proved做定语吗?即是否能这么改写:
“the hydrogen itself was proved too expensive.” or
"the hydrogen is too expensive which was proved."
but后是另外一个分句,一个完整的句子,因为but是连接词。所以proved是分句的谓语动词,storing and supplying the hydrogen itself 是此分句的主语,ing形式做主语。