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JANUARY 1, 2021 VOLUME 11 NUMBER 1 PUBS.ACS.ORG/ACSCATALYSIS
www.acs.org
pubs.acs.org/acscatalysis Research Article
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
Green hydrogen generation by low-temperature water
to reduce internal resistance. Using this configuration with a
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
between industrial sectors.1,2 In particular, green hydrogen zero-gap solid electrolyte assembly also allows for high-voltage
has a unique capability to eliminate the carbon emissions of efficiency, large current density, fast dynamic response (on the
industries that are otherwise difficult to decarbonize, such as order of milliseconds instead of seconds, like slower AELs),
ammonia synthesis, steel refining, and transportation, notably and the ability to operate at differential pressures.6,7
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−
alkaline electrolyte. However, for water-fed HEMELs to
40 wt % KOH or NaOH electrolytes have served as the
achieve high performance, an advanced hydroxide exchange
commercial technology since 1927.3,4 AELs exhibit a long
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
through the electrolyzer. Wang et al.8 reported the perform-
low capital cost.4 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 IrO2 in the anode) and an
that form within the liquid electrolyte and adsorb onto the
unstable commercial HEM and HEI. They achieved a current
electrode surface, as well as thick diaphragms, especially at high
density of 399 mA cm−2 at 1.8 V with poor durability in pure
current densities.5 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 (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). compared to a dark red FexNiyOOH layer firmly 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 Figure 1c show the typical diffraction peaks (2θ = 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 with the appearance of Fe(III)−OH/O and Ni(II)−OH
and can continuously run at 200 mA cm−2 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 FexNiyOOH-20F (Figure 1d),25 as confirmed 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 FexNiyOOH and decreases to 2.0 when the
configuration: (i) the self-supported electrode serves as both a F− concentration is increased to 30 mM in the reactants
catalyst support and a gas diffusion 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 constant (Kf) of 5.88 × 1015 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 transmission electron microscopy
first 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
■ RESULTS AND DISCUSSION
Figure 1a schematically shows the formation mechanism of a
TEM image in Figure 1g confirms the ultrathin nanosheet
structure with a thickness of 2−3 nm, and the lattice fringes
with d = 0.52 nm in high-resolution TEM image are
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
To investigate the OER activity and durability, cyclic crystalline FexNiyOOH,30,31 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).
cycles (Figures S6 and S7), while the F 1s XPS peak More specifically, the overpotential at 100 mA cmgeometric area−2
corresponding to the metal−fluorine bond disappears. For (η100) of FexNiyOOH-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
redox reaction becomes more irreversible. This is likely due to the electrochemical kinetics,26 as seen from the electro-
the formation of metal (oxy)hydroxide species with lower chemical impedance spectroscopy (EIS) in Figure S10.
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
oxygen evolution condition, consistent with literature results (66.7 mV dec−1) for FexNiyOOH-20F, in comparison with
on fluoride-incorporated NiFe hydroxide.26 The Ni(II)/ 110.0 mV dec−1 for FexNiyOOH and 82.2 mV dec−1 for an Ir/
Ni(III) oxidation peak area represents the Ni(II)/Ni(III) C catalyst, shows further evidence of improved OER kinetics
transformation degree and is proposed as an index of the with F− incorporation and leaching (Figure S12). Figure 2c
resultant NiOOH active species after the Ni(II)/Ni(III) summarizes the η100 and specific current density at 1.55 V vs.
oxidation.27−29 FexNiyOOH-20F exhibits a more obvious RHE normalized with respect to the ECSA (jECSA@1.55 V).
oxidation peak than FexNiyOOH, especially after 20 repetitive The jECSA@1.55 V values of FexNiyOOH-nF are all higher than
cycles (Figure 2a). This is because the Ni percentage increases that of FexNiyOOH, confirming that the reconstruction
from 17.4% for FexNiyOOH to 29.1% for FexNiyOOH-20F, induced by F− leaching remarkably boosts the intrinsic OER
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 (
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
TP-85 HEI is loaded at a self-supported FexNiyOOH-20F as high IEC PAP-TP-85-MQN ionomer is loaded at a self-
electrode via a dip-coating method to provide OH− transport. supported FexNiyOOH-20F anode via the similar dip-coating
Ir/C or FexNiyOOH-20F powder catalysts and PAP-TP-85 method. The current density achieved at 1.8 V is 810 mA cm−2
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
comparison. FexNiyOOH-20F and Ir/C powder catalysts are of the series resistance (Rs) and the interfacial resistance (Rint)
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 x Ni y OOH-20F and Pt/C//Ir/C-based HEMELs
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 polystyrene ionomer (TMA-70, 3.3 mequiv g−1) recently
increased to 540 mA cm−2 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
Figure 3. Single-cell performance of water-fed HEMELs. (a) I−V high Pt loading (2.0 mgPt cm−2) 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. even superior to those previously reported to operate with
Membrane: PAP-TP-85 (20 μm); cathode: Pt/C (47 wt %, 0.94 mgPt potassium carbonate aqueous electrolytes51,52 and can be
cm−2); anode: FexNiyOOH-20F powder (4.8 mg cm−2), Ir/C (20 wt
%, 4.8 mg cm−2), or self-supported FexNiyOOH-20F (4.8 mg cm−2).
ascribed to the following factors: (i) The ohmic resistance
The ionomer in the cathode is PAP-TP-85 with a loading of 30 wt %. (∼0.19 Ω cm2) is lower than 0.23 Ω cm2 for previously
The ionomer in the anode is PAP-TP-85 with a loading of 30 wt % for reported water-fed HEMELs using PGM catalysts8 and 0.30 Ω
Ir/C and FexNiyOOH-20F powder catalysts and is PAP-TP-85 or cm2 for Zirfon membrane-based AELs operated with KOH
PAP-TP-85-MQN with a loading of 0.8 mg cm−2 for the self- aqueous electrolytes.41 It is even comparable to that (0.10−
supported FexNiyOOH-20F catalyst. 0.13 Ω cm2) of PEMELs;53 (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 (
ACS Catalysis pubs.acs.org/acscatalysis Research Article
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■ ACKNOWLEDGMENTS
This work was supported by the ARPA-E program of the U.S.
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