Bidirectional Electron Transfer Research Articles

Microbial community acclimation via polarity reversal supports extensive dechlorination and anaerobic mineralization of 2,4,6-trichlorophenol in biocathode

In the anaerobic biocathode, reductive dechlorination of 2,4,6-trichlorophenol (2,4,6-TCP) mainly proceeded to produce 4-monochlorophenol (4-MCP) and phenol, and anaerobic mineralization was challenging. In this study, biocathodes microbial communities were first acclimated via potentials repeatedly adjusted between −0.278 V and +0.200 V (T-200), −0.278 V and +0.400 V (T-400), and −0.278 V and +0.600 V (T-600). Subsequently, all biocathode potentials were stabilized at −0.278 V for the degradation of 2,4,6-TCP. The results revealed that the T-600 achieved the highest degradation rate of 2,4,6-TCP (0.43 d−1), outperforming T-200 (0.051 d −1), T-400 (0.020 d −1), and open-circuit bioreactor (OC, 0.038 d−1) groups. In T-600, pare-dechlorination was facilitated, allowing for the conversion of 2,4,6-TCP to 2-monochlorophenol (2-MCP). Additionally, the detection of 4-hydroxybenzoic acid, an anaerobic mineralization intermediate of 2,4,6-TCP, indicated that anaerobic mineralization was also occurring. Polarity reversal occurred only in T-600 during the acclimation period, resulting in alterations to the electrical properties of the biocathode and microbial community. The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) results indicated that there were more electroactive sites in biocathode of T-600, and the bidirectional extracellular electron transfer (EET) might be related to cytochrome. Mycobacterium (24.33 % in slurry and 19.92 % in biofilm), Thiobacillus (2.47 % in slurry and 17.85 % in biofilm), and Arenimonas (4.23 % in slurries and 4.32 % in cathode biofilm) were identified as the dominant electroactive bacteria in T-600, with Mycobacterium also playing an important role in the degradation of chlorophenols. These findings suggested that polarity reversal may represent a viable strategy for constructing microbial communities capable of achieving simultaneous dechlorination and mineralization of 2,4,6-TCP.

Bidirectional extracellular electron transfers by Stenotrophomonas sp. and by Serratia marcescens via tunable extracellular polymeric substances at low or at high solution conductivity

Unearthing robust electrochemically active bacteria (EAB) with bidirectional extracellular electron transfer (EET) is increasingly attractive for the treatment of wastewaters characterized by a diversity of ions and limited by either carbon/electron sources or electron acceptors in single-chamber bioelectrochemical systems (BESs). Here, both Stenotrophomonas sp. YS1 and Serratia marcescens Q1 biofilms were demonstrated to be switchable and thus capable of bidirectional EET metabolizing either organic (acetate) or inorganic species (NaHCO3) in single-chamber BESs. The electron uptake (inward EET) by Q1 was more efficient than that of YS1 while, YS1 was more efficient than Q1 for outward EET. Higher solution conductivity significantly increased the EET activities of both strains, although, Q1 was more affected than YS1 during outward EET, while YS1 was more impacted than Q1 during inward EET. Experiments performed at low (5.8 mS/cm) and at high (103 mS/cm) solution conductivity with inward and outward EET evidenced the variable amount and compositional diversity of extracellular polymeric substances (EPS) released by Q1 and YS1 biofilms and plankton cells during inward or outward EET, demonstrating the regulative manipulation of these EAB via adaptive electrochemically-tunable EPS components. This study demonstrates EET between Stenotrophomonas sp. or S. marcescens and the electrodes is bidirectional and switchable with dependence on solution conductivity via the response of EPS amount and compositional diversity, giving a comprehensive appreciation of the bacteria behaviour through tunable EPS in response to the altered solution conductivity widening our knowledge on EAB bidirectional EET in single-chamber BESs.

Bidirectional Electron Transfer Strategies for Anti-Markovnikov Olefin Aminofunctionalization via Arylamine Radicals.

Arylamines are common structural motifs in pharmaceuticals, natural products, and materials precursors. While olefin aminofunctionalization chemistry can provide entry to arylamines, classical polar reactions typically afford Markovnikov products. Nitrogen-centered radical intermediates provide the opportunity to access anti-Markovnikov selectivity; however, anti-Markovnikov arylamination is unknown in large part due to the lack of arylamine radical precursors. Here, we introduce bidirectional electron transfer processes to generate arylamine radical intermediates from N-pyridinium arylamines: Single-electron oxidation provides arylamine radicals that engage in anti-Markovnikov olefin aminopyridylation; single-electron reduction unveils arylamine radicals that engage in anti-Markovnikov olefin aminofunctionalization. The development of bidirectional redox processes complements classical design principles for radical precursors, which typically function via a single redox manifold. Demonstration of both oxidative and reductive mechanisms to generate arylamine radicals from a common N-aminopyridinium precursor provides complementary methods to rapidly construct and diversify arylamine scaffolds from readily available radical precursors.

Breaking the scaling relations of effective CO2 electrochemical reduction in diatomic catalysts by adjusting the flow direction of intermediate structures.

The electrocatalytic carbon dioxide reduction reaction (CO2RR) is a promising approach to achieving a sustainable carbon cycle. Recently, diatomic catalysts (DACs) have demonstrated advantages in the CO2RR due to their complex and flexible active sites. However, our understanding of how DACs break the scaling relationship remains insufficient. Here, we investigate the CO2RR of 465 kinds of graphene-based DACs (M1M2-N6@Gra) formed from 30 metal atoms through high-throughput density functional theory (DFT) calculations. We find that the intermediates *COOH, *CO, and *CHO have multiple adsorption states, with 11 structural flow directions from *CO to *CHO. Four of these structural flow directions have catalysts that can break the linear scale relationship. Based on the adsorption energy relationship between *COOH, *CHO and *CO, we propose the concepts of linear scaling, moderate breaking, and severe deviation regions, leading to the establishment of new descriptors that identify 14 catalysts with potential superior performance. Among them, ZnRu-N6@Gra and CrNi-N6@Gra can reduce CO2 to CH4 at a low limiting potential. We also discovered that DACs have independent bidirectional electron transfer channels during the adsorption and activation of CO2, which can significantly improve the flexibility and efficiency of regulating the electronic structure. Furthermore, through machine learning (ML) analysis, we identify electronegativity, atomic number, and d electron count as key determinants of catalyst stability. This work provides new insights into the understanding of the DAC catalytic mechanism, as well as the design and screening of catalysts.

Bidirectional extracellular electron transfers in Serratia marcescens and Stenotrophomonas sp. correlate to EPS and Cr(VI) removal in single‐chamber bioelectrochemical systems

AbstractBACKGROUNDElectrochemically active bacteria (EAB) capable of bidirectional extracellular electron transfer (EET), either outward or inward EET, largely control the efficiency of interactions and electrical communication between biofilm and electrode and, thus, control the performance of bioelectrochemical systems (BESs) for heavy metals removal. However, the behavior of such metallurgical EAB capable of bidirectional EET has yet to be investigated, and the role of extracellular polymeric substances (EPS) in these switchable EAB with bidirectional EET and in the presence of heavy metals remains unexplored in the single‐chamber BESs treating heavy metal‐based wastewaters that are limited by carbon/electron sources or electron acceptors.RESULTSThe biofilms of the Cr(VI)‐tolerant EAB Stenotrophomonas sp. YS1 and Serratia marcescens Q1 exhibited bidirectional EET metabolizing either organic (acetate) or inorganic (HCO3−) species with simultaneous removal of Cr(VI) in single‐chamber BESs. Q1 inward EET uptake of electrons was more efficient than that of YS1 (165 μA vs. 118 μA); meanwhile, YS1 outward EET was more efficient than Q1 (8.0 μA vs. 4.7–5.2 μA). The adaptive electrochemically‐tunable EPS in both biofilm strains was regulated by the direction of the EET (inward or outward) in the presence of Cr(VI) and circuital current.CONCLUSIONThis study demonstrates the switching properties of EAB, such as Stenotrophomonas sp. or S. marcescens, that are capable of bidirectional EET to or from the electrodes, and it displays the regulation of such responses with the amount and compositional diversity of the biofilms’ EPS, giving a comprehensive appreciation of tunable EPS for Cr(VI)‐wastewater treatment in single‐chamber BESs. © 2024 Society of Chemical Industry (SCI).

Enhanced nitrate removal through autotrophic denitrification using microbial fuel cells via bidirectional extracellular electron transfer

The novel biological denitrification process with electrochemical system using the bio-cathode of microbial fuel cells (MFC) presents a promising approach to reduce electron donor consumption during wastewater treatment. This method of autotrophic denitrification can utilize inorganic compounds (such as H2, S2-, S2O32- etc.) or electrodes (at a constant potential) to provide electrons for the denitrification process. This study focuses on an isolated autotrophic denitrifier strain Lyy (belonging to Stutzerimonas) and its application in bio-cathode denitrification. It was found that the extracellular electron transfer (EET) pathway between strain Lyy and electrode is facilitated via the electron shuttle phenazine-1-carboxylic acid (PCA). Especially, at a potential of −0.6 V (vs. SCE), strain Lyy can mediate inward EET from electrodes or inorganic compound into cells, enhancing denitrification with a NO3--N removal efficiency of 98.68 ± 0.68 % within 48 h. On the other hand, effective denitrification was achieved by the bio-cathode MFC without organic carbon sources. Meanwhile, the MFC exhibited a maximum output voltage of 172 ± 6 mV and a maximum power density of 77.46 ± 6.49 mW m−2 at a load resistance of 1 kΩ, with a NO3--N removal efficiency higher than 95 % and with no detecteable NO2--N, which implied that bio-cathode denitrification contributes to the electricity generation. This study showed bidirectional EET phenomena and expands the potential applications for treating low-carbon, high-nitrogen wastewater using strain Lyy.

Frequency-modulated alternating current-driven bioelectrodes for enhanced mineralization of Alizarin Yellow R

The alternating current (AC)-driven bioelectrochemical process, in-situ coupling cathodic reduction and anodic oxidation in a single electrode, offers a promising way for the mineralization of refractory aromatic pollutants (RAPs). Frequency modulation is vital for aligning reduction and oxidation phases in AC-driven bioelectrodes, potentially enhancing their capability to mineralize RAPs. Herein, a frequency-modulated AC-driven bioelectrode was developed to enhance RAP mineralization, exemplified by the degradation of Alizarin Yellow R (AYR). Optimal performance was achieved at a frequency of 1.67 mHz, resulting in the highest efficiency for AYR decolorization and subsequent mineralization of intermediates. Performance declined at both higher (3.33 and 8.30 mHz) and lower (0.83 mHz) frequencies. The bioelectrode exhibited superior electron utilization, bidirectional electron transfer, and redox bifunctionality, effectively aligning reduction and oxidation processes to enhance AYR mineralization. The 1.67 mHz frequency facilitated the assembly of a collaborative microbiome dedicated to AYR bio-mineralization, characterized by an increased abundance of functional consortia proficient in azo dye reduction (e.g., Stenotrophomonas and Shinella), aromatic intermediates oxidation (e.g., Sphingopyxis and Sphingomonas), and electron transfer (e.g., Geobacter and Pseudomonas). This study reveals the role of frequency modulation in AC-driven bioelectrodes for enhanced RAP mineralization, offering a novel and sustainable approach for treating RAP-bearing wastewater.

In situ coupling of reduction and oxidation processes with alternating current-driven bioelectrodes for efficient mineralization of refractory pollutants

The effective elimination of aromatic compounds from wastewater is imperative for safeguarding the ecological environment. Bioelectrochemical processes that combine cathodic reduction and anodic oxidation represent a promising approach for the biomineralization of aromatic compounds. However, conventional direct current bioelectrochemical methods have intrinsic limitations. In this study, a low-frequency and low-voltage alternating current (LFV-AC)-driven bioelectrode offering periodic in situ coupling of reduction and oxidation processes was developed for the biomineralization of aromatic compounds, as exemplified by the degradation of alizarin yellow R (AYR). LFV-AC stimulated biofilm demonstrated efficient bidirectional electron transfer and oxidation–reduction bifunctionality, considerably boosting AYR reduction (63.07% ± 1.91%) and subsequent mineralization of intermediate products (98.63% ± 0.37%). LFV-AC stimulation facilitated the assembly of a collaborative microbiome dedicated to AYR metabolism, characterized by an increased abundance of functional consortia proficient in azo dye reduction (Stenotrophomonas and Bradyrhizobium), aromatic intermediate oxidation (Sphingopyxis and Sphingomonas), and electron transfer (Geobacter and Pseudomonas). The collaborative microbiome demonstrated a notable enrichment of functional genes encoding azo- and nitro-reductases, catechol oxygenases, and redox mediator proteins. These findings highlight the effectiveness of LFV-AC stimulation in boosting azo dye biomineralization, offering a novel and sustainable approach for the efficient removal of refractory organic pollutants from wastewater.

A biocompatible electrode/exoelectrogens interface augments bidirectional electron transfer and bioelectrochemical reactions

Bidirectional electron transfer is about that exoelectrogens produce bioelectricity via extracellular electron transfer at anode and drive cytoplasmic biochemical reactions via extracellular electron uptake at cathode. The key factor to determine above bioelectrochemical performances is the electron transfer efficiency under biocompatible abiotic/biotic interface. Here, a graphene/polyaniline (GO/PANI) nanocomposite electrode specially interfacing exoelectrogens (Shewanella loihica) and augmenting bidirectional electron transfer was conducted by in-situ electrochemical modification on carbon paper (CP). Impressively, the GO/PANI@CP electrode tremendously improved the performance of exoelectrogens at anode for wastewater treatment and bioelectricity generation (about 54 folds increase of power density compared to blank CP electrode). The bacteria on electrode surface not only showed fast electron release but also exhibited high electricity density of extracellular electron uptake through the proposed direct electron transfer pathway. Thus, the cathode applications of microbial electrosynthesis and bio-denitrification were developed via GO/PANI@CP electrode, which assisted the close contact between microbial outer-membrane cytochromes and nanocomposite electrode for efficient nitrate removal (0.333 mM/h). Overall, nanocomposite modified electrode with biocompatible interfaces has great potential to enhance bioelectrochemical reactions with exoelectrogens.

Biological Self-Assembled Transmembrane Electron Conduits for High-Efficiency Ammonia Production in Microbial Electrosynthesis.

Usually, CymA is irreplaceable as the electron transport hub in Shewanella oneidensis MR-1 bidirectional electron transfer. In this work, biologically self-assembled FeS nanoparticles construct an artificial electron transfer route and implement electron transfer from extracellular into periplasmic space without CymA involvement, which present similar properties to type IV pili. Bacteria are wired up into a network, and more electron transfer conduits are activated by self-assembled transmembrane FeS nanoparticles (electron conduits), thereby substantially enhancing the ammonia production. In this study, we achieved an average NH4+-N production rate of 391.8 μg·h-1·L reactor-1 with the selectivity of 98.0% and cathode efficiency of 65.4%. Additionally, the amide group in the protein-like substances located in the outer membrane was first found to be able to transfer electrons from extracellular into intracellular with c-type cytochromes. Our work provides a new viewpoint that contributes to a better understanding of the interconnections between semiconductor materials and bacteria and inspires the exploration of new electron transfer chain components.

Artificial and Biosynthetic Nanoparticles Boost Bioelectrochemical Reactions via Efficient Bidirectional Electron Transfer of Shewanella loihica.

Bioelectrochemical reactions using whole-cell biocatalysts are promising carbon-neutral approaches because of their easy operation, low cost, and sustainability. Bidirectional (outward or inward) electron transfer via exoelectrogens plays the main role in driving bioelectrochemical reactions. However, the low electron transfer efficiency seriously inhibits bioelectrochemical reaction kinetics. Here, a three dimensional and artificial nanoparticles-constituent inverse opal-indium tin oxide (IO-ITO) electrode is fabricated and employed to connect with exoelectrogens (Shewanella loihica PV-4). The above electrode collected 128-fold higher cell density and exhibited a maximum current output approaching 1.5mAcm-2 within 24h at anode mode. By changing the IO-ITO electrode to cathode mode, the exoelectrogens exhibited the attractive ability of extracellular electron uptake to reduce fumarate and 16 times higher reverse current than the commercial carbon electrode. Notably, Fe-containing oxide nanoparticles are biologically synthesized at both sides of the outer cell membrane and probably contributed to direct electron transfer with the transmembrane c-type cytochromes. Owing to the efficient electron exchange via artificial and biosynthetic nanoparticles, bioelectrochemical CO2 reduction is also realized at the cathode. This work not only explored the possibility of augmenting bidirectional electron transfer but also provided a new strategy to boost bioelectrochemical reactions by introducing biohybrid nanoparticles.

Retraction for Yadav et al., "The bidirectional extracellular electron transfer process aids iron cycling by Geoalkalibacter halelectricus in a highly saline-alkaline condition".

In our paper, we reported the bidirectional extracellular electron transfer capability of Geoalkalibacter halelectricus based on biochemical (i.e., with insoluble Fe-oxide and Fe(0)) and bioelectrochemical (i.e., with electrodes) experimental approaches. We noticed some issues and limitations of the methods and techniques that were used to analyze Fe species, particularly the reduced Fe ions in insoluble and precipitated Fe(II) minerals that led to incorrect interpretation of the results, specifically the reduction of Fe(III) to Fe(0) in this study. We were made aware of thermodynamic constraints that would make the biological reduction of Fe(III) to Fe(0) implausible. Hence, the conclusion about microbial Fe(III)-oxide reduction to Fe(0) is invalid. We also noticed errors in estimating the protein-based iron reduction/oxidation rates and faradic efficiencies, besides the limitations of the methods used for Fe analysis. For these reasons, we retract this article and sincerely apologize for the inconvenience it may have caused to the readers.

Localized surface plasmon resonance-induced bidirectional electron transfer of formic acid adsorption for boosting photocatalytic hydrogen production on Ni/TiO2

Photocatalytichydrogenproduction from formic acid (FA) is a daunting challenge, yet an essential taskforthedevelopmentofhydrogenenergy. In this study, a p-NiO/n-TiO2 heterojunction incorporating 7-nm metallic Ni was fabricated, which demonstrated a remarkable localized surface plasmon resonance (LSPR) effect. Notably, 5 wt% Ni/TiO2 exhibited 1271-fold higher photocatalytic activity (2416 μmol⋅g−1⋅h−1) than TiO2 alone under light radiation at room temperature. The experimental investigations revealed the excitation of distinct components via irradiation by different light sources. Visible light-driven hydrogen production was predominantly influenced by the LSPR-induced hot electrons and holes effects of Ni. Further, FA molecules simultaneously lost and accepted electrons at the Ni0–Ti3+ and Ni0–O2− sites, respectively, generating a bidirectional electron transfer behavior with “valley-shaped” gas-sensitive responses, which was crucial toboosttheactivity. Moreover, the photocatalytic activity was mainly attributed to the heterojunction and defects structure under UV light irradiation, and Ti3+, VOs, and O2− as adsorption sites for FA. Thus, the synergistic interplay among different light sources could effectively boost the photocatalytic hydrogen production performance. Significantly, this research reveals that the LSPR effect of metallic Ni can effectively regulate electron transfer behavior and enhance visible light-driven photocatalytic activity.

Bidirectional electron transfer boosts Li–CO2 electrochemistry

Regarding the controversial issue of CO2 activation on metal-based catalysts, a novel “bidirectional electron transfer” mechanism was unraveled.

Bidirectional extracellular electron transfer pathways of Geobacter sulfurreducens biofilms: Molecular insights into extracellular polymeric substances

The basis for bioelectrochemical technology is the capability of electroactive bacteria (EAB) to perform bidirectional extracellular electron transfer (EET) with electrodes, i.e. outward- and inward-EET. Extracellular polymeric substances (EPS) surrounding EAB are the necessary media for EET, but the biochemical and molecular analysis of EPS of Geobacter biofilms on electrode surface is largely lacked. This study constructed Geobacter sulfurreducens-biofilms performing bidirectional EET to explore the bidirectional EET mechanisms through EPS characterization using electrochemical, spectroscopic fingerprinting and proteomic techniques. Results showed that the inward-EET required extracellular redox proteins with lower formal potentials relative to outward-EET. Comparing to the EPS extracted from anodic biofilm (A-EPS), the EPS extracted from cathodic biofilm (C-EPS) exhibited a lower redox activity, mainly due to a decrease of protein/polysaccharide ratio and α-helix content of proteins. Furthermore, less cytochromes and more tyrosine- and tryptophan-protein like substances were detected in C-EPS than in A-EPS, indicating a diminished role of cytochromes and a possible role of other redox proteins in inward-EET. Proteomic analysis identified a variety of redox proteins including cytochrome, iron-sulfur clusters-containing protein, flavoprotein and hydrogenase in EPS, which might serve as an extracellular redox network for bidirectional EET. Those redox proteins that were significantly stimulated in A-EPS and C-EPS might be essential for outward- and inward-EET and warranted further research. This work sheds light on the mechanism of bidirectional EET of G. sulfurreducens biofilms and has implications in improving the performance of bioelectrochemical technology.

Carbon source priority and availability limit bidirectional electron transfer in freshwater mixed culture electrochemically active bacterial biofilms

This study investigated, if a mixed electroactive bacterial (EAB) culture cultivated heterotrophically at a positive applied potential could be adapted from oxidative to reductive or bidirectional extracellular electron transfer (EET). To this end, a periodic potential reversal regime between − 0.5 and 0.2 V vs. Ag/AgCl was applied. This yielded biofilm detachment and mediated electroautotrophic EET in combination with carbonate, i.e., dissolved CO2, as the sole carbon source, whereby the emerged mixed culture (S1) contained previously unknown EAB. Using acetate (S2) as well as a mixture of acetate and carbonate (S3) as the main carbon sources yielded primarily alternating electrogenic organoheterotropic metabolism with the higher maximum oxidation current densities recorded for mixed carbon media, exceeding on average 1 mA cm−2. More frequent periodic polarization reversal resulted in the increase of maximum oxidative current densities by about 50% for S2-BES and 80% for S3-BES, in comparison to half-batch polarization. The EAB mixed cultures developed accordingly, with S1 represented by mostly aerobes (84.8%) and being very different in composition to S2 and S3, dominated by anaerobes (96.9 and 96.5%, respectively). S2 and S3 biofilms remained attached to the electrodes. There was only minor evidence of fully reversible bidirectional EET. In conclusion the three triplicates fed with organic and/or inorganic carbon sources demonstrated two forms of diauxie: Firstly, S1-BES showed a preference for the electrode as the electron donor via mediated EET. Secondly, S2-BES and S3-BES showed a preference for acetate as electron donor and c-source, as long as this was available, switching to CO2 reduction, when acetate was depleted.Graphical

The bidirectional extracellular electron transfer process aids iron cycling by Geoalkalibacter halelectricus in a highly saline-alkaline condition.

Bidirectional extracellular electron transfer (EET) is crucial to upholding microbial metabolism with insoluble electron acceptors or donors in anoxic environments. Investigating bidirectional EET-capable microorganisms is desired to understand the cell-cell and microbe-mineral interactions and their role in mineral cycling besides leveraging their energy generation and conversion, biosensing, and bio-battery applications. Here, we report on iron cycling by haloalkaliphilic Geoalkalibacter halelectricus via bidirectional EET under haloalkaline conditions. It efficiently reduces Fe3+ oxide (Fe2O3) to Fe0 at a 0.75 ± 0.08 mM/mgprotein/d rate linked to acetate oxidation via outward EET and oxidizes Fe0 to Fe3+ at a 0.24 ± 0.03 mM/mgprotein/d rate via inward EET to reduce fumarate. Bioelectrochemical cultivation confirmed its outward and inward EET capabilities. It produced 895 ± 23 µA/cm2 current by linking acetate oxidation to anode reduction via outward EET and reduced fumarate by drawing electrons from the cathode (‒2.5 ± 0.3 µA/cm2) via inward EET. The cyclic voltammograms of G. halelectricus biofilms revealed redox moieties with different formal potentials, suggesting the involvement of different membrane components in bidirectional EET. The cyclic voltammetry and GC-MS analysis of the cell-free spent medium revealed the lack of soluble redox mediators, suggesting direct electron transfer by G. halelecctricus in achieving bidirectional EET. By reporting on the first haloalkaliphilic bacterium capable of oxidizing and reducing insoluble Fe0 and Fe3+ oxide, respectively, this study advances the limited understanding of the metabolic capabilities of extremophiles to respire on insoluble electron acceptors or donors via bidirectional EET and invokes the possible role of G. halelectricus in iron cycling in barely studied haloalkaline environments. IMPORTANCE Bidirectional extracellular electron transfer (EET) appears to be a key microbial metabolic process in anoxic environments that are depleted in soluble electron donor and acceptor molecules. Though it is an ecologically important and applied microbial phenomenon, it has been reported with a few microorganisms, mostly from nonextreme environments. Moreover, direct electron transfer-based bidirectional EET is studied for very few microorganisms with electrodes in engineered systems and barely with the natural insoluble electron acceptor and donor molecules in anoxic conditions. This study advances the understanding of extremophilic microbial taxa capable of bidirectional EET and its role in barely investigated Fe cycling in highly saline-alkaline environments. It also offers research opportunities for understanding the membrane components involved in the bidirectional EET of G. halelectricus. The high rate of Fe3+ oxide reduction activity by G. halelectricus suggests its possible use as a biocatalyst in the anaerobic iron bioleaching process under neutral-alkaline pH conditions.

Iodanyl Radical Catalysis.

ConspectusHypervalent iodine reagents find application as selective chemical oxidants in a diverse array of oxidative transformations. The utility of these reagents is often ascribed to (1) the proclivity to engage being selective two-electron redox transformations; (2) facile ligand exchange at the three-centered, four-electron (3c-4e) hypervalent iodine-ligand (I-X) bonds; and (3) the hypernucleofugacity of aryl iodides. One-electron redox and iodine radical chemistry is well-precedented in the context of inorganic hypervalent iodine chemistry─for example, in the iodide-triiodide couple that drives dye-sensitized solar cells. In contrast, organic hypervalent iodine chemistry has historically been dominated by the two-electron I(I)/I(III) and I(III)/I(V) redox couples, which results from intrinsic instability of the intervening odd-electron species. Transient iodanyl radicals (i.e., formally I(II) species), generated by reductive activation of hypervalent I-X bonds, have recently gained attention as potential intermediates in hypervalent iodine chemistry. Importantly, these open-shell intermediates are typically generated by activation of stoichiometric hypervalent iodine reagents, and the role of the iodanyl radical in substrate functionalization and catalysis is largely unknown.Our group has been interested in advancing the chemistry of iodanyl radicals as intermediates in the sustainable synthesis of hypervalent I(III) and I(V) compounds and as novel platforms for substrate activation at open-shell main-group intermediates. In 2018, we disclosed the first example of aerobic hypervalent iodine catalysis by intercepting reactive intermediates in aldehyde autoxidation chemistry. While we initially hypothesized that the observed oxidation was accomplished by aerobically generated peracids via a two-electron I(I)-to-I(III) oxidation reaction, detailed mechanistic studies revealed the critical role of acetate-stabilized iodanyl radical intermediates. We subsequently leveraged these mechanistic insights to develop hypervalent iodine electrocatalysis. Our studies resulted in the identification of new catalyst design principles that give rise to highly efficient organoiodide electrocatalysts that operate at modest applied potentials. These advances addressed classical challenges in hypervalent iodine electrocatalysis related to the need for high applied potentials and high catalyst loadings. In some cases, we were able to isolate the anodically generated iodanyl radical intermediates, which allowed direct interrogation of the elementary chemical reactions characteristic of iodanyl radicals. Both substrate activation via bidirectional proton-coupled electron transfer (PCET) reactions at I(II) intermediates and disproportionation reactions of I(II) species to generate I(III) compounds have been experimentally validated.This Account discusses the emerging synthetic and catalytic chemistry of iodanyl radicals. Results from our group have demonstrated that these open-shell species can play a critical role in sustainable synthesis of hypervalent iodine reagents and play a heretofore unappreciated role in catalysis. Realization of I(I)/I(II) catalytic cycles as a mechanistic alternative to canonical two-electron iodine redox chemistry promises to open new avenues to application of organoiodides in catalysis.

Bidirectional electron transfer in Triboelectrification caused by friction-induced change in surface electronic structure

Triboelectrification (TE) plays fundamental roles in a wide range of industrial and scientific fields but its charge transfer mechanisms still remains elusive. It has been widely believed that the electron transfer in TE between two homogeneous surfaces should be always unidirectional. But we found here that the electron transfer during friction process will change its direction as the position of the contact point on the friction surface changes, indicating that bidirectional electron transfer occurs between the contacting surfaces. The experiments further show that the electron transfer direction at any given contact position on the friction surface keeps unchanged over time during the whole friction process if the relative sliding direction keeps unchanged, and when the sliding direction is reversed, the electron transfer direction at a given contact position will also be reversed if the friction surface is crystalline. To explain the observed non-unidirectional electron transfer, a theoretical model for the electron transfer in TE has been proposed, assuming that the changes in the electron transfer direction can be attributed to the changes in the surface work function caused by the friction-induced tangential deformation on the friction surface. The theory has further been validated by the first principles calculations about the influence of the tangential deformation on the surface electronic structure. The present work may offer a new perspective for understanding the charge transfer mechanism in TE and may provide potential applications in numerous fields involving TE such as triboelectric generator, petrochemical industry, power electronics and textile industry.

Microbial electrosynthetic nitrate reduction to ammonia by reversing the typical electron transfer pathway in Shewanella oneidensis

Ammonia production is a critical industrial process, and mild routes to recycle nitrates in wastewater could be a promising route to ammonia synthesis. In this study, ammonia production is demonstrated in a microbial electrosynthesis system with nitrate and an electrode as electron acceptors and donors, respectively. Based on the bidirectional extracellular electron transfer capability of Shewanella oneidensis MR-1, our microbial electrosynthetic system achieves a maximum ammonia production rate of 24.3 μg h−1·mg protein−1 with 82.5% selectivity and 33.1% cathodic efficiency and functions for several cycles over 30 days. Electrochemical analysis suggests that cytochromes c, flavins, and the flavin/c-cytochrome combination play a pivotal role. Charge transfer resistance weakens over the course of weeks, resulting in easier electron transfer. Parallel reaction monitoring proteomics suggest that reversing a typical “Mtr pathway” plays a role, and a dissimilatory nitrate to ammonia pathway is used. This work proposes a progressive route to carry out ammonia synthesis under mild conditions.