for engineering ligands, we report herein an innovative strategy for leveraging metal nodes to improve both the catalytic activity and the stability. Our electrolysis cell with a NiRh-MOF||NiRh-MOF configuration exhibited 10 mA cm-2 at an ultralow cell voltage of 0.06 V in alkaline seawater (with 0.3 M N2H4), outperforming its counterpart benchmark Pt/C||Pt/C cell (0.12 V). Impressively, the incorporation of Rh into a MOF secured a robust stability of over 60 h even when working in the seawater electrolyte. Experimental results and theoretical calculations revealed that Rh atoms serve as the active sites for hydrogen evolution while Ni nodes are responsible for the hydrazine oxidation during the hydrazine oxidation assisted seawater splitting. This work provides a paradigm for green hydrogen generation from seawater.

Atomically Dispersed Pt-Doped Co3 O4 Spinel Nanoparticles Embedded in Polyhedron Frames for Robust Propane Oxidation at Low Temperature.

This study adopts a facile and effective in situ encapsulation-oxidation strategy for constructing a coupling catalyst composed of atomically dispersed Pt-doped Co3 O4 spinel nanoparticles (NPs) embedded in polyhedron frames (PFs) for robust propane total oxidation. Benefiting from the abundant oxygen vacancies and more highly valent active Co3+ species caused by the doping of Pt atoms as well as the confinement effect, the optimized 0.2Pt-Co3 O4 NPs/PFs catalyst exhibits excellent propane catalytic activity with low T90 (184 °C), superior apparent reaction rate (21.62×108 (mol gcat -1 s-1 )), low apparent activation energy (Ea = 17.89 kJ mol-1 ), high turnover frequency ( 811×107 (mol gcat -1 s-1 )) as well as good stability. In situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory calculations indicate that the doping of Pt atoms enhances the oxygen activation ability, and decreases the energy barrier required for CH bond breaking, thus improving the deep oxidation process of the intermediate species. This study opens up new ideas for constructing coupling catalysts from atomic scale with low cost to enhance the activation of oxygen molecules and the deep oxidation of linear short chain alkanes at low temperature.

Lewis Acidic Support Boosts C-C Coupling in the Pulsed Electrochemical CO2 Reaction.

Copper-oxide electrocatalysts have been demonstrated to effectively perform the electrochemical CO2 reduction reaction (CO2RR) toward C2+ products, yet preserving the reactive high-valent CuOx has remained elusive. Herein, we demonstrate a model system of Lewis acidic supported Cu electrocatalyst with a pulsed electroreduction method to achieve enhanced performance for C2+ products, in which an optimized electrocatalyst could reach ∼76% Faradaic efficiency for C2+ products (FEC2+) at ∼-0.99 V versus reversible hydrogen electrode, and the corresponding mass activity can be enhanced by ∼2 times as compared to that of conventional CuOx. In situ time-resolved X-ray absorption spectroscopy investigating the dynamic chemical/physical nature of Cu during CO2RR discloses that an activation process induced by the KOH electrolyte during pulsed electroreduction greatly enriched the Cuδ+O/Znδ+O interfaces, which further reveals that the presence of Znδ+O species under the cathodic potential could effectively serve as a Lewis acidic support for preserving the Cuδ+O species to facilitate the formation of C2+ products, and the catalyst structure-property relationship of Cuδ+O/Znδ+O interfaces can be evidently realized. More importantly, we find a universality of stabilizing Cuδ+O species for various metal oxide supports and to provide a general concept of appropriate electrocatalyst-Lewis acidic support interaction for promoting C2+ products.

Highly efficient overall urea electrolysis via single-atomically active centers on layered double hydroxide.

Anodic urea oxidation reaction (UOR) is an intriguing half reaction that can replace oxygen evolution reaction (OER) and work together with hydrogen evolution reaction (HER) toward simultaneous hydrogen fuel generation and urea-rich wastewater purification; however, it remains a challenge to achieve overall urea electrolysis with high efficiency. Herein, we report a multifunctional electrocatalyst termed as Rh/NiV-LDH, through integration of nickel-vanadium layered double hydroxide (LDH) with rhodium single-atom catalyst (SAC), to achieve this goal. The electrocatalyst delivers high HER mass activity of 0.262 A mg-1 and exceptionally high turnover frequency (TOF) of 2.125 s-1 at an overpotential of 100 mV. Moreover, exceptional activity toward urea oxidation is addressed, which requires a potential of 1.33 V to yield 10 mA cm-2, endorsing the potential to surmount the sluggish OER. The splendid catalytic activity is enabled by the synergy of the NiV-LDH support and the atomically dispersed Rh sites (located on the Ni-V hollow sites) as evidenced both experimentally and theoretically. The self-supported Rh/NiV-LDH catalyst serving as the anode and cathode for overall urea electrolysis (1 mol L-1 KOH with 0.33 mol L-1 urea as electrolyte) only requires a small voltage of 1.47 V to deliver 100 mA cm-2 with excellent stability. This work provides important insights into multifunctional SAC design from the perspective of support sites toward overall electrolysis applications.

Metal-Organic Frameworks Offering Tunable Binary Active Sites toward Highly Efficient Urea Oxidation Electrolysis.

Electrocatalytic urea oxidation reaction (UOR) is regarded as an effective yet challenging approach for the degradation of urea in wastewater into harmless N2 and CO2. To overcome the sluggish kinetics, catalytically active sites should be rationally designed to maneuver the multiple key steps of intermediate adsorption and desorption. Herein, we demonstrate that metal-organic frameworks (MOFs) can provide an ideal platform for tailoring binary active sites to facilitate the rate-determining steps, achieving remarkable electrocatalytic activity toward UOR. Specifically, the MOF (namely, NiMn0.14-BDC) based on Ni/Mn sites and terephthalic acid (BDC) ligands exhibits a low voltage of 1.317 V to deliver a current density of 10 mA cm-2. As a result, a high turnover frequency (TOF) of 0.15 s-1 is achieved at a voltage of 1.4 V, which enables a urea degradation rate of 81.87% in 0.33 M urea solution. The combination of experimental characterization with theoretical calculation reveals that the Ni and Mn sites play synergistic roles in maneuvering the evolution of urea molecules and key reaction intermediates during the UOR, while the binary Ni/Mn sites in MOF offer the tunability for electronic structure and d-band center impacting on the intermediate evolution. This work provides important insights into active site design by leveraging MOF platform and represents a solid step toward highly efficient UOR with MOF-based electrocatalysts.

In situ probing the dynamic reconstruction of copper-zinc electrocatalysts for CO2 reduction.

Unravelling the dynamic characterization of electrocatalysts during the electrochemical CO2 reduction reaction (CO2RR) is a critical factor to improve the production efficiency and selectivity, since most pre-electrocatalysts undergo structural reconstruction and surface rearrangement under working conditions. Herein, a series of pre-electrocatalysts including CuO, ZnO and two different ratios of CuO/ZnO were systematically designed by a sputtering process to clarify the correlation of the dynamic characterization of Cu sites in the presence of Zn/ZnO and the product profile. The evidence provided by in situ X-ray absorption spectroscopy (XAS) indicated that appropriate Zn/ZnO levels could induce a variation in the coordination number of Cu sites via reversing Ostwald ripening. Specifically, the recrystallized Cu site with a lower coordination number exhibited a preferential production of methane (CH4). More importantly, our findings provide a promising approach for the efficient production of CH4 by in situ reconstructing Cu-based binary electrocatalysts.

Construction of N, P Co-Doped Carbon Frames Anchored with Fe Single Atoms and Fe2 P Nanoparticles as a Robust Coupling Catalyst for Electrocatalytic Oxygen Reduction.

A coupling catalyst of highly dispersed N, P co-doped carbon frames (NPCFs) anchored with Fe single atoms (SAs) and Fe2 P nanoparticles (NPs) is synthesized by a novel in situ doping-adsorption-phosphatization strategy for the electrocatalytic oxygen reduction reaction (ORR). The optimized Fe SAs-Fe2 P NPs/NPCFs-2.5 catalyst shows a superior ORR activity and stability in 0.5 m H2 SO4 and 0.1 m KOH, respectively. Theoretical calculations reveal a synergistic effect, in that the existence of Fe2 P weakens the adsorption of ORR intermediates on active sites and lowers the reaction free energy. The doped P atoms with a strong electron-donating ability elevate the energy level of Fe-3d orbitals and facilitate the adsorption of O2 . The active Fe atoms exist in a low oxidation state and are less positively charged, and they serve as an electron reservoir capable of donating and releasing electrons, thus improving the ORR activity. Operando and in situ characterization results indicate that the atomically dispersed FeN4 /FeP coupled active centers in the Fe SAs-Fe2 P NPs/NPCFs-2.5 catalyst are characteristic of the different catalytic mechanisms in acidic and alkaline media. This work proposes a novel idea for constructing coupling catalysts with atomic-level precision and provides a strong reference for the development of high-efficiency ORR electrocatalysts for practical application.

Using Exciton/Trion Dynamics to Spatially Monitor the Catalytic Activities of MoS2 during the Hydrogen Evolution Reaction.

The adsorption and desorption of electrolyte ions strongly modulates the carrier density or carrier type on the surface of monolayer-MoS2 catalyst during the hydrogen evolution reaction (HER). The buildup of electrolyte ions onto the surface of monolayer MoS2 during the HER may also result in the formation of excitons and trions, similar to those observed in gate-controlled field-effect transistor devices. Using the distinct carrier relaxation dynamics of excitons and trions of monolayer MoS2 as sensitive descriptors, an in situ microcell-based scanning time-resolved liquid cell microscope is set up to simultaneously measure the bias-dependent exciton/trion dynamics and spatially map the catalytic activity of monolayer MoS2 during the HER. This operando probing technique used to monitor the interplay between exciton/trion dynamics and electrocatalytic activity for two-dimensional transition metal dichalcogenides provides an excellent platform to investigate the local carrier behaviors at the atomic layer/liquid electrolyte interfaces during electrocatalytic reaction.

Strong Correlation between the Dynamic Chemical State and Product Profile of Carbon Dioxide Electroreduction.

Utilizing renewable electricity energy to convert CO2 into fuels and chemicals, namely, CO2 electrocatalytic reduction reaction (CO2RR), is becoming increasingly significant yet challenged by low activity and selectivity. Recently, a growing number of studies have demonstrated that oxidized species can surprisingly survive on the catalyst surface under highly cathodic CO2RR conditions and play crucial roles in affecting the product selectivity. However, dynamic evolutions of the surface chemical state together with its real correlation to the product selectivity are still unclear, which is one of the most controversial topics for CO2RR. Herein, we particularly resurvey recent CO2RR researches that are all based on advanced in situ/operando methodologies, aiming to clearly reveal the realistic variations in surface chemical state under the working conditions. Then, recent progress in the regulation of the surface chemical state toward specific CO2RR products in current state-of-the-art catalysts with varying metal centers is systematically summarized, which shows an impressive relation between the dynamic chemical state and product profile. Next, we further highlight the developed strategies to regulate the surface chemical state in catalysts and discuss the debates over the effects of chemical state on product profile during CO2RR. Finally, on the basis of previous achievements, we present major challenges and some perspectives for the exploration of the imperative chemical state sensitivity to product profile during CO2RR.