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Zero-gap-type reactors with gas diffusion electrodes (GDE) that facilitate the CO2 reduction reaction (CO2RR) are attractive due to their high current density and low applied voltage. These reactors, however, suffer from salt precipitation and anolyte flooding of the cathode, leading to a short lifetime. Here, using a zero-gap reactor with a transparent cathode end plate, we report periodic voltage oscillations under constant current operation. Increases in cell voltages occur at the same time as the reactor switches from the hydrogen evolution reaction (HER) to predominant CO2RR; decreases in cell voltage occur with the switch from the CO2RR to HER. Further, real time visual observations show that salt precipitation occurs during the CO2RR, whereas salt dissolution occurs during the HER. Slow flooding triggers the transition from the CO2RR to HER. A number of processes combine to slowly reduce the water content in the microporous layer, which triggers the transition back to the CO2RR.

Electrochemical CO2 reduction (CO2R) using heterogenized molecular catalysts usually yields 2-electron reduction products (CO, formate). Recently, it has been reported that certain preparations of immobilized cobalt phthalocyanine (CoPc) produce methanol (MeOH), a 6-electron reduction product. Here, we demonstrate the significant role of intermediate mass transport in CoPc selectivity to methanol. We first developed a simple, physically mixed, polymer (and polyfluoroalkyl, PFAS)-free preparation of CoPc on multiwalled carbon nanotubes (MWCNTs) which can be integrated onto Au electrodes using a poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) adhesion layer. After optimization of catalyst preparation and loading, methanol Faradaic efficiencies and partial current densities of 36% (±3%) and 3.8 (±0.5) mA cm-2, respectively, are achieved in the CO2-saturated aqueous electrolyte. The electrolyte flow rate has a large effect. A linear flow velocity of 8.5 cm/min produces the highest MeOH selectivity, with higher flow rates increasing CO selectivity and lower flow rates increasing the hydrogen evolution reaction, suggesting that CO is an unbound intermediate. Using a continuum multiphysics model assuming CO is the intermediate, we show qualitative agreement with the optimal inlet flow rate. Polymer binders were not required to achieve a high Faradaic efficiency for methanol using CoPc and MWCNTs. We also investigated the role of formaldehyde as an intermediate and the role of strain, but definitive conclusions could not be established.

Electrochemical CO2 reduction on Cu is a promising approach to produce value-added chemicals using renewable feedstocks, yet various Cu preparations have led to differences in activity and selectivity toward single and multicarbon products. Here, we find, surprisingly, that the effective catalytic activity toward ethylene improves when there is a larger fraction of less active sites acting as reservoirs of *CO on the surface of Cu nanoparticle electrocatalysts. In an adaptation of chemical transient kinetics to electrocatalysis, we measure the dynamic response of a gas diffusion electrode (GDE) cell when the feed gas is abruptly switched between Ar (inert) and CO. When switching from Ar to CO, CO reduction (COR) begins promptly, but when switching from CO to Ar, COR can be maintained for several seconds (delay time) despite the absence of the CO reactant in the gas phase. A three-site microkinetic model captures the observed dynamic behavior and shows that Cu catalysts exhibiting delay times have a less active *CO reservoir that exhibits fast diffusion to active sites. The observed delay times and the estimated *CO reservoir sizes are affected by catalyst preparation, applied potential, and microenvironment (electrolyte cation identity, electrolyte pH, and CO partial pressure). Notably, we estimate that the *CO reservoir surface coverage can be as high as 88 ± 7% on oxide-derived Cu (OD-Cu) at high overpotentials (-1.52 V vs SHE) and this increases in reservoir coverage coincide with increased turnover frequencies to ethylene. We also estimate that *CO can travel substantial distances (up to 10s of nm) prior to desorption or reaction. It appears that active C-C coupling sites by themselves do not control selectivity to C2+ products in electrochemical COR; the supply of CO to those sites is also a crucial factor. More generally, the overall activity of Cu electrocatalysts cannot be approximated from linear combinations of individual site activities. Future designs must consider the diversity of the catalyst network and account for intersite transportation pathways.

The origin of p-type conductivity and the mechanism responsible for low carrier mobility were investigated in pyrite (FeS2) thin films. Temperature-dependent resistivity measurements (10-400 K) were performed on polycrystalline and nanostructured thin films prepared by three different methods: (1) spray pyrolysis, (2) hot-injection synthesized and spin-coated nanocubes, and (3) pulsed laser deposition. The films have a high hole density (1018-1019) cm-3 and low mobility (0.1-4 cm2 V-1 s-1) regardless of the method used for their preparation. The charge transport mechanism is determined to be thermally activated conduction (TAC) at near room temperature, with Mott-type variable-range hopping (VRH) of holes via localized states occurring at lower temperatures. The density functional theory (DFT) predicts that sulfur vacancy induces localized defect states within the band gap and the charge remains localized around the defect. The data indicates that the electronic properties including hopping transport in pyrite thin films can be correlated to sulfur vacancy-related defects. The results provide insights into the electronic properties of pyrite thin films and their implications for charge transport.

Electrosynthesis of ethanol from carbon dioxide (CO2) is a promising route to generate a sustainable fuel and a convenient feedstock for chemical manufacturing. While significant progress has been achieved in boosting the selectivity of CO2 to ethanol, the subsequent ethanol separation remains a bottleneck, which prevents leveraging the laboratory results into large-scale systems. Here we report vacuum membrane distillation as a method that efficiently concentrates dilute ethanol streams produced by CO2 electrolysis (CO2R), yielding up to ∼40 wt% ethanol in pure water. In our design considerations, we include previously underappreciated thermodynamic properties of the catholyte (salting-out effect) and propose strategies allowing a more precise estimation of energy inputs to the separation processes. Our work provides the basis for the detailed design of complex systems which integrate flow reactors and liquid separations and supports scaling of the systems previously considered not optimized for industrial use.

Semiconductor photocatalyst approaches for solar CO2 reduction are attractive due to their simplicity but have lagged in efficiency compared to less-integrated photoelectrochemical (PEC) approaches and to electrolysis reactors. We identify poor mass transport and catalyst deactivation as key constraints. To address them, we have developed a continuous-flow photocatalytic reactor system allowing us to control the triple-phase interface on the photocatalyst surface using the liquid and reactant gas flow rates. With the goal of selectively producing CO, the reactor is optimized by controlling the pressure and flow rates of the reactant gas and electrolyte in contact with both sides with the intermediately placed catalyst. In comparison to batch reactors with an immobile photocatalyst bed and gas phase CO2 or CO2 purged water, 10-24 times higher production rates are achieved for photocatalysts such as TiO2, ZnO, C3N4, and CdS by simply changing to the designed flow-type photoreactor without any catalyst modification. In addition, CO selectivity (93.2%) and long-term stability (>780 min) using the designed reactor are significantly enhanced compared to using the batch reactors (71.7%,

Electron transport layers (ETLs) used as components of photocathodes for light-driven CO2 reduction (CO2R) in aqueous media should have good electronic transport, be stable under CO2R conditions, and, ideally, be catalytically inert for the competing hydrogen evolution reaction (HER). Here, using planar p-Si (100) as the absorbing material, we show that TaOx satisfies all three of the above criteria. TaOx films were synthesized by both pulsed laser deposition (PLD) and radio-frequency (RF) sputtering. In both cases, careful control of the oxygen partial pressure during growth was required to produce ETLs with acceptable electron conductivity. p-Si/TaOx photocathodes were interfaced with ca. 10 nm of a CO2R catalyst: Cu or Au. Under front illumination with simulated AM 1.5G in CO2-saturated bicarbonate buffer, we observed, for both metals, faradaic efficiencies for CO2R products of ∼50% and ∼30% for PLD TaOx and RF sputtered TaOx, respectively, at photocurrent densities up to 8 mA cm−2. p-Si/TiO2/Cu photocathodes were also evaluated but produced mostly H2 (>97%) due to reduction of the TiO2 to Ti metal under CO2R conditions. In contrast, a dual ETL photocathode (p-Si/TiO2/TaOx/Cu) was selective for CO2R, which suggests a strategy for separately optimizing selective charge collection and the stability of the ETL/water interface. The maximum photovoltage obtained with p-Si/TaOx/Cu devices was 300 mV which was increased to 430-460 mV by employing ion implantation to make pn+-Si/TaOx/Cu structures. Photocathodes with RF sputtered TaOx ETLs are stable for CO2R for at least 300 min. Techno-economic analysis shows that the reported system, if scaled, could allow for an economically viable production of feedstocks for chemical synthesis under the adoption of specific CO2 credit schemes, thus becoming a significant component of carbon-neutral manufacturing.

Because single catalysts cannot selectively convert CO2 into a specific reduced product more complex than CO either electrochemically or photoelectrochemically, an alternative is to use multiple catalysts organized into a cascade, which offers a means of generating complex chemicals with improved specificity by providing a channel for the reduction of CO and similar intermediates. The rules for how to select catalysts for this purpose and design systems that have a high degree of chemical control using them are not known however. The efficiency and selectivity for individual catalysts may be optimized, but the resulting efficiency of an entire system will depend on how well the timings of all the chemical steps in the cascade as well as transport between the catalysts are managed. In this Perspective, we discuss these challenges and examine the ways in which control is exerted in natural photosynthetic systems which use cascaded reactions to convert CO2 from the air into sugars using sunlight as the sole source of energy. These cascades take place within complex architectural elements that ensure that the chemistry is as efficient as possible. Adaptations of the functions of these natural design elements to artificial systems may offer ways to attain the promise of artificial photosynthesis.

Utilising carbon dioxide by synthesising commodity chemicals via electrocatalysis shows potential for long-term energy storage and industry decarbonisation. The latest copper-based gas-diffusion electrodes can operate at high currents, enabling large conversion rates. However, our incomplete understanding of active reaction paths in this system hinders us from designing catalysts with improved selectivities and reduced poisoning. Here, we identify and analyse ten previously unknown minor products of electrochemical CO2 reduction. Using an ultra-sensitive GC-MS setup, we report more than 20 products, including C5 species for the first time. From the trends in selectivity, we hypothesise two distinct reaction paths: while the coupling of oxygenated intermediates begins at very small potentials and favours double bond formation in the middle of carbon chains, coupling of highly-reduced methane precursors requires a large potential and leads to double bond formation at the chain end. This contribution represents a significant step towards the holistic comprehension of the mechanism for electrocatalytic CO2 reduction and calls for further mechanistic exploration via minor products and investigation of favourable reaction conditions.

The development of a highly selective, membrane-based ion separation technology could significantly improve the sustainability and energy efficiency of water treatment technologies and emerging applications, such as electrochemical CO2 reduction, extraction of valuable metals from seawater, and battery recycling. In this work, we show through computational modeling that an electronic flashing ratchet mechanism can be used for high-precision ion separation. The suggested ratchet-based ion pumps utilize a unique feature of electronic ratchets, frequency-dependent current reversal, to drive ions with the same charge, but different diffusion coefficient, in opposite directions. The model shows that ions whose diffusion coefficients differ by as little as 1% can be separated by driving them in opposite directions with a velocity difference as high as 1.2 mm/s. Since the pumping properties of the ratchet are determined by a time-varying electric input signal, the proposed ion pumps could be instrumental in realizing an efficient, large-scale, and fit-for-purpose system for selective ion separation. Examples of ratchet-driven systems for lithium extraction from seawater, lead removal from drinking water, and water desalination are discussed and analyzed.

In a recent issue of Nature, Zhou et al. report an artificial photosynthesis scheme that splits water into hydrogen and oxygen with an overall energy efficiency of nearly 10%, close to the value needed for commercial viability. This preview provides a technological context for this advancement and discusses needed next steps.

The development of a nonprecious, stable, and highly effective electrocatalyst for decomposition of water into oxygen and hydrogen is vitally important for sustainable energy conversion, but it still remains challenging to replace the noble metal electrocatalysts with more economically viable alternatives. Herein, a polyaniline (PANI) decorated hierarchical nickel cobalt thiospinel (NiCo2S4) hybrid catalyst (NCS-P) has been developed that shows enhanced dual catalytic activity for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) compared to pristine NiCo2S4 (NCS). Benefiting from the conductive PANI coating, the hierarchical NCS-P nanostructure exhibits outstanding electrocatalytic activity in alkaline solution with low overpotentials of 273 ± 3 and 77 ± 4 mV at 10 mA cm−2 and low Tafel slopes of 42.2 and 68.5 mV dec−1 for OER and HER, respectively, which are better than those of the benchmark noble-metal-based RuO2 and Pt/C.

Renewable fuel generation is essential for a low carbon footprint economy. Thus, over the last five decades, a significant effort has been dedicated towards increasing the performance of solar fuels generating devices. Specifically, the solar to hydrogen efficiency of photoelectrochemical cells has progressed steadily towards its fundamental limit, and the faradaic efficiency towards valuable products in CO2 reduction systems has increased dramatically. However, there are still numerous scientific and engineering challenges that must be overcame in order to turn solar fuels into a viable technology. At the electrode and device level, the conversion efficiency, stability and products selectivity must be increased significantly. Meanwhile, these performance metrics must be maintained when scaling up devices and systems while maintaining an acceptable cost and carbon footprint. This roadmap surveys different aspects of this endeavor: system benchmarking, device scaling, various approaches for photoelectrodes design, materials discovery, and catalysis. Each of the sections in the roadmap focuses on a single topic, discussing the state of the art, the key challenges and advancements required to meet them. The roadmap can be used as a guide for researchers and funding agencies highlighting the most pressing needs of the field.

Isotopically purified semiconductors potentially dissipate heat better than their natural, isotopically mixed counterparts as they have higher thermal conductivity (κ). But the benefit is low for Si at room temperature, amounting to only ∼10% higher κ for bulk ^{28}Si than for bulk natural Si (^{nat}Si). We show that in stark contrast to this bulk behavior, ^{28}Si (99.92% enriched) nanowires have up to 150% higher κ than ^{nat}Si nanowires with similar diameters and surface morphology. Using a first-principles phonon dispersion model, this giant isotope effect is attributed to a mutual enhancement of isotope scattering and surface scattering of phonons in ^{nat}Si nanowires, correlated via transmission of phonons to the native amorphous SiO_{2} shell. The Letter discovers the strongest isotope effect of κ at room temperature among all materials reported to date and inspires potential applications of isotopically enriched semiconductors in microelectronics.

Flooding of gas diffusion electrodes (GDEs) used in electrochemical conversion of CO2 to ethylene is caused by polymerization of minor products. The polymer so created reduces the hydrophobicity of the GDE, leading initially to a reduction in the ethylene yield and finally to complete failure.

Metal leaching during water oxidation has been typically observed in conjunction with surface reconstruction on perovskite oxide catalysts, but the role of metal leaching at each geometric site has not been distinguished. Here, we manipulate the occurrence and process of surface reconstruction in two model ABO3 perovskites, i.e., SrSc0.5Ir0.5O3 and SrCo0.5Ir0.5O3, which allow us to evaluate the structure and activity evolution step by step. The occurrence and order of leaching of Sr (A-site) and Sc/Co (B-site) were controlled by tailoring the thermodynamic stability of B-site. Sr leaching from A-site mainly generates more electrochemical surface area for the reaction, and additional leaching of Sc/Co from B-site triggers the formation of a honeycomb-like IrOxHy phase with a notable increase in intrinsic activity. A thorough surface reconstruction with dual-site metal leaching induces an activity improvement by approximately two orders of magnitude, which makes the reconstructed SrCo0.5Ir0.5O3 among the best for water oxidation in acid.

Cascade photoelectrocatalysis (PEC) is a possible method to improve the selectivity of solar-driven CO2 reduction (CO2R). This concept can be realized by coupling different CO2R catalysts to different subcells in a multijunction photovoltaic (PV) stack. Efficient implementation will require careful tuning of the photocurrents and design of the photovoltages provided by the subcells to the CO2R catalysts in such a way as to facilitate the target reaction. Here, we outline the design principles of the tandem PEC approach using two-step conversion of CO2 to ethylene in aqueous electrolyte, via a CO intermediate, as a model system. To perform this reaction, the first coupled PV-catalyst component should provide 4 electrons to reduce 2 molecules of CO2 to CO; the second component should provide 8 electrons to reduce 2 CO molecules to C2H4. Based on known CO2R catalysts, the overpotential required to produce CO can be less than that required to reduce it to ethylene, creating the opportunity for improved efficiency. Cascade PEC can be realized in a three-terminal tandem (3TT) configuration using III-V-semiconductor based subcells coupled to Au (produces CO intermediate) and Cu (converts CO to ethylene). The current to each catalyst can be controlled by the area of the subcell exposed to the electrolyte, and the photovoltage is determined by the materials selected and device configuration. Operating conditions are found by simulating the coupled system using the open-source circuit simulator SPICE (Simulation Program with Integrated Circuits Emphasis). We identify conditions under which a 3TT configuration can have a higher solar to chemical conversion efficiency compared to a two-terminal two-junction tandem (2T 2J) with the same absorbers and a Cu catalyst only. We also show that 3TT PEC devices can be less sensitive to variations in catalyst activity compared to 2T devices. Finally, we discuss the applications of cascade PEC to CO2 reduction, using different intermediates, and to other chemical networks.

Recycling of waste CO2 to bulk chemicals has a tremendous potential for the decarbonization of the chemical industry. Quantitative analysis of the prospects of this technology is hindered by the lack of flexible techno-economic assessment (TEA) models that enable evaluation of the processing costs under different deployment scenarios. In this protocol, we explain how to convert literature data into metrics useful for evaluation of the emerging electrolysis technologies, derive TEA models, and illustrate their use with a CO2-to-ethylene example. For complete details on the use and execution of this protocol, please refer to Barecka et al. (2021a).

Kinetic Monte Carlo (KMC) methods are frequently used for mechanistic studies of thermally driven heterogeneous catalysis systems but are underused for electrocatalysis. Here, we develop a lattice KMC approach for electrocatalytic CO2 reduction. The work is motivated by a prior experimental report that performed electroreduction of a mixed feed of 12CO2 and 13CO on Cu; differences in the 13C content of C2 products ethylene and ethanol (Δ13C) were interpreted as evidence of site selectivity. The lattice KMC model considers the effect of surface diffusion on this system. In the limit of infinitely fast diffusion (mean-field approximation), the key intermediates 12CO* and 13CO* would be well mixed on the surface and no evidence of site selectivity could have been observed. Using a simple two-site model and adapting a previously reported microkinetic model, we assess the effects of diffusion on the relative isotope fractions in the products using the estimated surface diffusion rate of CO* from literature reports. We find that the size of the active sites and the total surface adsorbate coverage can have a large influence on the values of Δ13C that can be observed. Δ13C is less sensitive to the CO* diffusion rate as long as it is within the estimated range. We further offer possible methods to estimate surface distribution of intermediates and to predict intrinsic selectivity of active sites based on experimental observations. This work illustrates the importance of considering surface diffusion in the study of electrochemical CO2 reduction to multi-carbon products. Our approach is entirely based on a freely available open-source code, so will be readily adaptable to other electrocatalytic systems.

If combined with renewably generated electricity, electrochemical CO2 reduction (E-CO2R) could be used as a sustainable source of chemicals and fuels. Tandem catalysis approaches are attractive for providing the product selectivity, which would be required for commercial applications. Here, we demonstrate a two-step tandem electrocatalytic E-CO2R with efficient conversion of the intermediate species. The catalyst scaffold is Si(100), which is etched to form a textured surface consisting of micron-sized pyramid structures with the {111} facets. Two metals are used in the electrocatalytic cascade: Ag is employed to perform a two-electron reduction of CO2 to the intermediate CO, and Cu performs conversion to more reduced products. Using high-angle physical vapor deposition, we form separated, micron-scale areas of the two electrocatalysts on opposite sides of the pyramids, with their relative surface coverages being tunable with the deposition angle. Compared to the textured scaffolds with blanket Ag and Cu used as controls, bimetallic pyramid tandem catalysts have higher current densities and much lower faradic efficiencies (FE) for CO. These effects are due to efficient conversion of the CO formed on Ag to more reduced products on Cu. Methane is the main product to be enhanced by the cascade pathway: a bimetallic catalyst with approximately equal coverages of Ag and Cu produces methane with a FE of 62% at -1.1 VRHE, corresponding to a partial current density of 12.7 mA cm-2. We estimate an intermediate conversion yield for the CO intermediate of 80-90%, which is close to the mass-transport limited value predicted by reaction-diffusion simulations.

Among metals used for CO2 electroreduction in water, Cu appears to be unique in its ability to produce C2+ products like ethylene. Bimetallic combinations of Cu with other metals have been investigated with the goal of steering selectivity via creating a tandem pathway through the CO intermediate or by changing the surface electronic structure. Here, we demonstrate a facile cation exchange method to synthesize Ag/Cu electrocatalysts for CO2 reduction using Cu sulfides as a growth template. Beginning with Cu2-x S nanosheets (C-nano-0, 100 nm lateral dimension, 14 nm thick), varying the Ag+ concentration in the exchange solution produces a gradual change in crystal structure from Cu7S4 to Ag2S, as the Ag/Cu mass ratio varies from 0.3 to 25 (CA-nano-x, x indicating increasing Ag fraction). After cation exchange, the nanosheet morphology remains but with increased shape distortion as the Ag fraction is increased. Interestingly, the control (C-nano-0) and cation exchanged nanosheets have very high faradaic efficiency for producing formate at low overpotential (-0.2 V vs. RHE). The primary effect of Ag incorporation is increased production of C2+ products at -1.0 V vs. RHE compared with C-nano-0, which primarily produces formate. Cation exchange can also be used to modify the surface of Cu foils. A two-step electro-oxidation/sulfurization process was used to form Cu sulfides on Cu foil (C-foil-x) to a depth of a few 10 s of microns. With lower Ag+ concentrations, cation exchange produces uniformly dispersed Ag; however, at higher concentrations, Ag particles nucleate on the surface. During CO2 electroreduction testing, the product distribution for Ag/Cu sulfides on Cu foil (CA-foil-x-y) changes in time with an initial increase in ethylene and methane production followed by a decrease as more H2 is produced. The catalysts undergo a morphology evolution towards a nest-like structure which could be responsible for the change in selectivity. For cation-exchanged nanosheets (CA-nano-x), pre-reduction at negative potentials increases the CO2 reduction selectivity compared to tests of as-synthesized material, although this led to the aggregation of nanosheets into filaments. Both types of bimetallic catalysts are capable of selective reduction of CO2 to multi-carbon products, although the optimal configurations appear to be metastable.

Producing hydrogen by water electrolysis suffers from the kinetic barriers in the oxygen evolution reaction (OER) that limits the overall efficiency. With spin-dependent kinetics in OER, to manipulate the spin ordering of ferromagnetic OER catalysts (e.g., by magnetization) can reduce the kinetic barrier. However, most active OER catalysts are not ferromagnetic, which makes the spin manipulation challenging. In this work, we report a strategy with spin pinning effect to make the spins in paramagnetic oxyhydroxides more aligned for higher intrinsic OER activity. The spin pinning effect is established in oxideFM/oxyhydroxide interface which is realized by a controlled surface reconstruction of ferromagnetic oxides. Under spin pinning, simple magnetization further increases the spin alignment and thus the OER activity, which validates the spin effect in rate-limiting OER step. The spin polarization in OER highly relies on oxyl radicals (O∙) created by 1st dehydrogenation to reduce the barrier for subsequent O-O coupling.

The chemical industry needs to significantly decrease carbon dioxide (CO2) emissions in order to meet the 2050 carbon neutrality goal. Utilization of CO2 as a chemical feedstock for bulk products is a promising way to mitigate industrial emissions; however, CO2-based manufacturing is currently not competitive with the established petrochemical methods and its deployment requires creation of a new value chain. Here, we show that an alternative approach, using CO2 conversion as an add-on to existing manufactures, can disrupt the global carbon cycle while minimally perturbing the operation of chemical plants. Proposed closed-loop on-site CO2 recycling processes are economically viable in the current market and have the potential for rapid introduction in the industries. Retrofit-based CO2 recycling can reduce annually between 4 and 10 Gt CO2 by 2050 and contribute to achieving up to 50% of the industrial carbon neutrality goal.

Perovskite oxides based on earth-abundant transition metals have been extensively explored as promising oxygen evolution reaction (OER) catalysts in alkaline media. The (electro)chemically induced transformation of their initially crystalline surface into an amorphous state has been reported for a few highly active perovskite catalysts. However, little knowledge is available to distinguish the contribution of the amorphized surface from that of the remaining bulk toward the OER. In this work, we utilize the promoting effects of two types of Fe modification, i.e., bulk Fe dopant and Fe ions absorbed from the electrolyte, on the OER activity of SrCoO3-δ model perovskite to identify the active phase. Transmission electron microscopy and X-ray photoelectron spectroscopy confirmed the surface amorphization of SrCoO3-δ as well as SrCo0.8Fe0.2O3-δ after potential cycling in Fe-free KOH solution. By further cycling in Fe-spiked electrolyte, Fe was incorporated into the amorphized surface of SrCoO3-δ (SrCoO3-δ + Fe3+), yielding approximately sixfold increase in activity. Despite the difference in remaining perovskites, SrCoO3-δ + Fe3+ and SrCo0.8Fe0.2O3-δ exhibited remarkably similar activity. These results reflect that the in situ developed surface species are directly responsible for the measured OER activity, whereas the remaining bulk phases have little impact.

Perovskite oxides based on earth-abundant transition metals have been extensively explored as promising oxygen evolution reaction (OER) catalysts in alkaline media. The (electro)chemically induced transformation of their initially crystalline surface into an amorphous state has been reported for a few highly active perovskite catalysts. However, little knowledge is available to distinguish the contribution of the amorphized surface from that of the remaining bulk toward the OER. In this work, we utilize the promoting effects of two types of Fe modification, i.e., bulk Fe dopant and Fe ions absorbed from the electrolyte, on the OER activity of SrCoO3-δ model perovskite to identify the active phase. Transmission electron microscopy and X-ray photoelectron spectroscopy confirmed the surface amorphization of SrCoO3-δ as well as SrCo0.8Fe0.2O3-δ after potential cycling in Fe-free KOH solution. By further cycling in Fe-spiked electrolyte, Fe was incorporated into the amorphized surface of SrCoO3-δ (SrCoO3-δ + Fe3+), yielding approximately sixfold increase in activity. Despite the difference in remaining perovskites, SrCoO3-δ + Fe3+ and SrCo0.8Fe0.2O3-δ exhibited remarkably similar activity. These results reflect that the in situ developed surface species are directly responsible for the measured OER activity, whereas the remaining bulk phases have little impact.

The previously developed bistable amphoteric native defect (BAND) model is used for a comprehensive explanation of the unique photophysical properties and for understanding the remarkable performance of perovskites as photovoltaic materials. It is shown that the amphoteric defects in the donor (acceptor) configuration capture a fraction of photoexcited electrons (holes) dividing them into two groups: higher-energy bright and lower-energy dark electrons (holes). The spatial separation of the dark electrons and dark holes and the k-space separation of the bright and dark charge carriers reduce electron-hole recombination rates, emulating the properties of an ideal photovoltaic material with a balanced, spatially separated transport of electrons and holes. The BAND model also offers a straightforward explanation for the exceptional insensitivity of the photovoltaic performance of polycrystalline perovskite films to structural and optical inhomogeneities. The blue-shifted radiative recombination of bright electrons and holes results in a large anti-Stokes effect that provides a quantitative explanation for the spectral dependence of the laser cooling effect measured in perovskite platelets.

Conversion of CO2 to reduced products is a promising route to alleviate irreversible climate change. Here we report the synthesis of a Co-based phthalocyanine with pyridine moieties (CoPc-Pyr), which is supported on a carbon electrode and shows Faradaic efficiency ∼90% for CO at 490 mV of overpotential (-0.6 V vs reversible hydrogen electrode (RHE)). In addition, its catalytic activity at -0.7 V versus RHE surpasses other Co-based molecular and metal-organic framework catalysts for CO2 reduction at this bias. Density functional theory calculations show that pyridine moieties enhance CO2 adsorption and electron affinity of the Co center by an inductive effect, thus lowering the overpotential necessary for CO2 conversion. Our study shows that CoPc-Pyr reduces CO2 at lower overpotential and with higher activity than noble metal electrodes, such as silver.

p-Type transparent conducting materials (p-TCMs) are important components of optoelectronic devices including solar cells, photodetectors, displays, and flexible sensors. Cu-Zn-S thin films prepared by chemical bath deposition (CBD) can have both high transparency in the visible range (>80%) as well as excellent hole conductivity (>1000 S cm-1). However, the interplay between the deposition parameters in the CBD process (metal and sulfur precursor concentrations, temperature, pH, complexing agents, etc.) creates a multidimensional parameter space such that optimization for a specific application is challenging and time-consuming. Here we show that strategic design of experiment combined with machine learning (ML) allows for the efficient optimization of p-TCM performance. The approach is guided by a figure of merit (FOM) calculated from the film conductivity and optical transmission in the desired spectral range. A specific example is shown using two steps of optimization using a selected subset of four experimental CBD factors. The ML model is based on support vector regression employing a radial basis function as the kernel function. 10-fold cross-validation was performed to mitigate overfitting. After the first round of optimization, predicted areas in the parameter space with maximal FOMs were selected for a second round of optimization. Films with optimal FOMs were incorporated into heterojunction solar cells and transparent photodiodes. The optimization approach shown here will be generally applicable to any materials synthesis process with multiple parameters.

Cascade catalytic processes perform multistep chemical transformations without isolating the intermediates. Here, we demonstrate a sequential cascade pathway to convert CO2 to C2+ hydrocarbons and oxygenates in a two-step electrocatalytic process using CO as the intermediate. CO2 to CO conversion is performed by using Ag, and further conversion of CO to C-C coupled products is performed with Cu. Temporal separation between the two reaction steps is accomplished by situating the Ag electrode upstream of the Cu electrode in a continuous flow reactor. Convection-diffusion simulations and experimental evaluation of the electrodes individually are performed to identify optimal conditions. With the upstream Ag electrode poised at -1 V versus reversible hydrogen electrode in a flow of CO2-saturated water in aqueous carbonate buffer, over 80% of the CO can be converted on the downstream Cu electrode. When the Ag electrode is on, a supersaturation of CO is achieved near the Cu electrode, which leads to a relative increase in the formation rate of C2 and C3 oxygenates as compared to ethylene.

Defects in conventional semiconductors substantially lower the photoluminescence (PL) quantum yield (QY), a key metric of optoelectronic performance that directly dictates the maximum device efficiency. Two-dimensional transition-metal dichalcogenides (TMDCs), such as monolayer MoS2, often exhibit low PL QY for as-processed samples, which has typically been attributed to a large native defect density. We show that the PL QY of as-processed MoS2 and WS2 monolayers reaches near-unity when they are made intrinsic through electrostatic doping, without any chemical passivation. Surprisingly, neutral exciton recombination is entirely radiative even in the presence of a high native defect density. This finding enables TMDC monolayers for optoelectronic device applications as the stringent requirement of low defect density is eased.

A current challenge in the processing of 2D materials, or "van der Waals (vdW) solids", is the transfer of 2D layers from source crystals and growth substrates onto target substrates. Transfer - as opposed to direct growth and patterning on the target - enables low-temperature processing of the target as well as the use of diverse target materials. These two attributes will allow the assembly of vdW heterostructures to realize devices exploiting the unique properties of vdW materials. Until now, however, there has been no effective method for transferring regions of monolayer material of controlled shape from a multilayer source. We introduce such a method and demonstrate its use in the spatially controlled transfer of arrays of single-layer MoS2 and WS2 sheets from multilayer crystals onto SiO2 substrates. These sheets have lateral sizes exceeding 100 μm and are electronically continuous. The method offers a scalable route to parallel manufacturing of complex circuits and devices from vdW materials.

Si photocathodes integrated with Ag-supported dendritic Cu catalysts are used to perform light-driven reduction of CO2 to C2 and C3 products in aqueous solution. A back illumination geometry with an n-type Si absorber was used to permit the use of absorbing metallic catalysts. Selective carrier collection was accomplished by a p+ implantation on the illumination side and an n+ implantation followed by atomic layer deposition of TiO2 on the electrolyte site. The Ag-supported dendritic Cu CO2 reduction catalyst was formed by evaporation of Ag followed by high-rate electrodeposition of Cu to form a high surface area structure. Under simulated 1 sun illumination in 0.1 M CsHCO3 saturated with CO2, the photovoltage generated by the Si (∼600 mV) enables C2 and C3 products to be produced at -0.4 vs. RHE. Texturing of both sides of the Si increases the light-limited current density, due to reduced reflection on the illumination side, and also deceases the onset potential. Under simulated diurnal illumination conditions photocathodes maintain over 60% faradaic efficiency to hydrocarbon and oxygenate products (mainly ethylene, ethanol, propanol) for several days. After 10 days of testing, contamination from the counter electrode is observed, which causes an increase in hydrogen production. This effect is mitigated by a regeneration procedure which restores the original catalyst selectivity. A tandem, self-powered CO2 reduction device was formed by coupling a Si photocathode with two series-connected semitransparent CH3NH3PbI3 perovskite solar cells, achieving an efficiency for the conversion of sunlight to hydrocarbons and oxygenates of 1.5% (3.5% for all products).