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Kimitake Kawakami, [Kentaro Yonesato](https://orcid.org/0000-0001-9143-6691), Shimon Akino, [Soichi Kikkawa](https://orcid.org/0000-0002-8193-6374), [Seiji Yamazoe](https://orcid.org/0000-0002-8382-8078), Takuma Kaneko, Tomoya Uruga, [Fumiaki Amano](https://orcid.org/0000-0003-2812-5799), Yoshiaki Honjo, Takaki Hatsui, [Koji Harano](https://orcid.org/0000-0001-6800-8023), [Kazuya Yamaguchi](https://orcid.org/0000-0002-7661-4936), [Kosuke Suzuki](https://orcid.org/0000-0002-8123-1462)

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[Countercation Manipulation Enables Selective Electrochemical CO                    <sub>2</sub>                    ‐to‐Formate Conversion Over Bi‐Polyoxometalate‐Derived Catalysts](https://mdr.nims.go.jp/datasets/303ecaf0-9cb2-4fd8-83d9-c067b265a976)

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Countercation Manipulation Enables Selective Electrochemical CO2‐to‐Formate Conversion Over Bi‐Polyoxometalate‐Derived Catalysts  Chemistry – A European JournalRESEARCH ARTICLE# Countercation Manipulation Enables Selective Electrochemical CO2-to-Formate Conversion Over Bi-Polyoxometalate-Derived CatalystsKimitake Kawakami1Kentaro Yonesato1,2Shimon Akino1Soichi Kikkawa3Seiji Yamazoe3Takuma Kaneko4Tomoya Uruga4Fumiaki Amano5Yoshiaki Honjo6Takaki Hatsui6Koji Harano7,8Kazuya Yamaguchi1Kosuke Suzuki1,21Department of Applied Chemistry, School of Engineering, The University of Tokyo, Tokyo, Japan2Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa-shi, Chiba, Japan3Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Tokyo, Japan4Japan Synchrotron Radiation Research Institute (JASRI), Hyogo, Japan5Department of Applied Chemistry for Environment, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, Tokyo, Japan6RIKEN SPring-8 Center, Hyogo, Japan7Center for Basic Research on Materials, National Institute for Materials Science (NIMS), Tsukuba-shi, Ibaraki, Japan8Research Center for Autonomous Systems Materialogy (ASMat), Institute of Integrated Research, Institute of Science Tokyo, Yokohama-shi, Kanagawa, JapanCorrespondence: Kosuke Suzuki ()Received: 18 February 2026Revised: 29 June 2026Accepted: 3 July 2026Keywords: bismuth electrochemical CO2 reduction reaction electrode catalysts nanocatalysts polyoxometalatesABSTRACT Electrochemical conversion of CO2 into formate (HCOO−) represents a promising strategy for mitigating global warming while producing an industrially important chemical. Polyoxometalates (POMs), anionic molecular metal-oxo clusters, offer unique opportunities as catalysts and catalyst precursors for the electrochemical CO2 reduction reaction (CO2RR) owing to their structural diversity, compositional tunability, and flexibility in countercation selection. Despite these advantages, effective strategies for developing POM-based CO2RR catalysts with high selectivity toward HCOO− remain limited. In this study, we demonstrate countercation manipulation as a powerful approach to achieving selective HCOO− production using a bismuth (Bi)-incorporated POM, \[Bi4O(γ-SiW10O36)2(OAc)\]7− (Bi4). Under gas-diffusion CO2RR conditions with a 2 M KHCO3 aqueous electrolyte, the Ba2+ salt of Bi4 immobilized on carbon (Ba-Bi4/C) exhibited high HCOO− selectivity with a Faradaic efficiency of 84.3% at −0.79 VRHE, whereas the corresponding tetra-n-butylammonium (TBA+) salt (TBA-Bi4/C) exhibited substantially lower selectivity (26.6% at −0.79 VRHE). Post-reaction analyses revealed that both catalysts transformed into highly dispersed Bi species protected by WOx nanoaggregates. Control experiments and detailed characterizations indicated that the countercations, in conjunction with the POM-derived catalyst structure, play a crucial role in governing CO2RR performance.## 1 IntroductionR e m e ecent global warming, caused primarily by anthropogenic missions of carbon dioxide (CO2), poses a serious threat to odern society \[1\]. To mitigate this climate crisis, considerable fforts have been devoted to developing technologies that utilizeCO2 as a chemical feedstock. Among the various CO2 utilization strategies, electrochemical CO2 reduction reaction (CO2RR) is particularly attractive because it enables the conversion of CO2 into value-added chemicals and fuels under mild conditions using renewable electricity \[2, 3\]. Among the possible CO2RR products, formic acid (HCOOH) and its conjugate base,T o his is an open access article under the terms of the Creative Commons Attribution Licen riginal work is properly cited. se, which permits use, distribution and reproduction in any medium, provided the© 2026 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbH.C hemistry – A European Journal, 2026; 32:e71437ksuzuki@appchem.t.u-tokyo.ac.jp \| \| \| \| https://doi.org/10.1002/chem.714371 of 10[https://doi.org/10.1002/chem.71437](https://doi.org/10.1002/chem.71437)[https://orcid.org/0000-0001-9143-6691](https://orcid.org/0000-0001-9143-6691)[https://orcid.org/0000-0002-8193-6374](https://orcid.org/0000-0002-8193-6374)[https://orcid.org/0000-0002-8382-8078](https://orcid.org/0000-0002-8382-8078)[https://orcid.org/0000-0003-2812-5799](https://orcid.org/0000-0003-2812-5799)[https://orcid.org/0000-0001-6800-8023](https://orcid.org/0000-0001-6800-8023)[https://orcid.org/0000-0002-7661-4936](https://orcid.org/0000-0002-7661-4936)[https://orcid.org/0000-0002-8123-1462](https://orcid.org/0000-0002-8123-1462)[mailto:ksuzuki@appchem.t.u-tokyo.ac.jp](mailto:ksuzuki@appchem.t.u-tokyo.ac.jp)[http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)[https://doi.org/10.1002/chem.71437](https://doi.org/10.1002/chem.71437)[http://crossmark.crossref.org/dialog/?doi=10.1002%2Fchem.71437\&domain=pdf\&date\_stamp=2026-07-22](http://crossmark.crossref.org/dialog/?doi=10.1002%2Fchem.71437&domain=pdf&date_stamp=2026-07-22)f i p \[ h l N C c d t o d p T t v h c o o a a m t a f a i s t i s c c b a c d H \[ o s P c a t u a i s a h P o s t p a ormate (HCOO−), are especially appealing. HCOOH is an mportant industrial chemical widely used in agriculture, leather rocessing, food preservation, and pharmaceutical industries 4\], and has recently attracted attention as a promising ydrogen carrier owing to its high stability, low toxicity, ow flammability, and moderate volumetric energy density. otably, the decomposition of HCOOH/HCOO− produces only O2 as a by-product, enabling a closed carbon cycle in which CO2 an be recycled to regenerate HCOOH—a feature that is often ifficult to achieve with other hydrogen carriers \[5\]. In addition, echno-economic analyses suggest that CO2RR-based production f HCOOH is considered economically viable \[6\]. Therefore, the evelopment of efficient CO2RR systems for HCOOH/HCOO− roduction is highly desirable. he catalytic performance of CO2RR is strongly governed by both he catalyst material and the electrolyte composition. Among arious metals, bismuth (Bi)-based catalysts are known to exhibit igh selectivity toward HCOO− while effectively suppressing the ompeting hydrogen (H2) evolution reaction owing to their high verpotential for H2 production \[7, 8\]. In parallel, the identity f electrolyte cations plays a crucial role in determining CO2RR ctivity and selectivity. Larger alkali metal cations, such as Cs+ nd K+, have been reported to enhance CO2RR performance by odulating the interfacial electric field, local pH, and stabiliza- ion of key reaction intermediates \[9–20\]. Although the effects of lkali metal cations have been extensively studied, considerably ewer investigations have focused on multivalent cations, such as lkaline earth metal cations \[13, 15, 17, 20, 21\], despite both exper- mental and theoretical indications that Ba2+ can outperform maller divalent cations such as Ca2+ \[13, 17, 20, 21\]. However, he use of alkaline earth metal cations at high concentration n CO2RR electrolytes—particularly in gas-diffusion electrode ystems—remains challenging, as the formation of insoluble arbonate or hydroxide species can hinder CO2 transport to the atalyst surface \[15\]. These insights suggest that combining Bi- ased catalysts with cations possessing large ionic radii represents promising approach for selective CO2RR. Recently, Chang and oworkers investigated cation effects using Bi nanosheets and emonstrated that larger cations, such as Cs+ and Ba2+, enhance COOH productivity and selectivity in acidic electrolytes (pH 2) 20\]. Nevertheless, a fundamental understanding of cation effects n Bi-based CO2RR catalysts remains limited, and strategies to ystematically exploit these effects are still at an early stage. olyoxometalates (POMs), anionic molecular metal-oxo clusters omposed of multiple {MOx} units (M = W, Mo, V, etc.), re characterized by their structural diversity, compositional unability, and flexibility in countercation selection, as well as nique physicochemical properties such as acid-base property nd multielectron redox property \[22–35\]. In particular, metal- ncorporated POMs enable atomic-level control over active metal pecies and have emerged as promising (pre)catalysts for CO2RR s well as components of CO2RR catalysts \[36–39\]. Our group as recently demonstrated that the catalytic performance of OM-derived CO2RR catalysts is highly sensitive to the identity f countercations, with Ba2+ significantly enhancing activity and electivity for carbon monoxide (CO) production \[40, 41\]. Despite hese advances, POM-based CO2RR catalysts that efficiently roduce HCOOH/HCOO− remain scarcely explored (Tables 1 nd S1) \[42–45\].image FIGURE 1 Preparation of POM-derived Bi catalysts and their application in CO2RR. (a) Synthetic method and the structure of Bi4 (\[Bi4O(γ-SiW10O36)2(OAc)\]7−). (b) Schematic illustration of the catalyst preparation and CO2RR using TBA-Bi4/C and Ba-Bi4/C.In this work, we demonstrate that the rational combination of a Bi-incorporated POM and a Ba2+ countercation provides an effective strategy for constructing HCOO−-selective CO2RR catalysts (Figure 1). A Bi-incorporated POM, Bi4 (\[Bi4O(γ- SiW10O36)2(OAc)\]7−, Figure 1a) \[46\], was synthesized by incorporating Bi3+ ions into a divacant lacunary POM, SiW10 (\[γ-SiW10O34(H2O)2\]4−) and subsequently immobilized on a carbon support (Figure 1b). The effect of countercation identity was investigated by comparing tetra-n-butylammonium (TBA+), Cs+, Mg2+, and Ba2+ salts. CO2RR experiments using a gas-fed electrolyzer revealed that the Ba2+-containing catalyst exhibited markedly enhanced HCOO− production compared with its TBA+ analogue (Figure 1b). Through a combination of electrochemical analyses, x-ray absorption fine structure analyses, and electron microscopy, we show that the Bi-incorporated POM serves as a molecular precursor that undergoes in situ transformation under CO2RR conditions to generate highly dispersed Bi species stabilized by WOx nanoaggregates, while the Ba2+ countercation plays a crucial role in directing the reaction selectivity. This study establishes countercation manipulation as a powerful approach for achieving efficient and selective electrochemical HCOO− production.## 2 Results and Discussion### 2.1 Preparation and Characterization of the CatalystsCarbon-supported Bi-incorporated POM catalysts were prepared by adapting a previously reported impregnation method for15213765, 2026, 40, D ownloaded from  https://chemistry-europe.onlinelibrary.w iley.com/doi/10.1002/chem .71437 by Koji H arano - National Institute For , W iley Online L ibrary on \[28/10/2026\]. See the Term s and Conditions (https://onlinelibrary.w iley.com/term s-and-conditions) on Wiley O nline Library for rules of use; O Aarticles are governed by the applicable C reative C omm ons License2of 10Chemistry – A European Journal, 2026|Catalyst|Electrolyte|Potential|FEHCOOH/HCOO− (%)|jHCOOH/HCOO− (mA cm−2)|Reference||---|---|---|---|---|---||Ba-Bi4/C|2 M KHCO3|−0.79 VRHE|84.3|33.5|This work||−0.87 VRHE|68.3|63.9||Bi-PMo nanosheets|0.5 M NaHCO3|−0.86 VRHE|93 ± 2|ca. 31|\[42\]||−1.16 VRHE|ca. 67|ca. 39||Indium sheet|0.1 M Na2SO4 containing 2 mM SiW9V3|−1.2 VAg/AgCl (−0.76 VRHE)|\<5|\<0.2|\[43\]||−1.6 VAg/AgCl|56.82|ca. 1.9||−1.7 VAg/AgCl|ca. 42|ca. 2.6||Cu9S5 single-unit-cell nanowiresa|0.1 M KHCO3|−0.8 VRHE|82.0|ca. 2.5|\[44\]||−0.9 VRHE|57.1|3.20||TBA3H\[(Ru(dmbpy)(ppt)Cl)2P2W17O57\]b|0.1 M TBAPF6- acetonitrile containing acetic acid|−1.56 VNHE|67|—|\[45\]|TABLE 1 Summary of POM-based CO2RR catalysts exhibiting large FEHCOOH/HCOO− (\> 50%).            aNanowires composed of Cu9S5 unit cell and PW12. bdmbpy = 4,4’-dimethyl-2,2’-dipyridyl, ppt = 4’-(4-phosphonophenyl)-2,2’:6’,2’’-terpyridine.  P f T B v t B T c ( e a t t E i o i e c B s a i \[ i m a b I s d t OM-derived electrocatalysts \[41\]. TBA-Bi4 (Figure 1a) \[46\] was irst immobilized on a carbon support (Vulcan XC 72R), affording BA-Bi4/C (Figure 1b). Subsequently, the countercation of i4 was exchanged from TBA+ to Ba2+, Mg2+, or Cs+ by igorously stirring TBA-Bi4/C in an acetone solution containing rifluoromethanesulfonate salts of Ba2+, Mg2+, or Cs+, yielding a-Bi4/C, Mg-Bi4/C, or Cs-Bi4/C, respectively (Figure 1b). o investigate the structure of Bi4 after immobilization and ation exchange, Bi and W L3-edge x-ray absorption fine structure XAFS) measurements were performed (Figure 2). At the Bi L3- dge, both TBA-Bi4/C and Ba-Bi4/C exhibited extended x-ray bsorption fine structure (EXAFS) oscillation patterns similar o that of TBA-Bi4, indicating preservation of the Bi coordina- ion environment (Figure 2a). Notably, the Fourier-transformed XAFS spectrum of TBA-Bi4/C displayed an increased peak ntensity at approximately 1.7 Å (Bi─O bond) relative to that f TBA-Bi4, whereas Ba-Bi4/C exhibited a slightly lower peak ntensity than TBA-Bi4/C (Figure 2b). Similarly, at the W L3- dge, the EXAFS oscillation patterns of TBA-Bi4/C and Ba-Bi4/C losely resembled those of TBA-Bi4 (Figure 2c). While TBA- i4 and TBA-Bi4/C showed similar Fourier-transformed EXAFS pectra, Ba-Bi4/C showed a higher intensity for the peak at pproximately 1.3 Å (terminal W═O bond) \[47\] and a lower ntensity for the peak at approximately 1.7 Å (W─O(─W) bond) 47\] compared with those of TBA-Bi4/C (Figure 2d). These results ndicated that immobilization and cation exchange induce only inor perturbations in the local coordination environment of Bi nd W, while the overall Bi4 structure remains largely intact in oth TBA-Bi4/C and Ba-Bi4/C. nfrared (IR) spectroscopy was further employed to assess the tructural integrity of Bi4 in TBA-Bi4/C and Ba-Bi4/C. However, ue to strong absorption from the carbon support, the charac- eristic IR absorption bands of Bi4 could not be observed forimage FIGURE 2 XAFS measurements of TBA-Bi4/C and Ba-Bi4/C. (a) k3-weighted Bi L3-edge EXAFS oscillation. (b) Fourier-transformed Bi L3- edge EXAFS spectra (k = 3–11 Å−1). (c) k3-weighted W L3-edge EXAFS oscillation. (d) Fourier-transformed W L3-edge EXAFS spectra (k = 3–13 Å−1).TBA-Bi4/C and Ba-Bi4/C. Therefore, IR spectra of unsupported TBA-Bi4 and Ba-Bi4 were measured. As shown in Figure S1, Ba-Bi4 exhibited characteristic absorption bands in the range of 300–1000 cm−1 that closely resemble those of TBA-Bi4. Impor- tantly, no bands attributable to TBA+ were observed for Ba-Bi4 in15213765, 2026, 40, D ownloaded from  https://chemistry-europe.onlinelibrary.w iley.com/doi/10.1002/chem .71437 by Koji H arano - National Institute For , W iley Online L ibrary on \[28/10/2026\]. See the Term s and Conditions (https://onlinelibrary.w iley.com/term s-and-conditions) on Wiley O nline Library for rules of use; O Aarticles are governed by the applicable Creative C omm ons LicenseChemistry – A European Journal, 20263 of 10t e B r s E B 4 S o d e he range of 1350–1500 cm−1, indicating successful countercation xchange without significant structural change to Bi4. Because a-Bi4 was prepared using the same protocol as Ba-Bi4/C, these esults support effective replacement of TBA+ with Ba2+ in the upported catalyst. lemental analysis further confirmed these findings (Table S2). oth TBA-Bi4/C and Ba-Bi4/C exhibited Bi:W ratios close to :20, consistent with the molecular composition of Bi4 (\[Bi4O(γ- iW10O36)2(OAc)\]7−). Moreover, Ba-Bi4/C showed a Ba:W ratio f 3.5:20. Considering the heptaanionic charge of Bi4 and the ivalent charge of Ba2+, this ratio supports nearly complete xchange of TBA+ with Ba2+.### 2 .2 CO2RR StudyT a t a B a t d l m C c a s g i h F a t p c B t s B w s 9 s p B a S a p m p e H r P p he CO2RR performance of the catalysts was evaluated using gas-fed electrolyzer equipped with a gas-diffusion elec- rode (Figure S2). In addition to the Bi4-derived catalysts, Ba2+ salt of SiW10 (\[γ-SiW10O34(H2O)2\]4−, a precursor of i4) supported on carbon (Ba-SiW10/C) was prepared as control sample (see Figure S3 and Table S2 for charac- erization). The catalyst-loaded electrodes were fabricated by rop-casting catalyst inks onto carbon fiber-based gas-diffusion ayer (Sigracet 39 BB, see Supporting Information for detailed ethods). O2 electrolysis was conducted at approximately −0.8 VRHE (iR- orrected) for 1 h under 80 mL min−1 CO2 flow with 2 M KHCO3 queous solution. Gaseous products (CO and H2) and the water- oluble product (HCOO−) in the catholyte were quantified by as chromatography and 1H NMR, respectively. As summarized n Figures 3a and S4a, and Table S3, Ba-Bi4/C exhibited the ighest partial current density (jHCOO− = 33.5 mA cm−2) and aradaic efficiency (FEHCOO− = 84.3%) for HCOO− production mong all catalysts tested. No CO or HCOO− was detected when he bare carbon support was used or when electrolysis was erformed under an Ar atmosphere using Ba-Bi4/C (Table S4), onfirming that these products originate from CO2RR. Notably, a-Bi4/C exhibited a jHCOO−value comparable to or higher than hose of previously reported POM-based CO2RR catalysts at imilar applied potentials (Table 1). In contrast, Mg-Bi4/C, Cs- i4/C, and TBA-Bi4/C exhibited lower CO2RR performance, ith TBA-Bi4/C predominantly producing H2 and showing ignificantly lower HCOO− productivity and selectivity (jHCOO− = .6 mA cm−2, FEHCOO− = 26.6%), highlighting the importance of electing appropriate countercations to achieve superior CO2RR erformance. Bismuth(III) acetate (Bi(OAc)3), a precursor of i4, exhibited an even lower jHCOO− value of 6.1 mA cm−2, lthough its FEHCOO− was moderate (70.4%) (Figures 3b and 4b and Table S3). In addition, a physical mixture of Bi(OAc)3 nd Ba-SiW10/C (Ba-SiW10/C+Bi(OAc)3) showed low CO2RR erformance. We also evaluated the CO2RR performance of onolayer bismuth tungstate (Bi2WO6), known for efficiently roducing HCOO− \[48\], and bulk Bi2WO6 \[49\], but both samples xhibited substantially lower productivity and selectivity for COO− production (Figures 3b and S4b and Table S3). These esults underscore the critical role of Bi incorporation into the OM framework in achieving high HCOO−-selective CO2RR erformance.image FIGURE 3 CO2RR performance of POM-derived catalyst-loaded electrodes with 2 M KHCO3 flow: FE (left axis) and jHCOO− (right axis). (a) Bi4/C possessing various countercations at a constant potential around −0.8 VRHE for 1 h. (b) Ba-Bi4/C and other benchmark catalysts at a constant potential around −0.8 VRHE for 1 h. (c) Ba-Bi4/C at different potentials for 1 h. (d) Ba-Bi4/C at −0.82 VRHE for 9 h. All potentials were iR-corrected.Potential-dependent measurements revealed a volcano-type rela- tionship between HCOO− selectivity and the applied potential for Ba-Bi4/C (Figures 3c and S4c and Table S5), with a maximum FEHCOO− of 84.3% observed at −0.79 VRHE. At more negative potentials, the jHCOO− increased further, reaching 63.9 mA cm−215213765, 2026, 40, D ownloaded from  https://chemistry-europe.onlinelibrary.w iley.com/doi/10.1002/chem .71437 by Koji H arano - National Institute For , W iley Online L ibrary on \[28/10/2026\]. See the Term s and Conditions (https://onlinelibrary.w iley.com/term s-and-conditions) on Wiley O nline Library for rules of use; O Aarticles are governed by the applicable C reative C omm ons License4of 10Chemistry – A European Journal, 2026a f t w B N d i k d r f w a u r n t t t t t −0.87 VRHE, representing the highest value reported to date or POM-based CO2RR catalysts (Table 1). Tafel analysis revealed hat the Tafel slope values in the low overpotential region ere 55, 54, 63, and 66 mV dec−1 for Ba-Bi4/C, TBA-Bi4/C, a-SiW10/C+Bi(OAc)3, and Bi(OAc)3, respectively (Figure S5). otably, these values are close to the theoretical value of 59 mV ec−1, indicating that the protonation of \*CO2 − to form \*OCHO s the rate-limiting step across these samples \[42, 50–52\]. This inetic similarity suggested that the enhanced HCOO− pro- uctivity of Ba-Bi4/C cannot be attributed to a change in the ate-limiting step but to other factors (as discussed in detail in the ollowing sections). Subsequently, long-term CO2RR electrolysis as conducted at −0.82 VRHE for 9 h using Ba-Bi4/C (Figures 3d nd S4d,e and Table S6). Although the selectivity to H2 grad- ally increased during the prolonged electrolysis, the FEHCOO− emained above 70% even after 9 h, and the jHCOO− remained early constant throughout the experiment. In addition, although he slight fluctuations were observed during the initial 4 h, the otal current density stabilized at approximately 40 mA cm−2 hereafter, demonstrating the robust electrochemical stability of he catalyst.### 2 E .3 Characterization of a Ba-Bi4/C-Loaded lectrode After CO2RRB e W b 1 s t s r v ( a a t f t T C e S w t ( d F o C p B w T s ased on the above results, we next discuss the structural volution of Ba-Bi4/C under CO2RR conditions. e first performed XAFS measurements on Ba-Bi4/C electrodes efore and after immersion in the catholyte (2 M KHCO3) for h to examine the effect of catholyte contact on the electronic tate and the structure of Bi4. As displayed in Figure 4, both elec- rodes exhibited x-ray absorption near edge structure (XANES) pectra (Figure 4a) and EXAFS oscillation patterns that closely esemble those of pristine Ba-Bi4/C (Figure 4b,d), although slight ariations in the peak intensities associated with the Bi─O bond peak around 1.7 Å, Figure 4c), the terminal W═O bond (peak round 1.3 Å, Figure 4e) \[47\], and the W─O(─W) bond (peak round 1.7 Å, Figure 4e) \[47\] were observed. These results suggest hat, although minor perturbations occurred during electrode abrication and catholyte contact, the overall electronic state and he molecular structure of Bi4 were largely retained. o identify the catalytically active Bi species generated under O2RR conditions, cyclic voltammetry (CV) of the Ba-Bi4/C lectrode after CO2RR at −0.8 VRHE for 1 h was conducted (Figure 6). A single cathodic peak appeared in the range of 0.1–0.2 VRHE, hich can be assigned to the reduction of Bi3+ to Bi0 \[53\]. Given hat CO2RR was conducted at much more negative potentials approximately −0.8 VRHE), Bi0 species were likely generated uring electrolysis and served as the truly catalytically active sites. urther insight into the post-reaction catalyst structure was btained by XAFS measurements of the Ba-Bi4/C electrode after O2RR (Figure 4). The Bi L3-edge XANES spectrum of the ost-reaction electrode closely overlapped with those of pristine a-Bi4/C and the electrode before the reaction, indicating that Bi as predominantly present as Bi3+ after the reaction (Figure 4a). his observation does not contradict the CV results, as the ex itu XAFS measurements were performed after exposure of theimage FIGURE 4 XAFS measurements of Ba-Bi4/C-loaded electrodes. (a) Bi L3-edge XANES spectra. (b) k3-weighted Bi L3-edge EXAFS oscillation. (c) Fourier-transformed Bi L3-edge EXAFS spectra (k = 3–11 Å−1). (d) k3- weighted W L3-edge EXAFS oscillation. (e) Fourier-transformed W L3- edge EXAFS spectra (k = 3–13 Å−1).electrode to air, which can readily oxidize metallic Bi. At the W L3- edge, the EXAFS oscillation pattern of the electrode after CO2RR more closely resembled that of WO3 than that of the electrode before the reaction; however, clear differences were observed in the k range of 7–9 Å−1 and in the Fourier-transformed spectra around 3–4 Å (Figure 4d,e). These features suggest the formation of WOx species distinct from bulk WO3 during CO2RR. To further elucidate the nanoscale structure of the catalyst after CO2RR, electron microscopy analysis was performed. We initially carried out atomic-resolution transmission electron microscopy (TEM) observation of the Ba-Bi4/C electrode after a 1 h CO2RR (Figure S7). However, the detailed analysis of catalyst morphol- ogy was difficult due to the low contrast between the carbon support and the catalyst. High-angle annular dark-field scan- ning transmission electron microscopy (HAADF-STEM) image (Figure 5a) and the corresponding STEM energy-dispersive X- ray spectroscopy (EDS) mappings (Figure 5b–e) suggest the formation of WOx nanoaggregates within which Bi species are15213765, 2026, 40, D ownloaded from  https://chemistry-europe.onlinelibrary.w iley.com/doi/10.1002/chem .71437 by Koji H arano - National Institute For , W iley Online L ibrary on \[28/10/2026\]. See the Term s and Conditions (https://onlinelibrary.w iley.com/term s-and-conditions) on Wiley O nline Library for rules of use; O Aarticles are governed by the applicable C reative C omm ons LicenseChemistry – A European Journal, 20265 of 10image FIGURE 5 STEM images of Ba-Bi4/C-loaded electrode after a 1 h CO2RR. (a) HAADF-STEM image and (b–e) STEM-EDS mappings (Bi, magenta; W, cyan; Ba, yellow).h c S h u r s l s T t p ighly dispersed. Although some Bi species formed nanoparti- les within WOx nanoaggregates (Figure S8), low-magnification TEM images also verified that most of the Bi species were ighly dispersed (Figure S9). Liu and coworkers reported that, nder CO2RR conditions, monolayer bismuth tungstate (Bi2WO6) econstructs to form metallic Bi stabilized by tungsten oxide pecies, as revealed by in situ characterization techniques \[48\]. In ight of this report, it is reasonable to conclude that, in the present ystem, Bi species are likewise stabilized by WOx nanoaggregates. aken together, the CV, XAFS, and STEM results demonstrate hat Ba-Bi4/C is transformed into highly dispersed Bi species rotected by WOx nanoaggregates under CO2RR conditions.### 2 P .4 Effect of Countercations on CO2RR erformanceA t w p s shown in the former section, TBA-Bi4/C exhibited substan- ially lower CO2RR performance than Ba-Bi4/C. In this section, e discuss the role of countercation in determining the CO2RR erformance.Post-reaction characterization of the TBA-Bi4/C electrode after CO2RR at −0.8 VRHE was first conducted. CV measurement revealed a cathodic peak in the range of 0.1–0.2 VRHE correspond- ing to the reduction of Bi3+ to Bi0, indicating that Bi0 is also generated as the active species under CO2RR conditions (Figure S6). XAFS analysis further clarified that the Bi L3-edge XANES spectra and W L3-edge EXAFS oscillation patterns of TBA- Bi4/C and Ba-Bi4/C electrodes after the reaction were largely comparable, with only minor differences observed in the Fourier- transformed W L3-edge EXAFS spectra (Figure S10). Consistent with these results, STEM analysis revealed the formation of highly dispersed Bi species protected by WOx nanoaggregates on the TBA-Bi4/C electrode after the reaction (Figure S11). These observations suggest that both catalysts underwent similar structural transformations during CO2RR. Elemental analysis was performed to quantify the Bi content remaining on the electrodes after reaction. As summarized in Table S7, although initial Bi loadings were nearly identical for both samples, the Ba- Bi4/C electrode after the reaction exhibited a slightly lower Bi loading than the TBA-Bi4/C electrode after the reaction. This result suggests that differences in Bi loading are unlikely to be the primary origin of the large performance difference. It has been reported that electron-rich Bi-based CO2RR catalysts tend to exhibit higher CO2RR performance, as electron-rich Bi sites facilitate either the reduction of \*OCHO intermediates or the formation of \*CO2 − intermediates \[54, 55\]. To assess the electronic state of Bi, CV measurements were performed on TBA-Bi4/C and Ba-Bi4/C electrodes after the reaction (Figure S6). Notably, the reduction peak of Bi3+ for the TBA-Bi4/C electrode appeared at a potential approximately 10 mV more negative than that for the Ba-Bi4/C electrode. Chen and coworkers reported a correlation among the reduction potential of Bi3+, the electronic state of Bi, and the CO2RR performance, demonstrating that more electron- rich Bi species exhibit more negative reduction potentials and higher CO2RR activity \[54\]. Accordingly, TBA-Bi4/C appears to generate slightly more electron-rich Bi species, consistent with our previous report \[40\]. This difference may originate from countercation-dependent electron-donating properties of the WOx nanoaggregates. Importantly, despite possessing slightly more electron-rich species, TBA-Bi4/C exhibited inferior CO2RR performance. This finding indicates that electronic effects cannot account for the enhanced CO2RR performance of Ba-Bi4/C. We therefore attribute the superior performance of Ba-Bi4/C primarily to cation effects. Although the origin of cation effects in CO2RR remains under debate, three main contributions have been proposed: (a) modulation of the electric field near the electrode surface; (b) buffering of the local pH near the electrode surface; and (c) stabilization of reaction intermediates \[56–59\]. While it is difficult to unambiguously identify the dominant factor in the present system, it is reasonable to conclude that Ba2+-induced cation effects play a critical role in enhancing the CO2RR performance of Ba-Bi4/C.### 2.5 Effect of Catalyst Structure on CO2RR PerformanceAs described in the CO2RR Study Section, Bi(OAc)3 exhibited significantly lower CO2RR performance than Ba-Bi4/C. In this15213765, 2026, 40, D ownloaded from  https://chemistry-europe.onlinelibrary.w iley.com/doi/10.1002/chem .71437 by Koji H arano - National Institute For , W iley Online L ibrary on \[28/10/2026\]. See the Term s and Conditions (https://onlinelibrary.w iley.com/term s-and-conditions) on Wiley O nline Library for rules of use; O Aarticles are governed by the applicable C reative C omm ons License6of 10Chemistry – A European Journal, 2026s C A d t i t t a t 2 s d t w t o C f s i f H e o r a i l t S B B l B e ection, we discuss the influence of the catalyst structure on O2RR performance. fter CO2RR at −0.8 VRHE for 1 h, CV analysis of the Bi(OAc)3- erived electrode revealed a reduction peak corresponding to he conversion of Bi3+ to Bi0 in the range of 0.0–0.3 VRHE, ndicating that Bi0 species also serve as the active species in his system (Figure S6). Ex situ XAFS measurement suggested hat in situ-generated Bi0 species were fully oxidized to Bi3+ fter the reaction (Figure S12). TEM analysis further revealed he formation of Bi nanoparticles with an average diameter of .9 ± 1.2 nm after the reaction (Figure S13). These outcomes all uggest that Bi(OAc)3 is transformed into small Bi nanoparticles uring the reaction. Elemental analysis (Table S7) showed that he Bi content remaining on the Bi(OAc)3 electrode after CO2RR as approximately twice that on the Ba-Bi4/C electrode after he reaction, suggesting that the inferior CO2RR performance bserved for Bi(OAc)3 may not stem from lower Bi loading. V measurements comparing the electronic states of Bi species ormed on Bi(OAc)3 and Ba-Bi4/C electrodes after reaction howed that the reduction peak for Ba-Bi4/C shifted by approx- mately 10 mV to a more negative potential, suggesting the ormation of slightly more electron-rich Bi species (Figure S6). owever, given the small magnitude of this shift, electronic ffects are unlikely to be the dominant factor governing the bserved performance difference. Instead, we attribute the supe- ior performance of Ba-Bi4/C primarily to its ability to generate nd stabilize highly dispersed Bi species. The electrochemical mpedance spectroscopy at −0.8 VRHE revealed that the cata- yst derived from Ba-Bi4/C exhibited significantly lower charge ransfer resistance than that derived from Bi(OAc)3 (Figure 14). The close proximity between Bi and {WO6} units in Ba- i4/C facilitates the formation of isolated or highly dispersed i species stabilized by WOx nanoaggregates, whereas Bi(OAc)3, acking such protection by WOx species, readily aggregates into i nanoparticles. As a consequence, Ba-Bi4/C showed faster lectron transfer kinetics and higher CO2RR performance.## 3 ConclusionI p m ( i a t a B d 3 V s p 6 a d W a n this study, we demonstrated that countercation manipulation rovides a facile and powerful strategy for tuning the perfor- ance of POM-based electrocatalysts for CO2RR. A bismuth Bi)-incorporated POM, \[Bi4O(γ-SiW10O36)2(OAc)\]7− (Bi4), was mmobilized on a carbon support via an impregnation method to fford TBA-Bi4/C, followed by countercation exchange with Ba2+ o yield Ba-Bi4/C. When evaluated as gas-diffusion electrodes in gas-fed CO2 electrolyzer using 2 M KHCO3 aqueous electrolyte, a-Bi4/C exhibited markedly enhanced formate (HCOO−) pro- uction, achieving a high partial current density (jHCOO− = 3.5 mA cm−2) and Faradaic efficiency (FEHCOO− = 84.3%) at−0.79 RHE. In contrast, TBA-Bi4/C showed significantly lower HCOO− electivity (FEHCOO− = 26.6% at −0.79 VRHE), while Bi(OAc)3, a recursor of Bi4, exhibited lower HCOO− productivity (jHCOO− = .1 mA cm−2). Post-reaction analyses revealed that both Ba-Bi4/C nd TBA-Bi4/C underwent in situ structural reconstruction uring CO2RR to form highly dispersed Bi species stabilized by Ox nanoaggregates, whereas Bi(OAc)3 was transformed into ggregated Bi nanoparticles. Systematic control experiments andcharacterizations highlighted the critical roles of both counter- cation identity and molecular structure of the precatalyst in achieving high CO2RR performance, underscoring countercation manipulation as a key design parameter. Given that HCOO−-selective POM-based CO2RR catalysts remain scarce and the role of cations in Bi-based CO2RR catalysis is still not fully understood, this work provides new insights into cation effects and establishes a generalizable strategy for designing efficient POM-based CO2RR catalysts. We anticipate that the concept of countercation engineering demonstrated here will inspire further development of molecularly defined electrocatalysts for selective and efficient CO2 conversion.## AcknowledgmentsThis work was financially supported by the JSPS KAKENHI (Grant JP24K01448, JP24H00463, JP24H02210, JP24H02217, JP22H04971, JP23H04874) and the JST FOREST (JPMJFR213M). A part of this work was supported by Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM) (JPMXP1225NM0211, JPMXP12 25UT0269) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). A part of this study was supported by Advanced Engineering and Services Division in National Institute for Materials Science (NIMS). The authors thank Dr. Fumihiko Uesugi (NIMS), Mr. Kouichi Okudaira (NIMS), Mr. Hiroyuki Oshikawa (The University of Tokyo), and Ms. Mari Morita (The University of Tokyo) for the assistance with TEM, HAADF-STEM, and EDS analyses. XAFS measurements using a CITIUS detector were performed at BL36XU in SPring-8 under an approved SACLA/SPring-8 Basic Development Program proposal (No. 202407 and No. 202505).## FundingJapan Science and Technology Agency. 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Supporting File 1: chem71437-sup-0001-SuppMat.pdf The authors have cited additional references within the Supporting Information \[60–96\]. The Supporting Information includes experimental section and supplementary figures and tables.15213765, 2026, 40, D ownloaded from  https://chemistry-europe.onlinelibrary.w iley.com/doi/10.1002/chem .71437 by Koji H arano - National Institute For , W iley Online L ibrary on \[28/10/2026\]. 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