There has been considerable research carried out into perovskite-related materials for use as electrode and electrolyte materials in solid oxide fuel cells. Traditional doping strategies to optimize electrolyte materials have involved doping a cation with a similar sized aliovalent cation e.g., LaGaO
3, doped with Sr on the La site and Mg on the Ga site [
1,
2,
3]. This doping strategy introduces oxide ion vacancies into the lattice, which allows the conduction of oxide ions or the incorporation of water to facilitate proton conduction [
1,
3,
4]. Ba(Ce/Zr)O
3 doped with Y on the Ce/Zr site has attracted considerable interest as a high temperature proton conductor in wet atmospheres, due to water incorporation into the oxide ion vacancies according to Equation 1 [
5,
6,
7].
While Y doped BaCeO
3 shows very high proton conductivity a major drawback with this system is its instability towards CO
2, forming BaCO
3 at the operating temperatures of solid oxide fuel cells (500–800 °C) [
8]. Y doped BaZrO
3 can be used as an alternative electrolyte, as it displays improved CO
2 stability, however this system typically exhibits a high grain boundary resistivity [
9,
10]. Recently there has been considerable interest in strategies to increase the grain size in Y doped BaZrO
3, through the use of sintering aids, or in the preparation of mixed Zr/Ce systems to try and resolve these issues [
11,
12]. Another system that has attracted a significant attention is the brownmillerite structured Ba
2In
2O
5 [
13,
14,
15,
16]. This system has a high level of oxygen vacancies and this leads to oxygen vacancy ordering such that there are alternating layers of octahedrally and tetrahedrally coordinated In, leading to a comparatively low ionic conductivity. Doping this system with higher charge aliovalent cations of similar size (e.g., Zr
4+ and Ti
4+ for In
3+) has been shown to introduce disorder onto the oxide lattice, and hence improve the conductivity. An alternative strategy developed in our group to optimize the conductivity of Ba
2In
2O
5 has been to introduce oxyanions (MO
4n−; M = Si, P, S), where the central cation of the oxyanion is located onto the perovskite cation B site, and the oxide ions of this group fill four of the available six oxide ion sites surrounding it (albeit displaced so as to achieve the tetrahedral coordination of the oxyanion group). This strategy was found to enhance oxide ion conduction by creating disorder on the oxide ion sublattice. Further enhancements in the conductivity were observed in wet atmospheres leading to water incorporation and a protonic contribution to the conductivity. Oxyanion doping was also shown to improve the stability of the system to CO
2 atmospheres attributed to a reduction of the basicity of the system with these acidic dopants [
17,
18,
19,
20]. This strategy was then extended to Ba
2Sc
2O
5 and Ba
2Sc
2−yGa
yO
5, with phosphate and sulfate doping showing improved stability to CO
2 [
21,
22]. In this paper, we extend this oxyanion doping strategy to In, Y, Yb doped BaCeO
3 with the hope of improving the CO
2 stability and analyzing the effect on the conductivity. As noted previously by Soares
et al., the effect of P additions in BaCeO
3 is also of relevance, since phosphate esters have been used as dispersants to prevent nanoparticle agglomeration in the preparation of related perovskite materials [
23]. In this previous work, Soares
et al. examined (1 −
x)BaY
0.15Zr
0.85O
2.925:
xP
2O
5 mixtures, and observed the presence of Ba
3(PO
4)
2 impurities, and had a negative effect on the conductivity. The addition of P
2O
5 to an as-prepared single phase BaY
0.15Zr
0.85O
2.925 perovskite, however meant that the examined compositions were effectively A site deficient, if one assume P as a B site dopant (as shown in our previous work on Ba
2(In/Sc)
2O
5), and this can explain the appearance of phosphate impurities. In this work, we have therefore examined cation stoichiometric compositions, assuming the incorporation of P on the B cation site,
i.e., BaCe
1−y−x(Y/Yb/In)
yP
xO
3−y/2+x/2, in order to determine whether some phosphate can be incorporated into the perovskite phase. We evaluate the effect of the trivalent cation dopant level on the incorporation of phosphate, and the resultant effect on conductivity and CO
2 stability.