JANUARY 1, 2021 VOLUME 11 NUMBER 1 PUBS.ACS.ORG/ACSCATALYSIS

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Water-Fed Hydroxide Exchange Membrane Electrolyzer Enabled by a Fluoride-Incorporated Nickel−Iron Oxyhydroxide Oxygen Evolution Electrode Junwu Xiao, Alexandra M. Oliveira, Lan Wang, Yun Zhao, Teng Wang, Junhua Wang, Brian P. Setzler, and Yushan Yan*

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ABSTRACT: Here, we have developed a dissolved oxygen and galvanic corrosion method to synthesize vertically aligned fluoride-incorporated nickel−iron oxyhydroxide nanosheet arrays on a compressed Ni foam as an efficient self-supported oxygen evolution electrode. It is integrated with poly(aryl piperidinium) hydroxide exchange membrane and ionomers with high ion exchange capacity into a hydroxide exchange membrane electrolyzer fed with pure water, which achieves a performance of 1020 mA cm−2 at 1.8 V and prevents the detachment of catalysts during continuous operation (>160 h at 200 mA cm−2). This work provides a potential pathway for massively producing low-cost hydrogen using intermittent renewable energy sources. KEYWORDS: hydroxide exchange membrane electrolyzer, oxygen evolution reaction, self-supported electrode, anion doping, electrocatalysis

■ INTRODUCTION to reduce internal resistance. Using this configuration with a Green hydrogen generation by low-temperature water hydroxide-conducting polymer membrane instead of the harsh electrolysis is considered a promising large-scale and long- acidic proton-conducting membrane of PEMELs, HEMELs duration technology for storage and movement of intermittent could remove the need for expensive PGM electrocatalysts and renewable wind and solar energy across continents and precious metal-coated titanium-based stack materials. The 1,2 zero-gap solid electrolyte assembly also allows for high-voltage between industrial sectors. In particular, green hydrogen ffi has a unique capability to eliminate the carbon emissions of e ciency, large current density, fast dynamic response (on the ffi order of milliseconds instead of seconds, like slower AELs), industries that are otherwise di cult to decarbonize, such as ff 6,7 ammonia synthesis, steel refining, and transportation, notably and the ability to operate at di erential pressures. One of the greatest improvements of HEMELs over AELs is with heavy-duty vehicles. the potential to operate with a water feed instead of a corrosive Traditional alkaline electrolyzers (AELs) operated with 25− Downloaded via UNIV OF DELAWARE on December 19, 2020 at 17:28:33 (UTC). alkaline electrolyte. However, for water-fed HEMELs to 40 wt % KOH or NaOH electrolytes have served as the 3,4 achieve high performance, an advanced hydroxide exchange commercial technology since 1927. AELs exhibit a long

See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. membrane (HEM) and hydroxide exchange ionomer (HEI) lifetime of 30−40 years, and their inexpensive platinum-group- are necessary to create stable hydroxide ion transport pathways metal (PGM) free catalysts and stack components give rise to a 8 4 through the electrolyzer. Wang et al. reported the perform- low capital cost. However, they suffer from low-voltage ance of a water-fed HEMEL single cell using PGM catalysts efficiency due to high internal resistance caused by gas bubbles (Pt black in the cathode and IrO in the anode) and an that form within the liquid electrolyte and adsorb onto the 2 unstable commercial HEM and HEI. They achieved a current electrode surface, as well as thick diaphragms, especially at high −2 5 density of 399 mA cm at 1.8 V with poor durability in pure current densities. The concentrated liquid electrolyte also water. Another HEMEL study with PGM-free catalysts (Ni− results in shunt currents, which cause efficiency losses, as well Mo in the cathode and Ni−Fe in the anode) and a self-made as hardware corrosion issues. Because of slow ion transport HEM and HEI demonstrated a current density close to 300 through liquid electrolytes, AELs also experience slow transient response, making it difficult to utilize intermittent renewable energy.5 Received: September 26, 2020 Hydroxide exchange membrane electrolyzers (HEMELs) Revised: December 4, 2020 provide an alternative solution that preserves the low-cost benefits of AELs while using the improved design of proton exchange membrane electrolyzers (PEMELs), which benefits from a solid electrolyte membrane and zero-gap configuration

© XXXX American Chemical Society https://dx.doi.org/10.1021/acscatal.0c04200 264 ACS Catal. 2021, 11, 264−270 ACS Catalysis pubs.acs.org/acscatalysis Research Article mA cm−2 at 1.8 V with a short-term durability of 8 h.9 In a more recent study, Li et al.10 reported a high-performance PGM-free HEMEL with a model quaternized polyphenylene HEM and quaternary ammonium polystyrene HEI with high ion exchange capacity (IEC, 3.3 mequiv g−1). Single-cell tests yielded a current density of 906 mA cm−2 at 1.8 V but even this showed short-term (<10 h) performance drops and instability in the long term. One of the main reasons is that catalysts are easily washed out during operation, since a high IEC HEI is often accompanied by increased solubility in water. Several commercial HEMs and HEIs have been developed recently, including Orion transition metal ion (TMI), a quaternary ammonium-functionalized aromatic polymer pro- duced by Orion Polymer.11 Ecolectro developed phospho- nium-functionalized polyethylene HEIs and HEMs,12 and Ionomr Innovations Inc. synthesized Aemion, a polybenzimi- dazolium conducting polymer.13 All experienced a point at which further increase in conductivity and IEC was impeded by dissolution in water. On the other hand, the HEMEL performance is dependent Figure 1. Formation and characterizations of fluoride-incorporated − on the catalytic activities of the electrodes employed, especially nickel iron oxyhydroxides (FexNiyOOH-20F) on Ni foam. (a) for the sluggish oxygen evolution kinetics in the anode.14 Schematic illustration of the formation mechanism via spontaneous Transition metal oxyhydroxides (MOOH, where M = Fe, Co, galvanic/dissolved oxygen corrosion processes; (b) digital images, (c) and Ni) are regarded as one of the most promising oxygen XRD patterns, and (d) high-resolution F 1s XPS spectra of evolution reaction (OER) candidates among PGM-free FexNiyOOH and FexNiyOOH-20F; and (e) scanning electron − fi catalysts in alkaline electrolyte.15 17 They are also proposed microscopy (SEM), (g) high-angle annular dark- eld scanning to be the realistic active species of the oxides, dichalcogenides, transmission electron microscopy (HAADF-STEM), (g) high- magnification TEM, and (h) high-resolution TEM (HRTEM) images nitrides, and phosphides that are generated from irreversible − of Fe Ni OOH-20F. surface reconstruction during the catalytic processes.18 24 x y However, a large overpotential (>400 mV) is still required to meet the level of industrial applications (>500 mA cm−2 due to (Figure 1b) reveal a uniform dark yellow FexNiyOOH-20F poor kinetics and electronic conductivity). fi compared to a dark red FexNiyOOH layer rmly grown on Herein, we present a water-fed HEMEL with a novel self- compressed Ni foam. X-ray diffraction (XRD) patterns in supported fluoride-incorporated nickel−iron oxyhydroxide ff θ − Figure 1c show the typical di raction peaks (2 = 44.5 and (FexNiyOOH-nF, where n indicates the F concentration in 51.8°) of Ni alongside three other diffraction peaks at 2θ = the reactants) oxygen evolution electrode that is able to stably 11.9, 16.9, and 35.3°. These are the characteristic peaks of incorporate poly(aryl piperidinium) (PAP) HEM and HEIs. It − FeOOH (JCPDS 01-075-1594), and they are in accordance shows a current density of 1020 mA cm 2 at 1.8 V and 90 °C − − −2 with the appearance of Fe(III) OH/O and Ni(II) OH and can continuously run at 200 mA cm for over 160 h species in high-resolution Fe 2p and Ni 2p X-ray photoelectron without the catalyst washing out. Aside from exhibiting spectroscopy (XPS) spectra (Figure S1). The F 1s peak at extraordinary catalytic activity in an alkaline electrolyte 684.0 eV reveals the existence of a (Fe, Ni)−F bond in the (Table S1), the oxygen evolution electrode grown on 25 fi FexNiyOOH-20F (Figure 1d), as con rmed by energy- compressed Ni foam via a dissolved oxygen and galvanic dispersive X-ray spectroscopy (EDS) elemental mapping corrosion mechanism provides several benefits over other (Figure S2), but not in the FexNiyOOH. The Fe/Ni molar electrodes fabricated using the catalyst-coated substrate (CCS) ratio is 4.6 for the Fe Ni OOH and decreases to 2.0 when the fi − x y con guration: (i) the self-supported electrode serves as both a F concentration is increased to 30 mM in the reactants ff catalyst support and a gas di usion layer (GDL) to replace the (Figure S3). This is because the strong coordination expensive titanium porous transport layer (PTL) found in interaction between F− anions and Fe3+ cations with a stability PEMELs; (ii) these catalytic active species are present × 15 ° constant (Kf) of 5.88 10 at 25 C results in a decreasing throughout the pores of the Ni foam instead of on the surface free Fe3+ concentration in the reactants. alone, which increases catalyst utilization; (iii) the unique Scanning electron microscopy (SEM) images in Figures 1e galvanic and dissolved oxygen corrosion mechanism promotes and S4 show a three-dimensional spongelike network structure, stable contact between the catalyst and PTL, reducing catalyst which is composed of vertically oriented and interpenetrating loss at a high current density and for long-term operation, and nanosheet arrays, as further illustrated by high-angle annular demonstrating 160 h of stability using a high IEC HEI for the dark-field scanning transmissionelectronmicroscopy fi rst time. With the stable architecture and high activity of this (HAADF-STEM) shown in Figure 1f. Moreover, the nano- oxygen evolution electrode, we were able to assemble a single- sheet thickness and sizes gradually decrease with increasing F− cell HEMEL that achieved high performance with excellent concentrations (Figure S4), which may be due to the lattice long-term durability. strain caused by the F− incorporation. The high-magnification TEM image in Figure 1g confirms the ultrathin nanosheet ■ RESULTS AND DISCUSSION structure with a thickness of 2−3 nm, and the lattice fringes Figure 1a schematically shows the formation mechanism of a with d =0.52nminhigh-resolutionTEMimageare self-supported FexNiyOOH-nF electrode. Optical images corresponding to the lattice distance of (200) planes of

265 https://dx.doi.org/10.1021/acscatal.0c04200 ACS Catal. 2021, 11, 264−270 ACS Catalysis pubs.acs.org/acscatalysis Research Article

FeOOH (Figure 1h). Moreover, this facile method can be and more metal (oxy)hydroxide species with low degree of − explored for preparing multimetallic oxyhydroxides, such as crystallinity induced by the F leaching are formed to greatly (Fe, Ni, Co)OOH (Figure S5). promote the exposure of OER active sites compared to highly 30,31 To investigate the OER activity and durability, cyclic crystalline FexNiyOOH, resulting in showing higher voltammetry (CV) cycling was initially performed in a O2- electrocatalytic activity. saturated 1.0 M KOH solution. Note that the vertically aligned The OER activity is further compared via polarization curves nanosheet structure and nickel, iron, and oxygen components with iR compensation. FexNiyOOH-20F shows the highest are conserved for FexNiyOOH-20F after 20 continuous CV OER activity among all FexNiyOOH-nF catalysts (Figure S9). fi −2 cycles (Figures S6 and S7), while the F 1s XPS peak More speci cally, the overpotential at 100 mA cmgeometric area η corresponding to the metal−fluorine bond disappears. For ( 100)ofFexNiyOOH-20F is 43 mV lower than that of FexNiyOOH-20F, the Ni(II)/Ni(III) oxidation peak moves FexNiyOOH and is even 90 mV lower than that of a PGM Ir/C more positive while the Ni(III)/Ni(II) reduction peak catalyst (Figure 2b). The extraordinary OER activity is mainly − becomes more negative during CV cycling, in contrast to no ascribed to two factors. First, the F leaching induces the obvious change for FexNiyOOH (Figure S8), suggests that the formation of a catalytic active layer at the surface to improve 26 redox reaction becomes more irreversible. This is likely due to the electrochemical kinetics, as seen from the electro- chemical impedance spectroscopy (EIS) in Figure S10. the formation of metal (oxy)hydroxide species with lower − crystallinity at the surface via the F− leaching-induced surface Second, the self-reconstruction caused by F leaching increases reconstruction process and the influence of the average the number of exposed active sites and the electrochemically oxidation valence state of Ni cations under the electrocatalytic active surface area (ECSA) (Figure S11). A smaller Tafel slope −1 oxygen evolution condition, consistent with literature results (66.7 mV dec ) for FexNiyOOH-20F, in comparison with 26 −1 −1 on fluoride-incorporated NiFe hydroxide. The Ni(II)/ 110.0 mV dec for FexNiyOOH and 82.2 mV dec for an Ir/ C catalyst, shows further evidence of improved OER kinetics Ni(III) oxidation peak area represents the Ni(II)/Ni(III) − transformation degree and is proposed as an index of the with F incorporation and leaching (Figure S12). Figure 2c η fi resultant NiOOH active species after the Ni(II)/Ni(III) summarizes the 100 and speci c current density at 1.55 V vs. 27−29 RHE normalized with respect to the ECSA (j @1.55 V). oxidation. FexNiyOOH-20F exhibits a more obvious ECSA oxidation peak than Fe Ni OOH, especially after 20 repetitive The [email protected] V values of FexNiyOOH-nF are all higher than x y fi cycles (Figure 2a). This is because the Ni percentage increases that of FexNiyOOH, con rming that the reconstruction induced by F− leaching remarkably boosts the intrinsic OER from 17.4% for FexNiyOOH to 29.1% for FexNiyOOH-20F, activity. Moreover, Fe is proposed to influence the average oxidation valence state of Ni cations under the catalytic conditions or alter the Fe/Ni−O bond length in the NiFe − catalyst,27 29,32,33 resulting in promoting the OER perform- ance, while the precise functions are still under debate. Inactive FeOOH species probably existed at high Fe contents (>25%), thus deteriorating the activity.29 However, even though the ∼ resultant FexNiyOOH-20F contains 70.9% of Fe, it shows overpotentials of 280 and 348 mV at geometric surface area current densities of 100 and 500 mA cm−2, respectively, which meets the requirement of industrial applications (<400 mV at 500 mA cm−2), and is comparable to the most optimized Ni− Fe catalysts grown on uncompressed metal foams by more − complex methods (Table S1).15,17,26,34 36 To demonstrate the perspectives of a fluoride-incorporated nickel−iron oxyhydroxide electrode in HEMELs, a single cell is assembled using a Pt/C catalyst (TKK) and FexNiyOOH-20F, as well as PAP-TP-85 HEM and HEIs we have reported previously with an IEC of 2.4 mequiv g−1.37,38 The Pt/C catalyst and PAP-TP-85 HEI are sprayed on the HEM to form −2 a porous cathode with a Pt loading of 0.94 mgPt cm and HEI loading of 30 wt % (Figure S13), where catalyst particles form an electron-conducting network, and the HEIs adsorbed at the catalyst surface form a OH− conducting network. Figure S14 shows the polarization curves of HEMELs working with KOH aqueous electrolyte at 80 °C. Note that the performance is improved with increasing the KOH concentration from 10 to 1000 mM, since externally supplying OH− ions improve the ionic conductivity of the HEM and HEI, decreases the ohmic resistance from 0.32 Ω cm2 for 10 mM KOH to 0.06 Ω cm2 for Figure 2. Electrochemical characterizations of FexNiyOOH and −1 1000 mM KOH, and speeds up the reaction rate toward the FexNiyOOH-20F electrodes. (a) CV curves at 10 mV s , (b) −2 −1 η OER. The performance is 1500 mA cm at 1.74 V as 1000 polarization curves at 5 mV s , and (c) 100 versus [email protected] V. Measurements were performed in an O2-saturated 1.0 M KOH mM KOH is fed into the anode and is almost 3 times higher solution at room temperature. (I: FexNiyOOH, II: FexNiyOOH-10F, than that of Zirfon membrane-based AELs under similar III: FexNiyOOH-20F, IV: FexNiyOOH-30F, and V: Ir/C (20 wt %)). experimental conditions (Figure S15), illustrating the high

266 https://dx.doi.org/10.1021/acscatal.0c04200 ACS Catal. 2021, 11, 264−270 ACS Catalysis pubs.acs.org/acscatalysis Research Article ionic conductivity of the PAP-TP-85 HEM. Moreover, the However, the high IEC ionomer cannot strongly hold the HEMEL performance is superior to most previously reported catalysts during continuous operation, leading to poor solid-state alkaline water electrolyzers using a 1.0 M KOH durability, especially at high current density.10 PAP-TP-85- − electrolyte (Figure S16)39 44 and approaches that of water-fed MQN ionomer provided by W7energy exhibits an IEC of 3.2 − − − PEMELs (Table S2). mequiv g 1 and OH conductivity of 150 mS cm 2 at room However, it is preferable to operate HEMELs with water temperature in hydroxide form, much higher than PAP-TP-85 instead of alkaline electrolytes to avoid electrolyte-induced and previously reported HEIs (Figure S18 and Table S3). corrosion. In the configuration of water-fed HEMELs, a PAP- Figure 3b shows the polarization curves of water-fed HEMELs as high IEC PAP-TP-85-MQN ionomer is loaded at a self- TP-85 HEI is loaded at a self-supported FexNiyOOH-20F − electrode via a dip-coating method to provide OH transport. supported FexNiyOOH-20F anode via the similar dip-coating −2 Ir/C or Fe Ni OOH-20F powder catalysts and PAP-TP-85 method. The current density achieved at 1.8 V is 810 mA cm x y ° ∼ HEI sprayed on compressed Ni foam with a catalyst loading of at 80 C, 1.5 times as much as that using PAP-TP-85 4.8 mg cm−2 and HEI loading of 30 wt % are given for ionomer at the anode (Figure 3b). This is due to the decrease of the series resistance (Rs) and the interfacial resistance (Rint) comparison. FexNiyOOH-20F and Ir/C powder catalysts are easily washed out from the anode outlet by water flow during between the catalyst layer and the membrane compared to that the measurement process due to the weak cohesive force of using PAP-TP-85 ionomer at the anode (Figure S19 and Table PAP-TP-85 ionomer (Figure S17). Hence, powder form Pt/ S4). Moreover, the amount of the self-supported FexNiyOOH- 20F catalyst washed out by water flow during the operation C//Fe Ni OOH-20F and Pt/C//Ir/C-based HEMELs x y process is negligible due to the unique in situ growth process showed poor electrochemical performance with current − (Figure S20), even when PAP-TP-85-MQN ionomer used in densities of 130 and 240 mA cm 2 at 1.8 V, respectively this study has a comparable IEC to a quaternary ammonium (Figure 3a). By comparison, the current density significantly −1 −2 polystyrene ionomer (TMA-70, 3.3 mequiv g ) recently increased to 540 mA cm with a self-supported FexNiyOOH- reported by Li et al.10 The current density at 1.8 V further 20F electrode, and the electrode is very stable during the increases to 1020 mA cm−2 as the cell is operated at 90 °C continuous operation process. − (Figure 3b), since the OER kinetics are improved with As is well known, the local OH concentration around the increasing cell temperature (Figure S21 and Table S4). catalysts, which is strongly dependent on the IEC of the HEIs, Water-fed HEMELs using the FexNiyOOH-20F/PAP-TP- is a critical factor to determine the HEMEL performance. 85-MQN anode show excellent performance in comparison to − most state-of-the-art of water-fed HEMELs (Figure 4),8,9,44 50

Figure 4. Comparison of cell performances (j1.8) of water-fed HEMELs composed of Pt/C and self-supported FexNiyOOH-20F − (1) in this study and the literatures (2: Pt black//IrO2;3:Ni Mo// − Ni Fe; 4: Ni//Li0.21Co2.79O4; 5: Pt black//Pb2Ru2O6.5; 6: Ni// Ce0.2MnFe1.6O4; 7: Acta 4030//Acta 3030; 8: Pt/C//CoS2-TiO2;9: Ni9Mo1C//Ni2Fe1; and 10: PtRu/C//Ni2Fe1).

with the exception of only the PtRu/C//Ni2Fe1 HEMEL developed recently by Li et al., which uses a unique in situ NaOH pretreatment prior to testing and PtRu/C catalyst with −2 Figure 3. Single-cell performance of water-fed HEMELs. (a) I−V high Pt loading (2.0 mgPt cm ) in the cathode. Moreover, the curves of a water-fed HEMEL using FexNiyOOH-20F (i: powder cell performance reported by Li et al. severely deteriorated catalyst; ii: self-supported catalyst) and Ir/C anode catalysts and PAP- with prolonged operation time and experienced failure within 8 TP-85 ionomer in the anode at 80 °C. (b) I−V curves of a cell with a h due to the catalyst loss issue.10 Moreover, the outstanding self-supported FexNiyOOH-20F catalyst and PAP-TP-85 or PAP-TP- performance of the HEMEL we have presented in this work is 85-MQN ionomer in the anode at cell temperatures of 80 and 90 °C. μ even superior to those previously reported to operate with Membrane: PAP-TP-85 (20 m); cathode: Pt/C (47 wt %, 0.94 mgPt 51,52 −2 −2 potassium carbonate aqueous electrolytes and can be cm ); anode: FexNiyOOH-20F powder (4.8 mg cm ), Ir/C (20 wt − − ascribed to the following factors: (i) The ohmic resistance %, 4.8 mg cm 2), or self-supported Fe Ni OOH-20F (4.8 mg cm 2). x y (∼0.19 Ω cm2) is lower than 0.23 Ω cm2 for previously The ionomer in the cathode is PAP-TP-85 with a loading of 30 wt %. 8 Ω The ionomer in the anode is PAP-TP-85 with a loading of 30 wt % for reported water-fed HEMELs using PGM catalysts and 0.30 2 Ir/C and Fe Ni OOH-20F powder catalysts and is PAP-TP-85 or cm for Zirfon membrane-based AELs operated with KOH x y 41 PAP-TP-85-MQN with a loading of 0.8 mg cm−2 for the self- aqueous electrolytes. It is even comparable to that (0.10− Ω 2 53 supported FexNiyOOH-20F catalyst. 0.13 cm ) of PEMELs; (ii) the superior OER activity and

267 https://dx.doi.org/10.1021/acscatal.0c04200 ACS Catal. 2021, 11, 264−270 ACS Catalysis pubs.acs.org/acscatalysis Research Article fast electron transport behavior of this self-supported oxygen supported FexNiyOOH-20F nanosheet arrays directly grown evolution electrode compared to the Ir/C and other nickel− on compressed Ni foam GDL as an efficient and robust iron electrode (Table S1); and (iii) high IEC and OH− electrode have excellent structural and chemical stabilities and conductivity of PAP HEM and HEIs (Table S3). show good catalytic activity and durability in the HEMEL Durability is another critical concern for commercial configuration, even when using a high IEC ionomer. Further applications. Most water-fed HEMELs reported previously improvements of water-fed HEMELs need to depend on showed short lifetimes (<100 h), and the performance rapidly strengthening the attachment of high IEC ionomers at the deteriorates during durability tests mainly due to the catalyst catalyst surface, constructing efficient PGM-free catalysts loss and irreversible chemical degradation of the HEIs and toward the hydrogen evolution reaction (HER), and HEM, especially for an HEI in intimate contact with the decreasing the resistance of the catalyst layer. catalysts.10,49,54,55 The short-term durability of a water-fed HEMEL was first investigated at different current densities. ■ CONCLUSIONS Note that the cell potential experiences almost no decay after fl − four continuous hours of operation at current densities of In summary, we report uoride-incorporated nickel iron 100−500 mA cm−2 and 80 °C(Figure S22). Figure 5 shows oxyhydroxide nanosheet arrays grown on compressed Ni foam as a self-supported PGM-free anode. The electrode not only exhibits high OER activity but also can be continuously operated without the obvious catalyst loss as it is constructed with a high IEC PAP HEM and HEIs into an anode feed water HEMEL, resulting in excellent performance (∼1020 mA cm−2 at 1.8 V) and good long-term durability. This work is believed to be an effective water electrolysis technology for narrowing the gap between lab and commercial-scale production of low- cost hydrogen using intermittent renewable energy sources. To achieve commercial targets, continued research efforts will focus on developing more stable HEIs with a low swelling Figure 5. Long-term stability performance of water-fed HEMELs at ratio, designing more efficient PGM-free hydrogen evolution 200 and 500 mA cm−2 and 80 °C. Membrane: PAP-TP-85 (20 μm); fi −2 reaction (HER) catalysts, and optimizing the con guration and cathode: Pt/C (47 wt %, 0.94 mgPt cm ); and anode: self-supported −2 operation conditions of HEMELs to further decrease the FexNiyOOH-20F (4.8 mg cm ). The ionomer is PAP-TP-85 in the overall cost and improve performance. cathode with a loading of 30 wt % and is PAP-TP-85-MQN in the anode with a loading of 0.8 mg cm−2. ■ ASSOCIATED CONTENT long-term durability performance at 80 °C. The cell potential *sı Supporting Information decreases from 1.71 to 1.63 V in the initial 3 h of operation at The Supporting Information is available free of charge at −2 − 200 mA cm due to the catalyst activation and full HCO3 / https://pubs.acs.org/doi/10.1021/acscatal.0c04200. OH− exchange of HEM and HEIs and slowly increases with the rate of 0.56 mV h−1 in the following 160 h of operation. Experimental procedures, electrochemical impedance Even if the current density was increased to 500 mA cm−2, the spectroscopy analysis, XPS analysis, and comparison of cell potential is still lower than 1.9 V after a continuous 70 h the performances of OER catalysts, HEM, HEI, as well operation at 80 °C, and the degradation rate is 1.81 mV h−1. as water-fed HEMELs (PDF) Compared with previously reported water-fed HEMELs (Table S5), long-term durability performance is significantly AUTHOR INFORMATION improved. ■ Figure S23 shows SEM images of the self-supported Corresponding Author Fe Ni OOH-20F/PAP-TP-85-MQN anode before and after − x y − Yushan Yan Department of Chemical and Biomolecular 160 h of continuous operation at 200 mA cm 2. PAP-TP-85- Engineering, University of Delaware, Newark, Delaware MQN ionomer at the catalyst surface is mostly lost, which is 19716, United States; orcid.org/0000-0001-6616-4575; supposed to be the key reason why the cell potential slowly Email: [email protected] increases with prolonged measurement time. However, the vertically oriented nanosheet array structure and crystal phase Authors of the oxygen evolution electrode (Figures S23 and S24), as Junwu Xiao − Department of Chemical and Biomolecular well as the chemical configurations (Figure S25a−c), are well Engineering, University of Delaware, Newark, Delaware preserved. High-resolution F 1s and N 1s spectra in Figure 19716, United States; Key Laboratory of Material Chemistry S25d,e reveal that the peaks centered at 688.0 and 402.2 eV, for Energy Conversion and Storage, Ministry of Education, which correspond to the C−F bond and ammonium group of Hubei Key Laboratory of Material Chemistry and Service PAP-TP-85-MQN ionomer, respectively,56,57 appear in the Failure, Department of Chemistry and Chemical Engineering, anode before and after the stability test, although intensities are Huazhong University of Science & Technology, Wuhan weakened.ThissuggeststhatafewPAP-TP-85-MQN 430074, P. R. China; orcid.org/0000-0002-2472-0569 ionomers are still attached at the catalyst surface to facilitate Alexandra M. Oliveira − Department of Chemical and the OH− transport during continuous operation, thus showing Biomolecular Engineering, University of Delaware, Newark, good long-term durability. Meanwhile, F− anions in the Delaware 19716, United States − FexNiyOOH-20F catalyst are leached during the catalytic Lan Wang W7energy LLC, Wilmington, Delaware 19803, process,26,58 just like that in alkaline electrolytes. Hence, self- United States; orcid.org/0000-0001-5616-4600

268 https://dx.doi.org/10.1021/acscatal.0c04200 ACS Catal. 2021, 11, 264−270 ACS Catalysis pubs.acs.org/acscatalysis Research Article

Yun Zhao − Department of Chemical and Biomolecular Membranes in Electrochemical Energy Conversion Technology. Acc. Engineering, University of Delaware, Newark, Delaware Chem. Res. 2019, 52, 2745−2755. (12) Kostalik, H. A.; Clark, T. J.; Robertson, N. J.; Mutolo, P. F.; 19716, United States ̃ Teng Wang − Department of Chemical and Biomolecular Longo, J. M.; Abruna, H. D.; Coates, G. W. Solvent Processable Engineering, University of Delaware, Newark, Delaware Tetraalkylammonium-Functionalized Polyethylene for Use as an Alkaline Anion Exchange Membrane. Macromolecules 2010, 43, 19716, United States; orcid.org/0000-0003-4927-5999 − − 7147 7150. Junhua Wang Department of Chemical and Biomolecular (13) Thomas, O. D.; Soo, K. J. W. Y.; Peckham, T. J.; Kulkarni, M. Engineering, University of Delaware, Newark, Delaware P.; Holdcroft, S. A Stable Hydroxide-Conducting Polymer. J. Am. 19716, United States Chem. Soc. 2012, 134, 10753−10756. Brian P. 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