Highly Active Nickel-Based Catalyst for Hydrogen Evolution in ... - MDPI

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Highly Active Nickel-Based Catalyst for Hydrogen Evolution in Anion Exchange Membrane Electrolysis Alaa Y. Faid 1, * , Alejandro Oyarce Barnett 2 , Frode Seland 1 1 2

*

and Svein Sunde 1, *

Department of Materials Science and Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway; [email protected] SINTEF Industry, Sustainable Energy Technology Department, New Energy Solutions Group, Trondheim, Norway; [email protected] Correspondence: [email protected] (A.Y.F.); [email protected] (S.S.)

Received: 6 November 2018; Accepted: 29 November 2018; Published: 3 December 2018

Abstract: Anion exchange membrane (AEM) electrolysis is hampered by two main issues: stability and performance. Focusing on the latter, this work demonstrates a highly active NiMo cathode for hydrogen evolution in AEM electrolysis. We demonstrate an electrolyzer performance of 1 A cm−2 at 1.9 V (total cell voltage) with a NiMo loading of 5 mg cm−2 and an iridium black anode in 1 M KOH at 50 ◦ C, that may be compared to 1.8 V for a similar cell with Pt at the cathode. The catalysts developed here will be significant in supporting the pursuit of cheap and environmentally friendly hydrogen fuel. Keywords: Nickel; HER; anion exchange membrane; electrolysis

1. Introduction Water electrolysis utilizing a solid polymer electrolyte membrane has been widely studied [1]. Compared to traditional alkaline water electrolysis that employs porous diaphragm separators with alkaline solution electrolytes, solid polymer electrolytes provide advantages such as lower gas crossover, improved efficiency, differential pressure operation, and improved operation dynamics [2]. Two types of solid polymer electrolytes are currently being pursued: proton exchange membranes (PEMs) and anion exchange membranes (AEMs) [3]. PEM water electrolysis (PEMWE) has matured considerably over the past decade, fulfilling many of the technical requirements for power-to-gas energy storage from renewables [4,5]. PEM electrolyzer technology still requires expensive catalysts based on noble metals, e.g., iridium and platinum, high cost perfluorinated polymers membranes such as Nafion [6]. AEM water electrolysis (AEMWE) has the potential to become a cheaper alternative to PEM water electrolysis systems, for example by allowing for the use of non-precious transition metal electrocatalysts [7]. Therefore, AEM water electrolysis aims to combine the low costs of alkaline electrolysis with the high power and flexibility of PEM electrolyzers [2]. However, the water splitting performance of AEM water electrolysis is currently much lower than that of PEMWE [8]. In general, the membrane electrode assembly (MEA) consists of a polymeric membrane with an anode and a cathode catalyst on each side of the membrane as shown in Figure 1. The catalyst can be coated on the membrane, thus forming a catalyst-coated membrane (CCM). Alternatively, catalyst ink can be coated on the porous substrate and compressed onto either side a polymer membrane forming catalyst-coated substrates (CCS) [9]. In AEM water electrolysis, hydrogen gas and hydroxide ions (OH− ) produced from water reduction at the cathode while AEM exchanges (OH− ) ions to the anode [10]. 2H2 O→2H2 + O2 (1) Catalysts 2018, 8, 614; doi:10.3390/catal8120614

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2H2O

2H2 + O2

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The overall reaction in Equation (1) requires catalytic activity, towards the oxygen evolution reaction (OER) at the anodeinand for the (1) hydrogen at the to form The overall reaction Equation requiresevolution catalyticreaction activity, (HER) towards the cathode, oxygen evolution the respective gases from theand electrode [11]. The overall reaction a theoretical free reaction (OER) at the anode for thesurfaces hydrogen evolution reaction (HER)requires at the cathode, to form the energy electrolysis voltage or thermodynamic cell The voltage of 1.23 V to get hydrogen and oxygen from respective gases from the electrode surfaces [11]. overall reaction requires a theoretical free energy water at 25 °C [12]. In practice, the cellcell voltage needed efficient hydrogen must beat electrolysis voltage or thermodynamic voltage of 1.23for V to get hydrogen and generation oxygen from water higher than In 1.23 V. Additional voltage is required to overcome associated 25 ◦ C [12]. practice, the cell voltage needed for efficient hydrogenover-voltages generation must be higherwith than electrode kinetics and the ohmic resistance of the electrolyte andassociated electrolyzers among 1.23 V. Additional voltage is required to overcome over-voltages withcomponents, electrode kinetics and others [2,13].resistance of the electrolyte and electrolyzers components, among others [2,13]. the ohmic

Figure Figure1.1.Catalyst Catalystelectrode electrodelayer layerand andmembrane membraneelectrode electrodeassembly assemblyfor foranion anionexchange exchangemembrane membrane (AEM) electrolyzer, is is mixed with anan ionomer, Reprinted from Artyushkova et et al.,al., (AEM) electrolyzer,where wherethe thecatalyst catalyst mixed with ionomer, Reprinted from Artyushkova License LicenseNumber Number4406040674790 4406040674790[14]. [14].

Performanceimprovement improvementthrough throughthe thedevelopment developmentofofnew newmaterials materialsand andoptimization optimizationofofthe the Performance MEA fabrication process is of high importance. AEMs with high ionic conductivity and stability, MEA fabrication process is of high importance. AEMs with high ionic conductivity and stability, as as well as catalysts improved activity durability in alkaline conditions have been studiedinin well as catalysts withwith improved activity andand durability in alkaline conditions have been studied variousreports reportsininrecent recentyears years[3,15,16]. [3,15,16].An Anetetal.al.[17] [17]developed developeda amathematical mathematicalmodel modeltotopredict predictthe the various performanceofofAEMWE. AEMWE.Their Theirresults resultsshowed showedthat thatananactivation activationpolarization polarizationofofthe thehydrogen hydrogenand and performance oxygenevolution evolutionreactions reactionsisisresponsible responsiblefor forthe theperformance performancereduction reduction(voltage (voltagetotoachieve achievespecific specific oxygen current)ininAEMWE. AEMWE.This Thispoints pointstotothe thenecessity necessityofofdeveloping developinghigh-performance high-performanceMEAs MEAsthrough through current) electrocatalystand andmembrane membraneoptimization optimization[17,18]. [17,18]. electrocatalyst Onlya few a few studies address the influence a non-precious metal catalyst cathode and hydroxide Only studies address the influence ofof a non-precious metal catalyst cathode and hydroxide ion-conductivity AEMWEdevices. devices.For For example, Scottetetal.al.[19] [19]investigated investigatedthe theperformance performanceofof ion-conductivity ininAEMWE example, Scott −2− AEMWEsusing usingdifferent differentcobalt-based cobalt-basedoxides oxides(2.5–3.0 (2.5–3.0mg mgcm cm ) as the OER catalyst and (2.0mg mg AEMWEs ) 2as the OER catalyst and NiNi(2.0 − 2 −2) as cm ) the as the HER catalyst. a 1.9 V cell voltage, achieved current densities ranging cm HER catalyst. At aAt1.9 V cell voltage, the the cell cell achieved current densities ranging fromfrom 65 2 (3.0 2 of2.3Cu −−2 2 (2.5 2 of −2 (3.0 −2 of cm −2 (2.5 65 cm mA cm−mg mA mg cm−Co Co O4 ). mA cmmg Cu− 0.7Co O4)0.7toCo 175 mA cm175 mgcm cm of Li-doped 3OLi-doped 4). Comotti et3al. 2.3 O 4 ) to Comotti et al. [20]the demonstrated thecatalyst effect of(Ni/(CeO HER catalyst (Ni/(CeO O3 )/C) loading on AEMWE [20] demonstrated effect of HER 2-La2O 3)/C) loading AEMWE performance, 2 -La2on 2 as thefrom performance, the current at 1.9 V increased 160−2 to cm−varied loading varied the current density at 1.9 Vdensity increased from 160 to 470from mA cm as470 the mA loading 0.6 to 7.4 − 2 −2 −2 from to 7.4 mginvestigated cm . Xiaohigh et al. investigated catalyst loadings for40 both mg cm 0.6 . Xiao et al. catalyst loadingshigh for both the HER (NiMo mg the cm HER ) and(NiMo OER −2) electrodes −2 at 1.9 V.[21]of 40 mg ) and OER (NiFe 40 mg cm−2 ) in electrodes resultedofin570 AEMWE (NiFe 40cm mg−2cm which resulted AEMWEwhich performance mA cmperformance 570 performance mA cm−2 at 1.9 [21]. This performance was comparable that observed using This wasV comparable to that observed using PGMtocatalysts for the HERPGM (Pt, catalysts 3.2 mg − 2 − 2 −2 −2 for),the HER (Pt, 3.22, mg ), and OER (IrOrespectively ) electrodes, respectively [21]. cm and OER (IrO 2.9 cm mg cm ) electrodes, 2 , 2.9 mg cm [21]. InInthis thispaper, paper,we weshow showthat thatour oursynthesized synthesizedNiMo NiMocatalyst catalystoffers offersa acathode cathodeperformance performance comparabletotoPtPtnanoparticle nanoparticlecatalyst catalyst AEMWE. Wealso alsoinclude includea adescription descriptionofofthe theinfluence influenceofof comparable inin AEMWE. We the theKOH KOHconcentration concentrationononthe theperformance performanceofofNiMo NiMoHER HERcatalysts catalystsinina areal realAEMWE AEMWEenvironment. environment. Resultsand andDiscussion Discussion 2.2.Results TheNiMo NiMocatalyst catalyst was was prepared solution containing the the Ni and Mo metal The prepared by byreducing reducingananaqueous aqueous solution containing Ni and Mo precursors in presence of sodium borohydride. SEM images of the amorphous catalysts are shown metal precursors in presence of sodium borohydride. SEM images of the amorphous catalysts are in Figure 2. The2.catalyst exhibited nanosheet-like structures. A similar nanosheet morphology has shown in Figure The catalyst exhibited nanosheet-like structures. A similar nanosheet morphology also been obtained for NiFe prepared by reduction of precursors with NaBH4 [22]. These nanosheets

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has also been obtained for NiFe prepared by reduction of precursors with NaBH 4 [22]. These are loosely stacked andstacked form sponge-like structures, structures, leading to leading high specific surface areas. Here we nanosheets are loosely and form sponge-like to high specific surface areas. used X-ray diffraction (XRD) to investigate crystal structure and phases present in the NiMo catalyst. Here we used X-ray diffraction (XRD) to investigate crystal structure and phases present in the NiMo The XRDThe pattern NiMofor in NiMo Figurein 2 shows broad peaks consistent with an amorphous catalyst catalyst. XRDfor pattern Figure two 2 shows two broad peaks consistent with an amorphous powder, a very small crystallite size, or both. The XRD pattern does not display any sharp peaks that catalyst powder, a very small crystallite size, or both. The XRD pattern does not display any sharp may be related to an extended periodicity of the lattice [23]. peaks that may be related to an extended periodicity of the lattice [23].

Figure 2.2. (a) (a) SEM SEM image image of of Ni Ni0.9 0.9Mo Mo0.1 0.1 nanosheets, field scanning scanning transmission transmission Figure nanosheets, (b) Inverted dark field electron microscopy microscopy(STEM) (STEM)Ni Ni0.9 0.9Mo Mo0.1 0.1 nanosheets (c) (c) Ni Ni0.9 0.9Mo0.1 prepared electron supportedin in Vulcan Vulcan XC72 prepared 0.1supported by nanosheets. by chemical chemical reduction, reduction, (d) (d)X-ray X-raydiffraction diffractionpattern patternof ofNi Ni0.9 0.9Mo0.1 0.1 nanosheets.

Raman spectroscopy was wasemployed employedtotoevaluate evaluate vibrational modes of the NiMo catalyst. Raman spectroscopy thethe vibrational modes of the NiMo catalyst. The The Raman spectra reproduced in Figure 3 contain peaks corresponding to the one-phonon (1P) and Raman spectra reproduced in Figure 3 contain peaks corresponding to the one-phonon (1P) and twotwo-photon Raman modes 570and and1090 1090cm cm−1−, 1respectively , respectively[24]. [24].Although Although Raman Raman phonon photon (2P)(2P) NiONiO Raman modes at at 570 phonon modes (1P and 2P) shown in Figure 3 are identical to those in the single-crystal, the 1P mode may also modes (1P and 2P) shown in Figure 3 are identical to those in the single-crystal, the 1P mode may be associated with bulk defects or surfaces [24]. also be associated with bulk defects or surfaces [24]. The spectroscopy (XPS) spectrum in Figure 3 of the3 NiMo displays The X-ray X-rayphotoelectron photoelectron spectroscopy (XPS) spectrum in Figure of thenanosheets NiMo nanosheets three peaks at 230.6, 402, and 410 eV, related to Mo3d, Mo3p , and Mo3p levels of molybdenum, displays three peaks at 230.6, 402, and 410 eV, related to3/2Mo3d, Mo3p1/2 3/2, and Mo3p1/2 levels of 2 states of 2) states of respectively peaks can be assigned presence of to thepresence Mo4+ (4d molybdenum . These molybdenum, respectively . These peaks canto be assigned of ) the Mo4+ (4d oxide [25]. The high[25]. resolution peaks of Mo3d eV) and Mo3d eV) are related molybdenum oxide The high resolution peaks of Mo3d 5/2 (230.1 eV) and Mo3d3/2 (233.3 eV) are 5/2 (230.1 3/2 (233.3 4+ oxidation 4+ to the Mo state (MoO ) on the surface [26]. In addition, the NiMo XPS spectra of the NiMo related to the Mo oxidation state of 2 (MoO2) on the surface [26]. In addition, the NiMo XPS spectra nanosheets contain peaks corresponding to the Ni2pto andNi2p Ni2p binding the NiMo nanosheets contain peaks corresponding 3/21/2 andlevels Ni2pwith 1/2 levels withenergies binding 3/2the 854 and 873 respectively, which suggests thesuggests presencethe of Ni in an oxidized [27]. Auger energies 854eV, and 873 eV, respectively, which presence of Ni in state an oxidized statepeaks [27]. of Ni appears in the binding energy range from 600 to 800 eV. A peak with a binding energy of eV Auger peaks of Ni appears in the binding energy range from 600 to 800 eV. A peak with a 187.0 binding corresponds to the the B1S level [28,29] The XPS data[28,29] are thus the energy of 187.0 eV of corresponds toof theelemental of the B1Sboron level of elemental boron Theconsistent XPS data with are thus presence of surface oxide states of Ni and Mo in the NiMo nanosheets. consistent with the presence of surface oxide states of Ni and Mo in the NiMo nanosheets. The electrocatalytic performance performanceofofthe theNiMo NiMo catalysts evaluated in a The electrocatalytic andand Pt/CPt/C catalysts were were evaluated in a threethree-electrode in N2 -saturated 1 Melectrolyte KOH electrolyte with a rotating disk electrode (RDE). electrode systemsystem in N2-saturated 1 M KOH with a rotating disk electrode (RDE). Figure 4 Figure 4 shows the current recorded during linear sweep voltammetry (LSV) for NiMo catalysts with a shows the current recorded during linear sweep voltammetry (LSV) for NiMo catalysts with a Mo Mo content 10%, respectively. canbebeseen seenfrom fromFigure Figure 4a, 4a, the current density content withwith 3%,3%, 5%,5%, andand 10%, respectively. AsAs can density normalized to the geometric surface area, and thus the catalytic activity is dependent on composition.


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normalized to the geometric surface area, and thus the catalytic activity is dependent on composition. Ni exhibitsthe thehighest highestelectrochemical electrochemicalactivity. activity.It It worth noting that bare glassy carbon Ni0.90.9Mo Mo0.1 0.1 exhibits is is worth noting that bare glassy carbon and and Vulcan XC/72 support have almost negligiblehydrogen hydrogenevolution evolutionactivity. activity. The Vulcan XC/72 support have almost negligible The evaluation evaluation of ofthe the ECSA of NiMo presented here is based on the double layer capacitance procedure [30,31]. For Pt/C, ECSA of NiMo presented here is based on the double layer capacitance procedure [30,31]. For Pt/C, the upd theECSA ECSAwas wascalculated calculatedfrom fromthe thearea areaunder underHH upd peak peak[32]. [32]. As Asfor forthe themass massactivity, activity,we wekept keptthe the 2 catalyst loading at the electrode at 0.25 mg/cm and the glassy carbon electrode geometric surface area 2 catalyst loading at the electrode at 0.25 mg/cm and the glassy carbon electrode geometric surface 2 . Mass activity (A/g) was calculated from the actual catalyst loading (in mg/cm2 ) ofarea S(geo) of=S0.196 (geo) = cm 0.196 cm2. Mass activity (A/g) was calculated from the actual catalyst loading (in 2 at a given overpotential [33]. and current density j (mA/cm 2) and 2) at a given overpotential [33]. mg/cm current density j )(mA/cm

Figure NiNi Mo Figure3.3.(a)(a)Raman Ramanspectrum spectrumofof Mo 0.1nanosheets, nanosheets,(b) (b)X-ray X-rayphotoelectron photoelectronspectroscopy spectroscopy(XPS) (XPS) 0.90.9 0.1 spectrum NiNi Mo nanosheets, (c) XPS peak fitting of Nickel peaks (d) XPS peak fitting of Mo peaks. spectrumofof 0.9 Mo 0.1 nanosheets, (c) XPS peak fitting of Nickel peaks (d) XPS peak fitting of Mo 0.9 0.1

peaks.

To reach the benchmark activity of −10 mA/cm2 Pt required an overpotential of −105 mV, 2 Pt while To Ni0.9 Mo0.1 required −185activity mV. The slope for NiMo is about −120 mV, which is larger reach the benchmark ofTafel −10 mA/cm required an overpotential of −105 mV, than while that −70 mV/dec. kinetics the NiMo catalyst thuswhich appears to bethan consistent Ni0.9for MoPt/C 0.1 required −185 mV. The The HER Tafel slope forof NiMo is about −120 mV, is larger that for with adsorption as the rate-determining step for thus the HER. The Tafel is therefore Pt/Celectrochemical −70 mV/dec. The HER kinetics of the NiMo catalyst appears to beslope consistent with consistent with both the Volmer − Heyrovsky and Volmer Tafel mechanisms [34]. electrochemical adsorption as the rate-determining step for the HER. The Tafel slope is therefore Electrochemical of NiMo andand Pt in half-cell measurements consistent with bothinvestigation the Volmer−Heyrovsky Volmer Tafel mechanismswere [34].done with the same mass loading and the weight ratio (Pt/NiMo = 1:1). The trend in Figure 4, shows that done Pt haswith higher Electrochemical investigation of NiMo and Pt in half-cell measurements were the HER than NiMo. Based on measurements cell, the mass loading of sameactivity mass loading and the weight ratio (Pt/NiMoin= the 1:1).electrochemical The trend in Figure 4, shows that Pt has Ni should be activity five times higher than thatonofmeasurements Pt to attain comparable activity. Based analysis, higher HER than NiMo. Based in the electrochemical cell,on thecost mass loading owing to the price difference between Ni (10 cents (¢) per gram), Mo (10 cents (¢) per gram), and (Pt of Ni should be five times higher than that of Pt to attain comparable activity. Based on cost analysis, (30 USDtoper a NiMo: Pt weight ratio of(¢) 5:1per thegram), HER catalyst cost would reduced owing thegm), pricewith difference between Ni (10 cents Mo (10 cents (¢) perbe gram), and at (Pt least 60 times [35]. Such a cost reduction cost is highly beneficial for applications in industry and (30 USD per gm), with a NiMo: Pt weight ratio of 5:1 the HER catalyst cost would be reduced at least energy systems. 60 times [35]. Such a cost reduction cost is highly beneficial for applications in industry and energy systems.

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Figure 4. Linear sweep voltammetry (LSV) of NiMo with different Mo composition and Pt/C normalized to (a) geometric surface area and (b) electrochemical surface area and (c) mass loading.

Electrochemical impedance spectroscopy (EIS) was carried out for investigation of the electrodeelectrolyte interface kinetics. Figure 5a shows the Nyquist plots of Ni0.9Mo0.1 and Pt/C at different applied potentials (−150 and −100 mV vs. RHE) in 1 M KOH. Pt and Ni0.9Mo0.1 have similar highFigure Linear (LSV) of with Mo composition Figure 4.4. Linear sweep sweep voltammetry (LSV) of NiMo NiMo with different different Mothe composition and Pt/C frequency resistances. Forvoltammetry Pt and NiMo at higher applied potential, radius ofand thePt/C semicircle normalized to (a) geometric surface area and (b) electrochemical surface area and (c) mass loading. normalized to (a) geometric surface area and (b) electrochemical surface area and (c) mass loading. decreases, signifying a lower charge transfer resistance (Rct) as expected if the current depends exponentially on potential [36,37].spectroscopy (EIS) was carried out for investigation of the Electrochemical impedance Electrochemical impedance spectroscopy (EIS) was carried out for investigation of the electrodeChronoamperometric were applying −200 electrode-electrolyte interfacemeasurements kinetics. Figure 5a performed shows the by Nyquist plotsconstant of Ni0.9 potentials Mo0.1 and of Pt/C electrolyte interface kinetics. Figure 5a shows the Nyquist plots of Ni 0.9Mo0.1 and Pt/C at different for 1200 min. From Figure(− 5b, Niand 0.9Mo0.1 in 1 M KOH displays short-term stability which is similar atmV different applied potentials 150 −100 mV vs. RHE) in 1 M KOH. Pt and Ni0.9 Mo0.1 have applied potentials (−150 and −100 mV vs. RHE) in These 1 M KOH. Pt and 0.9Mo0.1 have similar highto that of Pt/C at this applied overpotential. results thusNi indicate that the Niof 0.9Mo0.1 similar high-frequency resistances. For Pt and NiMo at higher applied potential, the radius the frequency resistances. For Pt and NiMo at higher applied potential, the radius of the semicircle electrocatalysts have good short-term stability [38]. semicircle decreases, signifying a lower charge transfer resistance (Rct ) as expected if the current decreases, signifying a lower charge transfer resistance (Rct) as expected if the current depends depends exponentially on potential [36,37]. exponentially on potential [36,37]. Chronoamperometric measurements were performed by applying constant potentials of −200 mV for 1200 min. From Figure 5b, Ni0.9Mo0.1 in 1 M KOH displays short-term stability which is similar to that of Pt/C at this applied overpotential. These results thus indicate that the Ni 0.9Mo0.1 electrocatalysts have good short-term stability [38].

Figure 5. (a) Nyquist plot of Ni0.9 Mo0.1 and Pt/C in 1 M KOH at −150 and 150 mV vs. RHE (b) Chronoamperometry of Ni Pt/C forPt/C 24 h in 11 M Figure 5. (a) Nyquist plot of 0.1 Niand 0.9Mo 0.1 and MKOH. KOH at −150 and 150 mV vs. RHE (b) 0.9 Mo Chronoamperometry of Ni0.9Mo0.1 and Pt/C for 24 h in 1 M KOH.

Chronoamperometric measurements were performed by applying constant potentials of −200 mV for 1200 min. From Figure 5b, Ni0.9 Mo0.1 in 1 M KOH displays short-term stability which is similar to that of Pt/C at this applied overpotential. These results thus indicate that the Ni0.9 Mo0.1 electrocatalysts have good short-term stability [38]. Figure 5. (a) Nyquist plot of Ni0.9Mo0.1 and Pt/C in 1 M KOH at −150 and 150 mV vs. RHE (b) Chronoamperometry of Ni0.9Mo0.1 and Pt/C for 24 h in 1 M KOH.


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Based on structural and the evaluation of catalyst activity in the electrochemical cell, (Ni0.9 Mo0.1 ) have the best HER activity and durability among the NiMo catalysts and have therefore been chosen to represent NiMo in AEM water electrolysis cell. In AEMWE cell Pt/C or NiMo supported on Vulcan x72 carbon (NiMo/X72) were used as cathode catalysts while Ir black served as the anode catalyst. Reinforced Fumatech membranes, Fumapem FAA-3-PE-30 and 10 wt. % Fumion FAA-3-solute-10 ionomer in NMP were utilized in the MEA preparation. The MEAs (shown in Figure S1 in the ESI†) were assembled in a modified Baltic cell hardware between two commercially available porous Ti transport layers for water electrolysis (Beakaert). The morphology of the catalyst layer is best described as catalyst particles covered with ionomer and electrolyte as illustrated in Figure 1. Compositional uniformity along catalyst layers was confirmed by energy dispersive X-ray (EDX) mapping as illustrated in Figure S2 in the ESI†; the elemental mapping shows a uniform distribution of Ni, Mo, O, and carbon. The ink is sprayed in the whole membrane area, Nickel percentage in the catalyst is 90% while Mo is only 10%, that is why by EDX we can see Nickel is more pronounced in Figure S2. The cross-sectional interface view of the cathode MEA demonstrated in Figure S2 (ESI†) confirmed the uniform dispersion of the catalyst along the MEA layer and its firm adherence to the membrane. SEM images (Figure S3, ESI†) show that the distribution of catalyst is also uniform across the surface of the AEM; the SEM images revealed no voids or cracks in the catalyst layer. Electrochemical impedance spectroscopy (EIS) during in-situ full cell testing was conducted to provide information about the uncompensated resistance and then to separate the ohmic resistance from other contributions to the voltage of the AEMWE cells. Figure 6 shows the impedance-plane plot at 0.4 A cm−2 for the cells with NiMo/X72 cathodes (Figure 6a) and Pt/C cathodes (Figure 6b) for both 0.1 and 1.0 M KOH. The impedance-plane plots appear to consist of two partly overlapping and depressed semicircles. (A high-frequency tail extending towards positive imaginary parts is considered to be due to the electronics and the experimental setup and is not considered further in this work). The low-frequency arcs are of a similar size for the two catalysts, whereas the high-frequency arc has a significantly larger radius for the NiMo catalyst than for Pt. The total ohmic resistance of the 25 cm2 cell was determined from the high-frequency resistance (HFR), i.e., from the intercept with the real (Re) axes of the impedance-plane plot [39]. For 1 M KOH, NiMo/X72 cell has an HFR of approximately 0.150 Ω cm2 , which is lower compared to the Pt/C based AEMWE cell (0.190 Ω cm2 ). This shows that despite having higher NiMo loadings resulting in thicker catalyst layers, the NiMo/X72 cell still shows excellent cell conductivity. However, a considerable increase in the HFR was observed when changing the KOH concentration to 0.1 M KOH; 0.310 Ω cm2 for NiMo/X72 and 0.290 Ω cm2 for Pt/C. This HFR increase at the lower KOH concentration may indicate insufficient ionic conductivity of the membrane [26]. The conductivity of the membrane is directly proportional to KOH concentration until reaches 5 M KOH. Beyond this concentration, the membrane conductivity tends to decrease as KOH concentration increases [40]. Stefania et al. [41] found similar behaviour for Pt/C. The HFR increases when the catalyst loading decreases, which might be associated with constriction resistances [42]. However, it is difficult to unequivocally relate the change in the HFR with KOH concentration for the two catalysts to constriction resistances. Newman et al. [43] showed that electrochemical reactions are nonuniformly distributed within the electrode. The distribution will be dependent on the ohmic resistance in pores as well as the charge transfer kinetics. For slow kinetics, the reaction is forced to be more uniformly distributed than for fast kinetics. The former case would correspond to the NiMo and the latter to Pt/C. The contribution to the changes in impedance with KOH concentration from the pore resistance would thus be expected to be more significant the more evenly the reaction is distributed. While this may contribute to the larger changes in the radius of the arcs in Figure 6 for the NiMo sample than for Pt/C, an interpretation of the changes of the high-frequency intercepts with concentration will require investigation going beyond the scope here.

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Electrochemical impedance is taken at 0.4 A in both 1 M and 0.1 M KOH at 50 ◦°C. C. (a) A 5 mg Figure 6. Electrochemical −2 NiMo loading compared to (b) 1 mg cm−−22 Pt. −2 Both catalyst catalyst coated cm NiMo loading compared to (b) a mg cm Pt. Cell active area: area: 25 25 cm cm−−22.. Both −22Ir-black. Ir-black.The Theimpedance impedancedata datarepresented represented by by symbols symbols and and the membranes (CCMs) using 3 mg cm cm− fitted impedance data represented by a solid line.

We emphasize layers, thethe two cells were constructed using the We emphasizethat thatapart apartfrom fromthe thecathode cathodecatalyst catalyst layers, two cells were constructed using same components, i.e., using the same type of bipolar plates, porous transport layers, anode catalyst the same components, i.e., using the same type of bipolar plates, porous transport layers, anode layers, and membranes. Therefore, Therefore, the differences the EIS of in thethe different attributed to are the catalyst layers, and membranes. the in differences EIS ofcells theare different cells cathode catalyst layer only. attributed to the cathode catalyst layer only. The low-frequency low-frequency arc arc at at around around 55 Hz Hz (Figure (Figure 6) 6) being being of of similar similar magnitude magnitude in in the the two two cases, cases, The may be attributed to mass transport [39,44] or the anode [44]. The much larger high-frequency arc, may be attributed to mass transport [39,44] or the anode [44]. The much larger high-frequency arc, on the other hand, indicates significant differences in the kinetic contributions to the cell voltage from on the other hand, indicates significant differences in the kinetic contributions to the cell voltage from the NiMo/X72 NiMo/X72and andPt/C Pt/Ccathodes. cathodes.For Foranalysis, analysis,we weconverted convertedthe therecorded recordedimpedance impedancedata data to to Tafel Tafel the impedance [33], i.e., the impedance multiplied with the steady-state current density at which it was impedance [33], i.e., the impedance multiplied with the steady-state current density at which it was obtained. For a kinetically limited process, the Tafel slope for the reaction can be found from the Tafel obtained. For a kinetically limited process, the Tafel slope for the reaction can be found from the Tafel impedance as as the the diameter diameter of of the the impedance impedance arc arc [45,46]. [45,46]. Assuming Assuming that that the the entire entire impedance impedance consists consists impedance of kinetic contributions in Figure 6, we thus estimate the Tafel slope in 1 M KOH to be 50 mV for Pt Pt of kinetic contributions in Figure 6, we thus estimate the Tafel slope in 1 M KOH to be 50 mV for − 2 and 95 95 mV mV for for NiMo NiMo (Table Evenififthe thelow-frequency low-frequency arc arc may may be be due due to to other other and (Table S1 S1 ESI†) ESI†) at at 0.4 0.4 A A cm cm−2..Even processes than that can be ascribed to the HER at the cathode, Figure 6, and (Figures S4 and S5, ESI†) processes than that can be ascribed to the HER at the cathode, Figure 6, and (Figures S4 and S5, ESI†) indicates that the Tafel slope for the NiMo cell is twice that of Pt under the same process conditions. indicates that the Tafel slope for the NiMo cell is twice that of Pt under the same process conditions. Thus, the the reaction reaction mechanism Thus, mechanism is is different different at at NiMo NiMo cell cell than than at at Pt Pt cell. cell. In Figure 6, we show the equivalent circuit that used to fit the In Figure 6, we show the equivalent circuit that used to fit theimpedance impedancedata dataeecircuit circuiton onFigure Figure6 ◦ C. Ni Mo is used to fit the impedance data taken at 0.4 A in both 1 M and 0.1 M KOH at 50 0.90.1 and 0.1 6 is used to fit the impedance data taken at 0.4 A in both 1 M and 0.1 M KOH at 50 °C. Ni0.9Mo and Pt/C cells. The fitted electrical circuit is comprised of two parallel circuits containing a resistor Pt/C cells. The fitted electrical circuit is comprised of two parallel circuits containing a resistor and a and a constant-phase element (CPE)ineach in with seriesa with a resistor. The R latter, Rel , is attributed to the constant-phase element (CPE) each series resistor. The latter, el, is attributed to the ohmic ohmic resistance of (current the cell collectors (current collectors and membrane). take the balance circuit resistance of the cell and membrane). We take theWe balance of the circuitoftothe represent to represent charge transfer, adsorption of intermediates and double layer capacitance in a porous charge transfer, adsorption of intermediates and double layer capacitance in a porous electrode layer electrode [39,41]. A detailedis, interpretation is, however, beyond [39,41]. A layer detailed interpretation however, beyond the scope here. the scope here. Figure 77 shows the potentiostatic polarisation curves curves of of both Figure shows the potentiostatic polarisation both iR-corrected iR-corrected and and uncorrected uncorrected voltages for the AEMWE at different KOH concentration, both for cells with cathode MEAs containing voltages for the AEMWE at different KOH concentration, both for cells with cathode MEAs −3 ) for three−3measurements). For the polarisation NiMo/X72 and Pt/C (standard deviation is