Catalytic H2 Production Research Articles

Antimony Complexes for Electrocatalysis: Activity of a Main-Group Element in Proton Reduction.

Main-group complexes are shown to be viable electrocatalysts for the H2 -evolution reaction (HER) from acid. A series of antimony porphyrins with varying axial ligands were synthesized for electrocatalysis applications. The proton-reduction catalytic properties of TPSb(OH)2 (TP=5,10,15,20-tetra(p-tolyl)porphyrin) with two axial hydroxy ligands were studied in detail, demonstrating catalytic H2 production. Experiments, in conjunction with quantum chemistry calculations, show that the catalytic cycle is driven via the redox activity of both the porphyrin ligand and the Sb center. This study brings insight into main group catalysis and the role of redox-active ligands during catalysis.

Antimony Complexes for Electrocatalysis: Activity of a Main‐Group Element in Proton Reduction

AbstractMain‐group complexes are shown to be viable electrocatalysts for the H2‐evolution reaction (HER) from acid. A series of antimony porphyrins with varying axial ligands were synthesized for electrocatalysis applications. The proton‐reduction catalytic properties of TPSb(OH)2 (TP=5,10,15,20‐tetra(p‐tolyl)porphyrin) with two axial hydroxy ligands were studied in detail, demonstrating catalytic H2 production. Experiments, in conjunction with quantum chemistry calculations, show that the catalytic cycle is driven via the redox activity of both the porphyrin ligand and the Sb center. This study brings insight into main group catalysis and the role of redox‐active ligands during catalysis.

High-Throughput Screening of Catalytic H2 Production.

Hydrogenases, ferredoxins, and ferredoxin-NADP+ reductases (FNR) are redox proteins that mediate electron metabolism in vivo, and are also potential components for biological H2 production technologies. A high-throughput H2 production assay device (H2 PAD) is presented that enables simultaneous evaluation of 96 individual H2 production reactions to identify components that improve performance. Using a CCD camera and image analysis software, H2 PAD senses the chemo-optical response of Pd/WO3 thin films to the H2 produced. H2 PAD-enabled discovery of hydrogenase and FNR mutants that enhance biological H2 production is reported. From a library of 10 080 randomly mutated Clostridium pasteurianum [FeFe] hydrogenases, we found a mutant with nearly 3-fold higher H2 production specific activity. From a library of 400 semi-randomly mutated Oryza sativa FNR, the top hit enabled a 60 % increase in NADPH-driven H2 production rates. H2 PAD can also facilitate elucidation of fundamental biochemical mechanisms within these systems.

Syntheses, crystal structures, and electrochemical studies of dinuclear coordination compounds with the Fe2(CO)6 core

Reaction of 2-C5H4 NCOSPh, generated from 2-C5H4NCO2H and PhSH in the presence of DCC, with Fe3(CO)12 affords (μ-κ2C,N-2-C5H4N)(μ-PhS)Fe2(CO)6 (1) and (μ-PhS)2Fe2(CO)6 (2). Reaction of (NC)2C=C(SMe)2, formed from NCCH2CN, CS2, and MeI in the presence of NaOH, with Fe3(CO)12 provides (μ-κ2C,S-(NC)2C=CSMe)(μ-MeS)Fe2(CO)6 (3) and (μ-MeS)2Fe2(CO)6 (4). All complexes have been fully characterized by EA, IR, 1H NMR, and 13C NMR spectroscopy and structurally determined by X-ray crystallography. In 1 and 3, the group attached to the bridging S is at the equatorial position. In 2, two phenyl groups are at equatorial positions. Two isomers of 4, ae-4 and ee-4, can be separated by thin-layer chromatography. DFT calculations reveal that the Gibbs energy difference between ae-4 and ee-4 is −2.17 kcal mol−1 in THF and −2.29 kcal mol−1 in benzene, and the isomerization barrier between ae-4 and ee-4 is 14.92 kcal mol−1 in THF and 16.84 kcal mol−1 in benzene. All these results suggest that ae-4 is more stable than ee-4 in either THF or benzene, and the two isomers do not interconvert. Electrochemical studies of 1 and 3 demonstrate that using HOAc as a proton source 1 and 3 can catalyze H2 production.

Efficient Low-Temperature H2 Production from HCOOH/HCOO– by [Pd0@SiO2-Gallic Acid] Nanohybrids: Catalysis and the Underlying Thermodynamics and Mechanism

Hybrid Pd0-based nanoparticles have been synthesized in aqueous solution by two routes: (a) reduction of Pd ions by gallic acid (GA) producing Pd0-GA and (b) Pd0 formed on SiO2-GA nanohybrids where GA was covalently grafted on SiO2 nanoparticles (Pd0@SiO2-GA). In both protocols, Pd0 nanoparticles were formed in situ, under alkaline pH, via reduction of Pd2+ ions by GA radicals formed by atmospheric O2. XRD and TEM data show that the Pd0@SiO2-GA consists of 6.5 nm Pd0 nanoparticles finely dispersed on the SiO2-GA nanosupport, whereas Pd0-GA consists of aggregated 12 nm Pd0 nanoparticles. The two families of Pd0 nanohybrids have been studied for catalytic H2 production from formic acid/sodium formate in aqueous solution at near ambient temperatures 40–80 °C. Pd0@SiO2-GA achieves H2 production from NaCOOH/HCOOH at 19 mL/min per mg of Pd. This outperforms by a factor of 400% the H2 production by (Pd0-GA) particles, as well as all Pd0-SiO2 catalysts, so far reported in the literature. The Pd0@SiO2-GA catalyst ...

Architecting Nitrogen Functionalities on Graphene Oxide Photocatalysts for Boosting Hydrogen Production in Water Decomposition Process

This study elucidates how nitrogen functionalities influence the transition and transfer of photogenerated electrons in graphene‐based materials. Graphene oxide dots (GODs) and Nitrogen‐doped GODs (NGODs) are synthesized by thermally treating graphene oxide (GO) sheets in argon and ammonia, respectively, and then ultrasonically exfoliating the sheets in nitric acid. The nitrogen functionalities of NGODs are mainly quaternary/pyridinic/pyrrolic, and the nitrogen atoms in these functionalities are planar to the GO sheets and repair the vacancy defects on the sheets. Hydrothermal treatment of NGODs in ammonia yields ammonia‐treated NGODs (A‐NGODs), with some pyridinic/pyrrolic groups being converted to amino/amide groups. The nitrogen atoms in the amino/amide groups are not planar to the GO sheets and are prone to donate their lone pair electrons to resonantly conjugate with the aromatic π electrons. The promoted conjugation facilitates the relaxation of photogenerated electrons to the triplet states and prolongs the electron lifetime. When deposited with Pt as the co‐catalyst, the samples catalyze H2 production from an aqueous triethanolamine solution under 420 nm monochromatic irradiation at quantum yields of 7.3% (GODs), 9.7% (NGODs), and 21% (A‐NGODs). The high activity of A‐NGODs demonstrates that architecting nitrogen functionalities effectively mediate charge motion in carbon‐based materials for application to photoenergy conversion.

Straining catalyst films boosts water-splitting

Adding stress and strain to the lives of busy researchers hardly sounds like a strategy for improving productivity. It can work wonders for catalysts, however. A study has revealed that straining catalytic sites in molybdenum disulfide films substantially boosts the reaction rate of a key step in the water-splitting process relative to the rate on unstrained films (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b01377). Splitting water could provide a nearly limitless supply of hydrogen to run clean-burning fuel cells if a suitable catalyst can be found. Platinum works well, but it’s expensive. In looking elsewhere, researchers have found that low-cost MoS2 films with microscopic defects catalyze H2 production. But complete details of the process have remained unknown, impeding further development. To sort out the kinetics, a team led by Xiaolin Zheng of Stanford University and Allen J. Bard of the University of Texas, Austin, exposed a MoS2 film to a

Uranium-mediated electrocatalytic dihydrogen production from water.

Depleted uranium is a mildly radioactive waste product that is stockpiled worldwide. The chemical reactivity of uranium complexes is well documented, including the stoichiometric activation of small molecules of biological and industrial interest such as H2O, CO2, CO, or N2 (refs 1 - 11), but catalytic transformations with actinides remain underexplored in comparison to transition-metal catalysis. For reduction of water to H2, complexes of low-valent uranium show the highest potential, but are known to react violently and uncontrollably forming stable bridging oxo or uranyl species. As a result, only a few oxidations of uranium with water have been reported so far; all stoichiometric. Catalytic H2 production, however, requires the reductive recovery of the catalyst via a challenging cleavage of the uranium-bound oxygen-containing ligand. Here we report the electrocatalytic water reduction observed with a trisaryloxide U(III) complex [(((Ad,Me)ArO)3mes)U] (refs 18 and 19)--the first homogeneous uranium catalyst for H2 production from H2O. The catalytic cycle involves rare terminal U(IV)-OH and U(V)=O complexes, which have been isolated, characterized, and proven to be integral parts of the catalytic mechanism. The recognition of uranium compounds as potentially useful catalysts suggests new applications for such light actinides. The development of uranium-based catalysts provides new perspectives on nuclear waste management strategies, by suggesting that mildly radioactive depleted uranium--an abundant waste product of the nuclear power industry--could be a valuable resource.

NiFeSe]-hydrogenase chemistry.

The development of technology for the inexpensive generation of the renewable energy vector H2 through water splitting is of immediate economic, ecological, and humanitarian interest. Recent interest in hydrogenases has been fueled by their exceptionally high catalytic rates for H2 production at a marginal overpotential, which is presently only matched by the nonscalable noble metal platinum. The mechanistic understanding of hydrogenase function guides the design of synthetic catalysts, and selection of a suitable hydrogenase enables direct applications in electro- and photocatalysis. [FeFe]-hydrogenases display excellent H2 evolution activity, but they are irreversibly damaged upon exposure to O2, which currently prevents their use in full water splitting systems. O2-tolerant [NiFe]-hydrogenases are known, but they are typically strongly biased toward H2 oxidation, while H2 production by [NiFe]-hydrogenases is often product (H2) inhibited. [NiFeSe]-hydrogenases are a subclass of [NiFe]-hydrogenases with a selenocysteine residue coordinated to the active site nickel center in place of a cysteine. They exhibit a combination of unique properties that are highly advantageous for applications in water splitting compared with other hydrogenases. They display a high H2 evolution rate with marginal inhibition by H2 and tolerance to O2. [NiFeSe]-hydrogenases are therefore one of the most active molecular H2 evolution catalysts applicable in water splitting. Herein, we summarize our recent progress in exploring the unique chemistry of [NiFeSe]-hydrogenases through biomimetic model chemistry and the chemistry with [NiFeSe]-hydrogenases in semiartificial photosynthetic systems. We gain perspective from the structural, spectroscopic, and electrochemical properties of the [NiFeSe]-hydrogenases and compare them with the chemistry of synthetic models of this hydrogenase active site. Our synthetic models give insight into the effects on the electronic properties and reactivity of the active site upon the introduction of selenium. We have utilized the exceptional properties of the [NiFeSe]-hydrogenase from Desulfomicrobium baculatum in a number of photocatalytic H2 production schemes, which are benchmark systems in terms of single site activity, tolerance toward O2, and in vitro water splitting with biological molecules. Each system comprises a light-harvesting component, which allows for light-driven electron transfer to the hydrogenase in order for it to catalyze H2 production. A system with [NiFeSe]-hydrogenase on a dye-sensitized TiO2 nanoparticle gives an enzyme-semiconductor hybrid for visible light-driven generation of H2 with an enzyme-based turnover frequency of 50 s(-1). A stable and inexpensive polymeric carbon nitride as a photosensitizer in combination with the [NiFeSe]-hydrogenase shows good activity for more than 2 days. Light-driven H2 evolution with the enzyme and an organic dye under high O2 levels demonstrates the excellent robustness and feasibility of water splitting with a hydrogenase-based scheme. This has led, most recently, to the development of a light-driven full water splitting system with a [NiFeSe]-hydrogenase wired to the water oxidation enzyme photosystem II in a photoelectrochemical cell. In contrast to the other systems, this photoelectrochemical system does not rely on a sacrificial electron donor and allowed us to establish the long sought after light-driven water splitting with an isolated hydrogenase.

Syntheses, crystal structures, and electrochemistry of novel Fe2SN and FeSN carbonyl complexes with pendant bases

Reactions of Fe2(CO)9 with thioacylhydrazones ArCH=NNHCSPh in THF afford Fe2(CO)6(μ-κ2S:κ2N-PhC(S)=NNCHArCHArN(CHAr)N=CSPh) (1, Ar = C6H5; 3, Ar = 4-CH3C6H4) and Fe(CO)3(κ2S:N-PhC(=S)NHNCHArCHArN(CHAr)N=CSPh) (2, Ar = C6H5; 4, Ar = 4-CH3C6H4). They have been characterized by elemental analyses, IR, 1H NMR, and 13C NMR and structurally determined by X-ray crystallography. Electrochemical studies reveal that when using HOAc as a proton source, they exhibit high catalytic H2-production.

Combined Spectroscopic and Electrochemical Detection of a Ni(I) ⋅⋅⋅H-N Bonding Interaction with Relevance to Electrocatalytic H2 Production.

The [Ni(P(R) 2 N(R') 2 )2 ](2+) family of complexes are exceptionally active catalysts for proton reduction to H2 . In this manuscript, we explore the first protonation step of the proposed catalytic cycle by using a catalytically inactive Ni(I) complex possessing a sterically demanding variation of the ligand. Due to the paramagnetic nature of the Ni(I) oxidation state, the protonated Ni(I) intermediate has been characterized through a combination of cyclic voltammetry, electron nuclear double resonance (ENDOR) spectroscopy, and hyperfine sublevel correlation (HYSCORE) spectroscopy. Both the electrochemical and spectroscopic studies indicate that the Ni(I) complex is protonated at a pendant amine that is endo to Ni, which suggests the presence of an intramolecular Ni(I) ⋅⋅⋅HN bonding interaction. Using density functional theory, the hydrogen bond was found to involve three doubly-occupied, localized molecular orbitals: the 3dxz , 3d z 2, and 3dyz orbitals of nickel. These studies provide the first direct experimental evidence for this critical catalytic intermediate, and implications for catalytic H2 production are discussed.

Synthesis, crystal structure and electrochemical study of (μ-κ2C:κ2S-NHC+-CS)[Fe2(CO)6]− generated from the reaction of NHC+-CS2− with Fe3(CO)12

Reaction of zwitterionic salt NHC+-CS2− (1), prepared from 1,3-diallylbenzimidazol-2-ylidene and CS2, with Fe3(CO)12 affords 2, (μ-κ2C:κ2S-NHC+-CS)[Fe2(CO)6]−. Two new compounds have been fully characterized by EA, IR, 1H NMR and 13C NMR spectroscopy and structurally determined by X-ray crystallography. Electrochemical study of 2 has demonstrated that using HOAc as a proton source complex 2 can efficiently catalyze H2 production.

On the promoting effect of H2S on the catalytic H2 production over Gd-doped ceria from CH4/H2O mixtures for solid oxide fuel cell applications

In order to understand the exceptional tolerance of ceria to high sulfur levels in the fuel for further SOFC applications, the interaction of sulfur with a commercial Ce0.9Gd0.1O2−x (CGO, from Praxair) and its influence on H2 production from CH4/H2O mixture was studied. The activity tests were performed at 750°C, in the presence of H2S and under gradual internal reforming conditions (large excess of CH4 with respect to water vapor, 10:1). The presence of H2S in the reactant mixture promoted the catalytic activity of CGO. The nature of various species formed by sulfur–ceria interaction is discussed based on CH4-TPR, XRD, XPS, and in situ FTIR results. H2S reacts with ceria fraction in CGO and generates new catalytic sites with improved reactivity toward CH4. The formation and stabilization of oxygen vacancies by S-insertion in ceria lattice are evidenced and directly related to the improved activity. The detrimental effect of Ce2O2S phase, when formed, on the catalytic activity of doped ceria is confirmed.

Co-catalytic Effect of Functionalized SiO2 Materials on H2 Production from Formic Acid by an Iron Catalyst

ABSTRACTSurface modified L@SiO2 particles bearing covalently attached functional groups (L) have been tested as co-catalysts for H2 production from Formic Acid (FA) by the homogenous FeII/P(CH2CH2PPh2)3 catalyst. The L@SiO2 particles induce remarkable increase of catalytic H2 production i.e. by 710 %, when L=a basic functionality such as Imidazoles, or NH2-groups. This effect is attributed to a thermodynamic promotion of FA deprotonation facilitating coordination of HCOO- anion on the FeII atom of active catalyst during catalysis.

Noncovalent Immobilization of Electrocatalysts on Carbon Electrodes for Fuel Production

We show that molecular catalysts for fuel-forming reactions can be immobilized on graphitic carbon electrode surfaces via noncovalent interactions. A pyrene-appended bipyridine ligand (P) serves as the linker between each complex and the surface. Immobilization of a rhodium proton-reduction catalyst, [Cp*Rh(P)Cl]Cl (1), and a rhenium CO2-reduction catalyst, Re(P)(CO)3Cl (2), afford electrocatalytically active assemblies. X-ray photoelectron spectroscopy and electrochemistry confirm catalyst immobilization. Reduction of 1 in the presence of p-toluenesulfonic acid results in catalytic H2 production, while reduction of 2 in the presence of CO2 results in catalytic CO production.

PH-Dependent Reduction Potentials and Proton-Coupled Electron Transfer Mechanisms in Hydrogen-Producing Nickel Molecular Electrocatalysts

The nickel-based P2(Ph)N2(Bn) electrocatalysts comprised of a nickel atom and two 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane ligands catalyze H2 production in acetonitrile. Recent electrochemical experiments revealed a linear dependence of the Ni(II/I) reduction potential on pH with a slope of 57 mV/pH unit, implicating a proton-coupled electron transfer (PCET) process with the same number of electrons and protons transferred. The combined theoretical and experimental studies herein provide an explanation for this pH dependence in the context of the overall proposed catalytic mechanism. In the proposed mechanisms, the catalytic cycle begins with a series of intermolecular proton transfers from an acid to the pendant amine ligand and electrochemical electron transfers to the nickel center to produce the doubly protonated Ni(0) species, a precursor to H2 evolution. The calculated Ni(II/I) reduction potentials of the doubly protonated species are in excellent agreement with the experimentally observed reduction potential in the presence of strong acid, suggesting that the catalytically active species leading to the peak observed in these cyclic voltammetry (CV) experiments is doubly protonated. The Ni(I/0) reduction potential was found to be slightly more positive than the Ni(II/I) reduction potential, indicating that the Ni(I/0) reduction occurs spontaneously after the Ni(II/I) reduction, as implied by the experimental observation of a single CV peak. These results suggest that the PCET process observed in the CV experiments is a two-electron/two-proton process corresponding to an initial double protonation followed by two reductions. On the basis of the experimental and theoretical data, the complete thermodynamic scheme and the Pourbaix diagram were generated for this catalyst. The Pourbaix diagram, which identifies the most thermodynamically stable species at each reduction potential and pH value, illustrates that this catalyst undergoes different types of PCET processes for various pH ranges. These thermodynamic insights will aid in the design of more effective molecular catalysts for H2 production.

Simultaneous catalytic H2 production and CO2 capture in steam reforming of toluene as tar model compound from biomass gasification

During biomass gasification in a fast internal circulating fluidized bed (FICFB) reactor hydrogen production is strongly influenced by the physicochemical properties and ability of the catalyst to transform the organic fraction (tar molecules), considered as pollutant and produced along the process. The hydrogen production can be also favored by the sorption of CO2 formed during biomass gasification. In this work, two types of bi-functional sorbents/catalysts were prepared taking into account the high attrition resistance required in this type of reactor. Olivine was chosen as catalyst and support of CaO or CaO–Ca12Al14O33 (CaO–CaAl) used as CO2 sorbent phases. The bi-functional materials and olivine were characterized by SEM, BET, XRD, TPR and Mössbauer spectroscopy. The SEM and BET analyses showed that the higher content of CaO–CaAl on olivine allowed having larger pore volume and smaller pore size than CaO/olivine. XRD and TPR showed that the formation of α-Fe2O3 by oxidation of Fe2+ from the olivine structure is favored in presence of the CaO–Ca12Al14O33 phase which improves iron oxide dispersion.The CaO–CaAl/olivine 1:2 ratio showed the best sorption properties at 700°C. The CO2 sorption capacity was attributed to the high CaO content and its large pore volume while the presence of Ca12Al14O33 provided CO2 sorption stability during carbonation–decarbonation cycles.The enhancement of catalytic activity in tar conversion for these bi-functional materials compared to olivine was demonstrated and attributed to the presence of well dispersed iron oxides and the high content of sorbent on olivine surface. This work presents the interest of a CaO–CaAl/olivine catalyst for hydrogen production improved by CO2 capture in a fast internal circulating fluidized bed biomass gasifier.

New Insights into [FeFe] Hydrogenase Activation and Maturase Function

[FeFe] hydrogenases catalyze H2 production using the H-cluster, an iron-sulfur cofactor that contains carbon monoxide (CO), cyanide (CN–), and a dithiolate bridging ligand. The HydE, HydF, and HydG maturases assist in assembling the H-cluster and maturing hydrogenases into their catalytically active form. Characterization of these maturases and in vitro hydrogenase activation methods have helped elucidate steps in the H-cluster biosynthetic pathway such as the HydG-catalyzed generation of the CO and CN– ligands from free tyrosine. We have refined our cell-free approach for H-cluster synthesis and hydrogenase maturation by using separately expressed and purified HydE, HydF, and HydG. In this report, we illustrate how substrates and protein constituents influence hydrogenase activation, and for the first time, we show that each maturase can function catalytically during the maturation process. With precise control over the biomolecular components, we also provide evidence for H-cluster synthesis in the absence of either HydE or HydF, and we further show that hydrogenase activation can occur without exogenous tyrosine. Given these findings, we suggest a new reaction sequence for the [FeFe] hydrogenase maturation pathway. In our model, HydG independently synthesizes an iron-based compound with CO and CN– ligands that is a precursor to the H-cluster [2Fe]H subunit, and which we have termed HydG-co. We further propose that HydF is a transferase that stabilizes HydG-co and also shuttles the complete [2Fe]H subcluster to the hydrogenase, a translocation process that may be catalyzed by HydE. In summary, this report describes the first example of reconstructing the [FeFe] hydrogenase maturation pathway using purified maturases and subsequently utilizing this in vitro system to better understand the roles of HydE, HydF, and HydG.

Genetic disruption of both Chlamydomonas reinhardtii [FeFe]-hydrogenases: Insight into the role of HYDA2 in H2 production

Chlamydomonas reinhardtii (Chlamydomonas throughout) encodes two [FeFe]-hydrogenases, designated HYDA1 and HYDA2. While HYDA1 is considered the dominant hydrogenase, the role of HYDA2 is unclear. To study the individual functions of each hydrogenase and provide a platform for future bioengineering, we isolated the Chlamydomonas hydA1-1, hydA2-1 single mutants and the hydA1-1 hydA2-1 double mutant. A reverse genetic screen was used to identify a mutant with an insertion in HYDA2, followed by mutagenesis of the hydA2-1 strain coupled with a H2 chemosensor phenotypic screen to isolate the hydA1-1 hydA2-1 mutant. Genetic crosses of the hydA1-1 hydA2-1 mutant to wild-type cells allowed us to also isolate the single hydA1-1 mutant. Fermentative, photosynthetic, and in vitro hydrogenase activities were assayed in each of the mutant genotypes. Surprisingly, analyses of the hydA1-1 and hydA2-1 single mutants, as well as the HYDA1 and HYDA2 rescued hydA1-1 hydA2-1 mutant demonstrated that both hydrogenases are able to catalyze H2 production from either fermentative or photosynthetic pathways. The physiology of both mutant and complemented strains indicate that the contribution of HYDA2 to H2 photoproduction is approximately 25% that of HYDA1, which corresponds to similarly low levels of in vitro hydrogenase activity measured in the hydA1-1 mutant. Interestingly, enhanced in vitro and fermentative H2 production activities were observed in the hydA1-1 hydA2-1 strain complemented with HYDA1, while maximal H2-photoproduction rates did not exceed those of wild-type cells.

Bioelectrochemical hydrogen production with hydrogenophilic dechlorinating bacteria as electrocatalytic agents

Hydrogenophilic dechlorinating bacteria were shown to catalyze H(2) production by proton reduction, with electrodes serving as electron donors, either in the presence or in the absence of a redox mediator. In the presence of methyl viologen, Desulfitobacterium- and Dehalococcoides-enriched cultures produced H(2) at rates as high as 12.4 μeq/mgVSS (volatile suspended solids)/d, with the cathode set at -450 mV vs. the standard hydrogen electrode (SHE), hence very close to the reversible H(+)/H(2) potential value of -414 mV at pH 7. Notably, the Desulfitobacterium-enriched culture was capable of catalyzing H(2) production without mediators at cathode potentials lower than -700 mV. At -750 mV, the H(2) production rate with Desulfitobacterium spp. was 13.5 μeq/mgVSS/d (or 16 μeq/cm(2)/d), nearly four times higher than that of the abiotic controls. Overall, this study suggests the possibility of employing dechlorinating bacteria as hydrogen catalysts in new energy technologies such as microbial electrolysis cells.