Earth-Abundant Oxygen Electrocatalysts for Alkaline Anion Exchange

Subscriber access provided by Kaohsiung Medical University

Article

Earth-Abundant Oxygen Electrocatalysts for Alkaline Anion Exchange Membrane Water Electrolysis: Effects of Catalyst Conductivity and Comparison with Performance in Three-Electrode Cells Dongyu Xu, Michaela Stevens, Monty Cosby, Sebastian Z. Oener, Adam Smith, Lisa J. Enman, Katherine E. Ayers, Christopher B. Capuano, Julie Renner, Nemanja Danilovic, Yaogang Li, Hongzhi Wang, Qinghong Zhang, and Shannon W. Boettcher ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04001 • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Earth-Abundant Oxygen Electrocatalysts for Alkaline Anion Exchange Membrane Water Electrolysis: Effects of Catalyst Conductivity and Comparison with Performance in Three-Electrode Cells

Dongyu Xuab, Michaela Burke Stevensa, Monty R. Cosbya, Sebastian Z. Oenera, Adam M. Smitha, Lisa J. Enmana, Katherine E. Ayersc, Christopher B. Capuanoc, Julie N. Rennerd, Nemanja Danilovice, Yaogang Lib, Hongzhi Wangb, Qinghong Zhangb, Shannon W. Boettchera* a Department

of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403,

United States b State

Key Laboratory for Modiﬁcation of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of China c

Proton OnSite, Wallingford, Connecticut 06492, USA

d Department

of Chemical and Biological Engineering, Case Western University, Cleveland Ohio 44106, United States e Lawrence

Berkeley National Laboratory, Berkeley, California, 94720, United States

Abstract: Anion exchange membrane (AEM) electrolysis is a promising technology to produce hydrogen through the splitting of pure water. In contrast to proton exchange membrane (PEM) technology, which requires precious metal oxide anodes, AEM systems allow for the use of earthabundant anode catalysts. Here we report a study of first-row transition-metal (oxy)hydroxide/oxide catalyst powders for application in AEM devices and compare physical properties and performance to benchmark IrOx catalysts as well as typical catalysts for alkaline electrolyzers. We show that the catalysts’ oxygen evolution activity measured in alkaline electrolyte using a typical three-electrode cell is a poor indicator of performance in the AEM system. The best OER catalysts in alkaline electrolyte, NiFeOxHy oxyhydroxides, are the worst in AEM electrolysis devices where a solid alkaline electrolyte is used along with a pure water feed.

*

Corresponding author. Tel: +1-541-346-2543. E-mail address: [email protected]. 1

ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

NiCoOx-based catalysts show the best performance in AEM electrolysis. The performance can be further improved by adding Fe species to the particle surface. We attribute the differences in performance in part to differences in the electrical conductivity of the catalyst phases, which are also measured and reported. Keywords: Oxygen evolution reaction, water electrolysis, alkaline exchange membrane, electrocatalysts, electrolyzer, electrical conductivity 1.

Introduction Hydrogen gas can be produced through water electrolysis to store electrical energy in the form

of a chemical fuel.1-4 Several types of water electrolysis systems are being employed or considered for large-scale hydrogen production, in particular alkaline liquid-electrolyte5-7 and polymerexchange-membrane electrolysis systems.8 The latter category can be further separated into proton-exchange-membrane (PEM)9-10 and anion-exchange-membrane (AEM) water electrolysis systems. PEM electrolyzers operate in pure water and use proton exchange membranes, such as Nafion, that support high current densities (~2 A cm-2) and separate the gas products.11 However, because of the acidic environment at the Nafion interface, OER electrocatalysts are limited to noble metal oxides, most prominently IrO2.12 In contrast, AEM electrolyzers, which offer an alkaline environment at the membrane interface, could offer the advantages of PEM systems while allowing the use of earth-abundant metal oxides as electrocatalysts.13-14 While the largest impediments to commercialization of AEM systems are membrane stability and ionic conductivity, an improved understanding of how to integrate catalysts into AEM systems is also needed to make the technology commercially competitive. Research on AEM systems has focused on electrocatalysts,15-24 membranes,20,

25-30

and

understanding operational mechanisms31-33 with the broad goal of achieving a high efficiency, lowcost, and stable AEM device assembly. Wu and coworkers demonstrated an early example of an AEM pure-water electrolysis system using a CuCo3-xO4 anode and Pt/C cathode.21 The relatively 2


Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

low performance (~200 mA at 2 V) was ascribed to absence of ionomer in the catalyst layers. Leng et al. used the AS-4 ionomer and A-201 AEM (Tokuyama) to improve the membrane/ionomer resistance and demonstrated pure water electrolysis at 399 mA cm-2 at a voltage of 1.8 V. IrO2 and Pt black served as OER and hydrogen evolution reaction (HER) catalysts, respectively.32 Xiao and coworkers integrated non-precious electrocatalysts with a self-crosslinked quaternary ammonium polysulfone (xQAPS) membrane, where electrodeposited Ni-Fe and Ni-Mo alloys were used as OER and HER electrocatalysts, respectively.16 Parrondo et al. developed a series of pyrochlorestructure OER catalysts showing overpotentials 0.1 ~ 0.2 V lower than the IrOx reference catalyst in an AEM electrolyzer (and achieved 500 mA cm-2 at 1.8 V).15 Although the above and other results17-18 are promising, a direct comparison between different systems is challenging. Many parameters can affect the performance of AEM electrolyzers (Figure 1), including the membrane type and conductivity,25, 34-35 membrane pretreatment method,26 gas diffusion electrode (GDE) preparation,17, 32 ionomers used,29, 31-32 catalyst ink formula,32 assembly technique31-32 and the water/electrolyte reactant feed method used.32 Here we are specifically concerned with how the OER catalyst used affects the performance and stability of AEM electrolysis systems. Because of the complexities described above, however, it is difficult to understand why one OER catalyst may work better than another. This hampers the design of catalysts for high-performance AEM electrolysis systems. Here we aim to understand how to design OER catalysts for use in AEM systems. We study, for a number catalyst compositions, the correlation between the dry catalyst electrical conductivity, the OER activity as measured in a three-electrode cell, and the performance as measured when the catalyst is incorporated in an AEM electrolyzer test station. A repeatable AEM electrolyzer baseline was established by using a commercial solid-polymer electrolyte (Fumatech FAA-3) with

3


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

IrOx and Pt/C as OER and HER catalysts, respectively. A series of first-row transition metal oxides/hydroxides were synthesized by a surfactant-free hydrothermal method. The AEM electrolyzer performance and stability was measured for each OER catalyst composition. We find that the AEM electrolyzer performance is strongly influenced by the catalyst’s dry electrical conductivity - for similar OER activities, as measured in a three-electrode cell, the largely varying electrical conductivity appears the determining factor of AEM electrolyzer performance.

Figure 1. (a) Schematic cross section of an AEM water electrolysis system. (b) AEM electrolyzer components (based on a modified fuel cell assembly from Fuel Cell Technologies).

2.

Experimental

2.1 Synthesis of transition metal oxides catalysts particles Metal oxide catalysts (except for IrOx obtained from Proton OnSite) were synthesized by a

4


Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

modified hydrothermal method.36 Typically 2 mmol metal acetate hydrate was dissolved in a mixture of 13.8 mL ethanol (98 %) and 1.2 mL ultrapure water; 2.5 mL of 25 % ammonia was then dropped into the solution under vigorous stirring (Table S1). The precursor solution was stirred for 15 min. The resulting suspension was transferred into a 45 mL Parr bomb, sealed, and then heated at 150 °C for 3 h. The as-synthesized nanoparticle metal oxides were centrifuged and washed with ethanol three times, then dried at 80 °C overnight. 2.2 Electrochemical measurement in a three-electrode system The catalyst powders were dispersed in isopropanol at a concentration of 1.0 mg mL-1. The suspension was sonicated by a horn sonicator (500 W) for 5 min with 50 % power. Then 1 mL of the suspension was sprayed onto Au/Ti-coated 5 MHz quartz-crystal microbalance (QCM) crystals (Stanford Research Systems QCM200). The spray-coating process was performed on a hotplate heated to 120 °C until the loading mass reached about 10 μg cm-2. The film masses were calculated according to the Sauerbrey equation: Δf = −Cf × Δm

(1)

Where Δf is the experimental frequency change, Cf is the sensitivity factor, 58.3 Hz cm2 μg−1, of the 5 MHz AT-cut quartz crystal in air, and Δm is the change in mass per area. The precondition to use the Sauerbrey equation to analyze the loading mass is that the film has a uniform thickness and is rigidly coupled to the QCM. The QCM conductance showed minimal changes (< 10 %) before and after the catalyst powder application indicating rigid coupling between the catalyst particles and the substrate.37 The SEM images on Au/Ti (Figure S3) demonstrate that the sprayed catalyst particles cover the substrate uniformly. Electrochemical measurements were performed using a BioLogic SP200/SP300 potentiostat in a home-built three-electrode electrochemical system in 1.0 M KOH (Sigma-Aldrich,

5


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

Semiconductor grade) electrolyte. High-purity oxygen was bubbled through the electrolyte 20 min before and during the electrochemical test. A Pt wire was used as the counter electrode (although the use of Pt as a counter electrode causes contamination issues when non-precious-metal hydrogen evolution or oxygen reduction catalysts are tested, this is not an issue when testing OER catalysts as Pt is a very poor OER catalyst and generally does not dissolve under cathodic counterelectrode conditions). An Hg/HgO reference electrode was used and its potential (vs. RHE) was calibrated against a reversible hydrogen electrode. All measured potentials (Emeasured) were converted to overpotential (η) according to: η = Emeasured - Erev - iRu

(2)

where Ru is the uncompensated series resistance and Erev the reversible oxygen potential (0.3 V vs. Hg|HgO). Ru was determined by equating it to the minimum impedance between 1 kHz and 1 MHz, where the phase angle was closest to zero. Typically, for the catalyst on QCM electrodes in 1.0 M KOH, Ru was ~5 Ω. Electrochemical impedance spectroscopy (EIS) was performed with an amplitude of 5 mV, at frequencies between 0.1 Hz and 1 MHz, and at a potential of 0.60 V vs Hg/HgO. Cyclic voltammetry, for the integration of the nickel redox wave, was performed with a scan rate of 10 mV/s. The average total-metal turnover frequency (TOF), i.e. the number of O2 formed per s per metal ion, was calculated based on the number total metal atoms and the steady-state current at 350 mV iR-compensated overpotential according to the Equation 3: TOF =

Current (A) 96485

( ) C mol

at η = 350 mV × 4

×

n

(3) n =

mass

× (a + b + MW (NiaCobFecOxHy)

c)

The total number of moles of metal cations n is calculated by dividing the mass measured by

6


Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

QCM by the estimated molecular weight. The chemical formula of each catalyst is determined by diffraction and x-ray fluorescence measurements. The estimated chemical formula used for each sample is IrO2 for IrOx, Co3O4 for Co3O4, (Co0.78Fe0.22)3O4 for CoFeOx, Ni0.88Fe0.12(OH)2 for NiFeOxHy,

(Ni0.21Co0.79)3O4

for

NiCoOx,

(Ni0.22Co0.77Fe0.01)3O4

for

NiCoOx:Fe,

(Ni0.17Co0.78Fe0.05)3O4 for NiCoFeOx. See SI for the detailed composition measurement.

2.3 Dry conductivity measurement The electrical conductivity of the catalyst powders was measured in a homemade twoelectrode conductivity cell. The catalyst powder was sandwiched between two copper cylinders inserted in a polypropylene block. The copper cylinders were then connected to the potentiostat by applying silver paint between a wire and the copper. To determine the powder electrical resistance (R), a current-voltage curve was measured while the assembly was held under constant pressure (500 psi) using a laboratory press. The thickness of the powder sample was measured by subtracting the height of the empty assembly from the height of the assembly with the sample. The electrical conductivity was calculated according to: 𝑙

σ = 𝐴𝑅

(4)

Where the σ is the electrical conductivity, l is the thickness of the compressed powder, A is the geometric area of copper electrodes and R is the measured electrical resistance. We note that the conductivity of powders is typically measured after sintering to achieve >95% of their theoretical density. We did not sinter the pellets so that the measured conductivity is more representative of the materials conductivity in the MEA. 2.4 Materials characterization The film morphology was measured via scanning electron microscopy (SEM) at 5 keV (Zeiss

7


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

Ultra 55). X-ray diffraction (XRD) analysis was carried out on a Rigaku D/max-2550 using Cu Kα radiation. The compositional information was measured using a Rigaku ZSX-II X-ray fluorescence (XRF) tool. To calibrate the compositions, metal oxides prepared from annealed metal nitrates with known stoichiometric ratio were characterized by this method (see details in SI). 2.5 Membrane preparation Prior to device assembly, dry non-reinforced FAA-3 membranes (130 μm thickness) with a bromide counterion (FumaTech) were hydrated and fully ion-exchanged to achieve sufficient OHion conductivity. To do this, the membranes were submerged in ultrapure water for three days (the water was changed each day). Carbon dioxide (CO2) from air has been found to degrade membrane performance in fuel cells and electrolyzers.38-39 Here CO2 contamination from air was minimized via N2 purging of the ultrapure water at least 15 min before submerging the membranes. The membranes were then soaked in 1.0 M KOH (Thermo Fisher, ACS grade) for 24 h and subsequently washed thoroughly with ultrapure water. 2.6 Alkaline anion exchange membrane electrolyzer The electrode assembly (gas diffusion layer/HER catalyst + membrane + OER catalyst/gas diffusion layer, with an active area of 4 cm2), and electrolyzer are shown in Figure 1b. The AEM electrolysis test station is shown in Figure S1. The electrode assembly performance was assessed in a home-made test station consisting of an alkaline electrolyzer (modified Single Cell Fuel Cell Hardware Assembly, Fuel Cell Technologies), coupled to a temperature-controlled reservoir of water (~15 L) that was pumped through the system during use. We replaced the original graphite anode plate with a corrosion-resistant stainless-steel plate (machined with a serpentine flow field with channels 1.5 mm wide spaced 1.7 mm apart) that was soaked in 1 M nitric acid prior to initial use.15

8


Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

The catalyst inks were prepared as follows. First, 3 mL of 5 wt % (FAA-3, FumaTech) ionomer solution was prepared by stirring dry ionomer and ethanol for 2 h. Second, 0.090 g catalyst and 2.15 g of a solvent mixture (water and 2-propanol with a mass ratio of 1:3.43) were added to a scintillation vial and stirred for 25 min to prepare the catalyst suspension. Finally, 0.95 g of 5 wt % AEM ionomer solution was added into 2.24 g of the catalyst suspension, followed by stirring for 120 min. Platinum black (Sigma-Aldrich, fuel cell grade, 25 ~ 34 m2 g-1) was used as the HER catalyst, while IrOx (Proton OnSite) or as-synthesized catalyst powders were used as OER catalyst. Gas diffusion electrodes (GDEs) were prepared by directly spraying the catalyst ink on the gas diffusion layers (GDLs). A Pt-coated (1 μg cm-2) sintered titanium frit (400 μm thickness, Baoji Yingao Metal Materials Inc.) was used for the anode. Carbon paper (Toray 090 without microporous layer, 20 wt % PTFE) was used for the cathode. The spray-coating process was performed on a hotplate heated to 80 °C until the loading mass reached 3.0 mg cm-2. After the ink spraying process, the 5 wt % ionomer solution was sprayed on top of the catalyst layer until its dry mass reached 10 % of the total dry catalyst/ionomer-blend sprayed layer mass. The anode and cathode were soaked in 1 M KOH for 1 h, then washed with nanopure water. Then the electrodes and membrane were assembled into a membrane-electrode assembly (MEA), which was integrated with other parts into the electrolyzer as shown in Figure 1b. A recirculating water tank was filled with nanopure water and heated to 50 °C. The 50 °C water was then flowed through the anode side of the AEM electrolysis system at a flow rate of 500 mL min-1. Before the polarization curves were obtained, the AEM electrolysis system was held at 100 mA cm-2 for 10 min to reach steady state. For the polarization curves, the power supply was scanned from 50 to 1000 mA cm-2 in 100 mA cm-2 steps. The voltmeter output (measured directly at the current collector plates to avoid the iR drop in the cables) was recorded and the average

9


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

value over 3 min was calculated. The measurement was stopped either by reaching a current density of 1000 mA cm-2 or a maximum voltage of 3.0 V. After the polarization curve was collected, the AEM electrolyzer was held at a constant current density of 200 mA cm-2 for 3 h. A second polarization curve was then collected to assess the stability of the system (again stepping from 50 to 1000 mA cm-2). Electrochemical impedance spectroscopy (EIS) measurements on the complete electrolyzer device were collected with an amplitude of 5 mV and at frequencies between 0.1 Hz and 1 MHz. The data was plotted on a Nyquist plot and the high frequency resistance was taken to be the intercept with the real impedance axis at high frequency.

3.

Results and Discussion To understand and optimize AEM electrolysis performance, the influence of the individual

components must be disentangled from the overall system response. In particular, the catalyst conductivity and catalytic activity in the absence of the AEM electrode assembly are important. Here we have characterized these parameters, then used the results to understand the system performance as a function of catalyst composition.

3.1

Characterization of as-synthesized catalyst powders Figure S2 shows the X-ray diffraction patterns of as-synthesized catalyst powders. Except for

NiFeOxHy which shows both nickel/iron hydroxides and magnetite, the samples appear to be single oxide phases. The samples containing mixtures of Ni, Co and Fe all crystallize in the spinel oxide structure. SEM images of powders sprayed on an Au/Ti substrate with a loading ~10 μg cm-2 are shown

10


Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

in Figure S3. The cobalt-containing particles had diameters from 10 ~ 100 nm and were distributed uniformly. The NiFeOxHy exhibits a larger flake morphology in addition to smaller particles. The phase, particle size, grain size and composition of each catalyst powder are summarized in Table S2.

Figure 2. Cyclic voltammetry (a) and steady-state Tafel measurements (b) of catalyst powders collected in 1.0 M KOH on a quartz crystal microbalance electrode at a loading of ~10 μg cm-2. The data shown in panel (b) is the average of three different electrodes.

11


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.2

Page 12 of 28

OER activity in liquid alkaline electrolyte The OER activity of the catalyst powders was studied in 1.0 M KOH with a dry loading of ~10

μg cm-2 (Figure 2). Among these samples NiFeOxHy is the most-active OER catalyst in the alkaline electrolyte, substantially better than IrOx, as is known.40 The remaining spinel catalysts perform worse than IrOx. NiCoOx:Fe represents NiCoOx particles that were treated with an Fe-containing aqueous solution as described in the experimental section. NiCoFeOx represents the ternary oxide catalysts synthesized through a one-step hydrothermal reaction. The compositions of these catalysts were measured by X-ray fluorescence as shown in Table S2. Performance metrics are listed in Table 1.

Table 1. Comparison of Metal-Oxide OER Activity in 1.0 M KOH Mass activity J @ η = 0.35 V @ η = 0.35 V (mA cm-2) (A g-1)

Materials

dry mass loading (μg cm-2)

η @ 10 mA cm-2 (mV)

TOF (s-1) @ η = 0.35 V

Tafel slope (mV dec-1)

IrOx

9.9 ± 1.4

393 ± 7

2.65 ± 0.63

257 ± 35

0.149 ± 0.021

46.7 ± 0.5

Co3O4

9.9 ± 0.9

434 ± 2

0.30 ± 0.03

30 ± 4.8

0.019 ± 0.003

48.7 ± 1.4

CoFeOx

11.0 ± 1.6

404 ± 1

0.65 ± 0.12

61 ± 16

0.038 ± 0.010

40.4 ± 0.2

NiFeOxHy

11.1 ± 0.7

348 ± 2

6.98 ± 0.31

633 ± 63

0.368 ± 0.037

41.5 ± 0.7

NiCoOx

10.6 ± 0.6

494 ± 4

0.09 ± 0.01

8.8 ± 1.0

0.006 ± 0.001

53.8 ± 1.6

NiCoOx:Fe

11.6 ± 2.2

457 ± 8

0.19 ± 0.03

17.2 ± 5.1

0.010 ± 0.002

55.0 ± 1.4

NiCoFeOx

11.2 ± 0.5

461 ± 7

0.20 ± 0.05

18 ± 3.3

0.010 ± 0.002

52.7 ± 1.1

Figure 2a shows voltammetry measurements for the different catalyst samples which are consistent with the respective steady-state Tafel plots (Figure 2b). NiCoOx shows the highest overpotential (494 ± 4 mV at 10 mA cm-2). The overpotentials of the NiCoOx-based materials decrease by ~40 mV after adding Fe on either the surface after synthesis or in the bulk during 12


Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

synthesis. It is well known that Fe enhances OER activity in first-row transition-metal (oxy)hydroxide/oxide catalysts in alkaline media.41-44

3.3

Catalyst electrical conductivity Electrical conductivity is a critical property for electrocatalysts, even though it is rarely

reported. For instance, in previous reports, FeOxHy has been identified as a very poor catalyst for OER,45 but it is also an electrical insulator. For very thin FeOxHy films on highly conductive substrates, FeOxHy is the best first-row “single” transition metal catalyst for OER in liquid alkaline electrolyte.5,

46

In an AEM electrolysis system the catalyst’s conductivity is likewise critical.

Because a conductive carbon binder is not stable under anodic conditions, especially at high current densities,47 the catalyst material must serve not only as the active site for OER but also conduct electrical current to those active sites. The electrical conductivity of dry as-synthesized catalyst powders (measured as a compressed, unsintered powder to more-closely match the environment in the MEA) is summarized in Figure 3. The NiFeOxHy is the most insulating of those tested with a conductivity of 6.3 × 10-9 S cm-1. The cobalt-based catalysts also show limited electrical conductivity – Co3O4 and CoFeOx have conductivities of 6.6 × 10-6 S cm-1 and 1.4 × 10-7 S cm-1, respectively, which are lower than the annealed Co3O4 from metal nitrate (5.5 × 10-3 S cm-1). This difference is presumably due to differences in crystallinity.48 The NiCo-based spinel oxide catalysts, however, show electrical conductivity of 1 ~ 10 S cm-1 – similar to the conductivity of the IrOx powder which is known to be a conductive metallic oxide.49 High conductivities for the mixed Ni-Co spinel oxides have been previously observed.50 The introduction of Fe lowers the electrical conductivity of NiCoOx-based catalysts, likely due to the addition of mid-gap defect states.51

13


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

Figure 3. Electrical conductivity of as-synthesized catalyst powders (striped). The solid-colored columns represent the conductivity of powders synthesized from metal nitrate salts heated at 400 °C for comparison. The higher electrical conductivity of the particles obtained by heating bulk nitrates is presumably due to their larger grain size (44 nm for Co3O4 and 28 nm for NiO, see Table S2). Each measurement was repeated at least once with similar results, except for CoFeOx which was only measured once due to the low yield of the synthesis.

3.4

AEM electrolysis measurements

3.4.1

Establishment of baseline electrolyzer performance

Because of the complexity of AEM electrolysis systems, we first established a repeatable baseline. Here we used a commercial solid polymer FAA-3 (FumaTech) as the membrane and ionomer and IrOx and Pt/C as the catalyst components. The preparation method (membrane, pretreatment, catalyst loading mass) and measurement parameters (temperature, electrolyte, operating pressure, current density, feeding mode) used in the baseline measurements were maintained with all subsequent measurements on other catalysts systems. Details including materials and preparation method can be found in the Experimental section and the supporting

14


Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

information, including discussion of hydrophobicity effects (Figure S5), the use of different types of membranes (Figure S6), the effect of catalyst mass loading (Figure S7) and the ink solvent (Figure S8). The AEM baseline system required an applied voltage of 2.29 ± 0.02 V to pass 500 mA cm-2 at 50 °C. While this performance is inferior to the best-reported AEM electrolysis devices reported previously, the device here is fabricated from a readily available commercial membrane material and provides a stable baseline from which to carefully assess the effect of the catalyst electrical conductivity and composition on the performance. 3.4.2

AEM electrolyzer performance with different catalysts

The polarization curves of the AEM electrolyzer with different OER catalysts are shown in Figure 4. The numerical values corresponding to the polarization curves are in Table S3. The electrolyzer was operated at 50 °C with nanopure water fed to the anode. The catalyst loading mass both for anode and cathode was kept at 3.0 mg cm-2 mixed with 15 wt % ionomer. To confirm reproducibility, each catalyst composition was tested in an AEM electrolyzer configuration at least twice (Figure S12). The AEM electrolyzer assembled from NiFeOxHy exhibits the worst performance, despite this catalyst having the best performance in alkaline media. This is explained by the fact that Ni-based (oxy)hydroxides are porous electrolyte-permeable materials that are only electrical conductors when oxidized.5 Because the AEM cell is operated in pure water, oxidation of the catalyst (and thus water oxidation) can only occur at the phase boundary with the ionomer. The bulk of the catalyst thus remains unoxidized, an electrical insulator (Figure 3), and unable to drive the OER. This leads to the high potentials > 2.5 V needed to drive moderate currents (200 mA cm-2) through the system. NiFeOxHy catalysts may also dissolve faster in the neutral water feed. For Co0.8Fe0.2Ox and Co3O4, the performance for AEM electrolysis is comparable to IrOx for

15


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

current densities < ~200 mA cm-2, but worse when operating at higher current densities. Figure 3 shows that Co0.8Fe0.2Ox and Co3O4 have low electrical conductivity compared to IrOx. Thus the poor performance of these Co-containing catalysts at higher current density can be attributed to series electrical resistance through the catalyst/ionomer layer. Figure 4b shows the strong correlation between catalyst electrical conductivity and the electrolyzer performance for all the catalysts studied. We note that no conductive binder is used for the OER catalysts in this system because of the instability of carbon under the conditions employed here and in relevant electrolysis devices. This correlation between electrical conductivity and performance is also evident in previously reported AEM water electrolyzers using pure water feed. Many systems to date have used IrO2 as the anode catalyst31-32, which has high electrical conductivity. Likewise the lead ruthenate pyrochlore catalysts used in high performance AEM electrolyzers also have metallic conductivity15. The non-precious metal CuCo3-xO4 catalyst used previously17, 21 also has relatively high electrical conductivity (~20 S cm-1 as a sintered thin film52), consistent with the reasonable performance even in the absence of ionomer. Finally the good performance of electrodeposited NiFe catalysts is also likely related to the fact it was deposited under strongly cathodic conditions onto Ni metal particles leading to metallic NiFe deposits with only a very thin active Ni-Fe oxyhydroxide catalytic surface.16 The NiCoOx, NiCoOx:Fe, and NiCoFeOx catalysts show AEM electrolysis performance that is superior to the baseline system that uses IrOx as the catalyst. The Ni-Co oxide catalysts are also better than Co0.8Fe0.2Ox and Co3O4 which is likely due to the much higher electrical conductivity. It is interesting to consider why the Ni-Co materials perform better than IrOx in the AEM environment. In the three-electrode liquid-electrolyte measurement, IrOx showed higher catalytic performance. IrOx also showed a higher electrical conductivity in the dry state than the Ni-Co

16


Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

oxides. It may be that the Ni-Co oxides interface better within the ionomer therefore improving the triple-phase boundary region. We also note that the NiCoOx catalyst shows better stability with time than the IrOx catalyst. To understand this better we collected two-electrode voltammetry data on each electrolyzer catalyst system before testing and after the polarization/stability tests at high current density. Interestingly IrOx does perform better than NiCoOx initially (Figure S16), but its performance degrades faster. The electrolyzer performance degradation occurs at the same as time as a loss in IrOx pseudocapacitance (Figure S16a), likely related to a loss in connectivity between the ionomer and IrOx catalyst. Stability is discussed in more detail below.

17


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

Figure 4. (a) Initial polarization curves of the AEM electrolyzer with different OER catalysts. Note that only IrOx and NiCoFeOx have error bars associated with the three different membraneelectrode assemblies tested. Two separate membrane-electrode assemblies were tested for the other compositions with similar results. (b) The current passed at a fixed applied voltage in the electrolyzer is correlated with the electrical conductivity (extracted from Figure 4a).

Fe was integrated into the NiCoOx catalyst using two methods. In the first Fe acetate was added during the hydrothermal reaction to incorporate it as part of, presumably, bulk NiCoFeOx. In the second method NiCoOx:Fe was obtained by dispersing 0.2 g of as-synthesized NiCoOx nanoparticles in 50 mL of 100 ppm FeCl2 aqueous solution for 30 min to adsorb Fe on the surface. Fe is known to be an essential component of first-row transition-metal (oxy)hydroxide OER electrocatalysts in alkaline liquid electrolyte and it is hypothesized to act as the active site.5-6, 41, 53 Here adding Fe increases the AEM electrolysis performance, whether in the bulk or at the surface. Because the addition of Fe only moderately decreased the electrical conductivity (Figure 3), but

18


Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

significantly increased the OER activity measured in the three-electrode cell (Figure 2), we attribute the better performance of the AEM system using such Fe-containing catalytic particles to an increased intrinsic OER activity.

3.4.3

Stability

The short-term stability of the AEM electrolyzers with different catalysts was evaluated via continuous operation at 200 mA cm-2 for 3 h while feeding nanopure water at 50 °C. Previous studies have reported limited stability due to a decay in membrane ionic conductivity associated with carbon dioxide contamination (which was characterized by measuring the high-frequency resistance, HFR, through electrochemical impedance).38 Here we minimize CO2 contamination by using nitrogen gas to protect the whole device and to degas the feed water (for > 1 h). As a result, the HFR measurements show a negligible conductivity decay (see also below and Figure S15). Figure 5 shows a comparison of the stability of the AEM electrolyzers prepared with different OER catalysts. None of the systems showed long-term stability. All catalysts, including IrOx, show a similar increase in voltage of 150 ~ 250 mV over the 3 h test period at 200 mA cm-2, with the exception of NiCoOx:Fe which shows a more pronounced change of ~ 450 mV. This suggests that the AEM systems studied here are affected by a common degradation mechanism irrespective of the exact catalyst composition while NiCoOx:Fe is affected by an additional mechanism. This common degradation mechanism is most-likely related to the ionomer which binds the catalyst particles together (as opposed to degradation of the membrane or catalyst), as is supported by the observation that the system performance and stability depends sensitively on the ionomer distribution (Figure S11) and weight percentage in the catalyst ink (Figure S10). This is discussed in more detail below.

19


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

Figure 5. Durability of different OER catalysts during operation in the AEM electrolyzer at a current density of 200 mA cm-2 and nanopure water feed at 50 °C. The anode catalysts (labelled) have a constant loading mass (3.0 mg cm-2). The cathode catalyst is platinum black (the loading is also 3.0 mg cm-2). The cell voltage efficiency is calculated by dividing the applied voltage by the combustion enthalpy of hydrogen (1.48 V).

To further investigate the degradation mechanism, we first studied the possible influence of changing membrane properties over time. We used high frequency impedance measurements to determine the series resistance before and after the stability test for each catalyst group (Figure S15). While the results show slight changes in resistance (typically decreasing after the electrolysis test), those changes in resistance cannot explain the observed performance degradation shown in Figure 5 over the short 3 h time scale. Another often-observed degradation mechanism is related to metal ion poisoning of the membranes, ionomer and even of the active sites of the Pt cathode catalyst. For example, from stability tests on PEM fuel cells it is known that Nafion is susceptible to Fe and that the active

20


Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

surface sites of the Pt black cathode catalyst can be poisoned by metal impurities.54 In contrast to PEMs, however, AEMs generally suffer from OH- nucleophiles, not from the same cation contamination mechanism as PEMs.38 As mentioned above, we do not observe significant degradation of the membrane conductivity that might be associated with metal ion poisoning (Figure S15). Furthermore, the degradation mechanisms seem catalyst unspecific, which is unlikely in the case for ion-specific membrane or cathode catalyst poisoning mechanisms. Therefore, we rule out those mechanisms as the main reasons for the degradation. We then performed two-electrode voltammetry of the complete AEM assembly to further interrogate the loss in activity (Figure S16). Significant pseudocapacitive features are evident in the voltammograms at voltages prior to the onset of electrolysis current. This pseudocapacitance is presumably related to redox chemistry occurring on the OER catalyst material, in analogy to the pseudocapacitive features shown in Figure 2a observed in alkaline solution. After the 3 h stability test at 200 mA cm-2 these pseudocapacitance features are significantly reduced. This observation suggests a loss of connectivity of the anode catalyst material with either the ionomer (i.e. the triple phase regions get smaller) or the back electrode contact (i.e. the catalyst is no longer electrically connected to the metallic gas diffusion electrode / current collector), or an overall dissolution of the catalyst. Given that AEMs show a substantially lower structural stability compared to Nafion, largely due to the detrimental impact of hydroxide on the polymer network, ionomer degradation/dissolution may also be occurring in the catalyst region.55 The performance degradation of the NiCoOx:Fe system is more severe than for the other catalysts which suggests an additional degradation mechanism. Here we prepared NiCoOx:Fe catalyst by absorbing Fe ions from solution without post-annealing treatment. These surfaceabsorbed Fe species may be dissolved under OER conditions into the neutral water electrolyte.

21


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

This proposed mechanism is supported by the fact that both NiCoOx:Fe and NiCoOx AEM catalyst systems show similar performance after 3 h of testing.

4.

Conclusions An AEM electrolysis baseline was established using a commercial solid-state alkaline anion

exchange electrolyte (FAA-3) as the membrane and ionomer along with IrOx and Pt/C catalysts. The system performed water electrolysis at 500 mA cm-2 with a potential of 2.29 ± 0.02 V. Firstrow transition metal (oxy)hydroxides/oxides catalyst powders with different OER activity (measured in alkaline electrolyte) and electrical conductivity (measured in the dry state) were synthesized through a simple surfactant-free hydrothermal method and their performance in an AEM electrolyzer was studied. NiFeOxHy showed the worst performance in the AEM electrode assembly despite showing the best OER performance in liquid alkaline electrolyte. This result is explained by the very poor electrical conductivity of NiFeOxHy when in the unoxidized hydroxide phase and by the fact that there is no liquid electrolyte to permeate the porous oxyhydroxide and allow internal sites to participate in the OER. In contrast, dense oxide particles that are electrical conductors in the dry state and also contain putative Fe active sites, such as NiCoFeOx, performed the best in AEM electrolysis systems. These catalysts substantially outperform IrOx. All systems studied have poor stability. Performance degradation is not due to membrane resistivity increasing, but instead appears likely due to a loss of interconnection between the catalyst and ionomer. The addition of Fe-species only at the surface of NiCoOx initially enhances activity, but the activity rapidly decays presumably from Fe leaching into the neutral water flow. In sum, these findings help disentangle the catalyst properties from the overall AEM electrolysis performance and underline the importance of electrical conductivity for earth-abundant OER

22


Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

catalysts in an AEM electrode assembly environment.

Notes The authors declare no competing financial interest. Supporting Information Available: Additional data regarding nanoparticle synthesis, nanoparticle properties, and electrolyzer performance. Acknowledgement This work was supported by the National Science Foundation under grant CHE-1566348. D.X. thanks the Special Excellent PhD International Visiting Program and the Fundamental Research Funds for the Central Universities (16D310603) for additional support. The work made use of shared facilities in the Center for Advanced Materials Characterization in Oregon.

References (1) Marini, S.; Salvi, P.; Nelli, P.; Pesenti, R.; Villa, M.; Berrettoni, M.; Zangari, G.; Kiros, Y., Advanced Alkaline Water Electrolysis. Electrochim. Acta 2012, 82, 384-391. (2) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F., Combining Theory and Experiment in Electrocatalysis: Insights Into Materials Design. Science 2017, 355, eaad4998. (3) Hunter, B. M.; Gray, H. B.; Muller, A. M., Earth-abundant Heterogeneous Water Oxidation Catalysts. Chem. Rev. 2016, 116, 14120-14136. (4) Armaroli, N.; Balzani, V., Solar Electricity and Solar Fuels: Status and Perspectives in the Context of the Energy Transition. Chem.-Eur. J. 2016, 22, 32-57. (5) Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S.; Boettcher, S. W., Oxygen Evolution Reaction Electrocatalysis on Transition Metal Oxides and (Oxy)hydroxides: Activity Trends and Design Principles. Chem. Mater. 2015, 27, 7549-7558. (6) Stevens, M. B.; Enman, L. J.; Batchellor, A. S.; Cosby, M. R.; Vise, A. E.; Trang, C. D. M.; Boettcher, S. W., Measurement Techniques for the Study of Thin Film Heterogeneous Water Oxidation Electrocatalysts. Chem. Mater. 2017, 29, 120-140. (7) Zeng, K.; Zhang, D., Recent Progress in Alkaline Water Electrolysis for Hydrogen Production and Applications. Prog. Energy Combust. Sci. 2010, 36, 307-326. (8) Grigoriev, S.; Porembsky, V.; Fateev, V., Pure Hydrogen Production by PEM Electrolysis for 23


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

Hydrogen Energy. Int. J. Hydrogen Energy 2006, 31, 171-175. (9) Muller, F.; Ferreira, C. A.; Azambuja, D. S.; Aleman, C.; Armelin, E., Measuring the Proton Conductivity of Ion-exchange Membranes Using Electrochemical Impedance Spectroscopy and Through-plane Cell. J. Phys. Chem. B 2014, 118, 1102-1112. (10) Zhao, S.; Yu, H.; Maric, R.; Danilovic, N.; Capuano, C. B.; Ayers, K. E.; Mustain, W. E., Calculating the Electrochemically Active Surface Area of Iridium Oxide in Operating Proton Exchange Membrane Electrolyzers. J. Electrochem. Soc. 2015, 162, F1292-F1298. (11) Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A Comprehensive Review on PEM Water Electrolysis. Int. J. Hydrogen Energy 2013, 38, 4901-4934. (12) Ayers, K. E.; Anderson, E. B.; Capuano, C.; Carter, B.; Dalton, L.; Hanlon, G.; Manco, J.; Niedzwiecki, M., Research Advances Towards Low Cost, High Efficiency PEM Electrolysis. ECS Trans. 2010, 33, 3-15. (13) Sun, K.; Liu, R.; Chen, Y.; Verlage, E.; Lewis, N. S.; Xiang, C., A Stabilized, Intrinsically Safe, 10% Efficient, Solar‐Driven Water‐Splitting Cell Incorporating Earth‐Abundant Electrocatalysts with Steady‐State pH Gradients and Product Separation Enabled by a Bipolar Membrane. Adv. Energy Mater. 2016, 6, 1600379. (14) Vincent, I.; Bessarabov, D., Low Cost Hydrogen Production by Anion Exchange Membrane Electrolysis: A Review. Renew. Sust. Energ. Rev. 2018, 81, 1690-1704. (15) Parrondo, J.; George, M.; Capuano, C.; Ayers, K. E.; Ramani, V., Pyrochlore Electrocatalysts for Efficient Alkaline Water Electrolysis. J. Mater. Chem. A 2015, 3, 10819-10828. (16) Xiao, L.; Zhang, S.; Pan, J.; Yang, C.; He, M.; Zhuang, L.; Lu, J., First Implementation of Alkaline Polymer Electrolyte Water Electrolysis Working Only With Pure Water. Energy Environ. Sci. 2012, 5, 7869-7871. (17) Pavel, C. C.; Cecconi, F.; Emiliani, C.; Santiccioli, S.; Scaffidi, A.; Catanorchi, S.; Comotti, M., Highly Efficient Platinum Group Metal Free Based Membrane-electrode Assembly for Anion Exchange Membrane Water Electrolysis. Angew. Chem. Int. Ed. 2014, 53, 1378-1381. (18) Fabbri, E.; Nachtegaal, M.; Binninger, T.; Cheng, X.; Kim, B. J.; Durst, J.; Bozza, F.; Graule, T.; Schaublin, R.; Wiles, L.; Pertoso, M.; Danilovic, N.; Ayers, K. E.; Schmidt, T. J., Dynamic Surface Self-reconstruction is the Key of Highly Active Perovskite Nano-electrocatalysts for Water Splitting. Nat. Mater. 2017, 16, 925-931. (19) Ahn, S. H.; Lee, B.-S.; Choi, I.; Yoo, S. J.; Kim, H.-J.; Cho, E.; Henkensmeier, D.; Nam, S. W.; Kim, S.-K.; Jang, J. H., Development of a Membrane Electrode Assembly for Alkaline Water Electrolysis by Direct Electrodeposition of Nickel on Carbon Papers. Appl. Catal., B 2014, 154, 197-205. (20) Ayers, K. E.; Anderson, E. B.; Capuano, C. B.; Niedzwiecki, M.; Hickner, M. A.; Wang, C.Y.; Leng, Y.; Zhao, W., Characterization of Anion Exchange Membrane Technology for Low Cost Electrolysis. ECS Trans. 2013, 45, 121-130. 24


Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(21) Wu, X.; Scott, K., CuxCo3−xO4 (0 ≤ x < 1) Nanoparticles for Oxygen Evolution in High Performance Alkaline Exchange Membrane Water Electrolysers. J. Mater. Chem. 2011, 21, 12344-12351. (22) Seetharaman, S.; Balaji, R.; Ramya, K.; Dhathathreyan, K.; Velan, M., Graphene Oxide Modified Non-noble Metal Electrode for Alkaline Anion Exchange Membrane Water Electrolyzers. Int. J. Hydrogen Energy 2013, 38, 14934-14942. (23) Pandiarajan, T.; Berchmans, L. J.; Ravichandran, S., Fabrication of Spinel Ferrite Based Alkaline Anion Exchange Membrane Water Electrolysers for Hydrogen Production. RSC Adv. 2015, 5, 34100-34108. (24) Joe, J. D.; Kumar, D. S.; Sivakumar, P., Production Of Hydrogen by Anion Exchange Membrane Using AWE. IJSTR 2014, 3, 38-42. (25) Park, E. J.; Capuano, C. B.; Ayers, K. E.; Bae, C., Chemically Durable Polymer Electrolytes for Solid-state Alkaline Water Electrolysis. J. Power Sources 2018, 375, 367-372. (26) Marino, M. G.; Melchior, J. P.; Wohlfarth, A.; Kreuer, K. D., Hydroxide, Halide and Water Transport in a Model Anion Exchange Membrane. J. Membr. Sci. 2014, 464, 61-71. (27) Faraj, M.; Boccia, M.; Miller, H.; Martini, F.; Borsacchi, S.; Geppi, M.; Pucci, A., New LDPE Based Anion-exchange Membranes for Alkaline Solid Polymeric Electrolyte Water Electrolysis. Int. J. Hydrogen Energy 2012, 37, 14992-15002. (28) Cao, Y.-C.; Wu, X.; Scott, K., A Quaternary Ammonium Grafted Poly Vinyl Benzyl Chloride Membrane for Alkaline Anion Exchange Membrane Water Electrolysers With No Noble-metal Catalysts. Int. J. Hydrogen Energy 2012, 37, 9524-9528. (29) Wu, X.; Scott, K., A Polymethacrylate-based Quaternary Ammonium OH− Ionomer Binder for Non-precious Metal Alkaline Anion Exchange Membrane Water Electrolysers. J. Power Sources 2012, 214, 124-129. (30) Zeng, L.; Zhao, T., Integrated Inorganic Membrane Electrode Assembly With Layered Double Hydroxides as Ionic Conductors for Anion Exchange Membrane Water Electrolysis. Nano Energy 2015, 11, 110-118. (31) Cho, M. K.; Park, H.-Y.; Choe, S.; Yoo, S. J.; Kim, J. Y.; Kim, H.-J.; Henkensmeier, D.; Lee, S. Y.; Sung, Y.-E.; Park, H. S.; Jang, J. H., Factors in Electrode Fabrication for Performance Enhancement of Anion Exchange Membrane Water Electrolysis. J. Power Sources 2017, 347, 283-290. (32) Leng, Y.; Chen, G.; Mendoza, A. J.; Tighe, T. B.; Hickner, M. A.; Wang, C. Y., Solid-state Water Electrolysis With an Alkaline Membrane. J. Am. Chem. Soc. 2012, 134, 9054-9057. (33) An, L.; Zhao, T.; Chai, Z.; Tan, P.; Zeng, L., Mathematical Modeling of an Anion-exchange Membrane Water Electrolyzer for Hydrogen Production. Int. J. Hydrogen Energy 2014, 39, 1986919876. (34) Arges, C. G.; Wang, L.; Parrondo, J.; Ramani, V., Best Practices for Investigating Anion 25


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

Exchange Membrane Suitability for Alkaline Electrochemical Devices: Case Study Using Quaternary Ammonium Poly(2,6-dimethyl 1,4-phenylene)oxide Anion Exchange Membranes. J. Electrochem. Soc. 2013, 160, F1258-F1274. (35) Wu, X.; Scott, K.; Xie, F.; Alford, N., A Reversible Water Electrolyser With Porous PTFE Based OH− Conductive Membrane as Energy Storage Cells. J. Power Sources 2014, 246, 225231. (36) Dong, Y.; He, K.; Yin, L.; Zhang, A., A Facile Route to Controlled Synthesis of Co3O4 Nanoparticles and Their Environmental Catalytic Properties. Nanotechnology 2007, 18, 435602. (37) Buttry, D. A.; Ward, M. D., Measurement of Interfacial Processes at Electrode Surfaces With the Electrochemical Quartz Crystal Microbalance. Chem. Rev. 1992, 92, 1355-1379. (38) Parrondo, J.; Arges, C. G.; Niedzwiecki, M.; Anderson, E. B.; Ayers, K. E.; Ramani, V., Degradation of Anion Exchange Membranes Used for Hydrogen Production by Ultrapure Water Electrolysis. RSC Adv. 2014, 4, 9875-9879. (39) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K., Anion-exchange Membranes in Electrochemical Energy Systems. Energy Environ. Sci. 2014, 7, 3135-3191. (40) Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W., Solution-cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution. J. Am. Chem. Soc. 2012, 134, 17253-17261. (41) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W., Nickel-iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation. J. Am. Chem. Soc. 2014, 136, 6744-6753. (42) Corrigan, D. A.; Bendert, R. M., Effect of Coprecipitated Metal Ions on the Electrochemistry of Nickel Hydroxide Thin Films: Cyclic Voltammetry in 1 M KOH. J. Electrochem. Soc. 1989, 136, 723-728. (43) Enman, L. J.; Burke, M. S.; Batchellor, A. S.; Boettcher, S. W., Effects of Intentionally Incorporated Metal Cations on the Oxygen Evolution Electrocatalytic Activity of Nickel (Oxy)hydroxide in Alkaline Media. ACS Catal. 2016, 6, 2416-2423. (44) Merrill, M. D.; Dougherty, R. C., Metal Oxide Catalysts for the Evolution of O2 From H2O. J. Phys. Chem. C 2008, 112, 3655-3666. (45) Subbaraman, R.; Tripkovic, D.; Chang, K.-C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M., Trends in Activity for the Water Electrolyser Reactions on 3d M (Ni, Co, Fe, Mn) Hydr(oxy)oxide Catalysts. Nat. Mater. 2012, 11, 550-557. (46) Zou, S.; Burke, M. S.; Kast, M. G.; Fan, J.; Danilovic, N.; Boettcher, S. W., Fe (Oxy)hydroxide Oxygen Evolution Reaction Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity, Substrate, and Dissolution. Chem. Mater. 2015, 27, 8011-8020. (47) Jang, S.-E.; Kim, H., Effect of Water Electrolysis Catalysts on Carbon Corrosion in Polymer 26


Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Electrolyte Membrane Fuel Cells. J. Am. Chem. Soc. 2010, 132, 14700-14701. (48) Sakamoto, S.; Yoshinaka, M.; Hirota, K.; Yamaguchi, O., Fabrication, Mechanical Properties, and Electrical Conductivity of Co3O4 Ceramics. J. Am. Ceram. Soc. 1997, 80, 267-268. (49) Marshall, A.; Børresen, B.; Hagen, G.; Tsypkin, M.; Tunold, R., Preparation and Characterisation of Nanocrystalline IrxSn1−xO2 Electrocatalytic Powders. Mater. Chem. Phys. 2005, 94, 226-232. (50) Windisch, C. F.; Ferris, K. F.; Exarhos, G. J.; Sharma, S. K., Conducting Spinel Oxide Films With Infrared Transparency. Thin Solid Films 2002, 420, 89-99. (51) Cornell, R. M.; Schwertmann, U., The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses. 2 ed.; Wiley: Weinheim, 2006. (52) Fradette, N.; Marsan, B., Surface Studies of CuxCo3-xO4 Electrodes for the Electrocatalysis of Oxygen Evolution. J. Electrochem. Soc 1998, 145, 2320-2327. (53) Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W., Cobalt-iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure And Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 2015, 137, 3638-3648. (54) Feng, Q.; Liu, G.; Wei, B.; Zhang, Z.; Li, H.; Wang, H., A Review of Proton Exchange Membrane Water Electrolysis on Degradation Mechanisms and Mitigation Strategies. J. Power Sources 2017, 366, 33-55. (55) Merle, G.; Wessling, M.; Nijmeijer, K., Anion Exchange Membranes for Alkaline Fuel Cells: A Review. J. Membr. Sci. 2011, 377, 1-35.

27


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

Table of contents figure

28


Earth-Abundant Oxygen Electrocatalysts for Alkaline Anion Exchange

Recommend Documents