the fuel cell model of abiogenesis: a new approach to...

The Fuel Cell Model of Abiogenesis:A New Approach to Origin-of-Life Simulations

Laura M. Barge,1,2 Terence P. Kee,3 Ivria J. Doloboff,1 Joshua M.P. Hampton,3 Mohammed Ismail,4

Mohamed Pourkashanian,4 John Zeytounian,5 Marc M. Baum,6 John A. Moss,6

Chung-Kuang Lin,1 Richard D. Kidd,1 and Isik Kanik1

Abstract

In this paper, we discuss how prebiotic geo-electrochemical systems can be modeled as a fuel cell and howlaboratory simulations of the origin of life in general can benefit from this systems-led approach. As a specificexample, the components of what we have termed the ‘‘prebiotic fuel cell’’ (PFC) that operates at a putativeHadean hydrothermal vent are detailed, and we used electrochemical analysis techniques and proton exchangemembrane (PEM) fuel cell components to test the properties of this PFC and other geo-electrochemical systems,the results of which are reported here. The modular nature of fuel cells makes them ideal for creating geo-electrochemical reactors with which to simulate hydrothermal systems on wet rocky planets and characterizethe energetic properties of the seafloor/hydrothermal interface. That electrochemical techniques should beapplied to simulating the origin of life follows from the recognition of the fuel cell–like properties of prebioticchemical systems and the earliest metabolisms. Conducting this type of laboratory simulation of the emergenceof bioenergetics will not only be informative in the context of the origin of life on Earth but may help inunderstanding whether life might emerge in similar environments on other worlds. Key Words: Astrobiology—Bioenergetics—Iron sulfides—Origin of life—Prebiotic chemistry. Astrobiology 14, 254–270.

1. Introduction

Identifying the specific processes responsible for theemergence of life on Earth and then convincingly simu-

lating these same processes in the laboratory are among themost intriguing challenges for contemporary biogeochemists.A large part of the difficulty is that plausible scenarios for theorigin of life must not only account for the fundamentalproperties of life today but also for the geochemical condi-tions on early Earth through which life arose. These condi-tions are such that we only have patchy knowledge of them,and they present challenging engineering problems duringattempts at simulation.

A primary process common to all extant life is the con-version of a pH gradient into energy-storing phosphate an-hydride bonds via chemiosmosis across a semipermeablemembrane. In prokaryotes, as well as in mitochondria, the‘‘power plant’’ of eukaryotes, electric potentials across the

membrane that induce a pH gradient between the membraneinterior and exterior (i.e., the proton motive force) drive thefundamental energy generation mechanism for life (Mitch-ell, 1961; Jagendorf and Uribe, 1966; Harold, 1986). As inlife today, the potential energy contained within theseelectron and proton gradients is transduced into usefulchemical energy and stored in metastable energy currencymolecules such as adenosine triphosphate (ATP) or inor-ganic pyrophosphate (PPi). It has been argued that the veryfirst microorganisms were autotrophic; that is, they gainedand assembled their building constituents from the simplemolecules available to them on early Earth (e.g., H2, CO2,CH4, NH3=NHþ2 , HPO2�

4 , and HS - /H2S) (Fuchs, 1989,2011; Berg et al., 2010; Say and Fuchs, 2010). The electrontransfer proteins involved in the generation of the protonmotive force—many of which host inorganic metal sulfide–containing active centers—have been suggested to beamong the most ancient catalysts (Eck and Dayhoff, 1966;

1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA.2Blue Marble Space Institute of Science, Seattle, Washington, USA.3School of Chemistry, University of Leeds, Leeds, UK.4Centre for Computational Fluid Dynamics, University of Leeds, Leeds, UK.5Molecular and Computational Biology Program, University of Southern California, Los Angeles, California, USA.6Department of Chemistry, Oak Crest Institute of Science, Pasadena, California, USA.

ASTROBIOLOGYVolume 14, Number 3, 2014ª Mary Ann Liebert, Inc.DOI: 10.1089/ast.2014.1140

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Baymann et al., 2003; Volbeda and Fontecilla-Camps,2006; Vignais and Billoud, 2007; Lill and Siegbahn, 2009;McGlynn et al., 2009; Nitschke et al., 2013; Schoepp-Cothenet et al., 2013).

Microbes are, therefore, essentially performing the samechemical redox processes as those in ion-exchange mem-brane fuel cells (Mitchell, 1977; Barbir, 2005), and like fuelcells, biology uses proton and ion gradients to generateenergy. In fact, recognition of this similarity within the fuelcell community has helped to implement the use of mi-crobes, their redox-active enzymes, and even mitochondriathemselves as components of electrodes for biofuel cells dueto their excellent catalytic ability to transfer electronsand promote environmentally significant redox reactions(Arnold and Rechnitz, 1980; Heller, 1992; Chang et al.,2006; Arechederra and Minteer, 2008; Huang et al., 2012;Tran and Barber, 2012). Certain geochemical environmentsalso constitute fuel cell–like systems, for example, at hy-drothermal vents where electrical and pH potentials aregenerated at the interface between reduced hydrothermalfluid and oxidizing seawater (Baross and Hoffman, 1985;Russell and Hall, 1997, 2006; Martin and Russell, 2007;Yamamoto et al., 2013). In the hydrothermal vent example,the ambient geochemical potentials can be maintainedand mediated across physical boundaries, for example,electrically conductive biofilms, sediments, or an inorganicchimney precipitate (Ludwig et al., 2006; El-Naggar et al.,2010; Nakamura et al., 2010a, 2010b; Yamamoto et al.,2013). A second geological example of fuel cell–like be-havior that is believed to have been of significance withinearly Earth environments is galvanic corrosion of iron-nickel alloys (associated with meteoritic infall) from contactwith seawater, which can in principle establish pH/Eh gra-dients (Bryant et al., 2013).

Because life universally depends on pH/Eh gradients todrive metabolism, geological environments that alreadygenerate similar electrochemical energy gradients are ofinterest for origin-of-life studies. One such example is hy-drothermal vents, which can be produced by magmatic ac-tivity (e.g., black smokers) or water-rock chemistry (e.g.,serpentinite-hosted alkaline vents). Alkaline hydrothermalvents produced by serpentinization (the process of olivine/pyroxene oxidation via seawater interaction in a series ofexothermic reactions) are of special interest to the origin oflife, as they can generate a very alkaline hydrothermal ef-fluent and, thus, an ambient pH gradient in addition to anelectrical potential difference across the fluid interface(Baross and Hoffman, 1985; Russell and Hall, 2006; Martinet al., 2008; Russell et al., 2010; Lane and Martin, 2012).Serpentinization is likely to have progressed on early Earth(Russell et al., 1989; Sleep et al., 2011; Arndt and Nisbet,2012) and may have been active elsewhere in the SolarSystem, for example, on early Mars, Europa, or other icymoons with a water-rock interface (Vance et al., 2007;Ehlmann et al., 2010; Wray and Ehlmann, 2011; Michalskiet al., 2013), possibly providing an environment for emer-gent prebiotic chemistry.

The operations of extant life are analogous to those of afuel cell, and some version of the fundamental componentsthat make the biological fuel cell function (ATP-synthase,ion-selective membranes maintaining pH/electrical gradients,the electron transport chain) was also likely present in LUCA,

the last universal common ancestor (Gogarten et al., 1989;Martin and Russell, 2007; Mulkidjanian et al., 2007;Schoepp-Cothenet et al., 2013). Because we consider thatLUCA also had these basic fuel cell properties (Russell andHall, 1997; Nitschke and Russell, 2009; cf. Huang et al.,2012; Lane and Martin, 2012), we suggest that some keysteps at the origin of metabolism may then be simulatedexperimentally within the context of the electrochemicalgradient architecture of a fuel cell. As life is an example of afar-from-equilibrium biological system, which presumablyemerged from a far-from-equilibrium geological and geo-chemical system, it seems logical that a far-from-equilibriumchemical system such as a fuel cell may provide an effectivelink between the two.

We set out to examine whether the fuel cell–like prop-erties of certain geochemical environments, such as seafloorinterfaces and hydrothermal vent systems, could be simu-lated in out-of-equilibrium electrochemical experiments andultimately modeled in the laboratory as a fuel cell. In ageological/geochemical fuel cell scenario, some mineral orother inorganic component would be required to act as anelectrocatalyst as well as an ion transfer ‘‘membrane’’ thatseparates contrasting chemical reservoirs. Depending on thescenario, this component could be a conductive mineral, gel,or other porous material. For our preliminary experiments,we chose to focus on (i) Fe sulfides and (ii) Fe-Ni phaseswithin iron meteorites. Both of these are viewed as be-ing potential electron transfer catalysts relevant for proto-metabolic reactions and are considered to have been read-ily available and accessible on early Earth (Williams, 1961,1965; Eck and Dayhoff, 1966; Russell et al., 1994; Bryantet al., 2009, 2013; Nitschke and Russell, 2011). Metal sul-fides in particular constitute an important part of the envi-ronmental fuel cell in modern hydrothermal vent systems asthey facilitate transfer of electrons (Nakamura et al., 2010b).The nature of mineral precipitation in a gradient is alsosignificant, as the chemical disequilibrium can result incomplex precipitate structures such as hydrothermal ‘‘che-mical garden’’ chimneys (Haymon et al., 1983; Russellet al., 1989, 1994; Ludwig et al., 2006) that may exhibitelectrochemical and ion-transfer capabilities (Ayalon, 1984;van Oss, 1984). To model these conditions in the laboratory,an iron sulfide electrocatalyst material was synthesizedby separating an acidic, Fe2 + -containing solution and an al-kaline, sulfide-containing solution across a permeable bar-rier to form a self-assembling precipitate membrane in agradient. This simulated geological precipitate separatedtwo contrasting solutions and could mediate gradients, asmight occur in a natural far-from-equilibrium system suchas chimney growth at a hydrothermal vent. Electrochemicalexperiments also were conducted by using geological ana-log materials (both synthetic and field samples) as catalysts,and a proton exchange membrane (PEM) fuel cell apparatuswas constructed to simulate an electrochemically activegeochemical interface. The fuel cell represents a holisticchemical system that is well understood, amenable tocomputational modeling, and open to sophisticated analyti-cal diagnostics. This systems approach has not yet beenexploited experimentally within the sphere of abiogene-sis and offers a potentially powerful theoretical and exper-imental model through which to explore such emergentphysicochemical systems. These experimental techniques

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can be applied to a variety of geochemical systems or analogmaterials and may even be used to simulate energetic pro-cesses at seafloor interfaces on other planets as well as todetermine whether prebiotic chemistry would be possible onother wet, rocky worlds.

2. Examples of Fuel Cell-like Systemsin Biology and Geology

To envision experimental ways by which to test prebioticchemistry within an electrochemical framework, it is usefulto define and constrain the functional parts of the naturalfuel cells of geology and biology, as well as a proposedprebiotic fuel cell (PFC) that may have powered inorganicand organic redox reactions on early Earth.

Fuel cells are primarily composed of an anode (the sur-face where oxidation occurs), a cathode (the surface wherereduction occurs), and an ion-conducting electrolyte thatphysically separates reservoirs containing the fuel and oxi-dant (Haile, 2003a, 2003b). The physical setup of the fuelcell that separates two chemically distinct reservoirs isessential, for it is this chemical/redox disequilibrium—chemical gradients maintained over some distance—fromwhich electricity is ultimately produced. The essentialcomponents include the electrolyte, which physically sep-arates the fuels and oxidants, allowing a voltage gradient tobe maintained; the electrodes, which facilitate oxidation/reduction and contain catalytic materials; and the fuel(electron donor) and oxidant (electron acceptor), whichtransfer electrons to and from the electrodes.

Mitochondria, the free energy factories of animal cells,are very efficient electrochemically, and they and their as-sociated enzymes have been successfully used as elec-trodes in manmade fuel cells (e.g., Arechederra and Minteer,2008). As in a PEM fuel cell, the flow of protons andelectrons is spatially separated in mitochondria, and life hasevolved precise chemical means by which to harness theenergy present in these potential gradients. The lipid bilayermembrane is the capacitor that separates contrasting che-mical environments, and the electrodes of mitochondriaare the ends of a series of electrochemically connected en-zyme complexes within the inner membrane, togetherknown as the electron transport chain. The iron-nickel-sulfide–containing centers of the enzymes act as anodes andcathodes (Berg and Holm, 1982; Baymann et al., 2003;Vignais and Billoud, 2007), while the membranes—capableas they are of selective ion transport—maintain chemicaldisequilibrium (Lane and Martin, 2012). Derived fromproteobacteria, the mitochondria bear certain similarities tochemiosmotic LUCA (Yang et al., 1985; Martin and Muller,1998; de Paula et al., 2013). LUCA’s biological batterywould have had some similar functional parts, such as thecomplex I, referred to as the ‘‘steam engine of the cell’’ byEfremov and Sazanov (2011), that drives protons to theperiplasm through redox disequilibria. These protons form apH gradient due to the disequilibrium between membraneexterior and interior and drive ATP synthase—the rotarybiomolecular motor—to make ATP (Yoshida et al., 2001;Branscomb and Russell, 2013).

A geological fuel cell phenomenon also can emergewithout biological mediation at seafloor interfaces, for ex-ample, in hydrothermal vents produced either by magmatic

activity or by water-rock chemistry. High-temperature hy-drothermal alteration on oceanic spreading ridges can pro-duce a hot ( > 350�C) acidic hydrothermal fluid driven bymagmatic intrusion that fuels black smokers, named afterthe black particles that form as the rapid temperature/pHchange induces precipitation of metal sulfides at the fluidinterface. In the absence of magmatic heating, reducinghydrothermal fluids can also be produced by serpentiniza-tion, a process in which seawater permeates through frac-tures in the Fe/Mg-silicate ocean crust and produces analkaline, H2- and CH4-enriched, moderately hot (*100–200�C) hydrothermal fluid (Russell et al., 1989; Kelleyet al., 2001, 2005; Bradley and Summons, 2010; Charlouet al., 2010; Klein et al., 2013), even in low-temperatureexhalations (Neal and Stanger, 1983; Coveney et al., 1987;Etiope and Sherwood-Lollar, 2013; Etiope et al., 2013).Both black smoker and alkaline vents can produce chimneyprecipitates where hydrothermal fluids feed back into theocean, and these naturally occurring mineral precipitates aresignificant in that they provide a semipermeable and semi-conducting barrier between the two contrasting fluids. Theredox-active fluid interface of a hydrothermal vent consistsof fuel/oxidant reservoirs separated by an inorganic mineralprecipitate and constitutes a naturally occurring fuel cell thatdoes not depend on biological processes to function. Theinside and outside surfaces of the hydrothermal chimneycould be considered as anode and cathode, respectively.Depending on composition and environmental conditions,the chimney itself may be able to catalyze the oxidation ofhydrothermal fuels and conduct resulting electrons to sea-water oxidants. The fuel cell–like properties of hydrother-mal vents have been directly demonstrated in field studies ofblack smoker vents, in situ by harnessing the electricalcurrent produced by the vent to power a small device on theseafloor (Yamamoto et al., 2013), as well as in the labora-tory, where it has been demonstrated that black smokerchimney material is electrically conductive and capable ofcatalyzing redox reactions (Nakamura et al., 2010b).

It is also perhaps pertinent to the PFC model that, as puremetals and metal alloys are frequently found to be highlyefficient electrocatalysts (Chen et al., 2011), such materialswere probably geologically present within the Hadean pe-riod through the reducing power of serpentinizing vents(e.g., awaruite, Ni3Fe; Ulrich, 1890; Frost, 1985; Kleinand Bach, 2009) and as Fe-Ni meteoritic deposits during thelate heavy bombardment (Buchwald, 1977; Cockell, 2006).The corrosion of metals and metal alloys by an electrolytesuch as seawater can give rise to a localized fuel cell–likesystem: anodic oxidation of Fe to Fe2 + with concomitantcathodic reduction of, for example, SO� 2

4- to elemental S or

S2 - ; H + to H2; or NO�3 to NO�2 (Bryant et al., 2013)—eachof which could, in principle, establish localized pH/Eh

gradients.

3. Creating a Fuel Cell Model of a PrebioticHydrogeological System

The energetic ‘‘essence’’ of extant anaerobic life could beviewed as quite similar to a fuel cell, in that ion gradientsand electron potentials maintained across a membrane pro-duce energy as long as fuels and oxidants are supplied. Thesediment/water or hydrothermal/ocean interfaces in modern

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marine systems also can constitute naturally occurring fuelcells, deriving from the electrochemical cell of Earth and itsatmosphere and mediated by microbes acting as redox cat-alysts. This has led to the proposal that ion/electron poten-tials also were the driving forces for life’s origin and thatthese gradients were initially provided by the naturally oc-curring pH/Eh potentials in alkaline hydrothermal vents(Russell and Hall, 1997; Russell et al., 2010). This alkalinehydrothermal origin-of-life model can be generalized to anywet rocky world with a chemically similar water-rock in-terface (Russell et al., 2014), so we chose to develop ourelectrochemical/fuel cell model using alkaline vents as oneexample of a hydrogeological origin-of-life scenario thatcould benefit from this concept. Similar fuel cell modelsalso could be applied to simulating electrochemically activewater-rock interfaces on early Mars, early Venus, or the icymoons of Jupiter and Saturn.

Earth’s oceans 4 billion years ago are thought to havebeen anoxic, mildly acidic (pH * 5–6 due to dissolvedatmospheric CO2, NO, and ephemeral SO2), and rich inFe2 + ; and they could have contained Ni2 + , Mn2 + , phos-phates, nitrate, nitrite, and some ferric and manganese ions(Macleod et al., 1994; Russell and Hall, 1997; Hagan et al.,2007; Martin and Russell 2007; Ducluzeau et al., 2009;Mloszewska et al., 2012; Nitschke et al., 2013). The hy-drothermal fluid produced by serpentinization would likelyhave been similar to modern-day hydrothermal fluids: al-kaline, containing some silicate, trace Mo and W, and dis-solved H2 and CH4 along with formate and a range ofhydrocarbons (Proskurowski et al., 2008; Konn et al., 2009;Lang et al., 2010, 2012; Mielke et al., 2010; Charlou et al.,2010). It is also thought that ancient hydrothermal springs inserpentinizing systems would have contained millimolarlevels of sulfide from dissolution of crustal sulfide minerals,as determined from laboratory reactor experiments simu-lating serpentinization in a Hadean system (Mielke et al.,2010, 2011). Vents driven by serpentinization produce rel-atively low-temperature fluids compared to the scaldingenvironment of black smokers, but between them, hydro-thermal vent systems on early Earth could have contributedmany of the fuels and materials that are thought to be rel-evant for an autogenic/autotrophic emergence of life. Theinterfacing of alkaline hydrothermal fluids and seawater onearly Earth might have produced a variety of inorganicprecipitates within the ambient geochemical/electro-chemical gradients, among them, silica gel, ferrous/ferricoxyhydroxides, and transition metal sulfides (Russell andHall, 1997). Hydrothermal precipitates formed by serpenti-nization on early Earth might therefore have had mineralcompositions and electrochemical properties in some wayssimilar to modern black smoker chimneys, for example,conductive iron sulfide minerals that catalyze redox reac-tions. Thus, electrochemical studies of modern blacksmokers (e.g., Nakamura et al., 2010b; Yamamoto et al.,2013) may also be relevant for understanding the energeticsof hydrothermal vent fuel cells on early Earth.

A putative Hadean hydrothermal vent with a chimneystructure would have operated somewhat like a flow-throughfuel cell. The contrasting reservoirs of reduced, H2/CH4-containing hydrothermal fluid and the relatively oxidized(containing minor concentrations of FeIII and dissolvedNO), CO2-rich ocean would have been separated by a pre-

cipitate of mixed composition, containing material such ascarbonates, silicates, and iron and other transition metalsulfides and mixed-valence layered iron ‘‘double’’ oxy-hydroxides (Russell et al., 2014; Fig. 1). Transition metalsulfides are one of several possible components of a prebi-otic hydrothermal chimney, but as one of the more elec-trochemically active materials likely to be present, it isuseful to characterize iron sulfide chimney growth andelectrochemistry in the laboratory, which various studieshave pursued (Russell et al., 1989; Mielke et al., 2010,2011; McGlynn et al., 2012; Barge et al., 2014). Manyproperties of iron sulfide precipitates relevant to a far-from-equilibrium geological system have been experimentallydetermined. For example, when initially precipitated be-tween contrasting solutions, self-assembling iron sulfidemembranes are capable of generating an electrical potentialof *0.6 to 0.7 V and maintaining the pH gradient betweenthe hydrothermal and ocean solutions (Filtness et al., 2003;Russell and Hall, 2006; Barge et al., 2014). The precipitatesproduced in out-of-equilibrium experiments have a complexstructure at the microscale; membranes vary in thicknessfrom *5 to *100 lm, exhibit and comprise a mix of ironsulfides and iron oxyhydroxides/silicates, and can also in-corporate dissolved ions such as other transition metals andphosphates (Mielke et al., 2011; Barge et al., 2012, 2014;McGlynn et al., 2012; Barge and Kanik, unpublished data).

In a hydrothermal system with a chimney structure, theprecipitate could facilitate diffusion of ions and electronconduction (Nakamura et al., 2010b); thus, it could functionas a selectively permeable electrolyte membrane betweenhydrothermal fuels and seawater oxidants. In a fuel cellconcept of prebiotic alkaline hydrothermal vents, the anodeand cathode would be the interior and exterior chimneymineral surfaces, respectively. The exterior and interior of achimney could have different mineral compositions, sincechemical garden–like structures tend to exhibit composi-tional variations across the membrane reflecting the chem-ical gradients in which they precipitate (Pagano et al.,2006; Russell and Hall, 2006; Cartwright et al., 2011; Bargeet al., 2012). Though a hydrothermal chimney would likelyalso contain components such as silica gel or carbonate, theability of the ‘‘electrodes’’ to drive redox reactions wouldprobably be dependent on the presence of electrocatalyticminerals such as metal sulfides and oxyhydroxides (cf.,Arrhenius, 2003; Antony et al., 2008). Minerals such asmackinawite (FeS) and greigite (Fe3S4) are electricallyconductive, can behave as capacitors, and are thought to becatalytically active in organic reactions (Huber and Wach-tershauser, 1997; Rickard et al., 2001). These iron sulfideminerals also are structurally similar to the Fe4S4 and FeSgroups in metalloenzymes that catalyze redox reactions inbiology, a similarity that has led to the suggestion that thesemetalloenzyme centers originally derived from inorganichydrothermal minerals (Russell and Hall, 1997, 2006;Rothery et al., 2008; Nitschke et al., 2013). In a putativeprebiotic hydrothermal fuel cell, transport of electrons fromfuel to oxidant could be mediated by a series of smallerredox steps, as in biological systems. Within this scenario,the fuels and oxidants would be indefinitely replenished aslong as serpentinization remained active, which is envisagedfor modern alkaline hydrothermal systems to last for overone hundred thousand years (Ludwig et al., 2011). The


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chimney membranes themselves could be continuously re-newed as the hydrothermal fluids produced by serpentini-zation continue to rise and interface with the ocean, feedingthrough and possibly disaggregating older membranes.

4. Experimental Studies: Exploiting ElectrochemicalTechniques and Fuel Cells as Planetary GeologyTest Beds

The large body of fuel cell work in which enzymes, mi-tochondria, or microbes are used as electrocatalysts is tes-tament to the importance of charge and concentrationgradients in biology. Because both hydrogeological envi-ronments (e.g., vents) and biological cells (at least, in theirfree energy generation mechanisms) can be conceptualizedas fuel cells, it is reasonable to pursue hydrothermal origin-of-life experiments in a setup that preserves these funda-mental aspects of pH and electrochemical gradients across amembrane. To this end, we conducted preliminary experi-ments to simulate hydrogeological systems with electro-chemical gradients and using geological materials aselectrode catalysts. We illustrate how these methods can beused to simulate geochemical free energy interfaces and testhypotheses regarding the transition to bioenergetics. It is ourintention to broaden these initial studies of simulated sea-floor/hydrothermal interfaces to explore application ofsimilar techniques to other hydrogeological environments,such as galvanic corrosion of iron meteorites or water/rockinterfaces on other planets, within the framework of a fuelcell reactor. We envisage the results presented here to be

preliminary indications of possible value in the model wepropose.

4.1. Electrochemical studies of simulatedhydrothermal precipitates

Though an ancient hydrothermal chimney would havehad a heterogeneous composition, some of the most elec-trochemically relevant minerals thought to be formed in thissystem are iron and iron/nickel sulfides. To investigate theelectrochemical properties of metal sulfide minerals thatcould be formed at alkaline vents on early Earth, we pre-cipitated inorganic membranes in out-of-equilibrium solu-tion interface experiments (Fig. 2; Filtness et al., 2003;Barge et al., 2014). We interfaced contrasting solutions thatrepresent the dissolved Fe2 + in the acidic primordial ocean(added as dissolved FeCl2�4H2O) and dissolved sulfide inthe alkaline hydrothermal fluid released from crustal sulfideminerals (added as dissolved Na2S�9H2O). When interfaced,these solutions self-assembled into mineral precipitates inthe same manner as the injection chemical garden experi-ments used previously to simulate Hadean hydrothermalchimney precipitates (Mielke et al., 2011; Barge et al.,2012, 2014). However, we have found that a glass fuel cellsetup yields a more structurally stable membrane precipitatethat can be used in subsequent electrochemical experiments(Barge et al., 2013, 2014). We have previously formediron sulfide membranes (and chimneys) using various Fe2 +

and sulfide concentrations and have found that, althoughmembranes can be formed by using geologically realistic

FIG. 1. Alkaline hydrothermal vent on early Earth modeled as a membrane fuel cell. The reservoirs of seawater (relativelyoxidizing, acidic) and hydrothermal fluid (reducing, alkaline) are physically separated across a hydrothermal chimney pre-cipitate, which functions as part electrolyte and part capacitor. The chimney precipitate would have contained (among otherthings) iron sulfide minerals, due to the solubility of Fe2 + in the early anoxic oceans and the influx of soluble bisulfide (HS - )from the hydrothermal fluid. The anode and cathode are the inside and outside surfaces of the chimney, which would havevariable composition reflecting the chemical gradients in which they precipitated. The chimney walls would have beensomewhat conductive to electrons, which would pass from fuels in the hydrothermal reservoir to oxidants in the ocean reservoir,catalyzed by electrochemically active minerals in the chimney wall.

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millimolar concentrations over 3–4 days, more robustmembranes form over shorter timescales when using higher(*50 mM) concentrations, and these still preserve the out-of-equilibrium nature of the precipitates. Therefore, we usedhigher reactant concentrations in this study to form precip-itates on shorter timescales that are reasonable for laboratoryexperimentation and to generate more precipitate materialfor use as an electrocatalyst. We recognize that, in a naturalsystem, similar precipitates might take much longer to formfrom the dilute Fe2 + - and sulfide-containing fluids.

4.1.1. Formation of membranes simulating hydrothermalchimney walls. A synthetic porous membrane template wasclamped between two fluid reservoirs in a two-chamberglass membrane fuel cell apparatus (Adams & Chittendenglassware, with high-temperature clamp stable up to 150�C).Most experiments were performed by using dialysis tubing(Fisher, 3500 MCW) as the precipitation template, butconductive carbon cloth (Sigracet, hydrophilic, no micro-porous layer) was also tested as a template material. Theiron and sulfide solutions were added to the half-cells re-spectively, and the cell was kept anoxic throughout withconstant N2 purging and allowed to react for 24 h at roomtemperature. High-resolution imaging and chemical analysiswere carried out on the iron sulfide membranes by using anenvironmental scanning electron microscope with an at-tached energy-dispersive X-ray tube. The electrical potentialgenerated across the inorganic membrane as it precipitatedwas recorded over the course of experiments by an AgilentLXI Data Acquisition/Data Logger Switch Unit with elec-trodes placed in each cell. pH measurements were recordedwith an Excel XL20 pH/conductivity meter (Fisher Scien-tific) with temperature calibration. The membranes formedwere used in subsequent electrochemical experiments, de-scribed below.

4.1.2. Membrane potential tests. We conducted mem-brane potential tests in which an iron sulfide membrane wasallowed to precipitate for 24 h, followed by removal of theiron and sulfide solutions. The membrane and both half-cells

were carefully rinsed with ddH2O; then NaCl or KCl saltsolutions of varying concentration were added to both sidesof the precipitated membrane. Salt concentrations were 10 ·greater on the ‘‘formerly alkaline’’ side than on the ‘‘for-merly acidic’’ side, and concentrations between 10 mM and1 M were tested for both NaCl and KCl. Electrodes wereplaced several millimeters from either side of the membranesurface to record the membrane potential. In previousstudies, this technique has been shown to generate concen-tration-dependent membrane potentials across inorganicprecipitate membranes in many different chemical systemsincluding sulfides (e.g., of Hg, Sn), although iron sulfidemeasurements were not reported (Beg et al., 1977, 1978,1979; Sakashita and Sato, 1977; Malik et al., 1980; Ayalon,1984; Siddiqi and Alvi, 1989; Kushwaha et al., 2001; Begand Matin, 2002).

4.1.3. Membrane galvanostatic tests. We conductedgalvanostatic tests in which an electrical current was appliedacross an iron sulfide membrane after it had formed (Honigand Hengst, 1970). Half-cells were filled with acidicFeCl2�4H2O solution and alkaline Na2S�9H2O solution,respectively, and a dialysis membrane was clamped betweenthe two reservoirs. Two electrodes were placed in each cell:one in the bulk solution to apply current and one very close(2–3 mm) to the membrane to measure membrane potential.After the membrane had precipitated for 24 h, a currentof – 0.75 mA/cm2 was applied across the membrane, and theresulting membrane potential relative to the rest potentialwas measured. The measurement also was repeated afterboth half-cells had been emptied and refilled with 0.1 MNaCl solution.

4.1.4. Voltammetry studies. To investigate the ability ofiron sulfide chimney precipitates to facilitate electron trans-fer from hydrothermal reductants to seawater oxidants in aprebiotic hydrothermal system, we applied simulated hy-drothermal iron sulfide–containing precipitate membranematerial as a catalyst on a glassy carbon electrode (GCE).Cyclic voltammetry half-cell experiments were conducted in

FIG. 2. (A) Precipitation of Fe/S membrane between two interfacing solutions in a glass fuel cell apparatus. Thereservoirs represent the Hadean ocean and alkaline hydrothermal fluid, and they are separated by a synthetic porousseparator that allows ion contact but not solution mixing. (B) Fe/S membrane precipitated on dialysis membrane (precipitatearea *4.9 cm2).


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which the GCE + catalyst was characterized in a Hadeanocean simulant containing oxidants of interest to the originof life in serpentinizing systems. In this experiment, a Fe/Sprecipitate membrane was formed in the fuel cell apparatusshown in Fig. 2, and after 24 h the membrane was removed,rinsed with ddH2O, and dried under N2. The dried precipi-tate was removed from the dialysis membrane template andattached to a GCE with conductive carbon tape. Potentialfrom – 1 V was applied (relative to Ag/AgCl, using a Ptcounter electrode) in a solution representing bicarbonateas an electron acceptor in the Hadean ocean (20 mMNaHCO3 + 0.6 M NaCl, titrated to pH *5). A control wasperformed such that no FeS catalyst was applied to theworking electrode, but ground FeS particles (Sigma) werestirred into the simulated ocean while potential was applied.Other controls had no FeS in the system. Experiments alsowere performed in which carbon cloth was used as themembrane separator in the two-cell precipitation apparatusdescribed above, so that Fe/S membranes were precipitateddirectly onto the carbon cloth and pieces of the Fe/S-coveredcloth (5 mm · 50 mm) could be used directly as workingelectrodes in voltammetry studies. Controls were also per-formed with plain carbon cloth (with no Fe/S) as workingelectrode.

4.2. Fuel cell reactors simulating prebioticgeo-electrochemical systems

4.2.1. Fuel cell design rationale. An origin-of-life fuelcell experiment designed to test a hydrogeological systemon early Earth or any wet rocky planet might involve acombination of techniques: synthesizing simulated mineralcatalysts in out-of-equilibrium systems (as in Barge et al.,2012, 2013, 2014; Mielke et al., 2010, 2011); electro-chemical studies of simulated precipitate materials to char-acterize their utility as electrocatalysts; and the use of thesesimulated prebiotic catalysts to create catalytic electrodesfor use in a fuel cell. There are various types of fuel cellexperiments that could be informative in this regard. Imi-tating the design of microbial fuel cells (MFCs), where thecatalytic electrodes are submerged in two liquid reservoirs

and separated by an ion-exchange membrane, would allowfor testing of different fluid chemistries and dissolved ionic,as well as gaseous, electron donors and acceptors. MFC-type experiments would also lend themselves well to testingother relevant biological components as catalysts, such asindividual enzymes or organisms. A geo-electrochemicalsystem also could be simulated by a proton exchangemembrane fuel cell (PEMFC), which could use gaseousreductants/oxidants under higher pressure or temperature,with the simulated geological catalysts embedded in the gasdiffusion layers (GDLs) within the membrane electrodeassembly (MEA). Either way, the modular engineering ar-rangement of fuel cell components makes for a convenientplanetary geology test-bed system in which components,such as the ion-exchange membranes, electrodes and cata-lysts (or electrolyte assembly unit), and the anode/cathodereactant feeds, may be substituted by materials more closelyconnected to geological environments. For example, ratherthan a commercial GDL catalyst layer, one could substitutesimulated hydrogeological precipitates similar to those de-scribed above. This type of experiment is not limited tosimulating hydrothermal vent environments; other geologicalelectrochemical processes of interest could be explored in asimilar experimental setup, for example, as mentioned inSection 1 above, the electrochemical corrosion of meteoriteson early Earth (Bryant et al., 2009). Field samples of catalyticminerals derived from reduced Fe/Ni in meteorites, whichhave been previously proposed to assist with early phospho-rus chemistry (Pasek and Lauretta, 2005; Pasek et al., 2013),could be treated in the same way as hydrothermal precipitatesin the GDL electrocatalyst layers (see below).

We conducted some preliminary proof-of-concept tests,using a PEMFC to simulate the catalytic action of geolog-ical mineral catalysts in a far-from-equilibrium system. Asample experimental arrangement for a PEMFC that simu-lates a geo-electrochemical system is shown in Fig. 3. In ourpreliminary tests, a commercial proton-exchange membraneand two GDLs containing electrocatalysts were sandwichedbetween two composite graphite bipolar plates, and the fuelcell was fed with humidified hydrogen (to represent hydrogen

FIG. 3. Outline of PEMFC subsystem construction.

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produced by serpentinization) and air at the anode andcathode sides, respectively. We examined the potential forefficient carbon-black-deposited platinum electrodes to bereplaced by geological materials, in this case Fe-Ni alloyspresent within iron meteorites, where taenite and kamaciteare the dominant phases. While these materials may at firstglance appear incompatible with terrestrial minerals, there issupport for them to have contributed a significant compo-nent of the early Earth lithosphere (Pasek and Lauretta,2008), and they are closely related to the hydrothermal Fe-Ni mineral, awaruite (Klein and Bach, 2009). Most impor-tantly, these meteorite samples represent a proof of conceptfor fabricating geologically analogous GDLs or electrodesfor a planetary geology fuel cell reactor by using fieldsamples rather than synthetic catalysts. Such a procedurecould be applied to other field samples of interest, for ex-ample, samples of hydrothermal precipitates, serpentinite,metal sulfides, or other minerals that might be important in aprebiotic system.

4.2.2. Preparation of geological fuel cell catalysts. TheFe-Ni ‘‘geological electrocatalyst’’ was prepared by usingshavings from the coarse octahedrite, group IAB-sLL To-luca meteorite that has a mean composition of 91% Fe and8.1% Ni (Fig. 4). This geological sample was ball-milled toan average particle diameter of ca. 5 microns before beingdispersed in ethanol, sonicated in an ultrasonic bath for10 min, mixed with Nafion polymer binder, and then spray-coated onto a GDL. For both electrodes, the amount of cat-alyst required was calculated based on a 1.0 mg/cm2 catalystloading within an 11.56 cm2 active area. Both the cathodeand anode electrodes were positioned on the faces of thecatalyst-coated Nafion membrane and hot-pressed (with ahydraulic press) to form a unit of MEA. The requiredamount of the catalyst was first ultrasonically mixed for afew seconds with a few drops of deionized water in order tobreak the catalyst powder into small pieces and prevent thecatalyst from being deactivated when blending it with thedispersion agent, ethanol. A calculated amount of 50 wt %ethanol based on 15 mL ethanol per 50 mg catalyst wasadded to the wet catalyst, and the resulting mixture wasultrasonically blended for about 3 min. The resulting blendwas then mixed with a 5 wt % Nafion solution (Sigma Al-drich) based on a loading of 0.1 mg Nafion/cm2. The Nafionsolution is added to create an ionic phase for proton trans-port from the anode to cathode. This final mixture was ul-trasonically blended for 15 min to form what is normallyknown as an ‘‘electrode ink.’’ The electrode ink was thenmanually sprayed onto the surface of the GDL that wasmounted on a PTFE plate heated to 80�C to evaporate thealcoholic components, until the desired 1.0 mg/cm2 catalystloading was reached.

4.2.3. Preparation of membrane electrode assembly. Amixture of 5 wt % Nafion solution, calculated based on0.5 mg Nafion/cm2, and acetone (about 3–5 mL) wassprayed onto the surface of the catalyzed GDL to enhancethe contact between the catalyst layer and the Nafionmembrane in the MEA being fabricated. The N115 Nafionmembranes were pre-treated before being used in MEAfabrication by heating at 80�C in 2 wt % hydrogen peroxidesolution for 1 h, rinsing several times with deionized water,

heating in 1 M sulfuric acid at 80�C for 1 h, and finallyrinsing several times with deionized water. Both the cathodeand anode electrodes were positioned on the faces of theNafion membrane and hot-pressed with a hydraulic press at130�C and 50 kg/cm2 for 3 min to form a unit of MEA.

4.2.4. Assembly and operation of the PEMFC. A singlePEMFC was assembled by sandwiching the MEA betweentwo composite graphite bipolar plates (Bac2) with an 11-turn and single-pass serpentine flow channel (2 mm · 2 mm,with a rib of width 0.8 mm). The PEMFC was placed be-tween two copper end plates and connected to an in-housefuel cell test station. The fuel cell was maintained at roomtemperature and fed with hydrogen at the anode side and airat the cathode side (Fig. 5). The flow rates of both hydro-gen and air were kept constant, equivalent to a stoichiom-etry of 2 calculated based on a 500 mA/cm2 current density.Dry hydrogen and air were fully humidified by flowingthrough bubbler-type humidifiers. The pressures of the inletand outlet gases were 1 and 0 barg, respectively. Electro-chemical tests were performed with a Gamry Reference3000 potentiostat/galvanostat.

FIG. 4. (A) Scanning electron microscope image (sec-ondary electron) of shavings from the type IAB-sLL Tolucameteorite with mean composition of 91% Fe and 8.1% Ni.(B) Energy-dispersive mapping spectrum (EDS) on thesame course-grained meteorite sample with elemental coloridentification showing a small degree of surface oxidation ofthe meteorite shavings.


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5. Results

5.1. Electrochemical characterization of simulatedhydrothermal precipitates

When the contrasting acidic/iron (ocean simulant) andalkaline/sulfide (hydrothermal simulant) solutions were ad-ded to the two-chamber cell, they interfaced across the porousseparating barrier and immediately began to produce a self-assembling iron sulfide–containing precipitate film in/onthe template. The characteristics of this inorganic mem-brane precipitate varied depending on compositions of thetwo solutions and on the nature of the membrane templatematerial, but commonly a complex landscape of micron-scale crystal structures formed (Fig. 6A)—not unlike thecrystal morphologies observed in the interior membranes ofchemical gardens formed in injection experiments (Cart-wright et al., 2002, 2011; Barge et al., 2012). When dialysistubing was used as the template, the membrane precipitateformed within the pores and also on the surface of the dialysistubing. Membranes precipitated on dialysis tubing proved tobe ideal for electrochemical characterization within theoriginal precipitation apparatus, since the dialysis templateis only permeable to ions and solution mixing was com-pletely prevented. Thus, precipitates only formed on thetemplate separating the two solution reservoirs, not in eithersolution. Dialysis templates also yielded membranes thatwere structurally stable enough to enable gentle rinsing andemptying/refilling of solutions, and when removed from thecell, the precipitate was a flaky solid film that could beeasily dried and removed from the dialysis tubing for lateruse as a catalyst. Carbon cloth was much more porous thanthe dialysis tubing, and when used as a precipitation tem-plate, it did not prevent solution mixing as effectively. Thus,in the precipitation fuel cell apparatus where carbon clothwas used as the separator, the two solutions often mixed

near the interface (forming Fe/S precipitates in the bulksolutions as well as on the carbon cloth), and it was notpossible to do electrochemical experiments with the mem-brane still in situ. We also found that it was necessary topretreat the carbon cloth by wetting it with ddH2O, rinsingto remove extraneous carbon material that was not containedwithin the woven fibers, and applying FeCl2�4H2O andNa2S�9H2O solutions directly to the cloth to form a‘‘starter’’ precipitate in the pores before placing it betweenthe two solution reservoirs. In any case, using carbon clothcaused the FeS precipitate to form on the carbon cloth fibers(Fig. 6B); and because carbon cloth itself is conductive, apiece of the cloth containing FeS precipitate could be di-rectly used as a working electrode.

We observed that, even in a system containing only ironand sulfide, these self-assembling inorganic membraneswere able to mediate chemical/pH gradients for severaldays, as observed by monitoring the pH of the acidic cell.The membranes also generated a diffusion potential of*300–600 mV as they formed, depending on experimentalconditions, as was observed in a similar experiment byFiltness et al. (2003). This potential declined over severaldays as ions diffused through the membrane, eventuallyreaching equilibrium. When the iron and sulfide solutionswere removed after a membrane had precipitated, and thesolution reservoirs were refilled with concentrated salt so-lutions, the iron sulfide–containing membranes still gener-ated a membrane potential of a few tens to a hundredmillivolts. Consistent with previous studies, this membranepotential was dependent on the concentrations of salts oneither side of the membrane and is likely due to the ad-sorption of cations and/or anions on the charged inorganicmembrane surfaces. In galvanostatic experiments in whichcurrent was applied across a precipitated iron sulfidemembrane, the I/V characteristic curves of these Fe/S

FIG. 5. (A) PEMFC test rig at Uni-versity of Leeds. (B) Close-up of com-plete MEA fuel cell assembly with theanodic surface in the foreground con-nected to H2 inlet and outlet supply. Thecathodic reverse side is supplied with air,within which O2 acts as ultimate electronacceptor.

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membranes showed that the electrical resistance across themembrane changed at an inflection point (Fig. 7), indi-cating that the Fe-S precipitate membranes formed in theseout-of-equilibrium systems are somewhat bipolar, that is,partially cation- and anion-selective. This is an impor-tant property of biological and also electrodialysis mem-branes (Honig and Hengst, 1970; Sakashita et al., 1983;Bauer et al., 1988; Strathmann et al., 1993; Aritomi et al.,1996).

The ability of simulated prebiotic iron sulfide chimneyprecipitates to facilitate electron transfer to a simulatedHadean ocean, with applied potentials between – 1 volt(relative to Ag/AgCl), is shown in Fig. 8. The simulatedhydrothermal FeS membrane catalyst (grown on dialysistemplate then attached to a GCE) on the electrode facilitated

*0.4–0.8 mA of both anodic and cathodic current. Nosignificant current ( > tens of microamps) was observed withjust the GCE, GCE with carbon tape, or in the ocean sim-ulant containing FeS particles. Similar results, though athigher current ranging to – 10 mA, were observed whencarbon cloth containing Fe/S precipitate was directly used asan electrode. As in the dialysis tubing experiments, thecarbon cloth without Fe/S did not facilitate significant cur-rent at these potentials. The higher current range of Fe/Smembranes on carbon cloth, compared to Fe/S membranematerial taken from dialysis tubing, is likely due to the factthat the active surface area is greater on the carbon clothworking electrodes.

5.2. PEMFC function using geological electrocatalystmaterials derived from iron meteorites

Electrochemical analyses of the PEMFC in operationalmode revealed a stable open circuit voltage difference acrossthe electrodes peaking close to 0.7 V (Fig. 9, upper panel). Inthe presence of Toluca electrocatalyst–impregnated GDLs,the current-voltage polarization curves (Fig. 9, lower panel)show distinct behavior confirming that the fuel cell is func-tional; and the current flow increases with decreasing voltageas expected, approaching a load of ca. 13 mA. In the absenceof the geological catalyst components, no such polarizationbehavior was observed.

6. Discussion

The iron sulfide membranes that formed at the interfaceof contrasting solutions in our experiments represent spe-cific mineral components that could have been containedwithin larger, carbonate- and silica-containing hydrothermalchimney precipitates on early Earth. Iron sulfide precipitatesin a hydrothermal chimney, formed in pH/ion gradientsvia diffusion and self-assembly, could contribute to theelectrocatalytic properties of the chimney if they were suf-ficiently conductive. Mackinawite (FeS) is usually the first

FIG. 6. Environmental scanning electron microscope (ESEM) images of iron sulfide membrane precipitates on differentmembrane templates: (A) dialysis tubing and (B) carbon cloth (the cylinders are carbon fibers). ESEM images were obtainedwith a voltage of 20 kV and a working distance of 10 mm.

FIG. 7. Current/voltage characteristics of an iron sulfideprecipitate membrane in two repeated experiments. Themembrane was formed at the interface between simulatedHadean ocean and alkaline hydrothermal solutions; thenthese solutions were removed and replaced with 0.1 MNaCl. Voltages are measured relative to a rest potential.


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metastable product that forms from the precipitation ofaqueous Fe2 + and HS - , and over time it can transform togreigite (Fe3S4), pyrrhotite, or pyrite (Kwon et al., 2011;Csakberenyi-Malasics et al., 2012; Sines et al., 2012).Mackinawite is a layered mineral composed of Fe-S tetra-hedra stabilized by van der Waals forces, and since theFe-Fe distance is only 2.60 A (similar to elemental a-iron:2.48 A), it is highly electrically conductive and has evenbeen shown to exhibit superconducting characteristics(Kwon et al., 2011). It has been demonstrated that blacksmoker chimney material, which is also mostly composed ofmetal sulfides, is capable of converting the geochemicalredox potential between hydrothermal fluid and seawater toelectrical current and catalyzing redox reactions (Nakamuraet al., 2010b). Even though a hydrothermal chimney at analkaline vent on early Earth may not have had such a highfraction of metal sulfides as a modern black smoker chim-ney, still it is possible that in a prebiotic system subject toelectrical potential gradients between hydrothermal fuels(e.g., H2, CH4) and seawater oxidants (e.g., CO2, NO�3 ,NO�2 , FeIII), deposits of conductive Fe/S precipitates, aidedby their large active surface area as seen in Fig. 6, mighthave been able to facilitate electron conduction and catalyzeredox reactions on the mineral surfaces.

Our preliminary experiments revealed that, due to theirfar-from-equilibrium nature, inorganic precipitate mem-branes analogous to hydrothermal chimney walls have manyunusual properties (van Oss, 1984) that may be biologicallyand/or prebiotically relevant. The growth of a hydrothermalchimney precipitate that separates contrasting solutionscould generate and maintain an electrical potential for longperiods of time yet still facilitate selective ion transfer

through the membrane. The precipitates formed wouldlikely exhibit a compositional gradient from exterior tointerior, as is observed in real hydrothermal chimneysand chemical garden lab experiments (Haymon, 1983;Cartwright et al., 2002; Barge et al., 2012), and the chargedmineral surfaces could adsorb and concentrate ions fromeither ocean or hydrothermal solution. Metal sulfide–containing chimneys formed within a temperature/pH gradi-ent could potentially host a range of mineral phases withvarious electrochemical properties. It is likely that these in-organic membranes could also incorporate silicates, phos-phates, Ni, and Mo into the precipitate (Barge et al., 2014;Barge and Kanik, unpublished data). More experimental workis needed to fully characterize the properties of geologicalmineral precipitates in far-from-equilibrium systems in orderto understand the reactions that may be possible in hydro-thermal chimney environments.

In some ways, the inorganic Fe/S precipitates that formedon carbon cloth in these interface experiments are similar tothe ion-exchange membranes and gas diffusion layers incommercial as well as experimental fuel cells (Kim et al.,2009; Paulo and Tavares, 2011). The successful use ofcommercially available fuel cell electrode/GDL material(carbon cloth) as a template to precipitate simulated hy-drogeological mineral catalysts is promising, as these fab-ricated electrodes can be directly used in PEMFCassemblies to catalyze redox reactions in a pressurized out-of-equilibrium geology test-bed reactor. Our preliminaryPEMFC tests showed that geological minerals applied to theGDLs as catalysts have the potential to facilitate currentflow and possibly catalyze redox reactions in putative geo-logical fuel cells. Our methods proved successful for

FIG. 8. Cyclic voltammograms of GCEs in N2-purged Hadean ocean simulant (20 mM NaHCO3 + 0.6 M NaCl, pH 5),with and without simulated Fe/S hydrothermal membrane as an electrocatalyst. Inset (A): anoxic voltammetry cell. Inset(B): Fe/S membrane precipitate dried and applied to GCE.

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creating PEMFC MEAs with geological materials (Tolucameteorite taenite and kamacite material) as electrode cata-lysts and should work for any field sample or syntheticmaterial of similar composition.

Combined, these electrochemical techniques and fuel cellreactor designs may allow for testing of specific reactionsthat are proposed to be relevant to the emergence of me-tabolism. For example, some metal sulfide minerals thatform under hydrothermal conditions are structurally similarto the inorganic active centers of certain fundamental redoxenzymes (Russell and Hall, 1997; Rothery et al., 2008;Baradaran et al., 2013; Nitschke et al., 2013), so synthetichydrothermal precipitates formed in the laboratory could beevaluated as ‘‘proto-metalloenzymes’’ in electrochemicaland fuel cell studies. Experimentally testing the effects oftrace chemical components on the electron transfer capa-bilities of a hydrothermal chimney could also be instruc-tive. To give one example of this, Nitschke and Russell(2013) proposed a denitrifying methanotrophic acetogenicpathway for the origin of metabolism, where the energy

barriers from CH4 to methanol and CO2 to CO could besolved through electron bifurcation by hydrothermallyproduced Mo precipitating in trace amounts within achimney wall (Nitschke and Russell, 2009, 2011; Helzet al., 2011). This is an interesting proposition, since, inbiology, energy barriers are also surmounted through elec-tron shuttles such as quinones, flavins, and molybdenumand tungsten pterins (Darrouzet and Daldal, 2002; Stanieket al., 2002; Herrmann et al., 2008; Li et al., 2008; Nitschkeand Russell, 2009, 2011, 2013; Kaster et al., 2011; Buckeland Thauer, 2012; Poehlein et al., 2012; Schuchmann andMuller, 2012). The end result of a fuel cell experimentsimulating a geo-electrochemical system would be to revealredox reactions in either half-cell as a function of thecomposition of electrodes and membrane. In a simulation ofa prebiotic alkaline hydrothermal vent, for example, elec-trical current could be produced by oxidation of hydro-thermal H2 to electrons and protons, thus driving thereduction of oceanic CO2 to formate and possibly further toformaldehyde. Alternatively, hydrothermal CH4 could beoxidized to methanol and formaldehyde perhaps by usingnitrate or nitrite as an electron acceptor (Ducluzeau et al.,2009; Nitschke and Russell, 2013; Russell et al., 2013).Organic redox cofactors such as quinones, flavins, NADP,FAD, GMP, and AMP that participate in electron transfer inmodern bioenergetics and were likely present in LUCA(Buckel and Thauer, 2012; Schoepp-Cothenet et al., 2013)could also be incorporated into fuel cell electrodes fororigin-of-life simulations.

Regardless of the specific parameters, approaching origin-of-life experiments within a fuel cell/electrochemical frame-work could allow experimenters to take advantage of themany rigorous analytical methods and diagnostics that areroutinely employed in fuel cell tests. Moreover, there arevarious computational models that can be used to model fuelcell systems, none of which (to the best of our knowledge)have been employed with an abiogenesis perspective inmind. There are many reactions of importance to the originof life aside from the proto-metabolic redox reactions dis-cussed above, such as RNA synthesis, formation of peptidesor other complex organics, and polymerization of phos-phates to create energy currency. Any of these could beincorporated into a fuel cell–like experiment and couldbenefit from electrochemical techniques. For example, it hasbeen recently shown that Fe2 + can cause some biologicalRNAs to become redox active (Athavale et al., 2012; Hsiaoet al., 2013). In an early Earth hydrothermal system, it ispossible that these redox-active RNAs might participate inthe electrocatalytic function of a hydrothermal chimneysystem along with the inorganic components (cf. McGlynnet al., 2012). Another example could be studying the gen-eration of condensed and/or activated phosphates, whichare necessary at the origin of life to function as energy cur-rency molecules similar to ATP (Baltscheffsky, 1996). It hasbeen proposed that green rust, a redox-active iron oxyhy-droxide mineral that efficiently absorbs phosphate (Antonyet al., 2008; Barthelemy et al., 2012), might be capable offorming pyrophosphate bonds via regulation of proton flowthrough mineral layers (Arrhenius, 2003; Russell et al.,2013); alternately, pyrophosphate could be formed via sub-strate phosphorylation within a hydrothermally precipitatedmembrane (Martin and Russell, 2007; Barge et al., 2014).

FIG. 9. Open circuit voltage (upper panel) and polari-zation (I/V) curves of the Toluca MEA fuel cell (lowerpanel). The negative sign in the I/V graph indicates that thecurrent is being taken from the functional fuel cell.


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Incorporating components such as these into electrodes in afuel cell experiment might yield new prebiotically plausibleways to synthesize necessary high-energy molecules.

7. Conclusions

There are many proven engineering design techniques andanalytical methods for building membrane fuel cells andcharacterizing the electrochemical properties of different or-ganic and inorganic catalysts. These techniques and methodscan offer much insight into the workings of bioenergetics andmetabolism if applied to biological and geological early Earthcatalysis. Considering that (1) microbes and mitochondriamay be viewed as highly evolved biological fuel cells and (2)that the earliest life on Earth was likely chemiosmotic, che-mosynthetic, and utilized energy sources such as thosecommonly found in modern alkaline hydrothermal vents, webelieve that the PFC modeling approach has potential. Weargue that the electrochemical methods that have been suc-cessful in the fields of fuel cell development and bioelec-trochemistry should be applied to simulating the origin of lifebecause of the conspicuous fuel cell–like properties of met-abolic and geo-electrochemical systems. The preliminaryexperiments presented here were successful in forming pu-tative electrocatalytic minerals in out-of-equilibrium chemi-cal systems, mimicking the formation of metal sulfideprecipitates at the interface between ocean and hydrothermalfluids on early Earth. These inorganic membranes that sim-ulate prebiotic submarine hydrothermal precipitates areshown to be structurally complex, capable of generating andmaintaining membrane potentials and pH gradients, andelectrically conductive. We also successfully formed simu-lated hydrothermal chimney precipitates on commercial fuelcell electrode/GDL material, a technique that can be used tocreate electrode components for various types of fuel cellexperiments. Finally, we developed a PEMFC for simulatinggeo-electrochemical processes and demonstrated that thePEMFC is operational when field samples of geologicalmaterial are used as the electrode catalysts. The methods usedin this work are easily generalized to other geochemical in-terfaces of interest, such as water-rock reactions on earlyMars or the icy moons. Moving toward this type of laboratorysimulation of the emergence of bioenergetics will not only beinformative in the context of the origin of life on Earth butmay help in understanding whether it is possible for life tohave emerged in similar environments on other planets.

Acknowledgments

This research was carried out at the Jet Propulsion Lab-oratory, California Institute of Technology, under a contractwith the National Aeronautics and Space Administration withsupport by the NASA Astrobiology Institute (Icy Worlds) andat the University of Leeds with support from the Centre forComputational Fluid Dynamics and Energy Leeds. We ac-knowledge useful discussions with members of the NAI-sponsored Thermodynamics Disequilibrium and EvolutionFocus Group, and we thank two anonymous referees for theirhelpful suggestions. J.M.P.H. is a Nuffield Foundation Bursar(2013), and L.M.B. is supported by the NAI through theNASA Postdoctoral Program, administered by Oak RidgeAssociated Universities through a contract with NASA.

Author Disclosure Statement

No competing interests exist.

Abbreviations

ATP, adenosine triphosphate; GCE, glassy carbon elec-trode; GDLs, gas diffusion layers; LUCA, last universalcommon ancestor; MEA, membrane electrode assembly;MFCs, microbial fuel cells; PEM, proton exchange mem-brane; PEMFC, proton exchange membrane fuel cell; PFC,prebiotic fuel cell.

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Address correspondence to:Laura M. Barge

Mail Stop 183-601Jet Propulsion Laboratory

California Institute of Technology4800 Oak Grove Dr.Pasadena, CA 91109

E-mail: [email protected]

Submitted 4 November 2013Accepted 3 February 2014

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