The alkaline stability of N‐heterocyclic ammonium (NHA) groups is a critical topic in anion‐exchange membranes (AEMs) and AEM fuel cells (AEMFCs). Here, we report a systematic study on the alkaline stability of 24 representative NHA groups at different hydration numbers (λ) at 80 °C. The results elucidate that γ‐substituted NHAs containing electron‐donating groups display superior alkaline stability, while electron‐withdrawing substituents are detrimental to durable NHAs. Density‐functional‐theory calculations and experimental results suggest that nucleophilic substitution is the dominant degradation pathway in NHAs, while Hofmann elimination is the primary degradation pathway for NHA‐based AEMs. Different degradation pathways determine the alkaline stability of NHAs or NHA‐based AEMs. AEMFC durability (from 1 A cm−2 to 3 A cm−2) suggests that NHA‐based AEMs are mainly subjected to Hofmann elimination under 1 A cm−2 current density for 1000 h, providing insights into the relationship between current density, λ value, and durability of NHA‐based AEMs.
The alkaline stability of N‐heterocyclic ammonium (NHA) groups is a critical topic in anion‐exchange membranes (AEMs) and AEM fuel cells (AEMFCs). Here, we report a systematic study on the alkaline stability of 24 representative NHA groups at different hydration numbers (λ) at 80 °C. The results elucidate that γ‐substituted NHAs containing electron‐donating groups display superior alkaline stability, while electron‐withdrawing substituents are detrimental to durable NHAs. Density‐functional‐theory calculations and experimental results suggest that nucleophilic substitution is the dominant degradation pathway in NHAs, while Hofmann elimination is the primary degradation pathway for NHA‐based AEMs. Different degradation pathways determine the alkaline stability of NHAs or NHA‐based AEMs. AEMFC durability (from 1 A cm−2 to 3 A cm−2) suggests that NHA‐based AEMs are mainly subjected to Hofmann elimination under 1 A cm−2 current density for 1000 h, providing insights into the relationship between current density, λ value, and durability of NHA‐based AEMs.
Improving the utilization of platinum in proton-exchange membrane (PEM) fuel cells is critical to reducing their cost. In the past decade, numerous Pt-based oxygen reduction reaction catalysts with high specific and mass activities have been developed. However, the high activities are mostly achieved in rotating disk electrode (RDE) measurement and have rarely been accomplished at the membrane electrode assembly (MEA) level. The failure of these direct translations from RDE to MEA has been well documented with several key reasons having been previously identified. One of them is the resistance caused by complex mass transport pathways in the MEA. Herein, we improve the proton and oxygen transportations in the MEA by building a thin and uniform distribution of ionomer on the catalyst surface. As a result, a PEM fuel cell design is capable of showing a current density improvement of 38% at the same voltage (0.6 V) under the H 2 /air operation.
Supporting
IrO2 with conductive oxides has proven to
be a practical way to increase the conductivity of the catalyst layer
as well as decrease the anode IrO2 loading in proton exchange
membrane electrolyzers. In this work, we proposed a high-throughput
aerogel synthesis method to fabricate Sn–Sb–Nb ternary
oxide supports. Their thermal stability, conductivity, and acidic
stability were then systematically investigated; the results show
that Nb addition decreases the ternary oxide’s conductivity
by eliminating charge carriers. At the same time, Nb doping improves
the thermal stability and increases the specific surface area of the
ternary oxides; acidic stability is also increased with 5 at % Nb
addition. IrO2 nanoparticles are deposited on selected
oxide aerogels via the Adams fusion method to synthesize 50 wt % IrO2/Sn
n
Sb
m
Nb
l
O
x
catalysts.
The catalytic performance and stability of catalysts with various
supports were compared, revealing a boosted intrinsic activity than
unsupported IrO2. The optimal ternary oxide support employed
in this work was Sn80Sb15Nb5O
x
. Its supported catalyst counterpart has
a mass activity of 467 A g–1 at 1.6 V (vs reversible
hydrogen electrode) and a Tafel slope of 43.43 ± 0.43 mv dec–1. Compared with other Nb doping amounts, 5 at % Nb
catalyst dissolves Ir the least during the oxygen evolution reaction
test, which we ascribed to Nb sacrifice. Moreover, the surface area
of the supporting materials shows more remarkable influence on the
resistance of the catalyst layer than their conductivity, which matters
only when the supported catalysts have an approximate surface area.
This finding also puts forward a strategy for the screening of supporting
materials and provides valuable data for the design and prediction
of supporting materials.
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