Ag and AgCu Based Electrocatalysts for

CO2 Reduction: Operando Techniques for

studying degradation mechanisms

01st July, 2025

 CO2: Challenge and an Opportunity
Why CO2 reduction is important? CO₂ Utilization vs. Storage
After capturing CO₂, we can either:

• Store it underground (Carbon Capture & Storage – CCS)

• Use it to make chemicals and fuels (Carbon Capture &
Utilization – CCU)

Figure 1

• Global warming and climate change • Industrial emissions & atmospheric CO₂ rise

Figure 2 Figure 4
Figure 3
Fig 1: Fig 2: Fig 3: "Melting Iceberg"-Bilder: Stock- Fig 4: https://www.researchgate.net/figure/Comparison- 2
https://www.statista.com/statistics/276629/ https://www.yourdictionary.com/articles/global- Fotos & -Videos. | Adobe Stock of-CCS-vs-CCU-adapted-from-
global-co2-emissions/ warming-climate-change-difference wwwwikipediacom_fig1_356085989
Electrochemical Conversion

Electrochemical conversion is a clean method

using electricity, turns CO₂ into useful products
like CO, formate, methanol.

CO₂ + 2H⁺ + 2e⁻ ⇌ CO + H₂O

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Add reference
Catalyst Materials for CO₂RR

Catalysts control which product forms from CO₂RR as

different metals produce different products:

• Cu: hydrocarbons (CH₄, C₂H₄)

• Ag, Au: carbon monoxide (CO)

• Sn, Bi: formate (HCOO⁻)

Fig: Joule paper (2019): Wang et al., Self-Selective Catalyst Synthesis for CO₂ Reduction, Joule 3 (1), 1–10. DOI: 10.1016/j.joule.2019.05.023—contains this classification in the introductory discussion and/or Table 1.
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Electrochemical CO₂ Conversion: Basics and Barriers

Why is CO₂ conversion challenging?

• CO₂ is a very stable molecule → hard to activate

• High energy input needed to reduce CO₂

• Competing reaction: H₂ evolution

• Multi-carbon products need difficult C–C coupling

Figure

Fig 1: https://www.researchgate.net/figure/Schematics-of-A-free-energy-diagrams-for-CO2-activation-B-thermochemical-CO2_fig1_359604741 5
Why is CO₂ conversion challenging?
Structural Ways to Improve
Activity vs Selectivity
Degradation Catalysts
• High activity doesn’t • Surface • Alloying (Cu–Ag, Cu–
guarantee selectivity restructuring under Zn): tune binding
• Competition: CO₂RR bias • Nanostructuring:
vs HER • Nanoparticles may boost selectivity
• Cu is active but agglomerate • Supports & coatings:
yields CH₄, C₂H₄, H₂ • Metal leaching in prevent leaching
aqueous solutions • Electrolyte: pH
buffering, ion effects
• Voltage pulsing:
minimize
degradation

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ELECTROCATALYSTS SYNTHESIS
ACTIVITY VS STABILITY
 Electrocatalyst Design Strategies

1.MOF pyrolysis → Ag–Cu/C 2: AAO templating → Ag nanowires 3: Galvanic replacement → Ag–Cu hybrids

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1: Faraz et al., Energy Advances 2024, 3, 2367–2376. 2: Luan et al., ACS Applied Materials & Interfaces 2018, 10, 17950–17956. 3: Du et al., Nature Communications 2023, 14, 6142.
 MOF-Derived Ag–Cu NPs/C

Synthesis: Benefits:
1.Assemble MOF “sponge”: grow Cu–BTC framework (HKUST-1) at
1. Uniform dispersion of Ag–Cu nanoparticles
room temperature
2.Infuse metals: soak MOF in Ag⁺ and Cu²⁺ solution, then dry 2. Tunable size by pyrolysis temperature

3.Pyrolyze under H₂: heat between 550–850 °C for 6 h → carbon flakes 3. High conductivity from carbon support
with Ag–Cu NPs

1. Faraz et al., Energy Advances 2024, 3, 2367–2376

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AAO-Templated Ag Nanowires

Synthesis:
Benefits:
1. Stack Ag + AAO template: heat to 400 °C 1. High aspect-ratio wires with massive surface area
2. Apply pressure (~500 MPa): forces Ag into template pores (30 nm 2. Abundant low-coordinated Ag sites for strong CO₂ binding
or 200 nm diameter) 3. Robust alignment for efficient electron pathways
3. Remove template: dissolve AAO → free-standing Ag nanowire
array
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Luan et al., ACS Applied Materials & Interfaces 2018, 10, 17950–17956.
 Tandem Ag/Cu Single-Atom + NP Hybrid

Synthesis:
Benefits:
1.Mix Cu nanoparticles + Ag precursor under mild heating/sonication 1. Tandem catalysis: Ag sites produce CO; adjacent Cu

2.Spontaneous exchange: Ag deposits as single atoms and sites couple *CO into C₂+

nanoparticles on Cu 2. Atomic-level control of active sites

3. Compact composite for high-rate, selective conversion
3.Clean & collect: yields a hybrid where Ag and Cu coexist

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Du et al., Nature Communications 2023, 14, 6142.
Electrochemical Setup

1: Ag–Cu/C & Ag Nanowire Arrays – H-Cell Setup 2: AgCu SANP Catalyst – Flow Cell Setup
•Setup: Three-electrode H-type cell •Setup: Three-chamber flow cell with gas diffusion
•Electrolytes: electrode (GDE)
• Ag–Cu/C: 0.1 M KHCO₃ •Electrolyte: 1 M KOH
• Ag Nanowires: 0.5 M KHCO₃ •CO₂ Source: Continuous CO₂ gas flow into cathode
•CO₂ Source: CO₂-saturated electrolyte stream

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1: MTX Labs Global. All About H-Type Electrochemical Cell (H-Cell). 2: Du et al., Nature Communications 2023, 14, 6142.
Key Evaluation Methods

Degrading
Catalyst

Stable Catalyst

LSV (Linear Sweep CA

(Chronoamperometry) FE (Faradaic Efficiency) tells us
Voltammetry) shows how what fraction of electrons form
effectively a catalyst drives tracks how stable the
catalyst is by measuring the desired CO₂ reduction
CO₂ reduction at varying product like CO or C₂⁺.
voltages. current over time at a
fixed potential.

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Figures generated by chatGPT
MOF-Derived Ag–Cu/C

LSV: curves for 550, 600, 700, 850 °C pyrolysis CA: trace at −0.79 V for 850 °C sample
(current vs. potential) (current vs. time)

• Higher pyrolysis → more positive onset & larger peak currents

• 850 °C sample: ~44 mA/cm² peak, retains ~98 % over 40 h

Faraz et al., Energy Advances 2024, 3, 2367–2376 14

Ag Nanowire Array

LSV: 30 nm vs. 200 nm wires CA (–0.6 V): 200 nm wire loses

→ 30 nm reaches ~6.8 <10 % of current over 24 h
mA/cm² at –0.6 V

• Thinner wires → more active sites → higher current

• <10 % current decay → good stability in 1 day test

Luan et al., ACS Applied Materials & Interfaces 2018, 10, 17950–17956. 15
Ag–Cu Hybrid (SANP)

Bar chart at –0.65 V: AgCu FE (–0.65 V): ~94 % C₂⁺ FE

SANP → ~720 mA/cm²; other
catalysts much lower

• Industry-scale current (~720 mA/cm²) → highest rate of conversion

• ~94 % FE to C₂+ → outstanding selectivity for multi-carbon products

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Du et al., Nature Communications 2023, 14, 6142.
Comparative Overview of all Catalyst Systems
Study Catalyst system Synthesis Electrolyte / Setup Main FE (%) Stability/
method product(s) Current

Faraz, Ag–Cu NPs on MOF pyrolysis 0.1 M KHCO₃, H-cell, CO ~90 40 h, 44.1
et al.5 carbon Nafion mA/cm²

Luan, et Ag nanowire Nanomolding 0.5 M KHCO₃, 2-comp CO ~91 24 h, ~6.8

al.12 arrays via AAO cell mA/cm²

Du, AgCu SANP (SAA Galvanic 1 M KOH, flow cell C₂H₄, EtOH ~94 10 min

et al.6 + Ag NPs) replacement cycles, 720

reaction mA/cm²

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 CHARACTERIZATION OF
ELECTROCATALYSTS WITH OPERANDO
TECHNIQUES
 Characterization Approaches for CO₂RR

Ex Situ vs In Situ vs Operando

Operando Techniques
Ex Situ In Situ Operando
• Analysis before/after • Real-time analysis • Real-time analysis
reaction, outside during reaction, during reaction, with
operating conditions. without performance performance data. Operando Techniques Measures Insight
• Post-mortem analysis data. • Mechanism +
• Example: Scanning • Structural tracking. performance insight.
Electron Microscopy • Example: In situ • Example: Operando
Catalyst shape/size changes during
(SEM). Raman Spectroscopy. XAS. LP-EM Real-time imaging in liquid
reaction. 1

Operando XAS Electronic/atomic structure Oxidation state & local structure shifts. 2

Why Operando? Operando XRD Crystal structure Phase changes under reaction. 3

Guides Design: Enables rational improvement in Raman/SERS Surface vibrations Adsorbed intermediates. 4
catalysts and devices
Electrochemical Monitoring
Current, voltage, resistance Activity & surface area loss. 5
No Artefacts: Avoids post-reaction changes (CV, CA, EIS)

Mechanistic Insights: Captures reaction intermediates

and transient phases.

Direct Correlation: Real-time tracking

(1) Wan et al. Front. Chem. 2025, 13, 1525245. (2) Firet et al. J. Mater. Chem. A 2019, 7, 2597–2607. (3) Favaro et al. Proc. Natl. Acad. Sci. USA 2017, 114, 6706–6711. 19
(4) Girotto et al. Ag–Cu Nanocatalyst CO₂RR, 2025. (5) Hori et al. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2309.
Operando EC-Stem of Cu NPs during the CO2RR

@ -0.8V vs. RHE

(Beam dose rate: ~5 e-/nm2s)

• At -16 – 0 s, control experiment suggested no

beam damage.

• At 0 – 20 s, Cu NPs aggregated into 50-100 nm

Cu nanograins.

• At 40-100 s, minor particle coalescence was

observed.

Video Ref: North American Catalysis Society, In Situ & Operando Characterization, YouTube, 2022. Link
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 Case Study Comparison

Aspect Yun et al. (Ex situ)1 Herzog et al. (Operando) 2 Wu et al. (Operando) 3

Techniques IL-TEM, SEM Operando XAS, XRD Fast Operando XAS

Catalyst Silver nanoparticles Ag on Cu₂O nanocubes Defect-rich silver nanocrystals

Focus Morphology changes post- Electronic & structural changes in Real-time defect formation
reaction real-time

Key Findings Catalyst degradation Reversible Cu redox cycling & phase Dynamic defect formation
(aggregation) changes linked to activity

Advantages High-res images, no beam Real-time oxidation and structure Tracks fast atomic changes
damage monitoring

Impact Limited mechanistic insight Informs catalyst design via redox Highlights defect engineering
control

(1) Yun et al. IL-TEM study on Ag nanoparticle degradation during CO₂RR. Electrochim. Acta 2021, 371, 137795.
(2) Herzog et al. Operando study of Ag-decorated Cu₂O nanocubes for CO₂RR. Angew. Chem. Int. Ed. 2021, 60, 7426–7435.
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(3) Wu et al. Fast operando spectroscopy revealing defect generation in Ag nanocrystals for CO₂RR. Adv. Mater. 2021, 33, 2103740.
Yun et al. (Ex situ)

Figure: Ex Situ IL-TEM and SEM of Ag Nanoparticles

(a) Setup of IL TEM; (b–d) IL-TEM images of Ag nanoparticles before
reaction, after 1 hour, and after 6 hour; (e–j) SEM images of Ag catalyst
showing morphology changes with time.

Figure Description:

• Electrochemistry: CO₂ reduction causes changes in catalyst surface

area and activity.

• Observation: Nanoparticles coalesce and agglomerate after

electrolysis.

• Result: Decreased electrochemically active surface area leads to

lower catalytic performance and durability.

Yun et al. IL-TEM study on Ag nanoparticle degradation during CO₂RR. Electrochim. Acta 2021, 371, 137795. 22
Herzog et al. (Operando)

Figure: Operando XAS & XRD of Cu and Ag catalysts

(a, c) Before reaction: Cu in +1 state (Cu₂O), Ag metallic
(b, d) During CO₂RR: Partial reduction of Cu₂O to Cu⁰, Ag redispersion
(e, f) XRD before reaction: Cu₂O peaks present
(g, h) XRD during CO₂RR: Cu₂O peaks disappear, metallic Cu peaks
appear.

Figure Description:

• Electrochemistry: Cu undergoes reversible redox reactions

(Cu⁰ ↔ Cu⁺/Cu²⁺) during CO₂RR

• Observation: Real-time cycling of oxidation states under

applied potential

• Result: Redox state fluctuations directly influence selectivity

and catalyst stability

Herzog et al. Operando study of Ag-decorated Cu₂O nanocubes for CO₂RR. Angew. Chem. Int. Ed. 2021, 60, 7426–7435. 23
Wu et al. (Operando)

Figure: Fast operando spectroscopy of Ag nanocrystals during eCO₂RR

(a) Experimental setup
(b) EXAFS spectra showing changes in Ag–Ag bonds, indicating defect formation
(c) Time evolution of Ag–Ag and Ag–O bonds showing dynamic defect generation
(d, e) HRTEM images of surface defects after CO₂RR
(f) Schematic of Ag(111) surface with atomic disruptions from defects

Figure Description:

• Electrochemistry: Dynamic creation and removal of surface defects influence reaction

sites

• Observation: Low-coordination defect sites form transiently and are highly active for
CO production

• Result: Surface defect dynamics are critical for tuning catalytic activity and product
distribution

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Wu et al. Fast operando spectroscopy revealing defect generation in Ag nanocrystals for CO₂RR. Adv. Mater. 2021, 33, 2103740.
Conclusion

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