Identification of Single-Atom Ni Site Active toward Electrochemical CO2 Conversion to CO.

Electrocatalytic conversion of CO2 into value-added products offers a new paradigm for a sustainable carbon economy. For active CO2 electrolysis, the single-atom Ni catalyst has been proposed as promising from experiments, but an idealized Ni-N4 site shows an unfavorable energetics from theory, leading to many debates on the chemical nature responsible for high activity. To resolve this conundrum, here we investigated CO2 electrolysis of Ni sites with well-defined coordination, tetraphenylporphyrin (N4-TPP) and 21-oxatetraphenylporphyrin (N3O-TPP). Advanced spectroscopic and computational studies revealed that the broken ligand-field symmetry is the key for active CO2 electrolysis, which subordinates an increase in the Ni redox potential yielding NiI. Along with their importance in activity, ligand-field symmetry and strength are directly related to the stability of the Ni center. This suggests the next quest for an activity-stability map in the domain of ligand-field strength, toward a rational ligand-field engineering of single-atom Ni catalysts for efficient CO2 electrolysis.

Morphological-Electrical Property Relation in Cu(In,Ga)(S,Se)2 Solar Cells: Significance of Crystal Grain Growth and Band Grading by Potassium Treatment.

Solution-processed Cu(In,Ga)(S,Se)2  (CIGS) has a great potential for the production of large-area photovoltaic devices at low cost. However, CIGS solar cells processed from solution exhibit relatively lower performance compared to vacuum-processed devices because of a lack of proper composition distribution, which is mainly instigated by the limited Se uptake during chalcogenization. In this work, a unique potassium treatment method is utilized to improve the selenium uptake judiciously, enhancing grain sizes and forming a wider bandgap minimum region. Careful engineering of the bandgap grading structure also results in an enlarged space charge region, which is favorable for electron-hole separation and efficient charge carrier collection. Besides, this device processing approach has led to a linearly increasing electron diffusion length and carrier lifetime with increasing the grain size of the CIGS film, which is a critical achievement for enhancing photocurrent yield. Overall, 15% of power conversion efficiency is achieved in solar cells processed from environmentally benign solutions. This approach offers critical insights for precise device design and processing rules for solution-processed CIGS solar cells.

Electrochemical Fragmentation of Cu2O Nanoparticles Enhancing Selective C-C Coupling from CO2 Reduction Reaction.

In this study, we demonstrate that the initial morphology of nanoparticles can be transformed into small fragmented nanoparticles, which were densely contacted to each other, during electrochemical CO2 reduction reaction (CO2RR). Cu-based nanoparticles were directly grown on a carbon support by using cysteamine immobilization agent, and the synthesized nanoparticle catalyst showed increasing activity during initial CO2RR, doubling Faradaic efficiency of C2H4 production from 27% to 57.3%. The increased C2H4 production activity was related to the morphological transformation over reaction time. Twenty nm cubic Cu2O crystalline particles gradually experienced in situ electrochemical fragmentation into 2-4 nm small particles under the negative potential, and the fragmentation was found to be initiated from the surface of the nanocrystal. Compared to Cu@CuO nanoparticle/C or bulk Cu foil, the fragmented Cu-based NP/C catalyst achieved enhanced C2+ production selectivity, accounting 87% of the total CO2RR products, and suppressed H2 production. In-situ X-ray absorption near edge structure studies showed metallic Cu0 state was observed under CO2RR, but the fragmented nanoparticles were more readily reoxidized at open circuit potential inside of the electrolyte, allowing labile Cu states. The unique morphology, small nanoparticles stacked upon on another, is proposed to promote C-C coupling reaction selectivity from CO2RR by suppressing HER.

Carbon Monoxide as a Promoter of Atomically Dispersed Platinum Catalyst in Electrochemical Hydrogen Evolution Reaction.

Carbon monoxide is widely known to poison Pt during heterogeneous catalysis owing to its strong donor-acceptor binding ability. Herein, we report a counterintuitive phenomenon of this general paradigm when the size of Pt decreases to an atomic level, namely, the CO-promoting Pt electrocatalysis toward hydrogen evolution reactions (HER). Compared to pristine atomic Pt catalyst, reduction current on a CO-modified catalyst increases significantly. Operando mass spectroscopy and electrochemical analyses demonstrate that the increased current arises due to enhanced H2 evolution, not additional CO reduction. Through structural identification of catalytic sites and computational analysis, we conclude that CO-ligation on the atomic Pt facilitates Hads formation via water dissociation. This counterintuitive effect exemplifies the fully distinct characteristics of atomic Pt catalysts from those of bulk Pt, and offers new insights for tuning the activity of similar classes of catalysts.

Hierarchical Cu pillar electrodes for electrochemical CO2 reduction to formic acid with low overpotential.

To achieve high performance of electrochemical CO2 reduction, a series of Cu pillar electrodes (Cu-2.5 h, Cu-5 h) were fabricated by using an electrodeposition method, and then their catalytic activities and reaction mechanisms were investigated. The series of Cu pillar electrodes exhibited improved electrocatalytic activities toward CO2 reduction to formic acid (HCOOH) as Cu pillars on electrodes developed. The Cu-5 h electrode performed well with a 28% Faradaic efficiency for formic acid at -0.5 V (vs. RHE). X-ray diffraction (XRD) analysis indicated that the enhanced catalytic activities were primarily attributable to the increased (111) facet, which is energetically favourable for the production of HCOOH. Also, ultraviolet photoelectron spectroscopy (UPS) and in situ electrochemical impedance spectroscopy (EIS) results suggested that the series of Cu pillar structure electrodes improved the electron transfer to adsorbed CO2 due to the decreased work function of the Cu pillar structure.

Highly Efficient, Selective, and Stable CO2 Electroreduction on a Hexagonal Zn Catalyst.

Electrocatalytic CO2 conversion into fuel is a prospective strategy for the sustainable energy production. However, still many parts of the catalyst such as low catalytic activity, selectivity, and stability are challenging. Herein, a hierarchical hexagonal Zn catalyst showed highly efficient and, more importantly, stable performance as an electrocatalyst for selectively producing CO. Moreover, we found that its high selectivity for CO is attributed to morphology. In electrochemical analysis, Zn (101) facet is favorable to CO formation whereas Zn (002) facet favors the H2 evolution during CO2 electrolysis. Indeed, DFT calculations showed that (101) facet lowers a reduction potential for CO2 to CO by more effectively stabilizing a (.) COOH intermediate than (002) facet. This further suggests that tuning the crystal structure to control (101)/(002) facet ratio of Zn can be considered as a key design principle to achieve a desirable product from Zn catalyst.

Exploring the influence of cell configurations on Cu catalyst reconstruction during CO2 electroreduction.

Membrane electrode assembly (MEA) cells incorporating Cu catalysts are effective for generating C2+ chemicals via the CO2 reduction reaction (CO2RR). However, the impact of MEA configuration on the inevitable reconstruction of Cu catalysts during CO2RR remains underexplored, despite its considerable potential to affect CO2RR efficacy. Herein, we demonstrate that MEA cells prompt a unique reconstruction of Cu, in contrast to H-type cells, which subsequently influences CO2RR outcomes. Utilizing three Cu-based catalysts, specifically engineered with different nanostructures, we identify contrasting selectivity trends in the production of C2+ chemicals between H-type and MEA cells. Operando X-ray absorption spectroscopy, alongside ex-situ analyses in both cell types, indicates that MEA cells facilitate the reduction of Cu2O, resulting in altered Cu surfaces compared to those in H-type cells. Time-resolved CO2RR studies, supported by Operando analysis, further highlight that significant Cu reconstruction within MEA cells is a primary factor leading to the deactivation of CO2RR into C2+ chemicals.

A unifying mechanism for cation effect modulating C1 and C2 productions from CO2 electroreduction.

Electrocatalysis, whose reaction venue locates at the catalyst-electrolyte interface, is controlled by the electron transfer across the electric double layer, envisaging a mechanistic link between the electron transfer rate and the electric double layer structure. A fine example is in the CO2 reduction reaction, of which rate shows a strong dependence on the alkali metal cation (M+) identity, but there is yet to be a unified molecular picture for that. Using quantum-mechanics-based atom-scale simulation, we herein scrutinize the M+-coupling capability to possible intermediates, and establish H+- and M+-associated ET mechanisms for CH4 and CO/C2H4 formations, respectively. These theoretical scenarios are successfully underpinned by Nernstian shifts of polarization curves with the H+ or M+ concentrations and the first-order kinetics of CO/C2H4 formation on the electrode surface charge density. Our finding further rationalizes the merit of using Nafion-coated electrode for enhanced C2 production in terms of enhanced surface charge density.

Toward economical application of carbon capture and utilization technology with near-zero carbon emission.

Carbon capture and utilization technology has been studied for its practical ability to reduce CO2 emissions and enable economical chemical production. The main challenge of this technology is that a large amount of thermal energy must be provided to supply high-purity CO2 and purify the product. Herein, we propose a new concept called reaction swing absorption, which produces synthesis gas (syngas) with net-zero CO2 emission through direct electrochemical CO2 reduction in a newly proposed amine solution, triethylamine. Experimental investigations show high CO2 absorption rates (>84%) of triethylamine from low CO2 concentrated flue gas. In addition, the CO Faradaic efficiency in a triethylamine supplied membrane electrode assembly electrolyzer is approximately 30% (@-200 mA cm-2), twice higher than those in conventional alkanolamine solvents. Based on the experimental results and rigorous process modeling, we reveal that reaction swing absorption produces high pressure syngas at a reasonable cost with negligible CO2 emissions. This system provides a fundamental solution for the CO2 crossover and low system stability of electrochemical CO2 reduction.

Boosting Solar Cell Performance via Centrally Localized Ag in Solution-Processed Cu(In,Ga)(S,Se)2 Thin Film Solar Cells.

Fabrication of Cu(In,Ga)(S,Se)2 (CIGSSe) absorber films from environmentally friendly solutions under ambient air conditions for use in solar cells has shown promise for the low-cost mass production of CIGSSe solar cells. However, the limited power conversion efficiency (PCE) of these solar cells compared with their vacuum-processed counterparts has been a critical setback to their practical applications. This study aims to fabricate solution-processed CIGSSe solar cells with high PCEs by incorporation of Ag into the precursor layer of the CIGSSe absorber films. The results showed that Ag doping promoted grain growth by accelerating Se uptake, irrespective of the location within the CIGSSe film. Nevertheless, uniform Ag doping formed crevices that lowered the PCE of the cells, while centrally localizing the doped Ag prevented the formation of crevices, resulting in high PCEs up to 15.3%. Our results demonstrate that carefully doping Ag into a selected area of the precursor layer of the CIGSSe films can realize solution-processed chalcopyrite solar cells with high PCE.

Rational Design of a Hierarchical Tin Dendrite Electrode for Efficient Electrochemical Reduction of CO2.

Catalysis is a key technology for the synthesis of renewable fuels through electrochemical reduction of CO2 . However, successful CO2 reduction still suffers from the lack of affordable catalyst design and understanding the factors governing catalysis. Herein, we demonstrate that the CO2 conversion selectivity on Sn (or SnOx /Sn) electrodes is correlated to the native oxygen content at the subsurface. Electrochemical analyses show that the reduced Sn electrode with abundant oxygen species effectively stabilizes a CO2 (.-) intermediate rather than the clean Sn surface, and consequently results in enhanced formate production in the CO2 reduction. Based on this design strategy, a hierarchical Sn dendrite electrode with high oxygen content, consisting of a multi-branched conifer-like structure with an enlarged surface area, was synthesized. The electrode exhibits a superior formate production rate (228.6 µmol h(-1) cm(-2) ) at -1.36 VRHE without any considerable catalytic degradation over 18 h of operation.