Scalable Ir-Doped NiFe2O4/TiO2 Heterojunction Anode for Decentralized Saline Wastewater Treatment and H2 Production

Vol.:(0123456789)

e-ISSN 2150-5551

CN 31-2103/TB

ARTICLE

Cite as

Nano-Micro Lett.

(2025) 17:51

Received: 30 June 2024

Accepted: 23 September 2024

https://doi.org/10.1007/s40820-024-01542-x

Scalable Ir‑Doped

NiFe

/TiO

Heterojunction

Anode for Decentralized Saline Wastewater

Treatment and H

Production

Sukhwa Hong

, Jiseon Kim

, Jaebeom Park

, Sunmi Im

, Michael R. Hoffmann

Kangwoo Cho

1,2,3

HIGHLIGHTS

•

Ir-doped

NiFe

(NFI) spinel with

TiO

heterojunction overlayer brought about outstanding chlorine evolution reaction in circum-

neutral pH.

•

Electroanalyses including operando X-ray absorption spectroscopy uncovered the active role of

TiO

for

−

chemisorption.

•

NFI/TiO

anode boosted both

-to-N

conversion and

generation in wastewater, and the practical applicability was confirmed

with scaled-up anodes and real wastewater.

ABSTRACT

Wastewater electrolysis cells (WECs) for

decentralized wastewater treatment/reuse coupled with

production can reduce the carbon footprint associated with

transportation of water, waste, and energy carrier. This study

reports Ir-doped

NiFe

(NFI,

~ 5 at% Ir) spinel layer with

TiO

overlayer (NFI/TiO

), as a scalable heterojunction

anode for direct electrolysis of wastewater with circum-

neutral pH in a single-compartment cell. In dilute (0.1 M)

NaCl solutions, the NFI/TiO

marks superior activity and

selectivity for chlorine evolution reaction, outperforming

the benchmark

IrO

. Robust operation in near-neutral pH

was confirmed. Electroanalyses including

operando

X-ray

absorption spectroscopy unveiled crucial roles of

TiO

which serves both as the primary site for

−

chemisorption and a protective layer for NFI as an ohmic contact. Galvanostatic electrolysis

-laden synthetic wastewater demonstrated that NFI/TiO

not only achieves quasi-stoichiometric

-to-N

conversion, but also

enhances

generation efficiency with minimal competing reactions such as reduction of dissolved oxygen and reactive chlorine. The

scaled-up WEC with NFI/TiO

was demonstrated for electrolysis of toilet wastewater.

KEYWORDS

Wastewater electrolysis cell; Ir-doped

NiFe

; Reactive chlorine species; Decentralized

production; On-site

wastewater treatment

Kangwoo Cho, kwcho1982@postech.ac.kr

Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790−784, Korea

Linde Laboratory, California Institute of Technology, Pasadena, CA 91125, USA

Institute for Convergence Research and Education in Advanced Technology (I-CREATE), Yonsei University International Campus, Incheon 21983,

Republic of Korea

Nano-Micro Lett. (2025) 17:51

Page 2 of 18

https://doi.org/10.1007/s40820-024-01542-x

The authors

Abbreviations

EOP

Electrochemical oxidation process

ClER

Chlorine evolution reaction

RCS

Reactive chlorine species

WECs

Wastewater electrolysis cells

HER

Hydrogen evolution reaction

BDD

Boron-doped diamond

OER

Oxygen evolution reaction

NiFe

NFI

Ir-doped

NiFe

FE-SEM

Field emission scanning electron microscope

EDS

Energy-dispersive X-ray spectrometer

XRF

X-ray fluorescence

XRD

X-ray diffraction

XPS

X-ray photoelectron spectroscopy

PAL

Pohang accelerate laboratory

XANES

X-ray absorption near-edge structure

OCV

Open circuit voltage

Current interruption

Cyclic voltammetry

LSV

Linear sweep voltammetry

CDL

Double-layer capacitance

ECSA

Electrochemically active surface area

EIS

Electrochemical impedance spectroscopy

Solution resistance

Films resistance

Charge transfer resistance

PZC

Potential of zero charge

M-S

Mott–Schottky

DPD

N,N-diethyl-p-phenylenediamine

HHV

Higher heating value

Total nitrogen

COD

Chemical oxygen demand

Ion chromatography

GC-TCD

Gas chromatography with thermal conductiv

ity detector

EEM

Excitation-emission matrix

DOM

Dissolved organic matter

RHE

Reversible hydrogen electrode

Current efficiency

Energy efficiency

TOC

Total organic carbon

1 Introduction

The current societal pursuit toward carbon neutrality

would necessitate self-contained systems, ultimately to be

independent on the existing water and energy grid. For

example, on-site wastewater treatment and reuse are ben-

eficial for a sustainable water cycle in adaptation to the

climate change [

]. In addition, a reduction in water and

waste transportation would decrease the carbon footprint

[

] for sanitation and hygiene to meet the Sustainability

Development Goals established by the United Nations. To

this end, electrochemical oxidation processes (EOPs) have

emerged as a promising way of decentralized treatment of

toilet wastewater and effluent reuse [

]. While achieving

adequate effluent level set by the International Organiza-

tion for Standardization (

e.g

., ISO 30500 [

]), the EOPs

could be advantageous with respect to ease of automation

and connection with renewable energy sources (

e.g

., using

photovoltaic panels) [

]. A long-term operation of a com-

bined anaerobic digester and EOP has been demonstrated

for a self-contained public toilet with a nonpotable water

reuse (flushing) [

The chlorine evolution reaction (ClER) on electrocatalysts

oxidizes chloride ion to reactive chlorine species (RCS), the

core mediator to degrade aqueous organic pollutants and

ammonium

(NH

) [

–

]. In particular, the efficient deam-

monification by the electrolytic RCS has been a subject of

significant attention [

–

], which has been rarely achieved

by conventional septic systems or other non-sewered sanita-

tion systems based on biological (de)nitrification, stripping,

ion exchange, and wet chemical treatments [

]. Almost

stoichiometric conversion of

by the

in situ

gener

ated RCS without a generation of N-containing greenhouse

gases (

e.g

., N

) should be environmentally sustain

able, while alleviating concerns related to

such as

eutrophication and odors.

On the other hand, a distributed electrolysis of nontra-

ditional water sources including wastewater (effluent) can

be involved within the

economy [

]. A local produc-

tion of deionized water by reverse osmosis is known to

contribute marginally to the overall

production cost by

electrolysis. However, it can compete with drinking water

production in the areas with surplus renewable energy

(

e.g

., desert). In this regard, contributions from Hoffmann

and coworkers [

] advocate wastewater electrolysis cells

(WECs) for localized conversion of renewable energy into

, reducing the costs and

emission for (waste) water

treatment and transportation. A usage of separator (

e.g

proton exchange membrane) in a direct wastewater elec-

trolysis could bring about proliferating ohmic losses and

Nano-Micro Lett. (2025) 17:51

Page 3 of 18

contamination of the separator in wastewater matrix. In

the single-compartment WEC, therefore, oxygen reduction

reaction competes with the hydrogen evolution reaction

(HER), substantially decreasing the current and energy

efficiency [

]. Relatively low-grade

60%) in mix-

ture with

(from deammonification) and

(from min-

eralization) can be utilized by combustion, in a decent

analogy with the existing chloralkali processes that gen-

erates

as a byproduct. This approach might be more

available and appropriate practice.

Nonetheless, the bottlenecks of WEC include require-

ments of precious element-based electrocatalysts and unsat-

isfactory selectivity of ClER. The current anode materials in

EOPs exclusively rely on dimensional stable anode (DSA;

IrTaO

and

RuTiO

) [

] and boron-doped diamond

(BDD) [

], unaffordable for a decentralized system. In

spite of the recent developments on electrocatalysts based

on earth-abundant elements (

e.g

., Ni, Fe, Co, Cu, Zn, Mo

among others) [

–

], their instability in near-neutral pH

required an alkalified wastewater, while inferior ClER selec

tivity with dominant oxygen evolution reaction (OER) ruled

out a concurrent pollutants degradation during the electroly

sis [

]. To this end, evidences have been presented that

TiO

outer layers in heterojunction with conductive Ir-based

DSA could enhance both the ClER selectivity and durability

[

–

], although the underlying mechanism remains ambigu-

ous. In addition, we recently reported

NiFe

(NF) elec-

trocatalysts with a tiny amount (5 mol%) of Ir doping (NFI)

could enable extraordinary OER activity and stability [

A scaling relation between OER and ClER on (mixed) metal

oxide electrocatalysts motivated us to further deploy the NFI

for ClER in circumneutral pH in combination with the

TiO

heterojunction layer.

Within the aforementioned context, this study reports that

NFI/TiO

heterojunction anode (prepared by a straightfor

ward solution casting) allows ClER activity superior to the

benchmark

IrO

and almost absolute ClER selectivity in

0.1 M NaCl solutions. Electrolysis of

-laden synthetic

wastewater demonstrated that the admirable ClER metrics

simultaneously enhanced the kinetics of pollutants degrada-

tion and

generation. Electroanalyses coupled with

oper

ando

X-ray absorption spectroscopy revealed active ClER

primarily on

TiO

, while the underlying NFI served as an

ohmic contact. The practical applicability was validated by

a scaled-up WEC with toilet wastewater.

2 Experimental Section

2.1 Preparation of NFI/TiO

Anode

Ti foils (Alfa Aesar, 3

× 1 cm

, 0.25 mm thick, 99.5%

purity) underwent pretreatments to remove impurities,

including SiC sandblasting, degreasing by ultrasonication

in a mixed solvent (with equal volumes of ethanol, acetone,

and deionized (DI) water (18.2 MΩ, Millipore)) for 0.5 h,

and immersion in 10 wt% boiling oxalic acid for 0.5 h. The

precursors for mixed Ni–Fe oxides were prepared using

nitrate salts (Ni(NO

)

·6H

O and Fe(NO

)

·9H

O, both

from Alfa Aesar in 99% purity) dissolved in DI water with

0.1 M urea, in variable molar ratios of Ni to Fe ([total

metal]

= 250 mM). In particular, the precursor with Ni-

to-Fe ratio of 1:2 was used for NF. For

IrO

preparation,

250 mM

IrCl

was dissolved in a mixed solution with

equi-volumes of ethanol, isopropanol, and 0.3 M HCl. A

calculated amount of the Ir-precursor was added to the NF

precursor ([Ir]

= 12 mM) for the NFI. Ti-glycolate precur

sor for

TiO

layer was prepared by a peroxo-method [

In short, 0.25 M Ti(C

was dissolved in 0.4 M gly

colic acid solution by addition of concentrated

, and

the final pH was adjusted to be circumneutral by addition

of concentrated

OH. All anodes interrogated in this

study were fabricated by drop-casting (1 μL

−2

), drying

for 15 min (80 °C), and annealing for 15 min (425 °C for

NF, NFI, and

TiO

; 525 °C for

IrO

). This sequence was

repeated up to total 6 coats which underwent final anneal-

ing for 1.5 h (Fig.

a). A commercial BDD electrode as a

control was provided by Wesco Electrode.

2.2 Anode Characterization

The surface morphology was observed by high-resolution

field emission scanning electron microscope (FE-SEM,

JSM 7800F PRIME). The elemental compositions were

estimated by energy-dispersive X-ray spectrometer (EDS,

LN2 Free SDD type) with FE-SEM, X-ray fluorescence

(ED-XRF, SII Nano technology Inc., SEA1200VX), and

glow discharge spectrometry (GDS, LECO GDS850A

with Radio Frequency Lamp). The crystalline struc

ture was analyzed by X-ray diffraction (XRD, Phillips

X’Pert Panalytical diffractometer) at 30 mA, 40 kV, and

Nano-Micro Lett. (2025) 17:51

Page 4 of 18

https://doi.org/10.1007/s40820-024-01542-x

The authors

monochromated Cu Kα1 radiation. Raman spectra were

collected by Alpha 300R (WITec) with a

× 50 objec-

tive and wavelength of 488 nm using an

excitation

source. The composition and oxidation states on surface

(up to

~ 10 nm) were investigated by K-ALPHA X-ray

photoelectron spectroscopy (XPS, Thermo Fisher Scien-

tific, UK) using a monochromated Al Kα (12 kV, 72 W,

1486.6 eV, 400 μm spot size). The bulk electronic struc-

ture was interrogated by X-ray absorption spectroscopy at

the 10C beamline in Pohang Accelerate Laboratory (PAL),

to give X-ray absorption near-edge structure (XANES)

spectra.

operando

XANES analysis proceeded with the

working electrode attached to the cell window by Kapton

tape, under open circuit voltage (OCV), pre-ClER, and

ClER condition at a minute interval.

2.3 Electroanalysis

A single-compartment cell (working volume: 35 mL)

was used with three-electrode configuration including an

anode under investigation (effective geometric area: 2

× 1

), a Pt coil cathode (BASi), and a reference electrode.

Ag/AgCl (BASi) and Hg/HgO (BASi) reference elec-

trodes were used for electrolyte with neutral and alkaline

pH, respectively. The spacing between the working and

counter electrode was maintained at 0.5 cm. The meas-

ured potentials were converted to RHE scale by

RHE

Ag/

AgCl

+ 0.197

+ 0.059

× pH

= E

Hg/HgO

+ 0.140

+ 0.059

× pH.

The working electrode potential (

) was compensated with

ohmic (

) drop, based on a current interruption (CI) method

at 85% level. The electrochemical activity and stability were

Fig. 1

Preparation and characterization of NFI/TiO

anode.

Schematic illustration of the synthesis procedure.

b‑c

Horizontal SEM images of

NFI and NFI/TiO

XRD profiles of NFI and NFI/TiO

with references.

e–g

Ex situ

XANES for Ni K-edge, Fe K-edge, and Ti K-edge of NFI/

TiO

in comparison with NFI or Ti/TiO

Nano-Micro Lett. (2025) 17:51

Page 5 of 18

evaluated based on cyclic voltammetry (CV), linear sweep vol-

tammetry (LSV), and chronopotentiometry using a potentio-

stat (VSP, BioLogic). Double-layer capacitance

), which

represents the electrochemically active surface area (ECSA),

was measured by CV in a non-Faradaic potential window at

variable scan rates (1 to 100 mV

−1

) in 0.1 M NaCl (

–

0.861

RHE

) and 1 M KOH (

–

at 1.17

RHE

). The electro-

chemical impedance spectroscopy (EIS) estimated solution

resistance (

), films resistance (

), and charge transfer resist-

ance (

), with fitting by EC

− Lab software (VSP, BioLogic).

The baseline potential for EIS was 1.3 V Ag/AgCl in 0.1 M

NaCl, while the sinus amplitude of 10 mV and frequency scan

range of 100 kHz to 100 mHz were used. The potential of zero

charge (PZC) was determined based on the potential where the

capacitance was minimized [

]. The changes in capacitance

at different potentials were tracked using EIS operated through

a potentiostat (VSP, BioLogic). These impedance measure-

ments, which varied with the applied potential, were taken in

0.05 M NaCl solutions, using a frequency of 150 mHz and a

sinus amplitude of 5 mV. By applying 6th-order polynomial

fitting to the spectra, the point where F was at its minimum was

established as the PZC [

]. The Mott–Schottky (M-S) plots

were obtained by EIS (sinus amplitude: 10 mV, frequency

range: 10 kHz to 10 Hz, and potential range: 0 to 1 V versus

reference electrode). The M-S slope from the following equa-

tion was used to comparatively evaluate the electrical conduc-

tivity [

where

is the space charge capacitance (F),

is the

dielectric constant,

is the permittivity of the vac-

uum (8.854 ×

−12

−1

is the elementary charge

(1.602 ×

−19

C),

is donor density

−3

is active sur

face area

is the applied potential to the working

electrode (V),

is the flat band potential (V),

is Boltz-

mann’s constant (8.62 ×

−5

eV K

−1

), and

is the absolute

temperature (K). Prior to all electroanalyses, the cell was

rested in open circuit for 15 min.

2.4

RCS Generation by Galvanostatic Bulk Electrolysis

The ClER in aqueous electrolyte generates free chlorine spe-

cies including HOCl and

OCl

−

by pH-dependent hydrolysis

[

]. The performance of ClER (RCS generation) was

evaluated by galvanostatic electrolysis of 0.1 M NaCl solutions

(1)

1

2

=

2

휀휀

0

2

−

−

at variable current density (

, 10 to 50 mA

−2

). The evolu-

tion of [RCS] was periodically quantified with DPD reagents

for initial 7 min, where

ClO

−

ClO

−

generations were neg-

ligible. This study used the following metrics for fair compari-

son of anodes [

]. The

ClER

, EE

ClER

, and

ClER

of ClER

were estimated by the equations below [

where

represents the volume of the electrolyte (0.035 L),

is Faraday constant (96,485.3 C

mol

−1

), d[RCS]/dt is RCS

generation rate (M

−1

is current density (A

−2

electrolysis time (s), and

is cell voltage (V).

2.5

RCS‑Mediated Wastewater Treatment Coupled

with H

Generation

Using the aforementioned cell, bulk galvanostatic

(30 mA

−2

) electrolysis experiments for RCS-medi-

ated conversion of

with simultaneous

HER proceeded in synthetic wastewater samples. The

[Cl

−

]

was fixed at 0.1 M, while

[NH

]

was varied

([NH

]

:[Cl

−

]

= 2:1, 1.5:1, 1:1, 1:1.5, 1:2, 1:3, and 1:4

in molar basis) by mixing

Cl, NaCl, and

(NH

)

The CE of pollutants oxidation as well as CE and EE for

HER were estimated by following equations [

where

is the number of electron transfer for oxidation of

aqueous pollutants (3 for

-to-N

conversion),

dC/dt

decreased pollutants concentration per unit electrolysis time

−1

is the observed

production rate (mol

−1

and HHV is higher heating value of

(78 Wh

mol

−1

(2)

CEClER

(%)=

VFd

[

RCS

]

jAdt

(3)

ClER



mmolWh

−



[

RCS

]

jAdt

3.6

(4)

ClER

(

mmolcm

−

Vd[RCS]

Adt

360

(5)

(

pollutantsoxidation

nVFdC

jAdt

(6)

HER

=

(7)

HER

=

HHVQ

c

Nano-Micro Lett. (2025) 17:51

Page 6 of 18

https://doi.org/10.1007/s40820-024-01542-x

The authors

To demonstrate practical applicability, a pilot-scale

WEC was manufactured in working volume of 10 L (with

internal circulation). Scaled-up NFI/TiO

anodes and com-

mercial stainless steel 304 cathode were prepared in size

of 35.8 × 26.6

. Three anodes and cathodes were alter

nately sandwiched (with inter-electrodes distance of 1 cm)

and connected to a power supply (ODA Tech, EX30-60)

in monopolar configuration. The geometric surface area

of the electrode module exposed to electrolyte was 0.191

. Toilet wastewater was mimicked by mixing livestock

excretion (collected from Gyungju wastewater treatment

plant, Korea), seawater (collected in Pohang, Korea), and

tap water by volume ratio of 5:20:75. The composition

of the toilet wastewater was summarized with 102 mgN

−1

, 104 mgN

−1

of total nitrogen (TN), 580

mgO

−1

of chemical oxygen demand (COD), 120 NTU

of turbidity, 121 mM of

−

, 7.9 of pH, and 14.7 mS

−1

of conductivity. The wastewater sample was subjected to

electrolysis at constant current of 52.5 A (corresponding

to 27.5 mA

−2

) for 3 h.

Quantification of anions (

e.g.

−

, ClO

−

, NO

−

and

−

) was carried out by ion chromatography (IC,

DX-120). The concentration of free chlorine and total

chlorine were measured using DPD (N,N-diethyl-p-phe-

nylenediamine) and DPD/KI reagents, respectively, based

on absorbance at 530 nm in UV–Vis spectrometer (DR

3900, HACH). Combined chlorine (e.g., chloramines) was

estimated from the difference between free chlorine and

total chlorine [

]. TN was quantified using alkaline per

sulfate digestion [

] based on absorbance of nitrate at

420 nm.

-N was analyzed by salicylate method with

commercial kits

(NH

-N TNT kit, HACH) and absorbance

at 610 nm [

]. [COD] was measured by dichromate diges-

tion with colorimetric detection at 348 nm [

]. Gaseous

, N

, and

in reactor headspace were streamed with

carrier Ar gas to pass through a gas flow meter (Ritter

MilliGascounter), and the composition was measured by

gas chromatography with thermal conductivity detector

(GC-TCD, 6890-N, Agilent Technologies). Turbidity of

toilet wastewater was measured using turbidity colorimeter

(HUMAS, TURBY 1000). The excitation-emission matrix

(EEM) with fluorescence spectrometer (FluoroMax-4) was

collected to qualitatively investigate the variations in dis-

solved organic matter (DOM).

3 Results and Discussion

3.1 Characterization of NFI/TiO

Heterojunction

Anode

The NFI/TiO

anodes were fabricated through a straightfor

ward drop-casting method (Fig.

a). The horizontal images

of FE-SEM discovered NFI nanoparticles sized in the range

of 50–100 nm (Fig.

b) which aggregated during the thermal

treatment to bring about tableland and ridge morphology on

the Ti substrate, as shown by EDS mapping (Fig. S1). The

M-edge signals of Ir and K-edge signals of Ni, Fe, Ti, and O

revealed even elemental distributions on the NFI aggregates.

The EDS-based molar fraction of Ir was 5.27% for NFI, in

decent agreement with 5.15% from ED-XRF analysis (Fig.

S2). These values close to the precursor composition (5%)

suggested homogeneous Ir doping on NFI. XRD patterns of

NF and NFI powders (physically abraded from the Ti sub-

strate) confirmed that the

NiFe

spinel crystalline lattice

(JCPDS No. 10–0325, 2θ

= 36°, 43°, and 63° corresponding

to (311), (400), and (440)) [

] of NF was retained for NFI

despite the Ir dopants (Fig. S3a). Raman spectra for NF and

NFI (Fig. S3b) both exhibited congruence with the

NiFe

spinel structure of space group Fd-3 m, as affirmed with

active bands including

(symmetric stretch),

(sym-

metric bend), and

(asymmetric stretch) for tetrahedral

and octahedral sites [

]. This evidence substantiated the

uniform doping of Ir into NF nanoparticles without insig-

nificant structural perturbation and segregation into

IrO

The SEM/EDS analysis on NFI/TiO

noticed a stacked

film of

TiO

nanoparticles sized by 10–20 nm (Fig.

c), to

allow more even deposition of

TiO

outer layer reducing

surface tortuosity (Fig. S4). The nanoporous property of

the

TiO

layer would allow diffusive penetration of reac-

tants such as

−

, and

−

[

]. The XRD patterns of

NFI and NFI/TiO

electrodes (Fig.

d) were dominated by

signals from the Ti metal substrate (JCPDS No. 44–1294,

2θ

= 35°, 38°, 40°, 52°, 62°, 70°, 76°, and 77°) that largely

overlapped with those of spinel

NiFe

peaks. NFI/TiO

showed additional diffraction peaks from anatase

TiO

(JCPDS No. 21–1272, 2θ

= 25° and 48° corresponding to

(101) and (200), respectively). These evidences character

ized the NFI/TiO

as Ir-doped

NiFe

in heterojunction

with nanoporous

TiO

layer. A GDS analysis (Fig. S5)

Nano-Micro Lett. (2025) 17:51

Page 7 of 18

estimated the thickness of

TiO

and NFI to be

~ 250 nm

and ~ 2.75 μm, respectively.

XPS for NF and NFI (Fig. S6) clarified partial charge

transfer from Ni and Fe to the Ir dopants. The fractions of

and

from deconvoluted Ni 2

3/2

(854.5 eV for

and 856 eV for

with two satellite peaks at 861

and 865 eV) [

] and Fe 2

3/2

[

] (710 eV for

and

711.5 eV for

) photoelectron spectra were elevated in

NFI, whereas the binding energy of Ir 4

peak was between

those of

(61.8 eV) and

(60.9 eV) [

]. The decon-

volution of O 1

spectra noted escalated fraction of oxygen

vacancy upon the Ir doping. These observations were in

agreement with the prior report on NFI coated on Ni foam

[

]. On the other hand, the Ti 2

photoelectron spectra for

NF/TiO

and NFI/TiO

both indicated a partial oxidation

of Ti (Fig. S7), in comparison with the

TiO

layers directly

coated on the Ti substrate (Ti/TiO

). However, the concur

rent shifts in electronic structure of the underlying layers

were intangible due to the limited analytical depth of XPS.

To this end, XANES unambiguously informed on the elec-

tronic interaction across the heterojunction, based on the

edge position of individual metal components at the half-

maximum intensity to represent the oxidation state. The Ti

K-edge position of NFI/TiO

was positively shifted com-

pared to Ti/TiO

(Fig.

f), whereas both Ni and Fe K-edge

region absorbance spectra for NFI/TiO

suggested decreased

valency compared to NFI (Fig.

e, f). The

ex situ

XANES

thus provides compelling evidence of charge transfer from

the outer

TiO

to the underlying NFI across the interface.

3.2

Electrocatalytic Behaviors of NFI/TiO

Heterojunction Anode

Given the scaling relation between OER and ClER interme-

diates for (mixed) metal oxide electrocatalysts [

], screen-

ing electrocatalysts in terms of OER activity could be a

precedent step to employ the OER intermediates as ClER

center [

]. Due to a composition-dependent instability of

1-x

electrocatalysts in acidic-to-neutral pH, moreo-

ver, it was inevitable to evaluate them in alkaline electrolyte

where OER would overwhelm ClER. LSV curves in 1 M

KOH confirmed extraordinary OER activity of

NiFe

spi-

nel oxide substantially outperforming the other

1-x

compositions (x

= 0, 0.2, 0.5, 0.67, 0.8, and 1) identically

synthesized by dip coating (Fig. S8) [

]. Mixing Ir

within the

NiFe

precursor at variable atomic ratio (0, 1%,

3%, 5%, 7%, and 10%) enhanced OER activity up to 5% Ir

on the modified electrocatalysts, judging from overpotential

(

) at 10 mA

−2

(Fig. S9). Further elevation in Ir contents

marginally influenced the current wave, in compatible with

the previous report [

Armed with the supreme OER activity of NFI (5% Ir-

doped

NiFe

), the electrochemical performances of NFI

and NFI/TiO

were assessed in 0.1 M NaCl electrolyte with

circumneutral pH, and compared with NF and

IrO

with or

without the

TiO

overlayer. It should be noted that ClER

and OER would occur in parallel in this experimental con-

dition. The XRD for the control group samples confirmed

crystallinity of spinel

NiFe

, rutile

IrO

(JCPDS No.

15–870), and anatase

TiO

. (Fig. S10). The voltammograms

(Fig.

a) estimated required potentials at 10 mA

−2

be 2.00, 2.07, 1.81, 1.83, 1.88, and 1.90 V versus revers-

ible hydrogen electrode (RHE) for NF, NF/TiO

, NFI, NFI/

TiO

, IrO

, and

IrO

/TiO

, respectively. The NFI exhibited

the most facile charge transfer kinetics, even outperforming

the benchmarked

IrO

. Although the

TiO

overlayer moder

ately lowered the anodic wave, NFI/TiO

still marked supe

rior activity compared to the

IrO

. The

was measured

by plotting charging current density (

−

) as linear func-

tions of scan rate (Figs.

b and S11). The

value, as a

surrogate of ECSA, showed analogous trend with the LSV.

Nyquist plot from EIS disentangled

and

[

], as shown

in Fig. S12. The

IrO

exhibited singular semicircle owing

to the conductor-like property, whereas the

was noted for

NF and NFI by additional semicircles in lower frequency

ranges. The

based on diameter of the higher frequency

semicircles agreed with the activity trends, while the

TiO

overlayers substantially increased the

for the heterojunc-

tion anodes. Therefore, the moderate current reduction by

the

TiO

layer was ascribed to resistance to charge migration

(due to an inferior electrical conductivity of

TiO

) and/or

pore diffusion through the nanoporous film that was incom-

pletely compensated by the CI method.

The overall activity trends were maintained in voltam-

mograms obtained in 1 M KOH (Fig. S13), because of the

scaling relation between adsorption energy for intermediates

of OER (*OOH) and ClER (*OCl) on metal oxide electro-

catalysts [

]. The OER

of NFI (330 mV) at 10 mA

−2

was lower than NF and

IrO

. We previously presented evi-

dences that Ir doping on

NiFe

could shift the active motif

from Fe–O–Fe to Ni–O–Fe to concurrently escalate ECSA

Nano-Micro Lett. (2025) 17:51

Page 8 of 18

https://doi.org/10.1007/s40820-024-01542-x

The authors

and intrinsic OER activity of NF [

]. If the porous

TiO

layers were electrochemically inert, on the other hand, the

reduction of ECSA by the

TiO

deposition would be inde-

pendent on the electrolyte while the mass transport resist-

ance through the

TiO

film could be alleviated in 1 M KOH.

However, the

TiO

layer reduced the

value (Fig. S14)

of NFI more significantly in 1 M KOH compared to those

in 0.1 M NaCl. It implicitly elucidated an active electro-

catalytic roles of

TiO

for ClER. Mott–Schottky slopes both

in 1 M KOH and 0.1 M NaCl (Fig. S15) further revealed

substantially elevated donor density and electrical conduc-

tivity for NFI, compared to NF. The observed p-type prop-

erty of

NiFe

[

] rationalized the electron withdrawing

from the

TiO

(well-known n-type semiconductor) through

Fig. 2

Electrochemical performances.

LSV curves (scan rate: 10 mV

−1

) with 85%

correction.

Capacitive

–

versus scan rate from CV

(potential range: 0–0.5 V vs. Ag/AgCl, scan rate: 10, 20, 50, and 100 mV

−1

) for NFI, NFI/TiO

, NF, NF/TiO,

IrO

, and

IrO

/TiO

electrocata-

lysts in 100 mM NaCl.

Chrono-potentiometric profile for long-term stability test of NFI, NFI/TiO

, NF, NF/TiO

in 0.5 M

NaClO

d‑i

ClER

and

ClER

during galvanostatic electrolysis of 0.1 M NaCl solutions for NF, NFI

IrO

, NF/TiO

, NFI/TiO

, and

IrO

/TiO