MEA Preparation for Direct Formate/Formic Acid Fuel Cell—Comparison of Palladium Black and Palladium Supported on Activated Ca

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Communication MEA Preparation for Direct Formate/Formic Acid Fuel Cell—Comparison of Palladium Black and Palladium Supported on Activated Carbon Performance on Power Generation in Passive Fuel Cell

Adrianna Nogalska 1,* , Andreu Bonet Navarro 1,2 and Ricard Garcia-Valls 1,2

1 Eurecat, Centre Tecnològic de Catalunya, C/Marcel lí Domingo, 43007 Tarragona, Spain; · [email protected] (A.B.N.); [email protected] (R.G.-V.) 2 Department of Chemical Engineering, Universitat Rovira I Virgili, Av. Països Catalans, 26, 43007 Tarragona, Spain * Correspondence: [email protected]; Tel.: +34-977-297-089

Received: 30 October 2020; Accepted: 17 November 2020; Published: 19 November 2020

Abstract: Membrane electrode assemblies (MEAs) with palladium catalysts were successfully prepared by using a home-made manual pressing system with Nafion glue application that contributed to a decrease of additional energy consumption. The catalyst coated membranes were prepared with supported palladium on activated carbon (PdC) and unsupported palladium black (PdB) for comparison. The performance of passive, air breathing, functioning under ambient conditions and with low concentration (1 M) formate/formic acid fuel cell was evaluated. Based on polarization curves, the best result was obtained with carbon supported catalyst and HCOOK fuel, achieving 21.01 mW/mgPd. Still, constant current discharge with PdC showed an energy generation efficiency of 14% with HCOOH over 3% with HCOOK caused by lower potassium ion conductivity and its permeability through the proton exchange membrane. The faradic efficiency of conversion in the cell is equal to the overall energy efficiency and makes the cell self-sufficient.

Keywords: formate fuel cell; formic acid fuel cell; palladium electro-catalyst; MEA; catalyst coated membrane

1. Introduction

Nowadays, the capture and conversion of CO2 has become a main social, economic and technologic priority due to the climate change emergency. For CO2 removal to become cost-effective, it is necessary to develop strategies to convert it into usable products. One of the simplest and arguably less energy demanding CO2 reduction paths involves its conversion to formic acid. This way, renewable energy, which is intermittent and unpredictable, could be stored as formic acid without any additional CO2 released into the atmosphere. Despite being easy to transport and store for long times, the use of formic acid in fuel cells has rarely been investigated as compared to hydrogen and direct alcohol fuel cells, probably due to its energy density (2.1 kWh/L) which is lower than that of methanol (5.9 kWh/L). However, formic acid has numerous advantages: (i) it is a non-flammable liquid at ambient temperature, (ii) allows for easy storage and (iii) the formic acid cross over the proton exchange membrane is six times lower compared to methanol [1], that means that it provides better mass transport improving energy density [2]. The disadvantage of the acid over its salt is its corrosive nature. This can be overcome by use of salt, such as potassium formate, with the same high theoretical cell potential (1.45 V) and fast oxidation kinetics. Thus, the potential use of formate salt in fuel cells has gained a lot of attention in the scientific community [3].

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The heart of the fuel cell, specifically the proton exchange membrane fuel cell, is the membrane electrode assembly. It is a combination of the gas diffusion layers and catalysts for redox reactions on the anode and cathode with a proton exchange membrane in a sandwich structure [4]. Selection of the catalyst is crucial and mainly depends on the fuel used in the system. Gao et al. [5] studied the possible use of a blended fuel of formate and formic acid on platinum. The authors show that the mechanism of the oxidation of mixed fuel is through direct oxidation, while HCOOH alone undergoes oxidation through an indirect mechanism that can lead to poisoning. Still, they reported achieving higher current densities when using formate compared to the acid alone. Initially, researchers were using platinum catalyst for the formate/formic acid oxidation, as it is used in direct alcohol fuel cells, yet studies show that palladium has better performance, it is cheaper (according to prices of metal catalysts in Sigma Aldrich on October 2020) and it is rarely poisoned by CO, increasing the stability of the catalytic reaction. Bartrom et al. [6] found that formate oxidizes rather efficiently and with no strong bound intermediate on palladium. Catalysts are often supported on activated carbon providing a high contact area, excellent electron conductivity, and improved mass transfer allowing lowering of noble metal usage [7]. Palladium, when supported on activated carbon, can give high reactivity with highly concentrated HCOOH [8]. The main objective of this work is to compare the performance of passive, air breathing formate/formic acid fuel cells using two catalysts, palladium black and palladium supported on activated carbon, with two different fuels, HCOOH and HCOOK, at ambient conditions (25 ◦C, 1 bar) using low fuel concentration (1 M) to demonstrate which catalyst is most efficient for future commercialization under those conditions. The performance towards formate/formic acid oxidation in a fuel cell system was evaluated by polarization curves followed by constant current discharge. Moreover, X-ray diffraction of the catalysts was performed to describe their crystalline structure, size and to understand the catalysis mechanism. The list of abbreviations can be found in Supplementary Material Table S1.

2. Materials and Methods Parts of membrane electrode assembly (MEA), such as: Toray paper 060, TGP-H-060, as gas diffusion layer (GDL); Nafion membrane 117; and cathode, cloth gas diffusion electrode (GDE) 4 mg/cm2 PtB; were purchased from Fuel Cell Store (https://www.fuelcellstore.com/, College Station, TX, USA). Both catalysts, palladium black (surface area 40–60 m2/g, 99.95% trace metals basis) and palladium on carbon (extent of labeling: 30 wt.% loading, matrix activated carbon support), and Nafion glue (Nafion 117 solution 5% in a mixture of lower aliphatic alcohols and water) were purchased from Sigma Aldrich (Madrid, Spain). Isopropanol used in catalytic ink preparation was purchased from Scharlau (Barcelona, Spain). Hydrogen peroxide 30% (v/v) (Sigma-Aldrich, Madrid, Spain) and 95–97% H2SO4 (Serviquimia, Tarragona, Spain) were used to prepare cleaning solutions for the Nafion membrane. Potassium formate (reagent plus 99%) and formic acid (for synthesis) used as fuels were purchased from Sigma Aldrich (Madrid, Spain). Figure1 shows a schematic description of the membrane electrode assembly preparation starting from the catalyst application. The anode was prepared by air brushing (3 bar) of Pd-based catalyst, PdB or PdC, on 9 cm2 of a 16 cm2 previously cleaned Nafion membrane (Figure S1) to create a catalyst coated membrane (CCM). The catalytic ink was composed of palladium catalyst wetted by Milli-Q water, i-propanol and Nafion glue. The solid part was 6% wt of the total ink while the catalyst to Nafion proportion was 10:1. Membranes 2020, 10, 355 3 of 10 Membranes 2020, 10, x FOR PEER REVIEW 3 of 10

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FigureFigure 1.1. SchematicSchematic illustrationillustration ofof membranemembrane electrodeelectrode assemblyassembly (MEA)(MEA) fabrication.fabrication.

BeforeBefore thethe deposition,deposition, thethe inkink waswas sonicatedsonicated (Ultrasons(Ultrasons SelectaSelecta 30006833000683 (Selecta,(Selecta, Barcelona,Barcelona, Figure 1. Schematic illustration of membrane electrode assembly (MEA) fabrication. Spain),Spain), 5050/60/60 kHz,kHz, 110110 W)W) forfor 3030 min.min. ForFor thethe MEAMEA preparation,preparation, (i)(i) TorayToray carboncarbon paperpaper (9(9 cmcm22 GDL),GDL), 2 2 (ii)(ii) anode anodeBefore CCM CCM (Naﬁonthe (Nafion deposition, membrane membrane the ink 16 16 was cm cm 2sonicatedcontaining containing (U sprayed ltrasonssprayed Selecta catalyst catalyst 3000683 of of 9 9 cm cm (Selecta,)2) and and (iii) Barcelona,iii) commercialcommercial 2 cathodecathodeSpain), (cathode(cathode 50/60 kHz, cloth 110 GDE W) for4 4 mg/cm mg30 min./cm2 PtB)ForPtB) the were were MEA glued glued preparation, with with iv) (iv)(i) 100 Toray 100 µL µofcarbonL Nafion of Naﬁon paper 117 (9 117solution cm solution2 GDL), 5% and 5% andpressed pressed(ii) anode for 2 for hCCM with 2 h (Nafion with 400 400psi. membrane psi.The Thepressure 16 pressure cm 2was containing wasapplied applied sprayed by a by manual catalyst a manual home-madeof 9 home-madecm2) and celliii) commercial cell(Figure (Figure S2) and S2) andit was itcathode was measured measured (cathode with cloth with the GDE the load load4 mg/cm compression compression2 PtB) were cell cellglued di distributedstributed with iv) 100 by by µL SENSING, SENSING, of Nafion 117 S.L S.L solution (AEP transducers,transducers,5% and Modena,Modena,pressed Italy).Italy). for 2 h with 400 psi. The pressure was applied by a manual home-made cell (Figure S2) and InitIn was all all measurements, measuredmeasurements, with eitherthe either load 1.0 1.0compression M formicM formic acid cell acid or di 1.0 stributedor M 1.0 potassium M by potassium SENSING, formate formate S.L were (AEP usedwere transducers, as used fuel sourceas fuel Modena, Italy). onsource the anode on the side anode with side capacity with capacity of 7.2 mL of in 7.2 the mL passive in the fuelpassive cell (Figurefuel cell2 ).(F Ambientigure 2). Ambient air was used air was as In all measurements, either 1.0 M formic acid or 1.0 M potassium formate were used as fuel oxygenused sourceas oxygen for the cathodicsource compartment.for the cathodic Fuel cell Monitorcompartment. 3.0 (https: Fuel//www.fuelcellstore.com cell Monitor 3.0) source on the anode side with capacity of 7.2 mL in the passive fuel cell (Figure 2). Ambient air was was(https://www.fuelcellstore.com) usedused foras the oxygen electrochemical source characterizationwasfor usedthe cathodicfor the of fuel electrochemicalcompartment. cells. Polarization charFuel andacterizationcell corresponding Monitor of fuel3.0 power cells. generationPolarization(https://www.fuelcellstore.com) curves and corresponding were obtained through powerwas used generation automatic for the modecurveselectrochemical starting were obtained fromcharacterization open through circuit of potentialautomatic fuel cells. (OCP), mode whereasstartingPolarization from the constant-current open and circuit corresponding potential discharge power (OCP), wasgeneration wherea measured scurves the constant-current in were manual obtained mode. through discharge The automatic working was measured currentmode inin galvanostaticmanualstarting mode. from conditions The open working circuit was potentialcurrent chosen atin (OCP), maximumgalvanostatic wherea powers the conditions constant-current point based was chosen on discharge the characteristic at maximum was measured polarizationpower in point curvesbasedmanual ofon each the mode. system. characteristic The working Additionally, currentpolarization we in galvanostatic performed curves measurements conditionsof each wassystem. chosen on 20 Additionally, mAat maximum in all systems power we performed(3point mA /mg formeasurements PdB;based 10 on mA the /onmg characteristic20 for mA PdC). in all The polarizationsystems galvanostatic (3 mA/mcurves tests gof for were each PdB; stopped system. 10 mA/mg whenAdditionally, for cell PdC). voltage we The performed reached galvanostatic 0.00 V. measurements on 20 mA in all systems (3 mA/mg for PdB; 10 mA/mg for PdC). The galvanostatic Alltests measurements were stopped were when performed cell voltag ate reached 25 ◦C and 0.00 1013 V. hPa.All measurements The cells were were cleaned performed with Milli-Q at 25 °C water and tests were stopped when cell voltage reached 0.00 V. All measurements were performed at 25 °C and and1013 1% hPa. H 2TheSO4 cellsbetween were experiments. cleaned with CurrentMilli-Q water and power and 1% density H2SO4 were between normalized experiments. by weight Current of 1013 hPa. The cells were cleaned with Milli-Q water and 1% H2SO4 between experiments. Current palladiumand power for density better were interpretation normalized of results.by weight of palladium for better interpretation of results. and power density were normalized by weight of palladium for better interpretation of results.

FigureFigure 2. Fuel 2. cellFuel exploded cell exploded view. view. 1. cathodic 1. cathodic compartment compartment (PMMA), (PMMA), air entrance; air entrance; 2. current 2. current collectors, stainlessFigurecollectors, 2. steel Fuel coveredstainless cell exploded withsteel covered silicon view. gasket;with 1. silicon cathodic 3. anodic gasket; comp compartment, 3. anodicartment compartment, (PMMA), fuel reservoir; fuel air reservoir; entrance; 4. MEA. 4. (PMMA).MEA.2. current collectors,(PMMA). stainless steel covered with silicon gasket; 3. anodic compartment, fuel reservoir; 4. MEA. The(PMMA). power production efficiency was calculated based on the theoretical faradic efficiency of The power production efficiency was calculated based on the theoretical faradic efficiency of conversion and experimental results from galvanostatic measurements by integration of the power conversion and experimental results from galvanostatic measurements by integration of the power vs. timevs.The time function. power function. production Theoretically, Theoretically, efficiency the the maximum maximum was calculated energyenergy our ourba sedcells cells on couldcould the prodtheoretical produceuce was was faradic0.5092 0.5092 Wh efficiency with Wh with of 100%conversion100% efficiency. efficiency. and Asexperimental theAs the fuel fuel cell cellresults (FC) (FC) system fromsystem galvanostatic isis passive,passive, air air measurementsbreathing breathing and and functioning by functioning integration under under ambientof the ambient power conditions,vs. timeconditions, function. the faradicthe faradicTheoretically, effi efficiencyciency ofthe of conversion conversionmaximum isis en equalequalergy to toour the the cells overall overall could energy energy prod efficiency eucefficiency was [9]. 0.5092 [9]. Wh with 100% efficiency. As the fuel cell (FC) system is passive, air breathing and functioning under ambient

conditions, the faradic efficiency of conversion is equal to the overall energy efficiency [9].

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The characterization of palladium catalysts’ crystalline structures was done by X-ray diffraction (XRD)Membranes (Siemens 2020 D5000, 10, x FOR di PEERffractometer; REVIEW Bragg-Brentano para focusing geometry and vertical4 of 10 θ-θ goniometer, Aubrey, Texas, USA). The angular 2θ diffraction range was between 5 and 70◦ Data was The characterization of palladium catalysts’ crystalline structures was done by X-ray diffraction collected with an angular step of 0.05◦ at 3 s/step and sample rotation. Cukα radiation was obtained (XRD) (Siemens D5000 diffractometer; Bragg-Brentano para focusing geometry and vertical θ-θ from a copper X-ray tube operated at 40 kV and 30 mA. Obtained spectra were analyzed with High goniometer, Aubrey, Texas, USA). The angular 2θ diffraction range was between 5 and 70° Data was Score Plus software. Based on the obtained diffractograms, particles size was calculated with TOPAS collected with an angular step of 0.05° at 3 s/step and sample rotation. Cukα radiation was obtained 6.1 software.from a copper X-ray tube operated at 40 kV and 30 mA. Obtained spectra were analyzed with High Score Plus software. Based on the obtained diffractograms, particles size was calculated with TOPAS 3. Results and Discussion 6.1 software. All the parameters of the fuel cell tests are gathered in Table1. The experiments were performed 3. Results and Discussion in passive mode and ambient conditions with no additional energy consumption. Moreover, the use of Nafion glueAll for the theparameters MEA preparation of the fuel cell and tests use are of ga home-madethered in Table manual 1. The experiments pressing system were performed allowed not usingin heat, passive contributing mode andto ambient the decrease conditions inthe with energy no additional consumption. energy consumption. Moreover, the use of Nafion glue for the MEA preparation and use of home-made manual pressing system allowed not using heat, contributing to the decreaseTable in 1. theFuel energy cell parameters. consumption.

ParameterTable 1. Fuel cell parameters. Value Fuel cellParameter nature PassiveValue FuelAnode cell nature PdC or PassivePdB, 1 mg /cm2 PEMAnode PdC or Nafion PdB, 1 117 mg/cm2 2 CathodePEM PtB,Nafion 4 mg 117/cm 2 Catalyst eCathodeffective area PtB, 94 cmmg/cm2 Fuel potassium formate or formic acid 1 M, 7.2 mL Catalyst effective area 9 cm2 Oxygen source ambient air Fuel potassium formate or formic acid 1 M, 7.2 mL Conditions 25 ◦C, 1013 hPa Oxygen source ambient air Conditions 25 °C, 1013 hPa Palladium black (PdB), was just precipitated palladium, while PdC corresponds to 30% of palladiumPalladium supported black on activated(PdB), was carbon. just precipitated Figure3 palladium,presents the while X-ray PdC di correspondsffractograms to of30% the of two commercialpalladium catalysts. supported The on analysis activated confirmed carbon. Figure the typical 3 presents face centeredthe X-ray cubicdiffractograms structure of of the crystalline two palladiumcommercial particles catalysts. [10]. The Diff ractionanalysis peaksconfirmed at 2theΘ =ty40.2,pical face 46.6 centered and 67.9, cubic which structure represent of crystalline the Bragg palladium particles [10]. Diffraction peaks at 2Θ = 40.2, 46.6 and 67.9, which represent the Bragg reflections from the (111), (200) and (220) planes, which are present in the diffraction patterns of both reflections from the (111), (200) and (220) planes, which are present in the diffraction patterns of both diffractograms.diffractograms. PdB PdB gives gives higher higher intensity, intensity, probably probably becausebecause of of its its purity. purity. Activated Activated carbon carbon was was foundfound to have to have a highly a highly amorphous amorphous state state and and low low crystallinity crystallinity [11], [11], thus thus it itwas was not not detected detected with with XRD. XRD. Furthermore,Furthermore, the size the ofsize the of catalyst’sthe catalyst’s particles particles were were calculated calculated toto bebe 7.8 ± 0.20.2 and and 7.1 7.1 ± 0.1 0.1nm nmfor PdB for PdB ± ± and PdC,and respectively,PdC, respectively, thus thus larger larger size size and and the the same same larger larger surfacesurface area area should should have have a positive a positive impact impact on theon PdB the performance.PdB performance.

. Figure 3. Diﬀractograms of catalysts. Figure 3. Diffractograms of catalysts.

Membranes 2020, 10, x FOR PEER REVIEW 5 of 10 Membranes 2020, 10, 355 5 of 10 Elnabawy et al. [12] studied the selectivity of platinum and palladium catalysts towards formic Membranes 2020, 10, x FOR PEER REVIEW 5 of 10 acid oxidationElnabawy to et CO al.2 [.12 The] studied suggested the selectivityanodic reaction of platinum mechanisms and palladium are depicted catalysts in Figure towards 4: direct formic via formate (blue), direct via carboxyl (black) and indirect via carboxyl (red) through CO*. Pd (100) is acid oxidationElnabawy to CO et2 al.. The [12] suggested studied the anodic selectivity reaction of platinum mechanisms and palladium are depicted catalysts in Figuretowards4 :formic direct via mainlyformateacid catalyzing (blue), oxidation direct theto CO reaction via2. The carboxyl suggested through (black) anodic carboxyl and reaction indirect and mechanismsmight via carboxylget slightly are depicted (red) poisoned through in Figure by CO*. CO*4: direct but Pd via (100)it does is notmainly leadformate catalyzing to permanent (blue),the direct reaction deactivation via carboxyl through due(black) carboxyl to itsand easy indi and rectremoval might via carboxyl get from slightly the (red) surface. poisoned through Pd CO*. by anodes CO* Pd (100) but generally itis does mainly catalyzing the reaction through carboxyl and might get slightly poisoned by CO* but it does offernot lead lower to permanenttendency to deactivation get poisoned due by toCO* its easythan removalPt anodes. from On thethe surface.other hand Pd, anodes Pd (111) generally can catalyze oﬀer not lead to permanent deactivation due to its easy removal from the surface. Pd anodes generally thelower reaction tendency through to get both poisoned paths, by formate CO* than and Pt anodes.carboxyl; On however, the other the hand, formate Pd (111) group can catalyzeis becomes the stabilizedoffer lowerand, as tendency result, tothe get HCOO* poisoned path by CO*has athan higher Pt anodes. contribution On the other to the hand overall, Pd (111) activity can catalyzeon Pd (111). reactionthe throughreaction boththrough paths, both formate paths, formate and carboxyl; and carboxyl; however, however, the formate the formate group group is becomes is becomes stabilized and, asstabilized result, theand, HCOO* as result,path the HCOO* has a higher path has contribution a higher contribution to the overall to the overall activity activity on Pd on (111). Pd (111).

FigureFigure 4. A 4.general A general formic formic acid acid oxidation oxidation mechanism mechanism on PdPd and and Pt Pt anodes anodes of of direct direct formic formic acid acidfuel fuel cells (DFAFCs) cells (DFAFCs) by Elnabawy by Elnabawy et al.et al. Paths: Paths: blue, blue, blue, direct directdirect via via formate;formate; formate; black, black, black, direct direct direct via via viacarboxyl; carboxyl; carboxyl; and andred, and red, red, indirectindirect via carboxyl. via carboxyl. Reproduced Reproduced Reproduced from from [12]. [12 [12].].

PerformancePerformance resultsresults results ofof the theof the prepared prepared prepared MEAs MEAs MEAs with withwith di diﬀerentfferentfferent fuels fuels fuels in in interms terms terms ofof polarizationpolarizationof polarization curves curves curves are are shown in Figure 5. Based on the polarization curves we can say that PdC performs better in OCP areshown shown in Figure in Figure5. Based 5. Based on the on polarizationthe polarization curves curves we can we saycan thatsay PdCthat Pd performsC performs better better in OCP in OCP and and gives higher current density as well. This might be ascribable to the better gas transport gives higher current density as well. This might be ascribable to the better gas transport properties of and givesproperties higher of PdC.current The densityuse of activated as well. carbon This formight the Pdbe supportascribable provi todes the a highbetter contact gas transportarea PdC. The use of activated carbon for the Pd support provides a high contact area between catalyst propertiesbetween of catalystPdC. The and use reagents. of activated Additionally, carbon PdC for particles the Pd aresupport much smallerprovides than a highPdB andcontact they area betweenand reagents.contain catalyst only Additionally, 30%and ofreagents. palladium PdC Additionally, particles on their aresurfac muchPdCes. Whenparticles smaller we thansprayedare much PdB unsupported and smaller they containthan palladium, PdB only and we 30% they of containpalladiumnoticed only on that 30% their it oftends surfaces. palladium to agglomerate When on wetheir (Figure sprayed surfac S3),es. unsupported which When may we cause palladium,sprayed an increase unsupported we in noticed the mass palladium, that transfer it tends we to noticedagglomerateresistance that it (Figure oftends gasses to S3), andagglomerate which fuels, maywhic h(Figure cause agrees an withS3), increase thewhich literature inmay the cause[13,14]. mass transferan increase resistance in the mass of gasses transfer and resistancefuels, which of agreesgasses withand fuels, the literature which agrees [13,14 with]. the literature [13,14].

Figure 5. Polarization curves of the developed formate/formic acid fuel cells.

Figure 5. PolarizationPolarization curves curves of of the developed formate/formic formate/formic acid fuel cells.

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Table 2 includes the open circuit potential values of all studied cases derived from Figure 3. The electrochemicalTable2 includes potential the openof formic circuit acid potential (or formate values salt) of oxidation all studied reaction cases is derived as follows: from Figure3. The electrochemical potential of formic acid (or formate salt) oxidation reaction is as follows: Table 2. Theoretical reaction potential and measured open circuit potentials of the studied cells. + Anode: HCOOH (aq) CO (g) + 2H (aq)+ 2e− 0.22 V (1) → 2 − Theoretical PdC OCP (V) PdB OCP (V) + Cathode: 1/2O (g) + 2H (aq)+ 2e− H O (l) 1.23 V (2) Potential (V) HCOOH2 1 M HCOOK 1→ M 2 HCOOH 1 M HCOOK 1 M Net1.45 reaction: HCOOH0.85 (aq) + 1/2O0.922 (g) CO2 (g)0.80+ H 2O (L) 1.450.85 V (3) →

Table 2. Theoretical reaction potential and measured open circuit potentials of the studied cells. Anode: HCOOH (aq) → CO2 (g) + 2H+ (aq)+ 2e− –0.22 V (1)

Theoretical PdC OCP (V) PdB OCP (V) Cathode: 1/2O2 (g) + 2H+ (aq)+ 2e− → H2O (l) 1.23 V (2) Potential (V) HCOOH 1 M HCOOK 1 M HCOOH 1 M HCOOK 1 M

Net1.45 reaction: HCOOH 0.85 (aq) + 1/2O 0.922 (g)→ CO2 (g) + 0.80 H2O (L) 1.45 0.85 V (3) Despite the theoretical OCP being 1.45 V, according to the net reaction of formate/formic acid oxidationDespite (3), the the theoretical experimental OCP results being were 1.45 lower. V, according This difference to the net between reaction the of formatetheoretical/formic potential acid oxidationvalue and (3),the themeasured experimental value is results imputable were to lower. the use This of diairff aserence oxygen between source the instead theoretical of pure potential oxygen valueas the and catalyst the measured type should value not is imputableinfluence tothe the OCP. use ofMoreover, air as oxygen HCOOK source has instead a slightly of pure higher oxygen OCP as thecompared catalyst to type the should acid for not both influence studied the catalysts. OCP. Moreover, The better HCOOK performance has a slightly of the higher salt might OCP be compared a result toof theits higher acid for dissociation. both studied Indeed, catalysts. the The disso betterciation performance constant for of the HCOOK salt might is 0.19 be amol/dm result of3, itswhile higher for 3 dissociation.HCOOH it is Indeed,only 1.46 the × 10 dissociation–4 mol/dm3constant [15]. Based for on HCOOK XRD analysis is 0.19mol we /statedm , that while both for catalysts HCOOH used it is only 1.46 10 4 mol/dm3 [15]. Based on XRD analysis we state that both catalysts used in this study in this study× are− Pd (111) where formate anions are precursors for the oxidation reaction. are PdFigure (111) 6 where includes formate the values anions of are power precursors density for as the a function oxidation of reaction.current density for all studied FC configurations.Figure6 includes Additionally, the values in ofTable power 3 all density data of as maximum a function power of current peak density is gathered for all based studied on FCthe configurations.cells’ performances Additionally, from Figure in Table 6. Once3 all data again, of maximum the use of power PdC peakresults is gatheredin better basedperformance on the cells’ and performanceshigher power from density Figure compared6. Once again,to PdB. the The use ofmaximum PdC results power in better peak performance with PdC appears and higher at powersimilar densitycurrent compareddensity for to both PdB. fuels, The maximum with the powerpeak being peak withhigher PdC for appears HCOOK. at similar Thus, currentfrom a densitykinetic forstandpoint, both fuels, its with oxidation the peak reaction being is higher clearly for more HCOOK. favorable. Thus, PdB from reveals a kinetic the standpoint, opposite tendency: its oxidation the reactionmaximum is clearlypower moredensity favorable. is almost PdB equal reveals for both the oppositestudied fuels, tendency: but the the current maximum density power on density the peak is almostis higher equal for HCOOH. for both studied fuels, but the current density on the peak is higher for HCOOH.

Figure 6.6. FormateFormate/formic/formic acid fuel cells’ performances.

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Table 3. Formate/formic acid fuel cells’ maximum power peak data.

Catalyst PdC PdB Fuel (7.2 mL, 1 M) HCOOH HCOOK HCOOH HCOOK Power max (mW/mgPd) 16.99 21.01 4.17 4.04 Current at PM (mA/mgPd) 36.87 43.01 11.15 7.43

Table4 represents the comparison of obtained results of power density normalized to catalyst area with literature findings. It summarizes the results of the formate/formic acid fuel cells working in different conditions, such as temperature, oxygen source or fuel concentration and addition of supporting electrolyte in the form of an alkaline solution. In our system, we are evaluating the performance of an FC in very mild and ambient conditions using a fuel of low molarity without an electrolyte, and we could still detect power even with an amount of catalyst as low as 1 mg/cm2. Considering that no additional energy was used for solution circulation, pure oxygen pumping or heating it is a promising start for development of self-sufficient fuel cells for CO2 recycling.

Table 4. Comparison of obtained results with literature ﬁndings.

Max. Power Anode Fuel and C Cathode Membrane Density T( C) O Source Ref. (mg/cm2) ◦ (M) 2 (mW/cm2) Commercial anion 4 M HCOOK/4 Fe–Co Nano-Pd/C, 4 258 60 Pure O [9] exchange M KOH 2 Quaternized Fe–Co PdC, 2 130 80 5 M HCOOK Pure O [15] polysulfone 2 Polymer anion PtB PdB, 4 105 60 1 M HCOOK Air (21%) [16] exchange 1 M Cation-exchange PtC PdC, 4 300 25 HCOONa/3 M H O [17] membrane 2 2 NaOH PtC PdC, 2 Naﬁon membrane 103 45 3 M HCOOH Pure O2 [18] PtC PdC, 1 Naﬁon membrane 5.6 25 1 M HCOOK Air (21%) This work

Moreover, power generation experiments were carried out to assess the performance of developed FCs in time. Performing the power generation experiments in constant current discharge, at maximum power peak, leads to unstable measurements due to fast potential losses [19]. Mikolajczuk et al., in his articles, proved the hypothesis that in high current densities the reaction never reaches the steady state and the CO2 bubble production is very fast, which leads to blocking of access of fuel to catalyst and deactivates the catalyst. In order to verify this assumption, in addition to the measurements at maximum power current, we performed experiments at 20 mA and indeed we observed higher efficiency in all studied cases (Figure7). Figure7 reports the values obtained for all studied fuel cells’ configurations in power generation studies. As expected, based on polarization curves (Figures5 and6), PdB has an overall lower e fficiency of constant current discharge compared to PdC. On the other hand, the salt had high power density and current density but the energy production efficiency in constant current discharge was much lower. The FC efficiency is limited by catalyst activity, fuel dissociation state and by the solute conductivity. Above we showed that the dissociation of the salt is higher than acid, thus these differences can be assigned to the conductivity of cations. Indeed, limiting ion conductivity at 298.15 K of K+ is 73.50 S cm2/mol, which is much lower than the one of H+ which is 349.85 S cm2/mol [20]. Thus, · · even though the salt can be dissociated more easily, the overall power generation in time will be lower. Boncina et al. [20] observed that the molar conductivity of the salt does not change with its concentration, while for acid it exponentially decreases with the increase of molarity. This means that lower energy generation of salt could be compensated for with higher concentration. Membranes 2020, 10, x FOR PEER REVIEW 8 of 10 Membranes 2020, 10, 355 8 of 10

Figure 7. Figure 7. ConstantConstant current current dischargin dischargingg efficiency efficiency at at 20 20 mA mA and and maximum maximum power power current current (MP). (MP). Moreover, Nafion is a proton exchange membrane; therefore protons would pass through it Moreover, Nafion is a proton exchange membrane; therefore protons would pass through it easily. Yet, potassium ions might contribute to the membrane clogging, ultimately poisoning it. easily. Yet, potassium ions might contribute to the membrane clogging, ultimately poisoning it. These These assumptions are supported by studies reported by Bogdanowicz et al. [21]. In their article, assumptions are supported by studies reported by Bogdanowicz et al. [21]. In their article, among among others, the authors studied the selectivity of cations’ permeability through Nafion 117 membranes others, the authors studied the selectivity of cations’ permeability through Nafion 117 membranes and reported a higher transport resistance of K+ over H+, but it did not block the entrance. and reported a higher transport resistance of K+ over H+, but it did not block the entrance. 4. Conclusions 4. Conclusions MEA preparation with a home-made manual pressing system without heating with the Nafion glueMEA application preparation contributed with a tohome-made decreases manual of additional pressing energy system consumption. without heating XRD analysiswith the showedNafion gluethat palladiumapplication black contributed and palladium to decreases on carbon of additional are (111) energy face centered consumption. cubic structures, XRD analysis which showed makes thatthem palladium appropriate black for and formate palladium/formic on acid carbon oxidation are (111) though face centered HCOO* cubic adsorption. structures, PdB which revealed makes low themperformance appropriate in polarization for formate/formic curves and acid exhibited oxidation a lowerthough energy HCOO* generation adsorption. efficiency PdB revealed compared low to performancePdC. Unsupported in polarization palladium curves agglomerated and exhibited during a lower the MEA energy preparation generation which efficiency caused compared decreases to in PdC.mass Unsupported transfer. Carbon palladium supported agglomerated Pd catalysts during demonstrate the MEA good preparation activity providingwhich caused high decreases contact area in mass transfer. Carbon supported Pd catalysts demonstrate good activity providing high contact area along with the potential for a more efficient Pd metal utilization with lower metal loadings. along with the potential for a more efficient Pd metal utilization with lower metal loadings. Simple characterization in terms of the polarization curve does give us information about the Simple characterization in terms of the polarization curve does give us information about the capability of the system towards power generation. In this case, HCOOK gave better results due to capability of the system towards power generation. In this case, HCOOK gave better results due to higher dissociation constant over HCOOH. However, in the long term constant current discharge higher dissociation constant over HCOOH. However, in the long term constant current discharge we we observed a faster FC efficiency decrease when using potassium formate compared to formic acid. observed a faster FC efficiency decrease when using potassium formate compared to formic acid. Based on the literature research it can be attributed to two possibilities, i) lower conductivity of Based on the literature research it can be attributed to two possibilities, i) lower conductivity of potassium ion and ii) membrane clogging caused by K+. Hence, we proved the potential use of formate potassium ion and ii) membrane clogging caused by K+. Hence, we proved the potential use of salt in passive, air breathing fuel cells, through HCOO* oxidation. formate salt in passive, air breathing fuel cells, through HCOO* oxidation. Supplementary Materials: The following are available online at http://www.mdpi.com/2077-0375/10/11/355/s1, SupplementaryTable S1: List of abbreviations,Materials: The Figure following S1: Nafion are available membrane online cleaning at www.mdpi.com/xxx/s1, procedure, Figure S2: Home-made Table S1: List system of abbreviations,for MEA fabrication, Figure FigureS1: Nafion S3: Catalytic membrane ink. cleaning procedure, Figure S2: Home-made system for MEA fabrication,Author Contributions: Figure S3: CatalyticConceptualization, ink. R.G.-V.; methodology, A.N.; investigation, A.N. and A.B.N.; writing—original draft preparation, A.N.; writing—review and editing, A.B.N. and R.G.-V.; visualization, AuthorA.B.N.; supervision,Contributions: R.G.-V. Conceptualization, All authors have R.G.-V.; read and methodology, agreed to the published A.N.; investigation, version of the A.N. manuscript. and A.B.N.; writing—original draft preparation, A.N.; writing—review and editing, A.B.N. and R.G.-V.; visualization, Funding: This research was funded by the Regional Agency for the Business Competitiveness of the Generalitat A.B.N.; supervision, R.G.-V. All authors have read and agreed to the published version of the manuscript. de Catalunya (EURECAT-ACCIÓ). Moreover, Vincente Lopez scholarship from EURECAT was provided for Funding:Andreu Bonet This research Navarro was pre-doctoral funded by studies. the Regional Agency for the Business Competitiveness of the Generalitat de Catalunya (EURECAT-ACCIÓ). Moreover, Vincente Lopez scholarship from EURECAT was provided for Andreu Bonet Navarro pre-doctoral studies.

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Acknowledgments: Authors would like to acknowledge Francesc Gispert i Guirado for technical support with X-ray diffraction analysis. Conflicts of Interest: The authors declare no conflict of interest.

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