Key Role of Cations in Stabilizing Hydrogen Radicals for CO2-to-CO Conversion via a Reverse Water-Gas Shift Reaction.

Electrochemically converting CO2 into valuable chemicals and fuels in acidic media is argued as a promising energy- and carbon-efficient route. Although several key roles of alkali cations have been unveiled, the alkali cation trends for CO2 reduction remain largely elusive. With decreasing cation size from Cs+ to Li+, here we show that the apparent proton diffusion coefficient in 3.0 M Li+ is tens-fold lower than in 3.0 M K+ and 3.0 M Cs+ acidic electrolytes. Although Li+ has the strongest inhibition ability for proton transport, it acts the worst for both the CO2-to-CO conversion and partial current density on Au catalysts. Unexpectedly, K+ with a higher proton transport performs the best for CO2-to-CO conversion. We thus revisit the roles of alkali cations and find that hydrated K+ can stabilize hydrogen radicals benefiting CO2 conversion at the electrode interface while for Li+ this is not the case. This study proposes that cation-stabilized atomic hydrogen assists in activating CO2 via a reverse water-gas shift route under electrochemical conditions.

Steering the Reconstruction of Oxide-Derived Cu by Secondary Metal for Electrosynthesis of n-Propanol from CO.

As fuel and an important chemical feedstock, n-propanol is highly desired in electrochemical CO2/CO reduction on Cu catalysts. However, the precise regulation of the Cu localized structure is still challenging and poorly understood, thus hindering the selective n-propanol electrosynthesis. Herein, by decorating Au nanoparticles (NPs) on CuO nanosheets (NSs), we present a counterintuitive transformation of CuO into undercoordinated Cu sites locally around Au NPs during CO reduction. In situ spectroscopic techniques reveal the Au-steered formation of abundant undercoordinated Cu sites during the removal of oxygen on CuO. First-principles accuracy molecular dynamic simulation demonstrates that the localized Cu atoms around Au tend to rearrange into disordered layer rather than a Cu (111) close-packed plane observed on bare CuO NSs. These Au-steered undercoordinated Cu sites facilitate CO binding, enabling selective electroreduction of CO into n-propanol with a high Faradaic efficiency of 48% in a flow cell. This work provides new insight into the regulation of the oxide-derived catalysts reconstruction with a secondary metal component.

Electrochemical Redox Conversion of Formate to CO via Coupling Fe-Co Layered Double Hydroxides and Au Catalysts.

Formate has been considered an inactive molecule and thus cannot be further reduced under CO2 reduction conditions, which limits its widespread application as feedstock. Here we present an electrochemical redox conversion of formate to CO through the potential-dependent generation of carbon dioxide radical anions (CO2 ⋅- ) on Fe-Co layered double hydroxides (Fe-Co LDHs) and the subsequent reduction of CO2 ⋅- to CO on Au catalysts. We present an electrodeposition protocol for the synthesis of Fe-Co LDHs with precise composition control and find that Fe1 Co4 exhibits a promising potential window for CO2 ⋅- formation between 1.14 and 1.4 V and an optimized potential at 1.24 V at a neutral pH condition. We further determined the formation of CO2 ⋅- at 1.24 V via electron paramagnetic resonance and CO2 at >1.4 V through differential electrochemical mass spectrometry. This work provides a redox chemistry route for converting formate into CO through a coupled slit parallel-plate electrode system.

Criteria for Efficient Photocatalytic Water Splitting Revealed by Studying Carrier Dynamics in a Model Al-doped SrTiO3 Photocatalyst.

Overall water splitting (OWS) using semiconductor photocatalysts is a promising method for solar fuel production. Achieving a high quantum efficiency is one of the most important prerequisites for photocatalysts to realize high solar-to-fuel efficiency. In a recent study (Nature 2020, 58, 411-414), a quantum efficiency of almost 100 % has been achieved in an aluminum-doped strontium titanate (SrTiO3 : Al) photocatalyst. Herein, using the SrTiO3 : Al as a model photocatalyst, we reveal the criteria for efficient photocatalytic water splitting by investigating the carrier dynamics through a comprehensive photoluminescence study. It is found that the Al doping suppresses the generation of Ti3+ recombination centers in SrTiO3 , the surface band bending facilitates charge separation, and the in situ photo-deposited Rh/Cr2 O3 and CoOOH co-catalysts render efficient charge extraction. By suppressing photocarrier recombination and establishing a facile charge separation and extraction mechanism, high quantum efficiency can be achieved even on photocatalysts with a very short (sub-ns) intrinsic photocarrier lifetime, challenging the belief that a long carrier lifetime is a fundamental requirement. Our findings could provide guidance on the design of OWS photocatalysts toward more efficient solar-to-fuel conversion.

Regulating reconstruction of oxide-derived Cu for electrochemical CO2 reduction toward n-propanol.

Oxide-derived copper (OD-Cu) is the most efficient and likely practical electrocatalyst for CO2 reduction toward multicarbon products. However, the inevitable but poorly understood reconstruction from the pristine state to the working state of OD-Cu under strong reduction conditions largely hinders the rational construction of catalysts toward multicarbon products, especially C3 products like n-propanol. Here, we simulate the reconstruction of CuO and Cu2O into their derived Cu by molecular dynamics, revealing that CuO-derived Cu (CuOD-Cu) intrinsically has a richer population of undercoordinated Cu sites and higher surficial Cu atom density than the counterpart Cu2O-derived Cu (Cu2OD-Cu) because of the vigorous oxygen removal. In situ spectroscopes disclose that the coordination number of CuOD-Cu is considerably lower than that of Cu2OD-Cu, enabling the fast kinetics of CO2 reaction and strengthened binding of *C2 intermediate(s). Benefiting from the rich undercoordinated Cu sites, CuOD-Cu achieves remarkable n-propanol faradaic efficiency up to ~17.9%, whereas the Cu2OD-Cu dominantly generates formate.

Electron Paramagnetic Resonance Tracks Condition-Sensitive Water Radical Cation.

Oxidizing species or radicals generated in water are of vital importance in catalysis, the environment, and biology. In addition to several related reactive oxygen species, using electron paramagnetic resonance (EPR), we present a nontrapping chemical transformation pathway to track water radical cation (H2O+•) species, whose formation is very sensitive to the conditioning environments, such as light irradiation, mechanical action, and gas/chemical introduction. We reveal that H2O+• can oxidize the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to the crucial epoxy hydroxylamine (HDMP=O) intermediate, which further reacts with the hydroxyl radical (•OH) for the formation of the EPR-active sextet radical (DMPO=O•). Interestingly, we uncover that H2O+• can react with dimethyl methylphosphonate (DMMP), 2-methyl-2-nitrosopropane (MNP), 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO), and α-phenyl-N-tert-butylnitrone (PBN) which contain a double-bond structure to produce corresponding derivatives as well. It is thus expected that both H2O+• and •OH are ubiquitous in nature and in various water-containing experimental systems. These findings provide a novel perspective on radicals for water redox chemistry.

Self-adaptive amorphous CoOxCly electrocatalyst for sustainable chlorine evolution in acidic brine.

Electrochemical chlorine evolution reaction is of central importance in the chlor-alkali industry, but the chlorine evolution anode is largely limited by water oxidation side reaction and corrosion-induced performance decay in strong acids. Here we present an amorphous CoOxCly catalyst that has been deposited in situ in an acidic saline electrolyte containing Co2+ and Cl- ions to adapt to the given electrochemical condition and exhibits ~100% chlorine evolution selectivity with an overpotential of ~0.1 V at 10 mA cm-2 and high stability over 500 h. In situ spectroscopic studies and theoretical calculations reveal that the electrochemical introduction of Cl- prevents the Co sites from charging to a higher oxidation state thus suppressing the O-O bond formation for oxygen evolution. Consequently, the chlorine evolution selectivity has been enhanced on the Cl-constrained Co-O* sites via the Volmer-Heyrovsky pathway. This study provides fundamental insights into how the reactant Cl- itself can work as a promoter toward enhancing chlorine evolution in acidic brine.

Cu+-Mediated CO Coordination for Promoting C-C Coupling for CO2 and CO Electroreduction.

Selective electrochemical upgrading of CO2 to multicarbon (C2+) products requires a C-C coupling process, yet the underlying promoting mechanism of widely involved Cu oxidation states remains largely unclear, hindering the subtle design of efficient catalysts. Herein, we unveil the critical role of Cu+ in promoting C-C coupling via coordination with a CO intermediate during electrochemical CO2 reduction. We find that, relative to other halogen anions, iodide (I-) in HCO3- electrolytes accelerates the generation of strongly oxidative hydroxyl radicals that accounts for the formation of Cu+, which can be dynamically stabilized by I- via the formation of CuI. The in situ generated CO intermediate strongly binds to CuI sites, forming nonclassical Cu(CO)n+ complexes, leading to an approximately 3.0-fold increase of C2+ Faradaic efficiency at -0.9 VRHE relative to that of I--free Cu surfaces. Accordingly, a deliberate introduction of CuI into I--containing HCO3- electrolytes for direct CO electroreduction brings about a 4.3-fold higher C2+ selectivity. This work provides insights into the role of Cu+ in C-C coupling and the enhanced C2+ selectivity for CO2 and CO electrochemical reduction.

Non-covalent ligand-oxide interaction promotes oxygen evolution.

Strategies to generate high-valence metal species capable of oxidizing water often employ composition and coordination tuning of oxide-based catalysts, where strong covalent interactions with metal sites are crucial. However, it remains unexplored whether a relatively weak "non-bonding" interaction between ligands and oxides can mediate the electronic states of metal sites in oxides. Here we present an unusual non-covalent phenanthroline-CoO2 interaction that substantially elevates the population of Co4+ sites for improved water oxidation. We find that phenanthroline only coordinates with Co2+ forming soluble Co(phenanthroline)2(OH)2 complex in alkaline electrolytes, which can be deposited as amorphous CoOxHy film containing non-bonding phenanthroline upon oxidation of Co2+ to Co3+/4+. This in situ deposited catalyst demonstrates a low overpotential of 216 mV at 10 mA cm-2 and sustainable activity over 1600 h with Faradaic efficiency above 97%. Density functional theory calculations reveal that the presence of phenanthroline can stabilize CoO2 through the non-covalent interaction and generate polaron-like electronic states at the Co-Co center.

Enabling Specific Photocatalytic Methane Oxidation by Controlling Free Radical Type.

Selective CH4 oxidation to CH3OH or HCHO with O2 in H2O under mild conditions provides a desired sustainable pathway for synthesis of commodity chemicals. However, manipulating reaction selectivity while maintaining high productivity remains a huge challenge due to the difficulty in the kinetic control of the formation of a desired oxygenate against its overoxidation. Here, we propose a highly efficient strategy, based on the precise control of the type of as-formed radicals by rational design on photocatalysts, to achieve both high selectivity and high productivity of CH3OH and HCHO in CH4 photooxidation for the first time. Through tuning the band structure and the size of active sites (i.e., single atoms or nanoparticles) in our Au/In2O3 catalyst, we show alternative formation of two important radicals, •OOH and •OH, which leads to distinctly different reaction paths to the formation of CH3OH and HCHO, respectively. This approach gives rise to a remarkable HCHO selectivity and yield of 97.62% and 6.09 mmol g-1 on In2O3-supported Au single atoms (Au1/In2O3) and an exceptional CH3OH selectivity and yield of 89.42% and 5.95 mmol g-1 on In2O3-supported Au nanoparticles (AuNPs/In2O3), respectively, upon photocatalytic CH4 oxidation for 3 h at room temperature. This work opens a new avenue toward efficient and selective CH4 oxidation by delicate design of composite photocatalysts.

Hydroxyl radicals dominate reoxidation of oxide-derived Cu in electrochemical CO2 reduction.

Cuδ+ sites on the surface of oxide-derived copper (OD-Cu) are of vital importance in electrochemical CO2 reduction reaction (CO2RR). However, the underlying reason for the dynamically existing Cuδ+ species, although thermodynamically unstable under reductive CO2RR conditions, remains uncovered. Here, by using electron paramagnetic resonance, we identify the highly oxidative hydroxyl radicals (OH•) formed at room temperature in HCO3- solutions. In combination with in situ Raman spectroscopy, secondary ion mass spectrometry, and isotope-labelling, we demonstrate a dynamic reduction/reoxidation behavior at the surface of OD-Cu and reveal that the fast oxygen exchange between HCO3- and H2O provides oxygen sources for the formation of OH• radicals. In addition, their continuous generations can cause spontaneous oxidation of Cu electrodes and produce surface CuOx species. Significantly, this work suggests that there is a "seesaw-effect" between the cathodic reduction and the OH•-induced reoxidation, determining the chemical state and content of Cuδ+ species in CO2RR. This insight is supposed to thrust an understanding of the crucial role of electrolytes in CO2RR.

A self-healing catalyst for electrocatalytic and photoelectrochemical oxygen evolution in highly alkaline conditions.

While self-healing is considered a promising strategy to achieve long-term stability for oxygen evolution reaction (OER) catalysts, this strategy remains a challenge for OER catalysts working in highly alkaline conditions. The self-healing of the OER-active nickel iron layered double hydroxides (NiFe-LDH) has not been successful due to irreversible leaching of Fe catalytic centers. Here, we investigate the introduction of cobalt (Co) into the NiFe-LDH as a promoter for in situ Fe redeposition. An active borate-intercalated NiCoFe-LDH catalyst is synthesized using electrodeposition and shows no degradation after OER tests at 10 mA cm-2 at pH 14 for 1000 h, demonstrating its self-healing ability under harsh OER conditions. Importantly, the presence of both ferrous ions and borate ions in the electrolyte is found to be crucial to the catalyst's self-healing. Furthermore, the implementation of this catalyst in photoelectrochemical devices is demonstrated with an integrated silicon photoanode. The self-healing mechanism leads to a self-limiting catalyst thickness, which is ideal for integration with photoelectrodes since redeposition is not accompanied by increased parasitic light absorption.

Efficient Alkaline Water Oxidation with a Regenerable Nickel Pseudo-Complex.

Efficient and robust electrocatalysts are required for the oxygen evolution reaction (OER). Photosystem II-inspired synthetic transition metal complexes have shown promising OER activity in water-poor or mild conditions, yet challenges remain in the improvement of current density and performance stability for practical applications in alkaline electrolytes in contrast to solid-state oxide catalysts. Here, we report that a nickel pseudo-complex (bpy)zNiOxHy (bpy = 2,2'-bipyridine) catalyst, which bridges solid oxide and molecular catalysts, exhibits the highest OER activity among nickel-based catalysts with a turnover frequency of 1.1 s-1 at an overpotential of 0.30 volts, even outperforming iron-incorporated nickel (oxy)hydroxide under an identical nickel mass load. Benefiting from the strong coordination between bpy and nickel, this (bpy)zNiOxHy catalyst exhibits long-term stability in highly alkaline media at 1.0 mA cm-2 for over 200 h and at 20 mA cm-2 for over 60 h. Our findings indicate that dynamically coordinating a small amount of bpy in the catalyst layer efficiently sustains highly active nickel sites for water oxidation, demonstrating a general strategy for improving the activity of transition metal sites with active ligands beyond the incorporation of metal cations to form double-layered hydroxides.

Sulfur-anchoring synthesis of platinum intermetallic nanoparticle catalysts for fuel cells.

Atomically ordered intermetallic nanoparticles are promising for catalytic applications but are difficult to produce because the high-temperature annealing required for atom ordering inevitably accelerates metal sintering that leads to larger crystallites. We prepared platinum intermetallics with an average particle size of <5 nanometers on porous sulfur-doped carbon supports, on which the strong interaction between platinum and sulfur suppresses metal sintering up to 1000°C. We synthesized intermetallic libraries of small nanoparticles consisting of 46 combinations of platinum with 16 other metal elements and used them to study the dependence of electrocatalytic oxygen-reduction reaction activity on alloy composition and platinum skin strain. The intermetallic libraries are highly mass efficient in proton-exchange-membrane fuel cells and could achieve high activities of 1.3 to 1.8 amperes per milligram of platinum at 0.9 volts.

Improved water oxidation with metal oxide catalysts via a regenerable and redox-inactive ZnOxHy overlayer.

We report a regenerable and redox-inactive ZnOxHy layer that was in situ deposited onto metal oxides MOz (M = Co, Fe, and Ni) in alkaline media containing [Zn(OH)4]2- species during water oxidation. An interface dipole was developed at the MOz/Zn interface, resulting in a decrease of the OER overpotential. Exemplified by the CoOz/ZnOxHy bilayer structure, it presented a 155 mV lower overpotential to deliver 10 mA cm-2 and long-term stability relative to the unmodified CoOz film.

Molecular-Scale Insights into Electrochemical Reduction of CO2 on Hydrophobically Modified Cu Surfaces.

Depressing the competitive hydrogen evolution reaction (HER) to promote current efficiency toward carbon-based chemicals in the electrocatalytic CO2 reduction reaction (CO2RR) is desirable. A strategy is to apply the hydrophobically molecular-modified electrodes. However, the molecular-scale catalytic process remains poorly understood. Using alkanethiol-modified hydrophobic Cu as an electrode and CO2-saturated KHCO3 as an electrolyte, we reveal that H2O, rather than HCO3-, is the major H+ source for the HER, determined by differential electrochemical mass spectrometry with isotopic labeling. As a result, using in situ Raman, we find that the hydrophobic molecules screen the cathodic electric field effect on the reorientation of interfacial H2O to a "H-down" configuration toward Cu surfaces that corresponds to the decreased content of H-bonding-free water, leading to unfavorable H2O dissociation and thus decreased H+ source for the HER. Further, density functional theory calculations suggest that the absorbed alkanethiol molecules alter the electronic structure of Cu sites, thus decreasing the formation energy barrier of CO2RR intermediates, which consequently increases the CO2RR selectivity. This work provides a molecular-level understanding of improved CO2RR on hydrophobically molecule-modified catalysts and presents general references for catalytic systems having H2O-involved competitive HER.

Phase-Segregated SrCo0.8Fe0.5-xO3-δ/FexOy Heterostructured Catalyst Promotes Alkaline Oxygen Evolution Reaction.

Perovskite oxide is a promising alternative to noble metal electrocatalysts for the oxygen evolution reaction (OER). However, as one of the most active oxide catalysts, cubic SrCoO3 presents poor OER performance relative to the theoretically predicted activity. Appropriate introduction of a guest component in the lattice and surface could largely promote the OER activity. Herein, we present a thermal-induced phase-segregation strategy to synthesize a heterostructured SrCo0.8Fe0.5-xO3-δ/FexOy (SC8F5) catalyst for OER. This novel perovskite/Fe3O4 heterostructure allows us to enhance the electrical conductivity ability, increase the Co oxidation state, and activate the surface oxygen to active oxygen species (O22-/O-) for efficient OER. In contrast to the poor stability of SrCo0.8Fe0.2O3-δ, we found that the SC8F5 heterostructure with segregated Fe3O4 on the surface can mitigate surface reconstruction and stabilize the catalyst structure, thereby increasing catalytic stability.

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.

Nickel Iron Diselenide for Highly Efficient and Selective Electrocatalytic Conversion of Methanol to Formate.

The electro-oxidation of methanol to formate is an interesting example of the potential use of renewable energies to add value to a biosourced chemical commodity. Additionally, methanol electro-oxidation can replace the sluggish oxygen evolution reaction when coupled to hydrogen evolution or to the electroreduction of other biomass-derived intermediates. But the cost-effective realization of these reaction schemes requires the development of efficient and low-cost electrocatalysts. Here, a noble metal-free catalyst, Ni1- x Fex Se2 nanorods, with a high potential for an efficient and selective methanol conversion to formate is demonstrated. At its optimum composition, Ni0.75 Fe0.25 Se2 , this diselenide is able to produce 0.47 mmol cm-2  h-1 of formate at 50 mA cm-2 with a Faradaic conversion efficiency of 99%. Additionally, this noble-metal-free catalyst is able to continuously work for over 50 000 s with a minimal loss of efficiency, delivering initial current densities above 50 mA cm-2 and 2.2 A mg-1 in a 1.0 m KOH electrolyte with 1.0 m methanol at 1.5 V versus reversible hydrogen electrode. This work demonstrates the highly efficient and selective methanol-to-formate conversion on Ni-based noble-metal-free catalysts, and more importantly it shows a very promising example to exploit the electrocatalytic conversion of biomass-derived chemicals.

Selective Methanol-to-Formate Electrocatalytic Conversion on Branched Nickel Carbide.

A methanol economy will be favored by the availability of low-cost catalysts able to selectively oxidize methanol to formate. This selective oxidation would allow extraction of the largest part of the fuel energy while concurrently producing a chemical with even higher commercial value than the fuel itself. Herein, we present a highly active methanol electrooxidation catalyst based on abundant elements and with an optimized structure to simultaneously maximize interaction with the electrolyte and mobility of charge carriers. In situ infrared spectroscopy combined with nuclear magnetic resonance spectroscopy showed that branched nickel carbide particles are the first catalyst determined to have nearly 100 % electrochemical conversion of methanol to formate without generating detectable CO2 as a byproduct. Electrochemical kinetics analysis revealed the optimized reaction conditions and the electrode delivered excellent activities. This work provides a straightforward and cost-efficient way for the conversion of organic small molecules and the first direct evidence of a selective formate reaction pathway.